SOLAR FLARE AND RADIO BLACKOUT: In the past 24 hours, sunspot AR2473 has quadrupled in size. On Dec. 23rd (00:40 UT) it erupted, producing an M4-classsolar flare. NASA's Solar Dynamics Observatory recorded the extreme ultraviolet flash:
UV radiation from the flare bathed the top of Earth's atmosphere, ionizing atoms and molecules, and altering the normal propagation of radio transmissions. Around the South Pacific Ocean, where the sun was high in the sky, a full-fledged shortwave radio blackout occured. This map from NOAA shows the affected area:
The type of people who would notice such a blackout are ham radio operators, mariners, and some aviators who use shortwave radio in their work. The event lasted approximately half an hour.
More flares and blackouts are possible in the days ahead as AR2374 turns toward Earth. NOAA forecasters estimate a 45% chance of M-class explosions on Dec. 23-24.