The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. II. Theory

Liu, Yi‐Hsin; Drake, J. F.; Swisdak, M.

doi:10.1063/1.3627147

Cited by 29 publications

(26 citation statements)

References 37 publications

(79 reference statements)

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“…2(b) based on B xm /B x0 0.55 are roughly twice these values, mainly because of an overestimation of the outflow speed. However, it is common for electron-proton plasmas to have outflow speeds about half of the Alfvén speed [54]; this is likely due to a self-generated firehosesense temperature anisotropy in reconnection exhausts, which reduces the outflow speed (e.g., [55,56]) but is not considered in the current model. Adjusting for this factor of two, the predictions agree quite well with the simulations.…”

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confidence: 99%

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Why does Steady-State Magnetic Reconnection have a Maximum Local Rate of Order 0.1?

et al. 2017

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Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling analysis of the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and non-relativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD-scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models. Introduction-Magnetic energy is abruptly released in solar and stellar flares [1][2][3], substorms in magnetotails of Earth and other planets [4,5], disruptions and the sawtooth crash in magnetically confined fusion devices [6], laboratory experiments [7], and numerous high energy astrophysical systems [8,9]. Magnetic reconnection, where a change in topology of the magnetic field allows a rapid release of magnetic energy into thermal and kinetic energy, is a likely cause. The reconnection electric field parallel to the X-line (where magnetic field lines break) not only determines the rate that reconnection proceeds, but can also be crucial for accelerating energetic superthermal particles. It was estimated that a normalized reconnection rate of 0.1 is required to explain time scales of flares and substorms [10].

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“…As discussed earlier, the self-generated pressure anisotropy in the exhaust can reduce the outflow speed [55,56]. The plasma pressure gradient force in the outflow direction can also affect the outflow speed [26,54,61].…”

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Why does Steady-State Magnetic Reconnection have a Maximum Local Rate of Order 0.1?

et al. 2017

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“…The lock-in of e to 0.25 provides a favorable condition for the formation SS-RD (DD) compound structure. 44 Case (c): SS-RD-SS compound structure with / ¼ 160 and b i0 ¼ 0: 5 As shown in Fig. 3(c), an RD is embedded inside an extended slow shock on each side of current sheet.…”

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confidence: 98%

“…Liu et al 43,44 carried out particle simulations and provided a theoretical explanation for the Riemann discontinuity/shock evolution of an initial current sheet. They also pointed out the importance of leakage of SS downstream particles to upstream region.…”

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confidence: 99%

Generation of shock/discontinuity compound structures through magnetic reconnection in the geomagnetic tail

et al. 2012

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We use 1-D hybrid code to simulate the generation and evolution of MHD discontinuities associated with magnetic reconnection in a current sheet. It is found that the leakage of slow shock (SS) downstream particles to upstream region tends to increase the ion parallel temperature and temperature anisotropy with b ijj =b i? ) 1, where b ijj ðb i? Þ is the ion parallel (perpendicular) beta. As a result, the propagation speed of rotational discontinuity (RD) is highly reduced and RD becomes attached to SS, leading to formation of various compound structures in the reconnection outflow region. Four types of compound structure are found in our simulations: (a) RD-SS compound structure: the RD is attached to the leading part of SS, (b) SS-RD (DD) compound structure: RD is attached to the rear part of SS, (c) SS-RD-SS compound structure: RD is trapped inside SS, and (d) switch-off slow shock (SSS) with a rotational wave train. The type of compound structure generated depends on initial ion beta b i0 and magnetic shear angle /. RD tends to move in front of SS to form an RD-SS compound structure for cases with low b i0 . RD stays behind SS and form an SS-RD (DD) compound structure for large b i0 . The SS-RD-SS compound structure is formed for intermediate values of b i0 . When the shear angle is 180 , SSS with a wave train is formed.

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“…According to previous kinetic simulations, 15,32 however, the downstream anisotropy parameter is somewhat larger than 0.…”

Section: Discussionmentioning

confidence: 99%

Magnetic reconnection under anisotropic magnetohydrodynamic approximation

Hirabayashi

Hoshino

2013

Physics of Plasmas

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We study the formation of slow-mode shocks in collisionless magnetic reconnection by using one-and two-dimensional collisionless MHD codes based on the double adiabatic approximation and the Landau closure model. We bridge the gap between the Petschek-type MHD reconnection model accompanied by a pair of slow shocks and the observational evidence of the rare occasion of in-situ slow shock observations.Our results showed that once magnetic reconnection takes place, a firehose-sense (p || > p ⊥ ) pressure anisotropy arises in the downstream region, and the generated slow shocks are quite weak comparing with those in an isotropic MHD. In spite of the weakness of the shocks, however, the resultant reconnection rate is 10 − 30% higher than that in an isotropic case. This result implies that the slow shock does not necessarily play an important role in the energy conversion in the reconnection system, and is consistent with the satellite observation in the Earth's magnetosphere.PACS numbers: 94.05.-a,

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The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. II. Theory

Cited by 29 publications

References 37 publications

Why does Steady-State Magnetic Reconnection have a Maximum Local Rate of Order 0.1?

Why does Steady-State Magnetic Reconnection have a Maximum Local Rate of Order 0.1?

Generation of shock/discontinuity compound structures through magnetic reconnection in the geomagnetic tail

Magnetic reconnection under anisotropic magnetohydrodynamic approximation

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