On Wednesday afternoon, a powerful X-class flare ripped through the Sun's lower atmosphere and sent a blast wave directly toward Earth that should arrive Friday and produce moderate-to-strong auroras over the weekend.
Solar physicists believe that the Sun's current 11-year-long activity cycle peaked weakly last year and has already started to decline. But today our star sent a powerful reminder that, paraphrasing Monty Python, "I'm not dead yet!"
On Wednesday afternoon, the Sun erupted with a powerful flare that peaked at 1:48 p.m. Eastern Daylight Time. From its perspective in space, NASA's Solar Dynamics Observatory captured the titanic explosion, which covered an area several times larger than Earth.
SDO time-lapse images, recording X-ray emission at 193 angstroms from highly ionized iron atoms, show the blast wave rippling across the entire solar disk:
According to NOAA's Space Weather Prediction Center (SWPC), the flare was an X-class event — the most powerful — and it unleashed a torrent of relativistic particles and X-rays that swept over Earth minutes later.
Meanwhile, observer Steven Souza (Williams College) happened to witness the flare while viewing the Sun through a telescope equipped with a hydrogen-alpha filter. "He called me a little after 1:30 to say I should hurry upstairs," recalls his colleague, solar expert Jay Pasachoff. "Looking through the telescope, I could see the normal red disk of the Sun, but in its midst I saw a tremendously bright 'S' shape. It was on the left side of a dark spot, presumably the underlying sunspot, and there was an arc more-or-less completing a circle on the right side. The S-shape reminded me of the 'sigmoids' that my Caltech post-doc advisor, Harold Zirin, had proposed as a sign of flare-worthy active regions."
A half hour later, the bright regions were still visible but less prominent. It was, Pasachoff notes, "the brightest H-alpha flare I have ever seen (in contrast with the rest of the solar disk)."
The X-1.6 eruption came from a sunspot group known as Active Region 2158, situated very near the center of the solar disk. "It's pretty clear there was a large coronal wave," notes C. Alex Young, an associate director in the Heliophysics Science Division at NASA's Goddard Space Flight Center, with a coronal mass ejection (CME) directed squarely at Earth.
Based on the initial SDO imagery, Young estimated that the CME could have a velocity of up to 2,000 miles (3,000 km) per second, very fast as solar storms go. Later imagery from the Solar and Heliospheric Observatory (SOHO) showed the expanding blast wave escaping as a rapidly expanding halo around the Sun's entire circumference — a clear indication that it's headed our way.
Later, however, Young sent Sky & Telescope an update: the CME's initial velocity was less than initially thought, about 875 miles (1,400 km) per second.
Timetable for Auroras
Therefore, the shock front now crossing interplanetary space should reach Earth on Friday morning. SWPC forecasters are predicting a moderate G2 geomagnetic storm on Friday and more robust G3 levels through Saturday. These could trigger displays of auroras over locations roughly northward of latitude 45° north (Oregon, Illinois, New England, and northern Europe).
Although the "northern lights" can appear anywhere in the sky, would-be aurora watchers should look first toward north. Most displays appear as shimmering greenish curtains of light suspended in the sky. Stronger auroras can appear tinged with blue or red.
AR 2158 was also responsible for a lower-intensity eruption yesterday. Although not headed directly at Earth, that predecessor could create a partial clearing in the interplanetary medium that would allow today's blast to reach us sooner that it otherwise would. The likely merger of these two shock fronts is the reason for the potentially stronger geomagnetic disturbance on Saturday.
Solar flares occur in areas of concentrated magnetic fields in and above active regions. Contorted bundles of field lines can reconnect suddenly and violently, releasing vast amounts of stored energy in an abrupt "explosion" that propels blobs of the solar atmosphere outward. A powerful "super storm" in July 2012, perhaps the most energetic in 150 years, directed a perilously strong CME into space that fortunately missed Earth.
Could Earth ever fall victim to a "solar superstorm"? Gets the odds of that happening — and the consequences if it does — in the February 2011 issue of Sky & Telescope.