The April 2001 X20 Flare

What happened

At the time of its occurrence, the most powerful solar flare ever directly measured — an X20-class event from active region 9393. The flare was not directly Earth-directed, so geomagnetic effects were limited, but the X-ray output saturated GOES instruments and set the modern benchmark against which the still-larger Halloween 2003 X28+ flare was later compared.