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 exam ined to find breakups that were accompanied by a preceding fading. To illustrate typical features of the fading effect we analyze three even ts in detail and discuss seven other events to find the spatial and te mporal behavior of the fading and the global conditions favoring this phenomenon, which is not associated with every breakup. In these ten e vents the precipitation diminished typically for about 2 min and a loc al breakup followed after 2-3 min. Usually the are which broke up had faded earlier. Comparison with geostationary electron flux recordings shows that in many cases the global onset had already taken place when the fading was recorded at a different longitude. Thus fading is not just a growth-phase phenomenon as often thought, but can also appear a s a precursor of the approaching auroral bulge. The AE index and solar -wind data reveal that the fading has a tendency to take place during magnetically disturbed conditions caused by continuous energy input fr om the solar wind. Furthermore, while a widely recognized phenomenon, we have found that the fading prior to breakup is not a very common fe ature 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 g radients in the tail magnetic field and/or plasma pressure. Possible s cenarios causing fading both before and after the onset are discussed based on a few previously presented theoretical auroral-arc models.