Mioara Mandea

High-resolution magnetic field modeling : application to Magsat and Orsted data

Abstract Launched on 23rd February 1999, the Orsted satellite opened the decade of geopotential field research. This is the first satellite to measure the three components of the Earth’s magnetic field since MAGSAT (1979–1980). Orsted orbital parameters are very similar to those of MAGSAT, allowing a first-order comparison of the 1979 and 2000 magnetic fields. Using the available vector and scalar data over the first 14 months of the Orsted mission and applying classical selection criteria (local time, external magnetic activity), we compute a 29-degree/order main-field model and a 13-degree/order secular-variation model for the period 1999–2000. The applied method and the accuracy of the derived model are discussed. We compare the resulting main-field model to a similar one derived from MAGSAT data. Results of this comparison are presented, such as (i) morphology and energy spectrum of the secular variation and (ii) morphology of the crustal magnetic field at MAGSAT and Orsted epochs.

Geomagnetic observations and models

1 The Global Geomagnetic Observatory Network Jean L. Rasson, Hiroaki Toh, and Dongmei Yang 2 Magnetic Satellite Missions and Data Nils Olsen and Stavros Kotsiaros 3 Repeat Station Activities David R. Barraclough and Angelo De Santis 4 Aeromagnetic and Marine Measurements Mohamed Hamoudi, Yoann Quesnel, Jerome Dyment, and Vincent Lesur 5 Instruments and Methodologies for Measurement of the Earths Magnetic Field Ivan Hrvoic and Lawrence R. Newitt 6 Improvements in Geomagnetic Observatory Data Quality Jan Reda, Danielle Fouassier, Anca Isac, Hans-Joachim Linthe, Jurgen Matzka, and Christopher William Turbitt 7 Magnetic Observatory Data and Metadata: Types and Availability Sarah J. Reay, Donald C. Herzog, Sobhana Alex, Evgeny P. Kharin, Susan McLean, Masahito Nose, and Natalia A. Sergeyeva 8 Geomagnetic Indices Michel Menvielle, Toshihiko Iyemori, Aurelie Marchaudon, and Masahito Nose 9 Modelling the Earths Magnetic Field from Global to Regional Scales Jean -Jacques Schott and Erwan Thebault 10 The International Geomagnetic Reference Field Susan Macmillan and Christopher Finlay 11 Geomagnetic Core Field Models in the Satellite Era Vincent Lesur, Nils Olsen, and Alan W.P. Thomson 12 Interpretation of Core Field Models Weijia Kuang and Andrew Tangborn 13 Mapping and Interpretation of the Lithospheric Magnetic Field Michael E. Purucker and David A. Clark Index

Will the magnetic North Pole move to Siberia

The magnetic dip poles, defined as the surface points in the Northern and Southern hemispheres where the Earths magnetic field is vertical, have been the topic of various research activities over time. In addition, their location, and especially their motion, has attracted a large amount of public interest, and this interest will probably increase during the next few months due to the focus on polar regions stimulated by the International Polar Year (IPY) that started in March of this year.

Recent changes of the Earth’s core derived from satellite observations of magnetic and gravity fields

To understand the dynamics of the Earth’s fluid, iron-rich outer core, only indirect observations are available. The Earth’s magnetic field, originating mainly within the core, and its temporal variations can be used to infer the fluid motion at the top of the core, on a decadal and subdecadal time-scale. Gravity variations resulting from changes in the mass distribution within the Earth may also occur on the same time-scales. Such variations include the signature of the flow inside the core, though they are largely dominated by the water cycle contributions. Our study is based on 8 y of high-resolution, high-accuracy magnetic and gravity satellite data, provided by the CHAMP and GRACE missions. From the newly derived geomagnetic models we have computed the core magnetic field, its temporal variations, and the core flow evolution. From the GRACE CNES/GRGS series of time variable geoid models, we have obtained interannual gravity models by using specifically designed postprocessing techniques. A correlation analysis between the magnetic and gravity series has demonstrated that the interannual changes in the second time derivative of the core magnetic field under a region from the Atlantic to Indian Ocean coincide in phase with changes in the gravity field. The order of magnitude of these changes and proposed correlation are plausible, compatible with a core origin; however, a complete theoretical model remains to be built. Our new results and their broad geophysical significance could be considered when planning new Earth observation space missions and devising more sophisticated Earth’s interior models.

Geomagnetic and Archeomagnetic Jerks: Where Do We Stand?

The Earths magnetic field is generated mainly by a self-sustaining dynamo in the fluid outer core. Known as the core or main field, the dynamos magnetic field is not constant but changes with time, a phenomenon denoted as secular variation. Unfortunately, no common agreement exists about the definition of secular variation: While some use this term for the temporal changes of the core field in general, others use the term only for its linear part (first time derivative).

