Substorm inner plasma sheet particle reduction

Geotail measurements from within the equatorial inner plasma sheet at r similar to 10-13 R-E are used to show that a reduction in equatorial plasma pressure within the current wedge is a general feature of substorms and that this decrease is simultaneous with the well-known expansion phase increase in energetic particle fluxes. The decrease in pressure is found to be due to a decrease in particle fluxes at lower energies that gives a significant decrease in plasma density. We also find that when viewed as a function of the adiabatic energy invariant for the plasma sheet lambda, there is a reduction in the number of particles for all lambda despite the enhancements seen when energetic particle fluxes are viewed at fixed particle energy. These observations demonstrate that the expansion phase is associated with a significant reduction in flux tube content of particles within the current wedge and that this reduction leads to the expansion-phase reduction in equatorial plasma pressure and cross-tail current.