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How to read an aurora alert from the Bureau

Outlook, watch, alert, and that mysterious K number. Nana Sol decodes it for anyone on the NSW coast.

Who actually issues aurora warnings in Australia?

The Bureau of Meteorology does, through its Space Weather Services, also known as the Australian Space Weather Forecasting Centre. It puts out three kinds of notice, and you can subscribe to all of them by email. They sit under the Bureau's products for a possible auroral event.

The trick is knowing which one means "maybe next week" and which one means "get the chair out now".

What's the difference between an outlook, a watch and an alert?

An aurora outlook has a lead time of three to seven days. It goes out when a large, active region on the Sun is expected to turn toward a position where it could fling a coronal mass ejection our way. Think of it as a forecast of a forecast.

An aurora watch has a lead time of up to 48 hours. It follows a significant coronal mass ejection, or a coronal hole, that is expected to stir up Earth's magnetic field. Something is on its way.

An aurora alert means conditions are favourable right now. It tells you the band of latitudes where an aurora may be seen under good conditions. This is the one that matters on the night.

What is the K index?

It's a number from 0 to 9 that describes how much Earth's magnetic field is being disturbed. NOAA's Space Weather Prediction Center explains that each station's K value comes from the biggest swings a magnetometer records over a three-hour block, compared with a quiet day.

Because each station reads a little differently, there's also a planetary version, Kp. That's a weighted average from a network of observatories around the world.

And how does that link to storms?

NOAA's G scale for geomagnetic storms starts where Kp reaches 5. Kp 5 is a G1 storm, the minor end. Kp 9 is G5, the extreme end. Anything from Kp 0 to 4 is below storm level.

The Bureau puts it simply: the larger the K index, the greater the chance of seeing an aurora at lower latitudes. Its aurora page links a table that matches K values to the latitudes where a display might be visible.

What do the colours on the dials mean?

A guide in The Conversation describes a common colour code. Green, under 5, is a mild disturbance. Yellow, up to 7, is moderate. Red, reaching 9, is a major storm.

The same guide mentions two solar wind readings worth a glance. Speed tells you how fast the wind is heading for Earth. Bz is the direction of its magnetic field, and a southward, negative Bz helps auroras along.

Where should I look from here?

South. In the southern hemisphere you generally need a clear view of the southern horizon. Tasmania and southern New Zealand have the best seats, so further north the glow sits lower and fainter.

Check the cloud forecast first, as the Bureau suggests. Find a dark spot away from streetlights. Then give it time: displays often last 15 to 30 minutes, and sometimes run for hours.

So what's the routine?

Subscribe to the Bureau's notices. Treat an outlook as interesting, a watch as worth planning for, and an alert as your cue to step outside. Then face south, wait, and keep the kettle handy.

Nana Sol · opinion

The sun's been busy, petal, but the sun doesn't read the alerts. I've had nights with a big number and thick cloud, and nights with a small number and a pink smudge low to the south. The question nobody asks: how many auroras have we missed just because we went to bed?

Sources

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