Observations of the geomagnetic field daily variation at Mario Z ucchelli Station, Antarctica L. Cafarella, S. Lepidi, A. Meloni, M. Pietrolungo and L. Santar elli Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy

Abstract: during the 1986 -87 austral summer a geomagnetic observatory was installed at the Italian Antarctic Base Mario Zucchelli Station (formerly Terra Nova Bay; the base changed its name in 2004; international geomagnetic observatory code TNB; ge ographic coordinates: 74.7 ºS, 164.1 ºE; corrected geomagnetic latitude: 80.0 °S; LT=UT+13). In the first three years the measurements of the geomagnetic field were carried out only during summer expeditions. Since 1991 an automatic acquisition s ystem, operating through all the year, was put in operation. We present some peculiarities of the daily variation as observed at TNB through twenty years (1987 -2006) covering two solar cycles (22 and 23). The availability of a long series of data has allowed the definition of seasonal as well as solar cycle effects on daily variations as observed at a cusp -cap observatory. Also a comparison of daily variation with observations at other Antarctic observatories (Dome C and Dumont D’Urville), which are located in pairs at the same geographic and corrected geomagneti c latitude, is shown. TNB geomagnetic observatory The Italian program for scientific research in Antarctica installed, during 1986/1987 austral summer, a geomagnetic observatory at Terra Nova Bay (TNB) at the Mario Zucchelli Station (formerly Terra Nova Bay). Variations of the Earth’s magnetic field are recorded by means of three-axis fluxgate magnetometers oriented according to the magnetic reference system. The intensity of the geomagnetic field F is measured by means of an overhauser magnetometer. At the beginning, measurements were carried out only during local summer, while since 1991 an automatic acquisition system, operating through all the year, was put in operation. Data inventory graph from 1987 to 2006: In the first ten years data sampling rate has been variable from 15-30 Absolute measurements of the angular elements D and I, s (during the station opening period) to 4 min (automatic acquisition are performed regularly during the station opening period during winter period) while since 1996, 1-min data are routinely produced. Starting from 2001, the sampling rate is 1 s and the original (austral summer). In the data inventory plot, reported on 1 s measurements are stored, and also filtered and averaged to the right, a schematic diagram of the data sampling rate produce 1 min data. and of the observatory data availability is reported. Daily variation at TNB Daily variation at TNB 0 The daily variation is generated in the polar cap by two different electric current systems: S q (the p extrapolation of the mid latitude S q driven by the ionospheric dynamo) and the polar cap S q , related to field aligned currents flowing along the geomagnetic field lines from the magnetosphere to the p ionosphere. The S q system is generated by external sources related to processes in the magnetosphere and its interaction with the Interplanetary Magnetic Field. It is believed to be the primary source for the high latitude daily variation, especially during local winter when the polar cap ionospheric ionization due to solar radiation is strongly reduced. A comparison of daily variation pattern at TNB for a year of high (2001) and one of low (2006) magnetic activity is reported in figure on the left. The dynamic maps show the variation of the hourly H and D values with respect to the average level as a function of julian days and UT. A clear seasonal dependence of the daily variation amplitude emerges through both years with a reduction during local winter. As expected, on average the signal amplitude for both elements is higher in 2001 than 2006. Moreover in the year of minimum solar activity the trend is very stable, while some peculiar structures emerge in the year of maximum activity (red arrows in the figure) in which the diurnal variation shows an enhanced UT dependence.

The hodogram representation of daily variation allows to visualize simultaneously several years and to compare quickly the effects of each solar cycle Solar cycle 22 started at the end of on geomagnetic field 1986 and reached its maximum in variations. The 24 values in 1991. It lasted unusually only 9.8 each plot are the median values years, but it showed some of H and D (from hourly data, extraordinary intervals of activity after removing the average (for example in June 1991). Solar level) over summer season cycle 23 started in May 1996 and (Jan-Feb-Nov-Dec). reached its maximum in April 2000; it had a second, smaller peak in November 2001. Now, in 2008 we In the hodogram representation the 24 values represent the dynamic behavior of the projection of the total field F through the day on the horizontal plane. are just in a minimum (January In the plots each curve is covered in counter-clockwise direction. From the figure the daily variation dependence on solar activity clearly emerges: for both 2008). Solar cycle 23 has shown on solar cycles the largest and smallest excursions are found in the years corresponding to maximum and minimum sunspot numbers, respectively. Moreover the overall a modest activity in the largest excursions are observed in solar cycle 22, and this is a direct consequence of the fact that solar cycle 23 was not particularly active. comparison with the two previous solar cycles 21 and 22. Comparison among three polar geomagnetic observatories

We compare the daily variation at three different Antarctic The figure on the left shows the daily distribution of observatories: TNB, Dome C (DMC, geographic latitude the horizontal components average hourly values in the 75.1ºS; geomagnetic 88.9ºS) and Dumont D’Urville (DRV, geographic and geomagnetic reference systems. Each 66.7ºS; 80.4ºS). The analysis has been performed for 2006 single point is the variation at a fixed hour averaged p when the influence of the S q current system is low. over the whole year, and the error bar is the standard DMC deviation; the left panels refer to the geographic reference system (with LT as sorting parameter), the TNB The observatories and the geographic (dashed) and corrected right panels to the geomagnetic reference system (with geomagnetic (solid) coordinate systems. DRV MLT). The variation of the X and Y components shows exactly the same LT dependence showing the same In order to investigate the solar wind-magnetosphere interaction, we considered amplitude at the three stations, while the variation of the IMF component parallel to the axial dipole (Bz); positive and negative Bz the H and D components is phase shifted between the values correspond to closed or open magnetospheric conditions: stations. Then the geographic reference system, with respect to the geomagnetic, is better suitable to describe the daily variation. A: IMF field line B,C: connected field lines D: magnetosph. closed field line N: neutral point In the figure on the right the daily variation at the three observatories, Open magnetosphere Closed magnetosphere separately for the three seasons, is reported. It is clear that the path of the diurnal variation is the same at the three observatories. The pattern of the diurnal variation does not show a seasonal dependence, while its amplitude depends on season, being larger during local summer (more than 100 nT) and smaller in winter (around 50 nT).

In the figure on top we show the daily variation pattern for positive and negative

values of Bz. It is evident that the amplitude of the variation is definitively larger for B <0, (open magnetospheric conditions) than for B >0 (closed magnetospheric Acknowledgements: The research activity at TNB is supported by Italian PNRA (Programma Nazionale di Ricerche in Antartide), at DMC by z z PNRA and French IPEV (Institut Polaire Français Paul Emile Victor) and at DRV by IPEV. We thank Dr. J.J. Schott (Ecole et Observatoire des conditions); also standard deviation takes on larger values for Bz<0. Sciences de la Terre, France) that operates the INTERMAGNET observatory DRV. ACE data are from CDAWeb, run by NASA GSFC; thanks also to the ACE team and the ACE Science Center.