Electron density extrapolation above F2 peak by the linear Vary-Chap model supporting new GNSS-LEO occultation missions†

The new radio-occultation (RO) instrument on-board the future EUMETSAT Polar System 2nd Generation (EPS-SG) satellites, flying at a height of 820 km, is primarily focusing on neutral atmospheric profiling. It will also provide an opportunity for RO ionospheric sounding, but only below impact heights of 500 km, in order to guarantee a full data gathering of the neutral part. This will leave a gap of 320 km, which impedes the application of the direct inversion techniques to retrieve the electron density profile. To contribute to overcome this challenge, we have looked for new ways (accurate and simple) of extrapolating the electron density (also applicable to other Low-Earth-Orbiting, LEO, missions like CHAMP): a new Vary-Chap Extrapolation Technique (VCET). VCET is based on the scale height behaviour, linearly dependent on the altitude above hmF2 . This allows the electron density profile extrapolation for impact heights above its peak height (this is the case for EPS-SG), up to the satellite orbital height. VCET has been assessed with more than 3700 complete electron density profiles obtained in 4 representative scenarios of FORMOSAT-3/COSMIC occultations, in solar maximum and minimum conditions, and geomagnetically disturbed conditions, by applying an updated Improved Abel Transform Inversion technique to dual-frequency GPS measurements. It is shown that VCET performs much better than other classical Chapman models, with 60% of occultations showing relative extrapolation errors below 20%, in contrast with conventional Chapman model extrapolation approaches with 10% or less of the profiles with relative error below 20%.