Aurora glossary: forecast and app terms explained, A to Z

Aurora forecasts and apps are full of abbreviations and jargon: Kp, Bz, nT, L1, OVATION. This glossary explains them briefly, in alphabetical order, with the Norwegian word in brackets where you are likely to meet it in Norway, and links to articles where you can read more. If you are completely new, what the aurora is and the Kp index are good places to start.

A–C

ACE and DSCOVR

Spacecraft stationed at the L1 point between Earth and the Sun that measure the solar wind before it reaches us. NOAA uses their data in real time, and newer missions (including SOLAR-1 and IMAP) are taking over from them. See solar wind, Bz and speed.

All-sky camera

A camera with a fisheye lens that photographs the whole sky at once. Several research stations in northern Norway publish the images online, so you can check for aurora before going outside. See aurora apps and data.

Arcs and bands

The most common auroral forms: calm arcs stretching from east to west, and bands that fold like curtains. According to Store norske leksikon (the Norwegian encyclopedia), they are typically seen in the evening, before activity may build.

Aurora australis

Latin name for the southern lights, the counterpart of the northern lights around the southern magnetic pole. Both together are called polar lights.

Aurora borealis

Latin name for the northern lights (Norwegian: nordlys).

Auroral oval (nordlysoval)

The oval-shaped belt around each magnetic pole where aurora occurs at any given moment. On the night side it lies at around 67 degrees magnetic latitude, and it expands towards the equator as activity rises.

Auroral zone (nordlyssonen)

The belt where aurora is statistically most common, about 23 degrees from the geomagnetic pole. The coast of Troms and Finnmark in northern Norway lies in this zone, where aurora can occur on more than half of all nights, even when solar activity is low.

Bt

The total strength of the magnetic field carried by the solar wind, measured in nanotesla (nT). A high Bt means potential for strong aurora, but the direction (Bz) decides whether the energy gets in.

Bz

The north–south component of the solar wind's magnetic field. When Bz points south (negative values), the solar wind couples better with Earth's magnetic field and the aurora usually gets stronger. Many forecasts therefore highlight periods with Bz below zero or below −5 nT.

Cloud cover (skydekke)

How much of the sky is covered by cloud. The aurora shines far above the clouds, so cloud cover is often the single biggest factor in whether you see anything. See clouds and clear skies.

CME (coronal mass ejection)

A huge cloud of plasma and magnetic field thrown out from the Sun's corona. NOAA gives speeds from below 250 to nearly 3000 km/s; the fastest can reach Earth in 15–18 hours, while slower ones take several days. CMEs drive most strong geomagnetic storms.

Corona (in the aurora)

When auroral rays are directly overhead, they seem to radiate from a single point high in the sky, because the rays follow the magnetic field lines. This is called an auroral corona and is one of the most spectacular sights. Not to be confused with the Sun's corona.

Coronal hole

Dark regions in the Sun's corona where the magnetic field is open and the solar wind escapes more easily. They are an important source of moderate, recurring aurora, even when overall solar activity is low.

D–K

Dayside aurora

Aurora on the day side of Earth, high up (above roughly 150–200 km) and often red. It is usually faint, but can be seen during the polar night on Svalbard, where it is dark even at midday.

Density (solar wind)

How tightly packed the particles in the solar wind are. Sudden jumps in density and speed can signal that a CME or a compression region is reaching Earth.

Diffuse aurora

Faint, veil-like aurora without sharp edges, often milky white or pale green. It can be hard to tell apart from thin cloud with the naked eye, but shows up clearly on camera.

Electrons and protons

The charged particles that create the aurora when they collide with gases in the upper atmosphere. Most of the visible aurora is caused by electrons.

Equinox effect

Statistically, geomagnetic activity is higher around the spring and autumn equinoxes than in midwinter. This is often called the Russell–McPherron effect. See the best time of year.

Forecast horizons: 30 minutes, 3 days and 27 days

NOAA produces aurora forecasts with different horizons. The 30-minute forecast (based on OVATION) gives 30–90 minutes' warning and is the most reliable. The 3-day forecast and 27-day outlook give Kp predictions further ahead, but accuracy drops the further out you look. See how to read an aurora forecast.

G scale

NOAA's scale for geomagnetic storms, from G1 (minor) to G5 (extreme). G1 corresponds to Kp 5, G2 to Kp 6, G3 to Kp 7, G4 to Kp 8 and G5 to Kp 9.

