The temperature on the surface of the sun is about 6,000°C, while the atmosphere reaches a million degrees! We tend to imagine, as on Earth, that we lose heat as we enter the atmosphere. How to explain this paradox?
Many scientists study this question. If the answer is not yet clear, several hypotheses exist about the origin of the energy that heats the Sun’s atmosphere, and this could be related to our star’s magnetic field.
The temperature of the sun
Heat is created in the center of the sun, in the core, where the temperature reaches 27 million degrees Celsius. And just like when you move away from a campfire, the temperature decreases as you move away from the core.
The sun’s surface temperature is around 6,000°C, meaning it is much colder than the core. In addition, the cooling continues for a short distance above the surface.
But higher up in the atmosphere, the temperature suddenly rises to more than a million degrees! So there must be something warming the sun’s atmosphere. But it’s hard to know what it is.
The magnetic field of the sun
The most popular hypothesis among experts is that our star’s magnetic field draws energy from the Sun’s interior through its surface to the atmosphere.
Just like the Earth, the sun has a magnetic field. Let’s imagine the magnetic field as invisible lines connecting the north and south poles of a star or planet.
We can’t see magnetic fields, but we know they exist because we have objects that interact with them. For example, a compass needle on Earth will always point to the North Pole because it is aligned with the Earth’s magnetic field.
The sun has a north pole and a south pole, but its magnetic field behaves differently from Earth’s and looks much messier. On the surface of the Sun, the magnetic field lines look like many loops rising from the surface into the atmosphere, and these loops are constantly changing.
When the loops touch, they can cause sudden bursts of enormous amounts of energy that heat the atmosphere. We also know that waves travel along magnetic field lines and transport energy. Could they be responsible for warming the atmosphere?
Is it a combination of waves and explosions, or something completely different? If we could measure the sun’s magnetic field, we could really understand what’s going on.
Measure the magnetic field
Magnetic fields are invisible, but we can measure them because they slightly alter the light coming from the sun. The surface is very shiny, so it is easy to measure changes in the light coming out of it and the magnetic field there.
But the sun’s atmosphere is so hot that no light is visible anymore. Instead, it produces x-rays, a type of light that we cannot see. Even when we use special X-ray telescopes, the X-rays coming from the Sun’s atmosphere are too weak to determine what the magnetic field in the atmosphere looks like.
The good news is that a probe, NASA’s Parker Solar Probe, is currently orbiting close to the sun (but not too close) and crossing its magnetic field to measure it. We should receive a lot of exciting information in the coming years.
These magnetic field measurements will allow us to better understand what makes the atmospheres of the Sun and other stars much hotter than their surfaces.
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The original version of this article was published on La Conversation, a nonprofit news site dedicated to sharing ideas between academic experts and the general public.