The northern lights are nature’s very own magnificent light show. They are the mesmerising end result of electrically charged particles from the sun colliding with the Earth’s upper atmosphere. Though more frequently witnessed from the polar regions, the UK and other places on similar latitudes are lucky enough for the aurora borealis to occasionally grace their night sky.
But recent reports now claim the phenomenon may no longer be visible from places such as the UK – instead confined to the North Pole. But is this correct?
The northern lights are driven by activity on the sun and the sun’s activity waxes and wanes over an 11-year period known as a solar cycle. The number of large-scale aurora events, the type that is visible from places such as the UK, tends to follow this cycle. But each solar cycle is different, with the maximum and minimum activity varying between each cycle.
Predicting solar activity
Current predictions suggest that we are headed for a period of particularly weak solar cycles, where the solar maximum of each cycle will not result in much solar activity. We call this a grand solar minimum.
Grand solar minimums can last for several decades or even centuries and have occurred throughout history. Although solar output does decline during these periods, it doesn’t mean that we are heading for a new ice age.
A study recently published in Nature has modelled the perhaps most well-known grand solar minimum, called the “Maunder minimum”. This particular grand solar minimum started in 1645 and finally ended 70 years later. During this time only 50 sunspots, structures on the sun that act as a measure of its activity, were observed. This is compared to the 40,000-50,000 that we would expect during a period of “normal” activity lasting that long.
The authors of the study found that during the Maunder minimum, the solar wind, which drives the aurora, dramatically weakened. They also illustrate that as the solar wind weakens, so too will the aurora.
If we are in fact heading into a new grand solar minimum, it stands to reason that we might see less of nature’s beautiful spectacle. But does that mean we’ll stop seeing it from the UK altogether as some have suggested?
Lessons from the past
Looking back at historical records of aurora sightings might provide the answer. Fortunately, a study has done just that. The authors analysed auroral observations during two grand solar minimums– including the Maunder minimum. They found that the number of aurora sightings from below 56° magnetic latitude (which is similar to geographic latitude but measured from the magnetic pole rather than the geographic pole) did indeed decrease. But they did not stop altogether.
That value of 56° magnetic latitude is actually quite important as it happens to coincide with the magnetic latitude of the UK (more specifically somewhere close to Lancaster, England).
So what’s my prediction for the aurora over the next century? If the models are correct and we do head into a grand solar minimum, then solar activity is going to decrease – and remain at very low levels for decades to come. With this decrease in solar activity, aurora sightings from outside the polar regions are going to become rarer. But that doesn’t necessarily mean they’ll stop altogether. It also isn’t certain that we are heading for a grand solar minimum or – even if we are – when it might occur.
So while that elusive light show might get even more elusive, don’t fret just yet: the northern lights aren’t going out anytime soon.
Amateur astronomers and aurora hunters alike have been reporting a green glow across the UK sky. Easily confused with the aurora borealis, or northern lights, the sightings were of another phenomena called “airglow”.
Airglow is the natural “glowing” of the Earth’s atmosphere. It happens all the time and across the whole globe. There are three types of airglow: dayglow, twilightglow and nightglow. Each is the result of sunlight interacting with the molecules in our atmosphere, but they have their own special way of forming.
Dayglow forms when sunlight strikes the daytime atmosphere. Some of the sunlight is absorbed by the molecules in the atmosphere, which gives them excess energy. They become excited. The molecules then release this energy as light, either at the same or slightly lower frequency (colour) as the light they absorbed. This light is much dimmer than daylight, so we can’t see it by eye.
Twilight glow is essentially the same as dayglow, but only the upper atmosphere is sunlit. The rest of the atmosphere and the observer on the ground are in darkness. So, unlike day glow, twilightglow is actually visible to us on the ground with the naked eye.
The chemistry behind nightglow is different. There is no sunlight shining on the nighttime atmosphere. Instead, a process called “chemiluminescence” is responsible for the glowing atmosphere.
Sunlight deposits energy into the atmosphere during the day, some of which is transferred to oxygen molecules (e.g. O₂). This extra energy causes the oxygen molecules to rip apart into individual oxygen atoms. This happens particularly around 100km in altitude. However, atomic oxygen isn’t able to get rid of this excess energy easily and so acts as a “store” of energy for several hours.
Eventually the atomic oxygen does manage to “recombine”, once again forming molecular oxygen. The molecular oxygen then releases energy, again in the form of light. Several different colours are produced, including a “bright” green emission.
In reality, the green nightglow isn’t particularly bright, it’s just the brightest of all nightglow emissions. Light pollution and cloudy skies will prevent sightings. If you’re lucky though, you might just be able to see it by eye or capture it on long-exposure photos.
