Non-thermal Broadening of IRIS Fe XXI Lines Caused by Turbulent Plasma Flows in the Magnetic Reconnection Region During Solar Eruptions

TL;DR

This study uses 3D MHD simulations to demonstrate that turbulent plasma flows in solar flare reconnection regions cause the non-thermal broadening of Fe XXI lines observed by IRIS, linking plasma dynamics to spectral line features.

Contribution

The paper introduces a 3D MHD model that explains the origin of non-thermal line broadening in high-temperature lines during solar eruptions, connecting plasma turbulence to spectral observations.

Findings

Turbulent plasma flows cause non-thermal broadening of Fe XXI lines.

Modeled non-thermal velocities match IRIS observations, ranging from tens to over two hundred km/s.

Simulated spectral maps show dynamic downflows with high non-thermal velocities.

Abstract

Magnetic reconnection is the key mechanism for energy release in solar eruptions, where the high-temperature emission is the primary diagnostic for investigating the plasma properties during the reconnection process. Non-thermal broadening of high-temperature lines has been observed in both the reconnection current sheet (CS) and flare loop-top regions by UV spectrometers, but its origin remains unclear. In this work, we use a recently developed three-dimensional magnetohydrodynamic (MHD) simulation to model magnetic reconnection in the standard solar flare geometry and reveal highly dynamic plasma flows in the reconnection regions. We calculate the synthetic profiles of the Fe XXI 1354 \AA~line observed by the Interface Region Imaging Spectrograph (IRIS) spacecraft by using parameters of the MHD model, including plasma density, temperature, and velocity. Our model shows that the turbulent bulk plasma flows in the CS and flare loop-top regions are responsible for the non-thermal broadening of the Fe XXI emission line. The modeled non-thermal velocity ranges from tens of km s$^{-1}$ to more than two hundred km s$^{-1}$, which is consistent with the IRIS observations. Simulated 2D spectral line maps around the reconnection region also reveal highly dynamic downwflow structures where the high non-thermal velocity is large, which is consistent with the observations as well.