TV observations of multiple arcs in pulsating and diffuse auroras

Abstract

We present two cases of TV observations of multiple auroral arc-like forms in the ionospheric region filled with pulsating and diffuse auroras. Each occurs between two successive breakups, showing several passes of a band of enhanced luminosity through the background auroras toward the auroral oval poleward boundary. The band velocity is about 0.3 km/s at ionospheric altitude, the repetition period is 2–5 minutes. The arc-like forms in the pulsating auroras pulsate at a frequency close to that of the surrounding auroral patches. We connect the periodic activity in the ordinary auroral arc located poleward the region of diffuse auroras with the passage of these disturbances. MHD waves propagating in the medium with a smooth plasma gradient are assumed to be the reason for these phenomena. We also discuss these events in the context of the substorm triggering problem.