X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory (SDO).
Image credit: NASA/JPL-Caltech/GSFC
The Psychedelic Anatomy of a Solar Flare: Photos
View Caption +
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.
NASA/SDO
View Caption +
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.
NASA/SDO
View Caption +
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.
NASA/SDO
View Caption +
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.
NASA/SDO
View Caption +
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.
NASA/SDO
View Caption +
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.
NASA/SDO
View Caption +
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.
NASA/ESA/SOHO
View Caption +
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.
NASA/ESA/SOHO
What’s the sun got in common with distant black holes? Well, at first glance, not a lot. But as this psychedelic solar portrait shows, there is one trait that the sun and black holes do have in common — the emission of high-energy X-rays.
Now NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, has turned its gaze from distant black holes and focused on our sun, producing the most sensitive measurement of high-energy solar X-rays ever achieved.
Long before NuSTAR was even launched in 2012, solar physicist David Smith, of the University of California, Santa Cruz, approached the NASA NuSTAR mission team to request that the space telescope spend some of its observing time looking toward our nearest star.
Shifting focus from the high-energy X-rays generated by supermassive black holes in the centers of galaxies millions of light-years away to the sun may seem strange, but only NuSTAR has the capability of sensing the faint high-energy X-ray flashes generated by small-scale solar flares — known as nanoflares — deep inside the sun’s atmosphere, or corona.
“At first I thought the whole idea was crazy,” said NuSTAR principal investigator Fiona Harrison of the California Institute of Technology in Pasadena, Calif. “Why would we have the most sensitive high energy X-ray telescope ever built, designed to peer deep into the universe, look at something in our own back yard?”
Staring at the sun is as an unhealthy proposition for space telescopes as it is for the human eye. NASA’s Chandra X-ray space telescope, for example, would be blinded if it turned its gaze toward the sun as our nearest star generates a broad spectrum of lower-energy X-rays. But NuSTAR is unique in that it only detects the highest energy X-rays (and doesn't see the low-energy X-rays Chandra is sensitive to) that are generated by powerful relativistic processes surrounding black holes.
And it is high-energy X-rays, which the sun very weakly radiates, that Smith is interested in. But why?
Solar physicists and space weather forecasters have been puzzled for decades as to why the sun’s corona is so hot. On comparison with the sun’s ‘surface’ — the photosphere — which has a temperature of a few thousand degrees Fahrenheit, the corona is (on average) 1.8 million degrees Fahrenheit (1 million Kelvin). That doesn’t make sense in our everyday experience; it would be like the air surrounding a light bulb being hotter than the bulb’s glass, a situation that completely violates basic thermodynamic laws — normally it gets cooler the further you step away from a heat source, not hotter!
So in an effort to explain this mysterious coronal heating phenomenon, solar physicists have arrived at two key theories that have some observational evidence. Magnetohydrodynamic waves — basically waves that travel from the sun’s interior and through the magnetized corona — are thought to resonate with the energetic coronal plasma, causing a heating effect. Another theory suggests that tiny ‘reconnection’ events in the magnetic field of the corona cause rapid heating of the coronal plasma, generating nanoflares. If these nanoflares occur throughout the corona, perhaps they act as sparks that maintain coronal heating to millions of degrees.
Nanoflares are predicted to generate high-energy X-rays, but they have so far proven illusive as we haven’t had the instrumentation to filter out all the noise.
“NuSTAR will give us a unique look at the sun, from the deepest to the highest parts of its atmosphere,” said Smith, who is also a member of the NuSTAR team. “NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares.”
As the above image shows, a weak signal of high-energy X-rays have been detected by NuSTAR, but much more observation time is needed before we can definitively say whether or not nanoflares are causing it.
In addition to studying the emissions from possible nanoflares, the NuSTAR team are also taking a punt at the possibility of detecting a dark matter candidate deep inside the sun. Known as axions, these hypothetical dark matter candidates may be generating anomalous high-energy X-ray emissions in the solar core. But that’s a long shot.
Source: JPL