From AMS Glossary
A prolonged period of fading or faded radio communications, primarily in the HF range from ionospheric changes because of increased solar activity, in particular solar flares of the C-class level or higher on the sunlit side of Earth. Radio blackouts due to solar flares can last from minutes to hours. Solar proton events can also cause long-term radio blackouts over the polar regions for days; these are known as polar cap absorption events (PCAs). Radio blackouts due to solar flares of the M-class level and higher are classified using the NOAA R-Scale.”
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.