Earth's Main Magnetic Field Research Articles

THE EARTH’S MAGNETIC FIELD AND THE LARGE-SCALE MAGNETIC FIELD OF THE SUN: THE SOLAR-TERRESTRIAL CONNECTION

The paper presents the results of a joint analysis of the Earth's main magnetic field (core field) BIGRF and the large-scale magnetic field (LSMF) of the Sun. Selected 11- and 22-year periods of LSMF and BIGRF variations are well manifested in both fields and are usually modulated by solar activity. Even 11-year cycles for which the direction of the Sun's magnetic field coincides with the direction of the Earth's magnetic field are characterized by the minimum values of sanspot numbers, and odd cycles with opposite directions of magnetic fields have larger values of sunspot numbers. The rotation rate of two- and four-sector structure of the Sun source of LSMF varied with about 11-year and 22-year cycle. Longer changes in the magnetic fields of the Sun and the Earth with a period of about 75 years have also been revealed. The rotation periods of the Sun source global field (28.0-28.5 days) were maximum at the middle of the 20th century in the period 1940÷1960 years. This maximum of solar activity corresponds to temporal gradient of geomagnetic field. It is shown that the gradient of the geomagnetic field BIGRF depends on the rate of change in the length of the day. So, according to the results of the study, the rotation modes of the Sun and the Earth cause different periodic changes in their magnetic fields.

Some algorithms related to the orientation of satellites and unmanned aerial vehicles

The stabilization of unmanned aerial vehicles and its orientation in a given direction is provided by the orientation system. One of the important stages in the development of UAV orientation systems is the development of object orientation algorithms. The problem of orientation of objects in the magnetic field is based on the estimation of the discrepancies between the model Bmod and the measured Bmeas components of the Earth's magnetic field. Accordingly, the paper discusses some algorithms related to the orientation of satellites and unmanned aerial vehicles. It is shown that limiting the model to the first 13 harmonics can lead to errors in the representation of the Earth's main magnetic field of about 15 nT. A mathematical proof of this fact is carried out based on the consideration of the equation of an ideal magnetic field meter. The work analytically proves that the Jacobian matrix of the magnetic field measurement vector in the earth's coordinate system has a determinant equal to zero, which means that, having knowledge of the magnetic field measurement vector H-1, it is impossible to uniquely determine the vector of the orientation angles of the meter {f, o, y }t-. Thus, the paper shows an algorithm for calculating the orientation angles depending on the values of the measured and model values of the Earth's magnetic field.

INTERPRETASI DATA GEOMAGNETIK UNTUK MENDETEKSI SEBARAN BATUAN MENGANDUNG MINERAL LOGAM DI AWANG BANGKAL, KABUPATEN BANJAR

Research has been carried out to detect the presence and distribution of rocks containing metallic minerals. Almost every object used every day contains metal elements, so the presence of metal is very important to support every human activity. The presence of metal minerals in nature is generally quite a lot, but the types of metallic minerals vary. The geomagnetic method is a geophysical method that can detect subsurface rocks containing metallic minerals based on the difference in the magnetic field between the rock and the surrounding rock. Processing of geomagnetic data obtained in the field begins with daily correction and correction of the earth's main magnetic field (IGRF). The correction results obtained are anomaly of the earth's magnetic field which is distributed horizontally on the surface. The results of the analysis in the form of local magnetic field anomalies (∆H) are divided into 3 (three) regions, the first having a value between -236.5 nT to -8 nT (nano tesla) as an area that has a high local magnetic field anomaly value. Second, the value of -377.2 nT to -236.4 nT as an area that has a moderate anomaly value. Third, the low anomaly values ranged from -778.4 nT to -377.1 nT. Areas that have high local magnetic field anomaly values are interpreted as ultramafic rocks, which are source rocks of nickel lateric deposits rich in ferromagnesian minerals, distributed in the southeastern part of the study area.

