Auroras: What Causes the Northern and Southern Lights

Few sights in nature match an aurora in full swing: curtains of green light rippling across the sky,

rays shooting upward, and edges tinged with red or purple, all shifting from minute to minute.

People travel across the world hoping to see one, and sometimes a strong display surprises observers far from the polar regions.

The science behind the northern and southern lights connects our planet to the Sun in a direct and visible way.

Here is how auroras form, what their colors mean,

where and when you have the best chance of seeing them and how to use forecasts without being fooled by them.

It Starts at the Sun

The Sun constantly releases a stream of electrically charged particles into space, called the solar wind.

It flows past Earth all the time, at speeds of hundreds of kilometers per second, carrying the Sun's magnetic field with it.

At times the Sun releases much more.

Coronal mass ejections are enormous clouds of charged gas and magnetic field thrown off the Sun, often associated with solar flares.

When one is aimed toward Earth, it typically arrives within one to a few days.

Fast streams of solar wind flowing from regions called coronal holes can also stir up activity.

These events cause geomagnetic storms, and storms produce the brightest and most widespread auroras.

Following solar activity is part of the fun of aurora hunting, but leave the Sun itself to the forecasters and spacecraft.

Never look at the Sun without certified solar filters,

and never through binoculars or a telescope unless a proper solar filter covers the front of the instrument.

How Earth's Magnetic Field Creates the Glow

Earth is surrounded by a magnetic field that shields us from most of the solar wind.

That protective bubble, the magnetosphere, is stretched and squeezed by the flow of particles.

When the solar wind's magnetic field lines up in the right direction,

energy pours into the magnetosphere and charged particles are driven down along magnetic field lines toward the regions around Earth's magnetic poles.

There, mostly between about 100 and 300 kilometers above the ground, the particles collide with oxygen and nitrogen in the thin upper atmosphere.

The collisions give the gas extra energy, which it releases as light, a little like the glow of a neon sign.

The result is a ring of auroral activity around each magnetic pole, called an auroral oval.

The aurora borealis in the north and the aurora australis in the south occur at the same time,

often as rough mirror images of each other.

What the Colors Mean

Each color comes from a particular gas at a particular height:

• Green is the most common color, produced by oxygen at around 100 to 250 kilometers up. The human eye is especially sensitive to it.
• Red also comes from oxygen, but higher up, above roughly 250 kilometers. It often tops tall green curtains and dominates during strong storms seen from lower latitudes, where observers see the upper parts of distant auroras.
• Blue and purple come from nitrogen and often appear along the lower edges of bright, active curtains.
• Pink and yellow tones are usually mixtures, such as red and green overlapping.

Auroras are not unique to Earth.

Jupiter and Saturn have powerful auroras around their poles too, observed by spacecraft and space telescopes.

Where to See Them

Your chances depend mostly on your distance from the auroral ovals.

The most reliable viewing regions sit beneath or near them:

• Northern Hemisphere: northern Norway, Sweden and Finland, Iceland, Greenland, Alaska and northern Canada.
• Southern Hemisphere: Tasmania, the southern part of New Zealand's South Island and, for the few who go there, Antarctica.

During strong geomagnetic storms, the ovals expand toward the equator and auroras can be seen from much lower latitudes,

including parts of Europe, the northern United States and southern Australia.

From these mid-latitude locations, the aurora usually appears low over the poleward horizon,

so look north in the Northern Hemisphere and south in the Southern Hemisphere.

When to Look

Auroras happen during the day as well, but you need darkness to see them.

Near the poles, summer nights are too bright,

so the northern aurora season runs roughly from late August or September to March or April,

and the southern season roughly from March to September.

The weeks around the equinoxes tend to be somewhat more active on average.

The Sun's activity rises and falls over a cycle of about 11 years.

Strong auroras are more frequent around the peak of the cycle and during the few years that follow,

but good displays can happen at any stage.

On a given night, activity often comes in bursts called substorms, when a quiet arc suddenly brightens,

breaks into moving curtains and fills the sky for a few minutes to an hour.

Many displays peak in the hours around local midnight, but they can start soon after dark.

Patience is essential: an hour of quiet sky can turn spectacular with little warning.

Reading Aurora Forecasts

Government space weather agencies and many apps publish aurora forecasts.

The most common number is the Kp index, which runs from 0 to 9 and measures global geomagnetic disturbance.

Higher values mean the auroral ovals are expanding, so auroras may be visible farther from the poles.

Near the Arctic Circle, even modest values can bring a display, while mid-latitude observers usually need a strong storm.

Treat forecasts days ahead as rough guides.

Scientists can see eruptions leaving the Sun, but how strongly they affect Earth depends on details,

especially the direction of the magnetic field they carry,

that are only measured shortly before arrival by spacecraft positioned between Earth and the Sun.

Short-term alerts based on those measurements typically give less than an hour of notice.

Check the cloud forecast too; no aurora is visible through thick cloud.

What It Looks Like in Real Life

Auroras take many forms: a quiet arc across the sky, rippling curtains and bands, vertical rays,

faint patches that pulse on and off, and, when activity is overhead,

a corona in which rays seem to converge on a single point above you.

Strong displays can move and change shape within seconds.

Set your expectations for color.

Faint auroras often look gray or whitish to the eye, because our color vision works poorly in dim light,

while a camera reveals green or red. Bright, active displays, on the other hand,

show vivid color to the naked eye.

If you see a pale, arching glow on the poleward horizon, take a quick photo to check whether it is aurora.

Photographing and Staying Safe

A camera on a tripod with a wide-angle lens is ideal.

Focus manually on a bright star, set a high ISO and try exposures of a few seconds for bright,

fast-moving displays and longer for faint ones. Many smartphones capture brighter auroras well in night mode when held steady.

Carry spare batteries in an inside pocket, since cold drains them quickly.

Aurora hunting often means cold, remote places at night.

Dress for serious cold, with insulated layers, a hat, gloves and warm boots, and watch for signs of frostbite.

Drive carefully on icy roads, never stop on the roadway to look up, and park only in safe, legal places.

Check local conditions, tell someone your plans, and consider an experienced local guide if you are new to winter travel.

Chasing the Lights

Seeing an aurora comes down to three things: being near the auroral zone or catching a strong storm,

having dark and clear skies, and waiting long enough.

Watch the forecasts, keep an eye on the cloud cover, face the poleward horizon and give the sky your time.

When the curtains finally start to move, the cold and the waiting will feel entirely worth it.