Why all the effort?
Direct measurements of the Earth's magnetic field are continuously made around the world at magnetic observatories, and are obtained at all kinds of exotic locations from various oceanographic, land, aircraft and even satellite surveys. All this activity has made some people to wonder "why all the effort?". There are two obvious reasons that stand out for me. In the study of the Earth's history, very few physical quantities have left any record that can be measured today. The study of seismology and the Earth's gravity field have revealed many secrets about the Earth's interior, but there is no timeline in which to examine how these measurements may have, or are, changing with time. Measurements that can be examined retroactively are extremely important to make inferences about what the Earth may have been like in the past. When measuring the magnetic field, what it is actually determined is a combination of fields produced by different sources. The main Earth's magnetic field is generated in the fluid outer core by a self-exciting dynamo process. In addition to sources in the outer core the magnetic field observable at the Earth's surface has sources in the crust, and in the ionosphere and magnetosphere. The geomagnetic field varies on a range of scales and a description of these variations is now provided, from low frequency to high frequency variations, in both the space and time domains. Earth's magnetic field is very dynamic, and the effects of these dynamics on short-time scales (linked to Sun's activity) can be seen on the ground, as they affect power grids and electronics, causing blackouts and interference with radios and telephones.
Measuring, mapping and understanding these different aspects of the Earth's magnetic field, was and is my own contribution to a large international effort on this topic. In what follows, I will provide a more detailed account of a few aspects of my work, to show how the Earth's magnetic field can be both a tool and a hazard to modern world, and why I'm interested in some other planetary magnetic fields such as Mars, Mercury or Venus. Of course, my publications list provides further details about my research area.
Ancient magnetic data
Magnetic observations prior to the establishment of observatories and an absolute method of measuring magnetic intensity by Gauss in the 1830s were made, amongst others, by mariners engaged in merchant and naval shipping. These are mainly of declination and extend the global historic data-set back to the beginning of the 16th century. In the course of these studies we discovered some new sources of magnetic data, and also prior attempts to describe the Earth's magnetic field (Nautonier's book, 1602). This part of my work brought to my attention old books and maps originating from the French Navy old measurements and old instruments, and maybe the first geomagnetic map. Moreover, in some other studies, an overview of the magnetic field variations in Western Europe, over the last centuries is given, with a sustainable work on data from Paris, London, Munich and Bucharest.
In a magnetic observatory absolute vector observations of the Earth's magnetic field are recorded accurately and continuously, with a time resolution of one minute or less, over a long period of time. There are two main categories of instruments that can do it: variometers which make continuous measurements of elements of the geomagnetic field vector but in relative units and absolute instruments which can make measurements of the magnetic field in terms of absolute physical basic units or universal physical constants. After strenuous research activities in France to maintain magnetic observatories on global scale, I was also involved in developing a German geomagnetic observatory network.
The Earth's magnetic field is observed in a number of other ways. These are repeat stations and surveys made on land, from aircraft and ships. Repeat stations are permanently marked sites where high-quality magnetic observations are made for a few hours, sometimes a few days, every few years. Their main purpose is to track changes in the core-generated magnetic field. I have a good experience in field campaign measurements, from Romanian to French and to German networks, as well as over the Southern African continent.
Some magnetic measurements are also done at "exotic locations". For geomagnetism, these locations could be considered as magnetic poles. In 2001 I participated in one campaign to localize the North Magnetic Pole. This expedition provided not only the chance to see the great north, but also the possibility to do absolute measurements in very extreme situations.
Since the 1960s the Earth's magnetic field has been observed intermittently by satellites. The first dedicated magnetic field mission with adequate vector instrumentation was MAGSAT (1979-1980). Shortages inherent to the MAGSAT mission were represented by the short duration and the sun-synchronous orbit. Thanks to the recent launch of Oesrted satellite, 20 years after the MAGSAT analogous mission, the geomagnetic field is probing continuously. In part due to its fairly high orbit, the achieved resolution falls somewhat behind the desired expectation. With its high inclination (87°) orbit CHAMP (2000 - 2010) covers all local times, and the planned active life-time of 5 years facilitates the study of secular variation. The low circular orbit, starting at 454 km altitude and decaying over the life-time to below 300 km, together with the greatly advanced instrumentation flown on CHAMP provided an improvement in accuracy of data by an order of magnitude compared to MAGSAT. This offered new perspectives on the role of magnetic field measurements in solid Earth studies.The Swarm mission is a constellation of three identical satellites to measure the Earth's magnetic field; it was decided by ESA as an Opportunity mission as part of the Earth Explorer Program. The three satellites are launched together in 2013. The scientific objective of the mission is to provide the best ever survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earth's interior and Sun-Earth connections. I’ve been involved in this mission from the very beginning, and now it is one of the far missions of the Solid Earth’s program I am in charge of. The French contribution to this mission consists in the CNES supplying the absolute magnetometers.