Collisionless Reconnection in Magnetohydrodynamic and Kinetic Turbulence

Loureiro, Nuno; Boldyrev, Stanislav

doi:10.3847/1538-4357/aa9754

Cited by 93 publications

(107 citation statements)

References 43 publications

(75 reference statements)

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“…Outcomes of this work have potential applications for explaining the explosive events in the strongly magnetized space and astrophysical plasmas mentioned in the introduction, and further extend results of previous works of recursive magnetic reconnection (Shibata & Tanuma 2001;Tenerani et al 2015b;Singh et al 2019). Moreover, the interplay between fast reconnection and turbulence can be crucial, as predicted by reconnection-mediated turbulence models (Boldyrev & Perez 2012;Loureiro & Boldyrev 2017;Mallet et al 2017) and by recent numerical simulations of the solar wind plasma retaining kinetic effects (Franci et al 2017), where the role of reconnection in driving the turbulent cascade at sub-ion scales through the destabilization of current sheets of thickness a ≃ d i is estabilished. However, a recent study by Papini et al (2019) has demonstrated that the role of reconnection in shaping the spectrum of solar wind turbulence, including the change of slope at the ion inertial length d i , may be captured even without resorting to (hybrid) particle-in-cell simulations, by just retaining the Hall effect within a macroscopic MHD description, as in the present study.…”

Section: Discussionmentioning

confidence: 99%

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: Discussionmentioning

confidence: 99%

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

Papini

Landi

Zanna

2019

ApJ

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“…For example, LV99 in their Appendix C considered this possibility and concluded that tearing instability would affect negligibly their estimates for turbulent reconnection rates in the inertial range and at larger scales. Recently, however, an extensive literature has developed suggesting that reconnection in MHD turbulence must necessarily be induced by tearing instability, (see Loureiro and Boldyrev, 2017b;Boldyrev and Loureiro, 2017;Loureiro and Boldyrev, 2017a;Walker, Boldyrev, and Loureiro, 2018;Mallet, Schekochihin, and Chandran, 2017;Mallet, Schekochihin, and Chand ran, 2017;Comisso et al, 2018;Vech et al, 2018). The authors explore the subject of tearing instability and its possible modification of the properties of MHD turbulence.…”

Section: Objections To the Concept Of "Reconnection-mediated Turbumentioning

confidence: 99%

“…A separate issue is whether the theoretical treatment of "tearing-mediated turbulence" is accurate in the cited papers (Loureiro and Boldyrev, 2017b;Boldyrev and Loureiro, 2017;Loureiro and Boldyrev, 2017a;Walker, Boldyrev, and Loureiro, 2018;Mallet, Schekochihin, and Chandran, 2017;Mallet, Schekochihin, and Chand ran, 2017;Comisso et al, 2018;Vech et al, 2018). Indeed these various works reach somewhat divergent conclusions among themselves.…”

Section: Objections To the Concept Of "Reconnection-mediated Turbumentioning

confidence: 99%

3D turbulent reconnection: Theory, tests, and astrophysical implications

Lazarían¹,

et al. 2020

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Magnetic reconnection, topological change in magnetic fields, is a fundamental process in magnetized plasmas. It is associated with energy release in regions of magnetic field annihilation, but this is only one facet of this process. Astrophysical fluid flows normally have very large Reynolds numbers and are expected to be turbulent, in agreement with observations. In strong turbulence magnetic field lines constantly reconnect everywhere and on all scales, thus making magnetic reconnection an intrinsic part of the turbulent cascade. We note in particular that this is inconsistent with the usual practice of regarding magnetic field lines as persistent dynamical elements. A number of theoretical, numerical, and observational studies starting with the Lazarian & Vishniac 1999 paper proposed that 3D turbulence makes magnetic reconnection fast and that magnetic reconnection and turbulence are intrinsically connected. In particular, we discuss the dramatic violation of the textbook concept of magnetic flux-freezing in the presence of turbulence. We demonstrate that in the presence of turbulence the plasma effects are subdominant to turbulence as far as the magnetic reconnection is concerned. The latter fact justifies an MHD-like treatment of magnetic reconnection on all scales much larger than the relevant plasma scales. We discuss numerical and observational evidence supporting the turbulent reconnection model. In particular, we demonstrate that the tearing reconnection is suppressed in 3D and, unlike the 2D settings, 3D reconnection induces turbulence that makes magnetic reconnection independent of resistivity. We show that turbulent reconnection dramatically affects key astrophysical processes, e.g. star formation, turbulent dynamo, acceleration of cosmic rays. We provide criticism of the concept of "reconnection-mediated turbulence" and explain why turbulent reconnection is very different from enhanced turbulent resistivity and hyper-resistivity, and why the latter have fatal conceptual flaws.

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“…The role of the tearing instability and reconnection at kinetic scales, however, remains significantly less understood. Our preliminary dimensional estimates [40] suggested that the spectrum of tearingmediated kinetic-Alfvén turbulence should become relatively steep, with the spectral exponents ranging from −8/3 to −3. In the present Letter we propose a selfconsistent phenomenological theory of tearing-dominated kinetic-Alfvén turbulence.…”

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confidence: 90%

Role of reconnection in inertial kinetic-Alfvén turbulence

Boldyrev

Loureiro

2019

Phys. Rev. Research

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In a weakly collisional, low-electron-beta plasma, large-scale Alfvén turbulence transforms into inertial kinetic-Alfvén turbulence at scales smaller than the ion microscale (gyroscale or inertial scale). We propose that at such kinetic scales, the nonlinear dynamics tends to organize turbulent eddies into thin current sheets, consistent with the existence of two conserved integrals of the ideal equations, energy and helicity. The formation of strongly anisotropic structures is arrested by the tearing instability that sets a critical aspect ratio of the eddies at each scale a in the plane perpendicular to the guide field. This aspect ratio is defined by the balance of the eddy turnover rate and the tearing rate, and varies from (de/a) 1/2 to de/a depending on the assumed profile of the current sheets. The energy spectrum of the resulting turbulence varies from k −8/3 to k −3 , and the corresponding spectral anisotropy with respect to the strong background magnetic field from kz k 2/3 ⊥ to kz k ⊥ .PACS numbers: 52.35. Ra, 52.35.Vd, 52.30.Cv Introduction. Large-scale low-frequency fluctuations in astrophysical systems such as the interstellar medium, the solar wind, and others, are associated with nearly incompressible magnetohydrodynamic (Alfvén) turbulence [e.g., 1-3]. The nonlinear Alfvén wave packets that compose such turbulence are three-dimensionally anisotropic: elongated along the strong background magnetic field and resembling current sheets in the perpendicular plane [1,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. At scales smaller than the plasma microscales, such as the ion gyroscale or ion inertial scale, the shear-Alfvén modes transform into kinetic Alfvén modes. The character of turbulence then changes qualitatively. Numerical and analytical studies suggest that the energy spectrum becomes relatively steep in the sub-proton range, with the spectral index between −7/3 and −8/3, and fluctuations become compressible, with density and magnetic field fluctuations comparable to each other [e.g., 22-27]. Available solar wind measurements broadly agree with these predictions, with the measured energy spectral slope scattered around a slightly steeper value −2.8 [28][29][30][31][32].

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Collisionless Reconnection in Magnetohydrodynamic and Kinetic Turbulence

Cited by 93 publications

References 43 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

3D turbulent reconnection: Theory, tests, and astrophysical implications

Role of reconnection in inertial kinetic-Alfvén turbulence

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