Evolution of auroral acceleration types inferred from two-satellite coincidences

摘要

The relationship between monoenergetic electron acceleration and broadband electron acceleration is uncertain, although some have speculated that the latter is a temporal transient, and may evolve into the former. Here we have taken advantage of DMSP satellite coincidences to investigate the issue. We consider 1668 cases where one DMSP satellite observed an electron acceleration event covering at least 2 s, hence as many discrete accelerated spectra, and a second satellite subsequently observed an electron acceleration at the same location. The spatial coincidence required was tight, with a maximum separation of 0.1 magnetic latitude and 0.15 h magnetic local time. Time separations of 0-10 min were considered in 1 min bins, with auroral acceleration flagged as either monoenergetic, broadband, or a mixture of both. Within the first temporal bin (0-1 min), the second satellite had a high probability of observing the same type of aurora as the first, establishing consistency. When the first satellite observed monoenergetic aurora, the second satellite also observed monoenergetic aurora (about 80% of the time), and this continued to up to about 10 min of UT separation. In most of the other 20% of the cases, the second satellite also recorded monoenergetic acceleration but with an additional mixture of broadband acceleration. Thus monoenergetic aurora does not seem to typically evolve on a time scale of minutes. However, when the first satellite encounter was with broadband acceleration, the second encounter was highly time-dependent, with broadband dominating the second satellite encounter only in the 0-1 min bin. Between 1 and 5 min, the probability of observing a mixture of auroral types jumped, and after 6 min, the auroral acceleration was nearly as likely to be monoenergetic as broadband. Finally, if the first satellite encountered a mixture of acceleration, the second encounter was progressively more likely to be entirely monoenergetic aurora as time increased. These results are consistent with the idea that broadband aurora may be inherently a transient, and often progresses to monoenergetic aurora, while the latter is quasi-static.