EarthSky
Glossary · The Sun & Space Weather
Kp index
A 0–9 scale of how disturbed Earth's magnetic field is right now.
The planetary K index is derived every three hours from magnetometers around the world, and compresses "how much is the magnetic field wobbling" into a number from 0 to 9. Below 4 is quiet. Kp 5 is a minor geomagnetic storm and puts aurora over Scotland and the northern United States; Kp 7 pushes it into England and the mid-latitudes; Kp 8 and 9 are rare and put it over places that never see it. It is the single most useful number for deciding whether to go outside and look, which is why it is on this site's sky page and updated with every gather.
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.
The solar cycle
An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field.
Sunspot
A patch of the Sun where the magnetic field is strong enough to hold the heat back.
Questions
What is Kp index?
A 0–9 scale of how disturbed Earth's magnetic field is right now.
Why does it matter?
Kp 5 is a minor geomagnetic storm and puts aurora over Scotland and the northern United States; Kp 7 pushes it into England and the mid-latitudes; Kp 8 and 9 are rare and put it over places that never see it. It is the single most useful number for deciding whether to go outside and look, which is why it is on this site's sky page and updated with every gather.