Numerical Study on Excitation of Turbulence and Oscillation in Above-the-loop-top Region of a Solar Flare

Abstract

Extreme-ultraviolet imaging spectroscopic observations often show an increase in line width around the loop-top or above-the-loop-top (ALT) region of solar flares, suggestive of turbulence. In addition, recent spectroscopic observations found the oscillation in the Doppler velocity around the ALT region. We performed 3D magnetohydrodynamic (MHD) simulations to investigate the dynamics in the ALT region, with a particular focus on the generation of turbulence and the excitation of the oscillatory motion. We found a rapid growth of MHD instabilities around the upper parts of the ALT region (arms of the magnetic tuning fork). The instabilities grow more rapidly than the magnetic Rayleigh–Taylor-type instabilities at the density interface beneath the reconnecting current sheet. Eventually, the ALT region is filled with turbulent flows. The arms of the magnetic tuning fork have bad-curvature and transonic flows. Therefore, we consider that the rapidly growing instabilities are combinations of pressure-driven and centrifugally driven Rayleigh–Taylor-type instabilities. Despite the presence of turbulent flows, the ALT region shows a coherent oscillation driven by the backflow of the reconnection jet. We examine the numerical results by reanalyzing the solar flare presented in Reeves et al. We find that the highest nonthermal velocity is always at the uppermost visible edge of the ALT region, where oscillations are present. This result is consistent with our models. We also argue that the turbulent magnetic field has a significant impact on the confinement of nonthermal electrons in the ALT region.

Three-dimensional Turbulent Reconnection within the Solar Flare Current Sheet

“…This is attributed to the fact that our model treats the whole causal chain of the flare reconnection in a more realistic and logical way. By comparison, in most previous 3D simulations, either ideal configurations or simplified physics were assumed in order to focus on a specific process (Nishida et al 2013;Cécere et al 2015;Edmondson & Lynch 2017;Shibata et al 2023). Furthermore, the spatial resolution in our simulation is higher than in recent 3D simulations of flare CS reconnection (Shen et al 2022;Ruan et al 2023).…”

“…We only use the data in |x| < 0.3 and y < 1 for analysis in order to reduce the influences of numerical boundaries. Compared with adaptive mesh refinement, the uniform spatial mesh is a better choice for the direct numerical simulations of turbulence, which can avoid numerical perturbations brought by mesh refining and provide uniform resolution in turbulent regions (Shibata et al 2023). We use the static mesh refinement technique to implement a uniform mesh in the core reconnection region and also save computational costs.…”

“…On the other hand, the reconnection-generated downflows will collide with the dense plasma at the flare loop top, even forming a termination shock (Chen et al 2015;Shen et al 2018). Due to the nonuniformity of magnetic reconnection caused by the CS fragmentation, the downflows are intermittent with varying velocity (Cheng et al 2018), which tend to cause a highly turbulent region between the termination shock and the flare loop top (Ye et al 2020;Shibata et al 2023).…”

Modeling Hadronic Gamma-Ray Emissions from Solar Flares and Prospects for Detecting Nonthermal Signatures from Protostars

“…This energy is higher than the pion production threshold, E p,thr ; 1.22 GeV, and thus these CRs can emit high-energy gamma rays via hadronuclear interactions. In reality, multiple termination shocks may exist above the loop-top region (Takasao & Shibata 2016;Shibata et al 2022), but we consider the CR particle acceleration at a single shock for simplicity. In addition, stochastic acceleration by turbulence can be effective at the shock downstream, which may affect the gamma-ray emission during the flare (e.g., Petrosian 2012; Kocharov et al 2015).…”

Energetic Electrons Accelerated and Trapped in a Magnetic Bottle above a Solar Flare Arcade

“…This magnetic bottle structure coincides with the location of the ALT HXR source and shows a strong concentration of microwave-emitting nonthermal electrons (Chen et al 2020b), implicating a key role it may be playing in the particle acceleration processes. This is also where the fast reconnection outflows-which carry the bulk of the released energycollide head-on against the flare arcade to create a plethora of energetic phenomena such as collapsing magnetic traps (Somov & Kosugi 1997;Karlický & Kosugi 2004), fastmode termination shocks (Forbes 1986;Tsuneta & Naito 1998;Aurass et al 2002;Aurass & Mann 2004;Mann et al 2009;Guo & Giacalone 2012;Chen et al 2015Chen et al , 2019Takasao et al 2015;Polito et al 2018;Shen et al 2018Shen et al , 2022Ye et al 2020;Luo et al 2021;French et al 2024), slowmode or gas dynamic shocks (Reeves et al 2007 and turbulence and oscillations (Takasao & Shibata 2016;Kontar et al 2017;Reeves et al 2020;Ye et al 2020;Shen et al 2022Shen et al , 2023Ruan et al 2023;Shibata et al 2023;Wang et al 2023b), serving as an ideal environment to heat flare plasma and accelerate charged particles.…”