Can seafloor voltage cables be used to study large-scale circulation? An investigation in the Pacific Ocean

Abstract. Marine electromagnetic (EM) signals largely depend on three factors: flow velocity, Earth's main magnetic field, and seawater's electrical conductivity (which depends on the local temperature and salinity). Because of this, there has been recent interest in using marine EM signals to monitor and study ocean circulation. Our study utilizes voltage data from retired seafloor telecommunication cables in the Pacific Ocean to examine whether such cables could be used to monitor circulation velocity or transport on large oceanic scales. We process the cable data to isolate the seasonal and monthly variations and then evaluate the correlation between the processed data and numerical predictions of the electric field induced by an estimate of ocean circulation. We find that the correlation between cable voltage data and numerical predictions strongly depends on both the strength and coherence of the model velocities flowing across the cable, the local EM environment, as well as the length of the cable. The cable within the Kuroshio Current had good correlation between data and predictions, whereas two of the cables in the Eastern Pacific Gyre – a region with both low flow speeds and interfering velocity directions across the cable – did not have any clear correlation between data and predictions. Meanwhile, a third cable also located in the Eastern Pacific Gyre showed good correlation between data and predictions – although the cable is very long and the speeds were low, it was located in a region of coherent flow velocity across the cable. While much improvement is needed before utilizing seafloor voltage cables to study and monitor oceanic circulation across wide regions, we believe that with additional work, the answer to the question of whether or not seafloor voltage cables can be used to study large-scale circulation may eventually be yes.

Kinematic-gravitational ion model of planetary dynamo.

Based on the solution of inverse problem for the main magnetic field of Earth, it is shown that the global sources of magnetic field are situated in the equatorial region of the Earth’s core. The estimates of magnetic moments, volumetric currents and their densities are obtained. The direction of the current flow is determined, which can be explained by the motion of the positively charged core matter under the influence of tidal forces, directed against the basic axial rotation of the Earth. The core is positively charged due to migration of electrons across the core-mantle boundary. For the planets of the Solar system estimates of the tidal forces produced by the Sun and the satellites of the planets are obtained. A close linear relationship is established between the magnitude of tidal forces and the observed magnetic field of the planets. As a result, we arrive to a simple geodynamo mechanism in which the electric current is the motion of weakly positively charged liquid core associated with the action of tidal forces.

Solar Wind and Seasonal Influence on Ionospheric Currents From Swarm and CHAMP Measurements

AbstractWe present a new climatological model of the ionospheric current system, determined from magnetic measurements taken by the Challenging Minisatellite Payload (CHAMP) and Swarm satellites. The model describes the horizontal currents in the ionosphere, below the satellites, and the field‐aligned (Birkeland) currents that connect the ionosphere with the magnetosphere. The model provides ionospheric current values at any location as continuous functions of solar wind speed, interplanetary magnetic field, dipole tilt angle, and the F10.7 index of solar flux. Geometric distortions due to variations in the Earth's main magnetic field are taken into account, thus allowing for precise comparisons between the two hemispheres. The model is the first of its kind to describe the full 3‐D electric currents and not only the field‐aligned or the equivalent horizontal current. We use this capability to demonstrate a key difference between seasons: During winter, the total horizontal current is almost entirely confined to the auroral oval, for all interplanetary magnetic field orientations, where it connects upward and downward Birkeland currents. During more sunlit conditions, the horizontal current extends beyond the auroral oval and is a sum of currents connecting Birkeland currents and currents that circulate in the ionosphere. The westward electrojet is the only large‐scale current structure that is persistent across seasons. Comparison with average convection maps suggests that it is comprised largely of Hall currents, which connect to Birkeland currents in the winter but not in summer.

