International Geomagnetic Reference Field Model Research Articles

Ørsted satellite captures high‐precision geomagnetic field data

Space‐based, high‐precision magnetometry is essential for understanding a variety of phenomena ranging from secular variation of the Earth's main field, through the signatures of crustal magnetism and the effects of plasma currents flowing externally to the Earth. Ørsted, Denmark's first satellite, was launched on February 23, 1999 into a polar, low‐Earth orbit to provide the first near‐global set of high‐precision geomagnetic observations since the Magsat mission of 1979–1980 (see Magsat Special Issue of Geophysical Research Letters., vol. 9, no. 4, pp. 239–379, 1982). With the new mapping of the Earth's magnetic field, the International Geomagnetic Reference Field model (IGRF), a standard model used for navigation, prospecting, and other practical purposes, has been determined with improved precision for epoch 2000 [Olsen et al., 2000a; Mandea and Langlais, 2000]. The satellite has routinely provided high‐precision vector data since August 1999, and the mission is continuing well beyond its nominal 14‐month lifetime into 2001.

First step towards tiger frequency management: A ray tracing exploration using both IRI and IGRF models

Now that a recent addition to the southern hemisphere Super Dual Auroral Radar Network (SuperDARN) - The Tasman International Geospace Environment Radar (TIGER) - has become operational, real-time frequency management and coordinates registration of both back- and ground-scattered echoes are needed. Currently, and certainly in the near future, TIGER will operate like a high-frequency direction finding, single-site location instrument given that no other SuperDARN instrument currently overlaps its field of view. Ray tracing can be used initially to devise an optimum choice of operating frequencies for maximum coverage and to locate geographically (or geomagnetically) the backscattering regions. For convenience, we relied on the International Reference Ionosphere (IRI) and Geomagnetic Reference Field models (IGRF) for the electron density profile and geomagnetic field as both models could easily be implemented in the ray-tracing code. Collisions were neglected. We also assumed that the radar echoes would mainly return from regions of the ionosphere where rays are nearly perpendicular to the local geomagnetic field. Throughout the study, we considered the magnetic aspect angle condition, | sin −1 ( K·B /kB )|≤05°, to be a necessary (yet insuffiient) condition for backscatter resulting from field-aligned irregularities. We retained as potential backscatter centres all points along the rays where the condition is fulfilled. Although our approach does not encompass all types of irregularities susceptible to produce significant radar returns and completely ignores the geographic distribution of irregularities by assuming their omnipresence, it provides a starting point for the real-time frequency management of the TIGER radar.

Unusual behaviour of the IGRF during the 1945–1955 period

The International Geomagnetic Reference Field (IGRF) for 1900–2000 is testedin this paper. A peculiar behaviour is found in the time variations of higher-degree coefficients of the IGRF models. During the 1945–1955 period the spherical harmonic coefficients g n m and h n m with n > 7 show unusual jumps, while their variations before and after this period are quite smooth. These irregular variations have little effect on the main features of the surface magnetic pattern. However, when we extrapolate the field pattern downward through the insulating mantle to the core-mantle boundary (CMB), the contributions of the higher-degree coefficients become more important and are likely to affect the shape of the geomagnetic energy spectrum, distribution of magnetic flux, and magnetic determination of the conducting core radius. It seems necessary to re-examine and revise these coefficients in the IGRF models.

Magnetic signatures of the distant polar cusps: Observations by Polar and quantitative modeling

