GLOBAL NUMERICAL-SIMULATION OF THE GROWTH-PHASE AND THE EXPANSION ONSET FOR A SUBSTORM OBSERVED BY VIKING

We report the first global magnetohydrodynamic (MHD) simulation of an actual magnetospheric substorm, which was recorded by the Viking space craft on October 19, 1986. The simulation is driven by IMP 8 solar win d parameters measured upstream of the Earth's bow shock. The substorm, which had expansion onset at 1132 UT, was caused by a brief period of southward interplanetary magnetic field (IMF) and two weak solar wind shocks. The simulation model includes a self-consistent auroral ionos pheric conductance depending directly on the MHD magnetospheric plasma parameters and magnetic field. Synthetic auroral emissions, derived f rom simulation results, are compared to the Viking images, which show considerable dayside activity preceding the substorm. We also compare model-derived synthetic AU and AL indices to geomagnetic measurements. The simulation results are seen to be in reasonable agreement with th e observations throughout the growth phase and expansion onset. Moreov er, the results allow us to form conclusions concerning which essentia l processes were responsible for the substorm occurence. These results are a highly encouraging first step leading toward development of a s pace weather forecasting methodology based on the directly measured so lar input.