The ratio of X-ray flux to radio flux, correlated to sunspot number
UV emissions at 304 angstroms, correlated to the solar flux index✓Bonne réponse
The solar wind velocity at an angle of 304 degrees from the solar equator, correlated to geomagnetic storms
The solar emission at 304 GHz, correlated to X-ray flare levels
Explication
The 304A figure measures solar output in the strong ionized-helium line at 304 angstroms in the extreme ultraviolet, which comes from the same active regions that drive the 10.7 cm radio emission and therefore tracks the solar flux index. The other options misread the number as a radio frequency, an angle, or a flux ratio.