Multistation observations of auroras: Polar cap substorms

Abstract

Double‐wavelength (630.0 and 427.8 nm) all‐sky imagers were operated at the Automatic Geophysical Observatory (AGO) sites P1 and P4, (magnetic latitude=80°), South Pole Station (magnetic latitude=74°), and P3 (magnetic latitude=72°) during the austral winter of 1994 and 1995. A new data format was developed to present the 24 hour auroral displays from the station arrays, which provided 24 hour optical data during the Antarctic winter night for several months. Auroras at these stations exhibit the typical magnetic local time (MLT) features of high‐latitude precipitation. On the nightside (MLT), substorm poleward expansions were observed to reach very high latitudes. A typical event (June 3, 1994, 0400–0500 UT) started equatorward from the South Pole and propagated to the high‐latitude limit of the field of view at P1 and P4. The morphological appearance of these high‐latitude auroral substorm features was similar to that of substorm auroras observed at lower latitudes. The intense 427.8 nm emission accompanied by a weak 630 nm emission showed that the particles were energetic. The interplanetary magnetic field (IMF) Bz component was slightly negative before and during this event, emphasizing that these were substorm auroras and not polar cap auroras, which tend to occur during times of prolonged positive IMF Bz. Statistical analysis of the 1995 AGO optical data shows that over 20% of substorms are seen to penetrate to regions greater than 80° of latitude.