RIOMETER ABSORPTION EVENTS AT SANAE, L=4.0

The absorption of cosmic radio noise passing through the ionosphere ma y be described as a function of radio wave frequency A(f(e)) proportio nal to f(e)(-n), with n approximate to 2.0 for spatially uniform preci pitation of electrons and n < 2.0 for spatially nonuniform precipitati on. Using multifrequency riometer recordings at SANAE, the following o bservations are reviewed: (1) The frequency distribution of the power index, n, obtained from 4 min averaged absorptions during 1983, shows a most probable value around n approximate to 1.5, indicating that mos tly energetic electrons are precipitated spatially structured onto the upper atmosphere, as in optical aurora. (2) Multifrequency riometer r ecordings suggest that field-aligned ionospheric irregularities have s cattered additional cosmic radio waves from the central region of the Galaxy into the fields of views of the riometer antennae during an aur oral absorption event in the early morning hours of 27 July 1982. With the power reflectivity by ionospheric irregularities inversely propor tional to the fourth power of radio wave frequency, as required by the Bragg condition, an estimated 70% increase in the 20 MHz radio flux a t 01:22 UT, at the strong absorption peak, can explain the strongly re duced absorption observed in 20 MHz relative to 30 and 51.4 MHz. (3) G radual increases in absorptions observed at all three riometer frequen cies from onset at 11:50 UT of the largest solar proton ground level e nhancement on 29 September, 1989, until 18:00 UT, suggest diffusion of the much more intense low energy protons from the polar cap to the L = 4.0 geomagnetic field shell and subsequent precipitation at SANAE du e to the South Atlantic Geomagnetic Anomaly. (4) The flux of electron energy deposited per second at SANAE is closely related to geomagnetic activity, but has a lower maximum during the years 1971 and 1980 of s olar polar magnetic reversals than in the years 1976 and 1986/87 of mi nimum solar activity. (5) A significant correlation has been found bet ween the arrival of single-hop whistlers and 30 MHz riometer absorptio n events, using point statistics. The maximum absorption at 30 MHz was similar to 0.04 dB with a delay of 3 +/- 2 s relative to the whistler .