What is the strongest class of solar flare?

The strongest class of solar flare is X-class. Solar flares are ranked by their peak soft X-ray output, measured by satellites such as NOAA’s GOES spacecraft. The scale runs from A through B, C, M, and X, with each class representing a tenfold increase in intensity.

X-class flares produce peak fluxes of at least 10⁻⁴ watts per square metre. A number follows the letter to show the flare’s strength within that class: an X2 flare is twice as intense as X1, while an X10 is ten times as intense as X1. The scale can continue above X10 for exceptionally powerful events.

A common mix-up is assuming that X-class is the highest possible numerical category. It is actually the highest lettered category, but X-class flares can have different numerical ratings. The classification measures X-ray intensity, while the effects on Earth also depend on whether a flare produces a coronal mass ejection and whether that eruption is directed toward our planet.

The system was introduced in 1970 with C, M, and X classes; weaker A and B classes were added later as instruments became more sensitive.