Polar cusps are an essential element of the magnetospheric topology. In the outermost regions of the cusp the magnetosheath plasma becomes entrained on magnetospheric field lines and penetrates to low altitudes along field lines on the dayside, causing a significant local depression of B, clearly visible in magnetometer data taken at high latitudes. This study addresses the spatial distribution of the magnetic depression associated with the polar cusp, as derived from a large set of Polar magnetometer data taken in 1996–1998. The depression ΔB was derived as the difference between the total magnitude of the observed field and the one calculated from the International Geomagnetic Reference Field model of the internal field, combined with an external field model. In the noon sector the cusp depression is clearly visible in plots of ΔB versus solar magnetic latitude as a relatively narrow region of large negative ΔB (down to ∼ −80 nT), extending from R ∼ 8 − 9 RE (Polar apogee) to R ∼ 5 – 6 RE. At closer geocentric distances the cusp depression fades out because of a rapid increase of the geomagnetic field toward Earth. The cusp depression also shallows as one moves away from the noon meridian and completely disappears at solar magnetic longitudes ±40 – 60°. The second part of the paper deals with a mathematical representation of the observed polar cusp depression in quantitative models of the geomagnetic field. A method is suggested on the basis of the field deformation technique, making it possible to incorporate a realistic structure of the polar cusps in global models of the magnetosphere.

A reexamination of the local time asymmetry of lobe encounters at geosynchronous orbit: CRRES, ATS 5, and LANL observations

The local time of occurrence of a geosynchronous plasma flux dropout or lobe encounter is dependent on the longitude of the observing spacecraft. Satellites located on or west of the International Geomagnetic Reference Field (IGRF) model magnetic dip prime meridian (magnetic dip meridian) observe almost exclusively (77%) dawnside lobe encounters, while satellites located east of the magnetic dip meridian observe mostly duskside lobe encounters (67%). However, the transition from observing dawnside or duskside lobe encounters is not symmetric about the magnetic dip meridian. Three satellites (ATS 5, 1991‐0180, and 1990‐095) located essentially at the magnetic dip meridian observed a majority (73%) of dawnside encounters. This study therefore concludes that there is a dawn‐dusk asymmetry of the synchronous orbit altitude magnetic equatorial plane during active geomagnetic periods with the dawnside magnetosphere experiencing more radical distortions than the duskside. A case study of near‐simultaneous lobe observations from CRRES and 1989‐046 supports this conclusion. Note that it is found that the IGRF model dip prime meridian better organizes the local time occurrence frequency than does the magnetic dipole magnetic meridian, which has been shown to organize the seasonal dependence of geosynchronous lobe encounters in previous studies (i.e., Thomsen et al., 1994). Hence this study emphasizes the importance of considering the relative position of satellites with respect to both the magnetic dipole equator and the geomagnetic equator when utilizing geosynchronous plasma, magnetic field, and energetic particle data as diagnostics of the energy state of the magnetosphere.

International Geomagnetic Reference Field: The Seventh Generation

A seventh-generation revision of the International Geomagnetic Reference Field (IGRF) was adopted by the International Association of Geomagnetism and Aeronomy (IAGA) at the XXI General Assembly of the International Union of Geodesy and Geophysics in July 1995. The new spherical harmonic models adopted are based on weighted averages of candidate models submitted by NASA's Goddard Space Flight Center, the Russian Institute of Terrestrial Magnetism, Ionospheric, and Radio Wave Propagation-IZMIRAN, and jointly by the US Naval Oceanographic Office and the British Geological Survey. The revised IGRF specifies the Earth's main field from 1900 to 2000 and is declared to be definitive from 1945 to 1990. This paper lists the IGRF coefficients, describes the derivation of the new IGRF models, and examines aspects of the IGRF's accuracy, continuity, and behaviour during this century.

Geomagnetic transmission of solar energetic protons during the geomagnetic disturbances of October 1989

We present orbit‐averaged geomagnetic transmission measurements during the large solar energetic particle events of October 1989 using proton data from the NOAA‐10 and GOES‐7 satellites. We compare the measurements to geomagnetic transmission calculations determined by tracing particle trajectories through the combination of the International Geomagnetic Reference Field (IGRF) model and the 1989 Tsyganenko magnetospheric magnetic field model. We modify the effective ‘ring current’ parameter in the 1989 Tsyganenko model based on the Dst data. We compare our results to calculations employing only the IGRF and to a parameterization of geomagnetically quiet‐time cutoff rigidities derived from Cosmos/Intercosmos observations. Our 3‐hour orbit‐averaged results have ∼15% accuracy during the October 1989 events.

