Using solar ultraviolet irradiance measurements from GOES/EXIS to nowcast flare X-ray properties.

X-rays emitted by solar flares change the properties of the Earth's ionosphere and can damage technological systems. Space weather forecasters monitor the X-ray irradiance from the Sun and issue warnings to mitigate these potentially harmful effects. Other wavelengths of solar irradiance are also observed operationally and might provide advance information about a flare's X-ray properties. We investigate whether extreme ultraviolet (EUV) emissions can provide advance information about the strength and duration of 0.1–0.8 nm soft X-ray (SXR) solar flares. Specifically, we assess the predictive capability of He I (121.6 nm; Lyman-alpha), He II (30.4 nm), and the Mg II index in relation to SXR flare characteristics, with the goal of improving flare nowcasting. We analyzed operational spectral irradiance measurements from the Extreme Ultraviolet Sensor (EUVS) and the X-ray Sensor (XRS) aboard the Geostationary Operational Environmental Satellite-16 (GOES-16). The dataset includes all M- and X-class flares observed between 2017 and May 2025. We examined correlations between the timing and magnitude of EUV and SXR peaks, including lead times, flare durations, and peak intensities. EUV emissions peak before SXR emissions in approximately 76% of cases. For He II, the average lead time is 4.69 minutes for X-class flares and 4.09 minutes for M-class flares. Lyman-alpha leads SXR emissions by approximately 4.98 minutes for X-class and 4.90 minutes for M-class flares, while Mg II leads by an average of 5.45 minutes for X-class and 4.75 minutes for M-class flares. He II shows the strongest correlation with SXR properties: flare durations correlate at r = 0.63 for M-class and r = 0.72 for X-class flares, while peak strength shows a moderate correlation (r = 0.53). He II enhancements exceeding 20% above background are strong indicators of X-class flares, with fewer than 1% of M-class flares exhibiting such increases. Lyman-alpha demonstrates moderate predictive value for flare duration (r = 0.49 for M-class and r = 0.63 for X-class) and a weak correlation with flare strength (r = 0.32). The Mg II index similarly correlates moderately with SXR duration (r = 0.48 for M-class and r = 0.67 for X-class) and weakly with flare strength (r = 0.43).