Notes · Aurora
How to Read an Aurora Forecast from New Zealand
Every few weeks the phones light up with screenshots of a red forecast map and the question "is it on tonight?" Sometimes it is. More often the numbers look exciting at lunchtime and fizzle by dark. Learning to read the forecast yourself saves a lot of wasted nights.
Start with the Kp index, but don't stop there
The Kp index is a 0 to 9 scale of global geomagnetic disturbance, worked out in three-hour blocks. It is the number everyone quotes because it is simple, and it does give a rough guide to how far from the pole the auroral oval has spread.
For New Zealand observers a rough rule of thumb looks like this:
| Kp | What it tends to mean here |
|---|---|
| 3 to 4 | Faint glow on camera from Southland, Otago and Rakiura with a clear southern horizon |
| 5 (G1 storm) | Camera aurora across much of the lower South Island, sometimes visible to the eye in the deep south |
| 6 to 7 | Possible across the South Island and the southern North Island on camera |
| 8 to 9 | Rare. The May 2024 storm was seen and photographed as far north as Northland |
The catch is that Kp is largely a measurement of what has already happened. The planetary Kp chart from the NOAA Space Weather Prediction Center is useful for confirming a storm is underway, less so for predicting the next hour.
The solar wind numbers that actually drive it
Satellites sitting about 1.5 million kilometres sunward of Earth measure the incoming solar wind, giving roughly 30 to 60 minutes of warning. Three values matter most.
Bz: the switch
Bz is the north-south component of the interplanetary magnetic field. When it points south (a negative number) it couples with Earth's field and lets energy in. When it points north, even a fast, dense wind often does very little. A sustained Bz of -10 nT or lower for an hour or more is where things get interesting. A Bz that flips back and forth every few minutes usually produces short, patchy bursts.
Speed
Background solar wind runs at around 300 to 400 km/s. Above 500 km/s is lively, and the big storms from coronal mass ejections can push well past 700 km/s. Speed amplifies whatever Bz is doing.
Density
Density (particles per cubic centimetre) adds pressure. A sudden jump in both density and speed often marks the arrival of a CME shock front, and that is the moment to watch Bz very closely.
The real-time solar wind page shows all three on one chart. The most recent data is on the right, and the last hour or two is what matters.
Local watches and alerts
The Australian Bureau of Meteorology's Space Weather Services aurora page issues products aimed at our part of the world, which makes them more relevant than northern-hemisphere apps:
- Aurora Outlook: activity possible within the next few days, usually tied to a CME or a coronal hole stream.
- Aurora Watch: activity expected within roughly 48 hours. Worth checking the cloud forecast and planning.
- Aurora Alert: geomagnetic activity is happening now and aurora may be visible from suitable dark sites. This is the one to act on.
Their email notices beat waiting for social media to tell you.
The 30-minute oval map
NOAA's aurora 30-minute forecast uses the OVATION model to draw a probability map of the oval. Switch to the southern hemisphere view. If the green band reaches the bottom of the South Island, a camera pointed south from a dark spot will probably catch something. Keep in mind the "view line" idea: you don't need the oval overhead, because aurora reaches hundreds of kilometres up and can be seen low on the horizon from well north of the oval itself.
Why the North Island needs a big one
Aurora australis sits around the south geomagnetic pole, which lies off the coast of Antarctica south of Australia. Every degree of latitude you move north pushes the display lower on the southern horizon. From Ōtepoti Dunedin a moderate storm can climb a good way up the sky. From Pōneke Wellington the same event might be a faint glow just above the sea, and from Tāmaki Makaurau Auckland it may not show at all. That is why North Island observers usually want Kp 6 or 7 and a strongly southward Bz before heading out, while Southlanders can be rewarded at Kp 4.
Don't forget the clouds and the moon
A perfect solar wind is useless under overcast. Check the MetService rural forecasts for cloud, and look for a clear run to the south specifically. Clear, cold air behind a southerly change can be excellent if you catch the gap. A bright moon washes out faint aurora but won't hide a strong storm.
Camera first, eyes second
Most New Zealand aurora is faint to the naked eye. The eye's rod cells aren't sensitive to colour in low light, so a display that photographs as vivid green and magenta may look like a grey-white glow or pale pillars. Take a test shot facing south: a few seconds at ISO 1600 to 3200 on a wide lens, or phone night mode on a tripod. If the image shows colour, look again with dark-adapted eyes and you will often pick out structure and movement.
A simple routine
- Watch for a BoM Outlook or Watch, or news of a sizeable CME.
- Check cloud for your area and a backup site.
- After dark, glance at Bz and speed every 15 minutes or so.
- Head out when Bz goes and stays south, or when an Alert is issued.
- Give it at least an hour. Substorms come in pulses, often around local midnight.
There is more background on the phenomenon itself on our long-running aurora page and the aurora australis page.