Multi-fluid approach to high-frequency waves in plasmas. III. Nonlinear regime and plasma heating [SSA]

The multi-fluid modelling of high-frequency waves in partially ionized plasmas has shown that the behavior of magnetohydrodynamics waves in the linear regime is heavily influenced by the collisional interaction between the different species that form the plasma. Here, we go beyond linear theory and study large-amplitude waves in partially ionized plasmas using a nonlinear multi-fluid code. It is known that in fully ionized plasmas, nonlinear Alfv\’en waves generate density and pressure perturbations. Those nonlinear effects are more pronounced for standing oscillations than for propagating waves. By means of numerical simulations and analytical approximations, we examine how the collisional interaction between ions and neutrals affects the nonlinear evolution. The friction due to collisions dissipates a fraction of the wave energy, which is transformed into heat and consequently rises the temperature of the plasma. As an application, we investigate frictional heating in a plasma with physical conditions akin to those in a solar quiescent prominence.

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D. Martinez-Gomez, R. Soler and J. Terradas
Fri, 23 Feb 18

Comments: 26 pages, 15 figures, 3 movies. Accepted for publication in The Astrophysical Journal

On the Rates of Steady, Quasi-steady and Impulsive Magnetic Reconnection [CL]

Magnetic reconnection (MR) is considered as a major source of particle energization in astrophysical plasma. In the past, analysis of MR often assumes the magnetostatic condition, i.e. $\partial_t = 0$. We show that under the Sweet-Parker-Petschek framework, steady state is an over-constraint and is not achievable. On the other hand, the quasi-steady state defined as $\partial_t \mathbf{E} = 0$ but $\partial_t \mathbf{B} \neq 0$ or equivalently $\partial_t\mathbf{j}\neq 0$ better describes the asymptotic behaviour of MR without turbulence. The upper limit of MR rate for quasi-steady MR is found to be $\sim 1/3\sqrt{3} \sim 0.19$. The limit does not apply to impulsive or turbulent MR of which $\partial_t\mathbf{B} \neq 0$ and $\partial_t\mathbf{E} \neq 0$. In impulsive MR the rate can be higher or lower than 0.19 depending on the state of the turbulence. Our results may explain the apparent discrepancy in observations of solar flare MR rates. The analysis is independent of mass ratio and thus the results are applicable to pair plasma.

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H. Che
Fri, 23 Feb 18

Comments: submitted to ApJ Letter

Evolution of 3-dimensional Relativistic Current Sheets and Development of Self-Generated Turbulence [HEAP]

In this paper, the temporal evolution of 3-dimensional relativistic current sheets in Poynting-dominated plasma is studied for the first time. Over the past few decades, a lot of efforts have been conducted on studying the evolution of current sheets in 2-dimensional space, and concluded that sufficiently long current sheets always evolves into the so-called “plasmoid-chain”, which provides fast reconnection rate independent of its resistivity. However, it is suspected that plasmoid-chain can exist only in the case of 2-dimensional approximation, and would show transition to turbulence in 3-dimensional space.
We performed 3-dimensional numerical simulation of relativistic current sheet using resistive relativistic magnetohydrodynamic approximation. The results showed that the 3-dimensional current sheet evolve not into plasmoid-chain but turbulence. The resulting reconnection rate is $0.004$ which is much smaller than that of plasmoid-chain. The energy conversion from magnetic field to kinetic energy of turbulence is just 0.01\% which is much smaller than typical non-relativistic cases. Using the energy principle, we also showed that the plasmoid is always unstable for a displacement in opposite direction to its acceleration, probably interchange-type instability, and this always results in seeds of turbulence behind the plasmoids. Finally, the temperature distribution along the sheet is discussed, and it is found that the sheet is less active than plasmoid-chain. Our finding can be applied for many high energy astrophysical phenomena, and can provide a basic model of the general current sheet in Poynting-dominated plasma.

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M. Takamoto
Thu, 22 Feb 18

Comments: 9 pages, 10 figures, accepted for publication in MNRAS

Implosive collapse about magnetic null points: A quantitative comparison between 2D and 3D nulls [SSA]

Null collapse is an implosive process whereby MHD waves focus their energy in the vicinity of a null point, forming a current sheet and initiating magnetic reconnection. We consider, for the first time, the case of collapsing 3D magnetic null points in nonlinear, resistive MHD using numerical simulation, exploring key physical aspects of the system as well as performing a detailed parameter study. We find that within a particular plane containing the 3D null, the plasma and current density enhancements resulting from the collapse are quantitatively and qualitatively as per the 2D case in both the linear and nonlinear collapse regimes. However, the scaling with resistivity of the 3D reconnection rate – which is a global quantity – is found to be less favourable when the magnetic null point is more rotationally symmetric, due to the action of increased magnetic back-pressure. Furthermore, we find that with increasing ambient plasma pressure the collapse can be throttled, as is the case for 2D nulls. We discuss this pressure-limiting in the context of fast reconnection in the solar atmosphere and suggest mechanisms by which it may be overcome. We also discuss the implications of the results in the context of null collapse as a trigger mechanism of Oscillatory Reconnection, a time-dependent reconnection mechanism, and also within the wider subject of wave-null point interactions. We conclude that, in general, increasingly rotationally-asymmetric nulls will be more favourable in terms of magnetic energy release via null collapse than their more symmetric counterparts.

