Solar energetic electron events over one solar cycle

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Revision as of 02:46, 2 May 2012

Electrons accelerated at/close to the Sun are observed in situ as the velocity-dispersed increase in particle fluxes well above plasma energies, as expected if the electrons of all energies were simultaneously accelerated at the Sun and then traveled the same distance along the interplanetary magnetic field line to reach the spacecraft. This phenomenon is referred to as Solar Energetic Particle (SEP) events. We survey the statistical properties of 1191 solar electron events observed by the WIND 3DP instrument from 1995 through 2005, and we find that 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: ~10/year at solar minimum and ~190/year at solar maximum. After correcting for periods of high background, the inferred occurrence frequency of solar electron events exhibits a power-law distribution (see Figure 1): Failed to parse (PNG conversion failed; check for correct installation of latex, dvips, gs, and convert): dN/dJ = A \times J^{-\gamma} , with the index γ of ~1.08 to 1.63 (~1.02 to 1.38) at 40 keV (2.8 keV) for different years, similar to the frequency distributions of solar proton events with γ of ~1.1 to 1.5 (e.g., Cliver et al. 1991; Belov et al. 2007). These electron frequency distributions are significantly flatter than the frequency distributions of SXR flares with γ of ~2.2 (e.g., Hudson 2007; Belov et al. 2007), but similar/close to the frequency distributions of microflares with γ of ~1.4 to 1.6 (Christe et al. 2008a) and of CME kinetic energies with γ of ~1 (Vourlidas et al. 2002) and ~1.7 (Yashiro et al. 2008). At 40 keV and 2.8 keV, the integrated occurrence rate near the Earth is up to one order of magnitude larger than the observed occurrence rate (see Figure 2). This implies that the observed event number near 1 AU is likely strongly underestimated since many small electron events are missed due to high background or limited instrument sensitivity. Since these events typically extend over ~45◦ in longitude, the occurrence rate over the whole Sun is inferred to be �104/year near solar maximum. The observed solar electron events have a 98.75% association with type III radio bursts, indicating all type III bursts may be associated with a solar electron event. Solar electron events have a close (�76%) association with the presence of low-energy (�0.02-2 MeV/nucleon), 3He-rich (3He/4He > 0.01) ion emissions measured by the ACE ULEIS instrument. For these electron events, only �35% are associated with a reported GOES SXR flare, but �60% appear to be associated with a CME.