Sun with spot on central meridian

Sunspot region 10808 (formerly known as 10798) is a huge blot easily visible to the naked eye through a safe solar filter. This image was taken by the Solar and Heliospheric Observatory at 6:24 p.m. EDT (22:24 UT) on September 13th.


(Updated Sept. 16). It's been an amazingly busy week for solar observers, considering that we're several years past the peak of the 11-year sunspot cycle. During the last nine days, a large, intensely active sunspot complex known as Region 10808 (or 808 for short) has exploded with nine X-class flares and many smaller M-class flares. These have altered the near-Earth environment and disrupted radio communications, and they set off beautiful auroras over much of North America on the morning of the 11th.

The spot group itself is large enough to see with the unaided eye through a safe solar filter. The Sun's rotation has carried it to the western side of the Sun's disk as of the 16th, and will carry it out of sight around western limb on about the 21st.

Prominence on limb

On September 7th the Sun's rotation was just bringing the active region around the eastern limb. At 5 p.m. EDT, Sky & Telescope assistant editor Sean Walker imaged this grand loop prominence above the region in hydrogen-alpha light, using a Coronado Personal Solar Telescope. The glowing gases trace magnetic lines of force.

Photo by Sean Walker.

The show began on September 7th, when the spot group first came into view around the eastern limb and promptly erupted with a very strong X17 flare. "This is a significant event," noted Cary Oler of Solar Terrestrial Dispatch in a Sky & Telescope AstroAlert. "Very few solar flares ever reach the X-class rating, and only a few each solar cycle exceed an X10 rating." In fact it was the fourth-largest flare recorded in the last 15 years.

"The sunspot complex is an old 'friend' by the name of Region 10798, which was responsible for producing periods of severe geomagnetic storming last month," continued Oler. After the region rotated out of view in late August and came back around the eastern limb two weeks later, it was assigned its new number. Few spot groups last for a full solar rotation.

Various magnetic and high-energy-radiation effects from strong solar flares can damage spacecraft, induce crippling current surges in power lines, and black out long-distance radio communications. And, of course, they can cause grand displays of the aurora borealis (northern lights), due to charged particles streaming down magnetic lines of force into Earth's uppermost atmosphere.

X-class solar flare in H-alpha light

While imaging the Sun in hydrogen-alpha light on Saturday, September 10th, Sky & Telescope assistant editor Sean Walker caught an X-class flare erupting in Active Region 10808. These three images were taken just a few minutes apart, at at 17:36, 17:41, and 17:53 Universal Time.

S&T: Photos by Sean Walker.

Oler again: "Region 10808 appears to be a magnetically complex monster, with at least one very probable strong delta magnetic configuration visible (opposite-polarity umbrae in a single penumbra). Such configurations are inherently less stable and are often associated with energetic solar flares."

The most visible effects have been the recent auroras. September and March are the best aurora seasons, due to favorable orientation of Earth's magnetic field with respect to the Sun's, so any coronal mass ejections that the Sun sends our way during these times can produce fine displays.

"The next 3 to 4 days will be critical," Oler predicted as of September 14th, "as this is the time that Region 808 is 'pointed' most directly toward the Earth. For spacecraft and their sensitivity to energetic protons (the radiation
environment), the most vulnerable period will be later this week when Region 808 begins to approach the western solar limb. At that time, the Earth should be best 'connected' to the magnetic field lines that emanate from Region 808."

For e-mail aurora-watch notices, sign up for our solar activity AstroAlerts.

New Hampshire aurora

On the morning of September 11th a coronal mass ejection from the Sun buffeted Earth's magnetic field, sparking displays of northern lights — like this one in the dark skies over central New Hampshire. 'The aurora rose and fell in intensity from around midnight all the way to dawn,' says Sky & Telescope editor in chief Rick Fienberg. 'It was quite a show! All shots are 10-second exposures with my Canon 20Da at ISO 1600 with the zoom lens set around 17 mm.' Click the image for more pictures.

S&T: Richard Fienberg.


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