Global simulation of the Geospace Environment Modeling substorm challenge event

We use a global model of Earth's magnetosphere and ionosphere to simulate t he Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996. We compare our results to International Monitor for Auroral Geom agnetic Effects (IMAGE) ground magnetometer data, assimilative mapping of i onospheric electrodynamics (AMIE) polar cap potential and field aligned cur rent patterns, Polar Visible Imaging System (VIS) estimates of the polar ca p magnetic flux, GOES 8 geosynchronous magnetometer data, IMP 8 magnetomete r data, and Geotail plasma and magnetic field data. We find generally good agreement between the simulation and the data. The modeled evolution of thi s substorm generally follows the phenomenological near-Earth neutral line m odel. However, reconnection in the tail is very localized, which makes esta blishing a causal relation between tail dynamics and auroral dynamics diffi cult, if not impossible. We also find that the model results critically dep end on the parameterization of auroral Hall and Pedersen conductances and a nomalous resistivity in the magnetosphere. For many combinations of paramet ers that enter these parameterizations, no substorm develops in the model, but instead the magnetosphere enters a steady convection mode. The main dev iation of the model from the data is excessive convection, which leads to a strong, driven westward electrojet in the growth phase, only partial tail loading, and a reduced recovery phase. Possible remedies are a better model for auroral conductances, an improved anomalous resistivity model, and a m ore realistic treatment of the ring current.