Physics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997jastp..59.2281b&link_type=abstract
Journal of Atmospheric and Terrestrial Physics, v. 59, p. 2281-2293.
Physics
6
Scientific paper
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 fN to the gyro frequency fH. Certain stimulated phenomena only occur, or are greatly enhanced, when fN/(fH) ≅ n 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 fZ (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 fN/(fH) (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 fH or fN (whichever is greater) and the upper hybrid frequency fT. These latter echoes occur for smaller values (not necessarily near-integer values) of fN/(fH) 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 fZ, on the other hand, are here attributed to the ducting of sounder-generated Z mode waves in sounder-stimulated (or sounder-enhanced) 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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