Fluctuation and fading of auroral arcs preceding auroral substorn onsets

Studies involving magnetometer and high time resolution photometer data from the Alaska Meridian Chain Network have been initiated to document the characteristics of optical emissions leading up to an auroral substorm. Fluctuations of the peak intensity of 5577A [OI] and 4378A [N2+] emissions are observed to occur with a period of 1 to 4 minutes prior to a 4 to 7 minute fade which leads into a substorm. These fluctuations usually occur for twenty minutes or more prior to break-up and at times appear to be part of a cyclic sequence of similar events (i.e. fluctuations of other periods of amplitudes), which may indicate the initiation of magnetospheric and/or plasmaspheric activity prior to substorn onset. In the majority of cases a minimum of intensity takes place just before the abrupt brightening associated with the start of the substorm. When more than one arc is present the sky frequently does not fade as a whole, but some arcs will dim while others brighten. Aside from a slight decline in the magnitude of the H-component of the geomagnetic field some 15 to 20 minutes prior to negative bays associated with the onset of the substorms, no characteristic magnetic activity has been correlated with the observed fluctuations