Geomagnetic latitude

Latitude measured from Earth's magnetic pole rather than the geographic one. It says more about your aurora chances than ordinary latitude, because the auroral oval follows the magnetic field.

Geomagnetic storm

A period of strong disturbance in Earth's magnetic field, defined as Kp 5 or higher. During storms the aurora moves south and can be seen across Norway. See geomagnetic storms.

GFZ

The German Research Centre for Geosciences in Potsdam, which produces the official Kp index. NOAA's real-time Kp is an estimate of the same quantity.

Guovssahas

North Sámi for the aurora (plural guovssahasat). Read about the word and the traditions in myths and Sámi traditions.

High-speed stream

Fast solar wind flowing out of a coronal hole. When one reaches Earth it can boost aurora activity, and it may return roughly every 27 days, once the Sun has rotated.

Ionosphere

The part of the upper atmosphere where the gas is partly ionised by sunlight and particles. The aurora occurs in this layer.

K index and Kp index

The K index describes how much the magnetic field varies over three hours, on a scale from 0 to 9. Kp ("planetary K") is an average from 13 observatories at mid latitudes. The name comes from the German Kennziffer ("characteristic digit"), and Julius Bartels introduced the index in 1938. Read the Kp index explained.

L–O

L1 (Lagrange point 1)

A point about 1.6 million km from Earth towards the Sun, where spacecraft can hold station and measure the solar wind. The time the solar wind takes to travel from there to us gives forecasts a lead time of about 30–90 minutes.

Light pollution

Artificial light from towns, roads and houses that brightens the sky and hides faint aurora. Driving a few kilometres out of town makes a big difference.

Magnetic midnight

The time when a location sits directly under the night side of the auroral oval, measured relative to the magnetic field. The most dynamic aurora often appears around magnetic midnight. See the best time of night.

Magnetometer

An instrument that measures Earth's magnetic field. Kp and local activity indices are calculated from magnetometers, and rapid changes in their readings are a sign the aurora is active.

Magnetosphere

The region around Earth where Earth's magnetic field dominates over the solar wind. It is compressed on the day side and stretched into a long tail on the night side, where much of the aurora's energy is stored.

Midnight sun (midnattssol)

When the Sun stays above the horizon around the clock. The opposite of polar night, and the reason the aurora cannot be seen in northern Norway in summer.

Nanotesla (nT)

The unit for the solar wind's magnetic field and for variations in Earth's magnetic field. Bz and Bt are given in nT.

Nitrogen

The gas that gives the bluish and violet tones at the lower edge of strong aurora. See aurora colours.

OVATION

A model that calculates where the auroral oval lies and how intense it is, using solar wind data from L1. NOAA's 30-minute aurora forecast is based on it.

Oxygen

The gas behind the most common aurora colours: green at 557.7 nanometres and red at 630.0 nanometres. The red light sits higher up than the green.

P–Z

Polar lights

Collective term for the northern and southern lights.

Polar night (mørketid)

The period when the Sun does not rise above the horizon. In Tromsø it lasts from about 26 November to 17 January, and in Longyearbyen on Svalbard from about 26 October to 16 February. It is not pitch dark all day, but the long nights give many hours of aurora chances.

Pulsating aurora

Patches of aurora that blink on and off, often for hours, typically late at night and towards morning after an outburst.

Red aurora

Red light from oxygen high up, typically above the green. Under special conditions, especially during strong storms, red can dominate the whole display.

Solar cycle

The Sun's roughly 11-year activity cycle, from minimum to maximum and back. Strong storms can also happen in the declining phase. See the solar cycle.

Solar maximum

The period with the most sunspots in a solar cycle. For cycle 25, the smoothed sunspot number peaked in October 2024, according to the SIDC in Belgium.

Solar wind (solvind)

A continuous stream of charged particles and magnetic field from the Sun. Its speed, density and magnetic field direction (Bz) decide how much energy reaches Earth.

Substorm

An auroral outburst: calm arcs suddenly brighten, spread and dance across the sky, then activity settles into diffuse and pulsating light. This sequence can repeat several times in one night.

Sunspot

Dark regions on the Sun's surface with strong magnetic fields. The number of sunspots is the most common measure of how active the Sun is in the solar cycle.

Verlys

An old Norwegian folk name for the aurora (also "vêrlys" and "vêrljos"), roughly "weather light", linking the lights to the weather.

In short

The key things to remember: Kp tells you the activity level, Bz tells you whether the energy is actually getting in, and cloud cover decides whether you see anything. For all of it in one answer, check tonight's verdict for your location.