INTRODUCTION

It is a well-known fact that, in general, the Earth’s magnetic field (EMF) is constantly changing. In recent centuries, the planet’s dipole moment has been steadily decreasing. Just in 450 years, the Earth’s magnetic field strength has dropped by almost 20% (Dyachenko, 2003). Not only does the amplitude of the field change, but so does its morphology, including the movement of the Earth’s magnetic poles. The velocity at which the poles shift worries not only scientists, but also ordinary people who are often scared by an inevitable and forthcoming change (reversal) in the pole positions.

The information on drifting of the poles may now be obtained from models of the Earth’s normal magnetic field. The most frequently applied ones are International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM) (Thébault et al., 2015; NCEI…, 2020; Chulliat et al., 2020). The models are based on satellite data, findings of magnetometric observatories and records of magnetic surveys and have been accumulated in the form of spherical harmonics expansion coefficients for several decades now. The models annually become more accurate thanks to continually working magnetic satellite missions (Swarm project, etc.) (Magnitorazvedka..., 2016). As the models also contain retrospective information on the normal magnetic field, they enable calculation of the drift of the Earth’s magnetic poles. Unlike the north magnetic pole (NMP), there is no shift acceleration for the south magnetic pole, and it moves at a lower velocity (Mandea and Dormy, 2003). The NMP drift velocity reaches 48 km/year, while for the SMP it is 18 km/year. Magnetic pole fluctuations are not coaxial (Minligareev et al., 2020). To ensure a definite determination of the position of the magnetic poles, their regular instrumental verification is required, and it is, of course, an important global task of applied research.

The round-the-world Antarctic expedition dedicated to the 200th anniversary of the discovery of Antarctica and the 250th birthday anniversary of Admiral Ivan Kruzenshtern lasted from December 3, 2019 to June 8, 2020 on board the Russian Navy ORV Admiral Vladimirsky (Fig. 1). It was organized by the Russian Navy and involved the Russian Geographical Society (RGS). One of the expedition’s main objectives was to instrumentally determine the SMP position which was last verified in 2000 (Osipov et al., 2020). The prompt planning of magnetometric research, its monitoring and processing of obtained data were performed by members of the Chair of Geophysical Methods of the Earth’s Crust Study of the Lomonosov MSU Department of Geology and the Fedorov Institute of Applied Geophysics (Arutyunyan et al., 2020).

Fig. 1.
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ORV Admiral Vladimirsky in Antarctica.

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HISTORY OF THE EARTH’S SOUTH MAGNETIC POLE POSITION DETERMINATIONS

The first Russian Antarctic expedition of 1819—1821 led by Russian naval officers Thaddeus Bellingshausen and Mikhail Lazarev proved that the assumption about the existence of the sixth continent—was right. The Russian Antarctic expedition was a complete success and became (after James Cook) the second expedition to travel round the entire Antarctica and the first to prove that this continent existed (Bellingshausen, 2008). Observations by means of the ship compass during this expedition helped make the first instrumental measurement of the SMP position (Fig. 2, Table 1).

Fig. 2.
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Change in the position of the South Magnetic Pole according to instrumental measurements and the IGRF-13 model: (а)—against the background of the declination chart for the year 2020 based on (Chulliat et al., 2020); (b)—against the background of the anomalous magnetic field (EAMF) based on the ADMAP project (Golynsky et al., 2018).

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Table 1.   Instrumental determination of the SMP position (according to (Chulliat et al., 2020) as revised)
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In 1841, during a sea expedition aimed at determining the position of the magnetic pole in the Southern Hemisphere, James Clark Ross managed to reach a point 250 km away from the magnetic pole and determine its position by a series of magnetic dip measurements (Dyachenko, 2003). A year before that, the expeditions led by D’Urville and Wilkes had also provided measurements of the SMP position. All the four determinations indicated that the SMP should have been within Victoria Land in the 133°—155° E sector and 71°—76° S sector, east of the Ross Sea (Table 1, Fig. 2). It is important to note that, although the SMP position was obtained in the 19th century without a direct visit, the scatter in coordinates turned out to be small relative to the Antarctic area (a sector of 500 km in longitude and 600 km in latitude) and shifted about 1800 km away from the South Geographical Pole. These determinations are important not only from a historical perspective (Table 1, Fig. 2). The pole position determined by the IGRF-13 model (Alken et al., 2021) is not more than 200 km away from the point indicated by J. Ross almost 200 years ago (Fig. 2).

