Why Is the Sun's Outer Atmosphere Hotter Than Its Surface?
– A Thin 1,000,000°C Layer Sits on a 6,000°C Surface
A campfire gets less hot the farther you move away. On the Sun, it works the other way round. The glowing surface is about 6,000°C, yet the atmosphere beyond it tops 1,000,000°C. What carries the energy out there is thought to be not the heat of flames, but the Sun's magnetic force.
The air in a sauna can top 90°C. Yet people sit in it for minutes. Put your hand in 90°C water and you are scalded in an instant.
The temperature is the same, but the air is thin and has little heat to hand over to your body. "High temperature" and "holding a lot of heat" are, in fact, two different things.
This difference leads to the biggest mystery of the nearest star. During a total solar eclipse, a white crown of light spreads around the black Moon. That is the 1,000,000°C corona.
There are just two reasons
If heat simply flowed upward from the surface, the outer layer could never become hotter than the surface. The corona is thought to be heated by energy carried along magnetic fields (the paths of magnetic force) that reach up from beneath the surface.
The corona has only about one hundred-millionth as many particles as the gas near the surface. So even a little energy makes each particle move wildly, and the temperature climbs to 1,000,000°C.
In other words, the corona is less "something extremely hot" than "a place where a few particles fly around very fast." Let's take it step by step.
How temperature changes from the surface outward
Look at Figure 1. The horizontal axis is height above the Sun's surface, and the vertical axis is temperature. Each step up the vertical axis is a factor of 10.
About 500 km above the surface, the temperature first drops to a little over 4,000°C. So far it is like the campfire: farther away means cooler. But beyond that the temperature starts to rise, reaching about 20,000°C at a height of around 2,000 km.
Then, in just a few hundred kilometres more, it leaps to nearly 1,000,000°C. This steep slope is called the "transition region." Beyond it lies the corona, spreading out into space.
What carries the heat is magnetic force
Heat flows only from hot places to cold ones. This is a basic law of nature. So the idea that heat from the surface "seeps out" and warms the corona does not work, because the outer layer is the hotter one.
That is why magnetic fields have drawn attention. Just below the Sun's surface, hot gas churns up and down as if boiling. Bundles of magnetic field poke out of it into the space above the surface, in the shape of loops.
Shaken by the motion of the gas, these bundles twist and tremble. Two main mechanisms are considered. In one, waves travel up along the magnetic bundles and give out their energy in the corona, turning into heat. In the other, twisted magnetic fields reconnect in small ways, and each reconnection sets off a tiny explosion.
Both carry "motion," not "heat," upward, and turn it into heat higher up. That does not break the rule. Think of clay getting warm as you knead it by hand: even something cooler can heat its target by handing over motion.
Because it is thin, a little energy makes it hot
Look at Figure 2. The left box is near the surface, and the right box is the corona. Each dot stands for one gas particle.
Temperature is how wildly each single particle moves. In the corona, where particles are very few, sharing out just a little energy lets each one move very fast. Measured by the total amount of heat in the same volume, the corona holds only a few hundred-thousandth of what the gas near the surface holds (the calculation is in the fold-out at the end).
Thinness has another effect. Hot gas cools by giving off light, and how strongly it does so depends on how often particles collide. In the corona, with so few particles, collisions are rare and the gas does not cool easily. So even energy added bit by bit keeps the temperature high.
During the total eclipse of 1869, a green light was found in the corona that matched no element on Earth. For a while, people thought a new element, "coronium," existed. Around 1940, the light was traced to iron that had lost as many as 13 electrons. Stripping off that many electrons takes a temperature of about 1,000,000°C. The mystery element turned out to be evidence of a mystery heat.
Compared with the light the surface sends into space, the energy needed to maintain a quiet corona is put at about one 200,000th. In other words, a tiny share of the Sun's activity is enough to keep it at 1,000,000°C.
Summary
The Sun's outer corona is hotter than its surface not because heat seeps up from below. Motion of the gas beneath the surface shakes the magnetic field, and that energy is thought to turn into heat higher up. And because the corona is so thin, a small amount of energy makes each particle move fiercely, reaching 1,000,000°C. Still, which of the two mechanisms matters, and by how much, remains unclear today.
The corona is not a "hot lump" but "a few particles moving too fast."
High temperature and a lot of heat are different things.
Why space does not instantly freeze a human body is explained in "Is space really cold?", and what the gas blowing out of the corona does to Earth is in "Can a huge solar eruption black out the Earth?". For how the Sun itself keeps shining, see "Why can stars shine for billions of years?".
- Open the freezer and hold your hand in the air inside (only for a few seconds). It is cold, but not unbearable.
- Next, hold a chilled ice pack from the same freezer for a few seconds, through a towel. Though it is the same temperature as the air, it should feel much colder. The air is thin and has little power to draw heat from your hand.
- Search online for images from a solar observation satellite. Compare the Sun in visible light with the Sun in invisible short-wavelength light: in the latter, loops of the corona's magnetic field glow.
* Gripping an ice pack with bare hands for long can damage the skin. Use a towel and stop after a few seconds. Never look directly at the Sun itself. Even during an eclipse, use special eclipse glasses.
For those who want to know more – terms, formulas and links to textbooksEach part is labelled by level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics or earth science
- High school+Advanced high school material, or a textbook sidebar
- UniversityNot taught in high school; university courses (solar physics, plasma physics)
- ResearchNot taught even at university as settled fact; being studied by researchers now
Middle schoolTerms: this phenomenon has names
- Photosphere: the glowing surface of the Sun that we normally see. Its temperature is about 6,000°C.
