Nanoflares and Coronal Heating

Up here in the nosebleed section where we live (9200 feet or 2818 m) in the mountains, the evenings are getting distinctly chilly — reminding us that autumn for Northern Hemisphere folks is just around the corner (well, officially in September).  Fortunately, it warms up during the day, due to that local star known as the Sun. Sol. Or, in the names of some of the ancient religions: Amaterasu, Apollo, Helios, Freyr, Garuda, Huitzilopochtli, Inti, Liza, Lugh, Ra, Tonatiuh.  Or, like I said, the Sun.

Our star doesn’t exactly have a scientific designation like other stars do.  For example, Sirius — one of the closer stars to us, is also called Alpha Canis Majoris. Betelgeuse, the giant star in the constellation Orion, which lies somewhere around 600 light-years from us, is also called Alpha Orionis.  Other stars have simply letter and number designations, such as HD 189733 — a star with a planet that has methane in its atmosphere. Despite the lack of an official name, scientists have been studying the Sun as diligently as they do other stars — all in an effort to understand what makes it tick. What they learn helps them understand other stars — and conversely, sometimes what they see going on at other stars helps them figure out things about our nearest star.

Even though they’ve charted its cycles and measured the Sun’s surface temps for years and years, some aspects of the Sun’s behavior and characteristics have been tough for astronomers to understand as well as they’d like. Take its corona, for example. The corona is this diaphanous (that is, thin) region of rarefied VERY, VERY hot gases that stretch out from well above the surface of the Sun. The corona is made up of huge coronal loops that are shaped by magnetic fields. Those fields form something like a “bottle” or “tube” that guides superheated gases called plasmas.  Still, there’s only so much heating that can be caused by these flux tubes.  How did scientists explain the ten-million-degree temperatures commonly measured in the corona?  Keep in mind that the surface of the Sun — the part we can see — is only 5700 degrees Kelvin. Something wasn’t adding up. Clearly there were processes causing the corona to heat up so much, but what were they?

To unravel the mystery, scientists began looking for things that would cause heating — and one culprit is the action of magnetic fields. The corona is made up of loops of hot gas that arch high above the surface. The loops themselves are actually bundles of smaller, individual magnetic tubes or strands. The action of twisting magnetic fields can heat gas to incredibly high temperatures very fast. Add in something called nanoflares, and suddenly there’s an understandable reason why the corona gets so hot.

Nanoflares are small, sudden bursts of energy that occur inside the thin magnetic tubes in the corona. These flares can’t be seen through the usual panoply of satellite detectors and ground-based solar telescopes because they are too small to be detected. Solar astronomers have to measure the combined effect of many nanoflares occurring at the same time.  A group of astronomers at the NASA Goddard Space Flight Center studied the corona using the X-Ray Telescope and Extreme Ultraviolet Imaging Spectrometer on the Japanese Hinode satellite. They were able to measure the effects of the nanobursts and then created a computer model to explain how such bursty little flares can heat the corona.

The idea is that when a magnetically bound tube or strand erupts in a nanoflare, which releases a great deal of energy, the plasma in nearby low-temperature, it kind of sets off a feedback reaction that involves heat flows between regions of low and high-density gas. Low-density magnetic strands become very hot—around 10 million degrees K—very quickly.  The density remains low, so the emissions from the flare aren’t very bright — which is why they are difficult to detect using conventional means.  During the process, heat flows from up in the strand, where it’s hot, down to the base of the coronal loop, where temperatures are not as hot.  But, they get hot pretty quickly at the base, where things are a bit denser. Eventually the base temperature reaches about a million K, and begins to flow up the strand.  What you end up with is a coronal loop that is really a collection of faint, very hot (5-10 million-degree K) strands and some accompanhing 1 million degree K strands that are much brighter.

So, why do these nanoflares matter to solar scientists?  For one thing, it’s very cool (no pun intended) to solve the mystery of why the corona gets so hot because it helps us understand our star. But, there’s another, more selfish reason:  what happens on the Sun doesn’t just stay on the Sun — it affects us here on Earth, too.

Nanoflares are responsible for changes in the x-ray and ultraviolet (UV) radiation that are emitted as an active region evolves  on the Sun. Those emissions come blasting out through the solar system and eventually reach our planet.  X-ray and UV get absorbed by Earth’s upper atmosphere, which heats up and expands. Changes in the upper atmosphere can affect the orbits of satellites and space debris by slowing them down, an effect known as “drag.” It is important to know the changing orbits so that maneuvers can be made to avoid space collisions. The x-ray and UV also affect the propagation of radio signals and thereby adversely affect communication and navigation systems.  So, we’ve got at least two good reasons for wanting to know about what happens on the Sun — and I’m sure there’ll be more as astronomers unravel more mysteries about our star’s ongoing behavior.