High‐latitude, ground‐based observations of ULF plasma waves in the magnetosphere

summary . Recent theoretical work has clarified the spatial characteristics of ULF (1–20 mHz) plasma waves in the magnetosphere, and the magnetic perturbations that these waves cause at the Earth's surface. This paper uses ground‐based observations from seven, three‐component, flux‐gate magnetometers ranging in latitude from 58.7 to 77.7° N corrected geomagnetic latitude in order to evaluate the relevance of these theories. During the day, the sense of polarization and direction of the major axis of the pulsations in the horizontal plane follow the trend predicted by recent theories proposing a Kelvin—Helmholtz instability at the magnetopause as a source for these pulsations. The H component shows a distinct trend for the power to peak at lower latitudes as the frequency of the pulsation train increases. This feature is consistent with recent theoretical work which predicts pronounced latitudinal resonances in the D component in the magnetosphere, which are then rotated to the H direction on passing through the ionosphere to the ground. The experiment shows that at the ground the latitudinal resonance is most clearly marked by sharp peaks in the H and Z components, and by pronounced latitudinal phase shifts in the H and Z components. These phase shifts are not always accompanied by a latitudinal reversal in the sense of polarization in the horizontal plane.