Introduction to ionospheric heating at Tromsa-I

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md
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Vol. 55, No. 4/S, pp. 577-599, 1993.
Introduction to ionospheric heating at Tromsa-I.
Experimental overview
M. T. RIETVELD,* H. KOHL,? H. KOPKA~ and P. STUBBE?
*EISCAT, N-9027 Ramfjordbotn, Norway;
tMax-Planck-Institut fiir Aeronomie, D-341 1 Katlenburg Lindau, Germany
(Received in final form 2 April 1992 ; accepted 2 June 1992)
HF ionospheric modification (heating) facility at Ramfjordmoen will become a part of the
EISCAT association from January 1993. This paper, which is intended for the new user, describes the
technical capabilities of the facility and the broad range of geophysical and plasma physical experiments
which are possible. An overview is presented of the physical effects that a powerful HF electromagnetic
wave incident on the ionosphere can produce on timescales ranging from tens of microseconds to minutes
in height regions ranging from 50 to hundreds of km. Emphasis is placed on the practical implementation
of ionospheric heating experiments using the EISCAT incoherent scatter radars as the main diagnostic,
but other diagnostic techniques using ground-based radars, radio links, radio receivers, photometers,
rocket and satellite instrumentation are also described. A companion paper presents in greater depth some
Abstract-The
of the current
scientific issues being addressed
in ionospheric
1. INTRODUCTION
modification
research.
power waves on the ionosphere. The papers by KOHL
et al. (1993, this issue), ROBINSON (1989a), THIDB
(1989), STUBBE et al. (1992) provide more detailed
discussions of some of the outstanding scientific problems. The papers by STUBBEet al. (1985), ROBINSON
(1989b) provide good reviews of much of the work
already done at Tromss. The emphasis here is on high
latitude heating so that the results from other facilities
are only briefly referred to.
The heating facility at Ramfjordmoen
near Tromsa,
Norway (69.6’N, 19.2”E, L = 6.2, magnetic dip angle
I = 78”), was built by the Max-Planck-Institut
fiir
Aeronomie
in co-operation
with the University
of
TromsB. Experiments
have been performed
since
1980, resulting in over 70 publications
and at least
10 masters and doctoral theses. Much of this work
resulted
from collaborations
between
the MaxPlanck-Institut
fiir Aeronomie and other groups. The
experiments
have ranged from deducing ionospheric
and geophysical parameters
by perturbing the ionosphere in various ways, to fundamental
plasma
physics.
After an offer to EISCAT from the Max-PlanckInstitut fi.ir Aeronomie,
the scientific rationale for
taking over the facility and continuing
heating
research was made by a working group and presented
to the EISCAT council by ROBINSON (1989a). As a
result the EISCAT council decided that from January
1993 the Heating facility would be owned and run by
the EISCAT association.
This paper is intended as an introduction
to heating
science for the new user. Section 2 provides a technical
description
of the facility, Section 3 introduces
the
variety of physical effects which a powerful HF wave
has on the ionosphere,
and Section 4 describes some
of the diagnostic
techniques
used to detect these
effects. A historical overview of HF ionospheric modification research
can be found in GORDON and
DUNCAN (1990). The book by GIJREVICH (1978)
describes much of the physics of the effects of high
2. TECHNICAL DESCRIPTION OF THE
HEATING FACILITY
In this section we summarize the important technical parameters as of Summer 1990. Earlier descriptions by STLJBBEand KOPKA (1979) and STUBBEet al.
(1982a) are thereby updated.
2.1. Transmitters
The facility generates up to 1.2 MW of CW power
in the frequency range from 3.85 to 8 MHz from 12
linear class AB tetrode amplifiers (100 kW) each
driven by a solid state wideband exciter (1.5 kW), so
that only the transmitter output stage has to be tuned and
matched. Although originally specified to be 125 kW,
each transmitter
actually has a maximum
output
power of 100 kW. The output pi-L circuit (see Fig. 1)
is designed for a VSWR < 2:1, output impedance
50 R and 40 dB harmonic suppression.
The tuning
and matching are done automatically,
using a phase
discriminator
between grid and anode voltage and a
577
578
M. T. RIETVELV et al.
llkV!lZ5A
1200WZA
dir.
refl.
paver power
Fig. 1. Schematic of one of the 12 transmitters.
voltage discriminator
between the output and a part
of the anode voltage.
The overall level of harmonic
radiation
is suppressed much more than 40 dB compared to the fundamental,
because
of the antenna
impedance.
Measurements
near the antenna arrays indicate that
the second harmonic is about 55 dB and the third
harmonic
is about 73 dB below the fundamental.
Fifty Hz mains hum in sidebands of the heater frequency is also suppressed by about 55 dB.
The RF source for each transmitter is generated by a
Hewlett Packard HP 3325 A frequency synthesizer/
function generator
(Fig. 3a,b). The 12 synthesizers
are locked to a common frequency reference which
is locked to the EISCAT cesium clock. Each of the
12 synthesizers
can be programmed
in frequency,
amplitude
and phase over an HP-IB bus from a
Commodore
PET microcomputer.
Because of the
close coupling between the antennas, the process of
tuning and matching is done iteratively, with the microcomputer adjusting the amplitude and phase of the
RF to one transmitter
while all the others are kept
constant and then repeating this for all transmitters.
The process converges after about 4-5 steps, which
means a few minutes. Figure 1 shows a schematic of
a transmitter.
A pair of transmitters
feeds a row of crossed pairs
of linear full-wave dipole antennas with one linear
antenna of each pair being fed by one transmitter
and the orthogonal linear antenna fed from the other
transmitter.
The 90” phase shift between the two
halves of each row (to give circular polarization)
is
made at the transmitter
input, that is, at the synthesizer stage. Similarly, pointing the antenna beam
is done by appropriately
adjusting the phases between
pairs of transmitters.
Apart from tuning all the transmitters
to one frequency, it is possible to tune 2 frequencies in groups
of 6 transmitters,
3 frequencies
in groups of 4,
or 6 frequencies in groups of 2 transmitters
while retaining
circular
polarization.
For linear polarization 12 different frequencies
can be transmitted
simultaneously.
2.2. Antennas and transmission lines
There are three antenna arrays which were originally designed to cover the frequency range 2.758 MHz.
Arrays 2 and 3 each contain 6 rows of 6 crossed
dipoles (see Fig. 2) to cover the frequency ranges 3.855.65 and 5.5-8 MHz, respectively,
with a gain of
24 dB (5 1 dB, dependent on frequency). This produces a beamwidth of 14.5” and a maximum effective
radiated power (ERP) of 300 MW. Each full-wave
dipole is rhombically
broadened
(see Fig. 2) in the
horizontal plane, and is a quarter wavelength above
Ionospheric
heating
519
at Trams-I
Array-l
(5.5-8 MHz)
El
12Tx
Array-2
(3.85565
MHz)
Fig. 2. Schematic of the antenna arrays showing how a row of crossed dipole antennas is driven by two
transmitters.
At the left is shown an individual crossed dipole antenna in greater detail.
the ground at the centre frequency of each array. Each
row of dipoles connected to a transmitter is East-West
oriented so that the beam can be tilted in the NorthSouth plane by changing the phase between rows.
The second array did have additional phased lengths
which could be connected between antennas within a
row to allow the beam to be directed westward
towards Andlaya rocket range (ROSEet al., 1985) but
these have since been removed. The antennas were
designed to have a standing wave ratio of less than 2,
and in fact they are normally less than 1.5.
The 50 R coaxial transmission line from each transmitter is fabricated from a 10 cm diameter aluminium
tube. After being switched through a coaxial switch
to the appropriate
antenna
array, the 50 fl line
carrying 100 kW is transformed
to a 25 R line using
two 1./4 transformer
lines which also act as a power
splitter to three 75 R lines each carrying 35 kW to a
pair of antennas in the row. Each 75 Q (unbalanced)
line, made from 60 mm diameter tube, goes to a balun
which has 2 outputs of 150 Q in parallel each of
which is connected to an antenna of 300 Q impedance
through two L/4 transformers.
These last transformers
constitute, in fact, the vertical aluminium-tube
mast
supporting the centre of each antenna. The ends of
the antennas are supported by wooden masts. Figure
3(a) shows the power distribution
to the antennas in
arrays 2 and 3 in greater detail.
Array 1, which was the lowest frequency array until
it was severely damaged in a storm in October 1985,
was rebuilt in 1990 with 12 rows of 12 antennas to
give a gain of 30 dB, or 1200 MW of ERP in the
frequency range 5.558 MHz. The beam width corresponds to 7”. In this array, a transmitter
drives a
pair of antenna rows and the final feed arrangement
differs from that described above only in that the
output of the 75 fi balun is connected to four i./4
transformers.
Each output from the transformer
is
580
M. T. RETVELD
Wm
75almmhw
et al.
2quMawawlranEs~4fs
Unbalanmd con8
(5oIo25ohm)
krry2
(a)
75 Ohm
I
12
11
10
0
8
7
(w
rCanpurr
I
I
5
4
El
+-I
3
2
1
Fig. 3. Schematic
of the transmission
line system and one row of antennas
(a) for arrays 2 and 3, (b) for array 1.
driven by a pair of transmitters
Ionospheric
heating
power split into two and again into two before going
to the fullwave dipoles as shown in Fig. 3(b).
Each transmitter can be connected to any one of
the three antenna arrays through remotely controlled
coaxial switches. This enables subsets of the various
arrays to be used simultaneously. The whole transmission line system is fabricated from aluminium tube
connected with flanges on the outside and plug-in
connectors for the inner conductor. The whole system
is kept under a continuous low air pressure of 0.24.3
bar by a rotary compressor and air drier, which has
been very effective in preventing water affecting the
connections. Nevertheless, it is sometimes necessary
after a Winter season to repair a few connections in
the transmission line system that have been damaged
by snow, thaw or frost.
2.3. Modulation
The main HF wave parameters such as frequency,
polarization, beam direction, and maximum power
are chosen and set up at the time of tuning up. During
an experiment one usually wants to modulate the wave
with a particular and sometimes complicated on-off
pattern, and possibly vary some parameters such as
power, polarization, or even beam angle in a controlled fashion. Various ways of modulating the
heater are described below. Most of the techniques
use the fact that the synthesizers producing the RF
for each transmitter allow the amplitude and phase
of the RF signal to be changed from those values
programmed through the HP-IB interface at the time
of tuning up, by applying suitable voltages (- 5 to
+ 5 V) to two external inputs.
2.3.1. On-of modulation. The lowest possible synthesizer voltage is 37.5 dB below full power, which is
off for most purposes. By applying +5 V or -5 V
from any source to all 12 amplitude modulation inputs
of the synthesizers one may switch the heater off or
on, respectively. For applications where the residual
synthesizer output or noise for this ‘heater off’ state
is unacceptable, such as in an HF radar set-up, the
‘heater off’ power level can be reduced by a further
70 dB with attenuators switched in behind the synthesizers. In principle, any external source such as the
EISCAT radar controller or a pulse generator could
be used to modulate the heater on or off.
