OUR SUN IS GETTING HOTTER
As our nearest star enters its next solar cycle, physicists have revealed what the future holds – and it’s not what they expected
As a life-giver that warms and lights up our world, it is easy to forget the true, violent nature of the Sun. As the Sun enters a new cycle of surface activity, we are only now beginning to fully appreciate the wide-ranging ways our star’s changeable nature can impact our planet and modern lives.
Solar weather describes the influence of the
Sun on the Earth-space environment. Back in 2011 it was added to the UK government’s National
Risk Register and placed on a similar level to the emergence of a new disease due to the number of people it could potentially impact on Earth.
“The Sun is very dynamic,” says Helen O’Brien, lead engineer on the European Space Agency’s
Solar Orbiter mission. “It has different moods, it is very explosive and it has the potential to damage our modern infrastructure.” As well as providing heat and light, our star is constantly throwing out more deadly material. The solar wind is the name given to this constant stream of energised, charged particles, primarily electrons and protons.
On Earth we are shielded by our planet’s magnetic field while high-energy X-rays and ultraviolet light are absorbed high up in the atmosphere. They electrify their surroundings to create the Earth’s ionosphere and simultaneously excite constituents of our own atmosphere so they glow and create the famous aurorae.
While the aurorae are harmlessly enjoyed by polar communities and tourists, the Sun’s own magnetic field can throw far more violent eruptions our way. Its much larger field is composed of a series of magnetic lines that connect distant points on the surface. Over time these lines can become twisted as the Sun’s compositional fluidity sees material at its equator rotate faster than at its poles, and the magnetic field gets wrapped around the star. “When you distort a magnetic field it is like stretching an elastic band,” says Chris Scott, professor of Space and Atmospheric Physics at the University of Reading. “You are storing up energy.”
Those magnetic distortions cause complex knots to form, which burst to the surface as sunspots. When the Sun is very active you have lots of energy stored up in these knots, and occasionally the system will reconfigure itself through solar flares that throw out vast quantities of high-energy plasma like a cloud from the Sun's atmosphere.
These eruptions can be incredibly violent. The largest, known as coronal mass ejections, can contain billions of tonnes of material, which travels out at speeds of several million miles per hour.
“[The Sun] is very explosive and has the potential to damage our modern infrastructure” Helen O'Brien
“A strike today could bring down radio communications and upset electronics on the ground, causing power grids to fail”
If Earth is in the crosshair of these large storms the consequences can be both spectacular and costly. This was evident even back in September 1859 on the night of the most famous direct hit, known as the Carrington event, which bathed almost the entire surface of the Earth in beautiful aurorae. Though Carrington was spectacular in its scale and spectacle, it was also the first example of solar weather impacting our technology – recently rolled out telegraph systems in America and Europe were hit by fires and gave people electric shocks.
In today’s information age of integrated power networks and satellite communications, a similar strike today could bring down radio communications and upset electronics on the ground, causing long-distance power grids to fail. In 1989 a coronal mass ejection blacked out the entire Canadian province of Quebec, while last year an economic risk assessment by researchers from the University of Oxford found that a Carringtonstyle event could leave the UK with £15.9 billion (approximately $20.5 billion) worth of damage.
In general, a direct threat to human life on the Earth's surface is low. However, a small proportion of our population are spending more and more time higher up, and that does create risks. Storms increase the radiation impacting spacecraft to levels that could threaten astronaut health, while more transatlantic flights are crossing the poles where solar wind material is constantly funnelled by Earth’s magnetic field.
Exposure from a single flight during normal solar conditions will be tiny, but there is concern for flight staff working up there year round. Also, recent research from Clive Dyer of the University of Surrey Space Centre suggests flying in modern aircraft
during a solar storm could expose passengers to radiation levels equivalent to the annual working limit for air crews. This threat has left satellite companies, aircraft operators and power companies monitoring the solar cycle for clues as to when the threat level will be at its highest.
By counting sunspots on the Sun’s surface scientists have for some time known of 11-year cycles of increasing and decreasing solar activity and surface eruptions, driven by the tangling and untangling of the magnetic field lines. These plots indicate we are approaching the latest solar minimum, and therefore entering a new cycle.
Recent magnetic field evolution models developed by the Center of Excellence in Space Sciences in India concluded that the solar maximum of the next cycle, solar cycle 25, will occur around 2024. They also suggested the cycle could buck a wider trend of decreasing solar maximum intensities since the early 1990s, though perhaps not in a way that would greatly threaten ground- or space-based infrastructure.
“It is unlikely that this will affect big solar storms, as these can happen at any stage of the solar cycle,” says Scott. However, anticipating the timing and severity of the coming solar cycle could help us prepare for the more local effects such solar variability that effect our lives and which have only come to light in the last decade or two.
