7.09.2017

Two big, naked-eye sunspot groups are putting on a splendid show this week. We're also in the crosshairs for a strong geomagnetic storm and possible auroras.

Filtered by thick fire haze, several large sunspots were easily seen through a telephoto lens shortly before sunset September 3rd.
Bob King

Wait a minute. Giant sunspots, multiple M-class flares, and a coronal mass ejection forecast to send the magnetosphere into a tizzy of Northern Lights? Wasn't the Sun supposed to be easing toward solar minimum?

Last Saturday, a solitary sunspot metastasized into a sprawling cluster big enough to see with the naked eye and solar filter in just 24 hours. Named Active Region (AR) 2673, it grew rapidly in magnetic complexity. Most sunspot groups are bipolar, like the two poles of a horseshoe magnet; one end of the group is magnetic north, the other end magnetic south, with their polarities clearly separated. AR 2673 developed a potentially explosive Beta-Gamma-Delta structure, where bits of dark umbra of opposite polarity mingle close together inside a spot's penumbra.

In the broiling, convective environment of the Sun's photosphere, their proximity increases the chances they'll reconnect — positive to negative, negative to positive — and release their pent up magnetic energy as powerful solar flares. That's exactly what happened Monday, when no fewer than seven moderate or M-class flares erupted inside the group followed by five more on Tuesday.

A coronal mass ejection (CME) spawned by sunspot AR 2673 began around 20h UT September 4th. This photo was taken with the LASCO C3 coronagraph on the orbiting Solar and Heliospheric Observatory at 22:30 UT on the 4th. Particles from the blast are expected to sweep by Earth this evening and whip up a strong geomagnetic storm beginning in the afternoon and continuing through the night. Click the image to see a time-lapse of the CME.
NASA / ESA

During one of these flares, AR 2673 blasted a coronal mass ejection (CME) into space in Earth's direction. When it arrives later this morning (September 6th), space weather forecasters expect it to couple with our planet's magnetic field and send a torrent of high-speed electrons and protons into the upper atmosphere to spawn moderately-strong to strong (G2 to G3) geomagnetic storms.

NASA's Solar Dynamics Observatory (SDO) snapped this photo of the Sun on Tuesday evening, September 5th. Sunspot regions are labeled. Click the photo to see the latest SDO photos.
NASA / SDO

You know what that means? A good shot at seeing the aurora borealis from the northern border states to as far south as Illinois and Oregon. While timing couldn't be worse — it's full Moon and western skies filled with smoke — a strong storm would likely still show up as bright arcs or balletic rays dancing across the northern sky. If AR 2673 continues spouting flares, auroras could also appear later in the week, when the Moon won't be as much of a problem. That's why I'd advise you to keep a lookout through the weekend.

Auroras or not, the Sun's looking lively these days in both white light and H-alpha. Another more benign but no less impressive sunspot group, AR 2674, has been "crawling" across the disk like a large caterpillar with a prominent, naked-eye leader spot and two large "back legs."

Still have your eclipse glasses? I hope you didn't make the mistake my mom did and toss them out after the eclipse, thinking they were for one-time use. I've been watching the progress of both AR 2673 and the leader spot of AR 2674 as they've inched across the rotating solar disk. Both appear as dark, round blemishes on an otherwise pristine Sun and should remain visible for another couple days. Seeing naked-eye sunspots reminds us that the ancient Chinese astronomers also used their unaided vision to record the comings and goings of sunspots; their first observations reach back to as early as the 4th century B.C.

Astrophysicist Karl Battams calls this his Earth-ometer. It's a handy way to visualize the sizes of the sunspots in AR 2673 and AR 2674. The big spot in AR 2673 is one of the year's largest.
Karl Battams

Large regions of the western and central U.S. have been under a veil of smoke haze from forest fires in Oregon, Washington, and Canada throughout the summer. On Sunday night, smoke so attenuated the near-setting Sun, I could see the spots directly without a filter.

The M5.5 flare that erupted in AR 2673 on September 4th is seen in multiple wavelengths of light, most in far UV through the SDO.

Views through even a 3-inch telescope capped with a safe solar filter are jaw-dropping; AR 2673 looks like a fiddler crab or a bird taking flight with an ever-changing array of umbrae, while AR 2674 possesses a great variety of spots from tiny pores to the leader with a diameter three times that of Earth! Check out our white-light solar guide to help you find additional features.

Steve Lemieux captured the start of the M5.5 flare in AR 2673 in H-alpha light. Click to see an animation that gives a good sense of what a flare looks like in an amateur scope.
Steve Lemieux

Even in white light, both groups change in subtle and sometimes not-so-subtle ways each day. If you have an H-alpha scope, changes in fibrils, filaments, and the sudden appearance of flares will keep you hopping minute to minute. But don't delay — the regions are already turning toward the western hemisphere, with only about a week of good viewing remaining.

To monitor possible auroras and their extent, be sure to check out the Aurora — 30 Minute Forecast.

** UPDATE: AR 2673 erupted with an extremely powerful X9.3 flare at 12:02h UT on September 6th that produced a coronal mass ejection partially directed toward Earth. Space weather forecasters have also extended the G2 and G3 storm alerts to Thursday and Friday nights September 7th and 8th. Auroras are highly likely before the week is out.

The X9.3 flare in AR 2673 produced a powerful flash in far ultraviolet light photographed here by SDO at 12:02h UT on September 6th.
NASA / SDO

Quelle: Sky&Telescope