The rise time for the transmitter output power is
about 5 pts, which sets the lower limit for the length
of pulses to about 20 PLS.There is no duty cycle limit,
so ‘on’ times can be hours.
For regular modulation sequences with minimum
‘on’ or ‘off’ times of 1 s a programmable relay can be
used to apply a common modulating voltage to all
synthesizers, but normally a Texas-Instruments micro-
581
at TromsPr-1
computer is used for complex modulation patterns.
The same microcomputer can be used to program two
other Schneider gf106 synthesizers to provide amplitude modulating waveforms of fixed frequency in the
range 0.001-9999 Hz or swept frequencies from 200
to 6550 Hz. Because of transmitter power supply resonances, modulation frequencies between 15 and
200 Hz should not be used. The digital clock used for
most pulse sequence timing is locked to EISCAT’s
cesium frequency standard and is synchronized with
EISCAT’s clock.
2.3.2. Power stepping. In many experiments one
wants to vary the heater power in regular steps. Apart
from tuning up repeatedly with a different full power
every time, which takes several minutes, the same
result can also be achieved by varying the amplitude
modulation voltage to each RF synthesizer. Because
the transfer characteristic of each transmitter tube is
not linear and slightly different voltages need to be
applied to each synthesizer, the Texas-Instruments
computer calculates the necessary voltages in a calibration process, enabling 40 power intervals in 2.5%
steps to be subsequently programmed at will. In order
to get even smaller power steps, the initial tune-up
needs to be made at a correspondingly lower ‘full’
power level.
2.3.3. Polarization modulation. In order to change
between the two circularly polarized modes it is only
necessary to reverse the phase of the appropriate 6
transmitters (see Fig. 3). A voltage of 1 V applied under
the control of the Texas-Instruments computer to the
6 phase modulation inputs of the RF synthesizers
achieves this practically instantaneously.
Also the
PET computer can do this by changing the phases
through the HP-IB interface to the synthesizers.
2.3.4. Beam direction and width modulation. It
is possible to point the antenna beam in meridional
directions away from that set at tuning up, by varying
the phases between pairs of transmitters driving each
row of antennas. Applying suitable voltages to the
phase modulation inputs of the appropriate pairs of
synthesizers is not under the control of the TexasInstruments computer, but can be done in an ad hoc
manner with a specialiy built voltage source. The output power should be reduced for larger deflections
because the reflected power and hence the voltage
standing wave ratio increases with increasing deflection. In a similar way the shape of the beam could be
altered (made wider).
3. IONOSPHERIC
EFFECTS
In this section we first describe the basic principles
of linear (low power) HF propagation in the iono-
M. T.
582
&ETVELD
Table 1. Field strength and power density for the different
arrays
Array 1
Height
(km)
100
250
Arrays 2 and 3
(VTm) (mwFjm’)
2.8
1.0
10.0
1.5
(V;“m) (mW;m2)
1.4
0.5
2.4
0.4
sphere which are the basis for many of the design and
choice of operating parameters of the heating facility.
We then describe the various known nonlinear effects
that high power waves can cause in the ionosphere,
leaving the experimental techniques to Section 4.
3.1. Linear HFpropagation
For radio wave propagation in free space the wave
energy flux, F (W/m’) and the electric field E (V/m),
at a range R (km) from a transmitter with effective
radiated power, ERP (kW) are given by
E = 0.25@i5/R
F=
7.96 WSERP/R2.
From the Tromss facility typical values for the two
types of antenna array at two typical heights of interest are given in Table 1. The power actually delivered
to the F-region depends on the amount of absorption
in the underlying D-region. The absorption can be
substantial during aurora1 distrubances and may
effectively prevent F-region heating experiments, but
allows another class of D-region modification experiments which will be described in Section 3.2.1.
The height at which the incident HF wave is
reflected depends on the ionospheric
plasma
frequency, or electron density profile. Reflection
requires the wave frequency to be below the critical
frequency which, in cases of intense sporadic E layers
may well be in the E-region. At times during solar
minimum, the maximum F-region plasma frequency
(or critical frequency) may not be higher than about
5 MHz, limiting the range of experiments that can be
done. Heating by radio waves with frequencies greater
than the plasma frequency is termed underdense heating and with frequencies less than the local plasma
frequency, overdense heating.
HF electromagnetic waves propagate in the ionosphere in two modes (Fig. 4), sometimes called the
ordinary and extraordinary modes. The ordinary
wave is reflected from the level where X = 1, where
X = (JJfO)* and f, is the plasma frequency and f0 the
wave frequency. The extraordinary wave is reflected
from the level where X = 1 f Y, where Y = fh/fO.
.fh = eB,/m (the gyrofrequency), B,, is the Earth’s mag-
ef 01.
netic field strength, and e and m are the electron charge
and mass, respectively. For waves originating from
the Earth’s surface in low and middle latitudes the
extraordinary wave is reflected from the lower level
X = I- Y, but in high latitudes it is possible for the
energy to be ‘coupled’ to a wave, called the z-mode,
which can reach the level X = 1 + Y. (The names
extraordinary and ordinary imply that the wave
propagation perpendicular to the magnetic field is
affected or not by the magnetic field.) This z-mode
is discussed in more detail by MJDLHUS(1990) and
references therein. Figure 4 shows schematically the
ray paths through the ionosphere for the various HF
modes and at various angles of incidence.
At the reflection level the group velocity of the HF
wave decreases to zero resulting in a large increase in
the electric field strength (see Fig. 5). Furthermore the
standing wave set up by the reflected wave results in
a spatially periodic wave field which can be approximately described by an Airy function. The increase in
field strength at the maximum of the standing wave
pattern is called swelling. This wave field varies slowly
in amplitude, and for the Tromser situation has been
accurately calculated by LUNDBORGand THIDB(1986)
who found that swelling is more pronounced at higher
latitudes than at lower latitudes. Figure 5, taken from
their paper, shows the wave field for a linear density
profile of the F-region (a) at Arecibo, (b) at Tromso,
and (c) the E-region at Tromsn The reflection height
and its distance to the base of the ionosphere were
250 and 50 km for the F-region, and 113.8 and 5 km
for the E-region, respectively. The angle between the
magnetic field and the downward vertical is 42” at
Arecibo and 13” at Tromso. Swelling can increase the
amplitude by more than an order of magnitude and
has the consequence that the thresholds for various
plasma instabilities are easily exceeded.
Close to the ordinary mode reflection level (X = 1)
the ray turns rapidly such that the electric field
becomes magnetic field aligned and the propagation
vector, k,is perpendicular to B,,. Figure 5b shows the
calculated wave electric field pattern for the F-region
at Tromso compared to that at lower latitudes like
Arecibo, shown in Fig. 5a. Here we see that swelling
is most important for the electric field component
parallel to the geomagnetic field. Because of this field
orientation near the X = 1 level, the electromagnetic
wave can easily couple to plasma wave modes such as
Langmuir waves and ion-acoustic waves resulting in
various instabilities to be discussed below. Because
the extraordinary wave does not normally reach the
X = 1 level, it is less interesting for many F-region
ionospheric modification experiments.
Further below the reflection height the angle
Ionospheric heating at TromstiI
583
Ray paths for HF radio waves
I
\
.\ __.
Bottomof ionoapham
._. ... ... ..... . ... . . . .....
\
t
\
Heating TX
\
L
Noflh
Fig. 4. HF radio wave paths in the ionosphere from the heating transmitter for various angles of incidence
and polarizations.
between the electric field and the geomagnetic field
successively. At the upper hybrid resonance
height, where the wave frequency equals the upper
hybrid resonance frequency, fu,where f ,”= f i + f ,“,
the electric field is directed essentially perpendicular
to the geomagnetic field. At this height, typically 6 km
below the reflection height, wave modes which are
unique to a magnetised plasma, such as Bernstein
modes, may be enhanced by the electromagnetic wave.
When there is a steep plasma frequency profile, such
as far below the F-region peak or in the E-layer, the
difference between the upper hybrid height and the
reflection height becomes small, which can make it
difficult to determine ex~rimentally
which of these
two height regions is important for the various physical processes. Figure SCalso illustrates how the standing HF wave pattern at the steeper E-region gradient
increases
falls off over a much shorter height range than in the
F-region (Fig. 5b). The extraordinary mode also does
not reach the upper hybrid height, as illustrated in
Figure 4.
Ordinary mode rays incident on the ionosphere
with an angle greater than the critical angle (sometimes called ‘Spitze angle’), 4,, defined by
&, = sin-’ [JyI(I+y)
cos II
are also reflected below the level where X = 1. For
Tromss with I = 78” and fn = 1.35 MHz, c$~varies
from 6” at 4 MHz to 4.5” at 8 MHz. That the HF
wave is largely within this angle was one of the criteria
used for choosing the angular width of the two
antenna arrays of 1.5” (+7S” between the 3 dB
points). Figure 4 illustrates reflection at the critical
angle.
584
(4
M. T.
hETVELD
et
al.
Temporal Development of Modifiition
5 1-r,
Effects
w
0
249
250.2
km
WI
,cus1wu6
Ima
lammmm
I*
101 1oos1ooos
Timescale
(b) IO
Fig. 6. Timescales of heating-induced phenomena as a function of height.
b
The northward deflection of ordinary mode waves
near reflection illustrated in Fig. 4 can be of importance for some experiments. The horizontal deviation
can be about 30 km so that the heater beam is sometimes tilted by S-10” southward to measure the modification effects overhead.
0
249
250.2
km
3.2. Nonlinear effects
0
112.8
114
km
Fig. 5. Standing wave structure of an o-mode HF wave
below the reflection height for three different parameter sets
described in the text corresponding to (a) Arecibo F-region,
(b) Tromss F-region, and (c) Tromss E-region. The corresponding wave frequencies were 5.13, 5.423 and 3.515
MHz, respectively. The curve labelled E,, represents the parallel to B. component while the other two full-drawn curves
are the perpendicular component El and the East-West
component, the former always being the smallest of the two.
The field obtained for an unmagnetized plasma is given by
the dot-dashed curve. The vertical scale is in V/m if the
upgoing wave is normalized to 1 V/m at 100 km height (from
fig. 3 of LUNDBERGand THIDI?,1986).
Figure 6 shows schematically the timescales as a
function of height for the various nonlinearities
experienced by a powerful HF wave in the ionosphere.
A distinction can be made between different characteristic types of nonlinearity in a plasma. The first is
collisional heating of electrons in the electric field of
the wave, resulting in Ohmic, or non-deviative
(because the refractive index, n g 1, and there is little
bending of the ray path) absorption of the wave, and
was first noticed by TELLEGEN (1933) in what is now
called the Luxembourg effect. This thermal effect,
which is largest in the D-region where the product of
electron density (NJ and electron-neutral
collision
frequency (ve) is largest, causes v, and therefore the
electron temperature (T,) to increase, so that the
absorption increases with power at heights where
fi >>vi (70-80 km), but decreases with power where
fi CCv:. The time constant for this process is short,
of the order of tens of microseconds in the lower Dregion, increasing to about a millisecond at 90 km.
