Entropy antidiffusion instability and formation of a thin current sheet during geomagnetic substorms

In our recent study of the equilibrium configuration of the near‐Earth magnetotail, it is found that a steeper profile of entropy function S leads to the formation of a thinner current sheet. Here S = p V γ , 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 S 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.