It is generally believed that the SMP was reached by Shackleton’s expedition team in 1909. Despite the Herculean efforts of the team, given the circumstances of the time, the vertical inclination was determined once and without proper verification of the measurement results (Dyachenko, 2003). The SMP was more than 200 km away from the next determination in 1912, which corresponds to a linear movement velocity of 69.6 km/year during three years, which is unlikely according to the studied dynamics of the SMP shift. It is an interesting fact that the point of Shackleton’s expedition is in the epicenter of a positive magnetic field anomaly (Fig. 2b), which could have caused an additional error in determining the magnetic pole coordinates. The 1912 coordinates have to be considered unreliable, because they give an estimated velocity of 72 and 3.9 km/year from 1909 to 1912 and from 1912 to 1931, respectively.

According to subsequent instrumental determinations (in 1931, 1952, 1962), the SMP movement occurred at a strictly aligned azimuth (308 degrees, northwest) and at an average velocity of 15.4 km/year. The pole shift curve according to the IGRF-13 model runs along a more complex trajectory and at a variable velocity, but near the points of instrumental measurements. Just like Shackleton’s point, the 1952 and 1962 determinations gravitate toward local positive magnetic field anomalies.

In the first half of the 1960s, the SMP moved from land to the D’Urville Sea. The magnetic pole coordinates were first determined under marine conditions by the USSR Navy ships during the USSR Black Sea Fleet Hydrographic Service expedition on board the ORV Admiral Vladimirsky and Thaddeus Bellingshausen (1983–1984) according to a special program (Minligareev et al., 2020) (Fig. 3).

Fig. 3.
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ORV Admiral Vladimirsky in the round-the-world Antarctic expedition of the USSR Black Sea Fleet (1982–1983). Port of Wellington, New Zealand.

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The fact of determining the pole coordinates is shown in a historical photograph of a buoy with the inscription: “South Magnetic Pole. February 3, 1983” taken by the Thaddeus Bellingshausen hydrographers (Zolotaykin, 2007) (Fig. 4).

Fig. 4.
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Placing of a buoy where the SMP was determined in February 3, 1983.

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At that time, the SMP was about 70 km away from the mainland and about 20 km away from the glacier edge. From 1962 to 1983, the average movement velocity was 10.7 km/year at an azimuth of 355 degrees (Fig. 2, Table 2).

Table 2.   Technical specifications of the ORV Admiral Vladimirsky
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Two subsequent instrumental determinations of the SMP were made in 1986 and 2000 by Australian scientists on board the Sir Hubert Wilkins (Barton, 2002). In the latter case, the problem was solved with a specially designed magnetometer which could measure the horizontal components of the magnetic field. The equipment was towed behind the ship on a nonmagnetic structure and was enclosed in Helmholtz coils to compensate for the ship’s interference with the magnetometer readings (Fig. 5). Charles Barton and his colleagues in their paper by (Barton, 2002) note that the only drawback of their methodology was that there was no accounting for variation, which caused the pole to “run” over the measurement area from day to day. And this is a very important fact, because during the expedition on board the ORV Admiral Vladimirsky (2019–2020), the EMF variations were taken into account based on magnetic observatories in Antarctica and on differential magnetometric measurements.

Fig. 5.
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Magnetometer in determination of the SMP in 2000 by Australian geophysicists in the D’Urville Sea (Barton, 2002).

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From 1983 to 2000, the SMP shifted 245 km at an average velocity of 6.6 km/year. At each successive time segment at sea, there is a consistent decrease in the movement velocity, in fact, from 18 to 6.5 km/year. This means that over the past 60 years, the SMP movement velocity has significantly decreased.