Researcher Pablo Mauas has published a series of papers analysing river flows of the Paraná River, as well as measuring snow accumulation and counting tree rings to establish a remarkable agreement between local precipitation rates and the number of sunspots, tracked back over many decades. “I can quite believe there is an 11-year cycle in the flow rates of the river,“says Scott, who points to evidence of similar solar-influenced systems closer to home.
During recent low periods of solar activity it seems the jet streams become more meandering, and you get more ‘blocking events’ where airpressure systems get stuck over a certain location. These phenomena are thought to account for some of the very cold recent winters in northwest Europe, but perhaps this trend may reverse slightly if the next solar cycle is more active, as the Indian research team suggests.
In his own research Scott has shown that fast jets of solar wind passing the Earth, associated with more active solar periods, can result in a substantial increase in lightning strikes across Europe for up to 40 days as a result of disturbances to the electrical properties of the atmosphere. While communities and populations may need to adapt to changes in these localised weather systems, a better way of predicting larger scale solar weather on a more detailed day-to-day basis is an urgent priority.
This becomes more pressing if Carrington events prove to be more common than that ‘once-in-acentury’ tag. Reanalysis of magnetic behaviour measurements in the Earth’s atmosphere by Scott’s colleague Mike Lockwood has found storms in
1941 and 1972 that may have been as big, if not bigger, than Carrington, but had surprisingly little impact. “It might be that the biggest parts occurred over parts of the world where there wasn't the technological infrastructure to be disturbed,” says Scott.
There was also a storm in July 2012 that narrowly missed the Earth and fortunately hit a solar-observation spacecraft from the Solar TErrestrial RElations Observatory (STEREO) mission. It was travelling fast enough that if it had been on target it would have triggered a modernday Carrington-like event. If we are not to be so fortunate during the next solar cycle we will need to investigate ways to provide more detailed forecasts of what is coming at us.
“It’s like on Earth; we can say the winter will be colder than the summer and we will get more rain,” says O’Brien. “But what you really want to know is if it is going to rain on the day of your party.“However, there are challenges replicating our ability to predict Earth weather in space.
Meteorologists utilise a vast network of monitors collecting data 24/7 as satellites constantly track weather systems from above in order to run their increasingly sophisticated simulations.
And while we have models of the solar wind and how it propagates through space and interacts with the Earth, we can't look down on the entire system as we can when tracking tropical storms or rain fronts. “Imagine yourself as a meteorologist back in the pre-space age in the 1950s, and you are trying to make sense of all these spot measurements without the benefits of a satellite picture. That's probably where we are with space weather,” says Scott.
Reliable space weather forecasts will also require a greater understanding of the relationship between what we see on the Sun’s surface and what is in line to hit us several hours or days later. To help in this endeavour we have sent up a community of craft to monitor the Sun. However, they are all primarily scientific missions sent up to answer scientific questions. “They are proving useful, but they are not optimal,” says Scott, whose STEREO mission can only provide data at the end of each day, which isn't much use when really powerful, fast-moving storms can get to Earth in 17 hours.
The scientific community are in regular contact with industry and space agencies who are working to ensure they have spares at the most
“A lot of satellite operators choose not to worry about forecasts because they do not have sufficient accuracy”
Professor Chris Scott
vulnerable parts of their grids, safe modes for their satellites, back-up routes for transatlantic airlines and safe houses for orbiting astronauts. However, today’s rudimentary early warning systems make preparedness a significant economic risk.
“A lot of satellite operators choose not to worry about space weather forecasts because they do not have sufficient accuracy to make it worth their while,” says Scott, who calls for a new observationfocused mission to put a spacecraft out far enough to see the Sun and the Earth in the same field of view. It would be stationed near enough to us to provide continuous real-time observations.
Further notice could be provided by looking for signature surface behaviour that proceeds major eruptions. This is where two of the latest additions to the Sun’s community of human-made companions could prove useful. O’Brien’s ESAfunded Solar Orbiter mission is due to launch in 2020. It combines solar wind particle and magnetic field measurements with direct surface observation, all from inside the orbit of Mercury.
Key to the Solar Orbiter’s ability to spot impending eruptions will be its highly elliptical orbit, which will allow it to spend 10 to 15 days co-rotating with the Sun, providing uninterrupted coverage of sunspot, flare and storm development.
While the Solar Orbiter will take direct solar observation closer than ever before, NASA’s Parker Solar Probe is pushing the boundary yet further. On its journey to ‘touch’ the Sun the probe will eventually fly as close as 6.1 million kilometres (3.8 million miles), meaning it will pass through the Sun’s outer atmosphere.
At that distance it hopes to measure the pristine solar wind – what it looks like when it leaves the Sun before it gets jumbled up in the 150-million-kilometre (93-million-mile) journey to Earth. “We will be able to couple together unprecedented details on what is happening on the dynamic, bubbling, boiling surface of the
Sun with what is going in interstellar space,” says O’Brien, who believes these new data sets and monitoring stations provide hope for our ability to give fair warning of future eruptions during the next solar cycle.