Field-aligned currents distribution as derived from the Swarm satellite constellation

Abstract:

The seminal studies of Iijima and Potemra have outlined the major distribution features of field-aligned currents (FACs) in a local time versus magnetic latitude frame. The related plot showing the Region 1 and 2 FAC belts has been the reference in many studies as the baseline configuration during the past decades. What did we learn more since then?

ESA’s constellation mission Swarm provides the opportunity to derive more reliable FAC estimates from multi-satellite magnetic field measurements. We make use of the Swarm A/C satellite pair, which flies side-by-side at a separation of 1.4° in longitude. By considering along-track differences over 5 s the four readings at the corners of an almost symmetrical quad are used for calculating the mean vertical current density flowing through the encircled area. FACs are estimated by mapping the vertical current component onto the field direction. Within the auroral oval current estimates from single and dual-satellite solutions agree generally well. Significant differences are frequently observed in the polar cap. Here underlying assumptions for single-satellite solutions are obviously not well satisfied. Another characteristic derived from the multi-satellite observations: FACs can be divided into two classes. For scale sizes up to some ten kilometers rapid temporal variations are observed. These FACs are related to kinetic Alfvén waves. The other class with scale lengths of more than 150 km can be regarded as stationary current systems lasting for more than a minute.