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The Sun
The Sun constantly emits a stream of charged particles — electrons and protons — called the solar wind, traveling at 400–800 km/s. During periods of high solar activity, the Sun also produces solar flares and Coronal Mass Ejections (CMEs).
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Earth's Magnetic Field
Earth's magnetosphere acts as a protective bubble, deflecting most solar wind particles around the planet. However, at the polar cusps, magnetic field lines converge and funnel particles inward toward the atmosphere.
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The Atmosphere — How Colors Form
When charged particles spiral down the magnetic field lines, they collide with oxygen and nitrogen molecules in the atmosphere. These collisions excite the molecules and emit light — the aurora.
Understanding Activity
The KP Index Explained
The Planetary K-index (Kp) measures global geomagnetic activity on a scale from 0 to 9. It is derived from magnetometers at 13 stations worldwide and updated every 3 hours.
KP 0 – 1
No aurora
Geomagnetically quiet. No visible aurora even at polar latitudes.
KP 2 – 3
Faint aurora possible
Faint arcs may be visible from northern Iceland in dark skies. Best seen by camera.
KP 4
Good aurora activity
Curtains and bands visible to the naked eye from Iceland. A good night for viewing.
KP 5 – 6
Geomagnetic storm
Bright, active aurora with rapid movement. Visible from most of Iceland and northern Europe.
KP 7 – 9
Severe storm
Dramatic full-sky displays. Aurora visible at mid-latitudes in Scotland, Germany, and even further south.
Key Terms
Aurora Glossary
Bz
The north-south component of the interplanetary magnetic field (IMF). Negative Bz allows solar wind energy to flow into Earth's magnetosphere, triggering aurora.
Solar Wind Speed
The velocity of charged particles streaming from the Sun. Normal is 350-500 km/s. During solar storms it can exceed 800 km/s.
OVATION Model
A model from NOAA that predicts the intensity and location of the auroral oval in real time using solar wind data.