What is the solar flux index?

The solar flux index (SFI) is a direct measure of the radio noise emitted by the sun at a wavelength of 10.7 centimeters, which corresponds to a frequency of 2800 MHz. This specific wavelength is monitored because it correlates highly with the amount of ultraviolet and X-ray radiation the sun produces. These UV and X-ray emissions are responsible for ionizing the Earth's upper atmosphere to create the ionosphere. Therefore, a higher solar flux index generally indicates stronger ionization in the F-region, leading to better long-distance high-frequency (HF) propagation. Distractors such as the American sunspot number or the highest useful frequency describe related but distinct propagation concepts, not the physical measurement of the 10.7-centimeter solar radiation.