by Lucie Green
The idea is that the dust is levitating because it is electrically charged. On the dayside of the Moon photons of sunlight that fall on the dusty lunar surface have enough energy to knock electrons out of the atoms making up the dust grains. Without a complete set of electrons the dust becomes positively charged. On the nightside there is no sunlight, though, and the question as to how the dust becomes charged is harder to answer. But this is where the nature of the space environment around the Moon comes into play. The dust elucidates the invisible.
The dust on the nightside could be given an electrical charge if the space around the Moon has electrically charged particles – electrons, for example. Electrons could attach themselves to the dust, making the grains negatively charged.
Along the terminator there will be a sharp change between dust that has a positive charge and dust that has a negative electrical charge and this creates an electric field. The electrically charged dust particles will feel a force from this field, again lifting them up off the surface. This could be the reason why Apollo astronauts in lunar orbit saw the horizon glow and dust fountains as they flew towards the sunrise. You can see this region for yourself – just look up at any time other than full moon to see the line between the lit and unlit regions. The dust only levitates to a height of 100 kilometres and there isn’t enough for us to see it from the Earth. But it’s there and it tells us that the Moon is moving through a cloud of electrically charged particles that are trapped inside our magnetic field and one aspect of our space weather.
The Moon isn’t always inside the Earth’s magnetic field though. Whether the Moon is inside or outside it depends on where the Moon is in its orbit. It’s not that the Moon dramatically changes its distance from us – its orbit is very nearly circular; instead it’s the Earth’s magnetic field that’s highly distorted and the Moon moves in and out of it. From the solar wind alone, the magnetic field is stretched out to a distance of as much as 7 million kilometres on the nightside of the planet, well outside the Moon’s approximately 380,000 kilometres’ orbital distance. On the dayside the magnetosphere is compressed to 70,000 kilometres above the Earth.
So, once a month, around the time of the new Moon, our natural satellite pops outside the Earth’s magnetic field for a few days and finds itself sitting in the flow of the solar wind. The next time you catch a glimpse of the new Moon, think about its exposed place in the Solar System, being buffeted by the solar wind. When the Apollo 17 lunar module touched down, the Moon was just coming back inside the Earth’s magnetic field. The preceding Apollo missions kept inside the safety of the magnetic field too.
With NASA, ESA and even some private companies planning to send humans to the Moon again and also to Mars, understanding and predicting space weather is only going to get more and more urgent in the decades to come.
14. What Comes Next?
The Sun has been around for about 4.6 billion years and has about that long again left on the clock. Our observations over the past few hundred years are just a blink in the Sun’s lifetime. But the Sun is a changeable star and, of course, we want to know what the Sun is likely to do next, and how it will affect us.
EARTH FACING A MINI-ICE AGE ‘WITHIN TEN YEARS’ DUE TO RARE DROP IN SUNSPOT ACTIVITY
This was a UK newspaper headline that ran in 2010. I’m a solar physicist so it immediately caught my attention – partly because it talked about sunspot numbers, a subject very dear to me, but largely because of the sweeping claim that, because of the Sun, a mini-Ice Age was on the cards.
There does appear to be a background for this headline though. The idea that a drop in sunspot numbers could lead to a mini-Ice Age on Earth comes from an interval known as the ‘Maunder minimum’ – a time when sunspots were pretty much absent for around seventy years. It was Gustav Spörer, the German astronomer, who discovered this break in the normal pattern of the sunspot cycle. He died in 1895, though, before the solar physics community had accepted his proposal.
Spörer’s ideas were taken seriously by one British astronomer, Edward Walter Maunder. He worked at the Royal Observatory in Greenwich, London, the leading institute for solar observations at that time, and he had a keen interest in sunspots. His analysis of the historical sunspot data led him to the same conclusion as Spörer. Together, their work established that during the years 1645–1715 there were virtually no sunspots on the Sun. History has forgotten about Spörer’s contribution, though, and this seventy-year period when the Sun apparently turned off is named solely after Maunder.
The plot thickens though because the Maunder minimum is often said to have coincided with the Earth going through a period known as the ‘Little Ice Age’. Evidence quoted to support this global cooling often includes the River Thames freezing over so deeply that frost fairs were held on them between 1600 and 1814, converting the river into an impromptu market to capitalize on the otherwise lost trade of the country’s most economically important city.
Even William Herschel, who didn’t know what sunspots were and didn’t directly measure solar radiation across the Sun’s cycle, pondered a possible link between temperatures here on Earth and the sunspot number. In 1801 Herschel turned to economics and saw that wheat prices were higher at times of low sunspot numbers. He published a paper where he reasoned that high wheat prices were driven by the scarcity of wheat, which in turn was driven by low terrestrial temperatures, resulting from a low number of sunspots. But are things really this simple?
