Van Allen Probes investigation of the large-scale duskward electric field and its role in ring current formation and plasmasphere erosion in the 1 June 2013 storm

Using the Van Allen Probes, we investigate the enhancement in the large-scale duskward convection electric field during the geomagnetic storm (Dst~-120nT) on 1 June 2013 and its role in ring current ion transport and energization and plasmasphere erosion. During this storm, enhancements of ~1-2mV/m in the duskward electric field in the corotating frame are observed down to L shells as low as ~2.3. A simple model consisting of a dipole magnetic field and constant, azimuthally westward, electric field is used to calculate the earthward and westward drift of 90°pitch angle ions. This model is applied to determine how far earthward ions can drift while remaining on Earth's nightside, given the strength and duration of the convection electric field. The calculation based on this simple model indicates that the enhanced duskward electric field is of sufficient intensity and duration to transport ions from a range of initial locations and initial energies characteristic of (though not observed by the Van Allen Probes) the earthward edge of the plasma sheet during active times (L~6-10 and ~1-20keV) to the observed location of the 58-267keV ion population, chosen as representative of the ring current (L~3.5-5.8). According to the model calculation, this transportation should be concurrent with an energization to the range observed, ~58-267keV. Clear coincidence between the electric field enhancement and both plasmasphere erosion and ring current ion (58-267keV) pressure enhancements are presented. We show for the first time nearly simultaneous enhancements in the duskward convection electric field, plasmasphere erosion, and increased pressure of 58-267keV ring current ions. These 58-267keV ions have energies that are consistent with what they are expected to pick up by gradient B drifting across the electric field. These observations strongly suggest that we are observing the electric field that energizes the ions and produces the erosion of the plasmasphere.