This means that high power waves modulated in the
audio frequency range experience distortion, as well
as modulating other radio waves passing through the
same region. The effect of D-region absorption on the
field strength of a modifying wave reaching the F-
Ionospheric heating at TromseI
region is an important, undesirable and often poorly
known factor in heating experiments. Such collisional
absorption also occurs in the F-region, but there
‘anomalous’ absorption may become more important,
a phenomenon described below and in much greater
depth by ROBINSON(1989b).
A second type of nonlinearity occurs in collisionless
plasmas, where the electron mean free path is much
larger than the perturbing wave scale. Of the various
nonlinearity mechanisms possible, one of the most
important is that caused by the electric field inhomogeneity, or ponderomotive
force which is proportional to VIE] 2.A more detailed discussion of these
effects is given in the companion paper by KOHL et al.
(1993, this issue). In the so called Parametric Decay
Instability (PDI), the powerful electromagnetic wave
couples to a Langmuir wave and an ion acoustic wave.
The threshold field for typical F-region parameters is
of the order of 0.1 V/m (STUBBEand KOPKA, 1980),
a value easily exceeded at Tromso especially when
one takes amplitude swelling into account. The ion
acoustic waves excited by the PDI gives rise to
enhanced ion lines, and the Langmuir waves enhance
the upshifted and downshifted plasma lines which
are observable with the EISCAT incoherent scatter
radars. The PDI acts as a starting point for other
processes because the powerful electromagnetic wave,
also called the pump wave, is usually so strong that
plasma turbulence results (KOHL et al., 1993, this
issue). The nature of the turbulence is still not fully
understood and is an active area of research (e.g.
STUBBEet al., 1992 and references therein). Because
the attenuation distance of Langmuir waves in the Fregion is shorter than that of electromagnetic waves
by a factor of about 1000, Langmuir waves dissipate
much more heat per unit volume than electromagnetic
waves with the same initial amplitude. Through the
PDI therefore, energy from the pump wave is eventually dissipated in the F-region electron gas resulting
in one type of anomalous absorption of the pump
wave. The timescale of this nonlinearity is N 1 ms.
A third type of nonlinearity with a timescale of
seconds can result from the electromagnetic wave
coupling to plasma waves which propagate perpendicular to the geomagnetic field. The coupling
occurs at short scale (meters to tens of meters) field
aligned density irregularities which themselves grow
by a spatially resonant dissipation of the energy in
the plasma waves. This nonlinearity leads to thermal
parametric instabilities.
3.2.1. Electron
heating
effects
in the lower
ionosphere. The D-region electron temperature can be
changed by more than an order of magnitude with the
heating facility. The effect of heating D- and E-region
585
electrons has led to one particular area of heating
research, namely that of modulating the conductivity
and therefore the current distribution flowing in a
localised region of the lower ionosphere. The modulated current distribution
radiates waves at frequencies not easily produced by other artificial means.
The amplitudes of waves received on the ground under
the heated region are up to several picoTesla in amplitude at a few kHz. Waves of kHz frequencies radiated
by this ionospheric antenna of about 20 km diameter
have been used to understand the generation mechanism itself (e.g. STUBBEet al., 1982b ; RIETVELDet
al., 1989), to deduce D-region electron heating and
cooling times (RIETVELDet al., 1986), and to estimate
ionospheric electric fields with a time resolution of a
fraction of a second. As a wideband source of
ELF/VLF waves for Earth-ionosphere
waveguide
propagation studies (e.g. BARR et al., 1986) or for
magnetospheric propagation studies (e.g. JAMESet
al., 1990) the heating facility surpasses other groundbased sources which usually have narrow band
antennas (see RIETVELDet al., 1990 for a review of
the work done in this area at Tromso). It should be
added, however, that ionospheric current modulation
is an inefficient way of producing ELFjVLF waves,
with an estimated radiated power of several watts (at
a few kHz) for an HF radiated power of 1 MW (BARR
and STUBBE,1984). There are ways of increasing the
ELFjVLF power (e.g. BARR et al., 1987, 1988) but
results from the experiments with the high gain
antenna show (BARR and STUBBE, 199la,b) that
increases by orders of magnitude which have been
predicted theoretically by PAPADOPOULOS
et al. (1989)
are not confirmed experimentally with the presently
available power densities.
Modulation of currents at frequencies from the PC
1 (several Hz) down to PC 5 (millihertz) range becomes
more interesting because there are essentially no artificial sources of such waves. Furthermore, electron density changes may contribute significantly to the conductivity changes because the time constant for electron
density changes, given by t[s] z 2.5 x lO”/N, [cmm3],
becomes comparable to the ULF periods. For ELF/
VLF waves only temperature modulation is effective and this affects only a part of the conductivity. For an NO+ and O+ dominated ionosphere the
electron recombination rate decreases with increasing
T,, resulting in an enhanced electron density, N,.
Because the density modulation occurs at higher Eregion altitudes (above about 90 km) than the largest
temperature changes at VLF frequencies (about 7(r
80 km), it is important to have little D-region absorption so as to deposit most energy in the E-region. The
modelling of the resulting currents and their ground
586
M. T.
RIETVELD et al
magnetic signature has not been as successful as in the
ELFjVLF
case for several reasons. Firstly, to correctly account for all the currents in the modified
ionosphere
is a highly involved problem (STUBBE et
al., 1982b). Secondly, the experiments
are difficult
because of the enhanced natural background
noise,
and because the longer periods of stable ionospheric
conditions
required so that the number of events
studied in detail is very small (MAUL et al., 1990).
The rather strong pulsations, of minutes-long
period
having amplitudes up to 10 nT excited in the early
days of the heating (STUBBEet al., 1985) were thought
not to be explainable by the theory based on electron
temperature-induced
density perturbations
(STUBBE
and KOPKA, 1977). Whether
other mechanisms
involvmg instabilities of various kinds (STUBBEet al.,
1985) are necessary to explain such data is still an
open question, since there were few other diagnostic
measurements
of the essential
ionospheric
parameters, such as electric field, electron density and
temperature profiles needed to test the theories.
Incoherent
scatter radar measurements
should be
made using the new ‘alternating codes’ during such
experiments,
to enable the temperature
and density
changes in the E-region
to be directly measured
and compared with theory. Such experiments during
sporadic E conditions should prove particulary fruitful because the density change should be proportional
to N,’ (STUBBE et al., 1985) when the modulation
period is much less than the electron density time
constant.
Modulating
the ionospheric
conductivity,
apart
from generating
low frequency
radio waves, also
changes the reflection coefficient of VLF waves and
partially-reflected
HF waves. The effect of continuous
heating of the D-region is to lower the collision frequency profile by about 10 km which corresponds
to
an increase of VLF reflection height of about 4 km
(BARR et al., 1984). Such an artificial ionospheric
‘pimple’ was created and moved about the ionosphere
using the beam steering capability (Section 2.3.4) to
test theories of VLF waveguide propagation
(BARR et
ul.. 1985) and to help in the interpretation
of VLF
phase and amplitude changes caused by lightninginduced electron density perturbations
(DOWDEN et
al., 1991). Slower, heating-induced
D-region electron
density changes caused by temperature-dependent
reaction rates may also occur and could be measured
by similar techiques but this has not been done yet.
The effects of a perturbed D-region electron-neutral
collision frequency, and possibly an electron density
decrease
were measured
with the University
of
Tromss
partial
reflection
experiment
(HOLT et
al., 1985). The electron density change was postu-
lated from a 0.1 s timescale decrease in HF amplitude on heater turn on. The suggested reaction was
e- +202 + 0; +02. Further experimental
work is
lacking in this area of heater-induced
perturbations
to D-region chemistry, both with partial reflection or
other techniques such as incoherent scatter or wavewave interaction.
Indeed,
apart from an observation by TURUNEN and TURUNEN (1985) there have
been no published measurements
of D-region incoherent scatter signals showing plasma parameter
modifications
by HF heating.
3.2.2. Electron
heating effects
in the upper
ionosphere. The heating of the F-region electron gas
is thought to be dominated (at least for o-mode waves)
by ‘anomalous’ processes involving conversion of the
HF electromagnetic
wave to electrostatic
waves
which, partly because of their low group velocity,
dissipate their energy efficiently in a localised region
where the wave frequency equals the upper hybrid
frequency. ROBINSON(1989b) measured a peak ATJT,
of about + 50% which is consistent with anomalous
heating being the dominant
mechanism
although
there are many uncertainties in both the ionospheric
parameters
and in the measurements
used in calculating the relative effects of collisional and anomalous heating.
The effect of the electron temperature
increase on
the electron density in the F-region varies depending on the altitude.
Below a certain height the
temperature-dependent
recombination
of molecular
ions dominates,
resulting in an electron density increase, such as that discussed for the E-region. At
higher altitudes where O+ dominates,
the enhanced
temperature leads to an increased conversion to NO+
which recombines with electrons more readily to produce a density decrease. In addition, at the higher
altitudes diffusion of the hot plasma causes a density
decrease. From the measured AT, one can model the
measured AN, assuming model input parameters such
as ambipolar
diffusion
coefficients
and chemical
recombination
coefficients (ROBINSON, 1989b). It is
found that there is often a narrow density depletion
of about 10% formed within a second of heater turn
on at the height of the temperature maximum which
is near the reflection height. This density depletion or
cavitation is caused by the diffusion of plasma out of
the heated region as a result of the strong temperature
gradients which immediately appear. Heater-induced
density changes may also develop structure aligned
with the magnetic field with perpendicular
scales from
tens ofmeters up to kilometres, through thermal instabilities which are discussed further in Section 3.2.3.
Results from Arecibo experiments (NEWMAN et al.,
1988) indicate that large temperature
enhancements
Ionospheric heating at TromsPr--I
(1000-2000 K) are possible at times of low electron
density because of the correspondingly low cooling
rates of the electron gas. These and the EISCAT
measurements and modelling of ROBINSON (1989b)
show that testing of aeronomic models can be done
with the aid of heating experiments.
In the standing wave structure of the heater wave
near reflection (Fig. 5) the ponderomotive force may
be so strong so as to expel the plasma, setting up
horizontally stratified, quasi-periodic density depletions. Such depletions may trap Langmuir waves with
important
consequences
for the plasma instabilities to be discussed below. The depletions can also
act as a regular scattering structure for probing HF
waves of another frequency and of the other magnetoionic polarization (BELIKOVICHet nl., 1986). This
technique provides a new method of measuring
plasma density between the E- and F-regions with
high altitude resolution and enables vertical velocity
measurements with values of a few tens of centimetres
per second. Other parameters such as electron and ion
temperatures, ion-neutral collision frequencies and
coefficients related to collisions in the D-layer CdII also
be determined. This technique has been used extensively in the Soviet Union but not yet at Tromss.