Observations of tree growth using the thickness of tree rings does indicate a period of globally reduced average temperatures. However, this seems to have lasted for at least 500 years, ending around the mid-1800s. This is six times longer than the duration of the Maunder minimum. And even taking into account that sunspots were only recorded by European astronomers with telescopes from the early 1600s onwards, meaning that probably only three sunspot cycles were seen before the Maunder minimum began, over ten obvious solar cycles played out in the decades after the Maunder minimum and before the Little Ice Age ended – the Sun was active again for a long time before the Earth came out of its little freeze.
In fact, we know that the Sun was active before regular sunspot observations began and during the first half of this Little Ice Age because we can also use so-called ‘cosmogenic isotopes’ to study solar activity. These are generated by cosmic rays coming from our Galaxy – particles that we met earlier and that were seen as flashes of light in the eyes of some of the Apollo astronauts. And I mentioned that when they hit the Earth’s atmosphere they produce nuclear reactions and it is these that generate the cosmogenic isotopes. They are then deposited in reservoirs such as tree trunks and ice sheets. And by drilling into those reservoirs we can measure their abundance in past times. The reason why this tells us about past solar activity is that the Sun’s magnetic field helps protect the Earth from these cosmic rays, so the abundance of cosmogenic isotopes goes down when solar activity goes up, and vice versa.
A few alarm bells ring at this point. What I have just told you includes some broad-brush statements without any detail. I know that when I hear ‘Little Ice Age’ I think of global temperatures dropping, and it’s easy to link stories of the Thames freezing over with a widespread drop in temperature. But we do need to be careful here – for a start, the tree ring measurements mostly tell us about the temperature in the growing seasons and almost nothing about temperatures in winter. And if one of the major rivers in a country froze over 200 years ago, and it doesn’t today, is it fair to say that the temperature back then must surely have been significantly lower? Also, it seems that the cold spells during the Ma
under minimum when the Thames froze were a feature of northern Europe only, and not a global event. And on top of that some of the hottest summers were recorded during that era. The temperature in London was slightly cooler during the winter of 1814 than it has been during recent winters. But there is more to the river freezing than just the temperature.
The main reason for the Thames’s freezing over is that it was quite a different river back then. At the time of the frost fairs the river was wide, meaning that the water flowed slowly. Since the last frost fair, a significant amount of the river has been reclaimed to form the embankments. These embankments made the river substantially narrower, causing the water to flow much more quickly. The bridges across the river were different then too and they helped the river freeze. The old (medieval) London Bridge sat upon a series of narrow arches, making it a weir as well as a bridge, and this meant that any ice which did manage to form in the slowly moving flow could get caught, creating an ice dam that eventually led to the river freezing over upstream of the bridge. It was much easier for the Thames to freeze over a few hundred years ago than it is today. Things aren’t quite as simple as the newspaper headline makes out. It’s not as straightforward as: few sunspots = mini-Ice Age.
BEHIND THE HEADLINE
But there is still some truth to this headline, even if the conclusion is incorrect.
We’ll get to the ‘rare drop in sunspot activity’ in a moment. First we need to find out: can variations in the Sun’s activity have a direct impact on the Earth’s climate? If not an Ice Age, can changes on the Sun at least cause the Earth to globally heat up or cool down?
It’s clear that the Sun is our major source of energy, so on a very simplistic level you might say that if the Sun becomes brighter we will have a warmer Earth and if the Sun dims the Earth will cool with it. We know that the majority of the Sun’s radiation is coming from the photosphere and that sunspots are regions on the photosphere that emit less radiation than their surroundings because they contain cooler plasma. So the simplistic view would be to think that more sunspots mean a dimmer Sun and less energy coming our way.
Around the magnetic-field-intensive sunspot regions are smaller and weaker patches of magnetic field. These are the regions that form as the sunspot magnetic field disintegrates and disperses, and they are called ‘faculae’. In the faculae, the upward convection of heat is inhibited, just as in sunspots, but their smaller size allows the radiation at their edges to keep the plasma in faculae hot. Sunspots are too large for this to happen. This combines with their relatively low plasma density, which enables us to see a little deeper into the Sun where temperatures are slightly higher. But since the walls of the faculae glow more brightly than the surrounding photosphere, faculae are brightest when they are seen close to the limb of the Sun.
The brightness of the faculae more than makes up for the dimming caused by the sunspots, resulting in a net increase in photospheric light being produced. At the maximum of the solar cycle the Sun is actually brighter than it is at minimum by about 0.1 per cent. At sunspot minimum the Sun is a tiny bit dimmer. So can a low level of sunspots affect our climate? And, if so, how?
It turns out that the number of sunspots can affect our climate. But not in the way we expect and not to a level that is significant. I hate to ruin the ending of the story, but it turns out that the Sun can cause the climate to change on the Earth, but to a much lesser extent than things like CO2 emissions. The 0.1 per cent change in incoming solar radiation is a very small perturbation to the energy budget of the Earth’s atmosphere compared to that caused by the increased trapping of heat by greenhouse gases. To put it plainly, the solar cycle’s impact on the Earth’s temperature is swamped by human-made climate change. If anything, despite the Sun’s trying to cool the Earth with a diminishing number of sunspots and faculae, humans have still caused it to heat up. The effects of the Sun are just one of many competing factors that influence the numerous complex processes which create our climate. And solar variability, at the moment, plays a very small part.
