Geospace Environment Modeling 2008-2009 challenge: Geosynchronous magnetic field

In this paper the metrics-based results of the inner magnetospheric magnetic field part of the 2008-2009 GEM Metrics Challenge are reported. The Metrics Challenge asked modelers to submit results for four geomagnetic storm events and five different types of observations that can be modeled by statistical or climatological or physics-based (e. g., MHD) models of the magnetosphere-ionosphere system. We present the results of 12 model settings that were run at the Community Coordinated Modeling Center and at the institutions of various modelers for these events. To measure the performance of each of the models against the observations, we use direct comparisons between the strength of the measured magnetic field (B), the sine of the elevation angle Θ(xz) (τ), and the spectral power of fluctuations for both quantities. We find that model rankings vary widely by type of variable and skill score used. None of the models consistently performs best for all events. We find that empirical models perform well for weak storm events, and physics-based (magnetohydrodynamic) models are better for strong storm events. Within a series of runs of the same model we find that higher resolution may not always improve results unless more physics of the inner magnetosphere, such as the kinetic description of the ring current, is included.