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Glossary · The Sun & Space Weather
The corona
The Sun's outer atmosphere, which is three hundred times hotter than the surface for reasons still argued over.
The visible surface is about 5,800 K. The corona above it reaches one to three million. Heat is not supposed to flow that way, and how the corona is heated has been an open question since the 1940s — magnetic reconnection and wave heating are the leading candidates, and Parker Solar Probe was built specifically to fly through it and find out. From the ground the corona is only visible during a total eclipse, which is why eclipses were scientifically valuable long after they stopped being frightening.
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 corona?
The Sun's outer atmosphere, which is three hundred times hotter than the surface for reasons still argued over.
Why does it matter?
Heat is not supposed to flow that way, and how the corona is heated has been an open question since the 1940s — magnetic reconnection and wave heating are the leading candidates, and Parker Solar Probe was built specifically to fly through it and find out. From the ground the corona is only visible during a total eclipse, which is why eclipses were scientifically valuable long after they stopped being frightening.