Collisionless magnetic reconnection in an asymmetric current sheet

Abstract

[1] The process of collisionless magnetic reconnection in an ion-scale current sheet containing strong gradients in the density and magnetic field strength across the layer is investigated using two-dimensional particle-in-cell simulations. Such a current sheet configuration contains a strong normal polarization electric field on the high field/low density (magnetospheric) side of the layer. In initial-value simulations for such an asymmetric sheet, the reconnection rate and saturation level are found to be smaller by factors of 2-3 compared with a similar-scale symmetric current sheet. These rates are probably too small to explain observations at the dayside magnetopause. The addition of an external-driving electric field increases the reconnection rate substantially. This driven reconnection configuration is characterized by a nearly parallel inflow of electrons along the magnetosheath separatrices as the electrons attempt to flow from the high density side to the low density side of the layer, a strong outward flow of Poynting flux along the magnetospheric separatrices associated with the normal electric field and out-of-plane magnetic field, and a strong ion outflow jet. The outflow region on the magnetospheric side also exhibits a patchy parallel electric field structure and parallel electron velocity distributions with a counterstreaming feature. The addition of a moderate uniform magnetic guide field component (shear angle^110°) has no appreciable effect on the reconnection rate but does produce a drift of the X line in the direction of the electron diamagnetic drift at a small fraction of the magnetosheath Alfvén speed.

Response to “Comment on ‘Scaling of asymmetric magnetic reconnection: General theory and collisional simulations’ ” [Phys. Plasmas 16, 034701 (2009)]

“…Our own simulations confirmed these results for Sweet-Parker reconnection in resistive magnetohydrodynamics ͑MHD͒ simulations for asymmetric fields 1 and asymmetric densities 3 and for Hall reconnection in two-fluid ͑Hall-MHD with electron inertia͒ simulations for all combinations of asymmetries. 4 In addition, the reconnection rate E ͑whose derivation depends on the relation in question͒ has been confirmed by simulations from other groups [5][6][7][8][9] in various settings using various simulation techniques.…”

Magnetic Null Points in Kinetic Simulations of Space Plasmas

“…The upper layer considers the continuous hyperbolic profiles typically adopted in theliterature (Pritchett 2008), while the lower layer describes a pure tangential discontinuity set with an extremely steep gradient. Therefore, the total profile reads …”

“…No current profile is initially set up, but a strong current layer is soon generated selfconsistently by particle motion. The upper inversion layer is initialized with the "gentler" magnetic field and density profile (Pritchett 2008) given by Equations (1) and (2) in Cazzola et al (2015). A consistent out-of-plane current is initialized assuming that all current is carried by electrons.…”

Aspects of collisionless magnetic reconnection in asymmetric systems

“…The second question is related to an observation seen in the reconnection rates of an earlier study: 12 Why does magnetic reconnection exhibit a higher rate in the presence of a guide field than without it, if the system is asymmetric? The common expectation is that the introduction of a significant guide field will reduce plasma compressibility, and hence the available evolution space of the system.…”

“…Beginning with MHD-based analyses of balance equations, 6 including the "Cassak-Shay" theory, 11 reconnection theory and modeling has since been extended into the kinetic realm. Kinetic studies find substantial differences between symmetric and asymmetric reconnection: Pritchett 12 describes an electric field structure very different from symmetric systems; and Mozer and Pritchett 13 question the relevance of the reconnection electric field in the context of much larger structure of much stronger electric fields.…”