Space weather glossary
The words space-weather apps use, in plain language. Each entry says what the term describes and links to the guide that goes deeper.
On the sun
Active region
An area of strong, tangled magnetic field on the sun, usually visible as a group of sunspots. Most solar flares start in active regions; NOAA numbers them, for example AR 3664, the region behind the storm of May 2024.
Solar flare
A sudden burst of radiation from an active region, lasting minutes to hours; its light reaches Earth in about eight minutes. Flares are classed by peak X-ray brightness as A, B, C, M or X, each class ten times stronger than the one before. A big flare can disturb shortwave radio on the day side of Earth, but on its own it does not bring aurora. Space Weather: How Solar Activity Reaches Earth
Coronal mass ejection (CME)
A cloud of magnetised plasma thrown out from the sun. When one is aimed at Earth it usually arrives one to four days later, and it is the most common trigger of strong geomagnetic storms.
Coronal hole
A region where the sun’s magnetic field opens into space. It looks dark in extreme-ultraviolet images and releases a fast stream of solar wind; because the sun turns about once every 27 days, the minor storms it causes can come back on a similar rhythm.
Solar cycle
Sunspots rise and fall over a cycle of roughly eleven years. Solar Cycle 25 reached its maximum phase in 2024, and flares and storms stay common for several years after the peak.
Between the sun and Earth
Solar wind
The steady stream of charged particles leaving the sun. Typical speeds are 300 to 500 km/s; fast streams and CMEs can push it beyond 700 km/s.
L1
A point about 1.5 million kilometres from Earth toward the sun, where spacecraft measure the solar wind before it arrives. Depending on its speed, the wind needs roughly 15 to 60 minutes from L1 to Earth, which is the warning time live dashboards work with. See today’s solar wind →
Bz
The north-south part of the magnetic field carried by the solar wind, measured in nanotesla. When Bz points south (negative) for hours, energy flows into Earth’s magnetic field far more easily and storms build; a strongly negative Bz is often the best short-term hint that aurora may follow.
At Earth
Kp index
A scale from 0 to 9 for how strongly Earth’s magnetic field was disturbed during a three-hour window, based on magnetometer stations at mid-latitudes. The definitive values come from GFZ Potsdam; apps show NOAA’s near-real-time estimate. Space Weather Dashboard Basics
G-scale
NOAA’s five-step storm scale: G1 (minor) corresponds to Kp 5, G2 to Kp 6, G3 to Kp 7, G4 to Kp 8 and G5 (extreme) to Kp 9.
Geomagnetic storm
A period of strong disturbance of Earth’s magnetic field, by definition Kp 5 or higher. Most last from a few hours to about two days. Geomagnetic Storms and HRV
Aurora oval
The ring around each magnetic pole where aurora appears. During storms it widens toward the equator; from Switzerland you usually need Kp 7 or more and a clear view to the north. Space Weather: How Solar Activity Reaches Earth
Earth’s electromagnetic background
Schumann resonance
Extremely low-frequency waves that travel around the globe in the cavity between the ground and the ionosphere, excited mainly by lightning. The strongest mode lies near 7.8 Hz, with further modes near 14, 21, 27 and 34 Hz. Schumann resonance today →
7.83 Hz
The textbook value of the first Schumann mode. Real measurements drift by a few tenths of a hertz with the time of day and the state of the ionosphere. 7.83 Hz explained →
Spectrogram
The colourful chart many Schumann stations publish: time runs across, frequency runs up, and colour shows signal strength. Sudden bright patches can come from nearby thunderstorms or local interference, so one station’s chart is not a reading of the whole planet.
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