Solar energetic electron events over one solar cycle

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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): dN/dJ = A x J, 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.

The observed 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. Since 3He-rich electron events (the majority of solar electron events) typically fill a solar longitude extent of <~45◦ (see Figure 3) , we infer that ~104 events/year may occur over the whole Sun at solar maximum. Out of the 1191 solar electron events, 1176 have a type III solar radio burst; the inferred burst rate ~104/year near solar maximum is the same order of magnitude as the occurrence rate of type III radio bursts detected below 14 MHz by WIND/WAVES near solar maximum. Studies of individual impulsive electron events (e.g., Lin 1990; Ergun et al. 1998; Wang et al. 2006) show that the Langmuir waves that scatter to produce the type III radio emission were observed in situ simultaneously with the arrival of ~2-10 keV solar electrons at 1 AU. These may suggest that all type III bursts are produced by a solar electron event, indicating a one-to-one correlation between solar electron beams and interplanetary type III radio emissions.

For 3He-rich electron events, only ~35% have a reported GOES SXR flare, with about 90% being impulsive flares, and most being C-class flares and located at longitude W30◦-75◦ (see Figure 3). But these solar 3He-rich electron events have a ~99% association with type III radio bursts and a ~60% association with west-limb CMEs, with ~50% of the associated CMEs being narrow (< 50◦). These suggest that in electron/3He-rich SEP events, these electrons are accelerated in narrow CMEs/jets originating from interchange reconnection (e.g., Shimojo & Shibata 2000); these electrons then generate waves that selectively accelerate sup>3</sup>He and heavy ions (e.g., Temerin & Roth 1992).

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