The deviation of the pole shift trajectory according to the IGRF-13 model in the 1983, 1986, and 2000 offshore SMP surveys is significantly less than that of the onshore determinations. This fact is certainly attributed to the use of a larger amount of updated and highly accurate information (satellite, magnetic variation) in constructing the normal-field model. At the same time, the offshore determinations themselves are more accurate, since, unlike onshore observations, they are made by continuous area measurements and in a much shorter time period, which minimizes the pole “wandering” effect in the presence of magnetic variations.

Beginning in 2016, the unusually high SMP movement velocity led to serious errors in the 2015 model calculations. To correct these types of errors, an early update of the EMF models began in early 2019. In February 2019, the US National Geophysical Data Center (NGDC) updated the international WMM model. In December 2019, the International Association of Geomagnetism and Aeronomy (IAGA) released another version of the IGRF-13 model (Minligareev et al., 2020).

HARDWARE OF THE ROUND-THE-WORLD ANTARCTIC EXPEDITION ON BOARD THE ORV ADMIRAL VLADIMIRSKY (2019—2020)

The round-the-world expedition was made on board the Russian Navy ORV Admiral Vladimirsky of the Baltic Fleet Hydrographic Service. On board the same ship that took part in the 1983–1984 Antarctic expedition. The main parameters of the ORV Admiral Vladimirsky are shown in Table 2.

The joint inter-agency geophysical group of scientists studying the EMF was tasked not only with instrumental determination of the SMP position, but also with investigating the Earth’s anomalous magnetic field (EAMF) in some parts of the World Ocean along the expedition route (Arutyunyan et al., 2020).

By the time the ship reached the shores of Antarctica, the expedition had at its disposal:

— marine proton-precession magnetometers SeaSPY (Marine Magnetics, Canada), Yuzhmorgeologiya JSC;

— a three-component flux-gate magnetometer MVC-2, a component magnetometer (Kopytenko et al., 1994) developed by the SPbF IZMIRAN;

— a ship compass.

SeaSPY marine proton-precession magnetometers are designed to measure the modulus of the magnetic induction vector (Fig. 6). At present, such equipment is widely used for mapping the magnetic field and its anomalies, the sources of which are magnetic objects located on the bottom or in the rock strata under the bottom (Sokolova et al., 2021). During the expedition, the magnetometric survey was performed according to the methodology of differential (two-sensor) hydromagnetic observations, as accepted in the Russian geophysical practice (Gordin et al., 1986; Melikhov et al., 1987; Gorodnitskii et al., 2004; Lygin, 2020; Minligareev et al., 2020; Kuznetsov et al., 2021), in order to account for magnetic field variations that may be particularly intense in polar latitudes. It should be noted that modular magnetometric measurements can only indirectly contribute to determining the magnetic pole coordinates.

Fig. 6.
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Gondola of the SeaSPY marine proton-precession magnetometer.

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Component measurements of the magnetic induction vector were performed with a set of instruments developed at IZMIRAN, which included a 3-sensor magnetovariation complex MVC-2 and a component magnetometer with magnetoresistive sensors (Kopytenko et al., 1994) (Fig. 7).

Fig. 7.
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Magnetovariation complex MVC-2 developed at IZMIRAN in the magnetometric laboratory on board the ORV Admiral Vladimirsky.

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The vector magnetometric instrumentation was in the laboratory located at the stern of the ship. The sensors were oriented along the longitudinal, transverse and vertical axes of the ship (Fig. 7). The MVC-2 equipment was calibrated and checked for proper operation throughout the entire survey. Before the ship sailed off on its course, the MVC-2 magnetovariation complex was calibrated at reference points in the Baltic Sea. It showed characteristics proving that the complex was working properly. When the ship was approaching the research site, the scientists took appropriate steps to correct the deviation and calibrate the component magnetometer to exclude the ship’s interference.

Near the SMP, the magnetic induction vector is directed almost vertically, and its horizontal component is close to zero. When there is no horizontal component of the Earth’s external magnetic field, the regular magnetic compass needle is in an unstable equilibrium state and rotates during small excitations. Under such conditions on board the ship, the magnetic needle is oriented along the magnetization vector of the ship itself and describes its trajectory. Due to the fact that the ORV Admiral Vladimirsky has been demagnetized and has had a relatively low intrinsic magnetic moment before the voyage and immediately before the research work, when it is approaching the area of the assumed SMP position and subjected to rolling and pitching, there are no magnetic forces keeping the magnetic needle stable, so it rotates chaotically, reacting to any inclinations. If there are no additional disturbing magnetic fields, the moment of the most unstable position of the magnetic needle determines the SMP position.

