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How to record high quality data on moving ice shelf? – The geomagnetic observatory at Neumayer Station III, Antarctica |
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Jürgen Matzka (1), Tanja Fromm (2), Alfons Eckstaller (2), Jölund Asseng (2), Achim Morschhauser (1), Markus Schiffler (3) |
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1 GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany 2 AWI Alfred-Wegener-Institut, 27515 Bremerhaven, Germany 3 Leibniz-Institut für Photonische Technologien (IPHT) Jena e.V., 07745 Jena, Germany |
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VNA (IAGA code) is the geomagnetic observatory of the German Antarctic research station Neumayer III located on the Ekström ice shelf at 70.7° S and 351.7° E. The station is the currently the only permanent station in Antarctica situated on an ice shelf and as such it raises unique challenges for geomagnetic data acquisition. The station, along with the ice shelf, drifts by 157 m per year to the north and rotates clockwise by 0.25° per year. But high-quality, absolutely calibrated geomagnetic field vector data depend on precise knowledge of the geodetic reference frame and – for the study of secular variation – stable conditions, especially with respect to artificial or crustal fields at the observatory. Here, we present the observatory and our approach to compensate for the observatory's movements for obtaining both near real-time and definitive geomagnetic observatory data. First, the azimuth of the azimuth mark has to be determined and updated regularly, as it also rotates with the ice. The observatory is located in an underground ice cave and therefore celestial objects cannot serve as a fixed reference and instead, the azimuth is determined on a monthly basis by a north-seeking gyro. Second, due to its northward drift, the observatory is moving across a significant gradient of the crustal magnetic field observed first in aeromagnetic surveys over the region as well as in ground profiles of total field strength along the drift path. This results in a temporal change of the crustal bias of approx. 10 nT per year. Based on the observatory data alone, this signal cannot be separated from the secular variation of the core field. In order to correct the observatory time series for this change in crustal bias, vector absolute measurements were performed along the drift path in austral summer 2016/2017. Additionally, the aeromagnetic data was downward continued to obtain the vector field along the drift path of the observatory. In the austral summer 2017/18, the observatory was extended by a high frequency induction-coil magnetometer to study ultra low frequency waves associated with interaction of solar wind and the magneto- and ionosphere. |
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