Variability of mid-latitude magnetic parameters used to characterize magnetospheric substorms

In another paper we showed how mid-latitude magnetograms could be used to derive quantitative parameters that describe a magnetospheric substorm (Clauer and McPherron, 1974). These parameters include the onset time of the magnetospheric substorm expansion, the meridian of first onset, the central meridian of the resulting expansion observed at mid-latitudes, and the extent of the event in local time. In this paper we report the results of determining these parameters for a group of 45 magnetospheric substorms. Each event selected was isolated and well defined at mid-latitude and occurred during periods of auroral zone magnetic activity. We find that the magnitude, central meridian, and extent of the magnetospheric substorm are highly variable. We interpret our results in terms of a simple field-aligned diversion of the tail current through the auroral ionosphere. Our results suggest that this field-aligned circuit can form at any local time in the night sector. Past controversy concerning the magnetospheric signature of magnetospheric substorms may, in part, be a consequence of neglecting this aspect of substorm behavior. For example, the finite extent of the disturbance at mid-latitudes suggests that a limited longitudinal sector of the tail is involved, and the variability of central meridians suggests that this sector may occur in different positions across the tail.