A Statistical Study of Solar Electron Events over One Solar Cycle

Abstract

We survey the statistical properties of 1191 solar electron events observed by the WIND 3DP instrument from <1 keV to gsim300 keV for a solar cycle (1995 through 2005). After taking into account times of high background, the corrected occurrence frequency of solar electron events versus peak flux exhibits a power-law distribution over three orders of magnitude with exponents between -1.0 and -1.6 for different years, comparable to the frequency distribution of solar proton events, microflares, and coronal mass ejections (CMEs), but significantly flatter than that of soft X-ray (SXR) flares. At 40 keV (2.8 keV), the integrated occurrence rate above ~0.29 (~330) cm^-2 s^-1 sr^-1 keV^-1 near 1 AU is ~1000 year^-1 (~600 year^-1) at solar maximum and ~35 year^-1 (~25 year^-1) at solar minimum, about an order of magnitude larger than the observed occurrence rate. We find these events typically extend over ~45° in longitude, implying the occurrence rate over the whole Sun is ~10⁴ year^-1 near solar maximum. The observed solar electron events have a 98.75% association with type III radio bursts, suggesting all type III bursts may be associated with a solar electron event. They have a close (~76%) association with the presence of low-energy (~0.02-2 MeV nucleon^-1), 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, with ~50% of these CMEs being narrow. These electrons are often detected down to below 1 keV, indicating a source high in the corona.