Beyond F10.7: A Comprehensive Validation of the Mg II Index as a Robust Solar EUV Proxy Using Multi-Satellite Observations

Solar Extreme Ultraviolet (EUV) irradiance is a critical driver of geospace variability, but its direct measurement is challenging. Consequently, proxy indices like the F10.7 cm radio flux have been used for decades. The Mg II core-to-wing ratio has been proposed as a superior proxy due to its formation in the chromosphere, closer to the origins of much EUV variability. This study reproduces and extends the seminal work of Viereck et al. (2001) by conducting a comprehensive, multi-decadal validation of the Mg II index using modern multi-satellite observations. We analyze data from the Bremen Composite, SORCE, and the GOES-R series to assess the Mg II index's performance against direct EUV measurements from TIMED/SEE and SDO/EVE, and compare it to the traditional F10.7 proxy. Our results confirm a strong correlation (R = 0.992) between the long-term Bremen composite and SORCE Mg II indices, validating their consistency. However, a significant discrepancy was identified with the GOES-R Mg II data (R = 0.108 with Bremen), indicating a critical calibration issue. The real-time proxy formula from Viereck et al. (2001) did not perform as expected in our expanded analysis, yielding a negligible correlation (R = -0.054). Rolling correlation analyses revealed that while both F10.7 and Mg II indices show moderate average correlations with EUV over a solar cycle (R ~ 0.40–0.47), their relationship with EUV irradiance is highly variable and complex. This study underscores the superiority of the Bremen and SORCE Mg II indices as reliable EUV proxies but highlights the necessity for careful cross-calibration between different instrumental sources and a re-evaluation of proxy algorithms for operational space weather forecasting.