A solar flare is an intense burst of radiative energy across the entire EM spectrum, with the largest enhancements in the X-ray, EUV and radio portions. Powerful flares are often, but not always, accompanied by a CME. Flares are labelled with a letter and number. The letter is the flare class and indicates the peak intensity in W/m2 of X-rays in the 1-8 Ångstrom wavelength range as measured at Earth by the GOES spacecraft. The weakest one is A class followed by the classes B, C, M and X, each letter represents a 10-fold increase in energy output. Within each letter class there is a finer scale from 1-9 although X-class flares could be higher.