Geomagnetic observatories in Antarctica; state of the art and a perspective view in the global and regional frameworks
Cafarella L., D. Di Mauro, S. Lepidi and A. Meloni
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
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1) Introduction
The Earth is immersed in a planetary magnetic field. The field is generated in the Earth's core and can be measured at its surface. It shows mainly a typical dipolar profile with the dipole axis roughly parallel to the Earth’s rotation axis (tilting about 12°). At low latitudes the field reaches its minimum, while its maximum intensity is observable in polar regions, reaching there almost three times its equatorial value. The region around the Earth where the geomagnetic field extends is known as the Earth's magnetosphere. This region contains a very low density gas of electrically charged particles and is the space around the Earth where many electric and magnetic phenomena happen.
A fast outflow of hot plasma is emitted from the Sun in all directions in the interplanetary space. This ‘solar wind’ at the Earth's orbit carries a variable low-intensity interplanetary magnetic field (IMF). When the IMF reaches our planet it encounters the magnetosphere and, owing to this interaction, most of the solar wind particles are deflected around the Earth, twisting the magnetosphere in a comet type shape. The Earth’s magnetic field extends for about 10 Earth’s radii in the solar direction and has a long tail on the other side, in the anti-sunward direction. Some solar wind particles leak through the magnetic barrier and are trapped inside. They can also rush through funnel-like openings (cusps) at the North and South polar regions, releasing tremendous energy when they hit the upper atmosphere. The northern and southern lights, known as auroras, are a visible evidence of this energy transfer from the Sun to the Earth. The polar regions are key areas for this energy transfer. The structure and behavior of the magnetosphere is determined not only by its internal source, the Earth’s main magnetic field, but also by the solar wind and their interaction.
On the Earth the magnetic field varies both in space and time. Spatial variations are related to the dipolar geometry (which is only the first order of the mathematical model) but also to higher order terms of core origin that generate very large scale (thousands km) magnetic field structures; spatial variations also include, at smaller scale, wavelengths due to a magnetic crustal contribution. Time variations are present in a wide frequency range. They have both internal and external origins with respect to the Earth’s surface. Variations of internal origin cover time scales longer than a few years (2-5 years or more, generally); they are known as secular variation and show up in unpredictable time patterns. The secular variation also causes a slow drifting of the magnetic poles, which can then be assumed constant in position only over a few years time interval.
The diurnal variation of the geomagnetic field is due to the photoionization of the upper atmosphere and to atmospheric tides, while the more rapid time variations are caused by the interaction between the solar wind and the magnetosphere. The solar wind is not constant being influensed by solar disturbances (flares, coronal mass ejections, etc…) and their fallouts in the interplanetary space. The magnetosphere dynamics can be heavily influenced by the solar wind and IMF variable conditions. In particular the electrical current systems within the magnetosphere can be enhanced causing a general perturbed state of the Earth’s magnetic field. Although generally irregularly distributed, the magnetic fluctuations on Earth can exhibit a 27-day recurrence because some magnetic perturbations are related to persistent active regions on the Sun and this is the period of the rotation of the Sun as seen from Earth. Magnetic pole positions show also a more rapid dynamic behavior: a daily motion of the pole is due to the diurnal variation of the Earth’s magnetic field and also to solar activity. The distance and speed of these displacements depend on the state of perturbation of the magnetosphere.
Several books on geomagnetism are available for the interested reader: see for example Chapman and Bartels 1940; Parkinson 1983; Backus et al. 1996; Merrill et al. 1996; Lanza and Meloni 2006, and for space physics aspects Hargreaves 1992; Kivelson and Russell 1996. Finally Campbell 2001 and 2003 have given a very nice summary of all geomagnetism aspects.
The continuous monitoring of the Earth’s magnetic field is the only mean to study the different features of the field and especially of its time variations. This activity is regularly undertaken by geomagnetic observatories all over the world. Observatory recordings reveal geomagnetic variation features which give important information about the geomagnetic field nature, its internal source dynamics and its interaction with the external sources. In this paper the important contribution of geomagnetic observatories in Antarctica is shown and a perspective view of their future is also discussed.
