Main field and secular variation models from CHAMP and Ørsted

Stefan Maus
GeoForschungsZentrum Potsdam, Germany

With its low orbit, long life time, high resolution magnetometers, dual head star camera, and an almost 100% data coverage, CHAMP determines the geomagnetic field with unprecedented resolution and accuracy. Having 2 star camera heads pointing into different directions significantly reduces the inherent uncertainty in satellite attitude, in particular the errors in the rotation angle about the star camera bore sight. As inferred from the degree spectrum, the secular variation of our first main field model from CHAMP-vector-only data is reliable to degree 11. Inclusion of further Ørsted data going back to1999 improves the resolvable secular variation to degree 13. We are also beginning to see the change of the secular variation from 1999 to 2002.

World Data Centre repeat station data collection and use of these data in global modelling

Susan Macmillan
British Geological Survey, Edinburgh, UK

We describe the background to the present-day data holdings at WDC Edinburgh and the continued efforts to maintain them up-to-date. The use of repeat station data in modelling the Earth's magnetic field on a global scale is described.

Modelling European repeat station secular variation

Monika Korte, Volker Haak, Richard Holme*
GeoForschungsZentrum Potsdam, Germany
*now at: University of Liverpool, UK

A long-standing question has been whether large-scale lithospheric conductivity anomalies can be observed as induced secular variation anomalies. A dense coverage in both space and time with data of high accuracy is necessary to study this problem. Small scale secular variation cannot be resolved by global models due to the sparse long-time data coverage in large parts of the world. Europe is the region with the densest network of geomagnetic observatories, regional surveys and repeat stations. Spherical cap harmonic analysis has been used to model European secular variation over four decades. The inhomogeneous distribution of available data in space and time is a serious problem in the process of obtaining reliable models. We present some results that we had obtained in our first efforts of modelling a combination of European repeat station and regional survey data. By showing improvements made through modifications of the modelling technique we hope to motivate European efforts to improve future data distribution and quality.

A new proposal for Spherical Cap Harmonic Modelling

Erwan Thébault*, Jean-Jacques Schott* and Mioara Mandea&,
*Ecole et Observatoire des Sciences de la Terre, Strasbourg, France & Institut de Physique du Gloobe de Paris, France

Spherical Cap Harmonic modelling is an attractive regional representation of the geomagnetic field, widely used for field or secular variation modelling since the pioneer work of Haines (1985). It should offer many of the properties of the global Spherical Harmonic Analysis, particularly a straightforward upper continuation allowing to incorporate easily satellite data in the modelling. Yet, it may be shown that the bases proposed by Haines do not allow properly upward continuation. In addition, the linear relationship that may be laid down between global and local Gauss coefficients depends on the radial distance. Yet, the calculation of such relations would be useful for estimating prior covariance for the local Gauss coefficients and, hence, would be helpful in the inverse problem consisting of estimating local Gauss coefficients from a set of data. Setting the problem as a Boundary Value Problem on a domain defined by the intersection of a cone and two spheres, and splitting it up into two sub-problems with simpler boundary conditions, we propose a new set of two bases which solves the difficulties mentioned above. The orthogonality properties of each base relies upon a Sturm-Liouville problem. In Haines's formalism, both bases are deduced from a Sturm-Louville problem comprising the Legendre differential equation associated to two different sets of boundary conditions. Beside one of the bases proposed by Haines, we introduce a new base defined by a Sturm-Liouville problem involving the radial distance. This base contains so-called conical or Mehler functions, with complex degrees. The decomposition of the initial problem is not unique. We will present 3 possibilities, two of them leading to the construction of two mutually orthogonal bases. The effectiveness of this formalism and numerical implementation may be shown by an accurate reconstruction of the main field on the conical domain, either using the main field as boundary condition or computing local Gauss coefficients from a set of main field data distributed on a regular three-dimensional grid. We will show also preliminary results about an application to the regional secular variation modelling on a small cap covering the French territory.

