Aurora Field Glossary: Every Term You'll Meet, Plainly

Every aurora forecast throws jargon at you. Here is the complete working vocabulary — each term in one or two sentences, no prerequisites, cross-linked to the deep-dive pages where they exist.

The numbers

Kp index — 0–9 planetary measure of geomagnetic activity, updated every 3 hours. The headline number. Full explainer →

G1–G5 — NOAA's geomagnetic storm scale; G1 starts at Kp 5, G5 is extreme (the May 2024 event). Each G-level has real-world impacts, from grid fluctuations to satellite drag.

Bz — the north–south tilt of the solar wind's magnetic field; sustained southward (negative) Bz is the trigger for aurora bursts. Full explainer →

The structures

Auroral oval — the ring around each magnetic pole where aurora lives; storms inflate it toward mid-latitudes. Full explainer →

Substorm — a sudden release of stored magnetospheric energy: the aurora explodes in brightness and movement for 30–60 minutes. The reason you check the sky every 20 minutes.

Coronal hole — a region of open solar magnetic field that streams fast solar wind toward Earth; the classic source of recurring Kp 4–6 periods every ~27 days as the Sun rotates.

CME — coronal mass ejection: a billion-ton bubble of plasma thrown off the Sun; the engine of the biggest storms (Kp 8–9).

The sky

Bortle class — 1–9 scale of night-sky darkness; cities are 8–9, wilderness is 1–2. Full explainer →

Astronomical twilight — the sun between 12° and 18° below the horizon; below −18° the sky is fully dark and the aurora gets its stage. Full explainer →

Midnight sun — polar summer, when the sun never gets low enough for darkness; the annual hard-off for aurora-zone viewing.

Aurora australis — the southern lights, ringing the south magnetic pole; best viewed from Tasmania, New Zealand's South Island and Antarctica.

Where to go from here

Pick your city from any page's navigation to see all of this applied live — or read the methodology to see exactly how we weigh each factor.

Using the vocabulary in practice

You don't need all of this to get outside — you need about four terms, and you already know where to apply them. Kp: check it against your city's threshold (the city page does it for you). Bz: watch it when the verdict is MAYBE and the sky is clear. Cloud: trust tonight, doubt next week. Moon: prefer new-moon weeks for trips. Everything else — substorms, CMEs, coronal holes, the oval — is the deeper game, and the linked explainers cover each one in the same plain style. The glossary earns its keep when you read space-weather alerts: once "G2 watch" and "negative Bz" parse naturally, NOAA's posts become actionable instead of noise.

How the pieces fit together

A typical strong night tells this story: a coronal hole rotated into an Earth-facing position days ago; the fast solar wind it streams arrives with sustained negative Bz; the Kp index climbs through the evening; the oval inflates and pushes its edge toward mid-latitudes; a substorm fires around local midnight and the display surges for 45 minutes under whatever clouds, moonlight and light pollution your sky adds to the mix. Every tool on this site watches a different link in that chain.