Abstract
The first part of this paper reviews methods using effective solar indices to update a background ionospheric model focusing on those employing the Kriging method to perform the spatial interpolation. Then, it proposes a method to update the International Reference Ionosphere (IRI) model through the assimilation of data collected by a European ionosonde network. The method, called International Reference Ionosphere UPdate (IRI UP), that can potentially operate in real time, is mathematically described and validated for the period 9–25 March 2015 (a time window including the well-known St. Patrick storm occurred on 17 March), using IRI and IRI Real Time Assimilative Model (IRTAM) models as the reference. It relies on foF2 and M(3000)F2 ionospheric characteristics, recorded routinely by a network of 12 European ionosonde stations, which are used to calculate for each station effective values of IRI indices \(IG_{12}\) and \(R_{12}\) (identified as \(IG_{{12{\text{eff}}}}\) and \(R_{{12{\text{eff}}}}\)); then, starting from this discrete dataset of values, two-dimensional (2D) maps of \(IG_{{12{\text{eff}}}}\) and \(R_{{12{\text{eff}}}}\) are generated through the universal Kriging method. Five variogram models are proposed and tested statistically to select the best performer for each effective index. Then, computed maps of \(IG_{{12{\text{eff}}}}\) and \(R_{{12{\text{eff}}}}\) are used in the IRI model to synthesize updated values of foF2 and hmF2. To evaluate the ability of the proposed method to reproduce rapid local changes that are common under disturbed conditions, quality metrics are calculated for two test stations whose measurements were not assimilated in IRI UP, Fairford (51.7°N, 1.5°W) and San Vito (40.6°N, 17.8°E), for IRI, IRI UP, and IRTAM models. The proposed method turns out to be very effective under highly disturbed conditions, with significant improvements of the foF2 representation and noticeable improvements of the hmF2 one. Important improvements have been verified also for quiet and moderately disturbed conditions. A visual analysis of foF2 and hmF2 maps highlights the ability of the IRI UP method to catch small-scale changes occurring under disturbed conditions which are not seen by IRI.












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23 November 2017
The sentence that reads as “It has been demonstrated…very disturbed periods.” should read as “It has been demonstrated (Mirò Amarante et al. 2007) that the ionosphere shows small scale variations that global or regional empirical models describe hardly, especially for very disturbed periods.”
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Acknowledgements
This publication uses data from 14 ionospheric observatories in Europe, made available via the public access portal of the Digital Ionogram Database of the Global Ionosphere Radio Observatory in Lowell, MA. The authors are indebted to observatory directors and ionosonde operators for heavy investments of their time, effort, expertise, and funds needed to acquire and provide measurement data to academic research. GAMBIT Consortium is acknowledged for providing access to IRTAM computations. The IRI team is acknowledged for developing and maintaining the IRI model and for giving access to the corresponding Fortran code via the IRI Web site (http://irimodel.org/).
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A correction to this article is available online at https://doi.org/10.1007/s10712-017-9453-z.
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Pignalberi, A., Pezzopane, M., Rizzi, R. et al. Effective Solar Indices for Ionospheric Modeling: A Review and a Proposal for a Real-Time Regional IRI. Surv Geophys 39, 125–167 (2018). https://doi.org/10.1007/s10712-017-9438-y
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DOI: https://doi.org/10.1007/s10712-017-9438-y