Three-dimensional polarization characteristics of high-latitude Pc 5 geomagnetic micropulsations

In 1969 and 1970, a line of magnetometer stations was operated in western Canada between 58.5° and 77.7°N corrected geomagnetic latitude and within 2° of 302°E corrected geomagnetic longitude. This paper deals with the three-dimensional polarization characteristics of low-frequency (f 250 sec) Pc 5 geomagnetic micropulsations recorded at these stations. The center of activity of these pulsations follows the auroral oval during the whole day. The early morning (0200–1000 local geomagnetic time) has the greatest number of pulsations, with almost circular polarization in the horizontal plane, whereas the afternoon normally has few sinusoidal pulsations and almost linear polarization in the direction of magnetic north. A reversal in the sense of polarization in the horizontal plane, occurring between 1130 and 1230 local geomagnetic time, suggests that the Pc 5 events derive their energy from a Kelvin-Helmholtz instability at the magnetopause. A latitudinal change in the sense of polarization, occurring near the latitude of the peak intensity, indicates that the energy of the pulsations may be distributed in a resonant hydromagnetic mode within the earth's inner dipole field or that field lines at high latitudes (λ > 70°) may pass into the magnetosheath. The vertical components of the Pc events are likely caused by spatial gradients in the ionospheric currents associated with the pulsations. During part of the pulsation cycle, strong shear currents flow to the east and the west in the ionosphere, giving a large vertical magnetic field near the latitude of the peak in the intensity of the horizontal components.