Solar flares and Kelvin-Helmholtz instabilities: A parameter survey

TL;DR

This paper investigates how Kelvin-Helmholtz instabilities triggered by chromospheric evaporation flows can generate turbulence in solar flare loops, potentially explaining hard X-ray sources via inverse Compton scattering.

Contribution

It provides a parameter survey demonstrating the conditions under which KHI occurs and turbulence develops in solar flare loops, highlighting the role of energy deposition asymmetry.

Findings

KHI develops routinely in typical M class flare conditions.

Turbulence spectra follow a -5/3 power law in velocity and magnetic field fluctuations.

Asymmetry in energy deposition influences turbulence location and periodic SXR signals.

Abstract

Hard X-ray (HXR) sources are frequently observed near the top of solar flare loops, and the emission is widely ascribed to bremsstrahlung. We here revisit an alternative scenario which stresses the importance of inverse Compton processes and the Kelvin- Helmholtz instability (KHI) proposed by Fang et al. (2016). This scenario adds a novel ingredient to the standard flare model, where evaporation flows from flare-impacted chromospheric foot-points interact with each other near the loop top and produce turbulence via KHI. The turbulence can act as a trapping region and as an efficient accelerator to provide energetic electrons, which scatter soft X-ray (SXR) photons to HXR photons via the inverse Compton mechanism. This paper focuses on the trigger of the KHI and the resulting turbulence in this new scenario. We perform a parameter survey to investigate the necessary ingredients to obtain KHI through interaction of chromospheric evaporation flows. When turbulence is produced in the loop apex, an index of -5/3 can be found in the spectra of velocity and magnetic field fluctuations. The KHI development and the generation of turbulence are controlled by the amount of energy deposited in the chromospheric foot-points and the time scale of its energy deposition, but typical values for M class flares show the KHI development routinely. Asymmetry of energy deposition determines the location where the turbulence is produced, and the synthesized SXR light curve shows a clear periodic signal related to the sloshing motion of the vortex pattern created by the KHI.