On long-term trends in European geomagnetic observatory biases

We investigated the European geomagnetic observatory biases over 42 years, considered as contributions of the crustal field, and generally assumed to be constant in time. To estimate these biases, we compared observatory annual means to predictions given by the continuous CM4 model, and to four other core field models for different epochs. Solar-cycle related external fields are clearly present in the residuals. Although well-known, no suitable model to minimise them exists. We found that an empirical approach, taking advantage of the homogeneity of the external influences in the European region, can minimise these influences. Their reduction is better than when the external field description included in the comprehensive CM4 model is used. At several locations clear long-term trends remain after subtraction of the core field and minimisation of external fields. We investigated whether they are due to an insufficient description of the core field secular variation by the CM4 model, or to changes in induced lithospheric fields.

Contributions of the external field to the observatory annual means and a proposal for their corrections

In this study we separate, interpret and explain magnetospheric and ionospheric signals present in the observatory annual means. The data from 46 European geomagnetic observatories collected over 42 years (1960–2001) are used. To characterise the various field components, we use predictions from latest magnetic field models. The core field and its secular variation are described by the CM4 model, and the magnetospheric contributions are successfully removed by parameterising the POMME model with the Dst index. We regard the remaining signal as being caused by ionospheric currents. The annual averages of the Sq variation estimated by the CM4 model are subtracted from the residuals. A remaining variation in anti-phase with the magnetic activity index Ap finally can be removed with a function properly scaled by Ap and Dst. We offer an objective procedure to suppress the external field contributions in the annual means to an uncertainty level of ±2 nT. Except for the ionospheric currents, this could be achieved by applying recent magnetic field models, which shows that the quality of present day models is sufficient to correct observatory data for average external field contributions. Understanding the signal contained in the annual means is a prerequisite for obtaining reliable and physically meaningful results when such data are used in studies of the core field and its secular variation.

Geomagnetic jerks from the Earth’s surface to the top of the core

Rapid changes in the magnetic field characterised by an abrupt change in the secular variation have been named “secular variation impulses” or “geomagnetic jerks”. Three of these events, around 1968, 1978 and 1990, occurred during the time-span covered by the comprehensive model CM4 (Sabaka et al., 2002, 2004). This model, providing the best temporal resolution between 1960 and 2002 as well as a fine separation of the different magnetic sources, can be used to study rapid phenomena of internal origin. In order to analyse these events all over the globe, synthetic time series were obtained from the CM4 model between 1960–2002. Geomagnetic jerks are detected here as a rapid movement of the zero isoline of the second field derivative. Analysis of the area swept out by this isoline as a function of time allows us to map the spatial extent of jerks though time, and to identify an event around 1985 that is localized in the Pacific area. At the core surface, we compute the fluid flows under the frozen-flux and tangentially geostrophic assumptions. The flows do not exhibit any special pattern at jerk times, but instead show a smooth temporal evolution over the whole time period. However, the mean amplitude of the dynamical pressure associated with these flows present maxima at each jerk occurrence and helps to confirm the identification of a jerk in 1985.

Automation of absolute measurement of the geomagnetic field

In this paper a device is presented to measure the geomagnetic field vector absolutely and automatically. In contrast to the standard DI-Flux measurement procedure our automation approach is based on the rotation of a three-component fluxgate magnetometer about precisely monitored axes without using a non-magnetic theodolite. This method is particularly suitable for automation because it only requires exact knowledge of the axes orientations. Apart from this, requirements on mechanical precision are moderate. The design of the facility is presented, and mechanical, optical and magnetic limitations are discussed. First promising results of measuring the Earth’s magnetic field absolutely and automatically with the new device at Niemegk observatory are discussed.

Use of Ørsted scalar data in evaluating the pre-Ørsted main field candidate models for the IGRF 2000

For describing the main field model at the 2000.0 epoch and the secular variation over the 2000–2005 time-span, three candidate models for the International Geomagnetic Reference Field (IGRF 2000) were proposed at the beginning of 1999, called in alphabetical order IPGP00 (proposed by IPGP), IZMI00 (proposed by IZMIRAN) and USUK00 (proposed by USGS/BGS). A fourth model, IGRF95 (the updated IGRF 1995), was suggested by the Working Group chairman. The modelling methods and the data used are presented by each team elsewhere in this special issue. This study is an attempt to test these models using the total field intensity provided by the Ørsted satellite, the only data available from that satellite at the time when the two tests describing here were done. The first test consists of evaluating the differences between the real and the synthetic data computed from the candidate models. The second test compares the capability of the candidate models to reduce the Backus effect, using a predictive dip-equator position and Ørsted data. Both tests show that the quality of the candidate models is far from being acceptable, and, therefore, a new candidate model for the main field, using vectorial Ørsted data, is required.