A rich palette of X-ray and ultraviolet colors bursting from Earth’s nearest star is on display in a new image of the sun.
The image is a composite, with high-energy X-rays from NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, shown in blue. Green shows lower-energy X-rays from the X-ray Telescope instrument on the Japan Aerospace Exploration Agency’s Hinode spacecraft. NASA’s Solar Dynamics Observatory contributed the yellow and green colours, representing ultraviolet light.
As it exits the apex of a cycle that takes place every 11 years, the sun is in an active period with an abundance of flares and sunspots.
Dr Iain Hannah, of the University of Glasgow, said:
“We can see a few active regions on the sun in this view. Our sun is quietening down in its activity cycle, but still has a couple of years before it reaches a minimum.”
Tho active areas of the sun are packed with flares, massive eruptions on the surface of the sun which eject charged particles and high-energy radiation. They happen when magnetic field lines become tangled and broken, and then reconnect.
NuSTAR’s telescope cannot view the larger flares, due to its extreme sensitivity, but it can help measure the energy of smaller microflares, which generate only one-millionth the energy of the larger flares.
Hypothesised nanoflares, which would be only one-billionth the energy of flares, may also be able to be directly detected by NuSTAR. Nanoflares—which may help explain why the sun’s atmosphere, or corona, is so much hotter than expected—would be hard to spot due to their small size.
But nanoflares may emit high-energy X-rays that NuSTAR has the sensitivity to detect. Astronomers suspect that these tiny flares, like their larger sisters, can send electrons zipping around at astounding velocities. As the electrons fly around, they give off high-energy X-rays.
Hannah explains that astronomers still need the sun to quieten down more over the next few years to have the ability to detect these nanoflare. As our sun is approaching the quiet finale of its roughly 11-year activity cycle, it has been showing specious stints of high activity.
NuSTAR normally spends its time investigating the enigmas of black holes, supernovae and other high-energy objects in space. It can also look closer to home, however, to study our sun.
As Brian Grefenstette, an astronomer on the NuSTAR team, said:
“What’s great about NuSTAR is that the telescope is so versatile that we can hunt black holes millions of light-years away and we can also learn something fundamental about the star in our own backyard.”
Illustration: NuSTAR data shows X-rays with energies between 2 and 6 kiloelectron volts; the Hinode data, which is from the X-ray Telescope instrument, has energies of 0.2 to 2.4 kiloelectron volts; and the Solar Dynamics Observatory data, taken using the Atmospheric Imaging Assembly instrument, shows extreme ultraviolet light with wavelengths of 171 and 193 Angstroms. Note the green Hinode image frame edge does not extend as far as the SDO ultraviolet image, resulting in the green portion of the image being truncated on the right and left sides. Credit: NASA/JPL-Caltech/GSFC/JAXA