A study of the power spectra of dPi's and Pi 2's

Abstract

A study of the power spectra of dPi's observed between geomagnetic latitudes 64° and 54°N shows that they exhibit clear harmonic structure. The fundamental period is predominantly in the range 300–360 sec and linear harmonics are observed up to the 9th or 10th. The 53 events studied showed no latitude dependence of fundamental period, but it was clear in some cases that the spectral energy was redistributed among the harmonics at the lower latitude stations. A similar study of Pi 2's showed much less harmonic structure in their spectra. There is evidence of a relationship between the trigger which initiates the event and Kpindex (by inference solar wind intensity) and the results are interpreted as strong support for the view that dPi's are the complete manifestation of the nighttime impulsive event and that they are triggered, as transverse free oscillations, on the ‘last closed field lines’ in the premidnight sector of the magnetosphere. It is inferred that when they are observed at low latitudes it is due to the down-coming wave (transmitted across the meridian plane) causing forced oscillations of lower latitude lines of force. Secondary resonance at lower latitudes accounts for the change in spectral distributions observed and for the apparently unconnected Pi 2 spectral forms.