Variation of electron-strahl width in the high-speed solar wind: Ulysses observations

Halo electron velocity distributions have been surveyed during the first full Ulysses southern polar pass from January 1994 through April 1995. The data examined range from approximately 50° S heliolatitude to the maximum poleward extent of the Ulysses orbit, approximately 80° S, and then back through the solar equator to 35° N heliolatitude. During this traversal the radial distance of Ulysses ranged from 3.8 to 1.3 AU, allowing the same heliolatitude to be sampled at two different distances. The angular width of the electron strahl is found to be broader throughout the coronal hole region than that reported for high speed streams by in-ecliptic spacecraft during the mid 1970's. The average strahl width (full width at half maximum) extrapolated to 1 AU is found to be 49° for 77 eV electrons. The strahl width broadens substantially between 1.3 and 2.3 AU after which the width is approximately constant. The broadening is greatest for the lower energy (77 eV) electrons. One might expect a broadening of the strahl 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 scattering of the electron strahl occurs, consistent with interpretations of the reduction in electron heat flux with increasing heliocentric distance measured by Ulysses.