Evidence of low-intensity magnetic events associated with seismic or volcanic activity has been found as early as the 1950s.
Understanding these phenomena requires the simultaneous development of ground experiments and satellite measurements. Ground experiments, located close to epicentres, are easier to perform and can include a great variety of complementary experiments. However, they present certain difficulties:
- location (until the geophysical conditions necessary for an emission source are determined);
- sensor positions (as long as the reasons behind variations between VAN measurements over a few kilometres aren’t accurately identified);
- and observation time.
As flawed and limited as they can be, satellite observations are currently the best method for a preliminary demonstration phase to characterise electromagnetic events. They help improve ground observations by determining the best areas in which to conduct experiments and the most suitable equipment to use in future ground experiments.
Satellite observations also have the significant advantage of covering almost every areas of high seismic activity around the globe in a fast and efficient manner.
But these observations are only relevant if one can prove their seismotectonic source, as well as determine their characteristics and their variability depending on the conditions and environment surrounding the seismic event. Unfortunately, before Demeter, such observations had only been made using instruments which weren’t specifically designed for this kind of survey. As such they presented some issues:
- they lacked continuity over time;
- they were performed over narrow and often ill-adapted frequency ranges;
- and they were generally limited to only one aspect of the electromagnetic field.
The primary objective behind Demeter’s observations was a systematic survey of electromagnetic waves emissions detected during earthquakes and volcanic eruptions, disturbances in the ionosphere or upper atmosphere, and associated particle precipitation (Parrot et al., 1993). This kind of experiment is motivated by initially fortuitous observations from the 1980s, first made by Russian and Japanese scientists, and followed by French scientists. Demeter represented the logical next phase for this endeavour, and constitutes a crucial step towards identifying and thoroughly understanding these phenomena.
Demeter’s secondary objective (within CNES’s National Sun-Earth Programme) was to carry out a global survey of the electromagnetic environment surrounding Earth. Since 1981 and the AUREOL-3 mission, no low-altitude scientific satellite had taken measurements in mid-latitude regions. Demeter was able to study the effects of storms on Sun-Earth interactions and to evaluate the impact of human activity on the ionosphere (Parrot, 1994, Parrot and Zaslavsky, 1996).