Status of the Geomagnetic Network in the Czech Republic

Pavel Hejda and Josef Horacek,
Geophysical Institute, Academy of Sciences, Prague, Czech Republic

A recent stage of the magnetic surveying activity in the Czech Republic was started in the fifties by setting up a fundamental network of the first order. It consists of 199 points, which have been stabilized by means of granite boundary stones. Measurements of the vertical and horizontal components were carried out in 1957-58 and supplemented by reduced values of declination obtained at a former campaign. The network was re-occupied in 1976-1978 when three components D (by Matting & Wiesenberg theodolite), H (QHM) and Z (BMZ) were completed by the measurements of the total field F (by PPM). The latter allowed to correct random errors. The azimuth was determined by the Sun observations. The last magnetic survey in 1994-96 was carried out by a fluxgate theodolite and a proton precession magnetometer. Since 1970 six selected point have been reoccupied approximately every 2 years. Last measurements were carried out in 2001. All measurements are reduced to the epoch yyyy.5 using data from geomagnetic observatory Budkov.

Spanish Repeat Stations Program

Isabel Socias
Instituto Geografico Nacional, Madrid, Spain

The Spanish repeat station network is made up of 41 stations, 38 in the Mainland and 3 in the Balearic Islands; this means a mean density of 12500m2 at the mainland and 17000m2 in the islands. During the last years, because of civil noise (railways, metallic fences, electromagnetic interference) some of them have been moved to a nearby place.
The time interval of repeat stations observations is of two years and in each 4 or 8 series are done, during two days. The instrumentation use is: Theodolite Zeiss-Theo 20B and Fluxgate Bartington for D and I and a Proton magnetometer for F. Data are reduce to mean values using as reference San Pablo de los Montes Observatory.
At the Canary Islands there are 9 repeat stations, but the observations of these present a lot of problems because of the bid gradients due to the volcanic origin of these Islands.

Instrumentation, Measurement Procedure and Data Reduction for the UK Repeat Station network

J. Carrigan
British Geological Survey, Edinburgh, UK

The procedure at a UK repeat station is described outlining the instruments used for measuring Declination, Inclination and Total Field. The use of a north seeking gyro, GPS, declinometer/inclinometer and proton precession magnetometer are detailed. The procedure of reducing repeat station data using reference observatories is outlined.

The Restoration Project of Geomagnetic Measurements in Latvia

J. Burlakovs
State Land Service of Latvia, Riga, Latvia

The aim of geomagnetic measurements is to study the geomagnetic field at global, regional as well as local scales. To determine secular changes of the geomagnetic field it is very important to do a lot of regular field work. Recalculation and comparison of measured data for corrections must be made using the observatory or magnetic station data collected nearby the investigated area in the real-time. Field geomagnetic measurements in Latvia have not been made since 1991. The State Land Service of Latvia, the Geodesy Board plans to restart such kind of measurements in Latvia. The repeat station network must be renewed, regular magnetic declination, inclination and total field intensity data must be gathered, compared with the observatory data and secular changes of the geomagnetic field discovered. It is also possible to do regional correlations for data to determine future trends of the geomagnetic field changes. The detection of geomagnetic anomalies and the reason of the existence of those at particular territories could be made. Such kind of measurements demands the highest precisity and therefore is necessary to cooperate with magnetometrical research network groups in neighbouring areas - Estonia, Finland and Poland, where permanent magnetic stations are situated. One permanent magnetic station also could be established in the central part of Latvia, for example, in Baldone, to do permanent recordings of geomagnetic field components which give the possibility to do regional corrections for separate measurement recordings in the field. Geomagnetic field studies are important for cartographical, navigational and military needs, also it is possible to use this information together with geological and geophysical data to create and specify the geological model for the territory. In future Latvia must participate within the framework of international projects, e.g. IMAGE and INTERMAGNET - it will be important step towards the geomagnetic observation network development in Europe.

Past and present of the Italian Magnetic Network and its future in Europe

De Santis A., Dominici G., Meloni A.
Istituto Nazionale Geofisica e Vulcanologia, Roma, Italy

Magnetic measurements in Italy can be dated back to the XVII century. Stable engagement in regularly repeated magnetic measurements started with the national unification at the end of XIX century. Repeat station measurements now include estimation of inclination and declination by means of a DI Flux magnetometer and total intensity by means of a proton precession magnetometer. A gyroscopic theodolite for determination of geographic north is used when necessary and an automatic three-component variograph is usually installed in the region, if no Observatory is close enough. The Italian Magnetic network is composed of 2 Observatories, 2 continuous magnetic stations and 114 stations and the latter are repeated at 5-year intervals. Based on these data and, when available, other indipendent datasets, both simple and sophisticated models are computed. Maps of the magnetic elements and CD-rom (the latter for 2000.0 survey) are produced and distributed. Although national results obtained in the past have been reliable and profitable, we strongly agree with the main objective of this meeting, i.e. to take a unified picture of the geomagnetic field of Europe. We must plan altogether the best and optimal way to reach this goal, taking into account both diversities and similarities belonging to each group, taking advantage of the former and improving the latter.

