The corona is the Sun's outer atmosphere, a halo of thin gas heated to more than a million degrees that stretches millions of kilometres into space. It is about a million times fainter than the Sun's surface, so it is normally lost in the glare. During totality it appears as a pearly white crown around the black Moon.
Understanding Solar Corona
Why the corona is hotter than the surface
Heat normally falls with distance from its source, yet the corona is hundreds of times hotter than the surface below it. The energy is thought to come from the Sun's magnetic field, carried up by waves, released in countless small magnetic explosions, or both. NASA's Parker Solar Probe flies through the outer corona to measure it directly, passing 6.2 million km from the surface in December 2024, while ESA's Solar Orbiter photographs it from close range.
How the corona changes with the solar cycle
The Sun's magnetic field reorganises over its roughly 11-year cycle, and the corona seen at totality changes with it:
| Part of the cycle | What the corona looks like |
|---|---|
| Near solar maximum | Roughly round, with streamers in every direction |
| Declining years | Large streamers at middle latitudes, uneven around the Sun |
| Near solar minimum | Long streamers along the equator, short plumes at the poles |
Seeing and photographing the corona
To the naked eye the inner corona is bright and pearly while the outer streamers fade gradually into the dark sky; binoculars show fine rays and the pink prominences at the Moon's edge. Its brightness falls so steeply with distance that photographs need a series of exposures to capture it all. Outside eclipses, instruments called coronagraphs block the Sun's disc to study it, and ESA's Proba-3 mission flies two spacecraft in precise formation to create artificial eclipses. During a natural eclipse, the corona may be watched without glasses only during totality.