Observations of the Earth's upper atmosphere during a solar eclipse represent a unique opportunity to study the response of this region of the atmosphere to sudden variations of the solar energy input. In the past years eclipse studies have been performed from ground based observations, leading partly to contradictory observations of the ionospheric conditions during the eclipse and to open questions as to the effect on the geomagnetic field. Furthermore, the effects on the thermosphere have never been thouroughly studied. With the launch of CHAMP in 2000 this situation improved considerably. Not only in-situ measurements of the upper atmosphere are now available, but also different types of eclipses in different regions can be studied with the same methods.

A first case-study of the solar eclipse on 8 April 2005 showed significant modifications of the ionosphere at F2 layer altitudes, indicating an enhancement of the plasma fountain at the equator resembling post-sunset conditions. A reduction of the equatorial electrojet strength by almost 40% was also observed. This motivated recent studies of equatorial solar eclipses, which seem to indicate a strong influence on the equatorial electrojet possibly causing a reversal of the currents in this region originating a counter electrojet. Future studies will include the effects of solar eclipses at mid-latitudes.

(Dr. Ana Teresa Tomas)