Ari Le

A review of pressure anisotropy caused by electron trapping in collisionless plasma, and its implications for magnetic reconnection

From spacecraft data, it is evident that electron pressure anisotropy develops in collisionless plasmas. This is in contrast to the results of theoretical investigations, which suggest this anisotropy should be limited. Common for such theoretical studies is that the effects of electron trapping are not included; simply speaking, electron trapping is a non-linear effect and is, therefore, eliminated when utilizing the standard methods for linearizing the underlying kinetic equations. Here, we review our recent work on the anisotropy that develops when retaining the effects of electron trapping. A general analytic model is derived for the electron guiding center distribution f¯(v∥,v⊥) of an expanding flux tube. The model is consistent with anisotropic distributions observed by spacecraft, and is applied as a fluid closure yielding anisotropic equations of state for the parallel and perpendicular components (relative to the local magnetic field direction) of the electron pressure. In the context of reconnec...

Formation of a localized acceleration potential during magnetic reconnection with a guide field

Magnetic reconnection near the surface of the sun and in the Earth’s magnetotail is associated with the production of highly energetic electrons. Direct acceleration in the reconnection electric field has been proposed as a possible mechanism for energizing these electrons. Here, however, we use kinetic simulations of guide-field reconnection to show that in two-dimensional (2D) reconnection the parallel electric field, E∥ in the reconnection region is localized and its structure does not permit significant energization of the electrons. Rather, a large fraction of the electrons become trapped due to a sign reversal in E∥, imposing strict constraints on their motions and energizations. Given these new results, simple 2D models, which invoke direct acceleration for energizing electrons during a single encounter with a reconnection region, need to be revised.

Electron dynamics in two-dimensional asymmetric anti-parallel reconnection

Kinetic simulations and spacecraft observations have documented strong anisotropy in the electron distribution function during magnetic reconnection. The level and role of electron pressure anisotropy is investigated for asymmetric geometries applicable to reconnection in the day-side magnetopause. A previously derived analytic model for the pressure anisotropy is generalized and is applied to the asymmetric geometry. In agreement with the results from a kinetic simulation, the generalized model predicts the strongest pressure anisotropy and parallel electric fields in the in-flow region characterized by low electron pressure.

Laboratory observations of electron energization and associated lower-hybrid and Trivelpiece–Gould wave turbulence during magnetic reconnection

This work presents an experimental study of current-driven turbulence in a plasma undergoing magnetic reconnection in a low-β, strong-guide-field regime. Electrostatic fluctuations are observed by small, high-bandwidth, and impedance-matched Langmuir probes. The observed modes, identified by their characteristic frequency and wavelength, include lower-hybrid fluctuations and high-frequency Trivelpiece–Gould modes. The observed waves are believed to arise from electrons energized by the reconnection process via direct bump-on-tail instability (Trivelpiece–Gould) or gradients in the fast electron population (lower-hybrid).

Equations of state in collisionless magnetic reconnection

Kinetic simulation as well as in situ measurement of reconnecting current sheets in the Earths magnetosphere show strong electron temperature anisotropy, with the parallel electron temperature becoming several times greater than the perpendicular temperature. This anisotropy is accounted for in a solution of the Vlasov equation recently derived for general reconnection geometries with magnetized electrons in the limit of fast transit time. A necessary ingredient is a magnetic field-aligned electric field extending over the ion inertial length scale. The parallel electric field maintains quasineutrality by regulating the electron density, traps a large fraction of thermal electrons, and heats electrons in the parallel direction. Based on the expression for the electron phase-space density, equations of state provide a fluid closure for the electrons that relates the parallel and perpendicular pressures to the density and magnetic field strength. The resulting fluid model agrees well with fully kinetic simulations of guide-field reconnection, accurately predicting the electron temperature anisotropy. In addition, the equations of state impose strong constraints on the electron Hall currents and magnetic fields that develop during antiparallel reconnection. The model provides scaling laws for the Hall magnetic fields and predicts the magnitude of the current in the electron layer.

Spontaneous onset of magnetic reconnection in toroidal plasma caused by breaking of 2D symmetry

Magnetic reconnection is studied in the collisionless limit at the Versatile Toroidal Facility (VTF) at MIT. Two distinct magnetic configurations are applied in the experiments; an open magnetic cusp and a closed cusp. In the open cusp configurations, the field lines intersect the the vacuum vessel walls and here axisymmetric oscillatory reconnection is observed. Meanwhile, in the closed cusp configuration, where the field lines are confined inside the experiment, the coupling between global modes and a current sheet leads to powerful bursts of 3D spontaneous reconnection. These spontaneous events start at one toroidal location, and then propagate around the toroidal direction at the Alfven speed (calculated with the strength of the dominant guide field). The three dimensional measurements include the detailed time evolution of the plasma density, current density, the magnetic flux function, the electrostatic potential, and the reconnection rate. The vastly different plasma behavior in the two configurati...

Magnetic flux array for spontaneous magnetic reconnection experiments

Experimental investigation of reconnection in magnetized plasmas relies on accurate characterization of the evolving magnetic fields. In experimental configurations where the plasma dynamics are reproducible, magnetic data can be collected in multiple discharges and combined to provide spatially resolved profiles of the plasma dynamics. However, in experiments on spontaneous magnetic reconnection recently undertaken at the Versatile Toroidal Facility at MIT, the reconnection process is not reproducible and all information on the plasma must be collected in a single discharge. This paper describes a newly developed magnetic flux array which directly measures the toroidal component of the magnetic vector potential, A(phi). From the measured A(phi), the magnetic field geometry, current density, and reconnection rate are readily obtained, facilitating studies of the three-dimensional dynamics of spontaneous magnetic reconnection. The novel design of the probe array allows for accurate characterization of profiles of A(phi) at multiple toroidal angles using a relatively small number of signal channels and with minimal disturbance of the plasma.