Numerical Simulation of Superhalo Electrons Generated by Magnetic Reconnection in the Solar Wind Source Region

TL;DR

This study uses numerical simulations to show how magnetic reconnection in the solar wind source region can accelerate electrons, producing a power-law energy spectrum similar to observed superhalo electrons in interplanetary space.

Contribution

It demonstrates that magnetic reconnection in the solar wind source region can generate superhalo electrons with observed spectral characteristics, a novel insight into their origin.

Findings

Electrons are accelerated from thermal energies to hundreds of keV by reconnection electric fields.

Upward-propagating electrons exhibit a power-law energy distribution similar to observations.

Spectral hardness varies with resistivity parameters in reconnection models.

Abstract

Superhalo electrons appear to be continuously present in the interplanetary medium, even at very quiet times, with a power-law spectrum at energies above $\sim$2 keV. Here we numerically investigate the generation of superhalo electrons by magnetic reconnection in the solar wind source region, using the MHD and test particle simulations for both single X-line reconnection and multiple X-line reconnection. We find that the direct current electric field, produced in the magnetic reconnection region, can accelerate electrons from an initial thermal energy of T $\sim10^5$ K up to hundreds of keV. After acceleration, some of the accelerated electrons, together with the nascent solar wind flow driven by the reconnection, propagate upwards along the newly-opened magnetic field lines into the interplanetary space, while the rest move downwards into the lower atmosphere. Similar to the observed superhalo electrons at 1 AU, the flux of the upward-traveling accelerated electrons versus energy displays a power-law distribution at $\sim$ 2 $-$ 100 keV, $f(E) \sim E^{-\delta}$, with a $\delta$ of $\sim$ 1.5 $-$ 2.4. For single (multiple) X-line reconnection, the spectrum becomes harder (softer) as the anomalous resistivity parameter $\alpha$ (uniform resistivity $\eta$) increases. These modeling results suggest that the acceleration in the solar wind source region may contribute to superhalo electrons.