Changes in solar quiet magnetic variations since the Maunder Minimum: A comparison of historical observations and model simulations

AbstractMagnetic measurements going back to the eighteenth century offer a unique opportunity to study multicentennial changes in the upper atmosphere. We analyzed measurements from Rome and Mannheim from May 1782 to May 1783 and measurements from Greenwich, St. Helena, Cape of Good Hope, and Singapore from May 1841 to May 1842. A comparison of the daily magnetic variations in these historical data with modern‐day observations from 2010 at nearby stations (where available) showed notable differences in the amplitude and/or phase of the X and Y components. Model simulations indicated that these can be explained at least to some extent by changes in the Earth's main magnetic field. Changes in the main field strength and the northwestward movement of the magnetic equator, in particular in the region of the South Atlantic Anomaly, have caused changes in the positioning, shape, and strength of the equivalent current vortices in the ionosphere that result in the magnetic perturbations on the ground. Differences in solar activity between the historical and modern epochs, which were all near solar minima, were too small to have a notable effect on the ground magnetic perturbations. However, in regions where main magnetic field changes have been relatively small for the last ~400 years, e.g., in Singapore, the effects of a long‐term increase in solar activity from Maunder Minimum conditions to normal solar minimum conditions (an increase in F10.7 of ~35 solar flux units) were comparable to the effects of geomagnetic main field changes.

Derivation and use of core surface flows for forecasting secular variation

AbstractImproving forecasts of the temporal and spatial changes of the Earth's main magnetic field over periods of less than 5 years has important scientific and economic benefits. Various methods for forecasting the rate of change, or secular variation, have been tried over the past few decades, ranging from the extrapolation of trends in ground observatory measurements to computational geodynamo modeling with data assimilation from historical magnetic field models. We examine the utility of an intermediate approach, using temporally varying core surface flow models derived from relatively short periods of magnetic field data to produce, by advection, secular variation estimates valid for the Earth's surface. We describe a new method to compute a core flow changing linearly with time from magnetic secular variation and acceleration data. We invert a combination of data from the CHAMP satellite mission and ground observatories over the period 2001.0 to 2010.0 for a series of such models. We assess their ability to forecast magnetic field changes by comparing them to CHAOS‐4, a state‐of‐the‐art model using data from 1997 to 2014.5. We show that the magnetic field predictions tend to have a lower root‐mean‐square difference from CHAOS‐4 than the International Geomagnetic Reference Field or World Magnetic Map series of secular variation models.

Filtering ionosphere parameters to detect trends linked to anthropogenic effects

The great concern about the global warming observed in the troposphere has generated a large interest in the study of long-term trends in the ionosphere since the early 1990s, which has now become a significant topic in global change investigations. Some research works link ionosphere trends to anthropogenic sources such as the increase in greenhouse gas concentration, and others to natural causes such as solar and geomagnetic activity long-term changes, and secular variations in the Earth's main magnetic field. In all the cases, in order to analyze ionospheric trends, solar activity effect must be filtered out first since around 90% of ionosphere parameter variance is due to solar variations. The filtering process can generate ‘spurious’ trends in the filtered data series which may lead to erroneous conclusions. foF2 data series which include solar cycle 23 are analyzed in the present work in order to detect the effect of different filtering procedures on the determination of long-term trends. In particular, solar cycle 23 seems to have had an extreme ultraviolet (EUV) emission greater than that deduced from traditional solar EUV proxies during the maximum epoch and lower during the minimum epoch. When solar activity is filtered assessing the residuals of a linear regression between foF2 and Rz, or between foF2 and F10.7, this fact may bias trend values especially because it is at the end of the time series. The length of the period considered for trend assessment, the saturation and hysteresis effect of some ionosphere parameters, and the solar EUV proxy used are also considered in this study in order to quantify a possible spurious trend that may result as a by-product of a filtering process. Since trends expected as a consequence of anthropogenic effects are relatively small, these spurious effects may surely mask, or enhance, trends expected from anthropogenic origins.

Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry

The nondipolar portions of the Earth's main magnetic field constitute substantial differences between the two hemispheres. Beside the magnetic flux densities and patterns being different in the Northern Hemisphere (NH) and Southern Hemisphere (SH), also the offset between the invariant magnetic and the geographic poles is larger in the SH than in the NH. We investigated the effects of this magnetic field asymmetry on the high‐latitude thermosphere and ionosphere using global numerical simulations and compared our results with recent observations. While the effects on the high‐latitude plasma convection are small, the consequences for the neutral wind circulation are substantial. The cross‐polar neutral wind and ion drift velocities are generally larger in the NH than the SH, and the hemispheric difference shows a semidiurnal variation. The neutral wind vorticity is likewise larger in the NH than in the SH, with the difference probably becoming larger for higher solar activity. In contrast, the spatial variance of the neutral wind is considerably larger in the SH polar region, with the hemispheric difference showing a strong semidiurnal variation. Its phase is similar to the phase of the semidiurnal variation of the hemispheric magnitude differences. Hemispheric differences in ion drift and neutral wind magnitude are most likely caused partly by the larger magnetic flux densities in the near‐polar regions of the SH and partly by the larger offset between the invariant and geographic pole in the SH, while differences in spatial variance are probably just caused by the latter. We conclude that the asymmetry of the magnetic field, both in strength and in orientation, establishes substantial hemispheric differences in the neutral wind and plasma drift in the high‐latitude upper atmosphere, which can help to explain observed hemispheric differences found with the Cluster/Electron Drift Instrument (EDI) and the Challenging Minisatellite Payload (CHAMP).

On the role of ozone in long-term trends in the upper atmosphere-ionosphere system

Abstract. Origin of long-term trends in the thermosphere-ionosphere system has been discussed since the beginning of trend studies. The two most prioritized explanations have been those via long-term increase of atmospheric concentration of greenhouse gases and long-term increase of geomagnetic activity throughout the 20th century. Secular changes of the Earth's main magnetic field play an important role in trends in a limited region. Recently, Walsh and Oliver (2011) suggested that the long-term cooling of the upper thermosphere (above 200 km) may be due largely to the stratospheric ozone depletion. Here, we show that the role of ozone is very important in the mesosphere and lower thermosphere but not in the upper thermosphere. The suggestion of Walsh and Oliver (2011) is based on historical (before 1988) data from Saint-Santin radar, whereas more recent data do not support their conclusion.

What Caused Recent Acceleration of the North Magnetic Pole Drift?

The north magnetic pole (NMP) is the point at the Earth's surface where the geomagnetic field is directed vertically downward. It drifts in time as a result of core convection, which sustains the Earth's main magnetic field through the geodynamo process. During the 1990s the NMP drift speed suddenly increased from 15 kilometers per year at the start of the decade to 55 kilometers per year by the decade's end. This acceleration was all the more surprising given that the NMP drift speed had remained less than 15 kilometers per year over the previous 150 years of observation.

Trends in the solar quiet geomagnetic field variation linked to the Earth's magnetic field secular variation and increasing concentrations of greenhouse gases

Long‐term trends in solar quiet geomagnetic field variation (Sq) are studied in connection to the Earth's magnetic field secular variations and increasing greenhouse gas concentrations. Sq is mainly caused by ionospheric current systems that flow in the E region and depends, among other variables, on the ionospheric conductivities. These conductivities in turn depend on the Earth's main magnetic field (B) and the electron concentration in the E region, for which foE is a measure of its peak value. Since B shows secular variation, induced long‐term changes in Sq might be expected. Another possible mechanism that would be able to induce Sq trends is the increasing concentrations of greenhouse gases that produce a cooling effect in the upper atmosphere and, according to model predictions and experimental results, an increasing trend in foE. To detect if both mechanisms mentioned are able to induce trends in Sq, the Sq variation of the horizontal intensity (H) of three magnetic observatories (Apia, Fredericksburg, and Hermanus), for which B is decreasing, is analyzed for the period 1960–2001. We find significant increasing trends (6.6%, 5.4%, and 9.9%, respectively) which may be partially accounted for by B secular variations in the respective sites. The Sq trend expected from the theoretically predicted foE increase is low (∼0.5%), although positive, as is the observed trend.

