ULYSSES OBSERVATIONS OF WHISTLER WAVES AT INTERPLANETARY SHOCKS AND IN THE SOLAR-WIND

This study of whistler wave emission observed by the Ulysses Unified R adio and Plasma Wave (URAP) experiment between 1 and 5 AU is a complem ent to previous studies of whistler waves observed by the Hellos space craft between 0.3 and 1 AU. The Hellos spacecraft continuously detecte d a background of whistlers close to the Sun, and this background was found to decrease in intensity with larger heliocentric distance. Ulys ses plasma wave observations confirm this trend. Within a heliocentric distance of approximately 2 AU, whistler waves are routinely observed . Beyond about 3 AU the waves are usually observed only downstream of interplanetary shocks. Moreover, whistler waves are routinely observed within about 2 AU at all heliographic latitudes of the Ulysses trajec tory (-80 degrees to +80 degrees). The combined observations from the Hellos and Ulysses spacecraft suggest that whistler emission is always present in the solar wind, although at larger heliocentric distances the wave amplitudes are often below the thresholds of the URAP instrum ent. Observations throughout the first 5 years of the Ulysses mission show a clear correlation of whistler emission intensity with magnetic field strength, or gyrofrequency, such that increases in wave intensit ies coincide with increases in gyrofrequency. This correlation is espe cially evident in observations of interplanetary shocks and high-speed streams. A possible cause of this correlation is increased whistler w ave growth due to enhanced electron temperature anisotropies in region s of compressed magnetic held. A shift of the background whistler spec trum as a function of gyrofrequency could account for the observed dec rease in whistler amplitudes with increasing heliocentric distance.