Solar Physics
Solar Corona
The Sun's outer atmosphere — millions of degrees, mysteriously hotter than the surface
The solar corona is the Sun's outermost atmospheric layer — extends millions of km into space. Temperature ~1-3 million K (vs 5778 K surface). Why hotter than below — open question for decades. Solved partially: magnetic field reconnection and Alfvén waves heat plasma. Visible during total solar eclipses (or with coronagraphs). Source of solar wind. NASA's Parker Solar Probe (2018+) flying through corona for direct study.
- Temperature1-3 million K (vs 5778 K photosphere)
- Density10⁻¹⁵ kg/m³ (extremely thin plasma)
- Visible duringTotal solar eclipses; from coronagraphs
- Heating mechanismMagnetic reconnection + Alfvén waves
- Solar wind originatesOpen magnetic field lines feed solar wind
- Parker Solar ProbeFirst mission to "touch" corona (2018+)
Interactive visualization
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Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why corona matters
- Space weather. Source of CMEs and solar wind.
- Stellar physics. Sun is paradigm of star atmospheres.
- Magnetic field physics. Most accessible reconnection lab.
- Solar wind. Affects entire solar system.
- Mission targets. SOHO, Parker, Solar Orbiter, current missions.
- Eclipse science. Public engagement with solar phenomena.
- Auroras. Caused by corona-Sun-Earth interaction.
Common misconceptions
- Corona just visible during eclipses. Coronagraphs see it constantly.
- Corona is hotter "throughout." Different layers have different T; corona varies 1-3 million K.
- Coronal heating is solved. Major aspects understood; details still active research.
- CMEs are rare. Common; ~1 per day at solar max.
- Solar wind comes from photosphere. From corona.
- Sun's atmosphere is thin and uniform. Highly structured by magnetic fields.
Frequently asked questions
Why is corona hotter than the surface?
Counterintuitive — going UP from Sun's surface, you'd expect cooling. Instead, T jumps from ~5778 K to ~1-3 million K in just a few thousand km. Mystery for decades. Now believed: magnetic field reconnection events heat plasma; Alfvén waves transport energy from photosphere upward. Combined effects sustain corona's high T.
How is corona observed?
Hard — corona is dim compared to photosphere. (1) Total solar eclipses — Moon blocks Sun's disk; corona visible naked eye. Spectacular. (2) Coronagraphs — telescopes with internal disk to mask Sun's disk. Used for daily observation. (3) Spacecraft — SOHO, Parker Solar Probe, Solar Orbiter image corona constantly.
What's a coronal mass ejection?
Massive eruption of corona material. Billions of tons of plasma launched at high velocity. Can take 1-3 days to reach Earth. Major space weather event. Disrupts radio, satellites, can cause power grid failures. Carrington Event (1859) was largest recorded — telegraph systems failed worldwide. Modern equivalent could damage power grids.
How does corona heat work?
Multiple mechanisms work together. Magnetic reconnection — when oppositely directed field lines snap together, kinetic energy → heat. Alfvén waves — magnetic waves carry energy from below into corona. Microflares (small reconnection events) — many small heating events sum up to corona heating.
What's the solar wind?
Continuous stream of charged particles flowing outward from Sun. Sourced from corona's open magnetic field regions ("coronal holes"). Speed: ~400 km/s (slow wind) to ~800 km/s (fast wind). Density: very low. Mass flux: 10⁹ kg/s. Carries Sun's magnetic field outward through solar system. Reaches Earth in 4-5 days.
How is Parker Solar Probe special?
Closest spacecraft to Sun ever. Multiple flybys, getting progressively closer. As of 2024 — within 9 solar radii (6 million km). Instruments measure plasma, magnetic fields, particles in corona directly. Confirmed many predictions; provides data to solve coronal heating mystery. Launched 2018; mission continues.
How does corona affect Earth?
Drives space weather. Solar wind compresses Earth's magnetic field. Coronal mass ejections cause geomagnetic storms. Auroras. Satellite/communications/power grid impacts. Astronaut radiation exposure depends on corona activity. Solar maximum (every 11 years) brings most events.