The Romanian Repeat Station Network Past and Future

Crisan Demetrescu (1), Daniela Nitoiu (1), Mirel Ene (1), Nadejda Rosieanu (1), Gabriela Cucu (2), Andrei Soare (2)
1. Institute of Geodynamics, Bucharest, Romania
2. Romanian Geological Survey, Bucharest, Romania

Systematic repeat measurements on the Romanian territory began in 1964 and have continued, more or less regularly, till present. The network consists of 21 stations chosen, as to avoid regional anomalies, from the several hundred ones which have been measured in the late '50s and early '60s of the twentieth century for the Romanian magnetic map project. Measured values were reduced to the desired geomagnetic epoch (July 1st) by comparison with Surlari observatory data. Frequent calibration of the magnetometers has been performed. Methods of data reduction and analysis, as well as result on the secular variation evolution in the study area are presented.

The SV of Declination in Eastern Europe for the 1941-2000 Interval

Elzbieta Welker, Andrzej Sas-Uhrynowski
Institute of Geodesy and Cartography, Warsaw, Poland

The SV of D in the 1941-2000 interval have been determined for the Kaliningrad Region and the territories of Lithuania, Balarus and Western Ukraine, which belonged to Poland before the Second World War. The data from 15 observatories have been used to analyse the variations. The results of analysis have allowed to bring up-to-date the old maps of magnetic deviation for these territories, which the German Military Topographic Service worked out before 1941. The magnetic measurements performed recently in Lithuania and Belarus have been used to verification the results of analysis.

Magnetic Repeat Stations Measurements in France: from 1948 to 2002

Mioara Mandea
Institut de Physique du Gloobe de Paris, France

During the last fifty years IPGP has carried out magnetic surveys in about 33 repeat stations, occupied routinely every 5 years, last time in 2002. The spacing between points is typically about 200km, and somewhat less in geologically variable regions. Stations have been installed in order to be representative of the surrounding regions, in places with low gradients, avoiding large magnetic anomalies and subsurface electrical conductivity structures. A summary of the long experience in repeat stations measurements is given, with examples from practice (observational procedures: instruments, frequency and duration of observations; data reduction procedures: reduction to a common epoch, direct reduction using data from a reference observatory, errors).
The benefit and the challenge of these measurements are related to the improvement of our knowledge on the geomagnetic field, in terms of spatial and time variations. The time evolution of the geomagnetic elements in this territory is also presented, with implications for the secular variation studies.

Geomagnetic Field Measurements at Repeat Stations on the Territory of Yugoslavia in the Period 1957-2003

Dragan B. Popeskov
Geomagnetic Institute, Grocka, Belgrade, Yugoslavia

A brief historical review of the geomagnetic field measurements on the territory of Yugoslavia will be given. It all began in 1957 with the foundation of the Geomagnetic Institute, when continuous recordings at Grocka geomagnetic observatory (GCK) as well as field measurements started, still being the only institution of this kind on the territory of former Yugoslavia. The frequency of repeat measurements and the instruments used will be discussed. New network of repeat stations was designed for the territory of Federal Republic of Yugoslavia and we will point out basic principals for the network planning. Some suggestions will be given that might help in establishing a unique standard for repeat station measurements, data reduction and the presentation of final values in numeric and graphical form

