Our Fierce Sun: 12 Months of Explosive Activity
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.
In a dramatic new composite image released by NASA, 23 high-definition observations of our nearest star have been compiled creating the mother of all solar portraits.
Observed by NASA’s Solar Dynamics Observatory (SDO), the images were collected between Jan 11, 2015 to Jan. 21, 2016 and it is very clear where the sun is most active. The bright band through the middle of the solar disk is 12 months worth of bright active regions in the lower corona — the multi-million degree solar atmosphere.
Through the sun’s approximate 11-year solar cycle, magnetic activity from the sun’s interior becomes more fierce, reaching “solar maximum” when solar flares and coronal mass ejections are commonplace. Space weather can have serious impacts on our planet around these times. These explosive events occur over active regions — regions of intense magnetic activity that cause intense heating of solar plasma. At the base of these active regions, sunspots dominate, where the hotter surface layers of the chromosphere are pushed aside, exposing the cooler interior. Active regions tend to erupt around the sun’s equator, a pattern that is highlighted here.
After the crescendo of solar maximum peaks, the sun’s global magnetic field begins to ebb, eventually reaching “solar minimum”, when the sun settles into a quiescent state. We are currently experiencing the downward slope to quiescence since the sun hit solar maximum around 2012-2013. But as you can see from this beautiful portrait, the sun certainly isn’t going quietly.
The Solar Dynamics Observatory has just started its sixth year of operations (it was launched on Feb. 11, 2010), continually staring at the sun, revealing never-before seen dynamics in the sun’s outermost layers and its extended corona. This particular series of observations were recorded using the AIA instrument’s 171 angstrom filter. The emissions from the corona at these wavelengths are produced by plasma heated to over 1 million degrees Kelvin (Celsius). Active regions are a hotbed of coronal heating and therefore appear bright to the 171A filter.
As a comparison to the composite image, here’s a raw single observation from SDO as captured earlier today: