<p>Chromospheric evaporations are frequently observed at the footpoints of flare loops in flare events. The evaporations flows driven by thermal conduction or fast electron deposition often have high speed of hundreds km/s. Since the speed of the observed evaporation flows is comparable to the local Alfven speed, it is reasonable to consider the triggering of Kelvin-Helmholtz instabilities. Here we revisit a scenario which stresses the importance of the Kelvin-Helmholtz instability (KHI) proposed by Fang et al. (2016). This scenario suggests that evaporations flows from two footpoints of a flare loop can meet each other at the looptop and produce turbulence there via KHI. The produced KHI turbulence can play important roles in particle accelerations and generation of strong looptop hard X-ray sources. We investigate whether evaporation flows can produce turbulence inside the flare loop with the help of numerical simulation. KHI turbulence is successfully produced in our simulation. The synthesized soft X-ray curve demonstrating a clear quasi-periodic pulsation (QPP) with period of 26 s. The QPP is caused by a locally trapped, fast standing wave that resonates in between KHI vortices.</p><div></div><div></div><div></div><div></div><div></div><div></div><div></div><div></div><div></div><div></div>

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In a Priority Program on Massively Parallel Computing, eight Dutch research groups (http://www.phys.uu.nl/~mpr/) cooperate since 1995 to develop new algorithms and software to simulate the non-linear dynamics of thermonuclear, geophysical, and astrophysical plasmas. As one result of this collaboration, the generalpurpose Versatile Advection Code (http://www.phys.uu.nl/~toth/) has demonstrated excellent scaling properties across a variety of shared and distributed memory architectures. VAC uses various shock capturing numerical methods with explicit, semi-implicit, or fully implicit time stepping, on 1, 2, or 3 dimensional nite volume grids. Portability to di erent hardware platforms is achieved by preprocessors that can translate the code from Fortran 90 both forwards to High Performance Fortran and backwards to Fortran 77. With this code, we simulate complicated magnetized plasma phenomena where simultaneous large-scale and small-scale structures evolve on di erent timescales. We investigate fundamental hydromagnetic instabilities in multi-dimensional, fully non-linear regimes, as well as astrophysically oriented applications. An overview of recently investigated phenomena includes: the role of the magnetic eld in Rayleigh-Taylor and Kelvin-Helmholtz instabilities, stellar wind models and solar coronal mass ejections, and accretion ows onto compact objects (black holes).

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<p>In order to study the evaporation of chromospheric plasma and the formation of hard X-ray (HXR) sources in solar flare events, we coupled an analytic energetic electron model with the multi-dimensional MHD simulation code MPI-AMRVAC. The transport of fast electrons accelerated in the flare looptop is governed by the test particle beam approach reported in Emslie et al. (1978), now used along individual field lines. Anomalous resistivity, thermal conduction, radiative losses and gravity are included in the MHD model. The reconnection process self-consistently leads to formation of a flare loop system and the evaporation of chromospheric plasma is naturally recovered. The non-thermal HXR emission is synthesized from the local fast electron spectra and local plasma density, and thermal bremsstrahlung soft X-ray (SXR) emission is synthesized based on local plasma density and temperature. We found that thermal conduction is  an efficient way to trigger evaporation flows. We also found that the generation of a looptop HXR source is a result of fast electron trapping, as evidenced by the pitch angle evolution. By comparing the SXR flux and HXR flux, we found that a possible reason for the “Neupert effect” is that the increase of non-thermal and thermal energy follows the same tendency.</p>