On the determination of ionospheric electron density profiles using multi-frequency riometry

Funding Information: Acknowledgements. The authors wish to thank Andreas Kvammen, Theresa Rexer and Björn Gustavsson for their useful discussions. The authors also thank the reviewers for their positive responses and constructive criticism. The work by Derek McKay is partly supported by the Academy of Finland (project no. 322535). Juha Vierinen is supported by the Tromsø Science Foundation. The work of Antti Kero is funded by the Tenure Track Project in Radio Science at Sodankylä Geophysical Observatory and the University of Oulu. Noora Partamies is supported by the Research Council of Norway under CoE contract no. 223252 and a research grant contract (no. 287427). Publisher Copyright: © Copyright:Radio wave absorption in the ionosphere is a function of electron density, collision frequency, radio wave polarisation, magnetic field and radio wave frequency. Several studies have used multi-frequency measurements of cosmic radio noise absorption to determine electron density profiles. Using the framework of statistical inverse problems, we investigated if an electron density altitude profile can be determined by using multi-frequency, dual-polarisation measurements. It was found that the altitude profile cannot be uniquely determined from a "complete"measurement of radio wave absorption for all frequencies and two polarisation modes. This implies that accurate electron density profile measurements cannot be ascertained using multi-frequency riometer data alone and that the reconstruction requires a strong additional a priori assumption of the electron density profile, such as a parameterised model for the ionisation source. Nevertheless, the spectral index of the absorption could be used to determine if there is a significant component of hard precipitation that ionises the lower part of the D region, but it is not possible to infer the altitude distribution uniquely with this technique alone.Peer reviewe