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J. Thurgood, D. Pontin and J. McLaughlin
Wed, 21 Feb 18

Comments: Accepted in ApJ, will be published gold open access, refer to main journal

Marginal Stability of Sweet-Parker Type Current Sheets at Low Lundquist Numbers [CL]

Magnetohydrodynamic simulations have shown that a non-unique critical Lundquist number $S_c$ exists, hovering around $S_c \sim 10^4$, above which threshold Sweet-Parker type stationary reconnecting configurations become unstable to a fast tearing mode dominated by plasmoid generation. It is known that the flow along the sheet plays a stabilizing role, though a satisfactory explanation of the non-universality and variable critical Lundquist numbers observed is still lacking. Here we discuss this question using 2D linear MHD simulations and linear stability analyses of Sweet-Parker type current sheets in the presence of background stationary inflows and outflows at low Lundquist numbers ($S\le 10^4$). Simulations show that the inhomogeneous outflow stabilizes the current sheet by stretching the growing magnetic islands and at the same time evacuating the magnetic islands out of the current sheet. This limits the time during which fluctuations which begin at any given wave-length can remain unstable, rendering the instability non-exponential. We find that the linear theory based on the expanding-wavelength assumption works well for $S$ larger than $\sim 1000$. However we also find that the inflow and location of the initial perturbation also affect the stability threshold.

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C. Shi, M. Velli and A. Tenerani
Wed, 21 Feb 18

Comments: 15 pages, 10 figures

Dual phase-space cascades in 3D hybrid-Vlasov-Maxwell turbulence [CL]

To explain energy dissipation via turbulence in collisionless, magnetized plasmas, the existence of a dual real- and velocity-space cascade of ion-entropy fluctuations below the ion gyroradius has been proposed. Such a dual cascade, predicted by the gyrokinetic theory, has previously been observed in gyrokinetic simulations of two-dimensional, electrostatic turbulence. For the first time we show evidence for a dual phase-space cascade of ion-entropy fluctuations in a three-dimensional simulation of hybrid-kinetic, electromagnetic turbulence. Energy spectra are largely consistent with a generalized theory for the cascade that accounts for the spectral anisotropy of critically balanced, intermittent, sub-ion-Larmor-scale fluctuations. The observed velocity-space cascade is anisotropic with respect to the magnetic-field direction, with linear phase mixing along magnetic-field lines proceeding mainly at spatial scales above the ion gyroradius and nonlinear phase mixing across magnetic-field lines proceeding at perpendicular scales below the ion gyroradius. Such phase-space anisotropy could be sought in heliospheric and magnetospheric data of solar-wind turbulence.

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S. Cerri, M. Kunz and F. Califano
Tue, 20 Feb 18

Comments: 6 pages, 3 figures, submitted for publication

Energy cascade rate in isothermal compressible magnetohydrodynamic turbulence [CL]

Three-dimensional direct numerical simulations are used to study the energy cascade rate in isothermal compressible magnetohydrodynamic turbulence. Our analysis is guided by a two-point exact law derived recently for this problem in which flux, source, hybrid, and mixed terms are present. The relative importance of each term is studied for different initial subsonic Mach numbers $M_S$ and different magnetic guide fields ${\bf B}_0$. The dominant contribution to the energy cascade rate comes from the compressible flux, which depends weakly on the magnetic guide field ${\bf B}_0$, unlike the other terms whose modulus increase significantly with $M_S$ and ${\bf B}_0$. In particular, for strong ${\bf B}_0$ the source and hybrid terms are dominant at small scales with almost the same amplitude but with a different sign. A statistical analysis made with an isotropic decomposition based on the SO(3) rotation group is shown to generate spurious results in presence of ${\bf B}_0$, when compared with an axisymmetric decomposition better suited to the geometry of the problem. Our numerical results are eventually compared with previous analyses made with in-situ measurements in the solar wind and the terrestrial magnetosheath.

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N. Andres, F. Sahraoui, S. Galtier, et. al.
Fri, 16 Feb 18

Comments: N/A