Reconnection Inside a Dipolarization Front of a Diverging Earthward Fast Flow

We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi-scale multipoint analysis methods. In order to study the large-scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X-line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter-rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi-point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in- and outflow structure, consistent with previous simulations on strong guide-field reconnection events. This study shows that magnetic reconnection may not only take place at large-scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field

Collisionless magnetic reconnection: analytical model and PIC simulation comparison

Magnetic reconnection is believed to be responsible for various explosive processes in the space plasma including magnetospheric substorms. The Hall effect is proved to play a key role in the reconnection process. An analytical model of steady-state magnetic reconnection in a collisionless incompressible plasma is developed using the electron Hall MHD approximation. It is shown that the initial complicated system of equations may split into a system of independent equations, and the solution of the problem is based on the Grad-Shafranov equation for the magnetic potential. The results of the analytical study are further compared with a two-dimensional particle-in-cell simulation of reconnection. It is shown that both methods demonstrate a close agreement in the electron current and the magnetic and electric field structures obtained. The spatial scales of the acceleration region in the simulation and the analytical study are of the same order. Such features like particles trajectories and the in-plane electric field structure appear essentially similar in both models

Current Sheet Bending as Destabilizing Factor in Magnetotail Dynamic

The problem of the magnetohydrodynamical stability of bent magnetotail current sheets is considered by means of 2.5-dimensional numerical simulations. This study is focused on the cross-tail transversal mode, modeling the magnetotail flapping motions, at the background of the Kan-like magnetoplasma equilibrium. It is found that in symmetrical current sheets, both stable and unstable branches of the solution may coexist; the growth rate of the unstable mode is rather small, so that the sheet may be considered as stable at the substorm timescale. With the increasing dipole tilt angle, the sheet bends and the growth rate rises. For sufficiently large tilt angles, the stable branch of the solution disappears. Thereby, the sheet destabilization timescale shortens for an order of magnitude, down to several minutes. The analysis of the background parameters has shown that stability loss is not related to buoyancy; it is controlled by the cross-sheet distribution of the total pressure

Inner and outer electron diffusion region of antiparallel collisionless reconnection: Density dependence

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Inner and outer electron diffusion region of antiparallel collisionless reconnection: Density dependence

We study inflow density dependence of substructures within electron diffusion region (EDR) of collisionless symmetric magnetic reconnection. We perform a set of 2.5D particle-in-cell simulations which start from a Harris current layer with a uniform background density nb. A scan of nb ranging from 0.02 n0 to 2 n0 of the peak current layer density (n0) is studied keeping other plasma parameters the same. Various quantities measuring reconnection rate, EDR spatial scales, and characteristic velocities are introduced. We analyze EDR properties during quasisteady stage when the EDR length measures saturate. Consistent with past kinetic simulations, electrons are heated parallel to the B field in the inflow region. The presence of the strong parallel anisotropy acts twofold: (1) electron pressure anisotropy drift gets important at the EDR upstream edge in addition to the ExB drift speed and (2) the pressure anisotropy term -(div Pe)/(ne) modifies the force balance there. We find that the width of the EDR demagnetization region and EDR current are proportional to the electron inertial length ~de and ~denb0.22, respectively. Magnetic reconnection is fast with a rate of ~0.1 but depends weakly on density as ~nb-1/8. Such reconnection rate proxies as EDR geometrical aspect or the inflow-to-outflow electron velocity ratio are shown to have different density trends, making electric field the only reliable measure of the reconnection rate

Cold ion energization at separatrices during magnetic reconnection

International audienceSeparatrices of magnetic reconnection host intense perpendicular Hall electric fields. The fields are produced by the decoupling of the ion and electron components and are associated with the in-plane electrostatic potential drop between the inflow and outflow regions. The width of these structures is typically less than the ion inertial length, which is small enough to demagnetize ions as they cross the layer. We investigate ion acceleration at separatrices by means of 2D particle-in-cell simulations of magnetic reconnection for two limiting cases: (1) a “GEM-like” setup (here GEM stands for geospace environmental modeling reconnection challenge) with the lobe ion thermal velocity equal to the thermal velocity of the initial current sheet ions, which is comparable to the Alfvén velocity and (2) a “cold” ion setup, in which the temperature of the background lobe ions is 1/100 of the initial current sheet temperature. The separatrix Hall electric field is balanced by the ion inertia term in the cold background simulations. The effect is indicative of the quasi-steady local perpendicular acceleration. The electric field introduces a cross field beam of unmagnetized particles, which makes the ion distribution function strongly non-gyrotropic and susceptible to sub-ion scale instabilities. This acceleration mechanism nearly vanishes in the hot ion background simulations. Our particle-in-cell simulations are complemented by one-dimensional test particle calculations. They show that the hot ion particles experience energy-scattering after crossing the accelerating layer, whereas cold ions are uniformly energized up to the energies comparable to the electrostatic potential drop between the inflow and outflow regions