 Award ID(s):
 1804048
 Publication Date:
 NSFPAR ID:
 10204595
 Journal Name:
 Journal of Plasma Physics
 Volume:
 85
 Issue:
 1
 ISSN:
 00223778
 Sponsoring Org:
 National Science Foundation
More Like this

We study the timedependent formation and evolution of a current sheet (CS) in a magnetised, collisionless, highbeta plasma using hybridkinetic particleincell simulations. An initially tearingstable Harris sheet is frozen into a persistently driven incompressible flow so that its characteristic thickness gradually decreases in time. As the CS thins, the strength of the reconnecting field increases, and adiabatic invariance in the inflowing fluid elements produces a fieldbiased pressure anisotropy with excess perpendicular pressure. At large plasma beta, this anisotropy excites the mirror instability, which deforms the reconnecting field on ionLarmor scales and dramatically reduces the effective thickness of the CS. Tearing modes whose wavelengths are comparable to that of the mirrors then become unstable, triggering reconnection on smaller scales and at earlier times than would have occurred if the thinning CS were to have retained its Harris profile. A novel method for identifying and tracking Xpoints is introduced, yielding Xpoint separations that are initially intermediate between the perpendicular and parallel mirror wavelengths in the upstream plasma. These mirrorstimulated tearing modes ultimately grow and merge to produce island widths comparable to the CS thickness, an outcome we verify across a range of CS formation timescales and initial CS widths. Our results maymore »

A heat flux in a high $\unicode[STIX]{x1D6FD}$ plasma with low collisionality triggers the whistler instability. Quasilinear theory predicts saturation of the instability in a marginal state characterized by a heat flux that is fully controlled by electron scattering off magnetic perturbations. This marginal heat flux does not depend on the temperature gradient and scales as $1/\unicode[STIX]{x1D6FD}$ . We confirm this theoretical prediction by performing numerical particleincell simulations of the instability. We further calculate the saturation level of magnetic perturbations and the electron scattering rate as functions of $\unicode[STIX]{x1D6FD}$ and the temperature gradient to identify the saturation mechanism as quasilinear. Suppression of the heat flux is caused by oblique whistlers with magneticenergy density distributed over a wide range of propagation angles. This result can be applied to high $\unicode[STIX]{x1D6FD}$ astrophysical plasmas, such as the intracluster medium, where thermal conduction at sharp temperature gradients along magneticfield lines can be significantly suppressed. We provide a convenient expression for the amount of suppression of the heat flux relative to the classical Spitzer value as a function of the temperature gradient and $\unicode[STIX]{x1D6FD}$ . For a turbulent plasma, the additional independent suppression by the mirror instability is capable of producing large total suppression factors (severalmore »

Magnetic reconnection, a plasma process converting magnetic energy to particle kinetic energy, is often invoked to explain magnetic energy releases powering highenergy flares in astrophysical sources including pulsar wind nebulae and black hole jets. Reconnection is usually seen as the (essentially twodimensional) nonlinear evolution of the tearing instability disrupting a thin current sheet. To test how this process operates in three dimensions, we conduct a comprehensive particleincell simulation study comparing two and threedimensional evolution of long, thin current sheets in moderately magnetized, collisionless, relativistically hot electron–positron plasma, and find dramatic differences. We first systematically characterize this process in two dimensions, where classic, hierarchical plasmoidchain reconnection determines energy release, and explore a wide range of initial configurations, guide magnetic field strengths and system sizes. We then show that threedimensional (3D) simulations of similar configurations exhibit a diversity of behaviours, including some where energy release is determined by the nonlinear relativistic driftkink instability. Thus, 3D current sheet evolution is not always fundamentally classical reconnection with perturbing 3D effects but, rather, a complex interplay of multiple linear and nonlinear instabilities whose relative importance depends sensitively on the ambient plasma, minor configuration details and even stochastic events. It often yields slower but longerlasting andmore »

We propose that pressure anisotropy causes weakly collisional turbulent plasmas to selforganize so as to resist changes in magneticfield strength. We term this effect ‘magnetoimmutability’ by analogy with incompressibility (resistance to changes in pressure). The effect is important when the pressure anisotropy becomes comparable to the magnetic pressure, suggesting that in collisionless, weakly magnetized (high $\unicode[STIX]{x1D6FD}$ ) plasmas its dynamical relevance is similar to that of incompressibility. Simulations of magnetized turbulence using the weakly collisional Braginskii model show that magnetoimmutable turbulence is surprisingly similar, in most statistical measures, to critically balanced magnetohydrodynamic turbulence. However, in order to minimize magneticfield variation, the flow direction becomes more constrained than in magnetohydrodynamics, and the turbulence is more strongly dominated by magnetic energy (a nonzero ‘residual energy’). These effects represent key differences between pressureanisotropic and fluid turbulence, and should be observable in the $\unicode[STIX]{x1D6FD}\gtrsim 1$ turbulent solar wind.

Abstract We demonstrate using linear theory and particleincell (PIC) simulations that a synchrotroncooling collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability, in a process that we dub the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into smallamplitude, kineticscale, magneticfield fluctuations, which pitchangle scatter the particles and bring the plasma to a nearthermal state of marginal instability. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. We compare the SFHI for electron–positron and electron–ion plasmas. As a byproduct of the growing electronfirehose magneticfield fluctuations, magnetized ions gain a pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they also experience cooling due to collisionless thermal coupling with the electrons, which we argue is mediated by a secondary ioncyclotron instability. We suggest that the SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black hole accretion flows and relativistic jets, where the redistribution of energetic electrons from low to high pitch angles may cause transient burstsmore »