Satellite and ground‐based observations of a fading transpolar arc

Abstract

Satellite and ground‐based observations of the end phase of a transpolar arc event on September 25, 1986, are presented. The event was recorded by the UV imager of the Viking spacecraft during the time period from 2000 to 2110 UT. The transpolar arc was formed during a period of more than one hour of northward directed interplanetary magnetic field BZ. The arc started to fade when the IMF turned southward and a localized brightening of the auroral oval west of its nightside foot point was observed. Within a few minutes the activation was strongly enhanced. This enhancement was also recorded by all‐sky cameras in Kevo and Kilpisjärvi. Magnetic disturbances below the activation were relatively weak, but at the EISCAT magnetometer stations a counterclockwise rotating equivalent current system, which can be interpreted as due to an upward directed filamentary field‐aligned current, was observed to last about 10 min. The magnetic disturbance was fairly stable and did not move across the magnetometer chain during its existence. EISCAT observed a clear enhancement of E region electron density after 2100 UT, which was not associated with hard precipitation, as indicated by the absence of riometer absorption over Scandinavia. The EISCAT scan data on ionospheric horizontal ion drift velocity showed a localized region of eastward drift just north of Tromsø, in agreement with the magnetometer recordings. Also the postmidnight auroral oval was brightened in a wide longitudinal range, and a weak westward current was observed flowing in the ionosphere by Soviet magnetometers below the bright region of the oval. Though the magnetic signatures below the activation west of the foot point indicate a cablelike upward field‐aligned current, we do not interpret the phenomenon as a substorm. After the activation discussed here the ionospheric convection increased, leading to the breakup of a strong substorm about one hour later.