Wind-Induced Magnetic Fluctuations

Abstract

Irregular magnetic fluctuations, with time scales of the order 10 minutes to 2 or 3 hours, provide a characteristic feature of magnetograms at most latitudes during magnetic storms, and at high latitudes even at times of relatively low worldwide magnetic activity. Their amplitude is, of course, great at times of high activity, by definition, and decreases with decreasing activity. They have long been believed to yield a measure of the interaction between solar plasma and the geomagnetic field, and this belief has been firmly substantiated by recent observations of a close correlation between the magnetic Kp index and the speed of the solar wind [Snyder and Neugebauer, 1964]. It would be a natural extrapolation of this general viewpoint to invoke the same solar-wind source to explain the residual magnetic fluctuations, of amplitude 3-6 γ, say, that appear on most ‘quiet’ days even at middle latitudes. The present note is concerned with the possibility that a distinct second source is operative at this low level of intensity, however, and that this source provides a natural background of magnetic noise into which the fluctuations induced by the solar wind may subside on occasion. The source in question is that provided by the irregular winds of the ionospheric dynamo region.