arXiv subjects

M. A. Shay

Publications and source records attributed to M. A. Shay.

34 records · Page 2Linked to original sources

arXiv2016

Kinetic signatures of the region surrounding the X-line in asymmetric (magnetopause) reconnection

Kinetic particle-in-cell simulations are used to identify signatures of the electron diffusion region (EDR) and its surroundings during asymmetric magnetic reconnection. A "shoulder" in the sunward pointing normal electric field (EN > 0) at the reconnection magnetic field reversal is a good indicator of the EDR, and is caused by magnetosheath electron meandering orbits in the vicinity of the x-line. Earthward of the X-line, electrons accelerated by EN form strong currents and crescent-shaped distribution functions in the plane perpendicular to B. Just downstream of the X-line, parallel electric fields create field-aligned crescent electron distribution functions. In the immediate upstream magnetosheath, magnetic field strength, plasma density, and perpendicular electron temperatures are lower than the asymptotic state. In the magnetosphere inflow region, magnetosheath ions intrude resulting in an Earthward pointing electric field and parallel heating of magnetospheric particles. Many of the above properties persist with a guide field of at least unity.

physics.space-ph↗

arXiv2015

Fast Magnetic Reconnection Due to Anisotropic Electron Pressure

A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.

physics.plasm-ph↗

arXiv2014

Electron Heating During Magnetic Reconnection: A Simulation Scaling Study

Electron bulk heating during magnetic reconnection with symmetric inflow conditions is examined using kinetic particle-in-cell (PIC) simulations. The degree of electron heating is well correlated with the inflowing Alfvén speed $c_{Ar}$ based on the reconnecting magnetic field through the relation $ΔT_e = 0.033 \,m_i\,c_{Ar}^2$, where $ΔT_{e}$ is the increase in electron temperature. For the range of simulations performed, the heating shows almost no correlation with inflow total temperature $T_{tot} = T_i + T_e$ or plasma $β$. An out-of-plane (guide) magnetic field of similar magnitude to the reconnecting field does not affect the total heating, but it does quench perpendicular heating, with almost all heating being in the parallel direction. These results are qualitatively consistent with a recent statistical survey of electron heating in the dayside magnetopause, which also found that $ΔT_e$ was proportional to the inflowing Alfvén speed. The net electron heating varies very little with distance downstream of the x-line. The simulations show at most a very weak dependence of electron heating on the ion to electron mass ratio. In the antiparallel reconnection case, the largely parallel heating is eventually isotropized downstream due a scattering mechanism such as stochastic particle motion or instabilities. The study highlights key properties that must be satisfied by an electron heating mechanism: (1) Preferential heating in the parallel direction; (2) Heating proportional to $m_i\,c_{Ar}^2$; (3) At most a weak dependence on electron mass; and (4) An exhaust electron temperature that varies little with distance from the x-line.

physics.plasm-ph↗

arXiv2014

Nonlinear and Linear Timescales near Kinetic Scales in Solar Wind Turbulence

The application of linear kinetic treatments to plasma waves, damping, and instability requires favorable inequalities between the associated linear timescales and timescales for nonlinear (e.g., turbulence) evolution. In the solar wind these two types of timescales may be directly compared using standard Kolmogorov-style analysis and observational data. The estimated local nonlinear magnetohydrodynamic cascade times, evaluated as relevant kinetic scales are approached, remain slower than the cyclotron period, but comparable to, or faster than, the typical timescales of instabilities, anisotropic waves, and wave damping. The variation with length scale of the turbulence timescales is supported by observations and simulations. On this basis the use of linear theory - which assumes constant parameters to calculate the associated kinetic rates - may be questioned. It is suggested that the product of proton gyrofrequency and nonlinear time at the ion gyroscales provides a simple measure of turbulence influence on proton kinetic behavior.

physics.space-ph↗

arXiv2013

On the Cause of Supra-Arcade Downflows in Solar Flares

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.

