ALBERT

Description: It is widly accepted that the occurrence rate of the post-sunset equatorial F region plasma instabilities, often named Equatorial Spread-F (ESF), show a typical seasonal/longitudinal (s/l) distribution. Supporting global observations are now possible with recent satellite measurements of ESF involved electron density depletions (e.g., on board ROCSAT-1 or DMSP) or of magnetic field enhancements inside ESF due to the diamagnetic effect (on board CHAMP). Different mechanisms have been proposed to explain the observed ESF occurrence rate structure. There are electromagnetic conditions of the ionosphere, such as the alignement of the geomagnetic field with the sunset terminator, the strength of the ionisation anomaly, and the strength of the vertical upward plasma drift during the Pre-Reversal Enhancement (PRE). The latter one enhances the ESF growth rate in two ways: it creates a force parallel to the density gradient and antiparallel to gravity, and it pushes the ionosphere to higher altitudes, where the ion-neutral collision frequency is decreased. This paper presents the comparison of two independent continuous data sets between 2001 and 2004; the CHAMP ESF occurrence rate and the vertical E×B plasma drift velocity derived from ROCSAT-1 observations. Special advantage lies in the full local time availability of drift observations which allow the investigation of the PRE peak value and the integrated effect of the entire PRE period. We find excellent agreement in the s/l distribution of ESF occurrence rate and the plasma drift with corellation coefficients above 0.8 in all seasons. These results emphasises the PRE E×B plasma drift velocity as the dominant contributor in ESF initiation. Going one step further, we investigate the local time of ESF occurrence and compare these findings with the change in local time of the PRE depending on season and longitude.

Description: Novel possibilities to detect Equatorial Spread-F (ESF) events are given by the magnetic signatures of the depleted plasma tubes. The diamagnetic effect enhances the total magnetic field in regions of low plasma density. We use 7 years (2001-2007) of almost continuous high quality magnetic field observations on board the CHAMP satellite to reconstruct the global climatology of the ESF magnetic signatures at about 400 km altitude. Our results compare very well with existing ESF climatologies based on satellite observations of electron density, especially in longitudinal and seasonal (S/L) dependences. We find an excellent agreement with the S/L distribution of the equatorial prereversal vertical plasma drift velocity derived from ROCSAT-1 observations. These results prove that the vertical plasma drift acting on the F-layer height is the dominant contributor to the ESF occurrence. The increasing CHAMP data set spanning different solar cycle phases reveals the existence of more, larger and stronger ESF events for high solar flux level conditions. These characteristics are reflected in the extended local time and the higher L-values where ESFs are observed at high solar flux levels. CHAMP also provides observations of the transverse magnetic ESF signatures. These deflections imply the existence of field-aligned currents. Such observations will be used for characterizing the electrodynamic state of the ESF event.