HR: 0800h
AN: SH21A-0292    [Abstracts]
TI: Evolution of suprathermal electron distributions with heliocentric distance
AU: * Owens, M J
EM: mjowens@bu.edu
AF: Center for Space Physics, Boston University, 725 Commonwealth Ave, Boston, MA 02215, United States
AU: Crooker, N U
EM: crooker@bu.edu
AF: Center for Space Physics, Boston University, 725 Commonwealth Ave, Boston, MA 02215, United States
AB: Although suprathermal electrons (> 70eV at 1 AU) form less than 1% of the total solar wind electron flux, they carry heat flux away from the Sun and provide an effective tracer of heliospheric magnetic field topology. As electrons travel antisunward, they are subject to two competing processes: Adiabatic focusing owing to the decreasing heliospheric magnetic field strength, creating a more field-aligned distribution, and pitch angle scattering by some unspecified mechanism, creating a more isotropic distribution. Close to the Sun, adiabatic focusing must dominate, as a field-aligned distribution ("strahl") is observed at 1 AU. However, scattering must become increasingly important further from the Sun, as the strahl is observed to broaden with heliocentric distance. We present a model for the evolution of suprathermal electron distributions with heliocentric distance which demonstrates the importance of the Parker spiral magnetic field: In a given amount of time, electrons of a given energy and pitch angle move the same distance along the heliospheric magnetic field, but at larger heliocentric distance the radial displacement will be smaller due to the increasing angle between the magnetic field and the radial direction. Thus scattering will begin to dominate focusing with increasing distance from the Sun without any change in the pitch-angle scattering rate. Furthermore, the rate of strahl broadening will decrease with increasing electron energy, as observed, without requiring an explicit energy-dependence in the pitch-angle scattering mechanism. We determine the scattering rate required to match observations.
DE: 2134 Interplanetary magnetic fields
DE: 2164 Solar wind plasma
DE: 2499 General or miscellaneous
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2007 Fall Meeting