Magnetic Repeat Station Measurements in Sweden

Gerhard Schwarz
Geological Survey of Sweden, Uppsala, Sweden

In recognising the need for more systematic geomagnetic observations in Sweden the Hydrographic Service established a net of sites in the 1920s (Ljungdahl, 1934). These were almost evenly distributed over the country and intended to serve as repeat stations for future re-occupations. In 1930 the net consisted of 86 stations. A number of them were installed in remote areas and the effort in surveying these sites periodically was rather large. Therefore, the extended net of observational points was reduced to only 10 sites in the early 1950s. The remaining sites were then revisited almost every other year until 1996. Obviously, the instrumentation for surveying the sites changed and was improved until today, i.e., from classical Carnegie of Washington combined magnetometers in the late 1920s to modern fluxgate- and proton precession magnetometers of today. Observational errors, i.e., errors in reproducing the magnetic elements in value properly were reduced considerably from about ? 50 nT in the beginning to about 5 nT of today, owing mostly to the improvements in instrumentation. But, even the latter value of about 5 nT is rather large compared to the absolute changes of the magnetic field in Scandinavia.
Until 1969 four to six absolute observations of the Earth's magnetic field were done at each secular variation site of the network during one day or so. Data were reduced to the magnetic level of quiet days by either using the nearest geomagnetic observatory or taking weighted mean data from a combination of different observatories. In 1972 a proton magnetometer was inserted into the measuring procedure and the measuring practice was changed, i.e., the magnetic elements at a particular site were measured every other minute for at least four hours of a day or so. This procedure provides hourly mean values of the geomagnetic elements that were corrected with the help of the recordings of the nearest observatory or a combination of them.
We will present the secular variation data of the network of the ten sites that were measured periodically until today and compare the data with the recordings of the Nordic observatories. The secular magnetic variations in Scandinavia are rather small as well as regionally smooth. Only the data of six Nordic observatories are used for describing the magnetic elements by a second order polynomial as well as predicting them on a short term. It turned out that these polynomials describe very well the secular variations at the field sites. Yet, with the present time series of magnetic data starting only in 1928 and the concerns on data quality until about 1950 it is questionable whether the ability of predicting geomagnetic data will continue as well as secular variations on a smaller and more regional scale will be resolved.
When continuing with magnetic secular variation measurements in Sweden we have to focus on better quality of data. A better knowledge of the magnetic diurnal variations in Sweden will be accomplished by installing temporary base stations at some distance to the observational points. Such field instruments are now available. They will also help to reduce observation times. As well, re-selecting some other observational sites from the original net in exchange for unsuitable ones will improve data. Hopefully, these measures will help to better resolve secular variations in time and space in Sweden.

D - Annual 1950-2000 Variations on Polish Territory

Marek A. Zóltowski
Institute of Geodesy and Cartography, Warsaw, Poland

On the Polish territory, in the second half of the XX century, magnetic declination D was observed at two observatories and at 19 secular variation repeat stations. On the grounds of the all obtained data, the annual variation values have been calculated in relation to respective values received from Belsk Observatory. Neighbouring observatories have been also taken into account. Linear regression function for all stations and observatories as well as residuals have been compiled in the form of tables. The tables and the example of calculations of D-variation between facultative epochs Ti and Tj have been presented.

Magnetic repeat station surveys in Germany

Monika Korte and Martin Fredow
Geomagnetic Observatroy Niemegk, GeoForschungsZentrum Potsdam, Germany

The current network of magnetic repeat stations in Germany consists of 49 sites, which fall in two categories: there are 13 high-accuracy stations where an on-site variometer is used for data reduction and 36 stations where only absolute measurements are done. The maximum distance between the additional stations and a variometer station or observatory does not exceed 150 km. Most stations have been reoccupied every two years since 1996. We present the development of German repeat station suveys over the last decades and describe the measurement methods. Data reduction has changed with the use of variometer recordings at the 13 stations in the 1999/2000 survey. We show the improvements to the field results when reducing the data by means of quiet night time values.

Methodology of magnetic observations at magnetic observatory Lviv

Yuri Sumaruk
Magnetic observatory Lviv of the Institute of Geophisics of National NASU Ukraine.

Methodology of magnetic observation at the observatory Lviv in past and present will be presented. Instruments for absolute and variate measurements are discribed. The plans for improvement magnetic observations are suggested.

On secular variations at magnetic observatory Lviv

Taras Sumaruk
Magnetic observatory Lviv of the Institute of Geophisics of National NASU Ukraine

Secular variations at magnetic observatory Lviv for 1966-2000 years are calculated. The peculiarities of its development are investigated. Comparisons to variations at neighbour observatories are made.