Quiet-time Interplanetary ∼2-20keV Superhalo Electrons at Solar Minimum

The average flux of the ~2-20 keV superhalo electrons in the solar wind during quiet-time periods, measured by the Suprathermal Electron instrument onboard the two STEREO spacecraft, slowly decreases with time from 2007 to 2009 and then increases in 2010, similar to the solar cycle variation around this solar minimum. We made a comprehensive study for a 2-year period from 2007 March through 2009 March, and found that the observed quiet-time ∼2-20 keV superhalo electrons have a nearly isotropic angular distribution and a power-law spectrum, f ∼ v^-γ, ranging from v^-5 to v^-8.7, with the average index of 6.69. The observed power-law spectrum varies significantly on spatial scales of >∼0.1 AU and/or temporal scale of >∼ days. There is no correlation (-0.1 < coefficient < 0.2) with the solar wind proton density, velocity and temperature, but the power-law index γ is weakly anti-correlated (coefficient -0.48) with the electron velocity distribution function at 14.8 keV. The origin of these quiet-time superhalo electrons remains unclear, but since they are present even in the absence of any solar activity, they may be due to resonant wave-particle interactions in the corona or the interplanetary space.