The view from NASA's Solar Dynamics Observatory (SDO) of the stressed coronal loops emerging from AR2192 during the solar flare on Friday.
The sun may be an average star when compared to the menagerie of stars that exist in our galaxy, but to Earth and all life on our planet, the sun is the most important object in the Universe. However, regardless of its importance and close proximity, our nearest star holds many mysteries that continue to fox solar physicists after decades of modern studies with cutting-edge observatories. One of the biggest mysteries surrounding the sun is the underlying mechanisms that drive solar flares and coronal mass ejections (CMEs). Monday evening (EST), the sun reminded us that it hasn't quite finished with the current solar maximum (of solar cycle 24), unleashing a powerful X4.9 solar flare -- the biggest of 2014. An armada of space telescopes witnessed the event, including NASA's Solar Dynamics Observatory that can spy the sun's temper tantrums in astounding high definition. Shown here, 5 of the 10 filters from the SDO's Atmospheric Imaging Assembly (AIA) instrument are featured, showing the sun's lower corona (the solar multimillion degree atmosphere) through 5 wavelengths; each wavelength of extreme ultraviolet light representing a different plasma temperature and key coronal features -- such as coronal loops (highlighted here in the 'yellow' 171A filter) and ejected plasma that formed a CME.
At 7:13 p.m. EST (00:13 UT, Feb. 25) -- pictured here on the far left -- the active region (AR) 1990 was crackling with activity. Then, as magnetic field lines from the sun's interior forced together and through the solar photosphere, large-scale reconnection events occurred. Reconnection is a magnetic phenomena where field lines "snap" and reconnect, releasing huge quantities of energy in the process. At 7:44 p.m. EST (00:44 UT) -- second frame from the right -- a kinked coronal loop can be seen rising into the corona. At 7:59 p.m. EST (00:59 UT) -- far right -- solar plasma contained within the magnetic flux is accelerated to high energy, generating powerful x-rays and extreme ultraviolet radiation, creating the X-class flare.
The X4.9 flare was caught through the range of SDO fliters, including this dramatic view as seen through the 131A filter. The flare was so bright that photons from the flare overloaded the SDO's CCD inside the AIA instrument, causing the signal to "bleed" across the pixels. This bleeding effect is common for any optical instrument observing powerful solar flares.
Intense coronal activity is often associated with active regions -- the active lower corona is pictured here, left. In this case, the flare erupted from AR1990, at the limb of the sun. Also associated with active regions are sunspots, dark patches observed in the sun's photosphere (colloquially known as the sun's "surface") -- pictured right. The sun's cooler photosphere has been imaged by a different SDO instrument called Helioseismic and Magnetic Imager (HMI), which detects the intensity of magnetic fields threading though the sun's lower corona and photosphere.
In the case of AR1990, a large sunspot can be seen at the base of the coronal loops that erupted to generate the powerful flare. This is a prime example of how sunspots can be used to gauge solar activity and how they are often found at the base of intense coronal activity and flares.
The HMI monitors magnetic activity across the disk of the sun and can also generate a picture on the direction of the magnetic field polarity. In this observation of the sun's magnetic field around the time of the recent X-class flare, other active regions can be easily seen -- intense white and black regions highlighting where magnetic field lines emerge and sink back into the sun's interior in active regions.
The joint NASA/ESA Solar and Heliospheric Observatory (SOHO), which has been watching the sun since 1996, also spotted the flare, tracking a CME that was generated shortly after. Seen here by SOHO's LASCO C2 instrument, that monitors the interplanetary environment surrounding the sun for CMEs and comets, a growing bubble of solar plasma races away from the sun.
Approximately an hour after the flare, the CME grew and continued to barrel into interplanetary space. Space weather forecasters don't expect that this CME will interact with the Earth's atmosphere as it is not Earth-directed. This observation was captured by SOHO's LASCO C3 instrument -- an occulting disk covers the sun to block out any glaring effect. By combining observations by the SDO, SOHO and other solar observatories, the connection between the sun's internal magnetic "dynamo", the solar cycle, flares and CMEs, solar physicists are slowly piecing together what makes our nearest star tick, hopefully solving some of the most persistent mysteries along the way.
UPDATE (6:41 p.m. EDT): At 17:09 UT (12:09 p.m. EDT) today (Saturday), active region (AR) 2192 erupted with another X-class flare directed at Earth. This is the second powerful eruption in less than 24 hours to be triggered from the large sunspot that occupies the region. Today's flare registered at X1 on the solar flare Richter Scale, the most powerful class of flare, but weaker than Friday's X3-class flare. Further radio black-outs have been recorded on the daytime side of the Earth, but, once again, today's flare did not generate a significant coronal mass ejection (CME).
ORIGINAL: There was already a high probability that active region (AR) 2192 was going to erupt with a powerful solar flare, so it came as little surprise when, yesterday, the huge sunspot fired a powerful X-class flare right at Earth. And we sure did feel its impact.
The sun has a myriad of effects on Earth during intense solar activity. When a flare erupts in the lower solar corona, the radiation generated can cause extreme ionization in the upper atmosphere, interfering with the propagation of high-frequency radio waves, meddling with global communications. Signals from global positioning satellites (GPS) can be interrupted, air traffic communications can get patchy and the interference can even be measured by amateur radio operators.
On Friday at 21:40 UT (4:40 p.m. EDT), AR2192 erupted with an X3-class flare as the huge sunspot was facing Earth. Like looking down the barrel of a solar gun, the region crackled with X-ray and extreme-utraviolet (EUV) radiation that immediately washed over the Earth’s ionosphere. A “radio blackout” was reported across the sun-facing side of our planet, including much of the US.
With the immediate effects of the X-class flare (which is the most powerful class of flare) subsiding, solar scientists monitored the region for any trace of a coronal mass ejection that may have been associated with the flare. Coronal mass ejections, or CMEs, are magnetic bubbles of highly-energetic particles that are hurled into space from the sun’s lower corona. They may take hours or days to reach Earth orbit, but their impact on our planet’s magnetosphere can be dramatic.
However, it appears that yesterday’s flare did not launch a CME. In fact, none of the dozens of flares (all of lesser energies than yesterday’s event) AR2192 has produced have generated a CME, which is interesting.
Although CMEs and flares are thought to be triggered by a common phenomenon (magnetic reconnection in the lower corona), they are not necessarily triggered at the same time. A flare may occur without a CME and vice versa. But for an active region not to generate any significant CMEs, and yet still generate a large number of flares, is rare.
Needless to say, space weather forecasters will be studying this large sunspot — the largest sunspot seen on the sun for 24 years — until it rotates out of view to understand what is going on.