Pressure changes in the plasma sheet during substorm injections

Abstract

Using data from the CHEM instrument on AMPTE CCE, data from the three‐dimensional plasma instrument and the SULEICA instrument on AMPTE IRM, and magnetometer data from both spacecraft, we have determined the particle pressure and total pressure as a function of radial distance in the plasma sheet for periods before and after the onset of substorm‐associated ion enhancements over the radial range 7–19 RE. We have chosen events occurring during times of increasing magnetospheric activity, as determined by an increasing AE index, in which a sudden increase, or “injection”, of energetic particle flux is observed. During these events the particle energy of maximum contribution to the pressure increases from about 12 to about 27 keV. In addition, the particle pressure increases, and the magnetic pressure decreases, with the total pressure only changing slightly. For radial distances of less than 10 RE the total pressure tends to increase with the injection, while outside 10 RE it tends to decrease or remain the same. Because the fraction of the pressure due to particles has increased and higher energies are contributing to the pressure, a radial gradient is evident in the postinjection, but not preinjection, flux measurements. These observations show that the simultaneous appearance of energetic particles and changes in the magnetic field results naturally from pressure balance and does not necessarily indicate that the local changing field is accelerating the particles. The changes in the total pressure outside 10 RE are consistent with previous measurements of pressure changes at substorm onset and can be understood in terms of the unloading of energy in the magnetotail and the resulting change in the magnetic field configuration. The increase in pressure at R ≤ 10 RE may also be due to the changing magnetic field configuration if in the region close to the Earth where the field becomes more dipolar, the angle at which the solar wind interacts with the magnetopause increases, while farther down the tail it decreases. Finally, the radial pressure gradient observed in the 7–12 RE region agrees very well with that necessary to maintain stress balance for an isotropic plasma.