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How is it possible that the area around the sun is about 200 times hotter than its surface?

This question is #6 from http://www.cracked.com/article_19668_6-scientific-discoveries-that-laugh-in-face-physics.html:

We intuitively understand the direction that energy travels -- from the thing with energy to the thing with less energy. That's why the second law of thermodynamics is among the first things you learn in science class that makes you say, "Well, I could have told you that." If you're too hot, you move away from the campfire, not toward it. You don't need science to tell you that heat energy travels from the hot thing to the less-hot thing. Well, everywhere in the universe except the sun.

There's a discrepancy between what science says should happen and what the sun actually does, and it's known as the sun's coronal heating problem. Essentially, when heat leaves the sun, the laws of thermodynamics just totally break down for a few hundred miles, and nobody can quite figure out why. The facts are pretty straightforward; the sun's surface sits comfortably at a blazing temperature of roughly 5,500 degrees Celsius. No problem there. However, as the heat travels from the sun's surface to the layer a few hundred miles away from its surface (known as the sun's corona), it rises to a temperature of 1,000,000 degrees Celsius. Which is 995,000 degrees Celsius, or 1,791,000 degrees Fahrenheit, or 1 billion gigawatts per 1/4 gigabyte jiggawatt hour (metric) hotter than it has any right to be.

He's a loose cannon! The heat source (the giant ball of nuclear explosions and plasma) should be the hottest thing, not the empty vacuum of space around it. This is the only instance in the known universe where the thing doing the heating is actually cooler than the thing it's heating. And it's been plaguing solar physicists worldwide since they discovered the little disagreement reality has with our universe in 1939. How is it possible that the area around the sun is about 200 times hotter than its surface?

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    $\begingroup$ The only instance in the known universe? How about a microwave oven? $\endgroup$
    – hdhondt
    Commented Apr 29, 2018 at 10:22
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    $\begingroup$ @hdhondt Those don't exist. $\endgroup$
    – Stack Exchange Supports Israel
    Commented Apr 30, 2018 at 0:23
  • $\begingroup$ "the only instance in the known universe where the thing doing the heating is actually cooler than the thing it's heating" doesn't make any sense. If I rub cold sandpaper on cold wood, they both heat up. Or a million other examples. $\endgroup$
    – StayOnTarget
    Commented Sep 27, 2018 at 11:42

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There is a nice article about this on the Scientific American web site. The simple answer is that we don't know why the corona is so hot though there are a few well established and plausible suggestions.

Some introduction:

The coronal temperature is not just a bit higher than the Sun's surface, it's getting on for a thousand times hotter - a few million kelvin as opposed to around $5,700$ K at the surface of the Sun. But what we mean by this is the atoms/ions in the corona have very high velocities. The corona is pretty tenuous. The pressure in the corona is around a million times lower than the pressure at the Earth's surface, so the corona is pretty close to a vacuum. Rather than thinking of the corona as a hot gas we should think of it as a region in which a low density of atoms/ions have been accelerated to high velocities. The question is then what is doing the accelerating?

An obvious suggestion is that the Sun's magnetic field is responsible for transferring energy to the corona. The magnetic field is subject to various forms of turbulence and since the ions in the corona interact strongly with the magnetic field it is quite plausible that turbulence in the field is accelerating the ions.

The other suggestion is that turbulence at the surface generates sound waves, in effect the sort of shock waves created by explosions on Earth, and it's these shock waves that transfer energy to the particles in the corona.

But right now we don't have the experimental data to tell which, if either, of these mechanisms is correct.

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  • $\begingroup$ It would make sense from an intuitive point of view to think that it was being coupled through magnetic fields from the interior of the sun where temperatures there exceed even 1E6K... but maybe that's not what's happening. $\endgroup$
    – Michael
    Commented Jun 4, 2021 at 22:40
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The "heating things hotter than itself" only applies to thermal radiation.

I'm not an expert, but the corona could be heated by absorption of high energy particles from within the sun, from it's magnetic field, or from a variety of non-thermal sources.

edit: Wikipedia has a description of the current theories http://en.wikipedia.org/wiki/Corona#Physics_of_the_corona

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Perhaps the outer layers of sun are transparent to thermal x-rays generated in the very hot core of the sun.

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    $\begingroup$ I don't think this holds together...the bulk of the star is a plasma, so it should be pretty opaque in all bands (due to Compton scattering off of the free electrons). $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jan 25, 2012 at 0:15
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Could be that the ejected plasma combined with intense magnetic field creates higher plasma velocities, pehaps momentarily greater than lightspeed, creating "lightless" sunspots while violently altering the electro-magnetic frequencies/pulses, resulting in a superheated corona. -AMFM-

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