Data from the Scandinavian magnetometer array, a multichannel scanning photometer, and all-sky cameras have been combined to study the carriers of the downward field-aligned current in the substorm current wedge during the first minutes after the substorm onset. The ions are slow and thus cannot contribute much to the downward current immediately after the substorm onset, whereas the precipitating magnetospheric electrons can quickly set up the upward current. On March 4, 1979, magnetometers and all-sky cameras recorded a substorm onset at similar to 2236 UT slightly to the west of the stations. A scanning photometer was located at the center of the downward current during the event. The observations suggest that the downward current in this event was set up less than a minute after the formation of the upward current and that at this stage the current was mainly carried by cold ionospheric electrons.
- GROUND-BASED OBSERVATIONS
- MAGNETOMETER ARRAY