New geomagnetic indices as proxies of the energy embedded in the thermosphere-ionosphere system by solar-wind and magnetospheric effects
Résumé
Planetary geomagnetic indices ap are classically used in models to quantify the geomagnetic forcing of the thermosphere/ionosphere system as a consequence of solar wind/magnetosphere coupling. These indices are based upon K indices measured at a non-evenly distributed network of 13 geomagnetic observatories. K indices are directly related to the range during 3-hour intervals of the irregular variations in the horizontal components of the magnetic field. There are accordingly 8 K-derived planetary indices per day, corresponding to UT intervals 00-03, 03-06 up to 21-24. K-indices are proxies of the energy related to the geomagnetic activity. This approach of 3-hour resolution is a strong limitation for precise modelling of the thermosphere/ionosphere response to geomagnetic forcing. We therefore introduce new indices based on another proxy of the magnetic energy, namely the root mean square (rms) of the irregular variations in the magnetic horizontal components. Using such proxy does not put constraints on the length of the time interval over which the indices are derived. From the irregular variations re-computed at each observatory, the new magnetic indices are derived following algorithms similar to those used for am, aa planetary geomagnetic indices derivation. Results from statistical studies of these new indices are presented during quiet, medium or stormy magnetic activity. The potential benefits for specific thermospheric studies or Space Weather applications are discussed.