Simulations reveal fast mode shocks in magnetic reconnection outflows

Workman, Jared C.; Blackman, Eric G.; Ren, C.

doi:10.1063/1.3631795

Cited by 13 publications

(15 citation statements)

References 34 publications

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“…In order to produce a low Mach number shock, as predicted by other numerical simulations, the inflow Alfven Mach number is taken to be M A0 = 1.0. After reflection at the right boundary, this produces a shock with an average Mach number of about 2.0 in the shock frame, consistent with the flare termination shock predicted by previous MHD simulations (Forbes 1986;Workman et al 2011). The grid sizes are ∆x = ∆z = 0.5c/ω pi and the time step is taken to be ∆t = 0.01Ω −1 ci , where c/ω pi is the ion inertial length and Ω −1 ci is the ion cyclotron period.…”

Section: Methodssupporting

confidence: 87%

Particle Acceleration at a Flare Termination Shock: Effect of Large-Scale Magnetic Turbulence

Guo

Giacalone

2012

ApJ

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We investigate the acceleration of charged particles (both electrons and protons) at collisionless shocks predicted to exist in the vicinity of solar flares. The existence of standing termination shocks has been examined by flare models and numerical simulations (e.g., Shibata et al. 1995;Forbes 1986). We study electron energization by numerically integrating the equations of motion of a large number of test-particle electrons in the time-dependent two-dimensional electric and magnetic fields generated from hybrid simulations (kinetic ions and fluid electron) using parameters typical of the solar flare plasma environment. The shock is produced by injecting plasma flow toward a rigid piston. Largescale magnetic fluctuations -known to exist in plasmas and known to have important effects on the nonthermal electron acceleration at shocks -are also included in our simulations. For the parameters characteristic of the flaring region, our calculations suggest that the termination shock formed in the reconnection outflow region (above post-flare loops) could accelerate electrons to a kinetic energy of a few MeV within 100 ion cyclotron periods, which is of the order of a millisecond. Given a sufficient turbulence amplitude level (δB 2 /B 2 0 ∼ 0.3), about 10% of thermal test-particle electrons are accelerated to more than 15 keV. We find that protons are also accelerated, but not to as high energy in the available time and the energy spectra are considerably steeper than that of the electrons for the parameters used in our simulations. Our results are qualitatively consistent with the observed hard X-ray emissions in solar flares.

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Section: Methodssupporting

confidence: 87%

Particle Acceleration at a Flare Termination Shock: Effect of Large-Scale Magnetic Turbulence

Guo

Giacalone

2012

ApJ

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“…2 and 3, the average value of the inferred Mach number is around 1.6, and the maximum can reach 2.0. Our measured density compression ratio and inferred Mach number of this termination shock case is similar to those predicted in the numerical modeling results of termination shocks (Forbes 1986;Workman et al 2011;Shen et al 2018). They are also comparable to those estimated from split-band features in coronal-shock-driven type II radio bursts (see, e.g., statistical studies in Vršnak et al 2002;Du et al 2015 and case studies in Liu et al 2009;Zimovets et al 2012;Zucca et al 2014Zucca et al , 2018Kishore et al 2016).…”

Section: Spatially and Temporally Resolved Shock Compression Ratiosupporting

confidence: 89%

“…In a steady-state picture, the shock is perceived to be a standing shock above the looptop and is usually referred to as a flare "termination shock." Flare termination shocks were long predicted in numeri-cal simulations of flares (Forbes 1986(Forbes , 1988Forbes & Malherbe 1986;Workman et al 2011;Takasao et al 2015;Takasao & Shibata 2016;Takahashi et al 2017;Shen et al 2018) and were frequently invoked in some of the most well-known schematics of the standard flare model (e.g., Masuda et al 1994;Shibata et al 1995;Magara et al 1996;Lin & Forbes 2000). They have also been suggested as an outstanding candidate for driving particle acceleration (Forbes 1986;Shibata et al 1995;Somov & Kosugi 1997;Tsuneta & Naito 1998;Mann et al 2009;Warmuth et al 2009;Guo & Giacalone 2012;Li et al 2013;Nishizuka & Shibata 2013;Park et al 2013) and plasma heating (Masuda et al 1994;Guidoni et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-band Feature as Evidence for Shock Compression

