Disruption of Alfvénic turbulence by magnetic reconnection in a collisionless plasma

Abstract

We calculate the disruption scale λ D at which sheet-like structures in dynamically aligned Alfvénic turbulence are destroyed by the onset of magnetic reconnection in a low-β collisionless plasma. The scaling of λ D depends on the order of the statistics being considered, with more intense structures being disrupted at larger scales. The disruption scale for the structures that dominate the energy spectrum is4/9 , where d e is the electron inertial scale, ρ s is the ion sound scale, and L ⊥ is the outer scale of the turbulence. When β e and ρ s /L ⊥ are sufficiently small, the scale λ D is larger than ρ s and there is a break in the energy spectrum at λ D , rather than at ρ s . We propose that the fluctuations produced by the disruption are circularised flux ropes, which may have already been observed in the solar wind. We predict the relationship between the amplitude and radius of these structures and quantify the importance of the disruption process to the cascade in terms of the filling fraction of undisrupted structures and the fractional reduction of the energy contained in them at the ion sound scale ρ s . Both of these fractions depend strongly on β e , with the disrupted structures becoming more important at lower β e . Finally, we predict that the energy spectrum between λ D and ρ s is steeper than k −3 ⊥ , when this range exists. Such a steep "transition range" is sometimes observed in short intervals of solar-wind turbulence. The onset of collisionless magnetic reconnection may therefore significantly affect the nature of plasma turbulence around the ion gyroscale.

The Radial Dependence of Proton-scale Magnetic Spectral Break in Slow Solar Wind during PSP Encounter 2

“…Considering that f b correlates with all three of f c , f d and f ρ , we cannot constrain the physical mechanisms which relate to the spectral break. For instance, the observations of Vech et al (2018) which suggest that magnetic reconnection may disrupt the inertial cascade (Mallet et al 2017) at a disruption scale which has a similar scaling to the cyclotron resonant scale, if proton and electron temperatures are similar. Due to our current lack of electron temperature we have not made any attempt to distinguish the disruption scale.…”

“…Furthermore, the inclusion of the Hall term in the MHD approximation has been proposed as the source of the break at scales d p k ⊥ ∼ 1 (Galtier 2006). Mallet et al (2017) and Loureiro & Boldyrev (2017) suggest that the inertial-range turbulence could generate sheet-like turbulent structures, which could be disrupted by reconnection below a disruption scale intermediate to d p and ρ p .…”

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

“…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.…”

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

“…The rapid conversion of magnetic energy into heat and particle acceleration is often observed in astrophysical environments, typically in the form of flares, for instance in solar and stellar coronae (Priest & Forbes 2002), magnetars (Lyutikov 2006), jet and accretion disk systems (Romanova & Lovelace 1992), gamma-ray bursts (Drenkhahn & Spruit 2002), pulsar winds (Kirk & Skjaeraasen 2003) and their nebulae (Cerutti et al 2014). On macroscopic scales, magnetized astrophysical plasmas are invariably modeled by using the MHD approximation, with a finite conductivity to be employed in Ohm's law.…”