Diurnal polarization pattern of ULF geomagnetic pulsations in the Pc5 band from low to polar latitudes

We conducted a statistical analysis of the ULF (similar to1-4 mHz) geomagnetic field pulsations recorded at 30 geomagnetic observatories from low to very high latitudes during two consecutive years ( 1998-99), focusing our attention on the latitudinal dependence of the diurnal polarization pattern. At latitudes lower than similar to65degrees, a reversal of the polarization sense from counterclockwise to clockwise appears in the local afternoon and this feature could be interpreted in terms of corotating solar wind discontinuities impacting the postnoon magnetopause; our results also show that the reversal is less clear at 55degrees-65degrees, suggesting the presence of a further wave source in the afternoon sector, especially during enhanced magnetospheric activity, possibly related to phenomena occurring in the outer plasmasphere. For latitudes between similar to65degrees and 70degrees, the polarization sense reverses around local geomagnetic noon, indicating an antisunward propagation and suggesting the Kelvin-Helmholtz instability at the magnetopause as a possible wave source. Between similar to70degrees and similar to80degrees, the polarization pattern is characterized by four reversals during the day, as expected for the resonance region. During daytime hours, especially in the prenoon sector, the polarization pattern allows to clearly identify the latitude of the resonance line, which slightly increases with increasing time from morning toward noon; moreover, the location of the resonance line shifts to lower latitudes for increasing frequency. (C) 2003 Elsevier Ltd. All rights reserved.