On a Quest to Improve the Solar Forcing in IRI

The International Reference Ionosphere (IRI) is an empirical model of the ionosphere based on a large volume of ground and space measurements that was developed under the auspices of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) and that earlier this year became an international standard of the International Standard Organization (ISO). IRI currently uses several solar and ionospheric indices to describe the variations of ionospheric parameters with solar variability. These indices are used at an averaging level of 81 days or even a whole year. We have investigated the performance of these different indices at different averaging lengths using over 30 years of ionosonde foF2 data from the three stations Boulder, Jicamarca, and Grahamstown employing daily and monthly averages of foF2. In addition to the indices currently used in IRI our study also included indices composed of measured EUV fluxes (Lyman alpha -121.5nm, MgII-core-wing-flux-ratio, Integral flux 0-105nm). However, coverage gaps during the last two solar cycle maxima introduce uncertainties for these indices. We get the best results with Lyman alpha fluxes at an averaging length of about 81 days (3 solar rotations). The ionospheric-effective solar index IG, which is based on ionosonde data from five selected stations, performs almost equally well as the Lyman-alpha flux index. Surprisingly, we find that the monthly IG index performs as well if not better than the 12-month running mean of monthly IG that is currently used in IRI. This opens interesting possibilities for using a GIRO-based IG index (IGiro) that could be determined by averaging across a global selection of ionosonde stations available on the Global Ionospheric Radio Observatory (GIRO) at a much higher time resolution (down to 15 minutes) in near real-time. Most importantly such a new index could be designed such that it would not be limited by the constraints of the current IG index, which is determined with only noon data and with using the CCIR maps and thus should not be applied for nighttime and/or URSI maps.