Not to be confused with aurora
The green night glow emission is very similar to the famous green we see in the northern lights. This is unsurprising since it is produced by the same oxygen molecules as the green aurora. But the two phenomena are not related.
Aurora form when charged particles, such as electrons, bombard the Earth’s atmosphere. These charged particles, which started off at the sun and were accelerated in the Earth’s magnetosphere, collide with the atmospheric gases. They transfer energy, forcing the gases to emit light.
But it isn’t just the process behind them that is different. The aurora form in a ring around the magnetic poles (known as the auroral oval); whereas nightglow is emitted across the whole night sky. The aurora are very structured (due to the Earth’s magnetic field); whereas airglow is generally quite uniform. The extent of the aurora is affected by the strength of the solar wind; whereas airglow happens all the time.
Why then did we get a lot sightings from the UK recently, rather than all the time? The brightness of airglow correlates with the level of ultraviolet (UV) light being emitted from the sun – which varies over time. The time of year also seems to have an impact on the strength of airglow.
To maximise your chances of spotting airglow, you’ll want to take a long-exposure photograph of a clear, dark, night sky. Airglow can be spotted in any direction that is free of light pollution, at about 10⁰-20⁰ above the horizon.
NASA has posted a feature article about how the Aurorasaurus project and a recent study by Nathan are helping scientists to understand the aurora and estimate where they might be visible from.
“Without the citizen science observations, Aurorasaurus wouldn’t have been able to improve our models of where people can see the aurora,” said the study’s lead author, Nathan Case, a previous Aurorasaurus team member and now a senior research associate at Lancaster University, United Kingdom. “The team is very thankful for our community’s dedication and are excited to have more people sign up.”
People in the polar regions of the world, such as Scandinavia and Canada, sometimes get to watch majestic, rainbow-coloured clouds drift across an otherwise grey winter sky. Over the past few days, observers from across the UK and Ireland have also been lucky enough to witness this phenomenon, known as “nacreous” (or polar stratospheric) clouds.
In fact, nacreous clouds are so unusual in Britain that AuroraWatch UK, a service that monitors the likelihood of auroal sightings, received reports that these colourful displays were “aurora borealis”, also known as the northern lights, which is caused by collisions of electrically charged particles from the sun colliding with particles in Earth’s atmosphere. However, the two phenomena are not related.
Nacreous clouds typically form in the winter polar stratosphere, a layer of our atmosphere around 15,000 to 25,000m in altitude. The stratosphere is generally very dry and so cloud formation is rare, but it seems as though recent storms may have driven moisture high into the atmosphere. Nacreous clouds will also only form when the temperature in the stratosphere is below a chilly -78°C, which turns any moisture in the air into super-cooled liquid or ice crystals. Such temperatures generally only occur in the winter at high latitudes.
During the hours of “civil twilight”, when the sun is between 1° and 6° below the horizon, the first or last rays of the day illuminate these high altitude clouds from below. This light is refracted by the ice crystals in the clouds, a process known as cloud iridescence, producing the shimmering rainbow effect.
As pretty as they may look, nacreous clouds have a darker side too. These clouds enhance the breakdown of the Earth’s ozone layer, a vital part of our atmosphere that provides protection from the sun’s harmful ultraviolet rays. The ice crystals in the clouds encourage a chemical reaction between the ozone layer, which is made up of a specific type of molecular oxygen (O3), and gases such as chlorine and bromine. In fact, it is estimated that just one atom of chlorine in the stratosphere can destroy over 100,000 ozone molecules.
The presence of these ozone-destroying gases in the stratosphere is a problem of our own making. Although phased out after the Montreal Protocol in 1987, the prime reason for their presence is our use of chlorofluorocarbons (CFCs) in products such as refrigerators and aerosol cans. While usage of CFCs has been significantly reduced, it is estimated that it may take another 50-100 years before the effects of CFCs in the atmosphere is diminished.
Current weather predictions suggest that further sightings of nacreous clouds may be possible in the UK until around Saturday. At this time, the polar vortex (which is responsible for the cold conditions currently in the stratosphere above the UK) moves northward to its usual position.
Witnessing an aurora first-hand is a truly awe-inspiring experience. The natural beauty of the northern or southern lights captures the public imagination unlike any other aspect of space weather. But auroras aren’t unique to Earth and can be seen on several other planets in our solar system.
An aurora is the impressive end result of a series of events that starts at the sun. The sun constantly emits a stream of charged particles known as the solar wind into the depths of the solar system. When these particles reach a planet, such as Earth, they interact with the magnetic field surrounding it (the magnetosphere), compressing the field into a teardrop shape and transferring energy to it.
Because of the way the lines of a magnetic field can change, the charged particles inside the magnetosphere can then be accelerated into the upper atmosphere. Here they collide with molecules such as nitrogen and oxygen, giving off energy in the form of light. This creates a ribbon of colour that can be seen across the sky close to the planet’s magnetic north and south poles – this is the aurora.
