Space weather forecasters had to estimate its intensity, X-22 on a scale that only goes to 20, after sensors on the NOAA satellite could no longer measure the flare that occurred at 5:52 p.m. EST.
The solar eruption caused static on radio frequencies used to navigate boats and planes.
Meanwhile, powerful clouds of charged gas particles from an intense solar storm arrived at Earth this weekend, producing aurorae at latitudes as far south as the Texas-Mexico border.
Aurorae, the result of the interaction between Earth's magnetic field and "plasma" or charged gas clouds emitted by the Sun, generally are best viewed in polar regions. But astrophotographer Chris Grohusko caught the northern lights on film near El Paso, Texas, around 4 a.m. local time on Saturday.
"The spikes would appear for a few seconds and then reappear," Grohusko said of the aurora, which frequently make unexpected and ghostly appearances. "I had to use averted vision sometimes for the spikes. But the overall glow looking north was a firm milky white glow lasting for at least an hour and a half."
The colors varied nationwide. Grohusko says an airline flight passenger traveling in the U.S. Southwest e-mailed him to say he saw a green glow in the sky and called this weekend's aurora "one of the most beautiful displays" of the northern lights.
Near Tifton, Georgia, Derwood Eadie's brother called him just before 2 a.m. local time Saturday to wake him up to look outside at the aurora. Eadie hopped in his truck to head north of town for better viewing.
"Just as I was passing under an I-75 overpass, I noticed three police cars pulled off the road with their lights flashing and the officers standing outside looking up at the glowing sky," Eadie wrote to SPACE.com. "I suppose a concerned citizen had called about the strange glow. I proceeded a little farther out and found a suitable dark dirt road, parked my truck, got out and was treated to an amazing sight."
"I could see a blurry streamer of red light coming up from the northern horizon, just east of Polaris and extending up about 45 to 50 degrees. Due east and west were additional bright areas as well. As the aurora faded, I noticed a dim silvery double streamer extending from near the northeastern horizon to a few degrees past the zenith. The two parts of the streamer seemed to converge overhead and gave the impression of Saturn's rings viewed from near edge-on, as the streamer seemed to curve in on itself where it came together.
"I only witnessed the last 15-or-so minutes of the aurora, but it is not one I will soon forget. As I headed home, I could just make out a very dim red glow to the east, but it was fading quickly."
And Jason Starr wrote SPACE.com to say he received a similar treat in San Jose, California with the northern lights cutting through the Big Dipper.
In the past few days, space weather forecasts have downgraded the chances for the geomagnetic storms that cause aurorae. But the Sun still is fairly active with solar flares and coronal mass ejections spewing more charged particles than usual, so auroral activity may not be over. The same goes for power-grid disturbances and possible disruptions in satellite function and service, although no reports of such troubles surfaced this weekend.
This weekend's solar storm is associated with the solar maximum, a peak in solar activity that recurs every 11 years. The Sun's activity during this period is hard to predict, but generally our star spews charged particles of a higher intensity and frequency for a period of one to two years around solar maximum.
This weekend's storm emerged when the largest sunspot group of the past decade developed on the solar surface facing Earth last week.
The sunspot group and its resulting solar flares and eruptions sent plasma traveling at speeds in excess of a million miles (1.6 million kilometers) per hour toward Earth. The size of the sunspot group, called sunspot 9393, is equivalent to the total surface area of 13 Earths.
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