Initial data for Atlas of Earth's main magnetic field

Initial data for Atlas of Earth's main magnetic field

Technology of creation of digital charts of the Earth's main magnetic field

Technology of creation of digital charts of the Earth's main magnetic field

Role of the Africa Magnetic Anomaly in Controlling the Magnetic Configuration and its Secular Variation

AbstractThe configuration of the Earth's main magnetic field and its secular variation are greatly controlled by the nondipole field. This portion of the main field exhibits as 5 planetary‐scale magnetic anomalies in Australia, Africa, Eurasia, North America, and South Atlantic Ocean, as well as a few minor anomalies. Only anomaly in the equatorial region is the Africa anomaly, which affects seriously the shape and position of the magnetic equator, pushing the equator northward for 15° at maximum. The fast westward drift of the Africa anomaly results in a major secular variation cell (Żmax > 200 nT/a for 1900~2005) in Central America on the west of the anomaly. In addition, the secular variations of the Africa and South Atlantic Ocean anomalies seriously distort the geomagnetic field in the western hemisphere, implying its causal relation with the polarity reversed patches on the core‐mantle boundary.

Secular variation of the Earth's magnetic field: induced by the ocean flow?

Secular variation of the Earth's main magnetic field is believed to originate in the Earth's core. (The main field is operationally defined as comprising spherical harmonics of degree l⩽10.) I propose a different mechanism of secular variation: ocean water being a conductor of electricity, the magnetic field induced by the ocean as it flows through the Earth's main field may depend on time and manifest itself globally as secular variation. This proposal is supported by calculation of secular variation using the induction equation of magnetohydrodynamics, the observed main field and the ocean flow field. The predicted secular variation is in rough agreement with that observed. Additional support is provided by the striking temporal correlation (hitherto unsuspected) between the intensity of the North Atlantic oceanic circulation and the rate of secular variation in Western Europe; this explains, in particular, the geomagnetic jerks, and the recently discovered correlation between secular variation and climate. Spatial correlation between ocean currents and secular variation is also strong.

Comment on “Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field?” by Stefan Maus, Luis Silva, and Gauthier Hulot

Comment on “Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field?” by Stefan Maus, Luis Silva, and Gauthier Hulot

Reply to comment by V. Lesur et al. on “Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field”

Reply to comment by V. Lesur et al. on “Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field”

Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field?

Geomagnetic main field models used for navigation are updated every 5 years and contain a forecast of the geomagnetic secular variation for the upcoming epoch. Forecasting secular variation is a difficult task. The change of the main magnetic field is thought to be principally due to advection of the field by flow at the surface of the outer core on short timescales and when large length scales are considered. With accurate secular variation (SV) and secular acceleration (SA) models now available from new satellite missions, inverting for the flow and advecting it forward could lead to a more accurate prediction of the main field. However, this scheme faces two significant challenges. The first arises from the truncation of the observable main field at spherical harmonic degree 13. This can however be handled if the true core flow is large scale and has a rapidly decaying energy spectrum. The second is that even at a given single epoch the instantaneous SV and SA cannot simultaneously be explained by a steady flow. Nevertheless, we find that it may be feasible to use flow models for an improved temporal extrapolation of the main field. A medium‐term (≈10 years) hindcast of the field using a steady flow model outperforms the usual extrapolation using the presently observed SV and SA. On the other hand, our accelerated, toroidal flow model, which explains a larger portion of the observed average SA over the 2000–2005 period, fails to improve both the short‐term and medium‐term hindcasts of the field. This somewhat paradoxical result is related to the occurrence of so‐called geomagnetic jerks, the still poorly known dynamical nature of which remains the main obstacle to improved geomagnetic field forecasts.