Farther From the Fire, Hotter: The Sun's Unsolved Crown

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Step away from a fire and you get colder. Step away from the Sun's surface and the temperature jumps a hundredfold — from five thousand five hundred degrees to over a million. For eighty years physics has been unable to explain why, and a probe had to fly into that heat to look for the answer.

The farther you stand from a fire, the colder you get. On the Sun, the opposite happens: as you climb away from the surface, the temperature, instead of falling, rises a hundredfold. Physics does not forbid this. But for a hundred and eighty years, nobody has been able to say for certain how it happens.

August seventh, eighteen sixty-nine. In the American Midwest, in the field-scented little towns of Iowa, a handful of people have turned their telescopes to the sky and are holding their breath. A total solar eclipse is about to cut across the continent from one edge to the other. The observers are carrying a new instrument: the spectroscope. It passes light through a prism, splits it into its colours, and from the thin lines hidden inside those colours it lets you read off the identity of matter. In those years, the device was rewriting astronomy from the ground up. For the first time, you could say what a star was made of without ever touching it.

The moment the Moon covers the Sun's brilliant disc completely, the sky sinks into twilight and something appears: a pearl-white, silken, ragged crown wrapped around the Sun. The corona. Under ordinary conditions it is invisible, because the surface of the Sun drowns it in light millions of times brighter. Only during those few minutes of totality does it show itself.

In Burlington, Charles Augustus Young. In Des Moines, William Harkness. Independently of each other, they turned their instruments on the spectrum of the corona. And both saw the same thing: a bright, sharp, vivid green line. Its wavelength, five hundred thirty point three nanometres. Because it fell at number fourteen seventy-four on the scale Kirchhoff was using at the time, it was known for years simply as the fourteen seventy-four line.

The trouble was this. That line belonged to nothing. No element known in any laboratory on Earth had such a line in its spectrum.

Looking back from today, the first reaction that comes to mind would be that it must be a measurement error. But in eighteen sixty-nine there was no reason to think so, because exactly one year earlier the same thing had happened and had ended in a magnificent triumph. During the eclipse of eighteen sixty-eight, Janssen in India and Lockyer in London had each seen an unrecognised line at the Sun's edge, had attributed it to a new element, and had named it helium, from the Greek root for sun. Helium would not be found on Earth for another twenty-seven years. So there it was: an element discovered in the sky and only afterwards confirmed underfoot. The recipe had already worked once.

And so the green line was given a name of its own: coronium. The corona element. A mysterious substance present only in the Sun's crown and encountered nowhere on Earth. Even Mendeleev, the architect of the periodic table, at one point seriously entertained clearing a place for coronium at the very head of the table, reasoning that it had to be lighter than hydrogen.

Coronium waited seventy years for a seat that never came. In the time that passed, quantum mechanics was born, the inner architecture of the atom was worked out, and it became clear how electron shells emit light. And in the periodic table, no gap was left into which coronium could be fitted. However hard anyone looked, there was no room for a new element behind that green line.

The answer arrived in nineteen thirty-nine, from the German astrophysicist Walter Grotrian, working at Potsdam. Grotrian realised that the corona's red line was not a new element at all, but ordinary iron. Except not ordinary in the least: iron that had lost nine of its electrons. Within a few years the Swedish spectroscopist Bengt Edlén finished the job and identified that famous green line too. The source was iron again. This time, iron stripped of thirteen electrons.

This looks like the solution to a chemistry puzzle. It is not. It is the sentence that opens the door to a far larger one.

Because tearing thirteen electrons off an iron atom is no small matter. For that to happen, atoms have to collide with a violence that is hard to imagine. The temperature required is around two million degrees.

Now let us make the comparison. The surface of the Sun that we see with our eyes, the photosphere, is about five thousand five hundred degrees. The core, where the energy is produced, is fifteen million. So the heat source sits at the very centre. As you move outward from the centre, the temperature falls, and falls, and drops to five thousand five hundred at the surface. Up to this point everything agrees with intuition: the farther you move from the fire, the cooler you get.

Then the strange thing happens. Just above the surface, in the layer called the chromosphere, the temperature first dips a little further, down to around four thousand degrees, and then begins to climb. It rises into the tens of thousands. And then, in a wafer-thin layer called the transition region, no more than a few hundred kilometres thick, the temperature explodes. It leaps from hundreds of thousands into the millions. By the time you reach the corona, you are standing in one to three million degrees.

You are moving away from the fire and getting hotter. You step backward from the campfire and start to burn.

Let us close off one misunderstanding right away, because the direction of the astonishment matters. The corona is millions of degrees, but it is also unbelievably thin. The matter inside it is so sparse that it competes with the best vacuums we can make in a laboratory. Temperature is a measure of the average speed of particles, not of the total quantity of heat. Sparks from a sparkler are a thousand five hundred degrees, yet they do not burn your hand, because each spark contains almost no matter at all. The corona is like that. So the problem is not how the corona carries so much energy. The problem is far sharper than that: how does the energy get up there in the first place?