2) Geomagnetic observatories in Antarctica
All over the world, when and where this was possible, geophysicists have installed permanent structures devoted to recording magnetic field time variations. The datasets obtained from these structures are the basis of many geomagnetic field investigations. For example, following Gauss spherical harmonic analysis methods, the regular measurements of the magnetic field from all over the world are systematically used for a mathematical representation of the geomagnetic field at each epoch (usually every 5 years), known as the International Geomagnetic Reference Field (IGRF).
The geomagnetic field is monitored and consequently recorded in a geomagnetic observatory by means of so called magnetic elements: the horizontal magnetic field intensity, H, the angular difference between geographic north and magnetic north, called Declination, D, and Inclination or dip, I, the angle that the field vector makes with the horizontal plane. Other recorded elements are the intensive elements indicating the total field intensity F and the three perpendicular components X (geographic South-North), Y (West-East) and Z (vertical, positive downward). The most commonly used magnetic field unit is nT.
Two main categories of instruments generally operate in an observatory. The first includes variometers used for continuous measurements of elements of the geomagnetic field vector in arbitrary units (for example electrical voltage in the case of fluxgate instruments). The second category comprises absolute instruments, which can make measurements of the magnetic field in terms of absolute physical basic units and universal physical constants. The most common kind of absolute instrument are the fluxgate theodolite for measuring D and I and the proton precession magnetometer for measuring the field intensity F. In the fluxgate the basic unit is a dimensionless angle, while the proton precession magnetometer is based on the use of the universal physical constant, the gyromagnetic proton ratio, and the basic unit is frequency. Measurements with a fluxgate theodolite can only be made manually whilst a proton magnetometer can operate automatically. A detailed report on magnetic observatory operations and instruments can be found in Jankowsky and Sucsdorff 1996.
In order to facilitate data exchanges and to make geomagnetic products available nearly in real time, an international coordination programme called INTERMAGNET is in operation among many world magnetic observatories (Kerridge 2001). The programme has allowed the establishment of a global network of cooperating digital magnetic observatories, adopting modern common specifications for measuring and recording equipments. An INTERMAGNET Magnetic Observatory (IMO) is a modern magnetic observatory, full fiiling the following requirements: to provide one minute magnetic field values measured by a vector magnetometer, and an optional scalar magnetometer, all with a resolution of 0.1 nT and an original sampling rate of 5 sec. Vector measurements performed by a magnetometer include the best available baseline reference measurement (see ermagnet.org/).
The global distribution of geomagnetic observatories is strongly unbalanced in favor of the northern hemisphere and leaves the southern hemisphere poorly covered. In Figure 1a the geomagnetic observatory world distribution is reported for INTERMAGNET observatories. In Figure 1b all the Antarctic geomagnetic observatories are reported in a polar view. In Table 1 the Antarctic observatories with their geographic and corrected geomagnetic coordinates (IGRF2005) are reported. As it is clear from figures and table, most of the Antarctic observatories are located, for practical and historical reasons, along the coast. For this reason they are consequently subject to coast effects (a strong influence from horizontal electrical conductivity contrast between continent and ocean) and crustal field contamination. To decrease the influence of coast effects on data a more uniform observatory distribution is necessary; it would be very important also to improve the definition of the mathematical global models of the field in polar areas. The only continental stations are South Pole and Vostok that through the years have produced very significant data.Recently (from 2004) a geomagnetic observatory is working at Concordia station (code DMC) built as the result of an agreement between the French and Italian Antarctic Programs (IPEV and PNRA respectively) at Dome C (Lepidi et al. 2003). This site is particularly interesting for geomagnetic studies for many reasons, in particular since it is very close to the present position of the geomagnetic south pole (its geomagnetic latitude is nearly 89°S) and because of its location on a very thick (>3000 m) ice cap. This observatory, being so far from the bedrock, is less sensitive to the crustal magnetic anomalies that always bias measurements, for example the vertical component of the field.
The importance of magnetic observatories in Antarctica, is therefore the capability of continuously monitoring the geomagnetic field. In fact, time variations of both internal and external origin in polar regions show very peculiar features that can be used to address general problems related to the Earth’s magnetic field. In the following, we will describe some examples of the important results obtained from the high latitude geomagnetic data set in the last years.
