The Sun appears hottest at its bright surface, but the outer atmosphere above it is far hotter. The photosphere, the layer we see as the Sun’s surface, is about 5,500 degrees Celsius. The corona can reach roughly 2 million degrees Celsius.
That upside-down temperature pattern has puzzled solar physicists for more than a century. Heat normally spreads away from a hot source and cools with distance. The Sun’s corona does not behave like a simple campfire. Energy is being transferred into the thin plasma above the photosphere by processes tied to the Sun’s magnetic field.
What counts as the Sun’s surface?
The photosphere is not a solid shell. It is the first layer of the solar atmosphere, where most visible light escapes. Beneath it, convection moves hot plasma upward. Above it lie the chromosphere, a thin transition region and the corona.
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The corona is much less dense than the photosphere. A thermometer placed there would not behave the way an everyday thermometer behaves in air, because temperature measures the energy of individual particles while the amount of material is tiny. The corona can contain very energetic particles without carrying the same total heat as the denser layers below.
This is one reason the phrase “hotter than the surface” needs care. The particles in the corona move with enormous energies, but there are far fewer of them. The puzzle concerns how those particles acquire their energy, not a solid outer shell that somehow glows hotter than the Sun beneath it.
Magnetic fields carry energy upward
Convection in the Sun’s outer layers twists and stretches magnetic-field lines. Those fields rise into the corona in loops and arches. Their changing shape can transfer mechanical energy into the plasma above the surface.
One leading idea involves Alfvén waves, disturbances that travel along magnetic fields through the plasma. As the waves move outward, they can accelerate particles and release energy as heat. The process is difficult to measure from Earth because the solar wind mixes with other particles during its journey across the 150 million kilometres between the Sun and our planet.
Small explosions may add to the heat
Another explanation involves nanoflares. These are tiny releases of energy caused by magnetic reconnection, when stressed field lines rearrange and snap into a lower-energy configuration. A single nanoflare would be too small to notice from Earth, but countless events could keep the corona hot.
The two ideas are not mutually exclusive. Magnetic reconnection can release heat directly and launch waves that carry energy farther into the corona. Researchers are still working out how often each process occurs and how much energy each contributes.
Why Parker Solar Probe matters
NASA’s Parker Solar Probe is flying through the corona to measure particles and magnetic fields close to where they are heated. That is a crucial advantage. Instruments near the Sun can sample the plasma before it has travelled through space and lost the details of its origin.
Total solar eclipses provide another valuable view. When the Moon blocks the photosphere, the faint corona becomes visible from Earth. Modern spacecraft now watch it every day, but eclipse observations still reveal large-scale structures and help connect the Sun’s magnetic activity with what Parker measures locally.
A mystery with several pieces
Scientists do not yet have one final answer that explains every observation. Waves, reconnection and small-scale eruptions may all be part of the same heating system. The evidence points to energy entering the corona through the Sun’s magnetic and convective activity, rather than through sunlight simply travelling outward from the photosphere.
That is why the corona can be millions of degrees hotter than the layer beneath it—and why the Sun remains an active laboratory rather than a solved textbook problem.
For the wider space-weather picture, Curiosmos has also examined what a powerful solar storm can do at Earth.
