Geomagnetic Storms of August 4-5, 1972
Nagai, M.
Abstract
Very interesting and characteristics storm with sudden commencement occurred during Aug. 4-5d, 1972. Here, studies of the storm are carried out on the following 4 points, using ordinary magnetograms at 35 world-wide observatories.
1) At Kakioka, decrease of 219γin H-component was observed at 22h 12m on Aug. 4d. After the lapse of 45 minutes, eastward movement of 210γin D-component was observed at 22h 57m.
Duration of each disturbance is only about 50-60 minutes, but these intense variations which occur soon after the sudden commencement are unusual in middle and low latitudes such as Kakioka.
2) Corresponding to above disturbance, very intense oscillation of -1,500γ～+2,500γ was observed at College in the dayside auroral zone. On the other hand, broad variation with maximum range of -1,000γ was observed at Sodankyla in the night side auroral zone. Equivalent current systems are drawn for three epochs, 21h 30m, 22h 10m and 22h 30m on Aug. 4d. The regions of clockwise and counter-clockwise current vortices are twisted, and changed rapidly in response to the oscillative disturbance in the dayside auroral zone.
3)Negative ⊿H of sudden commencement occurred at 14h 00m on Aug. 5d at San Juan, Tucson and Fredericksburg. This is different from the ordinary pattern of ssc current system, in the point that the region of clockwise voltex of DS(sc) extends to the low and middle latitudes, such as above 3 observatories.
Parallel overhead electric current flows towards the meridian corresponding to 12h local geomagnetic time in the current system at 15h 10m on Aug. 5d. Though this direction is outside the range of 8h-11h in most cases of instantaneous individual disturbances, it is not so much different from the usual current system of DS at high latitude. And it is shown that eastward current predominates over the wide region of low and middle latitudes.
On the other hand, the current system at 15h 30m on Aug. 5d shows westward current over the almost all region of auroral zone, and this is different from pattern of the current system at 15h 10m.
4) In addition to above variations, active PC 5's of great amplitude were observed even in the low and middle latitudes such as Kakioka. Such a large amplitude of PC 5 had not been observed at Kakioka, since Feb. 11, 1958, when great geomagnetic storm occurred.
