SPRG Seminars
March 8, 2011:
"Solar Impulsive Energetic Electron Events"
Linghua Wang, Space Science Lab, University of California at Berkeley
Solar impulsive electron events, the electron part of impulsive SEP events, are the most common solar particle acceleration phenomenon observed in the interplanetary medium. First, I will present a comprehensive statistical study of 1191 solar energetic electron events observed by WIND/3DP from 1995 through 2005 (almost one solar cycle). Similar to sunspots and Soft X-Ray (SXR) flares, the observed occurrence rate of solar electron events near the Earth shows a strong solar-cycle variation. After correction for the high foreground effect, the estimated occurrence frequency of solar electron events exhibits a good power-law distribution, and the estimated occurrence rate at 40KeV near the Earth is ~1000/year at solar maximum and ~30/year at solar minimum for the instrumental sensitivity of WIND/3DP, up to one order of magnitude larger than the observed occurrence rate.
The observed solar energetic electron events have a one-to-one association with type III radio bursts, and a close (~78%) association with low-energy, 3He-rich ion emissions. For 3He-rich (impulsive) electron events, ~25% are associated with reported GOES SXR flares, and ~60% are associated with LASCO coronal mass ejections. Second, I will present a case study to investigate the pitch-angle distributions of ~0.3 to 300 keV electrons in five solar impulsive electron events with strong scatter-free temporal profiles. We find that in the interplanetary medium within 1 AU, low-energy (<~10-40 keV) electrons propagate essentially scatter-free, while high-energy (>~10-40 keV) electrons experience more pitch-angle scattering, with scattering strength increasing with energy. Such scattering appears to be caused by resonance with waves/turbulence at scale greater than ~ the thermal proton gyroradius in the solar wind.