Three‐dimensional current flow and particle precipitation in a westward travelling surge (observed during the Barium‐Geos Rocket Experiment)

During the Barium‐GEOS rocket experiment on September 24, 1979 the passage of a westward travelling surge (WTS) was observed over Scandinavia. Extended instrument networks in that area, viz., the Scandinavian Magnetometer Array, the STARE radars, all‐sky cameras, and riometers, allowed simultaneous observation of the two‐dimensional character of magnetic and electric field disturbances and particle precipitation associated with the auroral feature. By combination of the different datasets and additional information from particle and electric field measurements aboard the Barium‐GEOS rocket it was possible to derive the two‐dimensional distribution of ionospheric electric fields and conductivities and to model the three‐dimensional current flow in the vicinity of the westward travelling surge. The main feature of the resulting model current system is the presence of a localized upward field‐aligned current directed out of the head of the surge and fed by a westward electrojet, which is composed of both Hall and Pedersen currents. Secondary ionospheric currents, e.g. a counterclockwise loop of mainly Hall currents around the leading edge of the surge, are found to be responsible for most of the transient effects observed by ground‐based magnetometers as the WTS passed overhead. The most energetic particle precipitation as inferred from cosmic noise absorption measurements and triangulation of auroral arc altitudes is found to be confined to the leading part and central regions of the surge and to travel westward with the visual auroral form.