Ionospheric Current Closure of the Pre-existing Auroral Arc

Friday, 19 December 2014
Feifei Jiang1, Margaret Kivelson2, Robert J Strangeway2, Krishan K Khurana2, Raymond J Walker3 and James M Weygand2, (1)Organization Not Listed, Washington, DC, United States, (2)University of California Los Angeles, Los Angeles, CA, United States, (3)University of California Los Angeles, Earth, Planetary, and Space Sciences, Los Angeles, CA, United States
An auroral substorm commences when a discrete auroral arc brightens and subsequently expands poleward and azimuthally. The arc that brightens is usually the most equatorward of several auroral arcs that remain quiescent for ~5 to ~60 minutes before the break-up commences. This arc is often referred to as the “pre-existing auroral arc (PAA)” or the “growth-phase arc”. Till now, the ionospheric electrodynamics of the PAA has been studied extensively by ground radar, rockets and low-altitude spacecraft, and it is well established that the field-aligned currents (FAC) associated with the PAA in the ionosphere are current sheets that are narrow in latitude and elongated in longitude. However, it remains a question whether the ionospheric currents that connect the FAC pair of the PAA are meridional or azimuthal. 

In this study, we have identified ~180 PAA events from FAST measurements in 1998 and 1999 and used the statistics to investigate the ionospheric current closure of the PAA. We calculate the height-integrated Pedersen currents from the electric fields measured by FAST using an empirical ionospheric conductance model and infer the FAC density from the divergence of the Pedersen currents. We find that in the vicinity of the PAA, the FAC density inferred from the divergence of perpendicular currents mimics the trend of the FAC density inferred from magnetic perturbations seen on FAST, and that the boundaries between the upward and the downward FAC sheets inferred from two different approaches lie very close together. Additionally, the latitudinal gradient of the azimuthal component of the magnetic perturbation is much larger than the azimuthal gradient of the meridional component of the magnetic perturbation in the vicinity of the PAA, indicating that the density of a meridional current is much larger than that of an azimuthal current. Our observational analysis strongly suggests that the perpendicular current that closes the FAC pair of the PAA is a north-south Pedersen current in the ionosphere.