Evidence for a fast electron density depletion of
4.4% was obtained from a chirped-frequency incoherent scatter experiment at Arecibo (BIRKMAYERet
al.. 1986), from the difference in Langmuir frequency
between heater-induced and natural photo-electron
enhanced electron plasma lines. From the timescale
(< 1 s) of the phenomenon it was deduced that the
scale size of the depletion must have been on the
order of 30 m. Such density depletions have not been
confirmed at Tromso, although in a similar experiment other puzzling phenomena were found such as
plasma lines at unexplained frequencies (ISHAMet al.,
1990). Further experiments along these lines should
be performed, since the EISCAT radar also has its
own chirp generator.
3.23. Thermal se&&using. The effect of initially
small electron density fluctuations in the F-region is to
refract the HE; waves such that even stronger heating
takes place, leading to the self-focusing instability.
The Ohmic heating and the electric field ponderomotive force expel the plasma from these focused
regions thereby amplifying the initial perturbation.
The density perturbations have been estimated to be
about 5% (DUNCAN and BEHNKE, 1978) and the
irregularities have scale sizes from hundreds of metres
to several kilometres. Like natural irregularities on
these scales, they cause scintillations on HF and VHF
signals propagating through the modified region. The
effect is usually seen on HF signals as spread-F on
587
ionograms or on HF Doppler link signals (STUBBE et
using the
EISCAT 933 or 224 MHz receivers (FREYet al., 1984).
One surprising and unexplained result is that the scale
length of the excited irregularities is reduced when the
heater power is reduced. This is in contradiction to
existing theories which are discussed by FARLEYet at.
(1983) and which predict a strong increase of
threshold with decreasing irregularity scale length due
to increasing heat conduction and diffusion losses.
Another likely example of this instability was a 15%
density depletion reported by WRIGHT et al. (1988)
using the dynasonde HF radar at Tromss. In this
case the seed depletion appeared to be a single, weak,
large scale (N 100 km) perturbation associated with
an atmospheric gravity wave. In this case the above
theories appeared to be supported since the radiated
power was low, but apparently large enough to exceed
the threshold which is predicted to be low for such
large scale sizes.
3.2.4. ~ara~e~~ic Alaska ~~s~ab~~~~~es.
On the time
scale of milliseconds the HF electromagnetic wave can
interact with various plasma waves in a three-wave
interaction which must satisfy the relations
al., 1982a) or as radio star scintillations
.fu =f,+fZ
and
k0 = k,+k?
in order to satisfy energy and momentum conservation. The subscript 0 stands for the mother wave
(usually the HF electromagnetic wave) and 1 and 2
are daughter waves. The problem of identifying the
waves that satisfy these conditions and their dispersion relation in the plasma is described simply by
WALKER (1979) and THID~: (1989). The companion
paper by KOHL et al. (1993, this issue), discusses the
physics of these interactions and the data and their
interpretation in greater depth. In the parametric
decay instability (PDI) the daughter waves are a
Langmuir wave, which can be detected by incoherent
scatter radars as an echo offset from the radar frequency by the heater frequency minus the ion acoustic
frequency, and an ion acoustic wave which appears
as an enhanced shoulder or line in the normal ion
line spectrum of incoherent scatter radars. The decay
instability can also exist in another form, called the
oscillating two-stream instability (OTSI) or the purely
growing mode. Here the second decay product is not
a propagating low frequency wave but a non-propagating periodic structure. This manifests itself in
the radar signals as an echo offset from the radar
frequency by just the heater frequency and a peak in
the ion line at zero frequency. The height region over
which these instabilities can occur is limited by the
reflection height, zO,and the height below that where
588
M. T. RIETVELDetal.
dB above the background noise level. Some of the
features in the spectra, examples of which are given
in Fig. 7, could be readily linked to parametric interactions involving electromagnetic waves, Langmuir
(z,-0.M)
< 2 < 20.
waves and ion acoustic waves (STUBBEet al., 1984).
Two features in the SEE spectra have been seen for
This height range is approximately 10 km thick below
a11heater frequencies : the continuum, a usually asymthe reflection altitude of an o-mode wave at vertical
metric spectrum with a peak at f0 being stronger on
incidence. In this range all wavelengths down to lOthe downshifted side, and ranging from a few kHz up
20 cm are excited, but only some of these which match
to as much as 100 kHz, and the downshifted maximum
the Bragg scattering condition (k = 2k, for a mono(DM), a spectral maximum of a few kHz width at a
static radar with wavenumber kr) are detected by a frequency offset from the pump given approximately
radar. The interval in which Langmuir waves with a by AfDM= -2.10- ‘fa. Other features, such as the
given k are excited is of the order of 100 m (STUBBE do~shifted
peak (DP) 1-2 kHz below the pump,
and KOPKA, 1980).
have shown a dependence on heater frequency which
Since the threshold for PDI is normally easily
is discussed further in Section 3.2.7.
exceeded, the daughter waves do not necessarily
The continuum is thoughtLto be caused by the interemain weak compared to the mother or pump wave.
grated contributions of electromagnetic waves proThe Langmuir wave produced by PDI may itself decay
duced by Langmuir waves (from the PDI) scattering
into another Langmuir wave of slightly lower fre- off ion acoustic waves. The DP has also been explained
quency and an ion acoustic wave. This cascading can
by the PDI in the first pump wave maximum below
continue until the threshold for instability is no longer
the reflection height (STUBBYet al., 1984). These ideas
reached and results in a spectrum with a distinctive
could be further tested by measuring the rise time of
and finite set of cascade lines, as illustrated in figs lsignals in the continuum and in the DP, both of which
3 of KOHL et al. (1993, this issue). This picture
arc expected to be very low (-ms). The theories have
describes the weak Langmuir turbulence (WLT) conbeen extended to include the effect of the PDI of
cept (see FEJ~R, 1979 for a review and STUBBEet al., pump-induced density depletions (LEYSERand THIDL,
1992) and explains some but not all the observations.
1988 ; LEYSER,1989). If the density depletions are
The number of observed cascades is in fact smaller
important, the rise times of these spectral features
than theoretically expected and sometimes a broad
should be similar to the depletion time, about 100 ms.
spectrum is observed instead of cascade fines, and
SEE has not only been observed in sidebands of
displaced plasma lines (e.g. fig 3 of NORDLING et al., the reflected pump wave, but also at harmonic and
1988) and anomalous plasma lines (ISHAMetal.,
1990) subharmonic frequencies (DERBLOMet al., 1989). The
are observed but not predicted. New theories of strong
subharmonic emission may be evidence for stimulated
Langmuir turbulence (SLT) are being developed (e.g.
Raman scattering where an electromagnetic wave
DuBois et ul., 1990) but there remain many outdecays into another one at half the frequency plus a
standing problems (STUBBEet al., 1992). To test the
Langmuir wave. The second harmonic emission may
theories improved measurements of the Langmuir
be produced by a combination of PDI and Raman
spectra at different wavenum~rs
(i.e. radar freupscatte~ng. So the range of possible plasma instaquencies or angles) with high spatial resolution (about
bilities that can be examined through ionospheric
100 m) are required. Such measurements have recently
heating experiments has expanded by the discovery of
been started at EISCAT using simultaneous VHF and
the SEE technique. Further aspects of SEE measureUHF radar transmissions.
ments are discussed in Sections 3.2.6 and 3.2.7.
Another consequence of the excited Langmuir
The energy transfer from the electromagnetic pump
waves is that they may also convert to HF electrowave to the parametrically excited electrostatic waves
magnetic waves by scattering off low or zero fre- gives rise to an anomalous absorption of the heating
wave which may significantly exceed collisional
quency electrostatic waves. These electromagnetic
absorption. The timescale here is about 10 ms (see
waves can then propagate out of the modified ionoFig. 6) and can be observed as an overshoot of about
sphere to be observable on the ground as StimuIated
10 dB of the ionospherically reflected wave (e.g. FEJER
Electromagnetic Emissions (SEE), a phenomenon disand KOPKA, 1981). In Section 3.2.6 another type of
covered at Tromss (THIDB et al., 1982). In the sideanomalous absorption with a longer time constant
bands of ionospherically reflected heater signal, emissions were found reaching an intensity of SO-75 dB caused by a thermal plasma wave excitation mechbelow the reflected pump wave and several tens of anism will be described.
Landau damping becomes excessive, such that for
a linear electron density profile with scale height H
(STUBBEand KOPKA, 1980),
Ionospheric heating at Tromsra-I
589
f. = 5.623t+iz
ERP=260
nW
Fig, 7. Stimulated Electromagnetic Emission (SEE) spectra showing several characteristic features discussed
in the text (from fig. 9 of STUBBE et al., 1984).
3.2.5. Energetic electrons. The strong heaterinduced Langmuir waves can accelerate thermal electrons to several eV which can manifest itself as
enhanced airglow preferentially of the red 6300 A
oxygen line (1.96 eV threshold) and to a lesser extent
of the green 5577 8, oxygen line (4.17 eV threshold).
Although there have been some measurements of
enhanced 6300 A airglow by about 50% at Tromss
(STUBBE et al., 1982a), there was a slight unexpected
decrease of about 15% in the green line intensity which
may be explainable as a reduced excitation caused by
a reduced, temperature-dependent
recombination rate
of electrons. Although difficult to perform in the
aurora1 region, further observations of artificial airglow with imaging systems could yield neutral wind
velocities as has been done at lower latitudes
(BERNHARDT rt al., 1989).
Triggering of sporadic E by F-region heating, possibly through energetic electrons, was observed at
Platteville (WRIGHT, 1975), but the energies involved
must be much higher than a few eV. It was postulated
that the precipitation involved cyclotron resonance of
electrons with VLF waves that may have propagated
in artificially created ionization ducts. Although dynasonde measurements at Tromsnr have shown indications of brief sporadic E layers during heating
experiments, it was not clear whether they were natural or heater-induced. Further systematic measurements in the future are required.
3.2.6. Thermal resonance instabilities. There is a
wealth of observations using HF and VHF radio
waves probing the ionosphere heated by o-mode
waves, which show changes in some measured parameter with a timescale from a fraction of a second
to several seconds after the heater is switched on.
A good example is an anomalous absorption, of a
different type to that discussed above, of about 10 dB
but up to a maximum of 15 dB which is experienced
by a low power HF wave probing the heated F-region
near the reflection height. These observations are
linked with the formation of short-scale F-region striations seen by HF backscatter radars as a narrow
spectrum of a few Hz at frequencies from 3 to 17 MHz
at distances ranging from 200 to 1100 km (HEDBERG
et ok, 1983). The growth and decay times are some
seconds and several tens of seconds, respectively,
590
M. T.
RIETVELD
being shorter for higher frequencies (i.e. shorter
irregularity scale lengths).
All these results can be explained by conversion of
the heating pump electromagnetic energy into electrostatic energy at the density gradients of field aligned
irregularities (FAI) or striations. The striations may
occur naturally to start with but can be enhanced
through the pressure gradient force by differential
heating of HF plasma waves excited by the heater.
This process resonates when the pump frequency is
close to the upper hybrid resonance frequency, which
may be typically 6 km below the reflection height,
depending on the density gradient. In a linear electron
density profile this height is fi(dh/dfi)
below the
reflection height.