Because heat does not flow from cold to hot on its own. That is one of the most fundamental laws of thermodynamics. A surface at five thousand five hundred degrees cannot heat a gas at two million degrees above it, not by radiation and not by conduction. You cannot boil water by putting it in the refrigerator. Which means the energy reaching the corona does not travel there as heat. It travels in some other guise, a guise that only turns into heat once it arrives. And what that guise is has been, for a century and a half, one of astrophysics' most stubborn open questions.

Let us begin the search for the culprit the way any investigation begins: by working out how much energy is actually required. How much power does it take to hold the corona at millions of degrees?

Surprisingly little. In quiet regions a few hundred watts per square metre is enough. In the active regions, where magnetic activity is concentrated, the figure climbs to something like ten kilowatts. Set against the total energy streaming off the Sun's surface, that amounts to a few parts in a million for the quiet corona, and even in the most turbulent places only about one part in ten thousand. In other words, the Sun does this job while setting aside almost nothing from its budget. The problem is not a shortage of energy. The problem is delivering that energy to the right place, in the right form.

On the question of where the energy originates, there is no serious argument. Beneath the Sun's surface lies the convection zone: an enormous boiling soup. Hot plasma rises, reaches the surface, cools, and sinks. At the surface this appears to us as a simmering texture of cells called granules, each one roughly the size of Turkey. And this plasma drags the magnetic field lines along with it. The footpoints of those field lines are shoved, jostled, twisted and tangled without pause. That is the corona's energy source: the boiling beneath the surface, pumped upward through the magnetic field.

The fight breaks out after that. How does this energy travel up, and how does it turn into heat once it is there?

The first suspect is the idea put forward by Eugene Parker: nanoflares. Parker's reasoning runs like this. Because their footpoints are in constant random motion at the surface, the magnetic field lines in the corona become steadily more tangled with one another. Rather like the headphone cable in your pocket knotting itself. That tangling stores tension in the field. But magnetic field lines inside a plasma cannot endure unlimited twisting. At some point a critical threshold is crossed, the lines snap and reconnect, and the stored tension is released all at once. This is called magnetic reconnection. Parker calculated that such events would carry about a billionth of the energy of a large solar flare, and he named them nanoflares. One alone is negligible. But if the corona is crackling with these tiny detonations everywhere, all the time, the sum of them is enough to account for millions of degrees.

The second suspect is waves. The root of this idea reaches back to the Swedish physicist Hannes Alfvén, who was awarded the Nobel Prize for work showing that plasma threaded by a magnetic field can behave like a stretched wire, which is to say that it can carry waves. Picture a guitar string, except this string is made of plasma and held taut by the magnetic field. The boiling at the surface plucks and shakes that string constantly, and the vibration travels upward as a wave, into the corona. Energy is carried that way quietly, without any explosion at all.

The trouble with waves is not the carrying. It is the letting go. By their nature these waves are extremely resistant to damping. Instead of depositing their energy in the corona, they tend to pick it up and haul it off into space. So the hard part of the theory is reining the waves in at exactly the right place. There are proposed mechanisms: waves colliding with each other and collapsing into turbulence; neighbouring magnetic tubes oscillating at different rates and shearing against one another on microscopic scales; energy cascading into ever smaller structures until it finally becomes particle motion. Each one works mathematically. Which of them dominates on the real Sun is another question entirely.

So why can we not simply tell the two suspects apart? Because both produce more or less the same outcome: a hot, churning corona at millions of degrees. There are distinguishing signatures, but every one of them sits right at the edge of our resolution. If nanoflares are the answer, the heating should be impulsive: sudden, brief, local. It should leave behind tiny pockets of plasma that briefly exceed ten million degrees, and non-thermal accelerated particles. If waves are the answer, the heating should be smoother and more continuous, and the swaying of the magnetic tubes should be directly visible.

Both sides have found their evidence. The Japanese solar observatory Hinode, along with ground-based spectrometers, genuinely demonstrated the presence of waves in the corona and the chromosphere; magnetic tubes were recorded swaying, spicules were recorded trembling. On the other hand, telescopes sensitive to hard X-rays caught traces of unexpectedly hot plasma in regions that looked perfectly quiet, and that is the fingerprint of impulsive heating. The European Space Agency's Solar Orbiter, which began operating in twenty twenty, showed in its very first images countless tiny flashes scattered across the whole face of the corona, ranging from a few hundred to a few thousand kilometres across. They were christened campfires. They looked like the visual counterpart of the nanoflare idea. Yet the measurements revealed that most of these fires occurred higher up than expected, and that the total energy they carried might not be sufficient, on its own, to heat the quiet corona.

And here is the most maddening part of the whole business: the two suspects are not obliged to be rivals at all. When magnetic reconnection takes place, the field lines that snap and settle into a new configuration lash about like whips, and in doing so they generate precisely those waves. Nanoflares give birth to waves. Waves, in their turn, can collapse into turbulence, create small-scale current sheets, and trigger reconnection. Both suspects were at the scene of the crime, and in all likelihood they knew each other.

