WHAT I REALLY WANT TO KNOW IS…
Why is the Sun’s corona so hot?
The Sun is the one star astronomers can study close up. Features such as sunspots and prominences are obvious. But one characteristic has remained a mystery since it was discovered by X-ray satellites in the 1970s: that the Sun’s outer atmosphere, or corona, is more than a hundred times hotter than the visible surface, or photosphere. That’s counterintuitive, surely?
The photosphere reaches temperatures of around 5,000°C, but this soars to a million degrees or more in the corona. The mystery is one of the greatest challenges for solar modelling, and I have been leading a team trying to understand the physical mechanisms behind it.
From Earth, we can only see the delicate glow of the Sun’s corona during a total solar eclipse because it is so faint. So solar astronomers turn to dedicated Sun-watching satellites in space, which are able to observe the Sun in a range of wavelengths. Our own work has largely used NASA’s Solar Dynamics Observatory (SDO) which has been investigating activity on the Sun since 2010.
In particular, we need to view the Sun in extreme ultraviolet (EUV) light to see how the temperature is distributed within the corona. In addition, we have to make measurements of the photosphere’s magnetic field to compute electric currents.
Invisible forces
The corona is not only hot, it also looks structured when observed in EUV. On any high-resolution photo of the corona you will see a lot of loops and bright, arc-like structures. Such loops trace the magnetic field structure in the Sun. What is puzzling is that next to a bright loop you can find similar magnetic field lines that are less visible because they are not filled with glowing plasma.
With the aid computer modelling and our SDO observations, we created a 3D replica of an active region on the Sun. This revealed regions in the corona known as magnetic flux tubes – concentrations of magnetic fields – which carry an electrical current. We were surprised to find that the flux tubes carrying the strongest electrical currents were bright in our model but very faint or invisible in actual observations of the Sun. Something unexpected was going on. The presence of strong electric currents tells us that there is more energy, and presumably more heat, in those flux tubes. So it was puzzling that the emissions were not brighter. We already knew that flux tubes contain elevated levels of heavy metal ions in considerably greater proportions than found in the photosphere. What we discovered was that in the loops carrying a reasonably strong electric current, the iron ions reside in what we call ‘ion traps’ at the base of the loops. The existence of these traps implies that there are other highly energetic coronal loops, depleted of iron ions, which have so far eluded detection in the EUV range. Only metal ions, with their fluctuating electrons, produce emissions which make them visible, which could be why the loops were dimmer than expected. Though there are various theories, no one can yet fully explain how energy is carried along magnetic field lines into the corona to produce such incredible levels of heat. One thing that’s clear is that before we can find out more about how this energy is generated, we must do more to map and quantify the corona’s thermal structure. Our observations suggest that the corona may contain even more thermal energy than is directly observed in the EUV range. However, this energy visible in other wavelengths. We plan to do more to investigate with the aid of instruments aboard two other solar observatories, Japan’s Hinode satellite and NASA’s Interface Region Imaging Spectrograph (IRIS), as well as millimeter and sub-millimeter data from the ALMA telescope in Chile. These will provide us with a more detailed picture of what is going on in the Sun’s corona, and help give us a greater physical understanding of the processes involved.