An analysis of simultaneous reconnection events in the near-Earth magnetotail and enhancements in the aurora is undertaken. Exploiting magnetospheric data from the Geotail, Cluster, and Double Star missions, along with auroral images from the IMAGE and Polar missions, the relationship between a reconnection signature and its auroral counterpart is explored. In this study of 59 suitable reconnection events, we find that 43 demonstrate a clear coincidence of reconnection and auroral enhancement. The magnetic local time (MLT) locations of these 43 reconnection events are generally located within ±1 h MLT of the associated auroral enhancement. A positive correlation coefficient of 0.8 between the two MLT locations is found. The enhancements are localized and short-lived (τ≤10 min) and are as likely to occur during the substorm process as in isolation of a substorm. No significant dependence of the reconnection or auroral enhancement location on the dusk-dawn components of the solar wind velocity (Vy), IMF (By) or local By or Vy, as measured by the reconnection-detecting spacecraft, is found.
The article is open source and free to download from the publisher’s site.
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.
The Aurorasaurus project harnesses volunteer crowdsourcing to identify sightings of an aurora (the “northern/southern lights”) posted by citizen scientists on Twitter. Previous studies have demonstrated that aurora sightings can be mined from Twitter with the caveat that there is a large background level of non-sighting tweets, especially during periods of low auroral activity. Aurorasaurus attempts to mitigate this, and thus increase the quality of its Twitter sighting data, by using volunteers to sift through a pre-filtered list of geolocated tweets to verify real-time aurora sightings. In this study, the current implementation of this crowdsourced verification system, including the process of geolocating tweets, is described and its accuracy (which, overall, is found to be 68.4%) is determined. The findings suggest that citizen science volunteers are able to accurately filter out unrelated, spam-like, Twitter data but struggle when filtering out somewhat related, yet undesired, data. The citizen scientists particularly struggle with determining the real-time nature of the sightings, so care must be taken when relying on crowdsourced identification.
The article is open source and free to download from the publisher’s site.
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.
Accurately predicting when, and from where, an aurora will be visible is particularly difficult, yet it is a service much desired by the general public. Several aurora alert services exist that attempt to provide such predictions but are, generally, based upon fairly coarse estimates of auroral activity (e.g., Kp or Dst). Additionally, these services are not able to account for a potential observer’s local conditions (such as cloud cover or level of darkness). Aurorasaurus, however, combines data from the well-used, solar wind-driven, OVATION Prime auroral oval model with real-time observational data provided by a global network of citizen scientists. This system is designed to provide more accurate and localized alerts for auroral visibility than currently available. Early results are promising and show that over 100,000 auroral visibility alerts have been issued, including nearly 200 highly localized alerts, to over 2000 users located right across the globe.
The article is open source and free to download from the publisher’s site.
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.”
An aurora may often be viewed hundreds of kilometers equatorward of the auroral oval owing to its altitude. As such, the NOAA Space Weather Prediction Center (SWPC) Aurora Forecast product provides a “view-line” to demonstrate the equatorial extent of auroral visibility, assuming that it is sufficiently bright and high in altitude. The view-line in the SWPC product is based upon the latitude of the brightest aurora, for each hemisphere, as specified by the real-time Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting (OVATION) Prime (2010) aurora precipitation model. In this study, we utilize nearly 500 citizen science auroral reports to compare with the view-line provided by an updated SWPC aurora forecast product using auroral precipitation data from OVATION Prime (2013). The citizen science observations were recorded during March and April 2015 using the Aurorasaurus platform and cover one large geomagnetic storm and several smaller events. We find that this updated SWPC view-line is conservative in its estimate and that the aurora is often viewable further equatorward than is indicated by the forecast. By using the citizen reports to modify the scaling parameters used to link the OVATION Prime (2013) model to the view-line, we produce a new view-line estimate that more accurately represents the equatorial extent of visible aurora. An OVATION Prime (2013) energy-flux-based equatorial boundary view-line is also developed and is found to provide the best overall agreement with the citizen science reports, with an accuracy of 91%.
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.
Nathan recorded the following clip for Lancaster University, and AuroraWatch UK, giving a few tips about how best to see the northern lights from the UK.
The tips include:
1 – Head north: the aurora will generally be visible on the northern horizon (in the northern hemisphere)
2 – Find an unobstructed view of the northern horizon: such as a hillside or a northward facing coast.
3 – Find dark skies: city light pollution will tend to be brighter than the aurora, especially when attempting to view by eye. Try seeking out local dark sky areas.
4 – Hope for clear skies: (this one is down to chance!) the aurora occur higher in altitude than clouds, so any clouds will obscure your view.
5 – Sign up to AuroraWatch UK for alerts of increased chance in auroral visibility from the UK.
Nathan also recorded a clip on “5 Surprising Facts about the Northern Lights” which you can watch below.