Aurora glossary
Aurora forecasts and photography forums are full of jargon. This glossary explains the terms you are most likely to meet, in plain language, grouped by topic. Where a term has a typical value that matters for aurora watching, it is included.
The Sun and solar wind
- Solar wind
- The continuous stream of charged particles, mainly protons and electrons, flowing out from the Sun. It usually travels at 300–400 km/s near Earth and carries the Sun's magnetic field with it.
- Solar wind speed
- How fast the solar wind is moving. Speeds above about 500 km/s deliver noticeably more energy; fast CMEs can exceed 1,000 km/s.
- Density
- The number of particles per cubic centimetre in the solar wind. Sudden jumps often mark the arrival of a CME shock.
- Interplanetary magnetic field (IMF)
- The Sun's magnetic field carried through space by the solar wind. Its strength is measured in nanotesla (nT) and is labelled Bt.
- Bz
- The north–south component of the IMF. Southward (negative) Bz links up with Earth's magnetic field and lets energy in; it is the most important short-term aurora signal.
- Coronal mass ejection (CME)
- A huge cloud of magnetised plasma thrown out from the Sun. Earth-directed CMEs arrive roughly 15 hours to 4 days later and cause most major storms.
- Solar flare
- A sudden flash of radiation from the Sun's surface, classed A, B, C, M and X. Flares themselves do not cause aurora, but large ones are often accompanied by CMEs.
- Coronal hole
- An area of open magnetic field in the Sun's outer atmosphere that releases fast solar wind. Long-lived holes can bring recurring activity every ~27 days.
- High-speed stream (HSS)
- A stream of fast solar wind from a coronal hole. Often causes several nights of minor to moderate storms.
- Sunspot
- A dark, magnetically intense region on the Sun's surface. Sunspot numbers track the solar cycle, and complex sunspot groups produce most large flares and CMEs.
- Solar cycle
- The roughly 11-year rise and fall of solar activity. Cycle 25 peaked around late 2024; its declining phase continues through 2026–2027.
- L1
- The Lagrange point about 1.5 million km towards the Sun, where spacecraft measure the solar wind 15–60 minutes before it reaches Earth.
Geomagnetic activity
- Kp index
- A global measure of geomagnetic disturbance over three hours on a quasi-logarithmic scale from 0 to 9, divided into thirds (for example 4−, 4o, 4+). Higher values push the aurora further from the poles.
- G-scale
- NOAA's geomagnetic storm scale: G1 minor (Kp 5), G2 moderate (Kp 6), G3 strong (Kp 7), G4 severe (Kp 8) and G5 extreme (Kp 9).
- Geomagnetic storm
- A major disturbance of Earth's magnetic field, usually defined as Kp 5 or above. Storms expand the auroral oval towards the equator.
- Magnetosphere
- The region of space controlled by Earth's magnetic field, which shields the planet from most of the solar wind and stretches into a long tail on the night side.
- Substorm
- A sudden release of energy stored in the magnetic tail, causing a burst of bright, fast-moving aurora that typically lasts 15–60 minutes.
- Dst index
- A measure of the strength of the ring current around Earth during storms. Strongly negative values indicate a major storm.
- Hp30 and Hp60
- Newer activity indices similar to Kp but calculated every 30 or 60 minutes, so they react faster and are not capped at 9.
- Magnetometer
- An instrument that measures changes in the local magnetic field. Sharp swings at a nearby station often mean aurora overhead.
- Sudden commencement
- A sharp jump in magnetometer readings when a CME shock hits Earth's magnetosphere, often followed by a storm.
- Equinox effect
- The tendency for geomagnetic storms to be more frequent around the March and September equinoxes, when Earth's field is oriented to couple more easily with the solar wind.
- 27-day recurrence
- The pattern of activity repeating about every 27 days, as long-lived solar features rotate back to face Earth.
The aurora itself
- Aurora borealis
- The northern lights, seen around the north magnetic pole.
- Aurora australis
- The southern lights, seen around the south magnetic pole from Antarctica, Tasmania, New Zealand and southern South America.
- Auroral oval
- The ring-shaped zone around each magnetic pole where the aurora is most frequent, usually around 65–72° magnetic latitude. It expands towards the equator during storms.
- Magnetic latitude
- Latitude measured from the magnetic rather than the geographic pole. It decides how often a place sees the aurora.
- Magnetic midnight
- The time when a location lies directly on the night side of Earth's magnetic field. Aurora is often most active within a few hours of it, typically around 22:00–00:00 local time in Scandinavia.
- Arc
- A calm, even band of aurora stretching across the sky from east to west. Displays often start this way.
- Curtains and rays
- Folded, wavy sheets of aurora and the vertical stripes within them, which follow Earth's magnetic field lines.
- Corona
- Rays seen from directly below, appearing to converge at a point overhead. A sign of strong activity at your location.
- Pulsating aurora
- Patches of aurora that flicker on and off every few seconds, common after a substorm, often late at night.
- Green line (557.7 nm)
- The main auroral colour, emitted by oxygen atoms at about 100–250 km altitude.
- Red line (630.0 nm)
- Red light from oxygen above about 200 km. Faint to the eye except during strong storms, when it can be seen far from the poles.
- Nitrogen emissions
- Blue, violet and deep red light from nitrogen, forming the pink or purple lower edge of active displays.
- SAR arc
- Stable auroral red arc: a uniform, deep-red band formed by heat from the ring current during strong storms, distinct from normal aurora.
- STEVE
- Strong thermal emission velocity enhancement: a narrow purple-white ribbon, sometimes with green 'picket fence' features, seen at lower latitudes during active nights. It is not aurora in the usual sense.
Watching and photographing
- Bortle scale
- A 1–9 scale of night-sky darkness, from pristine wilderness (1) to inner city (9). Faint aurora needs Bortle 4 or darker.
- Dark adaptation
- The 20–30 minutes your eyes need to reach good night vision. Bright light resets it; red light preserves it.
- Astronomical twilight
- The final stage of dusk, when the Sun is 12–18° below the horizon. The sky is fully dark once the Sun is more than 18° below.
- ISO
- A camera's sensitivity setting. Aurora photos typically use ISO 800–3200 with a wide-aperture lens.
- Exposure time
- How long the camera collects light. Typical aurora exposures are 2–15 seconds: shorter for bright, fast displays, longer for faint ones.
- Required Kp
- On this site, the Kp level at which aurora is expected to become visible from a given town. Each town page shows it, along with the direction to face.
Frequently asked questions
- What is the most important number in an aurora forecast?
- For planning a night, Kp compared with the Kp your town needs. For the final hour, Bz: a sustained southward (negative) Bz, especially combined with solar wind faster than about 500 km/s, is the best sign that a display is about to start. Cloud cover matters just as much as either.
- What is the difference between a CME and a solar flare?
- A flare is a burst of light and radiation that reaches Earth in about eight minutes and does not cause aurora. A CME is a cloud of plasma and magnetic field that takes about one to four days to arrive and can trigger a geomagnetic storm. Large flares and CMEs often happen together.
- Is STEVE a type of aurora?
- Not in the usual sense. STEVE is a narrow ribbon of light linked to fast-flowing hot gas in the upper atmosphere, rather than to electrons raining down from the magnetosphere. It appears during active nights, usually closer to the equator than the main aurora, and is often seen alongside it.
Last reviewed 16 October 2026