Abstract
Abstract. We conduct a statistical analysis of the coherence and phase difference of low frequency geomagnetic fluctuations between two Antarctic stations, Mario Zucchelli Station (geographic coordinates: 74.7° S, 164.1° E; corrected geomagnetic coordinates: 80.0° S, 307.7° E) and Scott Base (geographic coordinates: 77.8° S 166.8° E; corrected geomagnetic coordinates: 80.0° S 326.5° E), both located in the polar cap. Due to the relative position of the stations, whose displacement is essentially along a geomagnetic parallel, the phase difference analysis allows to determine the direction of azimuthal propagation of geomagnetic fluctuations. The results show that coherent fluctuations are essentially detectable around local geomagnetic midnight and, in a minor extent, around noon; moreover, the phase difference reverses in the night time hours, indicating a propagation direction away from midnight, and also around local geomagnetic noon, indicating a propagation direction away from the subsolar point. The nigh time phase reversal is more clear for southward interplanetary magnetic field conditions, suggesting a relation with substorm activity. The introduction, in this analysis, of the Interplanetary Magnetic Field conditions, gave interesting results, indicating a relation with substorm activity during nighttime hours. We also conducted a study of three individual pulsation events in order to find a correspondence with the statistical behaviour. In particular, a peculiar event, characterized by quiet magnetospheric and northward interplanetary magnetic field conditions, shows a clear example of waves propagating away from the local geomagnetic noon; two more events, occurring during southward interplanetary magnetic field conditions, in one case even during a moderate storm, show waves propagating away from the local geomagnetic midnight.
Highlights
The study of geomagnetic field variations in Antarctica is important in that local field lines reach magnetospheric boundary regions, such as the magnetopause and the polar cusp, where several phenomena which generate magnetospheric ULF waves take place (Arnoldy et al, 1988 and references therein)
For pulsations in the Pc5 frequency range, we found that the power level maximizes around local geomagnetic noon, when the station is closer to the polar cusp, and that the fluctuation power is well related to the solar wind (SW) speed (Santarelli et al, 2003), indicating a wave source related to the Kelvin-Helmholtz instability (KHI) on the magnetopause (Atkinson and Watanabe 1966)
We found that coherent fluctuations are mostly detected in the hours around local geomagnetic midnight and, in a minor extend, noon
Summary
The study of geomagnetic field variations in Antarctica is important in that local field lines reach magnetospheric boundary regions, such as the magnetopause and the polar cusp, where several phenomena which generate magnetospheric ULF waves take place (Arnoldy et al, 1988 and references therein). Interesting information on Pc5 pulsations have been obtained from a statistical study of the diurnal pattern of their polarization sense (Lepidi et al, 1999); the result are in agreement with a scenario of waves propagating away from the noon and midnight sector and with local field line resonance phenomena occurring at lower latitudes (Samson, 1972).
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