EVIDENCE FOR THE STIMULATION OF FIELD-ALIGNED ELECTRON-DENSITY IRREGULARITIES ON A SHORT-TIME SCALE BY IONOSPHERIC TOPSIDE SOUNDERS

Ionospheric topside sounders can be considered to act as mobile ionosp heric heating facilities. They stimulate a wide variety of plasma phen omena 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 ra tio of the plasma frequency f(N) to the gyro frequency f(H). Certain s timulated phenomena only occur, or are greatly enhanced, when f(N)/f(H ) congruent to n where n is an integer. One example is a diffuse signa l return that appears at a frequency just above the Z mode wave cutoff frequency f(Z) (the L = 0 condition in the notation of Stir). This si gnal, which is the subject of the present paper, is present only for m oderately large near-integer values for f(N)/f(H) (e.g., n > 3). It is a relatively short-duration echo- usually less than about 10 ms. Thes e characteristics are quite different from the Z mode echoes often obs erved between f(H) or f(N) (whichever is greater) and the upper hybrid frequency f(T). These latter echoes occur for smaller values (not nec essarily near-integer values) of f(N)/f(H) 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 field-aligned irregularities (FAI ). The short-duration echoes observed just above f(Z), on the other ha nd, are here attributed to the ducting of sounder-generated Z mode wav es in sounder-stimulated (or sounder-enhanced) FAI. These FAI are beli eved to be generated (or enhanced) on a very short time scale(much les s than 1 s) by the efficient absorption of sounder energy when the pla sma/gyro frequency ratio is nearly an integer value significantly grea ter than one. The most likely generation process is the filamentation instability driven by the ponderomotive force due to the high-power so under pulse. (C) 1997 Elsevier Science Ltd.