We report the first global magnetohydrodynamic (MHD) simulation of an actual magnetospheric substorm, which was recorded by the Viking spacecraft on October 19, 1986. The simulation is driven by IMP 8 solar wind 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 ionospheric conductance depending directly on the MHD magnetospheric plasma parameters and magnetic field. Synthetic auroral emissions, derived from simulation results, are compared to the Viking images, which show considerable dayside activity preceding the substorm. We also compare model-derived synthetic A U and AL indices to geomagnetic measurements. The simulation results are seen to be in reasonable agreement with the observations throughout the growth phase and expansion onset. Moreover, the results allow us to form conclusions concerning which essential processes were responsible for the substorm occurence. These results are a highly encouraging first step leading toward development of a space weather forecasting methodology based on the directly measured solar input.

[1] It has been observed that H + is the dominant ion species in the plasma sheet and the ring current during quiet times. However, the O + /H + density ratio increases with increasing geomagnetic storm and substorm activity. Energetic neutral atom (ENA) images from Imager for Magnetopause-to-Aurora Global Exploration/High Energy Neutral Atom (IMAGE/HENA) reveal the rapid increase of O + ring current at substorm expansion. Finding the cause of this substorm-associated O + enhancement is the main focus of this paper. Two possible sources are suggested: direct injection from the ionosphere and energization of the preexisting oxygen ions in the magnetosphere. We perform numerical simulations to examine these two mechanisms. Millions of O + are released from the auroral region during a simulated substorm by the Lyon-FedderMobarry MHD model. The subsequent trajectories of these outflowing ions are calculated by solving the full equation of particle motion. A few minutes into the substorm expansion phase, an enhancement in O + pressure is found on the nightside at $12 R E . After careful analysis, we conclude that this pressure peak is coming from energization of the preexisting O + in the plasma sheet. The direct injection mechanism will introduce a significant time lag between strong ionospheric outflow and magnetospheric enhancement, so that it cannot explain the observed O + bursts. Using the temperature and density established by the test-particle calculations as boundary conditions to a ring current model, we calculate the O + fluxes and the corresponding ENA emissions during the model substorm. We are able to reproduce observable features of oxygen ENA enhancements as seen by IMAGE/HENA.

Abstract.We show the response of the Earth's magnetosphere and ionosphere to a density pulse in the solar wind using a global 3D MHD simulation model. Flow vortices are generated in the ionosphere and they exhibit many properties similar to those observed during impulsive traveling convection vortices. Two oppositely rotating flow vortices are formed at about 70 ø magnetic latitude near noon. They separate and move down the morning and evening flanks, greatly weakening as they pass the terminator. Meanwhile a second pair of flow vortices appears at noon at a slightly higher latitude and with opposite flow directions than the first pair. The second pair follows the first and also fades as it reaches the nightside. The results are interpreted as a hydromagnetic wave propagating in the inhomogeneous magnetosphere plasma.

[1] During the EXL98 aircraft mission, sprites and blue jets were observed by narrow band cameras that measure the N 2 + 1NG (0,1) band at 4278Å and the N 2 2PG (0, 0) band at 3370Å . We discuss the observations ($1 km resolution), instrumental and atmospheric corrections, and altitude profiles of ionized (1NG) and neutral (2PG) emission observed during a specific sprite. The ratio of ionized-to-neutral emission indicates a relative enhancement of ion emission below 55 km. Characteristic electron energies (E Ch ) and electric fields (E ) are derived from these emission ratios using excitation rates computed from a model that solves the Boltzmann equation as a function of electric field. Up to 55km E follows the breakdown field (E k ) and E Ch is $2.2eV. Above 55 km E drops below E k and E Ch drops to $1.75eV near 60km.

Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. As the angle between the IMF and the geomagnetic field is changed at the bow of the magnetosphere, the topological record of the location of the reconnection region should be recorded in the magnetosheath and on the magnetopause along the flanks of the tail, because the super fast flow freezes strong magnetic gradients formed in the bow reconnection regions into the plasma downstream. In this report, we present results from a three‐dimensional, magnetohydrodynamic (MHD), global numerical simulation code for the location of the separatrix between unconnected IMF magnetosheath field lines and reconnected field lines which penetrate the magnetopause and connect to the polar ionosphere. The angle between the IMF direction and the line where the separatrix crosses the magnetopause is shown to be a sensitive function of the IMF clock angle. We also explain how this behavior can be used to derive an approximate relation for the dependence of the cross‐polar voltage on the IMF clock angle. We conclude with a note of caution concerning the importance of physical boundary conditions in magnetoplasma simulations.