Magnetospheric Current System During Disturbed Times

Pre-storm Proton Pressure Using Bounce-Averaged Particle Drift Simulations 1130 UT Magnetic Field Perturbation 0500 UT Magnetohydrodynamic Pressure 0800 UT 1130 UT October 19, 1998 Storm 0500 UT

Pressure and Temperature From Guiding Center Particles 0300 to 0600 UT

The October 1998 storm (minimum Dst = –125 nT) was selected as a study event. We applied global MHD simulation, particle trajectory calculations and ring current simulations to this event. Particle Drift Simulations Trace bounce-averaged drifts of ions that conserve the first two adiabatic invariants M and J. Dipole and uniform southward magnetic field were assumed in this model AMIE electric potentials were mapped analytically from ionosphere to anywhere in magnetic field model. Guiding center: A distribution of particle at energies above 5 keV are launched on the night side at r = 10 R E within 21º of the x axis. At 0300 and 0400 UT 600,000 particles were launched. Trace the guiding center drifts of ions that conserve the first adiabatic invariant M in the MHD electric and magnetic fields.

MHD Inner Magnetosphere The MHD simulation shows a pressure increase at the beginning of the main phase that is located on the dusk side. In general, however, the MHD simulation had a relatively low pressure in the inner magnetosphere during the storm (maximum about 2 nPa). Guiding Center Particles in MHD Fields The pressure due to the test particles was greater in the inner magnetosphere during the storm than the MHD model (maximum about 10 nPa). Substantial particle losses occur through the dusk side (tail and dayside) and on open field lines in the cusp region. Most of the particles that were launched formed a partial ring current on the dusk side. Particle Drift Simulations The ring current simulation showed greater pressures in the inner magnetosphere (the highest pressure about 30 nPa) than either the MHD or the guiding center particle simulations. At later times the particle drift simulation’s ring current became more symmetric and very intense.

Model Comparisons Guiding center particle calculations led pressure maxima greater than the MHD result but less than the particle drift model. The particles were strongly energized by the electric fields but substantial losses at the (mainly dusk side) magnetopause and near the cusp reduced the pressure due to these particles. The particle drift simulation showed the highest pressure but did not include loss mechanisms that were important in the guiding center particle calculations. To complete this study we will: Investigate the dynamics of the inner magnetosphere by self-consistently combining a particle-based ring current model with a global MHD model. Include particle pressure in the MHD momentum equation to account for the particle drift.