Evaluation of Models Proposed for the 1991 Revision of the International Geomagnetic Reference Field.

The 1991 revision of the International Geomagnetic Reference Field (IGRF) comprises a definitive main-field model for 1985.0, amain-field model for 1990.0, and a forecast secular-variation model for the period 1990-1995. The five 1985.0 main-field models and five 1990.0 main-field models that were proposed have been evaluated by comparing them with one another, with magnetic observatory data, and with Project MAGNET aerial survey data. The three secular-variation models that were proposed have also been compared with one another, and with estimates of recent rates of change at the observatories. The comparisons indicate that the main-field models proposed by IZMIRAN, and the secular-variation model proposed jointly by the British Geological Survey and the U. S. Naval Oceanographic Office, should be assigned relatively lower weight in the derivation of the new IGRF models.

The Geomagnetic Main Field and the Geodynamo

Information available on the geomagnetic main field and the geodynamo is presented. Attention is given to the process of mapping the magnetic field, the last version of International Geomagnetic Reference Field Model, and maps of the magnetic field at the core-mantle boundary and their interpretation. Particular consideration is given to the existing geodynamo theories, with special relation given to the Braginsky and Meytlis theory of core turbulence in which the turbulence differs fundamentally from classical turbulence of Kolmogorov type.

Testing recent geomagnetic field models via magnetic flux conservation at the core-mantle boundary

The 28 candidate main field models submitted for the International Geomagnetic Reference Field Model Revision 1985 reveal that a dramatic increase in the rate of decline of the Earth's total pole strength began around 1960. Nevertheless, all but one field model conserves the absolute magnetic flux linking the core-mantle boundary (CMB) fairly well during the 40 year time span 1945–1985. This supports the frozen-flux approximation on global length and decade time scales. Geostrophic fluid motion at the top of a frozen-flux core conserves the total magnetic flux linking either geographic hemisphere of the CMB. This constraint is poorly satisfied by the candidate secular variation (SV) models. Yet other SV models truncated at higher degree, and models based upon temporal interpolation rather than extrapolation, satisfy this constraint very well. These results lead us to make several recommendations for the 1985 and future IGRF revisions.

Cubic approximations of definitive geomagnetic reference field models

The definitive geomagnetic reference field (DGRF) models for 1965, 1970, and 1975 and the international geomagnetic reference field (IGRF) model for 1980 with linear interpolation between the four epochs is defined as the DGRF linear model. It has 480 spherical harmonic coefficients. This model contains a piecemeal linear representation of coefficients, and hence of the field itself, that results in discontinuities in the secular variation which may lead to difficulties if the model is used at different epochs to subtract values from observations to obtain anomaly fields. A new DGRF cubic model is defined by the coefficients of a cubic curve for the time dependence of each spherical harmonic coefficient. It exactly fits the DGRF linear model coefficients at 1965, 1970, 1975, and 1980 and has continuous first, second, and third derivatives throughout the interval 1965–1980. This DGRF cubic model also contains 480 coefficients which can replace the 480 coefficients of the DGRF linear model. Differences in field components computed using the two models for epochs between the key epochs of the DGRF linear model are considerable, amounting to over 60 nT in 1977.5 for the Z component. No significant differences were found in the comparison of the two models with observatory data. This comparison is difficult because of observatory anomaly biases which appear to be variable with time because of observatory measurement errors or inadequate models. Because the DGRF cubic model provides a model of continuous secular variation, it is recommended that it replace the DGRF linear model.

