SPRG Seminars

March 8, 2011:

"Solar Impulsive Energetic Electron Events"

Linghua Wang, Space Science Lab, University of California at Berkeley

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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.