Abstract
During the 1986-87 austral summer a geomagnetic observatory was installed at the Italian Antarctic Base Mario Zucchelli Station. In the first three years continuous time variation monitoring and absolute measurements of the geomagnetic field were carried out only during summer expeditions. Starting 1991 an automatic acquisition system, operating through all the year, was put in operation. We present here some peculiarities of the daily variation as observed for fourteen years (1987-2000). The availability of a long series of data has allowed the definition of seasonal, as well as solar cycle effects, on short time variations as observed at a cusp-cap observatory. In particular, contrary to mid latitude behaviour, a clear dependence of the daily variation amplitude on the global geomagnetic K index was well defined.
Highlights
It is well known that the interaction of the variable solar wind with the magnetosphere is the main cause of the magnetospheric dynamics and of some special geophysical phenomena such as auroral emission (e.g. Russell, 1986; Carlson and Egeland, 1996)
The daily variation (24-hour period and its harmonics), is generated in the polar cap by two different ionospheric current systems: Sq0 and the polar cap system Spq generated by external sources related to processes in the magnetosphere and its interaction with the Interplanetary Magnetic Field (IMF) (e.g. Nagata and Kokubun, 1962; Ractliffe, 1972; Brekke, 1997)
The mean daily variations at TNB observatory during years 1991 and 1996, one at the maximum and one at the minimum of solar activity, are shown in Figure 3 as box-plot statistics organised as a function of universal time (UT) for three seasons selected according to the Lloyd criteria (Lloyd, 1861): summer (Nov, Dec, Jan and Feb, left panels in the figure); equinoxes (Mar, Apr, Sep. and Oct center panels); winter (May, Jun, Jul and Aug, right panels)
Summary
It is well known that the interaction of the variable solar wind with the magnetosphere is the main cause of the magnetospheric dynamics and of some special geophysical phenomena such as auroral emission (e.g. Russell, 1986; Carlson and Egeland, 1996). In the generation of magnetic variation, with periods ranging from few seconds to tens of minutes, are played by the incidence of hydromagnetic waves and of density oscillations of the solar wind, the Kelvin Helmholtz instability on the magnetopause, the interaction with interplanetary shocks, field aligned currents and radiation belt charged particles falling from the magnetosphere onto the upper ionized atmosphere. The mean daily variations at TNB observatory during years 1991 and 1996, one at the maximum and one at the minimum of solar activity, are shown in Figure 3 as box-plot statistics organised as a function of universal time (UT) for three seasons selected according to the Lloyd criteria (Lloyd, 1861): summer (Nov, Dec, Jan and Feb, left panels in the figure); equinoxes (Mar, Apr, Sep. and Oct center panels); winter (May, Jun, Jul and Aug, right panels). The 24 values represent the dynamics through the day of the projection of the total field F on the horizontal plane
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