There are basically two types of striation growth
mechanisms proposed in the literature. The first type
requires that the power of the heating wave exceeds a
certain threshold but requires no additional striations
to be present (e.g. DAS and FEJER, 1979).
The second type has a threshold which depends on
the product of pump power and striation amplitude,
producing explosive striation growth when the
threshold is exceeded (e.g. INHESTERet al., 1981).
It has been suggested that the first mechanism may
initially amplify the striations until the threshold of
the second mechanism is reached, which then leads to
further growth.
The determination of the FAI scale lengths can be
made by using HF probing signals that differ only
slightly from the heater frequency in a bistatic HF
radio link such that the ray paths do not cross the
modified D-region and are reflected at slightly different
heights (JONESet al., 1984). By measuring the anomalous absorption as a function of reflection height,
scale lengths of between 27 and 52 km (ROBINSON,
1989b), have been estimated for the scale length of the
striations parallel to the magnetic field. The striation
amplitude, (InI 2)/N& was estimated to reach a peak
of 2% near the pump resonance height.
The timescale for the generation of these striations
agrees roughly with that of several other phenomena
such as the ‘main overshoot’ in the enhanced plasma
lines observed using incoherent scatter. This may
mean that the accompanying anomalous absorption
decreases the pump strength near the reflection height
and hence decreases the excitation of the plasma lines
there.
In another measurement of enhanced plasma lines
by a 140 MHz radar about 200 km away the results
were quite different. The plasma lines had a growth
time of between 15 and 20 s (HEDBERGet al., 1984)
which was probably caused by direct conversion of
the heating wave into Langmuir waves at the striations
et
al.
which were slowly building up in the modified ionospheric region.
Some of the spectral features seen in the SEE spectrum also have growth times of the order of seconds.
For example the broad upshifted maximum (BUM)
(THIDBet al., 1983) at several tens of kHz above the
pump frequency, and the downshifted maxima (DM)
have rise times of several seconds after turn on of
the pump, suggesting that they also depend on the
presence of density irregularities. THIDJ?et al. (1983)
also noted that the SEE were strong whenever HF
radar backscatter from striations was strong.
3.2.7. Gyroharmonic effects. Three of the spectral
features that have been found in SEE spectra appear
only when the pump frequency is near a harmonic of
the cyclotron frequency, fh(about 1.35 MHz) in the
F-region. The BUM and DP features mentioned above
and a recently discovered broad symmetrical structure
(BSS) (STUBBEand KOPKA, 1990), consisting of two
spectral maxima symmetrically located around the
pump frequency at offset frequencies of 15-30 kHz,
are found only near gyroharmonics. The frequency
offset of the BUM feature is approximately given by
AfB”,,.,= fo-nfh. Both the DP and BSS have only
been found when f0 z 3fh, and the pump or gyro
frequency need only change by about 1% for the
features to appear or disappear. These variations can
be observed as the excitation region moves to higher
altitudes, and correspondingly jjl is changed to lower
values, at sunset in the late afternoon.
The likely explanation
for these phenomena
involves electron Bernstein modes possibly incorporating upper hybrid waves. These waves, both of
which may be generated by a thermal resonance instability (previous section), attain a low group velocity
as f0 -+ nfh,and therefore have a long residence in the
interaction region. These phenomena are by no means
adequately explained, however, and there is scope for
more theoretical and experimental work.
3.2.8. Linear conversion. Beside the process of HF
waves converting to electrostatic waves at electron
density irregularities of the right wavelength as
described in Section 3.2.6, an o-mode wave may
directly convert to electrostatic waves in the region of
high-frequency
plasma resonances described by
1 > X > 1 - Y’, in a weakly inhomogeneous magnetised plasma (MJBLHUSand FL& 1984). In the ionosphere a resonance occurs on the z-mode or ‘slow
extraordinary mode’, but the o-mode can couple to
this z-mode when the angle of incidence is near the
critical angle given by 4?. Figure 4 shows
schematically the path of such a ray propagating
through the radio window above the X= 1 level to
reflect at the X = 1+ Y level and return to near the
591
Ionospheric heating at TromseI
X = 1 level where the electrostatic interactions can
take place. MJDLHUSand FLA (1984) have calculated
for the Tromso case the spatial region (radio window)
of the resonance which should be very localised and
displaced south of the heater in the Northern Hemisphere. A second region displaced North is also
expected (see MJDLHUS,1990, which also reviews this
topic). A VHF or UHF radar would be expected to
observe very narrow spectra, shifted by exactly the
heater frequency, with a growth time shorter than
those of the nonlinear processes described in Sections
3.2.5 and 3.2.6. No experimental evidence of such
resonances has so far been found in heating experiments probably because suitably designed experiments have not been performed so far under the right
conditions.
4. MEASURINGTHE EFFECTSOF MODIFICATION
In addition to using incoherent scatter radars to
measure the effects of ionospheric modification, there
are good reasons for employing a broader array of
complementary instrumentation.
Although the incoherent scatter technique is excellent for measuring
modification effects in the plasma waves and plasma
environment with high spatial resolution, it is not so
good for measuring effects related to large horizontal
dimensions, such as km-scale irregularities, or effects
related to HF or VLF electromagnetic waves which
are produced. A description of some of the techniques
and their strengths and weaknesses for examining
heating-induced phenomena is now presented.
A term which often arises in the literature associated
with heating experiments is preconditioning.
This
term is used to denote the different results obtained
when studying parametric instabilities in the F-region
when heating an undisturbed ionosphere compared
to heating in an ionosphere which has been heated
previously. At Tromso preconditioning has a negative
influence by reducing the strength of modification
effects, most likely due to enhanced anomalous
absorption of the HF wave. For this reason many
experiments are performed with a low duty cycle
heater on/off pulse scheme. At low latitudes like
Arecibo, however, precondition supports stronger
modification effects for reasons discussed in STUBBE
et al. (1992).
Modification experiments in the aurora1 F-region
suffer from the difficulty that they require extremely
quiet conditions. These are seldom met, and if so,
almost exclusively during the day-time. At night the
patchiness of the F-region, the frequent occurrence of
blanketing or sporadic E layers and the high D-region
absorption during aurora1 conditions
F-region experiments unsuccessful.
usually render
4.1. The EISCAT incoherent scatter radars
It was envisaged from the beginning of the Heating
project that the EISCAT radars would constitute the
main diagnostic of ionospheric modification effects.
Both bulk plasma parameter modification effects and
plasma wave excitation effects can be measured by the
radars although different signal processing techniques
are usually used. The unique constellation of two
radars which enable plasma wavelengths of 16 and
50 cm to be probed simultaneously has only recently
begun to be exploited fully in a series of crucial experiments to test the Langmuir turbulence theories
(STUBBEet al., 1992). Since March 1990 the simultaneous operation of the heater, the VHF transmitter
(with one klystron) and the UHF transmitter has
occurred regularly without problems.
The successful measurement of heater-induced
Langmuir waves by the radars requires that Landau
damping of the waves is small enough. The condition
for this is given by T,[Kj < 2.108f$f,2 wheref, is the
radar frequency [STUBBEet al, 1984, equation (41b)].
This equation explains the general statements that
high F-region critical frequencies (and therefore high
fO) and the VHF radar (low L) are more favourable
for plasma wave experiments, especially when T, is
naturally high.
4.1.1. Practical aspects. When the heating transmitters are switched on, the sudden load on the power
line results in a voltage drop of several percent. Since
the Heating facility and the EISCAT radars are supplied by the same power line this also results in the
radar transmitter power being slightly affected. This
means that the radar transmitted power needs to be
accurately monitored and accounted for when looking
for small heating-induced effects such as electron density changes in the D- or E-regions.
Because the EISCAT antennas have very narrow
beams and the HF beam is quite narrow, it is important to ensure that the radar antennas are pointing
into the modified ionosphere. To ensure this one may
have to compensate for the northward deviation of
the heater beam at reflection (see Fig. 4) either
by tilting the heater beam or the radar antennas
appropriately.
The investigation of heater-induced plasma waves
imposes very different requirements on the data acquisition routines than those used for normal incoherent
scatter. The signals are often very strong and coherent,
exhibiting larger variations in intensity than can be
handled by the present &bit digitisers. Consequently
M. T. RIETVELD et al.
592
gain settings need to be constantly monitored and
adjusted. The high time and altitude resolution
required to advance our understanding of the processes involved are of the order of 100 m and milliseconds, respectively. The high time resolution can be
achieved using the present correlators with special
programmes that process only a small fraction of the
available data stream. With the high signal-to-noise
ratios involved this is not always as bad as it may
appear. In some applications it is of interest to simply
record the raw signal and process it afterwards, something the present correlator only allows in very short
data segments (4096 samples every integration period
which can only be as short as 1 s). Alternatively,
special recording and processing equipment may be
temporarily installed to record the raw signal from
the receiver before it goes into the EISCAT analogue
to digital convertors.
The high spatial resolution may be achieved
through short pulses, or special coding schemes such
as Barker or random phase codes. To investigate the
Langmuir spectrum high frequency resolution is
required as well as high spatial resolution, which may
be achieved through random phase coding or using
pulse-to-pulse correlation techniques.
As in all plasma line work, frequency agility in the
received signals is very important. The software tools
to enable rapid oscillator changes all exist now. The
interference from the Nordic Mobile Telephone
(NMT-900) system on the UHF, and other interfering
signals on VHF impose some restrictions with regards
to maximum upshifted plasma frequencies which can
be observed. To alleviate the constraints on received
frequencies it is important to retain flexibility in the
choice of (the lower) transmitter frequencies.
Synchronization of heater pulses with the radar is
easily achieved through the common frequency standard and synchronized timing. For some experiments,
with short heating pulses for example, it may be useful
to be able to control the heating on/off pulsing directly
from the radar controller. This is possible even now in
principle through an unused bit in the radar controller
which could be used to modulate the heating
transmitters.
4.2. Mugnetometers, ELF and VLF receivers
Artificially modulated
large scale currents in the
D- and E-regions cause magnetic field perturbations
measurable on the ground by sensitive magnetometers
when the modulation frequency is less than about
10 Hz (MAUL et al., 1990). Successful measurements
in this frequency range have been made only relatively
close to the heated region, such as up to about
100 km. It is important that precautions are taken that
the instrumentation
is not influenced by modulated
HF fields from the transmitter (e.g. NOBLE and
GORDON, 1990) or by modulated currents in nearby
powerlines. For frequencies greater than about 100 Hz,
ELFjVLF receivers can be used to detect the
electric and magnetic fields of waves radiated from
the modified region and propagating in the Earthionosphere waveguide, up to distances of a few thousand kilometres in fact (BARRet al., 1991). ELF and
VLF waves generated by this technique have been
used to study the natural aurora1 ionosphere, (e.g.
natural electric field fluctuations) and the generation
and propagation theories of such waves (see STUBBE
et al., 1982b ; RIETVELDet al., 1990 and references
therein). For useful ELFjVLF measurements of this
type it is important to have phase stable receivers
which are locked in phase to the modulation frequency which is normally derived from EISCAT’s
cesium clock.
