The Origin of Electron Halo in the Solar Wind Electron Velocity Distribution Function: Connection to Nanoflares in the Solar Corona

Abstract:

The formation of electron halo in the solar wind electron velocity distribution function (EVDF) is a long standing puzzle. In this letter, we propose that the electron halo is produced by the kinetic-scale turbulence driven by the nanoflare-accelerated keV electrons in the inner solar corona. The solar wind advects the core-halo feature of EVDFs and kinetic fluctuations from the corona into interplanetary plasma through open field lines. Based on the heating mechanism proposed, we find the location of the velocity ``break" discerning the core and halo is at three times of the thermal velocity of core; the ratio of halo-core temperature is about 6; and a relic relative drift that is comparable to the core thermal velocity exists between the core and the halo, a signature of two stream instability. These predictions are consistent with existing observations of solar wind EVDFs.