Quiet-time interplanetary ∼2-20 keV superhalo electrons at solar minimum

Abstract

We present a statistical survey of 2-20keV superhalo electrons in the solar wind measured by the SupraThermal Electron instrument on board the two STEREO spacecraft during quiet-time periods from 2007 March through 2009 March at solar minimum. The observed superhalo electrons have a nearly isotropic angular distribution and a power-law spectrum, fv -γ, with γ ranging from 5 to 8.7, with nearly half between 6.5 and 7.5, and an average index of 6.69 ± 0.90. The observed power-law spectrum varies significantly on a spatial scale of ≳0.1AU and a temporal scale of ≳severaldays. The integrated density of quiet-time superhalo electrons at 2-20keV ranges from 10-8cm-3 to 10-6cm -3, about 10-9-10-6 of the solar wind density, and, as well as the power-law spectrum, shows no correlation with solar wind proton density, velocity, or temperature. The density of superhalo electrons appears to show a solar-cycle variation at solar minimum, while the power-law spectral index γ has no solar-cycle variation. These quiet-time superhalo electrons are present even in the absence of any solar activity - e.g., active regions, flares or microflares, type III radio bursts, etc. - suggesting that they may be accelerated by processes such as resonant wave-particle interactions in the interplanetary medium, or possibly by nonthermal processes related to the acceleration of the solar wind such as nanoflares, or by acceleration at the CIR forward shocks. © 2012. The American Astronomical Society. All rights reserved..