VARIATION OF ELECTRON-STRAHL WIDTH IN THE HIGH-SPEED SOLAR-WIND - ULYSSES OBSERVATIONS

Halo electron velocity distributions have been surveyed during the fir st full Ulysses southern polar pass from January 1993 through April 19 95. The data examined range from approximately 50 degrees S heliolatit ude to the maximum poleward extent of the Ulysses orbit, approximately 80 degrees S, and then back through the solar equator to 35 degrees N heliolatitude. During this traversal the radial distance of Ulysses r anged from 3.8 to 1.3 AU, allowing the same heliolatitude to be sample d at two different distances. The angular width of the electron strahl is found to be broader throughout the coronal hole region than that r eported for high speed streams by in-ecliptic spacecraft during the mi d 1970's. The average strahl width (full width at half maximum) extrap olated to 1 AU is found to be 49 degrees for 77 eV electrons. The stra hl width broadens substantially between 1.3 and 2.3 AU after which the width is approximately constant. The broadening is greatest for the l ower energy (77 eV) electrons. One might expect a broadening of the st rahl if scattering dominates magnetic focusing of electrons caused by the decreasing magnetic field magnitude with increasing solar distance , The results presented here suggest that substantial wave-particle sc attering of the electron strahl occurs, consistent with interpretation s of the reduction in electron heat flux with increasing heliocentric distance measured by Ulysses.