Bin

Shen

Reeves

et al. 2019

ApJ

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Solar flare termination shocks have been suggested as one of the promising drivers for particle acceleration in solar flares, yet observational evidence remains rare. By utilizing radio dynamic spectroscopic imaging of decimetric stochastic spike bursts in an eruptive flare, Chen et al. found that the bursts form a dynamic surface-like feature located at the ending points of fast plasma downflows above the looptop, interpreted as a flare termination shock. One piece of observational evidence that strongly supports the termination shock interpretation is the occasional split of the emission band into two finer lanes in frequency, similar to the split-band feature seen in fast-coronal-shock-driven type II radio bursts. Here, we perform spatially, spectrally, and temporally resolved analysis of the split-band feature of the flare termination shock event. We find that the ensemble of the radio centroids from the two split-band lanes each outlines a nearly co-spatial surface. The high-frequency lane is located slightly below its low-frequency counterpart by ∼0.8 Mm, which strongly supports the shock-upstreamdownstream interpretation. Under this scenario, the density compression ratio across the shock front can be inferred from the frequency split, which implies a shock with a Mach number of up to 2.0. Further, the spatiotemporal evolution of the density compression along the shock front agrees favorably with results from magnetohydrodynamics simulations. We conclude that the detailed variations of the shock compression ratio may be due to the impact of dynamic plasma structures in the reconnection outflows, which results in distortion of the shock front.

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“…3A). The location and morphology of this surface closely resemble those of a TS as predicted in numerical simulations when viewed edge on (6,(9)(10)(11); see also Fig. 3B).…”

supporting

confidence: 74%

Particle acceleration by a solar flare termination shock

Bin

Bastian

Shen

et al. 2015

Science

157

161

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Solar flares-the most powerful explosions in the solar system-are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well-reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.One Sentence Summary: A termination shock is captured in action during a solar flare using radio observations, which show that it is a source of energetic electrons. Main Text:The acceleration of charged particles to high energies occurs throughout the Universe. Understanding the physical mechanisms is a fundamental topic in many space, astrophysical, and laboratory contexts that involve magnetized plasma (1). For solar flares and the often associated coronal mass ejections (CMEs), it is generally accepted that fast magnetic reconnection-the sudden reconfiguration of the magnetic field topology and the associated magnetic energy release-serves as the central engine driving these powerful explosions. However, the mechanism for converting the released magnetic energy into the kinetic energy in accelerated particles has remained uncertain (2, 3). Competing mechanisms include acceleration by the reconnection current sheet, turbulence, and shocks (2-5). 2Of possible interest in this regard is the termination shock (TS), produced by super-magnetosonic reconnection outflows impinging upon dense, closed magnetic loops in a cusp-shaped reconnection geometry (6). Although often invoked in the standard picture of solar flares (7,8) and predicted in numerical simulations (6,(9)(10)(11), its presence has yet to be firmly established observationally and, because of the paucity of direct observational evidence, its role as a possible particle accelerator has received limited attention (2, 3). Previous reports of coronal hard X-ray (HXR) sources in some flares have shown convincing evidence of the presence of accelerated electrons at or above the top of flare loops (referred to as the "loop-top" hereafter, or LT) (7,12), where a TS is presumably located. The often cited observational evidence for a solar flare TS has been certain radio sources showing spectroscopic features similar to solar type II radio bursts (radio emission associated with propagating shocks in the outer corona), but with small drifts in their emission frequency as a function of time, which implies a standing shock wave (13-17). However, because of the limited spectral imaging capabilities of the previous observations, none of ...

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Simulations reveal fast mode shocks in magnetic reconnection outflows

Cited by 13 publications

References 34 publications

Particle Acceleration at a Flare Termination Shock: Effect of Large-Scale Magnetic Turbulence

Particle Acceleration at a Flare Termination Shock: Effect of Large-Scale Magnetic Turbulence

Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-band Feature as Evidence for Shock Compression

Particle acceleration by a solar flare termination shock

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