ProjectA6

Project Participants

• A. Chulliat (IPGP) – French principal investigator
• P. Kotzé (HMO) – South African principal investigator
• J.-L. Le Mouël (IPGP)
• M. Muundjua (Ph D student)
• E. Thébault (IPGP)
• A. Chambodut (EOST)

Background

Figure 1: INTERMAGNET magnetic observatories in France and on the African continent. Observatories run by France (alone or in cooperation with other countries) are denoted by blue dots; South African observatories are denoted by red dots. Conjugate points of the Chambon la Forêt (CLF‟) and Hermanus (HER‟) observatories are indicated by small circles.

Both South-Africa and France have a long tradition of measuring and studying the Earth‟s magnetic field. The first French magnetic observatory was established in 1883 in Parc Saint-Maur, near Paris, and was later moved to Val-Joyeux in 1900 and then to Chambon-la-Forêt in 1936. The Bureau Central de Magnétisme Terrestre, which is under the responsibility of IPGP, is currently in charge of 16 observatories throughout the world. The first systematic observations in South Africa were performed at Cape Town Observatory (1843 till 1852, and 1932 till 1940). The Hermanus Magnetic Observatory officially commenced operation in 1941. It is now running two other observatories in southern Africa: Hartebeesthoek (in South Africa) and Tsumeb (in Namibia). All French and South African observatories belong to INTERMAGNET, the global network of magnetic observatories transmitting their data in quasi-real time (www.intermagnet.org).

1- Investigating possible relationships between the magnetic fields of the Sun and the Earth and climate change

It has become more and more obvious in recent years that solar activity has a deep influence on the Earth‟s climate variability. Yet the relationships between both phenomena are still not well understood. Solar wind, solar magnetic field and solar radiations interact in a complicated manner with the Earth‟s atmosphere. Important elements of this interaction are the Earth‟s magnetic field of internal origin and the rapidly-varying electrical currents in the ionosphere and magnetosphere, which in turn generate magnetic fields detectable on the ground.

The goal of this project is to use the long magnetic data series of Chambon and Hermanus observatories to study relationships between geomagnetic field variations and climate variability. Magnetic data series from observatories are among the longest geophysical data series available. They have all time scales of the magnetic field embedded and thus provide information on both the slowly-varying internal field and the rapidly-varying external field on a long time span. Long temperature series are also available near Paris and Cape Town and will be compared to magnetic series.

We will first focus on the long-term evolution of rapid variations, whose time-scales range from one second to one month. Such variations correspond to various physical processes in the ionosphere and magnetosphere. The Chambon and Hermanus observatories are roughly at the same geomagnetic latitude within their respective hemispheres (about 42°), which could help compensating for seasonal variability. We will try to better characterize the long-term evolution, using appropriate mathematical tools in order to separate the various physical processes involved, and then compare it with temperature.

The study will form part of preparations for the next SWARM satellite mission to be launched in 2010. The project should strengthen historic ties between the two observatories and will investigate scientific questions with increasingly important societal consequences (space weather and global change).

2- Modelling the secular variation at a regional scale

The secular variation of the geomagnetic field has considerably accelerated during the last 7 years with an increase in its changing rate especially after the 2003 geomagnetic jerk. As a result of a poor worldwide data distribution, in particular in the Southern hemisphere, determining the geomagnetic acceleration of the changing field at high resolution and at a global scale is not possible. The best model predictions do not represent these changes with a better spatial scale of 3000km (spherical harmonic degree 14) for the secular variation and 6000km (n=7) for the acceleration. It has been observed that geomagnetic jerks could be prominent on a regional scale and, indeed, the most recent dramatic changes occur almost exactly under South Africa, therefore representing the ideal location to perform a high-resolution analysis.

South Africa has a long tradition of repeat station measurements that dates back to the early 1960. The spatial distance between two repeat stations is not more than 300-400 hundred kilometres. In addition, we now have 5 years of continuous satellite measurements of high quality. A joint analysis of satellite and repeat station data over the southern African region with a regional modelling technique (Thébault et al., 2006a, 2006b) should provide an unprecedented spatial view of the secular changes in this region. Collaborating with Hermanus Magnetic Observatory and GFZ, which have performed recent repeat station surveys in Southern Africa, will offer a unique opportunity to achieve this important goal, leading to a better understanding of the Earth‟s core dynamics.

Objectives and milestones:

Results will be communicated on a regular basis at workshops, and international conferences. Peer-reviewed articles will be published in international journals.

Capacity building:

A PhD student, Mr Manfriedt Muundjua, will be supervised by A. Chulliat and P. Kotzé for his thesis based on a time–series analysis of observatory data.

Recent bibliography:

• Chulliat A., Blanter E., Le Mouël J.-L. & Shnirman M., On the seasonal asymmetry of the diurnal and semidiurnal geomagnetic variations, J. Geophys. R., 110, A05301, doi:10.1029/2004JA010551, 2005.
• Le Mouël J.-L., Mayaud P.-N. & Shebalin P., Magnetic activity inside the auroral zones and field-aligned currents, C. R. Geoscience., 335, 935-941, 2003.
• Le Mouël J.-L., Blanter E., Chulliat A. & Shnirman M., On the semiannual and annual variations of geomagnetic activity and components, Ann. Geophys., 22, 3583-3588, 2004.
• Le Mouël J.-L., Kossobokov V. & Courtillot V., Long-term trend of geomagnetic variation, Earth Planet. Sci. Lett., 232, 273-286, 2005.
• Le Mouël J.-L., Shebalin P. & Chulliat A., The field of the equatorial electrojet from CHAMP data, Ann. Geophys., 24, 515-527, 2006.
• Thébault E., Schott J.-J. & Mandea, M., Revised spherical cap harmonic analysis (R-SCHA): Validation and properties, J. Geophys. Res., 111, B01102, doi:10.1029/2005JB003836, 2006.
• Thébault E., Mandea M. & Schott, J.-J., Modelling the lithospheric magnetic field over France by means of revised spherical cap harmonic analysis (R-SCHA), J. Geophys. Res., 111, B05102, doi:10.1029/2005JB004110, 2006.