Gas giant auroras
Using measurements from spacecraft, such as Cassini, or images from telescopes, such as the Hubble Space Telescope, space physicists have been able to verify that some of our closest neighbours have their own auroras. Scientists do this by studying the electromagnetic radiation received from the planets, and certain wavelength emissions are good indicators of the presence of auroras.
Each of the gas giants (Jupiter, Saturn, Uranus, and Neptune) has a strong magnetic field, a dense atmosphere and, as a result, its own aurora. The exact nature of these auroras is slightly different from Earth’s, since their atmospheres and magnetospheres are different. The colours, for example, depend on the gases in the planet’s atmosphere. But the fundamental idea behind the auroras is the same.
For example, several of Jupiter’s moons, including Io, Ganymede and Europa, affect the blue aurora created by the solar wind. Io, which is just a little larger than our own moon, is volcanic and spews out vast amounts of charged particles into Jupiter’s magnetosphere, producing large electrical currents and bright ultraviolet (UV) aurora.
On Saturn, the strongest auroras are in the UV and infrared bands of the colour spectrum and so would not be visible to the human eye. But weaker (and rarer) pink and purple auroras have also been spotted.
Mercury also has a magnetosphere and so we might expect aurora there too. Unfortunately, Mercury is too small and too close to the sun for it to retain an atmosphere, meaning the planet doesn’t have any molecules for the solar wind to excite and that means no auroras.
The unexpected auroras
On Venus and Mars, the story is different. While neither of these planets has a large-scale magnetic field, both have an atmosphere. As the solar wind interacts with the Venusian ionosphere (the layer of the atmosphere with the most charged particles), it actually creates or induces a magnetic field. Using data from the Venus Express spacecraft, scientists found that this magnetic field stretches out away from the sun to form a “magnetotail” that redirects accelerated particles into the atmosphere and forms an aurora.
Mars’s atmosphere is too thin for a similar process to occur there, but it still has aurora created by localised magnetic fields embedded in the planet’s crust. These are the remnants of a much larger, global magnetic field that disappeared as the planet’s core cooled. Interaction between the solar wind and the Martian atmosphere generates “discrete” auroras that are confined to the regions of crustal field.
A recent discovery by the MAVEN mission showed that Mars also has much larger auroras spread across the northern hemisphere, and probably the whole planet too. This “diffuse” aurora is the result of solar energetic particles raining into the Martian atmosphere, rather than particles from the solar wind interacting with a magnetic field.
If an astronaut were to stand on the surface of Mars, they might still see an aurora but it would likely be rather faint and blue, and, unlike on Earth, not be necessarily near the planet’s poles.
Most planets outside our solar system are too dim compared to their parent star for us to see if they have auroras. But scientists recently discovery a brown dwarf (an object bigger than a planet but not big enough to burn like a star) 18 light years from Earth that is believed to have a bright red aurora. This raises the possibility of discovering other exoplanets with atmospheres and magnetic fields that have their own auroras.
Such discoveries are exciting and beautiful, but they are also scientifically useful. Investigating auroras gives scientists tantalising clues about a planet’s magnetic and particle environment and could further our understanding of how charged particles and magnetic fields interact. This could even unlock the answers to other physics problems, such as nuclear fusion.
Connor Gaffey from Newsweek.com today published an article about last night’s spectacular auroral display that was visible from across the UK. Connor spoke with Nathan about how best to see the aurora in the UK and why the display last night was so strong.
Newsweek spoke to Nathan Case, a member of the Aurora Watch U.K. team at Lancaster University, who track geomagnetic activity around the British Isles and sends alerts to users when sightings of the Northern Lights are possible, to find out the best way to see the lights.
Twitter data can even indicate where an Aurora Borealis may be spotted helping scientists have more sightings. After an electromagnetic storm in 2011 brought a flurry of Tweets with spottings of the Northern Lights even far down in the south, NASA scientist Elizabeth MacDonald created Aurorasaurus as a way to document sightings and to verify Tweets. A team of researchers led by Nathan Case found that Twitter data not only is a great indicator for where Aurora Borealis will be published but helps identify key characteristics of the light show including color.
Betsy Mason from Wired.com today published a story about how Aurorasaurus is using citizen science data to map the aurora. She and Nathan discussed the project via email and Nathan provided her with information about the project and an interesting new result:
“An interesting result is that, during our case study, around 60 percent of the reported sightings occurred equatorward (southward in the northern hemisphere, and northward in the southern hemisphere) of where our current best estimate predicted,” Case told me in an email.
Monitoring tweets about auroras can provide accurate and timely alerts of when and where auroras are visible from the ground, said lead study author Nathan Case, a space weather scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.