More Like this

1. Onset of magnetic reconnection in a collisionless, high- plasma

   https://doi.org/10.1017/S0022377819000084  

   Alt, Andrew; Kunz, Matthew W. (February 2019, Journal of Plasma Physics)

   null (Ed.)

   In a magnetized, collisionless plasma, the magnetic moment of the constituent particles is an adiabatic invariant. An increase in the magnetic-field strength in such a plasma thus leads to an increase in the thermal pressure perpendicular to the field lines. Above a $$\unicode[STIX]{x1D6FD}$$ -dependent threshold (where $$\unicode[STIX]{x1D6FD}$$ is the ratio of thermal to magnetic pressure), this pressure anisotropy drives the mirror instability, producing strong distortions in the field lines on ion-Larmor scales. The impact of this instability on magnetic reconnection is investigated using a simple analytical model for the formation of a current sheet (CS) and the associated production of pressure anisotropy. The difficulty in maintaining an isotropic, Maxwellian particle distribution during the formation and subsequent thinning of a CS in a collisionless plasma, coupled with the low threshold for the mirror instability in a high- $$\unicode[STIX]{x1D6FD}$$ plasma, imply that the geometry of reconnecting magnetic fields can differ radically from the standard Harris-sheet profile often used in simulations of collisionless reconnection. As a result, depending on the rate of CS formation and the initial CS thickness, tearing modes whose growth rates and wavenumbers are boosted by this difference may disrupt the mirror-infested CS before standard tearing modes can develop. A quantitative theory is developed to illustrate this process, which may find application in the tearing-mediated disruption of kinetic magnetorotational ‘channel’ modes. 

   more » « less
2. Microphysically modified magnetosonic modes in collisionless, high-β plasmas

   https://doi.org/10.1017/S0022377823000429  

   Majeski, S.; Kunz, M.W.; Squire, J. (June 2023, Journal of Plasma Physics)

   With the support of hybrid-kinetic simulations and analytic theory, we describe the nonlinear behaviour of long-wavelength non-propagating (NP) modes and fast magnetosonic waves in high- $$\beta$$ collisionless plasmas, with particular attention to their excitation of and reaction to kinetic micro-instabilities. The perpendicularly pressure balanced polarization of NP modes produces an excess of perpendicular pressure over parallel pressure in regions where the plasma $$\beta$$ is increased. For mode amplitudes $$|\delta B/B_0| \gtrsim 0.3$$ , this excess excites the mirror instability. Particle scattering off these micro-scale mirrors frustrates the nonlinear saturation of transit-time damping, ensuring that large-amplitude NP modes continue their decay to small amplitudes. At asymptotically large wavelengths, we predict that the mirror-induced scattering will be large enough to interrupt transit-time damping entirely, isotropizing the pressure perturbations and morphing the collisionless NP mode into the magnetohydrodynamic (MHD) entropy mode. In fast waves, a fluctuating pressure anisotropy drives both mirror and firehose instabilities when the wave amplitude satisfies $$|\delta B/B_0| \gtrsim 2\beta ^{-1}$$ . The induced particle scattering leads to delayed shock formation and MHD-like wave dynamics. Taken alongside prior work on self-interrupting Alfvén waves and self-sustaining ion-acoustic waves, our results establish a foundation for new theories of electromagnetic turbulence in low-collisionality, high- $$\beta$$ plasmas such as the intracluster medium, radiatively inefficient accretion flows and the near-Earth solar wind. 

   more » « less
3. Reconnection Onset in Overstretched Thin Current Sheets: PIC Simulations

   https://doi.org/10.1029/2025JA033863  

   Sitnov, M I; Arnold, H (May 2025, Journal of Geophysical Research: Space Physics)

   Abstract Onset of reconnection in the magnetotail requires its current sheet (CS) to thin down to the thermal ion gyroradius (or thinner) to demagnetize ions (or even electrons) and to provide their Landau dissipation. However, in isotropic plasma models of the tail the ion‐scale CSs inflate too rapidly with the distance from Earth to remain ion‐scale beyond 20 Earth's radii, where most X‐lines are observed. A key to solving this problem was recently found due to the discovery of “overstretched” thin CSs (OTCSs): If an ion‐scale CS is embedded into a much thicker CS with even a weak field‐aligned ion anisotropy, its current density iso‐contours can be stretched far beyond the magnetic field lines. Here we investigate onset of reconnection in OTCS with their scales and features closer to the observed geometry and evolution of Earth's magnetotail: extension beyond 100 ion inertial lengths, magnetic flux accumulation, dipole field effects and weak external driving. 2‐D particle‐in‐cell (PIC) simulations with open boundaries show that OTCSs help explain the observed X‐line location in the magnetotail. The reconnection electric field strongly exceeds both the external driving field and the slow convection electric field caused by the latter. The magnetic topology change (onset of reconnection proper) is preceded by divergent plasma flows suggesting that the latter are produced by the ion tearing plasma motions. OTCS are also shown to form in isotropic CS after an even shorter driving period, but their transient nature may question universality of this onset scenario. 

   more » « less
4. PIC Simulations of Overstretched Ion‐Scale Current Sheets in the Magnetotail

   https://doi.org/10.1029/2023GL104534  

   Arnold, H.; Sitnov, M_I (August 2023, Geophysical Research Letters)

   Abstract Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness. 

   more » « less
5. Magnetogenesis in a Collisionless Plasma: From Weibel Instability to Turbulent Dynamo

   https://doi.org/10.3847/1538-4357/ad0b0f  

   Zhou, Muni; Zhdankin, Vladimir; Kunz, Matthew W.; Loureiro, Nuno F.; Uzdensky, Dmitri A. (December 2023, The Astrophysical Journal)

   Abstract We report on a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of “seed” magnetic fields, the formation of turbulence, and the inductive amplification of fields by the fluctuation dynamo. Driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary “seed” magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until energy equipartition with the turbulent flow is reached. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums. 

   more » « less