For measuring the effects of D-region perturbations
on VLF signals from navigation or communication
transmitters it is necessary to have a quiet site and a
means of coherent detection and integration so as to
improve the signal-to-noise ratio because the heater
effects are typically very small (a fraction of a dB and
about a degree in phase). One also needs to choose
the heater modulation appropriately such that one
samples the heated and unheated ionosphere in quick
succession to compensate for natural background
variation. The effects may be larger by about an order
of magnitude at times of modal interference but the
natural variability also increases (BARR et al., 1985).
The choice of receiver site is determined by the transmitter to receiver path in relation to the heated region,
and whether one wants to receive many waveguide
modes (for a short path) or only a few (long path).
4.3. HF receivers
One of the discoveries made at Tromss was the
detection of HF radiation (SEE) emitted from the
modified ionosphere at frequencies up to a few
hundreds of kHz away from the heater frequency
itself. The equipment required is basically an antenna
and a spectrum analyser of large dynamic range
(THIDB et al., 1982). The strength of such sidebands
may reach 50-70 dB below that of the ionospherically
reflected pump wave, and several tens of dB above the
background noise level. Such measurements can be
made relatively close to the heater provided that the
direct signal does not overload the receiver or spectrum analyser. Figure 7 shows an example of the SEE
spectrum with several of the features mentioned in
Section 3.2.6 labelled.
Ionospheric heating at Tromsa-1
Simple measurements of the be~nning of the ionospherically reflected heater signal itself can also provide valuable diagnostic information. For example
high time resolution measurements of an overshoot
by FEJERand K~PKA (1981) showed the timescale of
one type of anomalous absorption to be - 100 ms.
The reception of the trailing edge of a heater puise is
also rich in information, like a radar pulse. One discerns several ionospheric echoes and varying amplitudes as the beater-induced ionospheric absorption
relaxes to an unperturbed level (Stubbe, unpublished
results). All one needs is a tuneable, variable gain
receiver of wide enough bandwidth, a detected output
and recording medium.
4.4. HF radio paths
With an HF link using low power (typically 30 W)
CW transmissions with a geometry chosen such that
the F-region reflection point lies in the modified
volume, one can monitor several modification effects.
The spacing of the transmitter and receiver must be
sufficient to guarantee that the diagnostic wave does
not pass through the modified D-region. By
judiciously choosing the frequency (usually within a
few hundred kHz of the heater frequency) one can
probe regions dosely above or below the pump reflection region (JONESet al., 1984). By using a vertical
dipole as the transmitting antenna, both ordinary and
extraordinary
components
are transmitted
with
approximately equal intensity. The corresponding
receiving antennas are usually pairs of crossed dipoles
such that the amplitudes of the different polarizations
are measured independently. Heating effects measured using the two polarizations are generally different
because the reflection heights of the two polarized
waves are different.
The level of heater induced wide band absorption is
obtained by comparing the signal amplitudes between
heater on and off. The o-mode probe signal experiences an ‘anomalous absorption’ of up to 15 dB and
relatively independent of heater power in a quiet ionosphere, caused by o-mode heating-induced striations.
On the other hand, the absorption is considerably
weaker and displays a marked dependence on heater
power when the ionosphere is strongly variable, as
indicated by a high fading rate. An explanation for
and a description of these effects is given by STUBBE
et al. (1982~). By varying the probe frequency and
thereby the reflection height, one can estimate the
scale length of such striations (JONES et al., 1984;
ROBINSON,1989b).
An increase in N, in the ionosphere causes a
decrease in the plasma refractive index, reducing the
593
phase path of a radio wave passing through the modified region and thereby causing a phase advance on
the receiver. So the measured phase variation should
be directly proportional to the electron density variation. For such phase perturbations measured at
Tromso, JONESet al. (1982) deduced that the piasma
density in the F-region increased due to electron heating, and by comparing with the level of wide band
attenuation,
deduced that anomalous absorption
makes a more significant contribution to heating than
deviative absorption.
By amplitude modulating the heater wave at a frequency of about 1 Hz, the anomalous absorption
varies in a similar way causing sidebands on the
diagnostic signal. This F-region cross modulation,
demonstrated by STUBBEer al. (1982c), could also be
used to study the time constant of various parametric
instabilities through the dependence of cross modulation strength on modulating frequency.
By observing the spread in Doppler shifts of a diagnostic wave ~netrating the modified F-region, typically a few Hz, one can observe the excitation of
large scale irregularities presumably caused by the self
focusing instability. An example is presented in fig. IS
of STUBBEez al. (1982~).
4.5. HF radars
HF radars provide a very valuable diagnostic of the
modified ionosphere especially if an incoherent scatter
radar is not available. Firstly, partial reflection radars
(PRE) at about 2.7 MHz can be used to measure
the effects of electron-neutral collision frequency and
electron density changes in the D-region caused by
heating. The results from one such experiment using
the University of Tromss’s PRE facility, reported by
HOLT et al. (1985), can be understood qualitatively
but are difficult to interpret quantitatively [see STUBBE
and ENCELHARDT(1988) for further development of
the theory]. Further experiments, combined with
other diagnostic measurements of the D-region could
lead to information on D-region chemistry. It should
be noted that the dynasonde at Tromss can also be
used for partial reflection work (JONESand GRUBB,
1980).
Another radar technique used for studying the Dand E-regions which has never been tried at Tromso, is
to use radar echoes from periodic density irregularities
formed in the standing wave pattern of the heater
wave reflected in the E-region (BELIKOVKHet al.,
1986). By observing how the amplitude of the echoes
varies after heater switch off, as a function of time
and height, ionospheric parameters such as electron
density, the coefficient of ambipolar diffusion, and
594
M. T.
&ETVELD
possibly vertical motions may be deduced. The same
technique can also be applied to the irregularities from
the F-region (BELIKOVICHet al., 1979) although the
physics changes. For example the density irregularities
are formed by thermal expansion instead of by changing the recombination rate.
Standard or digital ionosondes are essential in providing the critical frequency for choosing the right
heater frequency in F-region experiments. Usually one
wants to choose the frequency such that the heater
wave is reflected from the F-region (overdense heating). But apart from choosing the frequency, ionosondes can measure heating effects such as the production of large scale (hundreds to thousands of
metres) irregularities, visible as spread-F on ionograms. This phenomena has not been commonly seen
at Tromso, although the irregularities have been
observed through phase changes on HF Doppler
paths, as described earlier. Furthermore, large scale
density depletions are detectable through changes in
the critical frequency, as demonstrated by WRIGHT et
al. (1988) and discussed in Section 3.2.3.
So far we have discussed only HF radars situated
close to the modified volume. HF backscatter off FAI,
using radars at frequencies from 3 to 17 MHz situated
up to 1100 km away from the heater site, have been
described by HEDBERG et al. (1983). The enhanced
backscatter is due to Bragg scattering off striations
with scale sizes of half the radar wavelength. In order
for the radar wave to be reflected perpendicular to
the irregularities, HF refraction in the ionosphere is
necessary. The backscatter spectrum is very narrow,
having a width of a few Hz. Hence irregularity drifts
can be measured with high precision, as demonstrated
by HANUISE etal.(1986). Striation growth and decay
times are some seconds and some tens of seconds,
respectively.
4.5.1. Magnetospheric sounding. A novel use of an
ionospheric modification facility as a magnetospheric
radar was reported by Gurevich et al. (1992) recently.
Using the ‘SURA’ HF modification facility they
reported echoes from 4000 km range supposedly from
ion acoustic turbulence excited by field-aligned currents. These echoes may be related to the recently
discovered enhanced ion acoustic lines observed at
933 MHz (RIETVELDet al., 1991) and 224 MHz (COLLIS et al., 1991) along or close to the geomagnetic field
direction at Tromso. Preliminary results from these
EISCAT data suggest that the echoes are longer lived
and perhaps more common at lower radar frequencies
and at altitudes higher than about 600 km. The HF
facility at Tromso could be used to examine such
echoes at frequencies of up to 8 MHz, either by suitable modifications allowing an antenna array to be
et al.
used as a receiving antenna, or using it as a transmitter
in a bistatic arrangement where a separate receiving
antenna is used. This is a new area which could
enhance the scientific use of the HF facility.
It may be noted that the Heating facility was once
used as a 2.75 MHz transmitter, while the University
of Tromss’s partial reflection antenna (HOLT et al.,
1985) was used as a receiving antenna in a MST radar
experiment (CZECHOWSKY et al., 1983).
4.6. Riometers
Riometers are instruments which measure the relative ionospheric opacity at frequencies usually in the
range 30-50 MHz by recording cosmic noise. The
specific absorption (dB/km) is proportional to Nev,
and maximises in the 70-l 10 km region depending on
frequency. Since heating of the D-region can increase
the electron temperature and hence v, by at least an
order of magnitude, one might expect that an effect
should be observable using a riometer. One detrimental factor is that riometer beams are usually wide
(about 30”) whereas the heated D-region typically
subtends an angle of about 14”. Furthermore the
difference between heating and riometer frequencies
means that the largest heater-induced change in v, is
usually much lower than the maximum in the riometer
specific absorption, reducing the sensitivity of the
effect.
Attempts were made to detect heating effects on a
5 1.4 MHz narrow beam riometer (E. Neilsen, private
communication, 1986) situated 17 km from the heater,
but without success. The reasons for the lack of success were never satisfactorily explained, but the possibilities include the lack of the correct ionospheric
conditions.
4.7. Scintillation measurements
Both signals from radio stars and beacons from
geostationary satellites have shown intensity fluctuations as they pass through kilometre scale density
irregularities generated in the heated region. Using
observations of scintillations observed at Tromss on
a 250 MHz satellite beacon signal passing through
the heated region, BASU et al. (1987) deduced irregularity amplitudes of A(N,
of 3.4% with a wavelength of about 750 m. They estimated an HF power
density of 0.3 mW m-* which was within a factor of
two of that predicted by the self-focusing theory of
CRAGIN et al. (1977). However, other measurements
of scintillations at 933 MHz on radio star signals by
FREY et al. (1984) using the EISCAT UHF system
passively, suggest disagreement with this theory
Ionospheric heating at Tromw-I
because s~intiilations were observed with power densities as low as 0.022 mW m-* for irregularities in the
range 150-450 km.
By cross-correlating the artificial scintillations from
spaced receivers it is possible to measure the ionospheric drifts caused by the combined effects of electric
fields and neutral winds.
4.8. VHF radars
Coherent VHF radars looking perpendicular to the
magnetic field are able to obtain echoes from E-region
irregularities, which may be excited by the heater via
thermal resonance instabilities (Section 3.2.6). Radars
sensitive to 1 m irregularities such as STARE at
144 MHz (HIBBERD et al., 1983), or other radars
at 47 MHz sensitive to 3.2 m irregularities (DJUTH
et al., 1985) observe echoes with growth times ranging from about 100 ms to several tens of seconds.
Increased probability and strength of the echoes are
for o-mode overdense heating but effects were also
seen for underdense and x-mode heating.
