Annales Geophysicae
Annales Geophysicae
ANGEO
Articles | Volume 43, issue 1
Articles | Volume 43, issue 1
https://doi.org/10.5194/angeo-43-91-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Special issue:
https://doi.org/10.5194/angeo-43-91-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Articles | Volume 43, issue 1
Regular paper
 | 
22 Jan 2025
Regular paper |  | 22 Jan 2025

Revisiting sunspot number as an extreme ultraviolet (EUV) proxy for ionospheric F2 critical frequency

Bruno S. Zossi, Franco D. Medina, Trinidad Duran, Blas F. de Haro Barbas, and Ana G. Elias

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Cited articles

Australian Bureau of Meteorology: Index of /wdc/iondata/au, Australian Bureau of Meteorology [data set], https://downloads.sws.bom.gov.au/wdc/iondata/au/ (last access: 16 January 2025), 2025. 
Balan, N., Bailey, G. J., and Moffett, R. J.: Modeling studies of ionospheric variations during an intense solar cycle, J. Geophys. Res.-Space, 99, 17467–17475, https://doi.org/10.1029/94JA01262, 1994. 
Bilitza, D., Pezzopane, M., Truhlik, V., Altadill, D., Reinisch, B. W., and Pignalberi, A.: The International Reference Ionosphere Model: A Review and Description of an Ionospheric Benchmark, Rev. Geophys., 60, e2022RG000792, https://doi.org/10.1029/2022RG000792, 2022. 
Brown, M. K., Lewis, H. G., Kavanagh, A. J., Cnossen, I., and Elvidge, S.: Future Climate Change in the Thermosphere Under Varying Solar Activity Conditions, J. Geophys. Res.-Space, 129, e2024JA032659, https://doi.org/10.1029/2024JA032659, 2024. 
Clette, F.: Is the F10.7 cm – Sunspot Number relation linear and stable?, J. Space Weather Spac., 11, 25 pp., https://doi.org/10.1051/SWSC/2020071, 2021. 
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This study explores how solar sunspot number (Sn) compares with other solar indicators like solar radio fluxes in predicting changes in Earth's ionosphere, particularly its critical frequency, over more than 60 years. The work finds that Sn, despite recent fluctuations in other proxies, remains the most stable predictor across all time periods. When adjusting for potential data saturation, Sn outperforms other proxies, providing a more accurate forecast of long-term ionospheric trends.
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Long-term trends in the stratosphere–mesosphere–thermosphere–ionosphere...