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Register a new userSolar flares and Kelvin-Helmholtz instabilities: A parameter survey
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Wenzhi Ruan
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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.
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In this paper we investigate whether Smoothed Particle Hydrodynamics (SPH), equipped with artificial conductivity, is able to capture the physics of density/energy discontinuities in the case of the so-called shearing layers test, a test for examining Kelvin-Helmholtz (KH) instabilities. We can trace back each failure of SPH to show KH rolls to two causes: i) shock waves travelling in the simulation box and ii) particle clumping, or more generally, particle noise. The probable cause of shock waves is the Local Mixing Instability (LMI), previously identified in the literature. Particle noise on the other hand is a problem because it introduces a large error in the SPH momentum equation. We also investigate the role of artificial conductivity (AC). Including AC is necessary for the long-term behavior of the simulation (e.g. to get $lambda=1/2, 1$ KH rolls). In sensitive hydrodynamical simulations great care is however needed in selecting the AC signal velocity, with the default formulation leading to too much energy diffusion. We present new signal velocities that lead to less diffusion. The effects of the shock waves and of particle disorder become less important as the time-scale of the physical problem (for the shearing layers problem: lower density contrast and higher Mach numbers) decreases. At the resolution of current galaxy formation simulations mixing is probably not important. However, mixing could become crucial for next-generation simulations.
Using data obtained by the high resolution CRisp Imaging SpectroPolarimeter instrument on the Swedish 1-m Solar Telescope, we investigate the dynamics and stability of quiet-Sun chromospheric jets observed at disk center. Small-scale features, such as Rapid Redshifted and Blueshifted Excursions, appearing as high speed jets in the wings of the H$alpha$ line, are characterized by short lifetimes and rapid fading without any descending behavior. To study the theoretical aspects of their stability without considering their formation mechanism, we model chromospheric jets as twisted magnetic flux tubes moving along their axis, and use the ideal linear incompressible magnetohydrodynamic approximation to derive the governing dispersion equation. Analytical solutions of the dispersion equation indicate that this type of jet is unstable to Kelvin-Helmholtz instability (KHI), with a very short (few seconds) instability growth time at high upflow speeds. The generated vortices and unresolved turbulent flows associated with the KHI could be observed as broadening of chromospheric spectral lines. Analysis of the H$alpha$ line profiles shows that the detected structures have enhanced line widths with respect to the background. We also investigate the stability of a larger scale H$alpha$ jet that was ejected along the line-of-sight. Vortex-like features, rapidly developing around the jets boundary, are considered as evidence of the KHI. The analysis of the energy equation in the partially ionized plasma shows that the ion-neutral collisions may lead to the fast heating of the KH vortices over timescales comparable to the lifetime of chromospheric jets.
Transport coefficients in highly ionised plasmas like the intra-cluster medium (ICM) are still ill-constrained. They influence various processes, among them the mixing at shear flow interfaces due to the Kelvin-Helmholtz instability (KHI). The observed structure of potential mixing layers can be used to infer the transport coefficients, but the data interpretation requires a detailed knowledge of the long-term evolution of the KHI under different conditions. Here we present the first systematic numerical study of the effect of constant and temperature-dependent isotropic viscosity over the full range of possible values. We show that moderate viscosities slow down the growth of the KHI and reduce the height of the KHI rolls and their rolling-up. Viscosities above a critical value suppress the KHI. The effect can be quantified in terms of the Reynolds number Re = U{lambda}/{ u}, where U is the shear velocity, {lambda} the perturbation length, and { u} the kinematic viscosity. We derive the critical Re for constant and temperature dependent, Spitzer-like viscosities, an empirical relation for the viscous KHI growth time as a function of Re and density contrast, and describe special behaviours for Spitzer-like viscosities and high density contrasts. Finally, we briefly discuss several astrophysical situations where the viscous KHI could play a role, i.e., sloshing cold fronts, gas stripping from galaxies, buoyant cavities, ICM turbulence, and high velocity clouds.
The Kelvin-Helmholtz (KH) instability is commonly found in many astrophysical, laboratory, and space plasmas. It could mix plasma components of different properties and convert dynamic fluid energy from large scale structure to smaller ones. In this study, we combined the ground-based New Vacuum Solar Telescope (NVST) and the Solar Dynamic Observatories (SDO) / Atmospheric Imaging Assembly (AIA) to observe the plasma dynamics associated with active region 12673 on 09 September 2017. In this multi-temperature view, we identified three adjacent layers of plasma flowing at different speeds, and detected KH instabilities at their interfaces. We could unambiguously track a typical KH vortex and measure its motion. We found that the speed of this vortex suddenly tripled at a certain stage. This acceleration was synchronized with the enhancements in emission measure and average intensity of the 193 AA{} data. We interpret this as evidence that KH instability triggers plasma heating. The intriguing feature in this event is that the KH instability observed in the NVST channel was nearly complementary to that in the AIA 193 AA{}. Such a multi-thermal energy exchange process is easily overlooked in previous studies, as the cold plasma component is usually not visible in the extreme ultraviolet channels that are only sensitive to high temperature plasma emissions. Our finding indicates that embedded cold layers could interact with hot plasma as invisible matters. We speculate that this process could occur at a variety of length scales and could contribute to plasma heating.
Recent observations support the propagation of a number of magnetohydrodynamic (MHD) modes which, under some conditions, can become unstable and the developing instability is the Kelvin--Helmholtz instability (KHI). In its nonlinear stage the KHI can trigger the occurrence of wave turbulence which is considered as a candidate mechanism for coronal heating. We review the modeling of tornado-like phenomena in the solar chromosphere and corona as moving weakly twisted and spinning cylindrical flux tubes, showing that the KHI rises at the excitation of high-mode MHD waves. The instability occurs within a wavenumber range whose width depends on the MHD mode number emph{m}, the plasma density contrast between the rotating jet and its environment, and also on the twists of the internal magnetic field and the jet velocity. We have studied KHI in two twisted spinning solar polar coronal hole jets, in a twisted rotating jet emerging from a filament eruption, and in a rotating macrospicule. The theoretically calculated KHI development times of a few minutes for wavelengths comparable to the half-widths of the jets are in good agreement with the observationally determined growth times only for high order (10 $mathrm{leqslant}$ emph{m} $mathrm{leqslant}$ 65) MHD modes. Therefore, we expect that the observed KHI in these cases is due to unstable high-order MHD modes.
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