VHF radars have also been used to detect scatter
off heater-induced plasma lines using a low power
140 MHz radar from Kiruna (HEDBERGet al., 1984)
in a similar way to the VHF and UHF plasma line
measurements
made with the incoherent scatter
radars. Such measurements give valuable information
on the wavelength and propagation angle dependence
of the heater-induced Langmuir waves.
The effect of suprathermal electrons (few eV) produced by the heater-excited Langmuir waves should
be observable as an enhanced airglow through their
enhanced collisional excitation of O(lD) (red line)
and 0( 1s) (green line). Such enhancements have been
observed at other facilities (BERNHARDTet al., 1989;
and references therein) using photometers and imaging TV systems. There have been a few preliminary
results obtained from Tromss using a photometer
(STUBBE et al., 1982a) but other experiments using
spectrophotometers
(HENRIKSEN et al., 1984) have
proved negative. At Tromsar one has to contend with
a possibly large and variable natural airglow and auroral background and overcast skies which make such
experiments with narrow angle photometers more
difficult. Further experiments using sensitive imaging
systems would, by identifying the modified airglow
region by its spatial extent, eliminate some of the
problems associated with relatively narrow beam systems, namely that of relying just on a temporal signature in the aurora1 region where there are significant
natural variations.
595
In order to fly instrumented rockets through the
heated volume the heater beam must be tilted such
that the rocket trajectory (from Andsya) passes over
the sea or is within a limited region north of
ESRANGE near Kiruna. This constraint is because
rockets are not allowed to fly over the Norwegian
mainland. Four rocket flights from And@ya were
made in late 1982 to probe the modified F-region.
For this purpose special switchable delay lines were
installed in antenna array 2 such that the beam could
be tilted towards the west by 10” (these are now
removed). The aim was to measure quantities not obtainable by ground-based means, such as the electric
and magnetic field of the heater wave, the electric field
of high and low frequency plasma waves excited by the
heater and the suprathermal electron energy spectrum
(ROSEet al., 1985). The results from these flights were
surprising in several respects. The expected swelling
of the heater wave was not observed and the electron
temperature near the reflection altitude was found to
closely follow the 1 s on/off modulation of the heater
power whereas a much longer time constant is
expected. Suprathermal electrons greater than 10 eV
were found but only in short bursts, and the heaterinduced Lan~uir peaks show spectral peaks at about
25 kHz, much higher than expected from the PDI.
These unexpected results show the need for further
theoretical and experimental work, preferably with
rockets, simultaneous incoherent scatter radar and
other diagnostic measurements.
To test the models of ELFjVLF wave generation
which have been developed, a rocket instrumented
with a VLF wave detector and Langmuir probe flying
through the modified D-region would be of much
interest. Such an experiment has never been performed yet.
To probe the heated F-region from either AndQya
or ESRANGE is not possible now because of the lack
of East-West steering ability. Considerable investment would be needed to have such an option in the
future. Even by tilting to the South the heated region
would still be about 80 km West of the ESRANGE
firing area.
4.11. Satellites
Satellites can be used for measuring the VLF and
ELF waves propagating into the upper ionosphere
and magnetosphere produced in the lower ionosphere
by modulated heating. Successful reception has been
obtained, on four different satellites at altitudes ranging from 800 to 11,000 km, of signals ranging from
500 Hz to 6 kHz (see RIETVELDet al., 1990; JAMESet
596
M. T. RIETVELVet al.
ul., 1990). These results were used to further examine
the effects of irregularities
on the transionospheric
propagation
of VLF signals.
Attempts have been made to find lower frequency
signals, even down to PC 5 frequencies (mHz) on geostationary satellites without success. The lack ofdetection of these signals is probably due to several factors :
weak excitation of these waves in the magnetosphere,
their localised region of excitation which maps to a
very small region in the equatorial plane, and a small
number of coincident satellite passes with excitation
experiments.
Satellites passing through the modified region may
be used to observe large scale irregularities
in the
hundreds to thousands of metres range, as has been
done at Arecibo (FARLEY et al., 1983) using the AEE satellite. Attempts to measure similar variations at
Tromso with the DE-2 satellite were without success.
5. SUMMARY
We have described the present technical status and
capabilities of the heating facility at Tromser which
will be available to the whole EISCAT community
from .January 1993. Through the wider access to the
facility it is hoped that new and innovative experiments will be performed which throw further light on
some ofthe problems outlined above. An introduction
has been given to the many experiments and physical
problems which may be addressed
by ionospheric
modification.
It will be through the use of improved diagnostic
techniques with better temporal and spatial resolution
as well as simultaneous complementary
measurements
that further experimental progress will be made. It is
to be hoped that efforts will be made to apply
improved and new diagnostics to the problems that
can be studied through ionospheric modification. Well
designed experiments that test the theories are necessary. One area of particular interest is that of the saturation of Langmuir turbulence where both further
theoretical and experimental developments are necessary to explain all the observations.
Many of the other
phenomena associated with high power radio waves,
however, are also not fully understood,
as has been
outlined above. We look forward to continued growth
in our understanding
of the ionospheric
plasma
through new initiatives brought in by a wider group
of users of the heating facility.
Acknowledgements-The
authors
are
grateful
to L.
Bemmann, K. Eulig and H. Gegner, for the successful operation and maintenance
of the heating facility. We gratefully
acknowledge
the financial support of the Deutsche Forschungsgemeinschaft
(DFG) in building the Heating facility.
We thank the Plan-og Utbyggingsavdelingen
(PLUT) of the
University of Tromss for extensive support.
REFERENCES
BARR R., RIETVELDM. T., KOPKA H.
and STUBBEP.
BARR R., RIETVELDM. T., STUBBEP.
and KOPKA H.
1984
BARK R., RIETVELDM. T., STUBBEP.
and KOPKA H.
BARR R., RIETVELDM. T., STUBBEP.
and KOPKA H.
1987
BARR R. and STUBBEP.
1984
BARR R. and STUBBEP
1991a
BARR R. and STUBBEP
1991b
BARR R., STUBBEP. and KOPKA H.
1991
BARR R., STUBBEP., RIETVELDM. T
and KOPKA H.
1986
BASU S., BASU S., STUBBEP., KOPKA H. and
WAAKAMAA J.
1987
BELIKO~ICHV. V., BENEVIKTOVYE. A.,
GULYAYEVAT. L. and TERINAG. I.
1979
1985
1988
The effect of a heated patch of aurora1 ionosphere on
VLF radio wave propagation.
Nature 309, 534.
The diffraction of VLF radio waves by a patch of ionosphere illuminated by a powerful HF transmitter.
J.
geophys. Res. 90,286l.
Ionospheric heater beam scanning : a mobile source of
ELF radiation. Radio Sci. 22, 1073.
Ionospheric heater beam scanning : a realistic model of
this mobile source of ELFjVLF radiation. Radio Sci.
23, 379.
The ‘polar electrojet antenna’ as a source of ELF radiation in the Earth-ionosphere
waveguide. J. atmos.
terr. Phys. 46, 3 15.
ELF radiation from the Tromso ‘super-heater’
facility.
Geophys. Res. Left. 18, 1035.
On the ELF generation efficiencv of the Tromss heater
facility. Geiphys. Res. Lett. 18, 1971.
Long-range
detection of VLF radiation
produced by
heating the aurora1 electrojet. Radio Sci. 26, 87 1.
ELF and VLF signals radiated by the ‘Polar Electrojet
Antenna’ : experimental
results. J. geophys. Res. 91,
4451.
Daytime
scintillations
induced
by high-power
HF
waves at Tromso,
Norway.
J. geophys. Res. 92,
11,149.
Determination
of the electron density profile from the
resonance scatter of radio waves and vertical sounding ionograms.
Geomagn. Aeron. 19, 68 1.
Ionospheric
heating
BELIKOVICHV. V., BENEDIKTOVE. A. and
TER~NA G. I.
1986
BERNHARDT P. A., DUNCAN L. M. and TEPLEY C. A
1989
BIRKMAYERW., HAGFORS T. and KOFMAN W.
1986
COLLIS P. N., HACGSTRBM I., KAILA K
and RIETVELD M. T.
1991
CRACIN B. L., FEJER J. A. and LEER E.
1977
CZECHOWSKY I’., SCHMIDT G. and KOPKA H.
1983
DAS A. C. and FEJER J. A
1979
DERBLOM H., THIDB B., LEYSERT. B.,
NORLDING J. A., HEDBERG A., STUBBE P.,
KOPKA H. and RIETVELD M.
DJUTH F., JOST R. J., NOBLE S. T., GORDON W. E.,
STUBBE P., KOPKA H., NIELSEN E.,
BOSTR~M R., DERBLOM H., HEDBERG A.
and THID~: B.
DOWDEN R. L., ADAMS C. D. D., RIETVELD M. T.,
STUBBE P. and KOPKA H.
1989
DuBors D. F., ROSEH. A. and RUSSEL D
1990
DUNCAN L. M. and BEHNKE R. A.
1978
FARLEY D. T., LAHOZ C. and FEJER B. G.
1983
FEJER J. A.
1979
FEJER J. A. and KOPKA H.
1981
FREY A., STUBBE P. and KOPKA H.
1984
GORDON W. E. and DUNCAN L. M.
1990
GUREVICH A. V.
1978
GUREVICH A. V., BABICHENKOA. M.,
KARASHTIN A. N. and RAPOPORT V. 0.
HANUISE C., HEDBERG A, OKSMAN J.,
NIELSEN I?., STUBBE P. and KOPKA H.
1992
HEDBERG A., DERBLOM H., THIDB B., KOPKA H.
and STUBBE P.
HEDBERG A., TIIID~ B., BOSTR~ R.,
1983
1985
1991
1986
1984
DERBLOMH., KOPKA H. and STUBBEP.
HENRIKSENK., STOFFRECEN
W., LYBEKK B.
and STEENA.
1984
HIBBERDF. H., NIELSENE., STUBBEP.,
KOPKA H. and RIETVELDM. T.
1983
at TromseI
591
Diagnostics
of the lower ionosphere by the method of
radio wave resonance scattering. J. atmos. terr. Phys.
48, 1247.
Heater-induced
cavities as optical tracers of plasma
drifts. J. geophys. Res. 94, 7003.
Small-scale plasma-density
depletions in Arecibo highfrequency modification experiments. Phys. Rev. Lett.
57, 1008.
EISCAT
radar
observations
of enhanced
incoherent scatter spectra: their relation to red aurora
and field-aligned
currents.
Geophys. Res. Lett. 18,
1031.
Generation
of artificial spread-F
by a collisionally
coupled purely growing parametric instability. Radio
Sci. 12, 273.
Medium frequency radar observations
in the middle
atmosphere.
J. atmos. terr. Phys. 45, 729.
Resonance
instability
of small-scale
field-aligned
irregularities.
J. geophys. Rex 84, 6701.
Tromsa heating experiments : stimulated
emission at
HF pump harmonic and subharmonic
frequencies.
