A Possible Classification of Pc5 Geomagnetic Pulsations into Two Sub-Groups (Pc5A and Pc5B) Based on Spectral Structure

Abstract

Ground magnetic data recorded in the auroral zone station at Fort Churchill, Canada (FCC) during 1985 were used to study the spectral content of Pc5 magnetic pulsations. It was found that spectral peaks tend to group into two separate bands of 1-3mHz and 3-6mHz. The power in both bands appears to be linearly related to global planetary geomagnetic activity as represented by the Ap index. While pulsations in both bands show a 27-day recurrence tendency, only intense pulsations in the low frequency band (1-3mHz) seem to recur in the morning a few days after an initial enhancement. On the other hand, pulsations in the high frequency band (3-6mHz) always have low amplitudes in the afternoon. In general, the low frequency band activity is more intense than the high frequency band activity. It is suggested that Pc5 pulsations can be classified into two sub-groups, with pulsations in the low frequency band of 1-3mHz classified as Pc5A and those in the high frequency band of 3-6mHz classified as Pc5B. Probably, Pc5A is associated with the “ringing” of the large-scale three-dimensional magnetospheric and ionospheric current systems and Pc5B is due to field line resonance. This classification scheme would provide a convenient mean of expressing and recognizing the frequency content of Pc5 events.