We all know the sensation the Sun’s rays warming our faces. Its bountiful light and heat make life on our planet possible.
But our nearest star also provides us with what is literally the biggest mystery in the Solar System, spanning millions of miles. It is a puzzle so profound that, on the face of it, it challenges one of our most basic laws of physics.
Solving it may be the key to protecting our electronics and technology, not to mention our astronauts, from the worst effects of space weather: the gigantic storms of radiation and energy that occasionally burst from the Sun and collide with the planets, including Earth.
The mystery is that the Sun’s atmosphere, a gigantic gaseous envelope known as the corona, is hotter than the surface of the Sun itself, and not by a small amount. The Sun’s brilliant surface has a temperature of around 6000°C, whereas the extended corona soars to millions of degrees.
“It is very strange,” says solar physicist Dr Miho Janvier from the European Space Agency. “If you think about what we experience every day, the further you are from a source of heat, the colder it is. And so you would think the same thing would happen in terms of stars.”
But in the case of the Sun, it clearly does not. So, where does the extra energy come from that heats the corona?
The Sun’s nuclear heart
Before we get to the drama in the Sun’s outermost layer, we first need to understand what’s going on at its heart.
The Sun’s core is a natural nuclear fusion reactor of epic proportions, liberating the energy that powers the star.
Although comprising just three per cent of the Sun’s volume, the core contains more than one third of its mass. Gravity crushes it together, raising the temperature to a staggering 15 million°C (27 million°F).
At this temperature and density, atoms of hydrogen gas fuse together to become helium, releasing energy in the process. This diffuses through the rest of the Sun, heating it as it goes.
The further a layer is from the core, the cooler it is, dropping to ‘just’ 6,000°C (10,800°C) at the surface. Above this is a layer of the Sun’s atmosphere called the chromosphere, which extends to an altitude of around 3,000-5,000 kilometres (1,860-3,100 miles).
Initially, it too plays nice; the temperature drops to less than 4,000°C (7,200°F) as expected, but then something changes, and the temperature shoots up.
At the top of the chromosphere, the temperature rises to 35,000°C (63,000°F).
Moving through a transitional layer and into the tenuous corona beyond, the temperature skyrockets to more than one million degrees.
Clearly, there is an invisible energy source at play – but what?
Superheated gas is known to solar physicists as plasma. In the 19th century, astronomer Angelo Secchi observed jets of plasma, called spicules, shooting upwards from the Sun’s surface, directly injecting energised plasma into the corona.
“You start thinking, okay, maybe this can be why there's some transport of heat into the corona,” says Janvier.
Ultimately, however, modern analysis has shown that although spicules are ubiquitous, there are simply not enough of them to account for the corona’s temperature. There must be something else, even more powerful, at work.
Magnetic attraction
Beyond light and heat, the one other thing that the Sun produces in abundance is magnetism. The Sun’s magnetic field is colossal, conjured in its interior, the invisible field rises to break through the surface and reaches out into space, flowing through the entire Solar System.
“Magnetic fields are strange things. We don't have a lot of intuition about how magnetic fields carry energy, and the release of that energy is one of the critical parts of this whole story,” says Prof Tim Horbury, a solar physicist at Imperial College, London.
Although magnetic fields themselves are invisible, they trap plasma, which radiates at ultraviolet and X-ray wavelengths. By taking images at these wavelengths, we can trace out the magnetic fields.
But to get close enough to the Sun to see these structures in detail, we need very special spacecraft.
The two spacecraft that are currently doing this are ESA’s Solar Orbiter and NASA’s Parker Solar Probe.
They can survive because even though the corona is over a million degrees in temperature, the plasma is thinly spread out – similar to how you can put your hand in an oven, even when it’s at 180°C (350°F).
They do, however, still have to face temperatures exceeding 500°C (930°F) from the intense solar radiation and energetic particles they feel in their orbits. As a result, their onboard computers and instruments are at constant risk of overheating.
To survive, the missions use advanced heat shields. Solar Orbiter has a multi-layered shield made of titanium foil, while Parker Solar Probe employs a carbon-composite shield about 11.5cm (4.5 inches) thick.
Because it goes closer to the Sun, Parker’s shield has to withstand a maximum of almost 1,400°C (2,550°F).
Both spacecraft have sophisticated control systems that automatically keep their shields pointed sunward. Behind the shield, specially designed radiators dissipate heat, and finally, the electronics are insulated in order to reject any heat that does make it through the shields.
As a result of these extraordinary heat shields, the pair are returning the most detailed data ever, including ultraviolet and X-ray images to reveal the Sun’s magnetic field.
Janvier is a project scientist for ESA’s Solar Orbiter, launched in 2020, and the earlier ESA/NASA mission SOHO (Solar and Heliospheric Observatory). Solar Orbiter in particular carries an instrument known as EUI, the Extreme Ultraviolet Imager.
“EUI is making extraordinary images of the corona… an incredibly tangled volume of space, which is dominated by the magnetic field,” says Horbury.
Making waves
There are two main ways that a magnetic field can release energy into the surrounding plasma: the action of waves and a process called magnetic reconnection, both of which come from the Sun's magnetic field.
Although invisible, we can imagine a magnetic field as a series of lines. Known as field lines, they are revealed in the classic school experiment of placing a magnet under a sheet of paper and then scattering iron filings over the page.
In the presence of plasma, these lines can bend and twist. They can also carry waves along their length. These waves are known as Alfvén waves, after Hannes Alfvén, who calculated the theory of magnetic waves in the 1940s.
