Magnetic Reconnection Null Points as the Origin of Semi-relativistic Electron Beams in a Solar Jet

TL;DR

This study uses high-resolution radio imaging spectroscopy to observe semi-relativistic electron beams in a solar jet, directly linking their origin to magnetic reconnection null points in the low solar corona.

Contribution

It provides unprecedented high-precision observations of electron beam trajectories and identifies reconnection null points as their origin in a solar jet.

Findings

Electron beams diverge from compact regions in the low corona.

Reconnection null points are located behind the jet spire.

Null points likely have high density inhomogeneities at 10-km scales.

Abstract

Magnetic reconnection, the central engine that powers explosive phenomena throughout the Universe, is also perceived as one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to $\textit{in situ}$ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular ($\sim$20$''$), spectral ($\sim$1 %), and temporal resolution (50 milliseconds). These observations allow us to derive detailed trajectories of semi-relativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision ($<0''.65$). We found that each group of electron beams, which corresponds to a cluster of type III bursts with 1-2-second duration, diverges from an extremely compact region ($\sim$600 km$^2$) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10-km scales. These results, at least in the present case, strongly favor a reconnection-driven electron acceleration scenario.