Substorm entropies

The specific entropy ( s = p / ρ γ ) and total entropy ( S = p 1/ γ V ) of the plasma sheet during substorms are investigated with DMSP observations using a method that assumes ion isotropy (as also assumed in the derivation of S = p 1/ γ V ) and empirical magnetic field models that capture the expected characteristics of substorm phases. Earthward reductions of S are found during quasi‐steady periods (e.g., growth phase) as well as during the transition from growth to expansion phases. During quasi‐steady periods, (1) S and flux tube content ( N ) decrease moderately at midtail, but more steeply at the inner edge of the plasma sheet, and (2) s appears roughly conserved in the X or convection direction, but closer to Earth, there is a duskward heat flux. Both 1 and 2 suggest that curvature/gradient drifts can play a significant role in the S and N losses. On the other hand, during the transition from growth to expansion phases, S is reduced by an order of magnitude earthward of 20 R E , which can be attributed mainly to the reduction in V from the dipolarization after onset, but s is roughly conserved. This result is consistent with a mechanism that reduces the flux tube volume/content without significantly altering s . Recent magnetohydrodynamic/particle‐in‐cell simulations of magnetic reconnection indicate that s tends to be conserved except in the small dissipation region, but the change in the field line topology for a reconnected field line can lead to a reduction in S with the remainder contained in a plasmoid that forms tailward of the X line.