The Pitch-Angle Distribution Width Between 100 eV to 500 keV During the Solar Electron Burst of 2002 March 22

A compilation of various spacecraft measurements made over the last 30 years suggests that the pitch-angle distribution (PAD) width of a solar electron burst may have a complex, non-monotonic energy signature. However, only a few studies have considered the PAD width over a broad energy range during a single solar electron burst. Here we use WIND/3DP data to examine the energy dependence of the PAD width between 100 eV to 500 keV during the solar electron burst of 2002 March 22. We find that the PAD width-energy profile during this event was quite dynamic, occasionally showing significant changes in the width-energy profile over a time span of only 15 min. There were times, early on in the burst, when the PAD width below 10 keV was independent of energy. At later times, the PAD width below 10 keV displayed a local maximum at 4—6 keV. This local maximum was also present before and after the burst, but usually at lower energies. Such a local maximum may be a signature of the electrostatic electron/electron instability [Gary and Saito, GRL 34, 2007; Saito and Gary, JGR 113, 2008]. The PAD width-energy profile above 10 keV also showed considerable variability. There were times when the width increased monotonically with energy above 10 keV, consistent with cyclotron-resonant scattering by turbulence in the inertial range. At other times, the width increased with energy up to approximately 40—60 keV and then became independent of energy. At yet other times, the width-energy profile displayed a maximum near 100 keV. These results imply that the details and effectiveness of the wave-particle scattering mechanism vary with energy.