Plasma sheet ion pressure profiles during substorm

Over 200 substorm events were carefully selected from observations made by Polar UVI, VIS, and an ensemble of ground instruments. From these events, more than 140 of them have simultaneous DMSP observations. Four substorm phases were defined, namely, growth, expansion, early recovery, and late recovery. Plasma sheet 2-D ion pressure, density, and temperature profiles inferred from DMSP satellite observations were constructed for each of these four substorm phases. The growth phase profiles show that the ion pressure is higher in the inner edge of the plasma sheet. The premidnight pressure enhancement can be attributed to the temperature enhancement while the postmidnight pressure enhancement can be attributed to the density enhancement. The temperature enhancement at premidnight has been previously reported and attributed to the curvature and gradient drift of the ions. The postmidnight density enhancement may result from the enhanced convection. The profiles show that the ion pressure near the midnight meridian in the midtail region increases right after the substorm onset, during the expansion phase. The pressure subsequently declines during the early recovery phase and declines further during the late recovery phase, but the late recovery pressure is still higher than that of the growth phase. This near midnight pressure enhancement during the expansion and recovery phases can be attributed to both temperature and density enhancements. The 2-D pressure, temperature, and density profiles provide observational constraints to the competing substorm theories.