Past a certain point, looking from a distance was no longer enough. Somebody had to go and measure it in place.

On August twelfth, twenty eighteen, NASA launched a spacecraft it had named the Parker Solar Probe. The name belonged to Eugene Parker, the man who had predicted the solar wind theoretically and proposed the nanoflare idea; it was the first time NASA had named a mission after a living scientist. Parker was there in person that day, at ninety-one years old, to watch it go.

The central design problem for the probe was survival. At its front sits a shield two point four metres across and about eleven centimetres thick, made of carbon foam pressed between two carbon plates. During the closest passes the front face of that shield climbs to one thousand four hundred degrees, while the instruments directly behind it carry on working at room temperature. The probe was programmed so that on each approach it would use the gravity of Venus to draw its orbit a little tighter.

On April twenty-eighth, twenty twenty-one, something historic happened. At a distance of eighteen point eight solar radii, the probe crossed inside a boundary known as the Alfvén critical surface. That boundary defines where the solar atmosphere ends and the solar wind begins: inside it, the magnetic field rules the plasma and the particles are still bound to the Sun; outside it, the plasma drags the field along with it. The probe spent about five hours beneath that threshold. For the first time, humanity had touched the atmosphere of a star.

Then it went closer still. On December twenty-fourth, twenty twenty-four, the probe passed within six point one million kilometres of the Sun's surface, travelling at six hundred ninety-two thousand kilometres per hour. The records for the closest and the fastest human-made object were broken in the same instant. For two days nobody knew whether the spacecraft had come through Christmas Eve alive. When the signal arrived on the evening of December twenty-sixth, the team learned that the probe was perfectly fine.

So what was the answer?

From its earliest approaches, the probe saw something nobody had expected. The magnetic field lines streaming away from the Sun, instead of running straight, were in places carving a sharp letter S and folding back on themselves. The field direction would flip in an instant, then flip back. These were named switchbacks, and they turned out to be widespread throughout the wind.

A study published in twenty twenty-three traced that fingerprint back to its source. The distribution of the switchbacks the probe had measured matched the boundaries of the vast convection cells on the Sun's surface known as supergranules. Which is to say, the wind was not emerging from the surface as one smooth exhalation, but in intermittent clumps, out of funnel-shaped mouths at the cell edges where the magnetic field is concentrated. In the observations, the presence of high-energy ions and the shape of the energy distribution pointed to those clumps being produced by magnetic reconnection. What is more, the amount of energy released was on the scale required to drive the fast solar wind.

That is powerful evidence for the nanoflare camp. Reconnection is no longer a theoretical possibility; it is a phenomenon measured in place, in the Sun's immediate neighbourhood, with an energy budget you can calculate.

But the story did not end there. Data from the same probe also showed that the corona and the inner heliosphere are saturated with waves. The undulating fluctuations it measured were so pervasive and so energetic that they could account for much of the acceleration of the fast solar wind. In other words, the probe brought back evidence that both suspects are guilty.

And that is exactly where matters stand today. For most of the twentieth century, the coronal heating problem was framed as the question of which one is right. The data of the twenty-first century has turned that into the wrong question. What is asked now is this: in which region of the solar atmosphere, in which magnetic structure, does which mechanism dominate? In the active regions, where field lines form closed arches, impulsive reconnection appears to take the lead. In coronal holes, where the field lines open out into space, wave turbulence is the stronger candidate. And the transition region, just above the chromosphere, may be a crossroads where both are at work at once. Searching for a single answer was rather like trying to explain the climate of an entire continent with one equation.

It is worth adding that this matter does not stop at the Sun. The corona is not some quirk peculiar to our own star. Around every star with a magnetic field and convection we see hot, tenuous, X-ray-emitting atmospheres, and around some of them the emission is thousands of times brighter than the Sun's. The ferocious coronae of young, rapidly rotating stars are powerful enough to strip the atmospheres clean off the newborn planets around them. So understanding the mechanism that heats the corona does not merely satisfy an astrophysical curiosity; it is part of working out which stars can host planets capable of holding on to their air.

Closer to home, there is a directly practical stake. The processes that heat the corona are the same processes that accelerate the solar wind and hurl out great eruptions of plasma. When those eruptions reach the Earth, they disturb power grids, satellites, flight routes and positioning systems. A model that can calculate the heating of the corona is a model that can genuinely forecast space weather. At present our predictions can tell us hours in advance whether a storm is coming; how hard it will strike is still something we struggle to say.

This is where the road that began with that green line seen over Iowa in eighteen sixty-nine has arrived. We thought we had found a new element. We had not. We had found an unfamiliar state of a familiar one. We have been hunting for the energy that makes that state possible for more than eighty years, and by now we know who is responsible; what remains is who does how much of it, and where. Most of science moves this way in any case: the mystery is not dissolved, it gives way to a sharper, better-defined, more answerable mystery.

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