The sun fired off an X-class solar flare, the most powerful classification, this afternoon, from an Earth-facing sunspot called Active Region 2158. The flare was powerful enough to cause a sustained blackout of high-frequency radio communication here on Earth.

NASA's Solar Dynamics Observatory spied the flare this afternoon, beginning at 1721 and peaking at 1745 UTC (13:21 and 13:45 EDT, respectively).

The flare was accompanied by a coronal mass ejection (CME). NOAA forecasters have issued a geomagnetic storm warning for this Friday, September 12th, when SpaceWeather.com reports the CME is "expected to deliver a glancing but potent blow to Earth's magnetic field":

The storm could reach moderate intensity (G2-class) with auroras visible across northern-tier US states such as Maine, Michigan, and Minnnesota. Another CME could be following close on its heels if today's X-flare also launched a cloud in our direction. It all adds up to a high probability of geomagnetic storms in the days ahead.

We'll see how this storm compares to some of the bigger flareups in recent memory.

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