Characteristics of the postnoon auroras inferred from EISCAT radar measurements

Svalbard European Incoherent Scatter (EISCAT) radar and Polar satellite ultraviolet imager data have been used to investigate structure and dynamics of the postnoon auroral oval (in the so‐called “14 MLT” region). Southward turnings of the interplanetary magnetic field were associated with bright afternoon and evening arcs which drifted equatorward. Increase in the equatorward arc velocity occurred about 27 ± 7 min after the IMF Bz turned southward at the subsolar point. The observed equatorward motion may occur independently of the convection: In one of the cases, the velocity of the drifting arc was observed to be about 100 – 200 m/s southward whereas the ionospheric plasma drifted northward at a velocity of 160 m/s. The arcs were located in the region of convection shear that was shown by the ground magnetometers and the EISCAT data on ion velocity and temperature. In the cases when electric field data were available the meridional component of the electric field varied from about −60 mV/m (southward) equatorward of the arc to +30 mV/m (northward) poleward of the arc. Ionospheric plasma parameters were measured during the passes of the arcs through the radar beam. Downward field‐aligned ion flow at the 300 – 550 km altitude was observed poleward of the arc, whereas upward flows were observed equatorward of the arc. This variation is explained by the difference in the vertical gradient of plasma density. Convection flows associated with arc, and differences in electron precipitation and sunlight conditions create the vertical gradients of plasma density that determine the ambipolar diffusion and observed ion motion.