“The waves carry energy, propagate upwards and dump that energy into the plasma like waves breaking on a beach,” says Horbury.
In other words, they can heat the corona by making the plasma there more energetic, although the exact way the waves transfer their energy into the plasma is still a matter of ongoing research.
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Prof Richard Morton, from Northumbria University, recently made a breakthrough in the study of Alfvén waves using new instruments at the Daniel K. Inouye Solar Telescope (DKIST), a ground-based telescope on the Hawaiian island of Maui.
The new instruments provided more detailed images than before, allowing Morton and his team to observe a kind of twisting wave that had long been suspected to exist but never seen until now.
“I hadn't really expected to find it,” says Morton. That was because many people had looked before, but never quite seemed to have detailed enough instruments. This time, however, the recently upgraded instruments were good enough.
But by Morton’s own admission, there is a lot of work ahead to understand if these events can supply enough energy to fully heat the corona.
An explosive reconnection
The other process, magnetic reconnection, is something more explosive. Magnetic fields carry energy in the form of tension, rather like a stretched elastic band.
Magnetic reconnection occurs when the field suddenly reconfigures itself to become more stable, releasing all the pent-up energy in the process.
Magnetic reconnection is writ large on the Sun. It powers the solar flares that trigger huge eruptions of plasma from the corona.
These huge explosions provide the plasma with so much energy that they propel it clean out of the solar atmosphere and into space.
When these coronal mass ejections happen to collide with Earth they trigger the colourful aurorae that light the skies.
Recent work, however, has shown that smaller reconnection events are taking place all the time.
The very first images returned by Solar Orbiter revealed a multitude of small reconnection events driving miniature solar flares.
The scientists called them campfires, and they are ubiquitous. They take place all over the Sun all the time and could potentially be constantly feeding energy into the corona.
However, the complete chain of events has still not been revealed, even by the state-of-the-art spacecraft available today. So without a slam-dunk from either theory, the question remains: is it waves or reconnection that heats the corona? The answer is probably a bit of both.
“When you have those small reconnection events, the plasma is going to move, and this is going to generate waves. So it is not necessarily one or the other,” says Janvier.
Shaping the Solar System
Understanding the way in which the corona is heated has implications across our entire Solar System. Far from being a simple atmosphere surrounding the Sun, the solar corona provides the hot plasma that then travels outwards through space in something called the solar wind.
This perpetual sleet of coronal particles carries the Sun’s magnetic field out into space, creating a magnetic bubble called the heliosphere that surrounds the planets of our Solar System.
Understanding the heliosphere will also help scientists to study space weather – the name given to the gusts and storms of particles in the solar wind created by the solar flares and coronal mass ejections.
The electrical and magnetic effects of space weather can damage spacecraft electronics and even technology on Earth. And these particles are also dangerous to the health of astronauts.
To protect ourselves from these effects, we must understand the way in which the solar wind is accelerated away from the Sun, and that is intimately connected with the coronal heating problem because the hot corona supplies the raw particles that are then accelerated outwards.
Understanding the details of this acceleration is where NASA’s Parker Solar Probe comes into its own. Every 88 days or so, it dips deep down into the corona itself. Grabbing as much data as possible during these fast but frequent fly-bys. The audacity of the mission still takes Horbury by surprise.
“We are flying a spacecraft through the atmosphere of a star… it is an extraordinary thing to be able to say,” he muses.
And the results have been revelatory. Parker Solar Probe has shown that the solar wind is accelerated by magnetic reconnection events known as switchbacks, in which the magnetic field kinks back on itself.
So, have we finally solved the problem of heating the solar corona?
“It depends on your level of satisfaction in the solution,” says Morton. “I would say that once you have identified the energy source, you are most of the way there.”
And with the detailed study of waves and connections that is now taking place, the energy sources are indeed identified. All that remains is measuring the contribution of these sources and how they vary across different regions of the Sun.
But other details, such as how waves deposit their energy into the coronal plasma, will remain a subject of investigation, mainly because this is a process that is expected to take place on scales measured in metres, far below the sizes that even the best solar telescopes and spacecraft can currently see.
So plenty of work for the scientists still to do, and another reason for the rest of us to simply enjoy the warmth of the Sun and marvel at our closest star’s awesome power.
The spacecraft keeping an eye on the Sun
Solar Orbiter
Launched on 10 February 2020, ESA’s Solar Orbiter studies how the Sun creates and controls the magnetic bubble, known as the heliosphere, that surrounds the Solar System.
Solar Orbiter’s highly elliptical orbit brings it closer to the Sun than the innermost planet, Mercury, allowing high-resolution imaging of the corona.
Parker Solar Probe
Launched on 12 August 2018, NASA’s Parker Solar Probe was the first mission to actually fly through the Sun’s corona.
Using a sophisticated heat shield to withstand the extreme temperatures, it can spend short periods of time at less than 10 solar radii above the Sun’s surface. Its instruments measure magnetic fields, plasma and other energetic particles.
Solar Dynamics Observatory (SDO)
NASA’s Solar Dynamics Observatory launched on 11 February 2010 and now provides continuous, high-resolution observations of the Sun from its orbit around the Earth. Its instruments image the solar atmosphere in multiple ultraviolet wavelengths, and measure magnetic fields on the Sun’s surface.
SOHO
The Solar and Heliospheric Observatory (SOHO), launched on 2 December 1995 by ESA and NASA, is stationed directly between the Sun and the Earth.
Its instruments image the solar atmosphere and monitor the solar wind and corona. SOHO has provided long-term datasets on solar variability.
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