3) Analysis and modeling of the Earth’s magnetic field and secular variation
Global mathematical models are used to make world maps of the Earth’s magnetic field. Models are made for different epochs, also to make maps of secular variation (isoporic charts). Antarctica is characterized by one of the isoporic foci (areas of maximum rate of change of the main field elements) so the monitoring of the absolute level of all magnetic field elements here is fundamental.
The International Geomagnetic Reference Field (IGRF) is a global model of the Earth's magnetic field based on the international co-operation among geomagnetic field data contributors and modelers. IGRF represents the main magnetic field (of core origin) without external sources. The model is composed of a series of spherical harmonics with their associated coefficients. The first three terms describe the geomagnetic field of a dipole. Every five years the IGRF is revised and during the 5-year intervals between consecutive versions of the model, linear interpolation of the coefficients is recommended. The most recent IGRF set of coefficients is reported in McMillan and Maus 2005. It is generally agreed that the IGRF achieves an overall accuracy of better than 1° in declination. To attain these accuracies must be taken into account possible crustal field contaminations, daily variations and magnetic storms. For this reason the contribution of magnetic observatories is very important.
Torta et al. 2002 and De Santis et al. 2002 used a slightly different technique to represent, in a more accurate way, the field in a cap area delimited by latitude 60° South encircling the Antarctic continent and large part of the Southern Ocean. The model is based on the use of the Spherical Cap Harmonic Analysis (SCHA). Introduced by Haines (Haines 1985; Haines 1990), SCHA is a regionalization of the global model reduced to a spherical cap by means of non integer Legendre polynomials and Fourier series. The Antarctic reference model based on the use of this technique was called Antarctic Reference Model (ARM). In the most recent ARM version (Gaya-Piqué et al. 2004, Gaya-Piqué et al. 2006), annual means of X, Y and Z components registered at Antarctica observatories as well as a selected subset of satellite total field value data, have been used to develop a model, formed by 123 coefficients. In Figure 2 the maps of the magnetic field components for 2005 epoch from this model are reported for Antarctica.
The Antarctic region is the area where the South geomagnetic and magnetic poles are located. The location of the first one is determined by the dipole part of the IGRF global model, whereas the second one is the point on the Earth’s surface where the IGRF magnetic field is purely vertical (i.e., inclination is –90°). In Figure 3 the location of the two poles from 1900 to 2010 (prediction) based on IGRF2005 model is reported; in the same figure also the position of the measured south magnetic pole is indicated by stars starting from 1840. Its measured coordinates and their observers are reported in Table 2.
Observations of secular variation in Antarctica, by magnetic observatories, show a rapid decrease in the total magnetic field, as recently reported for example by Rajaram et al. 2002. The authors accurately report on this effect noting that a large region of the southern hemisphere has suffered a strong decrease in F over the past five decades. The maximum decrease occurs in a belt covering Argentine island, Sanae, Maitri, Novolazarevskaya, Syowa and Hermanus. Some authors have speculated that this rapid decrease would be of global relevance implying a progress towards a dipole reversal as happened several times in the Earth’s history (see for example De Santis et al. 2004) although others have denied this possibility (Gubbins et al. 2006).
One of the most unusual features of the temporal change of the magnetic field for a given element, is the geomagnetic jerk. This is a rapid change in the secular variation slope that takes place in periods of one or two years. More precisely, it is an abrupt change (a step-function) in the second time derivative (the secular acceleration) of the geomagnetic field. Jerks are of internal origin (Malin and Hodder 1982) and represent a reorganization of the secular variation while their short time scale implies that they could be due to a change in the fluid flow at the surface of the Earth’s core (Waddington et al. 1995). Recent papers try to explain the physical origin of jerks: Bloxham et al. 2002 suggest that jerks can be explained by oscillatory flows (torsional oscillations) in the Earth’s core. Moreover, Bellanger et al. 2001 show a correlation between geomagnetic jerks and the Chandler wobble (the motion due to the flattening of the Earth, that appears when the Earth rotation axis does not coincide with the polar main axes of inertia). From the historical magnetic records, there is evidence of some jerks (mainly at European observatories) which have occurred during the last forty-five years in approximately 1969, 1978, 1991, and 1999.