physics.space-ph↗

arXiv2013

von Kármán energy decay and heating of protons and electrons in a kinetic turbulent plasma

Decay in time of undriven weakly collisional kinetic plasma turbulence in systems large compared to the ion kinetic scales is investigated using fully electromagnetic particle-in-cell simulations initiated with transverse flow and magnetic disturbances, constant density, and a strong guide field. The observed energy decay is consistent with the von Kármán hypothesis of similarity decay, in a formulation adapted to magnetohydrodyamics (MHD). Kinetic dissipation occurs at small scales, but the overall rate is apparently controlled by large scale dynamics. At small turbulence amplitude the electrons are preferentially heated. At larger amplitudes proton heating is the dominant effect. In the solar wind and corona the protons are typically hotter, suggesting that these natural systems are in large amplitude turbulence regime.

physics.plasm-ph↗

arXiv2011

Estimates of Densities and Filling Factors from a Cooling Time Analysis of Solar Microflares Observed with RHESSI

We use more than 4,500 microflares from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) microflare data set (Christe et al., 2008, Ap. J., 677, 1385) to estimate electron densities and volumetric filling factors of microflare loops using a cooling time analysis. We show that if the filling factor is assumed to be unity, the calculated conductive cooling times are much shorter than the observed flare decay times, which in turn are much shorter than the calculated radiative cooling times. This is likely unphysical, but the contradic- tion can be resolved by assuming the radiative and conductive cooling times are comparable, which is valid when the flare loop temperature is a maximum and when external heating can be ignored. We find that resultant radiative and con- ductive cooling times are comparable to observed decay times, which has been used as an assumption in some previous studies. The inferred electron densities have a mean value of 10^11.6 cm^-3 and filling factors have a mean of 10^-3.7. The filling factors are lower and densities are higher than previous estimates for large flares, but are similar to those found for two microflares by Moore et al. (Ap. J., 526, 505, 1999).

astro-ph.SR↗

arXiv2011

Super-Alfvénic propagation of reconnection signatures and Poynting flux during substorms

The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfvén wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfvénic (V_parallel ~ 1500 - 5500 km/s) and generates substantial Poynting flux (S ~ 10^-5 - 10^-4 W/m^2) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in the Earth's ionosphere.

physics.plasm-ph↗

arXiv2008

Kinetic dissipation and anisotropic heating in a turbulent collisionless plasma

The kinetic evolution of the Orszag-Tang vortex is studied using collisionless hybrid simulations. In the magnetohydrodynamic regime this vortex leads rapidly to broadband turbulence. Significant differences from MHD arise at small scales, where the fluid scale energy dissipates into heat almost exclusively through the magnetic field because the protons are decoupled from the magnetic field. Although cyclotron resonance is absent, the protons heat preferentially in the plane perpendicular to the mean field, as in the corona and solar wind. Effective transport coefficients are calculated.

physics.plasm-ph↗

arXiv2008

From Solar and Stellar Flares to Coronal Heating: Theory and Observations of How Magnetic Reconnection Regulates Coronal Conditions

There is currently no explanation of why the corona has the temperature and density it has. We present a model which explains how the dynamics of magnetic reconnection regulates the conditions in the corona. A bifurcation in magnetic reconnection at a critical state enforces an upper bound on the coronal temperature for a given density. We present observational evidence from 107 flares in 37 sun-like stars that stellar coronae are near this critical state. The model may be important to self-organized criticality models of the solar corona.

astro-ph↗

arXiv2007

Two-scale structure of the electron dissipation region during collisionless magnetic reconnection

Particle in cell (PIC) simulations of collisionless magnetic reconnection are presented that demonstrate that the electron dissipation region develops a distinct two-scale structure along the outflow direction. The length of the electron current layer is found to decrease with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. A surprise, however, is that the electrons form a high-velocity outflow jet that remains decoupled from the magnetic field and extends large distances downstream from the x-line. The rate of reconnection remains fast in very large systems, independent of boundary conditions and the mass of electrons.

