Statistics of auroral hiss and relationship to auroral boundaries and upward current regions

Abstract

AbstractAn 8 year database of VLF auroral hiss observations from South Pole station (invariant latitude of −74° with magnetic local time (MLT) = UT −3.5 h) is analyzed. There are three peaks in hiss occurrence as a function of MLT in the evening sector (19–23 MLT), afternoon sector (13–17 MLT), and morning sector (7–11 MLT). The geomagnetic and interplanetary magnetic field (IMF) drivers of hiss are examined in the three MLT sectors, and the results are interpreted using an empirical model of auroral boundaries and an empirical model of field‐aligned current patterns. Auroral hiss on the dayside occurs when the auroral oval is centered near the latitude of the station, and in the afternoon sector higher disturbance levels are required. The afternoon sector favors positive By when Bz is positive and negative By when Bz is strongly negative, while the morning sector favors the complementary conditions. In each case the preference for hiss occurrence follows the pattern of upward field‐aligned currents, and hiss is more likely in the configuration where the peak in the upward current is closer to the latitude of the station. IMF By does not play a role on the nightside where hiss is most likely to occur during moderately weak driving conditions as higher disturbance levels are expected to move the auroral oval and upward current systems to latitudes well equatorward of South Pole.