MAGNETIC OBSERVATIONS IN THE D’URVILLE SEA DURING THE SMP DETERMINATION ON APRIL 6–7, 2020

Since the round-the-world expedition was following the established schedule, the instrumental determination of the SMP position was limited to 48 hours. Therefore, given the ship’s average speed of 8 knots, an observation grid was designed, which consisted of 9 row profiles and 5 transverse profiles with a total length of 355 line km (Fig. 8a). The distance between the row profiles was 2 km, and the transverse profiles—3.5 km. The position of the survey area is coordinated with the retrospective SMP determinations, its predicted position estimates according to the IGRF-12, IGRF-13, and WMM models, and the supposed velocity and direction of its shift. The joint inter-agency geophysical group of scientists studying the EMF on board the ship included (Fig. 9):

Fig. 8.
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Parameters of the magnetic field in the vicinity of the SMP in the D’Urville Sea: (а)—position of the SMP in different years against the background of the EAMF based on the ADMAP project (Golynsky et al., 2018). Cross section of isolines 20 nT; (b)—the anomalous magnetic field according to the materials of the expedition in 2020. Cross section of isolines 10 nT; (c)—magnetic declination according to the materials of the expedition in 2020.

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Fig. 9.
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Participants of the joint inter-agency geophysical group studying the EMF as part of the round-the-world Antarctic expedition of the Russian Navy on board the ORV Admiral Vladimirsky (2019–2020). From left to right: Vadim Alekseevich Soldatov, Ilya Yuryevich Grushnikov, Mikhail Georgievich Kuzyakin.

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— Vadim Alekseevich Soldatov—a researcher of the SPbF IZMIRAN;

— Mikhail Georgievich Kuzyakin—an engineer of Yuzhmorgeologiya JSC, Rosgeologiya Group of Companies;

— Ilya Yuryevich Grushnikov—a Master’s student of the Physics Department of Moscow State University.

The ORV Admiral Vladimirsky crew provided the records of the ship’s magnetometer readings. During the survey on April 6–7, 2020, the weather conditions were difficult—wind speed exceeded 30 m/s, wave height was 7 m—causing a strong rocking of the ship (Minligareev et al., 2020; Maksimochkin et al., 2020).

The hydromagnetic survey was performed according to the differential observation methodology. The first magnetometer sensor was towed at a distance of 300 m from the stern of the ship, the second sensor—400 m (Fig. 10). Due to bad weather, the data of the long-range sensor were of insufficient quality on some of the profiles.

Fig. 10.
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Works with the marine proton-precession magnetometer in the D’Urville Sea aimed at finding the position of the SMP.

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To maximize the use of all materials and control the reconstructed variations, the scientists included data from the closest magnetic observatories of the INTERMAGNET network located in Antarctica: Dumont d’Urville (DRV), France; Casey Station (CSY), Australia. They are about 830 and 2870 km away from the research area, respectively, and are located in the auroral electrojet zone, while the research area is outside of it.

The considerable distance between the stations and the research area, and the difference in the geomagnetic conditions between them during the hydromagnetic surveys in the polar regions usually do not allow the variations in the observatory data to be taken into account directly (Lygin, 2020). At the same time, on April 6–7, the geomagnetic field on Earth was relatively calm. During the entire survey period, the geomagnetic activity according to the Kp-index did not exceed 1 point.Footnote 1 The total amplitude of magnetic field variations at the DRV and CSY observatories was about 100 nT with a variance of no more than ±15 nT.Footnote 2 With such a favorable geomagnetic situation, of course, the main source of information on magnetic field variations was the results of processing differential magnetometer measurements, but controlled by observatory data.

The final RMS error of the magnetic survey after entering all corrections and linear equalization was ±4.4 nT. Comparison of the obtained EAMF with the ADMAP model (Golynsky et al., 2018) shows their high convergence, but with better detail based on offshore measurements (Fig. 8b).

The important results of the 2020 differential hydromagnetic survey in terms of determining the SMP position are as follows.

