Evidence for the stimulation of field-aligned electron density irregularities on a short time scale by ionospheric topside sounders |
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| Authors: | Robert F Benson |
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| Institution: | Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. |
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| Abstract: | Ionospheric topside sounders can be considered to act as mobile ionospheric heating facilities. They stimulate a wide variety of plasma phenomena that suggests that significant plasma heating can be produced in the vicinity of the spacecraft following the short duration (0.1 ms) high-power (hundreds of watts) sounder pulse. Most of these phenomena are sensitive to the ambient plasma conditions, particularly to the ratio of the plasma frequency  to the gyro frequency  . Certain stimulated phenomena only occur, or are greatly enhanced, when  where n is an integer. One example is a diffuse signal return that appears at a frequency just above the Z mode wave cutoff frequency  (the L = 0 condition in the notation of Stix). This signal, which is the subject of the present paper, is present only for moderately large near-integer values for  (e.g., n > 3). It is a relatively short-duration echo—usually less than about 10 ms. These characteristics are quite different from the Z mode echoes often observed between  or  (whichever is greater) and the upper hybrid frequency  . These latter echoes occur for smaller values (not necessarily near-integer values) of  and are received during the entire 30 ms listening time period following the sounder pulse. They have been attributed to the scattering of sounder Z mode signals from naturally occurring electron density fieldaligned irregularities (FAI). The short-duration echoes observed just above  , on the other hand, are here attributed to the ducting of sounder-generated Z mode waves in sounder-stimulated (or sounderenhanced) FAI. These FAI are believed to be generated (or enhanced) on a very short time scale (? 1 s) by the efficient absorption of sounder energy when the plasma/gyro frequency ratio is nearly an integer value significantly greater than one. The most likely generation process is the filamentation instability driven by the ponderomotive force due to the high-power sounder pulse. |
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