Marginal Stability of Sweet–Parker Type Current Sheets at Low Lundquist Numbers

Shi, Chen; Velli, Marco; Tenerani, Anna

doi:10.3847/1538-4357/aabd83

Cited by 17 publications

(26 citation statements)

References 19 publications

(28 reference statements)

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“…Figure 9 (right panel) shows that, once rescaled to the local Alfvén time τ * A = L * /c * A , the growth rate is compatible with the value γ ≃ 0.63 of the ideal tearing instability, with the exception of few points, that seem to be more compatible with the SP scaling. Notice, however, that here the local Lunquist number is close to the threshold minimum value of 10 4 requested to allow super-tearing modes (see Shi et al 2018, for an exploration of lower values). Moreover, the agreement with the critical scenario of the ideal tearing seems to be improving with increasing S * , as expected, since we are moving towards the asymptotic regime (S * → ∞).…”

Section: Secondary Ideal Tearing Instabilitiesmentioning

confidence: 59%

Fast Magnetic Reconnection: Secondary Tearing Instability and Role of the Hall Term

Papini

Landi

Zanna

2019

ApJ

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Magnetic reconnection provides the primary source for explosive energy release, plasma heating and particle acceleration in many astrophysical environments. The last years witnessed a revival of interest in the MHD tearing instability as a driver for efficient reconnection. It has been established that, provided the current sheet aspect ratio becomes small enough (a/L ∼ S −1/3 for a given Lundquist number S ≫ 1), reconnection occurs on ideal Alfvén timescales and becomes independent on S . Here we investigate, by means of two-dimensional simulations, the ideal tearing instability in the Hall-MHD regime, which is appropriate when the width of the resistive layer δ becomes comparable to the ion inertial length d i . Moreover, we study in detail the spontaneous development and reconnection of secondary current sheets, which for high S naturally adjust to the ideal aspect ratio and hence their evolution proceeds very rapidly. For moderate low S , the aspect ratio tends to the Sweet-Parker scaling (a/L ∼ S −1/2 ), in order to fulfill the condition δ ≪ a necessary for the onset of a tearing instability. When the Hall term is included, the reconnection rate of this secondary nonlinear phase is enhanced and, depending on the ratio d i /δ, can be twice with respect to the pure MHD case, and up to ten times larger than the linear phase. Therefore, the evolution of the tearing instability in thin current sheets in the Hall-MHD regime naturally leads to an explosive disruption of the reconnecting site and to energy release on super-Alfvénic timescales, as required to explain astrophysical observations.

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Section: Secondary Ideal Tearing Instabilitiesmentioning

confidence: 59%

Fast Magnetic Reconnection: Secondary Tearing Instability and Role of the Hall Term

Papini

Landi

Zanna

2019

ApJ

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“…Within the context of turbulence, the present approach neglects the effect of flows (vorticity sheets) on the disruption of turbulent eddies due to the tearing instability. Some recent discussion on the effect of flows on tearing mode can be found for example in Loureiro et al (2013), Tenerani et al (2015b) and Shi et al (2018). The instability of combined vortex-current sheets is a vast topic (Dahlburg et al 1998).…”

Section: Discussionmentioning

confidence: 99%

Spectral signatures of recursive magnetic field reconnection

Tenerani

Velli

2019

Monthly Notices of the Royal Astronomical Society

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We use 2.5D viscoresistive Magnetohydrodynamic simulations to investigate the spectral signatures of the nonlinear disruption of a tearing unstable current sheet via the generation of multiple secondary current sheets and magnetic islands. During the nonlinear phase of tearing mode evolution, there develops a regime in which the magnetic energy density shows a spectrum with a power-law close to B(k) 2 ∼ k −0.8 . Such an energy spectrum is found in correspondence of the neutral line, within the diffusion region of the primary current sheet, where energy is conveyed towards smaller scales via a "recursive" process of fast tearing-type instabilities. Far from the neutral line we find that magnetic energy spectra evolve towards slopes compatible with the "standard" Kolmogorov spectrum. Starting from a selfsimilar description of the nonlinear stage at the neutral line, we provide a model that predicts a reconnecting magnetic field energy spectrum scaling as k −4/5 , in good agreement with numerical results. An extension of the predicted power-law to generic current sheet profiles is also given and possible implications for turbulence phenomenology are discussed.

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“…At present there are no analytical estimates for the aspect ratio ξ c /λ X , which might also depend on the noise level (e.g. Ni et al 2010;Huang et al 2017;Shi et al 2018). However, numerical simulations have found ξ c /λ X ∼ 50 in the collisionless regime (e.g.…”

Section: Plasmoid-mediated Disruption Of the Current Sheets And Efficmentioning

confidence: 99%

The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles

Comisso

Sironi

2019

ApJ

142

132

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Magnetized turbulence and magnetic reconnection are often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully-kinetic 2D and 3D particle-in-cell simulations, we investigate the interplay between turbulence and reconnection in generating nonthermal particles in magnetically-dominated (or, equivalently, "relativistic") pair plasmas. A generic by-product of the turbulence evolution is the generation of a nonthermal particle spectrum with a power-law energy range. The power-law slope p is harder for larger magnetizations and stronger turbulence fluctuations, and it can be as hard as p 2. The Larmor radius of particles at the high-energy cutoff is comparable to the size l of the largest turbulent eddies. Plasmoid-mediated reconnection, which self-consistently occurs in the turbulent plasma, controls the physics of particle injection. Then, particles are further accelerated by stochastic scattering off turbulent fluctuations. The work done by parallel electric fields -naturally expected in reconnection layers -is responsible for most of the initial energy increase, and is proportional to the magnetization σ of the system, while the subsequent energy gain, which dominates the overall energization of high-energy particles, is powered by the perpendicular electric fields of turbulent fluctuations. The two-stage acceleration process leaves an imprint in the particle pitch-angle distribution: low-energy particles are aligned with the field, while the highest energy particles move preferentially orthogonal to it. The energy diffusion coefficient of stochastic acceleration scales as D γ ∼ 0.1σ(c/l)γ 2 , where γ is the particle Lorentz factor. This results in fast acceleration timescales t acc ∼ (3/σ) l/c. Our findings have important implications for understanding the generation of nonthermal particles in high-energy astrophysical sources.

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Marginal Stability of Sweet–Parker Type Current Sheets at Low Lundquist Numbers

Cited by 17 publications

References 19 publications

Fast Magnetic Reconnection: Secondary Tearing Instability and Role of the Hall Term

Fast Magnetic Reconnection: Secondary Tearing Instability and Role of the Hall Term

Spectral signatures of recursive magnetic field reconnection

The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles

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