COSMIC RADIO NOISE ABSORPTION RELATED TO STRUCTURES IN AURORAL LUMINOSITY

Digitized all-sky white light images of auroral optical emissions, rec orded by a low-light level TV system at Sanae (70.3 degrees S, 2.4 deg rees W, L = 4.0), have been mapped onto the angular sensitivity functi ons of the inner four beams of a 16-element imaging riometer. Cosmic r adio waves of 5-10 m wavelengths appeared to be absorbed more strongly in directions through regions adjacent to discrete auroral arcs than through the are regions themselves. These stronger absorptions may be due either to enhanced electron temperatures, caused by the Farley-Bun eman two-stream instability in the presence of large electric fields a round amoral arcs in the E region, or to D region absorption caused by energetic electrons precipitating along magnetic field lines passing through regions adjacent to auroral arcs. The two auroral events befor e local magnetic midnight reported in this paper started with high lum inosity and small ionospheric absorption. The absorption increased as the event developed. Auroral luminosities and structures were changing in all four viewing directions during the event on May 10, 1992, with variations in absorptions following variations in luminosity after a delay of 30 s. The event on June 8, 1992, involved a pulsating are str ucture. Changes in absorption appeared to be delayed relative to chang es in luminosity that varied from 0 to 60 s in a particular viewing di rection. The auroral event after local magnetic midnight on April 14, 1993, differed fr-om the former two events in the appearance of pulsat ing auroral patches and in slower temporal variations, but, occasional ly, changes in absorption of cosmic radio noise in the four beam direc tions were still delayed relative to changes in luminosity. The observ ed spatial and temporal differences in regions of optical emissions an d ionospheric absorption should be of magnetospheric origin rather tha n ionospheric origin on account of the long delay times.