Plausibility of ultraviolet burst generation in the low solar chromosphere

TL;DR

This study uses advanced MHD simulations to demonstrate that UV bursts can originate in the low solar chromosphere through magnetic reconnection, heating plasma to over 20,000 K and producing observable spectral features.

Contribution

It introduces a realistic simulation framework including ionization and radiative cooling, showing UV burst generation in the low chromosphere is plausible and elucidating the heating mechanism involved.

Findings

UV bursts can be generated in the low chromosphere with magnetic fields >500 G.

Reconnection heating raises plasma temperature above 20,000 K.

Simulated spectral line profiles match observed features at high magnetic field strengths.

Abstract

Ultraviolet (UV) bursts and Ellerman bombs (EBs) are small-scale magnetic reconnection events taking place in the highly stratified, low solar atmosphere. It is still not clear whether UV bursts have to be generated at a higher atmospheric layer than EBs or whether both UV bursts and EBs can occur in the low chromosphere. We numerically studied the low $\beta$ magnetic reconnection process around the solar temperature minimum region (TMR). The time-dependent ionization degrees of hydrogen and helium are included in the MHD code, which lead to a more realistic magnetic diffusion caused by electron-neutral collision and ambipolar diffusion. A more realistic radiative cooling model from Carlsson & Leenaarts 2012 is included in the simulations. Our results in high resolution indicate that the plasmas in the reconnection region are heated up to more than $20,000$ K if the reconnecting magnetic field is as strong as $500$ G, which suggests that UV bursts can be generated in the dense low chromosphere. The dominant mechanism for producing the UV burst in the low chromosphere is heating, as a result of the local compression in the reconnection process. The thermal energy occurring in the reconnection region rapidly increases after the turbulent reconnection mediated by plasmoids is invoked. The average power density of the generated thermal energy in the reconnection region can reach over $1000$ erg cm$^{-3}$ s$^{-1}$, which is comparable to the average power density accounting for a UV burst. With the strength of the reconnecting magnetic field exceeding $900$ G, the width of the synthesized Si IV 1394 A line profile with multiple peaks can reach up to $100$ km s$^{-1}$, which is consistent with observations.