Recent Advances and Difficulties of Infrasonic Wave Investigation in the Ionosphere

Abstract

Acoustic waves have a remarkable ability to transfer energy from the ground up to the uppermost layers of the atmosphere. On the ground, there are many permanent sources of infrasound, and also pulsed and/or sporadic sources (e.g., sea waves, infrasonic and sonic noise of cities, lightning, earthquakes, explosions, etc.). The infrasonic waves carry away the major part of their energy upwards through the atmosphere. What are the consequences of the upward energy transfer? What heights of the atmosphere are supplied by energy from various sources of an infrasonic wave? In most cases, the answers to these questions are not well known at present. The only opportunity to monitor the propagation of an infrasonic wave to high altitudes is to watch for its influence on the ionospheric plasma. Unfortunately, most of standard equipment for ionospheric sounding, as a rule, cannot detect plasma fluctuations in the infrasonic range. Besides, the form of an infrasonic wave strongly varies during propagation due to nonlinear effects. However, the development of the Doppler method of radiosounding of the ionosphere has enabled progress to be made. Simultaneously, the ionospheric method for sensing aboveground and underground explosions has been developed. Its main advantage is the remote observation of an explosion in the near field zone by means of short radio waves, i.e., the radio sounding of the ionosphere directly above the explosion. The theory of propagation of an acoustic pulse produced by an explosion on the ground up to ionospheric heights has been developed better than the theory for other sources, and has been quantitatively confirmed by experiments. A review of some advances in the area of infrasound investigations at ionospheric heights is given and some current problems are presented.

Simultaneous infrasonic, seismic, magnetic and ionospheric observations in an earthquake epicentre

“…Model calculations show that Panská Ves is in the infrasonic ''shadow'' (silence) zone for the infrasound excited in the epicentre. These calculations were performed by a standard acoustic ray calculation technique used also by others; the technique is partly described by Krasnov et al (2006). Moreover the epicentral infrasound is not very strong.…”

“…(2) More important, infrasonic signals excited by the earthquake of 28 October 2008 were at frequencies, which were too high to penetrate into the ionosphere (e.g., Blanc, 1985;Krasnov et al, 2006Krasnov et al, , 2007. Typical ionospheric infrasound (periods tens of seconds to a few minutes) was not excited by this earthquake; this part of spectrum revealed undisturbed pattern.…”

Global pattern of trends in the upper atmosphere and ionosphere: Recent progress

“…Usually, wave activity is divided into three main categories: gravity waves (periods of tens of minutes to hours), tides with periods of 24 h plus harmonics, and planetary waves of periods of about 2-30 days. Infrasonic waves with periods of seconds to a few minutes (e.g., Blanc, 1983;Krasnov et al, 2006) are probably less important and no information is available about possible trends due to lack of measurements.…”

Peculiar transient phenomena observed by HF Doppler sounding on infrasound time scales

“…It is possible that in this case we observe either superposition (interference) of AGWs or disturbances propagating in other wave modes. We cannot exclude the fact that these disturbances are formed by infrasound excited at the surface by (quasi)point sources, the energy of which is focused by the troposphere and middle atmosphere upwards into a narrow cone (e.g., Blanc, 1985;Krasnov et al, 2006), which then broadens again in the ionosphere. Nevertheless, a source of the S-shapes at higher altitudes like the solar terminator or other mechanisms of their excitation is more probable as suggested by the following statistical study; the terminator can excite infrasonic waves similar to proven excitation of gravity waves (e.g., Bosˇka et al, 2003).…”

“…The HF Doppler technique is also an important observational tool used for studying the effects of infrasonic waves on the ionosphere. These effects have been reviewed by Blanc (1985), Pokhotelov et al (1995) and Krasnov et al (2006). There are many mechanisms and sources of infrasound excitation in the atmosphere such as various strong meteorological phenomena (Hedlin et al, 2002), bolides, meteors, solar eclipses, auroral activity, earthquakes, nuclear and strong chemical explosions, rocket and shuttle launches, aircrafts, etc.…”