A statistical study of intense low‐altitude electric fields observed by Freja

Abstract

Extremely intense (up to 2 V/m) and fine‐structured (1–5 km) electric fields have been observed by the Freja satellite at altitudes of 1400–1770 km. To study the occurrence and characteristics of these intense electric field events, a database was set up by searching 7 months' worth of Freja data for events with peak values greater than 200 mV/m. The intense electric field events are distributed over all local times, but they are mostly concentrated to the midnight and early morning sector of the auroral oval. The events are seen to be associated with low ambient ionospheric conductivities, but are not activity dependent. The most intense events (located in the midnight sector) are associated with the smallest scale sizes, whereas the less intense events on the day side have larger scale sizes.The findings are consistent with the interpretation that a majority of the intense electric field events observed by Freja are associated with the small‐scale, east‐west‐aligned, low‐conductivity bands devoid of auroral emissions, known as black auroral bands, or with the curls that may develop when these bands go unstable. Several observations of diverging electric fields of around 1 V/m at 800 km altitude, within the southern auroral oval, may serve as support for the prediction that intense electric fields should exist also at ionospheric altitudes.