But to come back to the surprising fact that the Sun is actually dimmer at sunspot minimum. The first part is that the drop in photospheric light during solar minimum might be something of a red herring.
These warnings about how to interpret what happened in the past have been highlighted for several years now by Mike Lockwood, a professor of physics at the University of Reading. He revived an interest among the UK solar community in looking back through the long-term archive of data and not just focusing on what we have seen during our space age careers. And he encouraged us to put things into context: learning from the Maunder minimum but with an eye on regional temperature changes rather than global.
Mike looked to see whether there might be a link between the solar magnetic field and changes in the Earth’s atmosphere that could lead to colder temperatures in northern European winters in the modern era. Using a whole range of space data he was able to see that changes in the strength of the magnetic field in the solar wind seemed to track winter temperatures in northern Europe. This got people thinking that the energy which flows out from the Sun might have a regional effect on our weather here on the Earth. As the saying goes, correlation doesn’t imply causality, but it does mean the issue is worth looking into.
This work is interesting because it moves us away from the main point of the newspaper headline we saw at the start of this chapter, which just made us think about energy coming to us through the light of the photosphere. This is the most apparent vehicle for the energy, but there are other forms that are coming from the solar atmosphere. The solar wind is one way the Sun transports energy to us, and the gusty flows of this wind are varying all the time. But any variation is ultimately coming from changes in magnetic features in the solar atmosphere; after all, the solar wind is the expansion of the solar atmosphere.
So when Mike found an intriguing link between the magnetic field strength in the solar wind and northern European winter temperatures, it was an indication that we need to investigate whether changes in the solar atmosphere might be related to our colder winters. And as the magnetic field in the solar atmosphere varies, so too does the Sun’s radiation across the wavelengths of the electromagnetic spectrum that our eyes cannot see.
This is an aspect that has been studied by Joanna Haigh at Imperial College London. Her work has focused on the ultraviolet part of the spectrum, where there is around a 10 per cent variation in the amount of light emitted between solar minimum and cycle maximum, with more ultraviolet light being emitted at solar maximum. The UV therefore has a fractional variation that is 100 times greater than the 0.1 per cent variation in visible light and heat from the photosphere. At X-ray wavelengths the variation across the cycle is even larger, being around 1000 times brighter at solar maximum than at solar minimum. Just as for the solar wind, this variation is driven by the evolving magnetic field in the corona. At solar maximum there are more regions of intense magnetic field emanating from sunspots, and the hot plasma trapped in these structures makes the Sun shine more brightly in ultraviolet and X-ray wavelengths.
We all know the photospheric heat and light penetrate to the surface of the Earth, and we all benefit from that. As we have seen right from the start of this book, most of the UV and the X-rays do not reach the surface, which is fortunate as they are harmful to humans. Most of the UV only reaches as far as the middle atmosphere (and in particular the stratosphere, about 20 kilometres up, where it is absorbed by ozone) and X-rays only reach the uppermost atmosphere (the so-called thermosphere, which is above 100 kilometres up).
Joanna’s work considered the var
ying amount of ultraviolet light that the Sun emits and how this could have a regional effect at the Earth: from the minimum of the solar cycle to the maximum. Increased amounts of ultraviolet light falling on our atmosphere create more ozone, which in turn absorbs more ultraviolet radiation. But the important point is that when increasing amounts of ultraviolet light are absorbed by molecules in our atmosphere, the energy that the photons carry is also absorbed. So when more radiation is absorbed at these wavelengths the Earth’s atmosphere in the stratosphere is heated.
The heating isn’t uniform, though, and the equatorial stratosphere is heated more than that near the poles. This causes a gradient in both temperature and pressure and that drives a poleward motion of the gases, particularly in the hemisphere in which it is winter. And because the Earth is spinning, the poleward motion in turn causes an eastward wind because of the ‘Coriolis force’ – the ‘jet stream’ that straddles the boundary between the troposphere (the atmospheric layer that generates our weather) and the stratosphere above it. Meteorological science has shown that the jet stream has great implications for the weather in certain areas. When the jet stream moves north, warmer air from the south comes up to us, and when it moves south the opposite happens. The science is complex, but cutting-edge modelling is beginning to show how the behaviour of the thin air in the stratosphere can influence the lowest regions of atmosphere, and hence our weather.
By thinking about how invisible sunlight affects our atmosphere we might be able to rescue something from the newspaper headline. Joanna’s work shows that a less active Sun could contribute to how many colder winters will be experienced by those of us living in northern Europe. And the models and the data both show that if that happens, other areas, like Greenland for example, would have a correspondingly higher fraction of warm winters. So far from causing a mini-Ice Age, a drop in solar activity could act to speed up the melting of the Greenland ice sheet! While the newspaper told us to think globally, we actually need to think regionally.
