An examination of inner-magnetosphere shielding by Region-2 Field-Aligned Currents

It has long been speculated that the inner-magnetosphere is shielded from the cross-tail electric field by a dusk-dawn directed shielding electric field under geomagnetically steady conditions. This shielding electric field is expected to be generated due to charge separation associated with region-2 Field Aligned Currents (FACs). However, during periods of large and rapid fluctuations of the Interplanetary Magnetic Field, the shielding balance should be perturbed, resulting in either under-shielding or over-shielding of the inner-magnetosphere. In this study, we present a methodology to use the relative imbalance between Region-1 and Region-2 FACs, derived from AMPERE measurements, as a proxy for inner magnetosphere shielding. Furthermore, we identify periods of rapid changes in the interplanetary electric field and utilize large-scale measurements of FACs from the AMPERE project in conjunction with electric fields in the mid-latitude ionosphere from SuperDARN to examine the extent to which the inner magnetosphere is under-/over- shielded and determine the time scales associated with shielding under different geomagnetic conditions.