Low and mid-frequency pulsations in the polar cap: polarization pattern and MLT dependence of the spectral power during the descending phase of the solar cycle

Abstract

We present a statistical analysis of ULF (1-100 mHz) geomagnetic measurements conducted during years 2003- 2006 at the Italian/French base of Concordia at Dome C, close to the geomagnetic pole, and at the Italian base «Mario Zucchelli» at Terra Nova Bay, also located in the polar cap, but at lower latitude. Our analysis shows that high latitude ULF pulsation power is largely controlled by the solar wind speed. At Terra Nova Bay the power shows a maximum at local noon, clearly related to cusp and closed field lines phenomena. At few mHz, the polarization pattern indicates field line resonances driven, just equatorward with respect to the station, by waves propagating tailward; the polarization of higher frequency pulsations, mostly originated from interplanetary upstream waves, suggests waves propagating sunward from the night sector. At Dome C the wave power shows a small enhancement in the local morning, more pronounced for mid-frequency pulsations; the polarization pattern, at all frequencies, appears to indicate waves propagating sunward from the night sector, suggesting a propagation channel to the ground via the magnetotail lobes.