Aurora glossary

Aurora borealis and australis

The northern and southern lights. SWPC describes both as the result of electrons colliding with the upper atmosphere, typically 80 to 500 km up. See what causes the aurora.NOAA SWPC, Aurora

Auroral oval

The ring where aurora forms around each magnetic pole. Storms push its equatorward edge toward the equator, about 2 degrees of geomagnetic latitude per Kp step by SWPC's rule of thumb.NOAA SWPC, Aurora

Equatorward edge

The side of the oval closest to the equator, and the line that matters for towns south of the oval. Glow's southern edge tool finds it in the live OVATION grid at 10, 30 or 50 percent.

Substorm

A burst of auroral activity. SWPC describes arcs that twist and sway near midnight, then expand to fill the sky, moving fast and turning bright, at the substorm's peak.NOAA SWPC, Aurora

Geomagnetic storm

A major disturbance of the magnetosphere driven by the solar wind. SWPC counts Kp 5 or more as a storm and grades storms G1 to G5. See the storm levels hub.NOAA SWPC, Planetary K-index

G-scale

NOAA's geomagnetic storm scale. G1 Minor is Kp 5, G2 Moderate Kp 6, G3 Strong Kp 7, G4 Severe Kp 8 (including a 9-) and G5 Extreme Kp 9. Each level lists where aurora has been seen and how often it occurs per 11-year cycle.

Coronal mass ejection (CME)

A cloud of solar plasma with its own magnetic field. SWPC links the largest storms to CMEs and says they typically take several days to reach Earth, as little as 18 hours in the most intense cases.NOAA SWPC, Geomagnetic storms

High-speed stream and CIR

Fast solar wind that runs into slower wind ahead of it, creating a co-rotating interaction region. SWPC says these storms are usually less intense than CME storms but can deliver energy over a longer interval.NOAA SWPC, Geomagnetic storms

Southward solar wind magnetic field

A solar wind magnetic field pointing opposite to Earth's own at the front of the magnetosphere. SWPC calls it the most important condition for transferring energy into the magnetosphere.NOAA SWPC, Geomagnetic storms

Solar cycle

The period SWPC uses for storm frequency on its scales page, set at 11 years. G3 storms, for example, average 200 per cycle on 130 days.NOAA Space Weather Scales

K-index

A single digit from 0 to 9 for one magnetometer, set by the largest swing in the horizontal field over 3 hours. The K stands for the German Kennziffer, characteristic digit. Julius Bartels introduced it in 1938.NOAA SWPC, Planetary K-index

Kp index

The planetary K-index: the mean standardized K from 13 observatories between 44 and 60 degrees geomagnetic latitude. See what the Kp index means.NOAA SWPC, Planetary K-index

Estimated Kp

SWPC's running estimate of Kp from 8 magnetometers, updated every minute. It can differ from the final value.NOAA SWPC, Planetary K-index

Watch, warning and alert

SWPC's three Kp products. A watch forecasts the day's highest Kp at 5 and up; a warning says Kp 4 or more is expected; an alert says it was reached. See the alerts guide.NOAA SWPC, Planetary K-index

OVATION

Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting: the model behind SWPC's aurora forecast. It turns L1 solar wind data into aurora values per 1 degree cell, 30 to 90 minutes ahead. See how to read it.NOAA SWPC, Aurora 30-minute forecast

L1 point

The position 1.6 million km upstream of Earth where the solar wind is measured before it arrives. Its travel time to Earth sets OVATION's lead.NOAA SWPC, Aurora 30-minute forecast

Kp-based fallback

Glow's label for an OVATION file driven by the current Kp because solar wind data were missing. SWPC says such a file has no forecast lead time.NOAA SWPC, Aurora 30-minute forecast (on Kp input when solar wind data are missing)

Geomagnetic latitude

Latitude measured from the geomagnetic pole. Glow uses the IGRF-14 centered dipole, which puts Minneapolis at 53.5 degrees against 45.0 on the map. Try the calculator.

Dipole pole

The pole of the best-fit centered bar magnet, at 80.79N, 72.76W on IGRF-14 for 2025.0. It differs from the north magnetic pole, which SWPC places about 400 km from the geographic pole.NOAA SWPC, Tips on viewing the aurora

IGRF-14

The 14th generation International Geomagnetic Reference Field, published by NOAA NCEI. Glow uses its three degree 1 coefficients for 2025.0, valid through 2030.

Civil twilight

Ends when the sun's center is 6 degrees below the horizon (US Naval Observatory). Glow counts it as dark enough from that point: civil night.US Naval Observatory, Rise, set and twilight definitions

Nautical twilight

Ends at 12 degrees below the horizon. Glow shows nautical night beside civil night for fainter aurora and photography.US Naval Observatory, Rise, set and twilight definitions

Astronomical twilight

Ends at 18 degrees below the horizon, when scattered sunlight falls below starlight, per USNO. See how dark it needs to be.US Naval Observatory, Rise, set and twilight definitions

Solar midnight

The sun's lowest point below the horizon. It rarely falls at 00:00 on the clock. The solar midnight tool computes it for any place and date.

No civil darkness

A night when the sun never reaches -6 degrees, from midnight sun or white nights. At Tromsø it runs from late April to mid August. See the midnight sun guide.

Dark season

Glow's own term: days with at least 4 hours of civil night between 18:00 and 06:00 local time. It is not an official season.

Moon-free darkness

Glow's term for civil-night minutes with the moon's center below the horizon. The moon planner counts it night by night.

Sky cover

The NWS forecast of how much of the sky cloud covers, in percent. Forecast words map to octants: clear 0/8, mostly clear 1/8 to 2/8, partly cloudy 3/8 to 4/8, mostly cloudy 5/8 to 7/8, cloudy 8/8.National Weather Service glossary, Sky condition

What does the K in Kp stand for?

SWPC says the K comes from the German word Kennziffer, meaning characteristic digit. The p stands for planetary.

Is geomagnetic latitude the same as magnetic latitude?

The terms are often used for the same idea: latitude measured from a magnetic pole instead of the spin axis. Glow computes it on the IGRF-14 centered dipole and calls it dipole latitude when precision matters.

What is the difference between civil and astronomical twilight?

Civil twilight ends with the sun 6 degrees below the horizon; astronomical twilight ends at 18 degrees. Glow treats the first as dark enough for bright aurora.

What does G3 mean in an aurora forecast?

A strong geomagnetic storm, Kp 7 on the NOAA scale. SWPC says aurora at that level has been seen as low as Illinois and Oregon, typically 50 degrees geomagnetic.

What is the OVATION model?

The empirical aurora model behind NOAA SWPC's 30 to 90 minute aurora forecast. It uses solar wind data measured at the L1 point.

• NOAA SWPC, Aurorahttps://www.swpc.noaa.gov/phenomena/aurora
• NOAA SWPC, Planetary K-indexhttps://www.swpc.noaa.gov/products/planetary-k-index
• NOAA SWPC, Geomagnetic stormshttps://www.swpc.noaa.gov/phenomena/geomagnetic-storms
• NOAA Space Weather Scaleshttps://www.spaceweather.gov/noaa-scales-explanation
• NOAA SWPC, Aurora 30-minute forecasthttps://www.spaceweather.gov/products/aurora-30-minute-forecast
• NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora
• US Naval Observatory, Rise, set and twilight definitionshttps://aa.usno.navy.mil/faq/RST_defs
• National Weather Service glossary, Sky conditionhttps://forecast.weather.gov/glossary.php?letter=s

Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.