Sensitivity of cosmic ray trajectory calculations to geomagnetic field model representations

Cosmic ray vertical cutoff rigidities at sea level have been calculated, using the trajectory-tracing method, for a number of different epochs. These calculations have been carried out for a world-wide grid of locations, and, in an effort to locate the cosmic ray equator, for a fine grid in the equatorial region. Comparison of the vertical cutoff rigidity values obtained using the International Geomagnetic Reference Field model for 1980.0 with those obtained from previous models shows systematic significant changes in the Atlantic Ocean region and over South America. The differences are greater than those predicted utilizing the older field models with their predicted secular change. The cutoff rigidity values calculated using the new IGRF 1980.0 field model appear to be in better agreement with data from cosmic ray latitude surveys in the Atlantic Ocean region. The changes in the cosmic ray equator are asymmetrical with essentially no changes in the equator position in the Asian and Pacific region, but with significant changes in the South American, Atlantic Ocean and West African regions. Calculations have also been undertaken for different directions of arrival for a satellite orbiting at 400 km altitude using the predicted 1980 field model and the interim 1980 field model adopted in 1981. Some differences have been found.

An assessment of the near-surface accuracy of the international geomagnetic reference field 1980 model of the main geomagnetic field

The new International Geomagnetic Reference Field (IGRF) model of the main geomagnetic field for 1980 is based heavily on measurements from the MAGSAT satellite survey. Assessment of the accuracy of the new model, as a description of the main field near the Earth's surface, is important because the accuracy of models derived from satellite data can be adversely affected by the magnetic field of electric currents in the ionosphere and the auroral zones. Until now, statements about its accuracy have been based on the 6 published assessments of the 2 proposed models from which it was derived. However, those assessments were either regional in scope or were based mainly on preliminary or extrapolated data. Here we assess the near-surface accuracy of the new model by comparing it with values for 1980 derived from annual means from 69 magnetic observatories, and by comparing it with WC80, a model derived from near-surface data. The comparison with observatory-derived data shows that the new model describes the field at the 69 observatories about as accurately as would a model derived solely from near-surface data. The comparison with WC80 shows that the 2 models agree closely in their description of D and I near the surface. These comparisons support the proposition that the new IGRF 1980 main-field model is a generally accurate description of the main field near the Earth's surface in 1980.

The current status of the IGRF and its relation to magnetic surveys

As aeromagnetic surveys covered increasingly larger areas and as the need arose for the compilation of adjacent surveys with significant time differences, there has been an increase in the use of geomagnetic field models for regional field definition. Indeed, few aeromagnetic contracts exist today without some provision for removal of the International Geomagnetic Reference Field (IGRF). Yet, as publications indicate (Regan and Cain, 1975a, 1975b; Reford et al, 1976; Regan, 1977, 1978), the IGRF is far from perfect for this application despite the considered attention of the IAGA (International Association of Geomagnetism and Aeronomy) working group charged with its upkeep and maintenance. One reason for this is that the model is meant to serve all areas of geomagnetism and, as such, is not specifically tailored to the needs of the exploration community. The recent summary report of the IAGA committee (Peddie, 1982) could lead to some confusion because there are seven IGRF models published. A rational question for those involved in aeromagnetic surveys is “Which model should be used for regional field definition?”

A preliminary assessment of international geomagnetic reference field models for Australia.

A preliminary assessment of international geomagnetic reference field models for Australia.

International geomagnetic reference field: The third generation.

In August 1981 the International Association of Geomagnetism and Aeronomy revised the International Geomagnetic Reference Field (IGRF). It is the second revision since the inception of the IGRF in 1968. The revision extends the earlier series of IGRF models from 1980 to 1985, introduces a new series of definitive models for 1965-1975, and defines a provisional reference field for 1975-1980. The revision consists of: (1) a model of the main geomagnetic field at 1980.0, not continuous with the earlier series of IGRF models, together with a forecast model of the secular variation of the main field during 1980-1985; (2) definitive models of the main field at 1965.0, 1970.0, and 1975.0, with linear interpolation of the model coefficients specified for intervening dates; and (3) a provisional reference field for 1975-1980, defined as the linear interpolation of the 1975 and 1980 main-field models. The new models are series of solid spherical harmonics up to and including the tenth degree and order for the main-field models, and up to and including the eighth degree and order for the secular variation model. The models were derived from three sets of proposed models by taking weighted means. The weights were chosen according to the apparent accuracy of the proposed models. A brief history of the IGRF, a review of basic formulas, and a set of world contour maps of the geomagnetic elements based on the IGRF 1980 model are included.