J. geophys. Res. 94, IO,1 11.
Observations
of E-region irregularities
generated
at
aurora1 latitudes by a high power radio wave. J.
geophys. Res. 90, 12,293.
Phase and amplitude perturbations
on subionospheric
signals produced
by a moving patch of artificially
heated ionosphere. J. geophys. Res. 96,239.
Excitation of strong Langmuir turbulence in plasmas
near critical density: application
of HF heating of
the ionosphere. J. geophys. Res. 95,21,221.
Observations
of self-focusing electromagnetic
waves in
the ionosphere. Phys. Rev. Let!. 41,998.
Studies of the self-focusing
instability
at Arecibo. J.
geophys. Res. 88,2093.
Ionospheric
modification
and parametric
instabilities.
Rev. Geophys. Space Phys. l?‘, 135.
The effect of oiasma instabilities on the ionosohericallv
reflected wave from a high power transmitter.
j.
geophys. Res. 86, 5146.
- _
First exoerimental
evidence of HF oroduced electron
densiiy irregularities
in the polar& ionosphere
diagnosed by UHF radio star scintillations. Geophys. Res.
Left. 11, 523.
Historical
overview of HF ionospheric
modification
research, AGARD Conf. Proc. No. 485, 3-l.
Nonlinear Phenomena in the Ionosphere. Springer, New
York.
HF sounding of the aurora1 magnetosphere.
J. geophys.
Res. 97, 8693.
Comparison
between the ionospheric plasma drift and
the motion of artificially
induced irregularities
as
observed by HF backscatter
radars. Ann. Geophys.
4,49.
Observations
of HF backscatter associated with heating
experiment at Tromsa. Radio Sci. 18, 840.
First observations
of 140-MHz plasma line backscatter
during heating experiments
at Tromsa. J. geophys.
Res. 89, 11,038.
Photometer
and spectrometer
search of the oxygen
green and red lines during artificial ionospheric heating in the aurora1 zone. Ann. Geophys. 2,73.
Production
of aurora1 zone E region irregularities
by powerful
HF heating.
J. geophys. Res. 88,
6341.
598
M. T. RIETVELDet al.
HILT O., BREKKEA., HANSENT., KOPKAH.
and STUBBE P.
1985
INHESTERB.,
DAS A. C. and FEIERJ. A.
1981
ISHAMB., KOFMANW., HAGFORST.,
NORDLING J., THIDBB., LAHOZ C. and STUBBE P.
JAMESH. G., INANU. S. and RIETVELD
M. T.
1990
JONE.SM. and GRUBBR. N.
1980
JONES1’. B., ROBIN~CIN
T. R., KOPKAH.
and STUBBEP.
1982
JONFST. B., ROBINSON
T. R., STUBBEP.
and KOPKAH.
1984
LEYSERT. B.
1989
LEYSERT. B. and THIDBB.
1988
LUNDBORG
B. and THIDBB.
1986
MAULA.-A., RIETVELD
M. T., STUBBEP.
and KOPKAH.
1990
MJBLHUSE.
1990
MJOLHUSE. and FLA T.
1984
NEWMANA. L., CARL~~NH. C. JR, MANTASG. P.
and DJUTH F.
1988
NOBLES. T. and GORDONW. E.
1990
NORLDINGJ. A., HEDBERGA., WANNBEKG
G.,
LEYSERT. B., DERBLOM
H., OPGEN~~RTHH. J.,
KOPKAH., KOHLH., STUBBEP.,
RIETVELD
M. T. and LAHOZ C.
PAPADOFQULOS
K., SHARMAA. S. and CHANGC. L.
1988
RIETVELDM. T., BARRR., STUBBEP.,
MAULA., KOPKAH. and DOWDENR. L.
RIETVELDM. T., COLLISP. N. and ST-MAURICEJ.-P
1990
RIETVELI)M. T., KOPKAH. and STUBBEP.
1986
RIETVELI)M. T., STUBBEP. and KOPKAH.
1989
ROBINSON
T. R.
1989a
ROBINSON
T. R.
1989b
ROSE G., GRANDAL
1985
1990
1989
1991
HF modification of the aurora1 D-region detected by a
partial reflection experiment. J. atmos. terr. Phys. 47,
537.
Generation of small-scale field-aligned irregularities in
ionospheric heating experiments. J. geophys. Res. 86,
9101.
New phenomena observed by EISCAT during RF ionospheric modification experiments. Radio Sci. 25,251.
Observations on the DE-l spacecraft of ELF/VLF
waves generated by an ionospheric heater. J. geophys
Res. 95, 12,187.
D-region partial reflection Doppler measurements with
the NOAA/MPAE digital HF radar. Report MPAEW-02-80-20, Max-Planck-Institut
fiir Aeronomie,
F.R.G.
Phase changes induced in a diagnostic radio wave passing through
a heated
region
of the aurora1
ionosphere. J. geophys. Res. 87, 1557.
Frequency
dependence
of anomalous
absorption
caused by high power radio waves. J. atmos. terr.
Phys. 46, 147.
Stimilated electromagnetic emission in the ionosphere.
Thesis. IRF Scientific Renort 198.
Effect ofpump-induced density depletions on the spectrum of stimulated electromagnetic emissions. J.
geophys. Res. 93,868 1.
Standing wave pattern of HF radio waves in the ionospheric reflection region 2. Applications.
Radio Sci.
21,486.
Excitation of periodic magnetic field oscillations in the
ULF range by amplitude modulated HF waves. Ann.
Geophys. 8,765.
On linear conversion
Sci. 25, 1321.
in a magnetized
plasma.
Radio
Direct access to plasma resonances in ionospheric radio
experiments. J. geophys. Res. 89,392 1.
Thermal response of the F region ionosphere for conditions of large HF-induced electron temperature
enhancements. Geophys. Res. Left. 15, 311.
The generation of GiF waves in the ionosphere.
AGARD Conf. Proc. No. 485. 38-l.
Simultaneous bistatic European Incoherent Scatter
UHF, 145-MHz radar and stimulated electromagnetic emission observations during HF ionospheric modification. Radio Sci. 23, 809.
On the efficient operation of a plasma ELF antenna
driven by modulation of ionospheric currents.
Comm. Plasma Phys. ControNed Fusion 13, 1.
VLF, ELF and ULF wave research using the Tromss
Heating facility. AGARD Conf. Proc. No. 485, 6-l.
Naturally enhanced ion-acoustic waves in the aurora1
ionosphere observed with the EISCAT 933 MHz
radar. J. geophys. Res. 96, 19,291.
D-region characteristics deduced from pulsed ionospheric heating under aurora1 electrojet conditions.
J. atmos. terr. Phys. 48, 3 11.
On the frequency dependence of ELFjVLF waves produced by modulated ionospheric heating. Radio Sci.
24,270.
B., NE~KEE., OTT W.,
SPENNERK., HOLTETJ., MISEIDEK.
and TR~IMJ.
An HF heating facility for EISCAT : the scientific case.
EISCAT Heating Working Group Report.
The heating of the high latitude ionosphere by high
power radio waves. Phys. Rept 179,79.
Experimental results from the HERO project : in situ
measurements of ionospheric modifications using
sounding rockets. J. geophys. Res. 90, 2851.
Ionospheric
heating
STUBBE P. and ENGELHARDT H.-G
1988
STURBEP., KOHI. H. and RIETVELU M. T
1992
STUBHEP. and KOPKA H.
1977
STUBBE P. H. and KOPKA H.
1979
STUBBE P. and KOPKA H.
1980
599
at TromseI
Modification
of D-region partial reflections. Lecture
notes from the second Suzdal URSI symposium,
BREKKE A. (ed.).
Langmuir turbulence and ionospheric modification.
J.
geophys. Res. 97, 6285.
Modulation
of the polar electrojet by powerful HF
waves. J. geophys. Res. 82, 2319.
Ionospheric
modification
experiments
in northern
Scandinavia-a
description
of the Heating project,
Report MPAE-W-02-79-04,
Max-Planck-Institut
fiir
Aeronomie, F.R.G.
Modification
of the F region by powerful radio waves.
Exploration of the Polar Upper Atmosphere, DEEHR
C. S. and HOLTET J. A. (eds), D. 83. D. Reidel.
Holland.
Stimulated
electromagnetic
emission in a magnetized
plasma : a new symmetric spectral feature. Phys. Rev.
Lett. 65, 183.
Wide band attenuation
caused by powerful HF waves.
J. geophys. Res. 87, 155 1.
Ionospheric
modification
experiments
in northern
Scandinavia.
J. atmos. terr. Phys. 44, 1025.
I.
STUBBE P. and KOPKA H.
1990
STUBBEP., KOPKA H., JONES T. B.
and ROBINSON T. R.
STUBBEP., KOPKA H., LAUCHE H.,
RIET~ELD M. T., BREKKE A., HOLT O.,
JONES T. B., ROBINSON T., HEDBERG A.,
THID~: B.. CROCHET B. and LOTZ H.-J.
STUBBE P., KOPKA H., R~ETVELDM. T
and DOWDEN R. L.
1982~
STUBBE P., KOPKA H., R~ETVELDM. T., FREY A.,
HOEG P., KOHL H., NIELSEN E., ROSE G., LAHOZ C.,
1985
1982a
1982b
ELF and VLF wave generation by modulated heating
of the current carrying lower ionosphere.
J. atmos.
terr. Phys. 44, 1123.
Ionospheric modification
experiments with the Tromse
heating facility. J. atmos. terr. Phys. 47, 1151.
BARR R., DERBLOM H., HEDBERG A., THIDB B.,
JONEST. B., ROBINSON T., BREKKEA., HANSEN T.
and HOLT 0.
STUBBEP., KOPKA H., THID~ B.
and DERBLOM H.
1984
TELLEGEN B. D. H.
THID~ B.
1933
1989
THID~ B., DERBLOM H., HEDBERG A.,
KOPKA H. and STUBBE P.
1983
THIDB B., KOPK.~ H. and STUBBE P.
1982
TURIJNEN E. and TURUNEN T.
WALKER J. C. G.
1985
1979
WRIGHT J. W.
1975
WRIGHT J. W., KOPKA H. and STUBBE P.
1988
Stimulated electromagnetic
emission : a new technique
to study the parametric decay instability in the ionosphere. J. geophys. Res. 89,7523.
Interaction
between radio waves? Nature 131,840.
Stimulated scattering of large amplitude waves in the
ionosphere : experimental
results. Phys. Scripta T30,
170.
Observations
of stimulated electromagnetic
emissions
in ionospheric
heating experiments.
Radio Sci. 18,
851.
Observations
of stimulated scattering of a strong highfrequency wave in the ionosphere.
Phys. Rez;. Lett.
49, 1561.
EISCAT Annual Report 1985, p. 41, S-98128 Kiruna.
Active experimentation
with the ionospheric
plasma.
Reo. Geophys. Space Phys. 17,534.
Evidence for precipitation
of energetic particles by
ionospheric ‘heating’ transmissions.
J. geophys. Res,
80,4383.
A large-scale depletion by intense radio wave heating.
Geophys. Res. Lett. 15, 153 1.