- Chromosphere and transition region: the thin layer about 2,000 km thick just above the photosphere is the chromosphere. The layer a few hundred kilometres thick above it, where temperature jumps, is the transition region.
- Corona: the Sun's atmosphere beyond the transition region, above 1,000,000°C. It is seen as a white crown during a total solar eclipse.
- Coronal heating problem: the question of why the corona is hotter than the surface, a puzzle in solar physics for over 70 years.
Middle schoolHigh schoolChecking with a formula: how much heat does the corona hold?
Temperature is how wildly one particle moves, and the amount of heat is the total over all particles. We compare how much heat one cubic metre holds near the surface and in the corona.
| In symbols | u = (3/2) × n × k × T |
| In words | Heat stored in 1 cubic metre = 1.5 × number of particles × Boltzmann constant × temperature |
| Where it comes from | The kinetic theory of gases. The average motion energy of one particle is (3/2)kT, proportional to temperature. Multiply by the number of particles and you get the total heat. |
| u | Amount of heat per cubic metre (unit: joules per cubic metre) |
| n | Number of particles per cubic metre |
| k | Boltzmann constant (the same value applies to both, so it cancels when comparing) |
| T | Temperature (unit: kelvin. At this scale, nearly the same number as °C) |
| Temperature near the surface | about 6,000 kelvin |
| Temperature of the corona | about 1,000,000 kelvin |
| Particle ratio (near surface : corona) | roughly 100 million : 1 |
| Light emitted by 1 square metre of surface | about 63 million watts |
| Energy needed to maintain a quiet corona | about 300 watts per square metre of surface (Withbroe & Noyes 1977) |
| Temperature ratio (corona ÷ surface) | 1000000 ÷ 6000 ≒ 167 |
| Heat ratio (particle ratio ÷ temperature ratio) | 100000000 ÷ 167 ≒ 600000 |
| Ratio of surface light to energy the corona needs | 63000000 ÷ 300 ＝ 210000 |
The corona is 167 times hotter, yet the heat stored in the same cubic metre is about 1/600,000 as much. And the energy needed to maintain it is only about 1/210,000 of the light the surface gives off. It is the same relationship as sauna air at 90°C holding far less heat than 90°C water.
High schoolHigh school+The second law of thermodynamics, and heating by "motion"
High schoolHeat never moves by itself from cold to hot (the second law of thermodynamics). But work (pushing something to move it) can move energy from a cold object to a hot one. A refrigerator pumps heat from inside to the warmer room in the same way. The heating of the corona is not contradictory if we think of gas motion near the surface carrying energy as "work" through the magnetic field.
High school+The corona's temperature can be found from its emission lines. Around 1940, Grotrian and Edlén showed that the green emission line (wavelength about 530 nanometres) comes from iron ions that have lost 13 electrons. Stripping off that many electrons needs particle collisions at the level of 1,000,000 kelvin.
UniversityWave heating and nanoflare heating
The corona is a plasma split into electrons and ions, and it moves along magnetic fields. There are two leading explanations. One is "wave heating": Alfvén waves and others travelling along field lines dissipate in the corona and become heat. The other is "nanoflare heating": field lines twisted by convection at the surface undergo "magnetic reconnection" (re-linking), and countless small flares heat the corona. The rate at which thin plasma cools by emitting light is described by "radiative cooling," proportional to the square of the density, so the thin corona hardly cools.
📖 Derivations and further reading: Corona (Wikipedia, in Japanese) ／ Magnetic reconnection (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Waves or reconnection: which plays the lead? Observations have found clues for both, but how much each contributes is unsettled. It may differ by place and by the phase of solar activity.
- What the probe that went near the Sun saw. Parker Solar Probe, launched in 2018, found many structures near the Sun where the magnetic field folds back in an S shape. How these relate to coronal heating is being studied.
- Small "campfires." In 2020, the Solar Orbiter probe imaged countless small brightenings near the surface. Whether they are part of what nanoflares are is under debate.
So the content of this article is also "an explanation within what is known now." The coronal heating problem is still considered one of the biggest open questions in solar physics.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "The solar system and stars," "How heat travels" | Names of the photosphere and corona; heat flows from hot to cold |
| High school | Physics: "Kinetic theory of gases," "Thermodynamics"; Earth science: "The Sun" | Temperature as energy per particle, and how it differs from amount of heat |
| High school+ | Physics: "Atomic spectra" | Finding temperature from the green emission line and iron ions |
| University | Solar physics, plasma physics | Alfvén waves, magnetic reconnection, radiative cooling |
| Research | Observations by solar probes | Shares of wave and reconnection contributions |
| ― | Links to daily life | Sauna air vs. hot water; freezer air vs. an ice pack |
- Wikipedia, "Corona" (コロナ)
- Withbroe, G. L. & Noyes, R. W. (1977) Mass and Energy Flow in the Solar Chromosphere and Corona. Annual Review of Astronomy and Astrophysics 15
- Vernazza, J. E., Avrett, E. H. & Loeser, R. (1981) Structure of the solar chromosphere. III. The Astrophysical Journal Supplement Series 45
- Edlén, B. (1943) Die Deutung der Emissionslinien im Spektrum der Sonnenkorona. Zeitschrift für Astrophysik 22
- Wikipedia, "Magnetic reconnection" (磁気リコネクション)
* This article is a general-audience science explanation. The figures given are guides and rough estimates meant to help you understand the mechanism. The temperature curve in Figure 1 is the outline of an average model; on the real Sun it varies greatly by place and time.