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

We present a statistical survey of ∼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∝v−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 ≳0.1 AU and a temporal scale of ≳several days. The integrated density of quiet-time superhalo electrons at 2–20 keV ranges from ∼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 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.