Why Has the Sun Gone So Quiet?
The sun's photosphere as observed by NASA's Solar Dynamics Observatory (SDO) on July 22. One sunspot is visible in the lower left and another sunspot group appears to be appearing on the western limb.
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.
Although the sun is our nearest star, we certainly don’t have it all figured out.
Take, for example, that last week or so — the sun’s disk has been mostly sunspot-free and the X-ray output of the sun has plummeted since the beginning of July. This is in stark contrast to all the fireworks in the first few months of 2014.
To make the whole matter even more confusing, the sun should be at its peak activity, but just one look at the various images from solar observatories show a quiescent, blank solar disk, save for one sunspot (as of July 22). What’s going on?
The best answer is: we don’t really know. However, that doesn’t come as a surprise to many solar scientists.
“It is weird, but it’s not super weird,” Tony Phillips, NASA solar physicist and author of SpaceWeather.com, told the LA Times. “To have a spotless day during solar maximum is odd, but then again, this solar maximum we are in has been very wimpy.”
The sun experiences 11-year cycles of activity. At its peak — known as “solar maximum” — the sun becomes a broiling mess of magnetism; huge arcs of magnetized plasma erupt from the surface, creating intense active regions. These regions peel back the hot chromosphere and photosphere (at the base of the sun’s multimillion degree corona), exposing the cooler plasma below. The contrast in temperature creates dark spots across the surface known as sunspots. The active regions are often the triggering points for explosive events like coronal mass ejections and solar flares — events that can impact Earth in subtle and not-so-subtle ways.
It is for this reason that solar astronomers count the number of sunspots to gauge how active the sun is. And at present, the sun ain’t that active. In fact, space weather forecasters give a mere 1 percent chance that the sun will kick off a powerful X-class flare any time soon.
If the sun undergoes fairly predictable cycles of activity, why haven’t we figured out why sometimes the sun just seems to run out of steam? According to C. Alex Young, solar physicist at NASA’s Goddard Space Flight Center, Md., it’s not necessarily something strange, we just haven’t been observing the sun for long enough.
“We’ve only been observing the sun in lots of detail in the last 50 years,” Young said. “That’s not that long considering it’s been around for 4.5 billion years.”
So although we know this is the weakest solar cycle on record, we may just be seeing part of a longer-term cycle that we haven’t been able to recognize as we haven’t been taking detailed notes of solar activity for long enough.
“It all underlines that solar physicists really don’t know what the heck is happening on the sun,” added Phillips. “We just don’t know how to predict the sun, that is the take away message of this event.”