QUIET-TIME INTERPLANETARY ∼2-20 keV SUPERHALO ELECTRONS AT SOLAR MINIMUM

We present a statistical survey of similar to 2-20 keV 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, f proportional to v(-gamma), with. ranging from gamma 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 greater than or similar to 0.1 AU and a temporal scale of greater than or similar to several days. The integrated density of quiet-time superhalo electrons at 2-20 keV ranges from similar to 10(-8) cm(-3) to 10(-6) cm(-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 gamma 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.