Abstract
Variation of the luminosity in two parallel auroral arcs before auroral breakup has been studied by using digitised TV-data with high temporal and spatial resolution. The intervals when a new arc appears near already existing one were chosen for analysis. It is shown, for all cases, that the appearance of a new arc is accompanied by fading or disappearance of another arc. We have named these events out-of-phase events, OP. Another type of luminosity variation is characterised by almost simultaneous enhancement of intensity in the both arcs (in-phase event, IP). The characteristic time of IP events is 10â20 s, whereas OP events last about one minute. Sometimes out-of-phase events begin as IP events. The possible mechanisms for OP and IP events are discussed.
Highlights
Research into the dynamics and morphology of auroras before auroral breakup may give information about possible reasons for substorm triggering
Kauristie et al (1995) reported the ionospheric plasma depletion observed by EISCAT radar together with the arc fading just before the auroral breakup
We concentrated on the moments when a new arc appeared near the already existing one
Summary
Research into the dynamics and morphology of auroras before auroral breakup may give information about possible reasons for substorm triggering. According to Koskinen et al (1993) and Nakamura et al (1994) the main dierence between pseudobreakups and the substorm expansion onsets might be intensity and size of the disturbance source Another prebreakup event is the auroral fading i.e. decrease in the discrete auroral arc luminosity a few minutes before an auroral breakup (Zaytseva et al, 1976; Pellinen and Heikkila, 1978). In another paper by Elphinstone et al (1995a), the possible role of periodic (in longitude) auroral structures in evolution of multiple onset substorms is discussed. Safargaleev et al (1996) have reported an interesting case of observation of two auroral arcs a few minutes before an auroral breakup They have found that activation of weak auroral arc leads to fading of more powerful equatorward arcs, and vice versa. Safargaleev et al.: Luminosity variations in several parallel auroral arcs before auroral breakup
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