Why are the sun's outer rings so much hotter than the inner core?

Surrounding the sun is a layer of gas called the corona in plasma form with temperatures reaching over 3 million degrees.

That the sun is hot is nothing new. The surface of the sun is about 10,000 degrees Fahrenheit, enough to fry anything. But surrounding the sun is a layer of gas called the corona, a plasma that reaches temperatures of more than 3 million degrees.

And scientists are still trying to figure out why this outer layer is so much hotter than what's inside.

Picture 1 of Why are the sun's outer rings so much hotter than the inner core?

After decades of observation, the sun remains a great mystery to us. Photo: NASA SVS.

What puzzles scientists is this: Since the sun's heat source is at its center, the temperature should decrease as you move away from the center. But that's not what scientists observe. They can't explain why the corona surrounding the sun is so much hotter than the other layers of gas in the interior. And before we go any further, remember that temperature is actually a measure of how fast atoms are moving. So the approach most solar physicists take is to find ways to accelerate this material the way it moves in the hot corona.

The Eternal Mystery

Picture 2 of Why are the sun's outer rings so much hotter than the inner core?

The corona during a solar eclipse, when the rest of the sun is obscured. Image: NASA / Rami Daud.

Despite its heat, the corona is often invisible due to the Sun's intense brightness. Even advanced instruments have difficulty studying it without being overwhelmed by the light from across the Sun's surface. But that doesn't mean the corona's existence is a recent discovery. The corona, seen during rare total solar eclipses, has fascinated humans for millennia. In 1869, astronomers took advantage of the eclipse to peer into the Sun's outermost layers. They even used a spectrometer to get a glimpse of the elusive material in the glare, and discovered a strange green line that appeared to be an entirely new element: coronium. Seventy years later, scientists realized it was actually the familiar element iron, heated to an unprecedented temperature of millions of degrees. This is hundreds of times hotter than the temperature measured at the surface of the sun, and it's really confusing.

The original theory was that sound waves (think of the sun's material compressing and expanding like an accordion) were responsible for stirring up the corona in the same way a wave might hurl water at high speed toward shore. But solar probes have been unable to find waves with enough energy to account for the corona's heat.

So for nearly 150 years, the corona has remained one of science's little nagging mysteries: scientists are pretty sure that the temperatures they observe on the surface of the sun and in the corona are correct, and even more certain that the basic physics behind it suggests that the farther you are from a heat source, like a campfire, the colder the temperature gets. It's a fact that's certain, but it doesn't offer a good explanation.

How do magnets work?

Picture 3 of Why are the sun's outer rings so much hotter than the inner core?

The Sun emits powerful solar flares, in a composite image from a year of observations. Image: NASA GSFC/SDO/S. Wiessinger.

Part of the reason is that we don't understand many of the small events that happen on the sun. We know that the sun warms the earth and how it does it. But the scale of the materials and forces on the sun doesn't exist in a laboratory, and getting close enough to study them in detail is difficult.

Today, to answer most questions about the sun, we simplify the sun to a complicated magnet. The Earth also spins around its own magnetic field. But the Earth, despite its oceans and underground magma, is much denser than the sun, which is just a ball of gas and plasma. So the Earth spins more or less like a solid object.

The Sun is not like that. The Sun spins, but because it is not solid, its poles and equator spin at different speeds. The Sun also blows material up and down through its layers, like a pot of boiling water. The result is a tangle of magnetic field lines. The charged particles that make up the Sun's outer layers travel along these lines like trains on a high-speed rail. These lines tangle and reconnect, releasing huge amounts of energy (solar flares) or sending spirals of charged particles flying free into space at incredible speeds (solar winds). But it's likely that underneath what we see are near-constant microbursts of heat—tiny flares of tens of millions of degrees that, when combined, can raise the temperature of the corona.

The long-accepted explanation is waves. Heat, after all, is particles moving very fast. The faster they move, the hotter they get. In the same way that an ocean wave can send water crashing against a beach at high speeds, scientists have thought that waves traveling through the sun's interior could throw the sun's outer layers off. But for decades, scientists have been able to measure that acoustic waves (think of vibrations traveling through the sun as sound waves travel through air) don't carry enough energy to be a source. But the sun has many different types of waves, so other types of waves are being studied—including the Alfvén waves, which travel through plasma and along magnetic field lines, and are of most interest to solar physicists.

We have many satellites observing the Sun, but the Parker Solar Probe only began its mission earlier this year. It will continue observing the Sun until 2025. Scientists hope that by observing the Sun from the closest possible distance, we will find answers to the question of microbursts or Alfvén waves, or perhaps a complex combination of both.

Update 06 January 2025
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