EarthSky
Glossary · The Sun & Space Weather
Coronal hole
An opening in the Sun's magnetic field that lets a fast stream of wind escape.
Most of the corona is threaded by closed magnetic loops that trap the plasma. A coronal hole is a region where the field opens into space instead, so the solar wind pours out at 700 km/s or more rather than the usual 400. In X-ray images the hole is dark because the trapped hot gas is missing. When one rotates to face Earth the fast stream arrives a few days later and produces a mild, long-lasting geomagnetic disturbance — the aurora that arrives on schedule rather than as a surprise.
Where it has come up lately
Related terms
Coronal mass ejection
A billion tonnes of solar atmosphere thrown into space, and the cause of most serious aurorae.
Solar flare classes
A, B, C, M, X — a logarithmic scale for how hard the Sun just flashed in X-rays.
Kp index
A 0–9 scale of how disturbed Earth's magnetic field is right now.
The solar cycle
An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field.
Questions
What is Coronal hole?
An opening in the Sun's magnetic field that lets a fast stream of wind escape.
Why does it matter?
In X-ray images the hole is dark because the trapped hot gas is missing. When one rotates to face Earth the fast stream arrives a few days later and produces a mild, long-lasting geomagnetic disturbance — the aurora that arrives on schedule rather than as a surprise.