Widths of suprathermal pitch angle distributions during solar electron bursts: ACE observations

摘要

Solar active processes frequently produce electron bursts which, as observed at 1 AU, extend down to energies less than 1.4 keV. The characteristics of these solar electron bursts vary considerably from event to event due in part to the physical processes involved in their propagation to Earth. We present a thorough survey of 71 low-energy solar electron bursts that extended down to least 713 eV as observed by ACE/SWEPAM in 2002. This study characterizes properties of these bursts, including the preburst and maximum burst intensity, and the burst duration, as well as properties of the solar electron pitch angle distributions, including preburst strahl width and maximum burst beam width. Approximately 60% of bursts observed down to at least 713 eV have, on average, broader beams than the preceding strahl. Our analysis shows that all bursts in this survey have similar preburst strahl widths and electron intensities. Consequently, we conclude that beam broadening does not depend on preburst conditions. Furthermore, we find preferential associations between bursts that exhibit increased beam widths and several burst characteristics, namely, (1) high-intensity bursts, (2) bursts observable below 519 eV, and (3) long-duration bursts. Broadening of the pitch angle distribution function during solar electron bursts almost certainly results from particle scattering. Since the broadening of the pitch angle distribution function during bursts does not depend on preburst conditions and it does depend on burst characteristics, we suggest that scattering may result from self-generated fluctuations which should increase in magnitude as the energy density of the burst beam increases. Finally, we describe two sources of wave-particle interactions that offer plausible explanations for broadening of the electron pitch angle distribution during solar bursts.