physics.plasm-ph↗

arXiv2006

A Model for Spontaneous Onset of Fast Magnetic Reconnection

We present a model for the spontaneous onset of fast magnetic reconnection in a weakly collisional plasma, such as the solar corona. When a current layer of macroscopic width undergoes collisional (Sweet-Parker) reconnection, a narrow dissipation region forms around the X-line. This dissipation region naturally becomes narrower during the reconnection process as stronger magnetic fields are convected toward the X-line. When the dissipation region becomes thinner than the ion skin depth, resistive magnetohydrodynamics breaks down as the Hall effect becomes important and the Sweet-Parker solution ceases to exist. A transition to collisionless (Hall) reconnection ensues, increasing the reconnection rate by many orders of magnitude in a very short time. Predictions of the model are consistent with constraints set by observations of solar flares.

physics.plasm-ph↗

arXiv2005

A catastrophe model for fast magnetic reconnection onset

A catastrophe model for the onset of fast magnetic reconnection is presented that suggests why plasma systems with magnetic free energy remain apparently stable for long times and then suddenly release their energy. For a given set of plasma parameters there are generally two stable reconnection solutions: a slow (Sweet-Parker) solution and a fast (Alfvénic) Hall reconnection solution. Below a critical resistivity the slow solution disappears and fast reconnection dominates. Scaling arguments predicting the two solutions and the critical resistivity are confirmed with two-fluid simulations.

physics.plasm-ph↗

arXiv2005

The Transition from Anti-Parallel to Component Magnetic Reconnection

We study the transition between anti-parallel and component collisionless magnetic reconnection with 2D particle-in-cell simulations. The primary finding is that a guide field \approx 0.1 times as strong as the asymptotic reconnecting field -- roughly the field strength at which the electron Larmor radius is comparable to the width of the electron current layer -- is sufficient to magnetize the electrons in the vicinity of the x-line, thus causing significant changes to the structure of the electron dissipation region. This implies that great care should be exercised before concluding that magnetospheric reconnection is antiparallel. We also find that even for such weak guide fields strong inward-flowing electron beams form in the vicinity of the magnetic separatrices and Buneman-unstable distribution functions arise at the x-line itself. As in the calculations of {\it Hesse et al.} [2002] and {\it Yin and Winske} [2003], the non-gyrotropic elements of the electron pressure tensor play the dominant role in decoupling the electrons from the magnetic field at the x-line, regardless of the magnitude of the guide field and the associated strong variations in the pressure tensor's spatial structure. Despite these changes, and consistent with previous work, the reconnection rate does not vary appreciably with the strength of the guide field as it changes between 0 and a value equal to the asymptotic reversed field.

physics.space-ph↗

arXiv2004

Three species collisionless reconnection: Effect of O+ on magnetotail reconnection

The nature of collisionless reconnection in a three-species plasma composed of a heavy species, protons, and electrons is examined. Besides the usual two length scales present in two-species reconnection, there are two additional larger length scales in the system: one associated with a "heavy whistler" which produces a large scale quadrupolar out-of-plane magnetic field, and one associated with the "heavy Alfven" wave which can slow the outflow speed and thus the reconnection rate. The consequences for reconnection in the magnetotail with an O+ population present are discussed.

physics.plasm-ph↗

arXiv2003

Diamagnetic Suppression of Component Magnetic Reconnection at the Magnetopause

We present particle-in-cell simulations of collisionless magnetic reconnection in a system (like the magnetopause) with a large density asymmetry across the current layer. In the presence of an ambient component of the magnetic field perpendicular to the reconnection plane the gradient creates a diamagnetic drift that advects the X-line with the electron diamagnetic velocity. When the relative drift between the ions and electrons is of the order the Alfven speed the large scale outflows from the X-line necessary for fast reconnection cannot develop and the reconnection is suppressed. We discuss how these effects vary with both the plasma beta and the shear angle of the reconnecting field and discuss observational evidence for diamagnetic stabilization at the magnetopause.

physics.space-ph↗