Flux-Tube Depletions During Substorms

We report on early results from an investigation of the role of interchange during substorm injections. This investigation uses observations taken onboard the Geotail satellite near perigee (-9 R_E > XGSE > -12 R_E) in the midnight sector of the plasma sheet near times of dipolarizations/injections. We illustrate a method to obtain the instantaneous specific entropy, pV^{5/3}, of flux tubes from single-satellite measurements of the plasma and magnetic field and present some early findings. Injections at times of dipolarizations are comprised of low-content flux tubes ("bubbles") that have lower pV^{5/3} than their pre- onset predecessors.