SUBAURORAL ELECTRIC-FIELDS OBSERVED BY THE FREJA SATELLITE - A STATISTICAL STUDY

Over 12 months of Freja electric field data have been scanned for suba uroral electric fields (SAEF) to enable a comprehensive study of the i onospheric signatures of such electric fields. SAEF are encountered fr om 1800 to 0200 MLT in agreement with an earlier study. However, a lar ge majority of the SAEF are encountered at a time slightly premidnight (2200 - 2300 MLT), with rather few occurrences before 2000 MLT and af ter 2400 MLT. Furthermore, the strength of the subauroral electric fie ld is generally much larger for events close to 2200 MLT than for othe r events. The data confirm that SAEF occur during the substorm recover y phase but also show that SAEF occur earlier during recovery when loc ated close to 2200 MLT than at other local times. The dependence on se ason and geomagnetic activity is studied, and it is found that the SAE F are more commonly observed at times of high activity when the subaur oral electric fields are also generally stronger, except close to wint er solstice, when strong electric fields are observed during low activ ity. The potentials associated with the SAEF and the relation to inter planetary magnetic field B-y are also studied. The observations are di scussed in context with substorm-related field-aligned currents and th e midlatitude trough, and we present evidence that support and refine one of the proposed production mechanisms.