A study has been made of high-frequency backscatter observed by the ISIS 1 topside sounder at orbital heights from approximately 600 to 800 km. Signals are detected during high-latitude passes at radar ranges x of up to about 400 km and at frequencies from about the X mode cutoff at the spacecraft (usually around 5 MHz) up to 10 MHz typically and 18 MHz occasionally. The signals are interpreted as coherent aspect-sensitive scatter. The absolute power of return signals at high latitudes usually varies as x-2 for x≤150 km. This implies that the scattering cross-section density &sgr; is constant for azimuths about the magnetic field and for ranges out to 150 km or, in other words, that irregularity patches have a constant &sgr; over cross-field dimensions of 300 km. At larger ranges the signal often falls off more sharply with x, indicating azimuthal variations in &sgr;. The cross section scaled from the data using the radar equation is found to have values centered near 10-8 to 10-7 m-1 for the most intense signals, referred to an assumed polar-aspect angle sensitivity. However, the magnitude of &sgr; drops by about 2 orders of magnitude in the frequency range 5--15 MHz.

Previous statistical studies have established that strong backscatter is restricted to auroral lattitudes. Here the polar distribution of backscatter has been plotted for two sets of passes over the south pole, one collected during austral winter and the other during summer. The strongest backscatter is located at invariant latitudes around 80¿. Backscatter is significantly stronger and distributed more extensively over the polar cap in winter than in summer. ¿ The American Geophysical Union 1989