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.

Read this paper on arXiv…

H. Che
Fri, 23 Feb 18

Comments: submitted to ApJ Letter

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.

Read this paper on arXiv…

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

The NWRA Classification Infrastructure: Description and Extension to the Discriminant Analysis Flare Forecasting System (DAFFS) [SSA]


A classification infrastructure built upon Discriminant Analysis has been developed at NorthWest Research Associates for examining the statistical differences between samples of two known populations. Originating to examine the physical differences between flare-quiet and flare-imminent solar active regions, we describe herein some details of the infrastructure including: parametrization of large datasets, schemes for handling “null” and “bad” data in multi-parameter analysis, application of non-parametric multi-dimensional Discriminant Analysis, an extension through Bayes’ theorem to probabilistic classification, and methods invoked for evaluating classifier success. The classifier infrastructure is applicable to a wide range of scientific questions in solar physics. We demonstrate its application to the question of distinguishing flare-imminent from flare-quiet solar active regions, updating results from the original publications that were based on different data and much smaller sample sizes. Finally, as a demonstration of “Research to Operations” efforts in the space-weather forecasting context, we present the Discriminant Analysis Flare Forecasting System (DAFFS), a near-real-time operationally-running solar flare forecasting tool that was developed from the research-directed infrastructure.

Read this paper on arXiv…

K. Leka, G. Barnes and E. Wagner
Wed, 21 Feb 18

Comments: J. Space Weather Space Climate: Accepted / in press; access supplementary materials through journal; some figures are less than full resolution for arXiv

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.

Read this paper on arXiv…

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

Comments: 15 pages, 10 figures

Extreme Value Analysis of Solar Flare Events [CL]


Space weather events such as solar flares can be harmful for life and infrastructure on earth or in near-earth orbit. In this paper we employ extreme value theory (EVT) to model extreme solar flare events; EVT offers the appropriate tools for the study and estimation of probabilities for extrapolation to ranges outside of those that have already been observed. In the past such phenomena have been modelled as following a power law which may gives poor estimates of such events due to overestimation. The data used in the study were X-ray fluxes from NOAA/GOES and the expected return levels for Carrington or Halloween like events were calculated with the outcome that the existing data predict similar events happening in 110 and 38 years respectively.

Read this paper on arXiv…

T. Tsiftsi and V. Luz
Tue, 20 Feb 18

Comments: 17 pages, 5 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.

Read this paper on arXiv…

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.

Read this paper on arXiv…

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

Comments: N/A