Separation of Cowling channel and local closure currents in the vicinity of a substorm breakup spiral

The substorm breakup spiral demarcates the westernmost terminator of the substorm current wedge. A long‐standing debate has been devoted to the question of to what extent the strong upward field‐aligned currents (FAC) in that spiral are either closed by downward FAC through Pedersen currents flowing radially to the center of the spiral (“local closure”) or by currents that flow westward though a Cowling channel which extends in the region eastward of the spiral (“Cowling closure”). In particular, studies using ground‐based and space‐based instrumentation came to systematically different results. In order to address this question, we reanalyze a pseudobreakup spiral event on 3 February 1999, ∼2204 UT, that has been observed with the MIRACLE network of ground‐based instruments in Fennoscandia and for which spatial results of electric fields, conductances, and currents have been calculated by Amm et al. (2001). We apply a new technique that allows, under certain geometrical assumptions, the current systems to separate in the vicinity of the breakup spiral into their local closure and Cowling closure parts. The result shows that for the event analyzed, ∼68% of the upward FAC in the spiral is closed via the local closure current system and the remaining ∼32% via the Cowling closure current system. Therefore, the local closure mechanism is the dominating provider of the upward FAC in the breakup spiral overall. However, a localized area at the western flank of the spiral, where conductance gradients are largest, exists in which the Cowling closure dominates and feeds more than 80% of the upward FAC in that area. Further studies are required in order to examine the generality of the results obtained for our specific event.