Substorm inner plasma sheet particle reduction

Abstract

Geotail measurements from within the equatorial inner plasma sheet at r ∼ 10–13 RE 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 λ, there is a reduction in the number of particles for all λ 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.