Auroral precipitation fading before and at substorm onset: ionospheric and geostationary signatures

Rapid fading of auroral activity a few minutes before substorm breakup has earlier been analyzed in case-studies. Here we report on a study in which all-sky camera (ASC) and magnetic data over 3 years were examined to ®nd breakups that were accompanied by a preceding fading. To illustrate typical features of the fading e€ect we analyze three events in detail and discuss seven other events to ®nd the spatial and temporal behavior of the fading and the global conditions favoring this phenomenon, which is not associated with every breakup. In these ten events the precipitation diminished typically for about 2 min and a local breakup followed after 2±3 min. Usually the arc which broke up had faded earlier. Comparison with geostationary electron ̄ux recordings shows that in many cases the global onset had already taken place when the fading was recorded at a di€erent longitude. Thus fading is not just a growth-phase phenomenon as often thought, but can also appear as a precursor of the approaching auroral bulge. The AE index and solarwind data reveal that the fading has a tendency to take place during magnetically disturbed conditions caused by continuous energy input from the solar wind. Furthermore, while a widely recognized phenomenon, we have found that the fading prior to breakup is not a very common feature in the spatio-temporal scale of auroral ASC recordings. In many cases the deepness of the fading had a longitudinal dependence, which leads to the suggestion that this phenomenon is related to azimuthal gradients in the tail magnetic ®eld and/or plasma pressure. Possible scenarios causing fading both before and after the onset are discussed based on a few previously presented theoretical auroral-arc models. 1 Introduction In the high-latitude ionosphere during the last few minutes before the substorm breakup the growth phase processes sometimes show puzzling behavior. During this ``trigger phase'', as ®rst named by Pellinen and Heikkila (1978, 1984), the slow, linear growth phase processes are interrupted when ionospheric activity decreases just before the breakup. Fading is observed both in soft and hard precipitation and in magnetic pulsation and westward electrojet activity (Kauristie et al., 1995). In contrast to the more global growthphase development, fading is typically localized to the onset region (Pellinen and Heikkila, 1978). In their pioneering work, Pellinen and Heikkila (1978) studied four fading events during the years 1975±76 using numerous ground-based instruments [all-sky cameras (ASC's), photometers, riometers, and backscatter receivers]. They concluded that in recordings with suciently high time resolution (some tens of seconds) the fading is ``a rather common feature''. They pointed out that the fading can appear either as a temporal decrease in the auroral activity prior to the onset or as a dark region travelling ahead of the auroral surge. Making a distinction between these two cases is as dicult as de®ning the time of the global onset: a longitudinally extended ASC network would be helpful. Morse and Romick (1982) reported on fading events recorded with a meridian scanning photometer. The durations of the fadings were 4±7 min, a few minutes longer than in the cases analyzed by Pellinen and Heikkila (1978). The photometer recordings revealed periodic ̄uctuations in the peak intensity of the arc during 1 h before the breakup. The ̄uctuations were typically a superposition of strong long-period variations …period 10min† and smaller, more rapid variations …period 1±4min†. An exceptionally clear fading event was recorded on 5 October 1986 by several di€erent instruments, includCorrespondence to: K. Kauristie Ann. Geophysicae 15, 967±983 (1997) Ó EGS± Springer-Verlag 1997