Mapping the local time-universal time development of magnetospheric substorms using mid-latitude magnetic observations

Contour maps of mid-latitude geomagnetic perturbations of both north-south and east-west field components have been created to study the variability of magnetospheric substorm development. The maps were obtained by subtracting quiet day variations from observations along a longitudinal chain of observatories; this procedure created smooth interpolations of the resulting local time profiles at equal increments of universal time. The results were then contoured in the local time-universal time plane. To isolate magnetic perturbations during substorm expansions, ‘normalized’ maps were created by first subtracting the local time profile at the onset of the expansion from all later profiles. Examples of substorm variability have been chosen to demonstrate multiple onsets and movement of bay activity either eastward or westward from the meridian of initial onset. The local time profile obtained from the normalized map at the peak of the expansion represents the mid-latitude effects of the final substorm current systems. These profiles match the magnetic effects of a simple model in which a portion of the tail current is diverted through the auroral ionosphere via field lines. These profiles are further used to compute substorm parameters that include the meridian of onset, the meridian of maximum bay development, the magnitude, and extent of the event. Such quantification allows for better description of substorm features than has previously been available.