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

Data sets

Index of /wdc/iondata/au Australian Bureau of Meteorology https://downloads.sws.bom.gov.au/wdc/iondata/au/

Digital Ionogram Data Base (DIDBase) Lowell GIRO Data Center https://giro.uml.edu/didbase/scaled.php

Nobeyama Radio Polarimeters National Astronomical Observatory of Japan https://solar.nro.nao.ac.jp/norp/index.html

Manually scaled parameters National Institute of Information and Communications Technology https://wdc.nict.go.jp/Ionosphere/en/archive/isdj_manual_txt.html

Solar radio flux - solar monitoring program Space Weather Canada https://spaceweather.gc.ca/forecast-prevision/solar-solaire/solarflux/sx-en.php

Sunspot Number Sunspot Index and Long-term Solar Observations https://www.sidc.be/SILSO/datafiles

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Short summary
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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Special issue
Long-term trends in the stratosphere–mesosphere–thermosphere–ionosphere...