Spectral characteristics of radio noise at low and medium frequencies in the Antarctic topside ionosphere

The ISIS topside sounder data obtained at Syowa Sation, Antarctica, for the period from April 1976 to November 1977 are examined with emphasis on the noise spectra appearing on the Automatic Gain Control (AGC) data and on the ionograms. The noise events were observed on 16 out of 88 ISIS-1 passes and on 8 out of 138 ISIS-2 passes. At high altitudes near ISIS-1 apogee, almost all of the noise events are due to auroral kilometric radiations (AKR). A special event of AKR observed in the dayside ionosphere is investigated in detail. The result shows that this cusp-associated AKR occurred in a large scale region of electron density depletion where the ratio of electron plasma frequency, f_N to electron gyrofrequency, f_H ranges from 0.1 to 0.05. At altitudes below 2900km, two types of noise were observed; the whistler mode noise and the noise band appearing between the local f_N and f_T (upper hybrid resonance frequency). These noises are examined in connection with the local characteristic frequencies. The dependence of these noise intensities on the relationship between f_N and f_H is found to be in a qualitative agreement to Maggs\u27 power flux calculation of the electrostatic noise using the plausible auroral electron beam models