What is the solar-flux index?

The solar-flux index (SFI) is a standardized measure of solar activity that is measured at a specific frequency, specifically 2800 MHz (a wavelength of 10.7 centimeters). It serves as a reliable proxy for the Sun's ultraviolet and X-ray emissions, which are responsible for ionizing the Earth's atmosphere. The SFI is a daily reading, not taken annually or averaged over six months. Furthermore, while the SFI highly correlates with the smoothed sunspot number (SSN), they are different metrics; the SFI measures actual RF energy at a specific band, whereas the SSN is a mathematical average of visible sunspots.