Quiet-time Solar Wind Superhalo Electrons at Solar Minimum

摘要

We survey the statistical properties of ~2-20 keV superhalo electrons in the solar wind measured by the STEREO/STE instrument during quiet-time periods from 2007 March through 2009 March at solar minimum. The observed quiet-time 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 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 it, as well as the power-law spectrum, shows no correlation with solar wind protons. The density of superhalo electrons decreases by approximately one order of magnitude between early 2007 and early 2009, probably reflecting the decay of solar cycle 23 and the approach to its unusually deep activity 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 resonant wave-particle interactions in the interplanetary medium, or by nonthermal processes related to the acceleration of the solar wind such as nanoflares.