How to Never Miss an Aurora

Learn exactly how and when to expect the next display of the northern lights with a few easy-to-use online tools.

I'm often asked when is the best time to see the northern lights. I usually point out that they're connected to solar flares and vast eruptions of solar plasma called coronal mass ejections or CMEs. While these violent events and the auroras they spawn are more common during the peak of the sunspot cycle, they're liable to happen anytime and can be as unpredictable as earthly weather.

Not a very satisfying answer, I'll admit. But before you throw up your arms and seek another seer, allow me to arm you with several essential tools. If consulted regularly, I guarantee your chances of seeing an aurora will be maximized. Don't flog me if clouds show up. They're not part of the deal.

Your first step is to drop by NOAA's Space Weather Prediction Center, register (it's free) and sign up to receive their alerts, forecasts and summaries of incoming blasts from the Sun otherwise known as space weather. Many e-mailed products are offered, but these are the ones I'd recommend to start:

* NOAA 3-day forecast: A plain-language look ahead at expected disturbances to Earth's geomagnetic field, the protective bubble of magnetism that sheds most the worst of what the Sun throws at us. Shocks to the field from arriving clouds of subatomic particles from the Sun can sometimes spark auroras. Issued twice a day. Access it directly here.

* Forecast discussion: A free-form summary covering recent and expected space weather. Included are brief explanations of what caused a particular disturbance in Earth's geomagnetic field. Solar flares, CMEs, a filament of hydrogen gas ejected from the Sun, and high speed streams of particles escaping through "holes" in the Sun's magnetic canopy (coronal holes) are the usual culprits. Occasionally, the forecasters won't know for sure — just like your local TV weathercaster.

* Geophysical alert message: Updated every 3 hours, or more often if all hell breaks loose. This message provides current and predicted space weather conditions.

All three of these sources will add some heft to your inbox, but I doubt you'll mind if it could mean standing stunned-face under one of nature's grandest sky spectacles.

Now that you're subscribed, you will need to familiarize yourself with two key concepts: the K_p  index and B_z. Every report includes a reference to them. The index, from the German planetarische Kennziffer or "planetary code number", indicates the severity of Sun-induced magnetic disturbances in Earth's vicinity. Rated on a scale from 0 to 9, it's compiled using magnetometers at 13 magnetic observatories around the globe and updated every 3 hours.

When the index is below 5 the chances of seeing aurora are low or nil. A K_p of 5 likely indicates a minor or G1 geomagnetic storm visible across southern Canada and the northern U.S. When the index hits 6 or 7, auroras intensify as the auroral oval spreads southward. Skywatchers across the middle U.S. then get a shot at seeing shimmering curtains of pink and green light dance across their normally aurora-free skies.

Every forecast that arrives in your e-mail includes the estimated K_p index. A quick glance is all it takes to see if any "fives" are on the way. "Sevens" indicate an intense storm.

Ready to move on to B_z? Embedded within the billowy clouds of solar plasma is the stamp of the Sun's magnetic field. Like a magnet, the swarm has regions of positive and negative polarity.

Most of the time, Earth's protective magnetic bubble deflects the disturbance and we're none the worse for wear. But if the south-pointing region of the cloud (said to have a southward or negative B_z) happens to brush against Earth's northward-pointing magnetic field, the two will link up like two magnets snapping together. A path now opens for electrons and protons to spiral down Earth's magnetic field lines and smack into atoms and molecules in the upper atmosphere above the polar regions. The atoms reach an excited state and then return to relaxing and smoking cigars by releasing photons of green and red light. This is what creates the aurora.

Wouldn't it be nice to know in advance if an incoming blast possessed a south-pointing (B_z)? Wish granted!

Since 1997, we've been getting help from NASA's Advanced Composition Explorer (ACE) which orbits at the L₁ libration point, one of five places near Earth where the Sun’s and Earth’s gravity are in balance, allowing a satellite placed there to remain relatively stationary.

ACE acts as a distant early warning station 932,000 miles (1.5 million km) from Earth in the direction of the Sun.The probe detects the direction, strength and magnetic field particulars of incoming blasts of particles from the Sun and provides about an hour's advance warning of dangerous storms.

All this rich information, including the direction of the B_z, is plotted in near real time (updates every few minutes) at the ACE Dynamic Plots site. You can choose to see the B_z ups and downs at intervals from 2 hours to 7 days. Choose an interval and then click the "MAG SWEPAM" link.

The top plot, shown as a squiggly red line, represents the B_z. When it's positive or above the midline, the incoming solar wind points north and is unlikely to connect with Earth's field. But when it dips below the line, and in particular, when it sinks to –10 or lower, auroras are in the offing.

One final tool I'm sure you'll find useful is a relatively new NOAA product called the Ovation Oval. Simply, it shows the extent of the auroral oval, the permanent caps of aurora centered on Earth's geomagnetic poles. Polar bears and other arctic residents get to experience auroras most dark nights of the year. The rest of us have to wait for more vigorous storms, when the oval expands southward to more populous latitudes.

I hope this little guide will help you in your planning. I've selected just a few of all the resources available on the Web — you will undoubtedly find more. However you plan, remember that space weather can often be as unpredictable as Earth weather. Happy forecasting!

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Learn about the science of aurora in our recent cover story.