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Plasma dynamics and momentum transport near an X line during time‐dependent magnetic reconnection in a collisionless plasma are investigated based on two‐dimensional particle simulations. We find that a weakly skewed velocity distribution is formed near the magnetic X line, leading to the presence of off‐diagonal elements of the plasma pressure tensor. Let the reconnection electric field be in the y direction. The gradients of the off‐diagonal elements of the pressure tensor can provide a transport of the y momentum. During the normal magnetic reconnection, the momentum transport associated with the off‐diagonal terms of the pressure tensor mediates a transfer of the y momentum from the region near the X line to regions outside the X line. A period of “reverse magnetic reconnection,” during which the plasma kinetic energy is converted into magnetic energy, is also observed in the simulation. When reverse reconnection occurs, the gradients of the off‐diagonal pressure tensor elements can mediate a transfer of y momentum into the X line. It is found that the inertial term also plays a significant role in the force balance near the magnetic X line. An explanation for the origin of the off‐diagonal pressure terms is also given in this paper.

A magnetofrictional method is used to construct two‐dimensional MHD equilibria of the Earth's magnetosphere for a given distribution of entropy function (S = pVγ), where p is the plasma pressure and V is the tube volume per unit magnetic flux. It is found that a very thin current sheet with Bz < 0.5nT and thickness < 1000km can be formed in the near‐earth magnetotail (x ∼ −8 to −20 Re) during the growth phase of substorm. The tail current sheets are found to become thinner as the entropy or the entropy gradient increases. We suggest that the new “entropy anti‐diffusion instability” associated with plasma transport across field lines leads to magnetic field dipolarization and accelerates the formation of thin current sheet, which may explain the observed explosive growth phase of substorms.

Abstract. In our recent study of the equilibrium configuration of the near-Earth magnetotail, it is found that a steeper profile of entropy function $ leads to the formation of a thinner current sheet. Here $ -pV v, where p is the plasma pressure and V is the volume of a unit magnetic flux tube. In the presence of density gradient at the edges of the current sheet, the lower-hybrid-drift instability may lead to pressure diffusion. Since the pressure generally decreases and the entropy function $ increases tailward, the diffusion of plasma pressure results in a steepening, or antidiffusion, of the entropy profile. This, in turn, leads to a thinner current sheet, which enhances the pressure diffusion. This positive feedback process is called the entropy antidiffusion instability. On the basis of MHD simulations with a pressure diffusion, we find that the entropy antidiffusion instability leads to a further thinning of the near-Earth current sheet and the onset of dipolarization of near-Earth geomagnetic field lines. The growth rate of the instability is linearly proportional to the diffusion coefficient. The growth time at the final stage for the formation of a very thin current sheet is -•1 min. This instability may explain the observed explosive growth phase of substorms and the onset of dipolarization of geomagnetic field lines.