The low-latitude ionosphere regularily undergoes severe weather during the hours after sunset. At that time, it is susceptible to local plasma instabilities which form magnetic flux tubes of depleted plasma density. Such events have been named after their discovery in disrupted ionograms in the 1930's to Equatorial Spread-F (ESF). Ionospheric instabilities seriously affect the vulnerability of advanced, but vital technologies for human society. Steep plasma density gradients such as involved in the ESF phenomena are especially critical for the reliability of trans-ionospheric radio wave based navigation and communication systems, such as GPS or the future Galileo.

Based on the multi-year data set of high precision magnetometer observations onboard the CHAMP satellite ESF magnetic signatures in the total field as well as in the components perpendicular to the main magnetic field direction have been identified and monitored. An immediate explanation for the total field deflection is the diamagnetic effect. Magnitudes of up to 5nT are observed in the CHAMP data at ~400km. Perpendicular deflections imply field-aligned currents.

Especially the total field signatures enable us to provide a climatology of the occurrence of ESF magnetic signatures. Their dependence on local time, magentic latitude, longitude, season and solar activity could be derived (Stolle et al., 2006).