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Magnetic reconnection is the key mechanism for energy release in solareruptions, where the high-temperature emission is the primary diagnostic forinvestigating the plasma properties during the reconnection process.Non-thermal broadening of high-temperature lines has been observed in both thereconnection current sheet (CS) and flare loop-top regions by UV spectrometers,but its origin remains unclear. In this work, we use a recently developedthree-dimensional magnetohydrodynamic (MHD) simulation to model magneticreconnection in the standard solar flare geometry and reveal highly dynamicplasma flows in the reconnection regions. We calculate the synthetic profilesof the Fe XXI 1354 \AA~line observed by the Interface Region ImagingSpectrograph (IRIS) spacecraft by using parameters of the MHD model, includingplasma density, temperature, and velocity. Our model shows that the turbulentbulk plasma flows in the CS and flare loop-top regions are responsible for thenon-thermal broadening of the Fe XXI emission line. The modeled non-thermalvelocity 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 mapsaround the reconnection region also reveal highly dynamic downwflow structureswhere the high non-thermal velocity is large, which is consistent with theobservations as well.

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