Mesoscale ionospheric electrodynamics observed with the MIRACLE network: 1. Analysis of a pseudobreakup spiral

A short-lived auroral spiral is observed on February 3, 1999, around 2204 UT, using the Multi-Instrument Array for Ionosphere-Magnetosphere Coupling Studies (MIRACLE) network of ground-based instruments in Fennoscandia, which consists of magnetometers, coherent scatter radars, and all-sky cameras. Four minutes later, after this spiral has subsided, the auroral bulge of a fully developed substorm that has set on earlier over Russia intrudes into the MIRACLE field of view from the east. Hence the auroral spiral precedes the substorm spatially (i.e., it is located westward of the substorm bulge) rather than temporarily. Since this spiral is associated with a localized and short-lived substorm-type auroral development, we interpret it as a "pseudobreakup spiral," which is subsequently covered by the intruding auroral bulge, during the decaying phase of the pseudobreakup activity. In contrast to growth phase pseudobreakups that occur temporally before a full breakup, this pseudobreakup is observed spatially ahead (westward) of an already evolving substorm bulge. Our instantaneous spatial analysis of the distributions of ionospheric electrodynamic parameters associated with this spiral shows that all key features are similar to the ionospheric electrodynamics of the westward traveling surge: sharp decrease of conductance toward the west, from Hall conductance values up to 30 S in the spiral to less than 2 S west of it, a cluster 2 of upward field-aligned currents (FACs) in the spiral area with magnitudes up to 5 A kM(-2), and anticlockwise spiraling horizontal currents around it. The upward FACs, diverging at least 100 kA of current in total, are fed to a large extent by a "pseudosubstorm electrojet" carried by westward flowing Hall currents. No signs of local current closure by downward FACs from the immediate vicinity of the spiral are found. Our results are inconsistent with the earlier conjecture that insufficient ionospheric conductance would prevent the pseudobreakup to evolve to a full breakup. Since the solar wind data do not provide clear evidence for being a driver of the pseudobreakup-breakup sequence, the driver has to be an internal magnetospheric one.