Advancements in empirical geomagnetic field modeling during the Van Allen Probes era

The advent of numerous spacecraft magnetometer missions has allowed for the development of empirical magnetic field models with unprecedented spatial and temporal evolution during geomagnetic storms. Empirical models have long been used to reconstruct the response of the magnetosphere as a function of the solar wind driving and serve as a ground truth complement to first-principles approaches, for example, by adjusting the equation of state in MHD models with the empirical plasma pressure. Until recently, empirical models had a rigid predefined structure for all current systems both spatially and temporally. Beginning with TS07D, ad-hoc equatorial current systems were replaced with basis-function expansions allowing the morphology of the equatorial currents to be inferred directly from data. Additionally, TS07D employs pattern-recognition techniques to dynamically bin magnetometer data determined by solar wind conditions which are used to fit the model, allowing for the reconstruction of the temporal evolution of storm-scale features also inferred directly from data. New spacecraft missions (THEMIS and the Van Allen Probes) make it possible to enhance the TS07D model in the inner-magnetosphere. The enhanced model findings include a highly asymmetric ring current during the main phase of storms that returns to a symmetric ring current during the recovery phase. Other findings include a westward rotation and of the Region-2 ring current system during the main phase of the storm and a closed banana current in the inner-magnetosphere closing through asymmetries in the westward and eastward ring current system. The dynamical nature of these new models make them useful tools for the investigation of space weather and its geo-effectiveness on the global and inner-magnetosphere.