FAST Observations of Electromagnetic Stresses Applied to the Polar Ionosphere

The Fast Auroral Snapshot Explorer (FAST), with its 83 inclination orbit and 4000 km apogee, is ideally suited for investigation of the high latitude perturbations to the geomagnetic field. These data can be used to determine field-aligned currents, but here we emphasize the perturbations themselves, rather than their spatial gradient. This allows us to more readily visualize the forces applied to the ionosphere by the magnetosphere (and vice-versa). Our basic framework for interpreting the magnetic field perturbations is one in which flows in the magnetosphere and at the magnetopause apply stresses to the ionosphere where the imposed flows must overcome the collisional drag. Thus field-aligned currents flow in response to a requirement for an ionospheric JxB force to overcome the drag. We will interpret two intervals of polar data acquired by FAST in this framework, showing how the overall structure of the field perturbations can be understood in terms of applied stresses. We discuss briefly one implication of this approach, that the ionosphere may be important in braking substorm-related flow bursts.