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 <1keV to ≳300keV 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 40keV (2.8keV), the integrated occurrence rate above ∼0.29 (∼330)cm -2s-1sr-1keV-1 near 1AU is ∼1000year-1 (∼600year-1) at solar maximum and ∼35year-1 (∼25year-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 ∼104year-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 (∼0.02-2MeV 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 1keV, indicating a source high in the corona. © 2012. The American Astronomical Society. All rights reserved.