Aurora forecasting: how to chase the northern lights
Real forecasting gives you about thirty minutes of warning. Everything longer is a probability.
5 min read · Published 24 August 2026
The short answer
Aurora forecasting works on two timescales. Long-range forecasts of a few days come from observing solar eruptions and estimating arrival, and are broadly unreliable. Short-range forecasts of roughly 15 to 45 minutes come from spacecraft measuring the solar wind upstream of Earth, and those are genuinely actionable. The key numbers are the Kp index for expected latitude reach, and the Bz component of the interplanetary magnetic field, which must be pointing south.
What causes it
The Sun emits a continuous stream of charged particles, the solar wind. Earth's magnetic field deflects most of it, but particles enter near the magnetic poles and collide with atmospheric gases high up.
Those collisions excite atoms, which release the energy as light. Oxygen produces the common green at around 100 to 150 km altitude, and a rarer deep red at higher altitudes above 200 km. Nitrogen produces blues and purples, often seen at the lower edge of a display.
So the colours tell you which gas and roughly what altitude. Green is by far the most common because oxygen at that altitude is abundant and the transition is efficient.
The numbers that matter
| Measure | What it means | How to use it |
|---|---|---|
| Kp index (0–9) | Global geomagnetic disturbance | Rough guide to how far from the poles aurora may be visible |
| Bz | North-south direction of the interplanetary magnetic field | Must be negative — southward — for strong coupling. The single best short-term indicator |
| Solar wind speed | Typically 300–800 km/s | Higher is better; above 500 is promising |
| Density | Particles per cubic centimetre | Higher density with southward Bz is a good combination |
| Hemispheric power | Energy deposited into the auroral zone | A direct measure of current activity |
Bz is the one to watch. Earth's field points north; when the arriving field points south, the two connect efficiently and energy pours in. A high solar wind speed with a northward Bz frequently produces nothing at all, which is why speed alone is a poor predictor.
Why forecasts beyond an hour are unreliable
Long-range aurora forecasts are based on watching a coronal mass ejection leave the Sun and estimating when it will arrive, typically one to three days later.
The problem is that its magnetic orientation on arrival cannot be determined in advance. A CME can hit Earth squarely and produce almost nothing if its field points north. The arrival time itself is also uncertain by many hours.
The genuinely actionable forecast comes from spacecraft positioned about 1.5 million kilometres sunward of Earth, at the L1 point, which measure the solar wind directly as it passes. That gives roughly 15 to 45 minutes of warning depending on wind speed — enough to get outside, not enough to plan a trip.
So the practical approach is: use long-range forecasts to decide which nights to pay attention, and short-range data to decide when to go out.
Practical chasing
What actually helps
- Get north. Latitude dominates everything else. From high latitudes a modest Kp is enough; from mid-latitudes you need a rare strong storm.
- Find a clear northern horizon. From mid-latitudes aurora usually appears low in the north, so a hill or building in that direction ends the attempt.
- Escape light pollution. Faint aurora is washed out exactly as deep sky objects are.
- Check the Moon. A bright Moon reduces contrast for a faint display.
- Watch Bz in real time, not just Kp. Kp is calculated in three-hour blocks and lags; Bz is live.
- Be patient and stay out. Displays surge and fade over minutes to hours. People frequently leave ten minutes before a substorm.
- Use your phone as a detector. A long exposure of a grey patch will tell you whether it is cloud or aurora.
The solar cycle
Solar activity rises and falls over roughly an eleven-year cycle, and aurora frequency follows it. Around solar maximum, displays reach lower latitudes more often and mid-latitude sightings become genuinely possible rather than exceptional.
During the strongest storms aurora has been recorded far outside its usual range. The Carrington Event of 1859 — the most intense geomagnetic storm on record — produced aurora visible near the tropics, and induced currents strong enough to disrupt telegraph systems.
A comparable event today would be a serious infrastructure problem, which is why space weather monitoring is funded as a practical concern rather than an astronomical curiosity.
Common questions
- What is the Kp index?
- A 0 to 9 scale of global geomagnetic disturbance, used as a rough guide to how far from the poles aurora may be visible. It is calculated in three-hour blocks, so it lags real-time conditions and should be used alongside live solar wind data.
- What does Bz mean for aurora?
- Bz is the north-south component of the interplanetary magnetic field. It must be negative — pointing south — for efficient coupling with Earth's field. A fast solar wind with a northward Bz frequently produces no aurora at all.
- How far in advance can aurora be forecast?
- Reliably, only about 15 to 45 minutes, from spacecraft measuring the solar wind 1.5 million kilometres sunward of Earth. Longer forecasts estimate a coronal mass ejection's arrival but cannot determine its magnetic orientation, which is what decides whether anything happens.
- Why does my camera see aurora when I cannot?
- Human night vision relies on rod cells, which see no colour, so faint aurora often looks grey to the eye. A camera integrating light over several seconds records the green that is genuinely there. Phones are useful for confirming whether a pale patch is aurora or cloud.