Firstly, it has been found that during the survey, the geomagnetic variation conditions in the SMP region were relatively quiet—in most of the survey profiles, the magnetic variations do not exceed 30 nT and are linear. This means that the compass readings are not affected by interference from variations.

Secondly, a map of the EAMF on the SMP position area has been obtained (Fig. 8b). There are no significant positive magnetic anomalies within the area, which could affect the calculated SMP position measurement, as was the case in the early onshore observations.

The stormy conditions during the survey had a positive impact on the analysis of the ship’s compass readings: when its readings were recorded. As mentioned above, the least stable compass reading in the pole area indicates the SMP position. The ship compass readings were recorded when the ship was approaching the detailed survey area and when it was moving away from it, as well as throughout the entire survey, so as to provide at least 1–2 measurements on each profile. The total number of observations by means of the compass in the research area is more than 50. Figure 8c clearly shows that when the ship is approaching the SMP area, the direction of the compass needle (declination) changes and correlates more and more with the direction of the ship’s movement. This indicates that the horizontal component of the Earth’s magnetic field takes values close to zero, which proves that the research site has been chosen correctly, since the most significant component at the magnetic poles is the vertical one (Fig. 8c). Compass readings corrected for the ship’s deviation component are used to determine the SMP position. The uncertainty of the SMP point is primarily caused by the detail level of the observation grid and the non-uniformity of the compass observations. The size of the uncertainty can be measured by a circle with a radius of about 5 km.

CONCLUSIONS

The team of the ORV Admiral Vladimirsky and the joint inter-agency geophysical team of Russian scientists studying the Earth’s magnetic field, 20 years after the last instrumental determination of the magnetic pole, conducted research in the area of the South Magnetic Pole position in the D’Urville Sea near Adélie Land of Antarctica.

For the first time in the world practice, the SMP position in the sea was determined by a combined use of:

– modular proton-precession differential magnetometers;

– vector three-component flux-gate magnetometers;

– a ship compass;

– magnetic observatories that take into account variations in the EMF.

The detailed processing of the component magnetometer observations is still underway, but the available materials already indicate that the South Magnetic Pole position can be considered determined.

The error in determining the SMP position depends on the detail level of the observation grid and is ±5 km. The distance from the SMP position according to the WMM model was 4.5 km, and according to the IGRF-13 model—3.5 km. The SMP shift relative to the previous instrumental determination in 2000 (Barton, 2002) is about 135 km, an average velocity of 6.5 km/year. During approximately the same time period (1983 to 2000), the SMP shifted by 115 km at an average velocity of 6.8 km/year. Thus, we can note the consistent velocities and direction of the shift over a period of almost 40 years. Unlike the North Magnetic Pole, the South Magnetic Pole is more stable in the 21st century and drifts more slowly than in the 20th century. These facts are in agreement with the conclusions made in the work by (Mandea and Dormy, 2003).

Given the importance and global nature of such research, it is advisable:

– to determine the prospects of combining the capabilities of interdisciplinary instrumental EMF measurements and monitoring in different regions of the World Ocean on a regular basis to improve the magnetometric study of our planet’s water area;

– to continue the development of methods for mathematical processing of anomalous EMF parameters, including retrospective surveys involving the use of neural network technologies;

– based on the gained experience, to develop a methodology for determining the Earth’s magnetic pole in the sea, including the use of promising marine Geomagnetic-Variation Systems;

– due to the accelerated movement of the magnetic poles, to reach agreements on the regular instrumental control of the magnetic poles to improve the world models within the International Association of Geomagnetism and Aeronomy IAGA;

– to include the instrumental SMP determination on board ORV Admiral Vladimirsky (1983–1984) and (2019–2020) into international databases. (Fig. 11).

Fig. 11.
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ORV Admiral Vladimirsky on the Navy Day parade on the outer roads of Kronshtadt, in July 26, 2020.

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The instrumental determination of the Earth’s South Magnetic Pole position is a major world achievement over the past 20 years, a serious contribution of Russian scientists to the world’s collection of achievements in the area of understanding the underlying geophysical processes occurring on our planet, important for solving a number of fundamental and applied problems.