Seeing the northern lights, or aurora borealis, is high on many people’s bucket list. Is it on yours? © Smit/ Shutterstock
By James Ashworth and Beatrice Boutayre
The northern lights, also called the aurora borealis, have transfixed humans for thousands of years.
Discover the best time and place to see this natural phenomenon as well as the science behind how we forecast it.
The northern lights is the name given to the colourful lights that can appear in the night sky over the northern hemisphere. When they appear in the southern hemisphere, they’re called the southern lights instead.
This phenomenon is known as an aurora. The northern lights are more formally known as the aurora borealis because borealis means north in Latin. The southern lights, meanwhile, are called the aurora australis.
Auroras don’t just happen on Earth. Scientists have also seen them on other planets, including Mars, Jupiter and Saturn, as well as on comets and even dwarf stars far beyond our solar system.
The northern lights are caused by the Sun’s powerful magnetic field. In this ultraviolet photo, the magnetic field appears as bright lines in the haze of charged particles above the star’s surface. Public domain image by NASA/ GSFC/ Solar Dynamics Observatory via NASA Image and Video Library.
The northern lights are caused by the interaction between the magnetic fields of the Sun and Earth. Dr Geoff Vasil, who studies the Sun’s workings at the University of Edinburgh’s School of Mathematics, explains how this creates an aurora.
“The Sun is a nuclear furnace, squeezing atoms together at its heart to release energy,” Geoff explains. “Ultimately, that’s where the power behind the northern lights comes from.”
“Over time, this energy gradually emerges from the core and leaks to the outside, where it starts to cause convection currents in the Sun’s upper layers. These currents generate the Sun’s magnetic field.”
“While we’re not exactly sure how everything works, the intense heat and magnetism is enough to launch energetic charged particles into space as the solar wind.”
These charged particles are released in all directions, with some heading towards Earth.
Earth generates its own magnetic field due to currents in its metallic outer core, and this helps to deflect most of the charged particles.
But not all the particles are deflected. Instead, some are funnelled towards the planet’s poles and into the atmosphere. It’s these charged particles that are responsible for auroras such as the northern lights.
The main colours of the northern lights include greens, blues, reds and pinks, with other shades resulting from different light mixing. © Maris Hinn/ Shutterstock
The colours of the northern lights result from the charged particles hitting different atoms and molecules in Earth’s upper atmosphere. Some of the energy from the charged particles is passed on during the collision, but the atoms and molecules can’t store it for very long.
Instead, this excess energy is released as light, and each element emits different colours:
Bursts of charged particles can be released by solar flares. When this happens, it leads to stronger northern lights. Public domain image by NASA/ Goddard/ SDO via NASA Image and Video Library.
The strength of the northern lights varies for many different reasons, including how active the Sun is. Our star goes through an 11-year-long solar cycle during which the strength of its magnetic field changes.
When the solar cycle peaks, known as the solar maximum, the Sun’s magnetic field is at its strongest. This leads to stronger solar storms and more of them, including events such as solar flares and coronal mass ejections.
These can throw large amounts of charged particles out into space towards Earth. This causes geomagnetic storms that increase the chance of the northern lights occurring and make it more likely that the lights will be strong.
“When the magnetic fields of the Sun reach our planet, they can push Earth’s magnetic field out behind it,” explains Geoff. “Once it is stretched out far enough, Earth’s magnetic field snaps back like an elastic band.”
“This is known as reconnection and releases a large amount of particles into the upper atmosphere. This causes the northern lights to grow bigger and brighter.”
Your best chance to see the northern lights is in countries in the far north, such as Iceland. © Piotr Krzeslak/ Shutterstock
The best place to see the northern lights changes depending on the solar cycle, the time of year and a variety of other factors. Generally, however, northern lights visibility is normally better the closer you are to magnetic north.
“The location of the northern lights can change depending on the solar wind,” Geoff says. “They tend to happen nearer the poles where particles can enter the atmosphere more easily, but the aurora can move closer to the equator depending on the conditions.”
The region where the northern lights are most common is known as the annulus. This is a ring-shaped region centred on the northern magnetic pole that’s around 3,000 kilometres across.
Countries inside the annulus, including Finland, Iceland and Canada, are generally the best places to see the northern lights.
When solar activity is strong enough, however, the annulus can expand and reach more southerly nations, such as the UK and USA. This is most common around the time of the solar maximum.
UK northern lights, as seen here over Brighton, are more common during periods of high solar activity. © Jamie O’Gorman/ Shutterstock
The best time to see the northern lights is generally during March and April as well as later in the year during September and October. It’s particularly common around the spring and autumn equinoxes, the two occasions each year when day and night are roughly the same length.
This is often known as the Russell-McPherron effect, named after the two scientists who noticed that auroras tend to get stronger around the equinoxes. Their explanation for this is based on the way the Sun’s and Earth’s magnetic fields interact.
Like all magnets, their magnetic fields have a north and a south pole. The polarity of these magnetic fields changes over the year. Around the equinoxes, the polarity of the Sun’s and Earth’s magnetic fields line up in opposite directions.
This allows more particles from the solar wind to get into Earth’s atmosphere, causing more intense northern lights. The magnetic poles are also at a right angle to the flow of the solar wind around the equinoxes, which further enhances the aurora.
The effect of equinoxes is just one of the factors scientists consider when forecasting the northern lights.
The northern lights can be hundreds of kilometres high as this photo from the International Space Station shows. Public domain image by JSC via NASA Image and Video Library.
Northern lights forecasts attempt to predict when and where auroras will take place. Forecasters combine information about conditions on Earth and the Sun to predict how visible the northern lights will be, but it’s a tricky process.
“We know what happens in almost every stage of the process of the northern lights forming, but the whys are mysterious,” explains Geoff. “The stages on Earth are much better understood than those in the Sun.”
“As we don’t fully understand how the Sun works, solar forecasting currently attempts to model what our star might do based on previous observations. For example, the size and position of sunspots can be used to suggest when they might erupt.”
“At the moment, our short-term forecasts are more accurate than the longer-term predictions, which will need us to have a better idea of what’s happening in the Sun.”