The Magnetical Observatory at Neumayer Station, Antarctica
The time variations of the NS, EW and Z component of the geomagnetic field are continuously measured with three orthogonal fluxgate sensors which are integrated into a single sensor-triple. The total intensity F is measured with two proton precession magnetometers (PPM, Elsec 7705 and Geometrics G-856A). The alignment of the fluxgate system's NS-component parallel to geographic North was achieved with a gyro-compass. Declination D and inclination I are measured in regular intervals (typically every second day) using a non-magnetic theodolite on which a single-axis fluxgate sensor is mounted in parallel to its optical axis. The geographic North direction is also determined in regular intervals using the gyro compass mounted on top of the theodolite. Fluxgate measurements are only relative measurements. Absolute values of the three field components can be calculated from the measurements of D and I and the corresponding total intensity F. With an appropriate stepwise linear regression of these single absolute data continuous base line values are established to which the relative maesurements are then referred to.
The magnetic field's components are sampled every second. From these raw data absolute mean values for every minute are calculated. Hourly means means of total intensity F and hourly means of the three field components are listed in monthly tables according to the recommendations of the International Association of Geomagnetism and Aeronomy (IAGA). The tables are sent to the World Data Center (WDC) after one month of recording is complete.
Digital recording of the time variations of the geomagnetic field at GvN and Neumayer has been carried out almost continuously since 1983. This basic geomagnetic observatory programm will be continued until the end of the expected life time of the base. Geomagnetic field data from GvN and Neumayer Station, recorded at the standard interval of one minute, comprise now almost two complete solar cycles. These long term time series are a valuable basis for various aspects in geomagnetic research, for example:
- studying the long term variations of declination, inclination and total intensity as part of the secular variation of the geomagnetic field
- analysis of the field's daily variations, their seasonal dependance and their relation to the state of solar activity
- investigations of special magnetic phenomena related to the polar electrojet
- pulsation studies using 1 Hz or even 10 Hz data
Some basic data
(field values are monthly means for December 2001)
The geomagnetic field at Neumayer Station in 2001
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These pictures show the variations of the tee field components NS, EW and Z and of the total intensity F in 2001. In the upper row the data from January to June and in the lower row the data from July to December are plotted. A striking feature in these plots is that all field elements exhibit significant daily variations during austral summer which appear as small sinusoidal ripples. These daily variations are clearly to see at "solar quiet days" when the Earth's magnetic field is not disturbed by "magnetic storms". Magnetic storms appear in records at irregular intervals as "noise bursts" with large amplitudes and can last for several days. The ampltudes of the "solar quiet variations" (SQ variations) decrease steadily from austral summer towards winter. They vanish almost completely during the polar winter and appear again towards the next summer.
Another important behaviour of the geomagnetic field is that the both total intensity F and the vertical component are steadily decreasing from month to month. The horizontal components do not exhibit such a distinct change of their mean values.
Daily variations on "solar quiet days" (SQ variations)
Selected daily records of the NS, EW and Z components at magnetic quiet days for the month December, ranging from 1996 to 2001, are plotted in the uper left figure. To eliminate the influence of long term changes of their means a linear regression analysis was made for every selected day and the regression values were subtracted from the data. The upper right figure shows the hourly mean values of the stacked selected records. The SQ variations of all components have almost the same amplitude of approx. 20 nT. But there is a distinct difference in their phases. The graphs of the NS and Z components and the total intensity F are almost symmetric around to noon (which is not very different from local time). The graph of the EW component is antisymmetric. The daily variations are caused by ionospheric current systems which apparently circle with the sun around the Earth. The amplitudes of the observed SQ variations are determined by geographical latitude and conductivity of the ionsphere. The conductivity of the ionosphere is caused by ionization of the high atmosphere through solar UV and X-ray radiation. This explains the seasonal dependence of the SQ variations.
Variation of Declination and Inclination
Declination D and inclination I are determined in regular inervals in order to establish base lines for the calculation of absolute values for each field component. The figures above show all measurements of D and I since the beginning of the observatory program in 1982. The left plot shows that the declination was steadily decreasing until 1994/95.
Then this trend reversed and until today the westward declination is increasing again. The small offset in D in 1992 is due to the observatory's new location some 8 km southeast of the first one and is caused by local magnetic anomalies in the upper crust below this area (basaltic layers with high magnetization). The plot of the measured D values might pretend a seasonal variation of declination with lower values during the austral summer. However, this effect is not real and is only caused by the time distribution of these measurements. More then two third of all measurements were made in the afternoon. Because they can only be made with the requested accuracy at magnetic quiet days they are strongly influenced by the SQ variations which are strongest in summer. In the afternoon, however, the negative and therefore westward pointing mean EW component of the magnetic field is decreased by the positve SQ variation on this component. This results in lower declination values.
The inclination I exhibits a fairly steadily decrease during all these years. This decrease is directly related to the decrease of the vertical field component. Because the field of the Z component is approx. 8 times greater compared to the EW component there is almost no influence of the SQ variations on these measurements.
Decrease of Total Intensity F and Z Component
These figures document the present trend of the geomagnetic field. Both total intensity and the vertcal field component are continuously decreasing with almost the same rate. The reason for this behaviour is the decrease of the Earth's field main dipole moment. For comparison the values of the International Geomagnetic Reference Field (IGRF) for the station's coordinates are also plotted, showing the same trend. The amount of decrease is roughly 100 nT per year. The offset of approx. 200 nT between the two graphs of monthly means of GvN and Neumayer Station is caused by the already mentioned local magnetic anomalies. Together with the recordings of the horizontal components and the whole set of DI measurements these data describe the present state of the long term or secular variation variation of the geomagnetic field. Because the Ekström Ice Shelf is moving with 150 m/year towards North the local magnetic anomalies also have some influence on these variations. This can be seen in the slight difference in the curvature of the graphs of the IGRF data and the measured values.
Example for a Magnetic Storm
Magnetic storms appear at irregular intervals in the recordings at Neumayer Station. Weak storms can last only for a few hours and very strong events can disturb the geomagnetic fields for several days with amplitudes of some 1000 nT. In many cases the storm event starts with abrupt changes of field strength on all components. This is called the "sudden storm commencement" (ssc). The amplitudes of these first disturbances are often relatively moderate. For a few hours after the ssc the horizontal components show often increased field values obove the normal means. In the main stage of the magnetic storm the amplitudes of all components are often very strongly decreasing. Recovery to normal values may then even last for several days. Magnetic storms are, contrary to the daily variations, caused by the impact of high energy particles from the sun. Therefore the number of magnetic storms and their amplitudes depend strongly on the sunspot activity. In wintertime during clear nights polar lights can often be observed.