Scaling of electron heating by magnetization during reconnection and applications to dipolarization fronts and super-hot solar flares

TL;DR

This paper develops a theory predicting electron ring distribution properties during magnetic reconnection based on upstream plasma conditions, validated by simulations, and relates these findings to phenomena like dipolarization fronts and super-hot solar flares.

Contribution

The paper introduces a predictive model for electron ring distribution radii and temperatures in reconnection exhausts, validated by PIC simulations, linking upstream conditions to electron heating.

Findings

Excellent agreement between predicted and simulated ring distribution parameters.

Confirmation that magnetic field compression causes electron ring distributions.

Predicted electron temperatures align with observations in dipolarization fronts and solar flares.

Abstract

Electron ring velocity space distributions have previously been seen in numerical simulations of magnetic reconnection exhausts and have been suggested to be caused by the magnetization of the electron outflow jet by the compressed reconnected magnetic fields [Shuster et al., ${\it Geophys.~Res.~Lett.}, {\bf 41}$, 5389 (2014)]. We present a theory of the dependence of the major and minor radii of the ring distributions solely in terms of upstream (lobe) plasma conditions, thereby allowing a prediction of the associated temperature and temperature anisotropy of the rings in terms of upstream parameters. We test the validity of the prediction using 2.5-dimensional particle-in-cell (PIC) simulations with varying upstream plasma density and temperature, finding excellent agreement between the predicted and simulated values. We confirm the Shuster et al. suggestion for the cause of the ring distributions, and also find that the ring distributions are located in a region marked by a plateau, or shoulder, in the reconnected magnetic field profile. The predictions of the temperature are consistent with observed electron temperatures in dipolarization fronts, and may provide an explanation for the generation of plasma with temperatures in the 10s of MK in super-hot solar flares. A possible extension of the model to dayside reconnection is discussed. Since ring distributions are known to excite whistler waves, the present results should be useful for quantifying the generation of whistler waves in reconnection exhausts.