Assimilation procedures for updating ionospheric profiles below the F2 peak

[1] This paper describes and compares two real-time assimilative ionospheric models, with an emphasis on their ability to provide accurate profiles of the electron density below the peak of the F2 layer at a midlatitude location, given automatically processed vertical incidence ionograms at a single location. The two models are specifically oriented toward several important practical applications of high-frequency (HF) radio propagation: HF communications, single station location of HF transmitters, and coordinate registration for OTHR. Both models start with the International Reference Ionosphere (IRI) as a background ionosphere and assimilate digisonde observations (either the ionogram or the profile) and available GPS total electron content observations. The digisonde data from one site in the Republic of South Africa (RSA) provide the ionosonde assimilation data, while the other three digisondes in the RSA provide the ground-truth observations of foF2, hmF2, and the plasma frequency profile. Since the four RSA digisondes receive each other's transmissions, maximum observed frequencies have also been used as ground truth. The models tested have both been found to provide significant improvements over the IRI and to have similar accuracies for the study interval (September 2011). The errors of the models are very close to the minimum achievable errors for all the validation parameters, which seem to be set by the ubiquitous traveling ionospheric disturbances being different at the different locations. For the optimum ground-truth location ~685 km from the digisonde providing the assimilation data, the RMS errors in foF2 were found to be 0.2 MHz (night) and 0.5 MHz (day).