Pre-noon high-latitude auroral arcs as a manifestation of the interchange instability

Abstract

On 7 December 2000, TV ASC camera in Barentsburg (Svalbard) observed pre-noon (at 09:00-10:00 MLT) rayed auroral arcs, which occurred at the poleward edge of the auroral oval after an IMF transition from By-dominated (By = +8.8, Bz = +4.3) to strongly northward dominated (By = +2.7. Bz = +8.6 . The arcs appeared from the area of enhanced luminosity seen in the western (nightside) horizon, and developed to the east, progressing at a velocity of about 1.5 km/s. Simultaneously, the arcs were drifting poleward at a velocity of 300-500 m/s, whose value was equal to the F-region ionospheric plasma drift velocity observed by the Incoherent Scatter Radar (ESR). The arc appearance and motion corresponded well to the poleward expansion of the auroral oval following the IMF shift, which was observed by the UVI on board the Polar satellite. The observed auroras were associated with closed LLBL indicated by the particle precipitation data from DMSP satelites showing also several-keV electrons of PS origin. The observations allow us to suggest that the arcs arise due to the interchange instability that starts to develop at the boundary between the magnetospheric plasma and the magnetosheath flux tubes entering the closed magnetosphere due to the reconnection beyond the cusp after the IMF changes. The interchange instability can be suggested as a possible mechanism for the formation of the LLBL.