IMF polarity and annual variations of the Dst index |
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| Authors: | J Oksman E Kataja |
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| Institution: | Department of Electrical Engineering, University of Oulu, Finland;Geophysical Observatory, Sodankylä, Finland |
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| Abstract: | Summary. The annual variations of the Dst index were studied separately for the towards and away polarities of the interplanetary magnetic field. It was found that: (a) the towards polarity is associated with a spring minimum and the away polarity with an autumn minimum of Dst; (b) that both polarities exhibit an asymmetry between the solstices, the Dst values for the December solstice being more negative than those for the June solstice; and (c) that a shift of the annual minima in Dst to earlier times with increasing level of activity seems to exist, irrespective of IMF polarity. The first effect is related to the similar effect found earlier in other measures of geomagnetic activity and is assumed to be caused by varying energy transfer from the solar wind into the magnetosphere depending on the sign of the north-south component of the IMF; the second effect is compatible with a proposed annual variation in the size of the magnetosphere, the reason of which is still unknown; and the third suggests to us that the solar magnetospheric coordinate system might not be adequate for describing the interaction between the IMF and the magnetosphere during disturbed times. |
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