Infrasonic Waves in the Atmosphere over East Siberia

E.A. Ponomarev and A.G. Sorokin
INFRASONIC WAVES IN THE ATMOSPHERE OVER EAST SIBERIA
Institute of Solar-Terrestrial Physics SD RAS,
Russia, 664033 Irkutsk, 126 Lermontov St.,
P.O.Box 4026, tel. (3952)461591, fax: (3952)462557
E-mail: [email protected], [email protected]
Introduction.
Because of its specific properties, the Earth's atmosphere is a medium where a broad class of atmospheric
oscillations is excited. Most of these oscillations of both natural and artificial origin have been studied fairly
thoroughly to date [1-3]. The goal of this report is to summarize previous studies of infrasonic waves in the East
Siberian region which have been carried out during 25 years.
Development of measuring facilities.
The infrasound measuring complex "Badary" was put into operation in 1974, 180 km south-west of
Irkutsk, in the Tunka valley, Buryatia. It included two three-position stations, respectively with the
baselines of ~0.5 and 10 km. First we shall attempt to categorize the infrasound studies into several
temporal stages.
The beginning of the first stage corresponds to the end of 1972. The objective of that period was to
create equipment comparing well, in senitivity and the width of the frequency and dynamic ranges,
with instrumentation available abroad, as well as to obtain all those known types of infrasonic signals
which are described in the literature. The main performance specifications of our developed
microbarograph were as follows [4]: sensitivity 0.05 Pa/Volts; frequency range 4-0.003 Hz; intrinsic
noise level 10-3 Pa; and working temperature range 10-30 C.
The short baseline was intended for recording short-period infrasound, microbaroms, as well as
infrasound from explosions and thunderstorms. The long baseline was designed for recording lowfrequency waves from distant sources in the auroral zone. It was impossible to achieve these
objectives in full measure; nevertheless, important information was obtained about signals from
distant explosions, in particular from Chinese nuclear explosions in the atmosphere.
Investigatin of impulsive sources.
A next stage in the development of acoustic investigations included the period when the scientific
interests of the research group were focused on the problem of infrasonic radiation effects on the
ionosphere. Of particular interest within this context were such eruptive sources as thunderstorms,
earthquakes and volcanic eruptions, and underground and atmospheric nuclear explosions [5]. The
group participated in comprehensive observations of atmospheric (ionospheric) responses to a surface
test explosion within the "MASSA" program, as well as to a series of industrial explosions. The
ionospheric effect from these explosions was clearly identified; it was most conspicuous in
observations of time variations of the phase path when measuring the F region-reflected signal on
weakly oblique paths. It was possible to construct a "Doppler portrait" model for the surface explosion
which described reasonably well the actual frequency variation of the sounding signal caused by the
influence of the acoustic wave from the explosion on the ionizing component [6].
Fig. 1. Auroral infrasonic wave AIW1.
Very interesting results were obtained by investigating the thunderstorm. Unlike the audible range,
in the infrasonic range the lightning discharge forms a plane rather than cylindrical wave. Also, as
follows from the theory reported by A.J.Dessler, infrasound is emitted by a cloud which seems to
Fig. 2. Wave packet structure of AIW2.
"give a start" during a lightning discharge. The initial phase of the infrasonic signal is negative in this
case. The observations [7] have confirmed the theory.
Investigations of auroral infrasound.
The polar expeditions (1976-1979) in Tixie obtained data on solitary waves that were described by
Wilson and were associated with the supersonic movement of auroral arcs (AIW1) (Fig. 1), as well as
new information about the existence in the polar zone of relatively long-lasting quasi-sinusoidal
oscillations that are closely associated with geomagnetic activity (AIW2) (Fig. 2) [8]. Their oscillation
spectrum has the form of a curve declining with frequency, showing the presence of intermediate
maxima on the periods of 20, 60, and 120 sec. Besides, noise-like
fluctuations of atmospheric pressure were identified, which are closely associated with short-period
Pi1-Pi2 pulsations of the Earth's electromagnetic field (SPP). These waves are anticipated
to be associated with a transformation of magnetospheric magnetosonic waves to ordinary infrasound
(Fig. 3) [9].
Investigations of microbaroms.
Scientifically, of great interest and advantage was the period of increased interest in seismoacoustic
phenomena and primarily in the use of microbaroms and microseisms in
diagnostics of the atmospheric acoustic channel.
Having a nearly identical spectrum, microbaroms and microseisms propagate in different media.
The Earth's crust can be regarded as a time-invariable medium. By comparing microbaroms and
microseisms, this permits a monitoring of acoustic channels to be carried out.
Fig. 3. Simultaneous recordings of geomagnetic pulsations (SPP) and auroral infrasound of AIW3 in the
frequency band of up to 4 Hz (top), and dynamic spectrum of AIW3 (bottom).
Microbaroms in East Siberia have the form of quasi-sinusoidal oscillations, with the central frequency
of about 0.2 Hz and the amplitude of 0.5-1 microbar. In the auroral zone, the microbarom spectrum is
a frequency band of 0.2-0.3 Hz, with the modulated upper frequency. It seems likely that the
modulation should be assigned to the effect associated with acoustic-gravity and tidal waves. The
amplitude of auroral microbaroms is somewhat higher compared with mid-latitude microbaroms and is
about 3 microbar. Microbaroms are observed virtually always when the atmospheric noise level is
below 0.5 microbar. Microbaroms arrive in East Siberia both from the North Atlantic Ocean and from
the Pacific Ocean. The probability of detecting microbaroms depends both on the amplitude in the
source and on propagation conditions and the noise level at the receiving point. We have demonstrated
that the decisive condition for receiving microbaroms is the wind direction on the axis of the sound
channel in the area of receiver location [10].
Microbaroms have been monitored at station "Badary" over a number of years, which enabled us to
study the seasonal properties of microbarom intensity deep in the interior of mainland at the distance
of 3-4 thousand kilometers from the source. It turns out that the intensity curve of microbaroms at the
inland station nearly faithfully copies that of microseisms. This suggests the conclusion that the
intensity of inland microbaroms is determined mainly by the source of their generation, and azimuthal
characteristics depend on the conditions along the propagation path.
Conclusion.
Both at the permanent station "Badary" and during the expeditions, it was possible to collect a
unique catalogue of waveforms of different kinds of atmospheric infrasound. We were successful in
developing the theory for the propagation of finite-amplitude infrasound in an exponential atmosphere
and to create, on this basis, a model of ionospheric response to a surface explosion. A diagnostic
technique for atmospheric acoustic channels was developed, on the basis of simultaneous reception of
microbaroms and microseisms. Dessler's theory as to the generation of infrasound from thunderstorm
has been confirmed. An explanation has been proposed for a number of generation and propagation
features of some kinds of ionospheric infrasonic waves.
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