Comparison of candidate IGRF models.

Three candidate models for the International Geomagnetic Reference Field (IGRF) for the years 1965, 1970, 1975, and 1980 are evaluated using magnetic observatory and satellite data. The models were submitted by the USGS, the IGS, and NASA. Comparison based on annual means data shows the differences between models to be small, although the NASA model appears slightly more accurate in 1965 and 1975 and the IGS model in 1970. Evaluation with Magsat scalar and vector satellite data at 1980 shows the USGS and NASA models to be of comparable accuracy, while the IGS model displays larger residuals. The NASA model best represents the scalar POGO data at 1965 and 1970. Contoured differences between the models in components X, Y, Z, and F at the earth's surface at 1965, 1970, 1975 and 1980 are displayed and discussed.

Displaced isorigidity contours in the North Atlantic for 1975

The results obtained from a neutron‐moderated detector aboard a commercial airline flight between Johannesburg and New York in 1976 revealed a mismatch between the counting rates in the two hemispheres when they were plotted against vertical cutoff rigidity values interpolated from the 1975 values of the 5° × 15° world grid. Cutoff rigidities in the North Atlantic had to be increased by approximately 0.5 GV for a good fit. Detailed vertical cutoff rigidity calculations by the trajectory‐tracing method along the flight path in the North Atlantic revealed deviations of up to 0.3 GV at ∼8 GV between these values and those calculated by interpolation of the 5° × 15° grid. These values resulted in a better, though by no means satisfactory, fit for the data in the two hemispheres. In order to investigate the extent of these deviations, vertical cutoff rigidities were calculated by trajectory tracing for a 1° × 1° grid in the area between 20° and 36° and 308° and 325°E. The isorigidity contours obtained for this area indicated that the size of the deviations could be ≳0.25 GV for a longitudinal extent of perhaps more than 12°. Furthermore, these deviations were found to extend for more than 1 GV in directions perpendicular to the isorigidity contours obtained by interpolation. This result is an indication that a 5° × 15° grid of calculated cutoff rigidities is not sensitive enough, at least for the North Atlantic. When the flight data were plotted against vertical cutoff rigidities calculated for 1980, a better fit was found between data for the two hemispheres at mid‐latitudes, thus casting doubt on the 1975 extrapolation from the 1965 international geomagnetic reference field model.

REVISION OF THE IGRF: A Summary Report on the Zmuda Memorial Conference on Geomagnetic Field Models

On March 24 and 25, 1975, the Zmuda Memorial Field Model Conference cosponsored by the Society of Exploration Geophysicists (SEG) was held at the Broadmoor Hotel, in Colorado Springs, Colorado. This meeting was designed to provide an opportunity for dialogue between those who derive geomagnetic field models and the various users and to serve as an opportunity to discuss the plans for the proposed revision of the International Geomagnetic Reference Field Model (IGRF). The conference included an objective appraisal of the use of field models in magnetic surveys and documented the need for a more accurate reference field. This report summarizes the conference results as they pertain to the members of SEG. A complete summary, including abstracts, is published in the July (1975) issue of EOS, Transactions of the American Geophysical Union.

Inclination of the geomagnetic field measured by an incoherent scatter technique

A technique to measure the direction of the geomagnetic field by using incoherent scatter techniques is described. The technique has been used to measure the inclination of the magnetic field at the Jicamarca Radar Observatory with an accuracy of the order of 1 minute of arc in the height range from 200 to 800 km. The corresponding inclinations obtained from the Goddard Space Flight Center (12/66) and the International Geomagnetic Reference Field (10/68) models are found to be about 1° in error.