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Genetic classification of Pc3 and Pc4 geomagnetic pulsations in mid-latitudes Ng, Tai Ping

Abstract

Dynamic spectra processed from data recorded on magnetic tape at the mid-latitude Ralston station (Alberta) in 1967 have been studied in detail. The Pc3,4 pulsations appear to behave in a much more complicated manner than reported by other observers at low-latitude stations. The variation of the Pc3,4 frequency at Ralston assumes different forms from one day to another, the pattern depending largely upon the general level of magnetic disturbance represented by the K(p)-index. It appears, however, that most of the Pc3,4 spectra analysed may be classified into one of, or a combination of, four well-defined diurnal patterns under steady magnetospheric conditions. An interpretation is offered to explain the existence as well as the fine structure of these four diurnal patterns. The crux of the present interpretation is that Ralston, under moderate magnetospheric agitation, may pick up micropulsation activities originating from the plasmasphere and/or the plasmatrough depending upon its position relative to the plasmapause. Eigen oscillations of modified Alfven mode (poloidal oscillation) in these two magnetospheric regions are considered to be the prime sources of the ground observed Pc3,4 magnetic pulsations. Such suggestion is reinforced by observations made simultaneously at other mid- and high-latitude stations. Other morphological properties of Pc3 and Pc4 are discussed in the light of the new interpretation.

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