Non-thermal Observations of a Flare Loop-top using IRIS Fe XXI: Implications for Turbulence and Electron Acceleration

TL;DR

This study uses IRIS observations to link plasma turbulence at the loop-top of a solar flare with non-thermal electron acceleration, providing insights into energy dissipation processes during flares.

Contribution

It presents spatially resolved measurements of loop-top turbulence and its correlation with electron acceleration, a connection previously limited by observational constraints.

Findings

Non-thermal velocities up to 65 km/s indicating plasma turbulence.

Temporal decay of turbulence correlates with energy dissipation rates.

Co-spatial and co-temporal evidence of turbulence and non-thermal electrons.

Abstract

The excess broadening of high-temperature spectral lines, long observed near the tops of flare arcades, is widely considered to result from magnetohydrodynamic (MHD) turbulence. According to different theories, plasma turbulence is also believed to be a candidate mechanism for particle acceleration during solar flares. However, the degree to which this broadening is connected to the acceleration of non-thermal electrons remains largely unexplored outside of recent work, and many observations have been limited by limited spatial resolution and cadence. Using the Interface Region Imaging Spectrometer (IRIS), we present spatially resolved observations of loop-top broadenings using hot (11MK) Fe XXI 1354.1 \r{A} line emission at ~9s cadence during the 2022 March 30 X1.3 flare. We find non-thermal velocities upwards of 65km/s that decay linearly with time, indicating the presence and subsequent dissipation of plasma turbulence. Moreover, the initial Fe XXI signal was found to be co-spatial and co-temporal with microwave emission measured by the Expanded Owens Valley Solar Array (EOVSA), placing a population of non-thermal electrons in the same region as the loop-top turbulence. Evidence of electron acceleration at this time is further supported by hard X-ray measurements from the Spectrometer/Telescope for Imaging X-rays (STIX) aboard Solar Orbiter. Using the decay of non-thermal broadenings as a proxy for turbulent dissipation, we found the rate of energy dissipation to be consistent with the power of non-thermal electrons deposited into the chromosphere, suggesting a possible connection between turbulence and electron acceleration.