Why The "304A" number is a wavelength, 304 angstroms, in the extreme ultraviolet part of the spectrum. That line comes from ionized helium in the Sun's chromosphere and is measured by satellites such as SOHO and SDO. Its intensity tracks the 10.7 cm solar flux index closely, so it serves as another proxy for the solar activity that ionizes the F region and supports HF propagation.
Watch out The choice mentioning 304 GHz swaps the unit: an angstrom is a unit of length (1e-10 m), not frequency, and nothing about this index involves X-ray flares directly. The 304 degrees option invents a geometry that does not exist.
304 = angstroms, not GHz or degrees. Helium UV line, an SFI stand-in.