What the aurora actually is
The aurora is high-altitude (80–500 km) emission from oxygen and nitrogen atoms in the upper atmosphere, energised when charged particles from the Sun get steered down magnetic field lines into the polar regions. The bright green colour is from atomic oxygen at ~100 km altitude; the red glow above is from oxygen at higher altitudes; pinks and blues come from nitrogen.
Activity is driven by the Sun's wind and especially by coronal mass ejections — and follows the ~11-year solar cycle. We're currently near the maximum of Solar Cycle 25, so 2024–2027 are unusually good years for aurora at mid-latitudes.
The Kp index
The Kp index is a 0-to-9 number summarising global geomagnetic activity, derived every three hours from ground magnetometer readings around the world. NOAA's Space Weather Prediction Center (SWPC) publishes a real-time estimated Kp every 5 minutes and a 3-day forecast every 30 minutes.
- Kp 0–3: Quiet. Aurora confined to the typical auroral oval at high latitudes (~67° geomagnetic and above).
- Kp 4: Active. Pushed equatorward — visible from the northern US border states.
- Kp 5 (G1 storm): Minor geomagnetic storm. Aurora reaches ~56° geomagnetic — northern Michigan, Maine, much of Scotland.
- Kp 6 (G2): Moderate storm. Reaches ~54° — Pennsylvania, Iowa, southern England.
- Kp 7 (G3): Strong storm. Reaches ~52° — Oregon, Illinois, central Europe.
- Kp 8 (G4): Severe storm. Reaches ~50° — northern California, much of mid-latitude US.
- Kp 9 (G5): Extreme storm. Aurora has been reported as far south as Texas and Hawaii — historically rare; happened in May 2024 and Oct 2024.
Geomagnetic vs geographic latitude
The auroral oval is centred on the geomagnetic pole, which is offset from the geographic pole by about 11°. The geomagnetic north pole is currently in the Canadian Arctic near Ellesmere Island. This means:
- Norway and northern Russia are at higher geomagnetic latitude than their geographic latitude suggests — aurora is easy.
- Alaska and northern Canada are at much higher geomagnetic latitude — aurora is very easy.
- The British Isles, despite being at high geographic latitude (54–60°), are at relatively modest geomagnetic latitude — aurora needs Kp 5+ to reach Edinburgh, Kp 6+ for London.
CVAN computes your geomagnetic latitude using the IGRF-13 dipole model and compares it directly to the Kp viewing threshold. That's why the verdict can say "viewable" at Kp 4 from one location and "too far from pole" from another at the same geographic latitude.
Kp thresholds by city
Approximate minimum Kp needed for aurora to be visible on the northern horizon from common viewing cities (assuming clear skies and dark site):
Other essential factors
It must be night
Aurora doesn't show against a daylight or twilight sky. The sun must be below about −12° altitude (nautical twilight) to start seeing colour; below −18° (astronomical night) is ideal. This is why summer in northern Scandinavia is hopeless for aurora despite massive solar activity — the sun never sets enough.
Clear skies
Cloud cover above ~70% blocks aurora completely. CVAN factors in the Open-Meteo cloud cover forecast and warns when you've got viewing latitude but lose the bet to weather.
Magnetic local time
Aurora is strongest around magnetic midnight — the time when your location is on the night side of the Earth's magnetic axis. For most of North America and Europe this corresponds roughly to 10 PM – 2 AM local time.
Solar activity
High Kp comes from coronal-mass ejections (CMEs) and coronal-hole high-speed streams. SWPC's 3-day Kp forecast — pulled into CVAN's Kp Forecast tab — is the most actionable signal; a CME that arrives in 2 days will already be in the forecast curve.
Using the live forecast
CVAN's Aurora tab presents:
- The current Kp index, refreshed every 5 minutes.
- Your geomagnetic latitude vs the current viewing threshold.
- An interactive auroral-oval map with the day/night terminator overlaid.
- The 3-day Kp forecast curve (NOAA SWPC), updated every 30 minutes.
- A combined verdict: VIEWABLE / POSSIBLE / TOO FAR / WAIT FOR NIGHT / CLOUDED OUT.