From AMS Glossary
A space weather term for a sudden eruption of energy in the solar atmosphere lasting from minutes to hours from which radiation and particles are emitted. Flares are officially measured and classified based on peak X-ray flux from 1-minute-averaged GOES X-ray Sensor (XRS) observations in the 1–8-angstrom passband. Classification generally begins at the "B" level (>0.000 000 1 W m−2) and increases by an order of 10 with each subsequent level to "C" flares, "M" flares, and "X" flares. Flare levels of B, C, and M are assigned multiplicative factors (1–9), which are appended to the letter (e.g., M4); flare levels of X are appended with a multiplicative factor from 1 to 20 (anything above 20 is estimated).
Space Weather Prediction Center, 2018: Solar flares (Radio blackouts). Accessed 14 August 2018. Available at https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts.
Term edited 14 August 2018.