GEOMAGNETIC FIELD OBSERVATIONS AT TERRA NOVA BAY ( ANTARCTICA )

During the 1986-87 austral summer a geomagnetic observatory was installed at the Italian Antarctic Base Mario Zucchelli Station (TNB, geographic coordinates: 74.7S, 164.1E; corrected geomagnetic coord.: 80.0S, 307.7E; magnetic local time MLT=UT-8). In the first years, measurements of the geomagnetic field variations were carried out only during summer expeditions. Since 1991 the recording was implemented with an automatic acquisition system operating through the year. In this work we present the most relevant results obtained from TNB observations coming from more than twenty years of observations, also including a comparison with measurements taken at other Antarctic stations.


INTRODUCTION
Geomagnetic field measurements at high latitudes are important to understand dynamical processes of the energy transfer from Solar

EXPERIMENTAL RESULTS
Based on TNB observations, several studies of the geomagnetic field variations were conducted (Cafarella et al., 2009 and references therein), including secular variation, daily variation and geomagnetic pulsations.The availability of long series of data (Figure 1a) has allowed to study long term geomagnetic field variations, such as the secular variation (Bloxham and Gubbins, 1985).In

Figure 3 -(a) TNB daily variation hodograms for solar cycle 22 (1987-1995) and (b) corresponding sunspot number; (c) daily variation a three stations.
Daily variation (Matsushita and Xu, 1982) was extensively studied at TNB and other polar geomagnetic observatories (Cafarella et al., 1998(Cafarella et al., , 2007;;Lepidi et al., 2003;Santarelli et al., 2007a;Pietrolungo et al., 2008;Lepidi et al., 2011a); it was found that its amplitude strongly depends on solar cycle, season, magnetospheric activity and interplanetary parameters; moreover, both its shape and amplitude show relevant changes approaching the polar cusp.In Figure 3a (adapted from Cafarella et al., 2009) the daily variation is represented by hodograms in the horizontal plane; in each plot the 24 values are median hourly values, computed over austral summer season from hourly data, after removing the average level; the curves are covered in counterclockwise direction.This odogram representation allows to easily compare different years; the dependence on solar activity clearly emerges: the smallest and largest excursions correspond to miminum and maximun sunspot numbers (Figure 3b), respectively.In Figure 3c the daily variation at TNB, SBA and TLD, averaged over about 2 months (Jan-Mar, 2008) is shown.
The diurnal variation is very similar at the three stations, with a clear time shift, corresponding to the Magnetic Local Time (MLT) difference: SBA is leading about 1-hr with respect to TNB and 2-hrs with respect to TLD (Table 1).Another reasearch field widely investigated from TNB data regards low frequency magnetic pulsations, mostly Pc5 pulsations (Santarelli et al., 2007b;Lepidi et al., 2007, 2011b andreferences therein), which are geomagnetic field variations of external origin with f=2-7 mHz.From a comparison between the observations at different stations it has been possible to investigate the spatial extent and the propagation of the observed waves.In Figure 4 some examples of pulsations simultaneously observed at TNB and at another Antarctic observatory are shown; in figure 4a (adapted from Santarelli et al., 2007b) it can be seen that the wave activity is very similar at TNB and SBA, but around 17 UT SBA, which is closer to its MLT noon, observes the signal in advance (and with larger amplitude), while the contrary happens around 21 UT, when TNB is closer to its MLT noon.Also the wave packets in Figure 4b,c are very similar at the two stations and in both cases TNB, which is closer to its MLT noon, observes the signal in advance (and with larger amplitude) with respect to the other station.
These observations are consistent with a wave propagation in the antisunward direction, away from the noon region.From a statistical analysis of coherent pulsations between couple of stations, with TNB as the reference, it has been possible to infer the preferential propagation direction of low frequency pulsations.In Figure 5 a schematic sketch of the observed average propagation direction between couple of stations is drawn; the subsolar point is at the bottom, so for the stations along the 80°S geomagnetic parallel, also MLT noon is at the bottom.The figure refers to eight different hours during the day for which the statistical propagation direction clearly emerges.
The black arrows connecting the position of two stations indicate the dominant propagaton direction for low frequency (about 1-5 mHz) pulsations; the purple arrows indicate that the propagation direction clearly emerges only for the lowest frequencies (up to 2-2.5 mHz).The results show that along the 80°S magnetic parallel, and also between this parallel and the geomagnetic pole, they propagate in the antisunward direction in the dayside hemisphere (particularly evident in the 19 and 21 UT schetches), indicating a generation mechanism related to the Kelvin-Helmholtz instability (Atkinson and Watanabe, 1966); conversely, in the nightiside hemisphere (particularly evident in the 07 and 09 UT schetches), the propagation is in the opposite direction, indicating a generation mechanism related to the dynamics of the magnetotail (Chen and Kivelson, 1991).
Wind (SW) to magnetosphere and to characterize several geomagnetic phenomena at high latitude and their relation with the SW and the Inteplanetary Magnetic Field (IMF).Local field lines at TNB are close to the magnetopause: indeed, it is usually in the polar cap and around local geomagnetic noon approaches the cusp, which separates sunward, closed field lines from tailward, open lines.Moreover, the location of TNB with respect to other Antarctic observatories is particularly interesting (Figure 1): it is located at the same geomagnetic latitude as two INTERMAGNET observatories, Scott Base (SBA) and Dumont D'Urville (DRV), as well as the recently installed magnetometer at Talos Dome (TLD); the displacement along a geomagnetic parallel allows to study the azimuthal propagation of geomagnetic pulsations.Also the relative position of TNB and the French-Italian observatory Dome C (DMC) is peculiar, in that they are approximately located at the same geographic latitude, but at different geomagnetic latitude, in that DMC is very deep in the polar cap, close to the geomagnetic pole.

Figure 1 -
Figure 1 -TNB and the other Antarctic stations used for a comparison in the geographic(dashed) and corrected (solid) geomagnetic coordinate systems.Note that TNB, SBA, DRV and TLD are along the 80°S geomagnetic parallel, while DMC is almost at the geomagnetic pole.

Figure
Figure 2b the geomagnetic field trend of the H and Z components, the declination D and the total field intensity F is shown; the yearly data values are average values computed over local summer period, when absolute measurements are regularly performed; for a comparison, the corresponding values from the International Geomagnetic Reference Field (IGRF; McMillan and Maus, 2005) are alsoshown; it can be seen the good agreement between the measured long term variation and the variation expected from the global model; in particular, F shows a quite steady decrease of about 50 nT/Y(Rajaram et al., 2002).

Figure 4 -
Figure 4 -Pulsation events (a) at TNB and SBA, red and blue arrows indicate 12 MLT at the two stations; (b) at TNB and DRV; (c) at TNB and DMC.

Figure 5 -
Figure 5 -Schetch of the average propagation direction of Pc5 pulsations at different hours inferred from a comparison between couple of stations, with TNB as the reference.TNB, SBA and DRV are along the 80°S, while DMC is almost at the geomagnetic pole.The subsolar point is at the bottom.