Magnetic substorms, December 5, 1968

Magnetograms from 26 auroral and polar cap observatories and total intensity data from Ice Island T3 were used in the analysis of an intense polar magnetic substorm which developed at 08 U.T. on December 5, 1968. The magnetic effects were interpreted as resulting from a double substorm. The changing orientation of the equivalent current vector at Thule, close to the geomagnetic pole, was found to represent very closely the time development of the storm. Equivalent currents, calculated from both the horizontal and vertical components of the magnetic perturbation vector, supported the Akasofu model of the auroral oval for the expansion and early recovery phases of the storm. Prior to the westward surge and following the end of the substorm there was some evidence for an SD-type (two-celled) current system, but the available data were insufficient to verify this. A velocity of 1.1 km/sec for the westward surge and an expansion of the auroral oval by about 15° in the midnight sector were inferred from the magnetic effects. The direction of current flow in the oval was parallel to the auroral zone a few hours after local midnight. The poleward contraction of the oval was very rapid in the recovery phase of the storm, with the storm centre returning to a position considerably east of the local midnight meridian. A class of intense substorms for the years 1962 - 1969 was identified from the occurrence on the Mould Bay magnetograms of indented positive H bays around local midnight. These substorms occurred mainly in the winter and equinox, and were most numerous in the years immediately preceding sunspot minimum. No clear relationship was found between the occurrence and intensity of these substorms and the Dst index.