Abstract
Abstract Turbulence has been observed in flare loops and is believed to be crucial for the acceleration of particles and in the emission of X-ray photons in flares, but how the turbulence is produced is still an open question. A scenario proposed by Fang et al. suggests that fast evaporation flows from flare loop footpoints can produce turbulence in the looptop via the Kelvin–Helmholtz instability (KHI). We revisit and improve on this scenario and study how the KHI turbulence influences extreme-ultraviolet (EUV) and X-ray emission. A 2.5D numerical simulation is performed in which we incorporate the penetration of high-energy electrons as a spatio-temporal dependent trigger for chromospheric evaporation flows. EUV, soft X-ray (SXR), and hard X-ray (HXR) emission are synthesized based on the evolving plasma parameters and given energetic electron spectra. KHI turbulence leads to clear brightness fluctuations in the EUV, SXR, and HXR emission, with the SXR light curve demonstrating a clear quasi-periodic pulsation (QPP) with period of 26 s. This QPP derives from a locally trapped, fast standing wave that resonates in between KHI vortices. The spectral profile of the Fe xxi 1354 line is also synthesized and found to be broadened due to the turbulent motion of plasma. HXR tends to mimic the variation of SXR flux and the footpoint HXR spectrum is flatter than the looptop HXR spectrum.
Highlights
Looptop hard X-ray (HXR) sources in flare loops are frequently reported in observations of solar flares (e.g., Masuda et al 1994)
Energetic electrons play a crucial role in this mechanism, as most of the energy of the HXR photons comes from electron bremsstrahlung
The production of Kelvin–Helmholtz instability (KHI) turbulence triggered by evaporation flows in a flare loop is simulated and extreme-ultraviolet (EUV), soft X-ray (SXR), and HXR views on the flare loop are studied
Summary
Looptop hard X-ray (HXR) sources in flare loops are frequently reported in observations of solar flares (e.g., Masuda et al 1994). Rare in solar flares (Nitta et al 2012), it is possible to get KHI triggered by evaporation flows in flare loops. Ruan et al (2018) surveyed the evaporation flow based trigger of KHI parametrically, and found that KHI turbulence is very likely to appear in short-duration, impulsive flares. The production of KHI turbulence triggered by evaporation flows in a flare loop is simulated and extreme-ultraviolet (EUV), soft X-ray (SXR), and HXR views on the flare loop are studied. We obtain a fully realistic, time- and space-dependent heating source that triggers the evaporation flows in the flare loop.
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