The fact that the Sun’s corona is significantly higher in temperature than expected is a long-standing puzzle. New research shows this problem could be solved by mini solar flares, nicknamed ‘campfires.’
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The influence of the Sun over every aspect of the solar system simply cannot be overstated. Without our star, life on Earth could not possibly exist. Whilst the Sun may not literally be the center of the Universe, as we once believed, it most certainly is the center of our existence in a figurative sense.
It is little wonder then that one of the main aims of astronomy and space science has been to learn more about our star. And yet, despite stellar efforts from space agencies such as NASA and the European Space Agency (ESA), the Sun still holds some mysteries that are proving hard to solve.
Chief amongst these is the puzzle of why the Sun’s corona — the outermost part of its atmosphere — is so hot. This nebulous outer envelope of plasma has a temperature of almost one million degrees. This makes it monumentally hotter than the photosphere — what can roughly be described as the surface of our star — which has a temperature of between 5000 and 6000⁰C.
Whilst many attempts have been made to explain this disparity in temperature, thus far solar scientists have been unable to reach a decisive conclusion. That is a situation that could be about change thanks to the ESA’s Solar Orbiter and its 2020 discovery of mini-solar flares.
A team of scientists, including Yajie Chen, a Ph.D. student from Peking University, China, and Professor Hardi Peter from the Max Planck Institute for Solar System Research, Germany, have developed computer simulations of these diminutive flares — nicknamed campfires — that reveal the physics behind them. The simulations show that these campfires are driven by a process that may also be responsible for heating the corona.
“Our model calculates the emission, or energy, from the sun as you would expect a real instrument to measure. The model generated brightenings just like the campfires. Furthermore, it traces out the magnetic field lines, allowing us to see the changes of the magnetic field in and around the brightening events over time, telling us that a process called component reconnection seems to be at work.”
Professor Hardi Peter, Max Planck Institute for Solar System Research
The results were discussed during a session held at European Geosciences Union (EGU) General Assembly on Tuesday, and are detailed in series of papers published in the latest edition of the journal Astronomy & Astrophysics.
The Coronal Heating Problem
The Sun’s corona is an ultra-hot envelope of plasma that extends millions of kilometers into space. Despite its impressive temperature it actually is not visible from Earth apart from under circumstances like an eclipse or with a device called a corona scope.
This is because the emission of light from the corona is usually overwhelmed by light emitted by the photosphere, meaning that in order to see it, something needs to block out that more powerful emission.
To consider why the higher temperature of the corona is such a puzzle, it is worth bearing in mind a few things. Conventionally, we would consider a body to be hot at its center and then gradually cool moving outwards, getting much cooler as the distance away from the body increases.
The Sun and its counterpart main sequence — hydrogen-burning — stars are not like this. The further from the surface you move; the hotter they get.
Not only do temperatures increase in step with distance from the main body of the sun, the density of particles steadily decreases. The atmospheric pressure in the corona is much less than even the pressure found in Earth’s atmosphere — around 0.1 to 0.6pa in comparison to about 100 thousand pa.
Researchers are pretty sure that the temperature disparity has something to do with the Sun’s magnetic fields, but the precise mechanism has proved evasive.
This has led to a major part of the ESA’s Solar Orbiter’s mission; probing deeper into the corona than ever before to finally solve this mystery.
The Solar Orbiter Discovers the Sun’s Campfires
The first images from the Solar Orbiter — the most complex scientific laboratory ever sent to the Sun — were released in July 2020. The images — taken by the Solar Orbiter’s Extreme Ultraviolet Imager (EUI) — revealed over 1500 small flaring loops and erupting bright spots on the Sun’s surface. These miniature eruptions had been speculated about before, but this was the first time we had caught stellar campfires in action.
From this data, researchers were able to determine that these campfires are short-lived — lasting between 10 and 200 seconds — and have a footprint that covers between 400 and 400 kilometers.
Solar scientists were also able to determine that small and weak events of this nature were the most common and reveal a hitherto unimagined fine structure on the surface of the Sun. And it is within this fine structure, that the solution to the mystery of coronal heating could be hidden.
The model suggested by the team involves a type of magnetic reconnection called component reconnection. Whilst normal magnetic reconnection occurs when magnetic field lines of opposite direction break and then reconnect releasing energy, component reconnection occurs between field lines that are near parallel.
This means reconnection occurs at small angles causing small flares, and the team’s simulations show that these flares release just enough energy to drive up the temperature of the solar corona.
Staring at the Sun
The team’s model has now been applied to seven bright events created in their simulation which match up with the largest campfires spotted by the solar orbiter.
The next step for the research will come in November 2021 when the ESA’s solar orbiter is fully operational. This will take the form of unification between the craft’s Polarimetric and Helioseismic Imager (PHI) and Spectral Imaging of the Coronal Environment (SPICE) spectrograph.
The PHI will monitor changes in the Sun’s magnetic fields whilst SPICE measures the corona’s temperature and density.
Further insight into solar campfires will be granted by collaboration with NASA’s Solar Dynamics Observatory with the craft uniting to triangulate the height these mini-flares reach into the Sun’s atmosphere.
“To our surprise, campfires are located very low in the solar atmosphere, only a few thousand kilometers above the solar surface, the photosphere,” says David Berghmans, Principal Investigator of EUI. “Even though campfires look like small coronal loops, their length is on average a bit short for their height, suggesting we only see part of these little loops. But our preliminary analysis also shows that campfires do not really change their height during their lifetime, setting them aside from jet-like features.”
“It is very early days, and we are still learning a lot about the campfire characteristics.”
Whilst the team is happy with their results and their connection to the coronal heating problem, they do caution that it is still very early days for this model. This means there is still a need to explore alternatives.
“In one of our case studies, we find that the untwisting of helical magnetic field lines winding around a common axis initiates the heating instead. It’s exciting to find these variations, and we’re looking forward to seeing what further insights our models bring to help us improve our theories on the processes behind the heating.”
Professor Hardi Peter, Max Planck Institute for Solar System Research
Chen. Y., et al, , ‘Transient small-scale brightenings in the quiet solar corona: A model for campfires observed with Solar Orbiter,’ Astronomy & Astrophysics, [DOI: 10.1051/0004–6361/202140638]
Berghmans. D., et al, , ‘Extreme-UV quiet Sun brightenings observed by the Solar Orbiter/EUI,’ Astronomy & Astrophysics, [DOI: 10.1051/0004–6361/202140380]
Green. S. F., Jones. M. H., , ‘An Introduction to the Sun and Stars,’ Cambridge University Press.