Solar radiation

Take a look at the power output of the Sun, and it is obvious that visible light accounts for most of it. After all, the Sun looks yellow. We have evolved to make the most of the available light, which is why most animals have eyes that work in light wavelengths. Indeed the colour to which the average eye is most sensitive is known to optics as “visual” and is a sort of straw yellow close in hue to the colour of the Sun.

But the Sun bombards the Earth with far more than light. During World War II, a British Army group looking at interference with radar discovered that the interfering was mainly being done by the Sun, not by the Germans. This discovery was one of the cornerstones of the science of radio astronomy.

As our knowledge has grown, we have realized that the Sun emits a wide range of energy and particles. The first step on this road was taken in 1800 by the German/English astronomer William Herschel, better known, as we saw in Chapter 1, for discovering the planet Uranus. He used a prism to separate out sunlight into the spectrum, much as Isaac Newton had done over a century before. But then he placed a thermometer beyond the red end of the spectrum and noted that it rose. He had discovered infrared light — light below the red end of the spectrum.

We now know that the Sun emits radiation at all wavelengths we can observe, from the gamma, X and ultraviolet rays with shorter wavelengths than light, to light itself, and on into the longer, less energetic end of the spectrum with infrared and radio waves.

These invisible forms of radiation from the Sun are low in power compared to the light it emits. But they have plenty of important effects. The most striking is that ultraviolet light and X-rays from the Sun react with the upper atmosphere and produce charged particles called the ionosphere. This layer has the useful property of reflecting radio waves so that, with some cunning, it can be used for worldwide wireless transmission without satellites or subsea cables.

At the same time, the Sun generates a constant flow of particles, called the solar wind, whose composition more or less reflects that of the Sun itself. So it contains large numbers of protons and alpha particles (nuclei of hydrogen and helium) along with electrons.

All these particles carry electrical charges, and they emerge in especially massive numbers when the Sun is at its most active — when there are many sunspots, which are magnetic in origin, along with flares and mass ejections from the Sun. During extreme conditions on the Sun, some satellites in orbit around the Earth are shut down and turned away from the Sun to avoid damage to their electronics and their solar-energy-driven power systems.

These effects stay at a safe distance because the Earths magnetic field diverts such charged particles away from the surface of the planet. This is just one more way in which the Earth looks after us. A planet whose surface was bombarded by the solar wind would be a far less inviting one.

Space weather

What happens when these particles arrive at the Earth is a stunning tale that goes by a number of names, of which the snappiest but perhaps least accurate is space weather.

The Earths magnetic field is a powerful one and its effects are felt deep into space. When the incoming particles are about 100,000km from the Earth, they arrive at a feature called the bow shock, where the Earths magnetism diverts them around the Earth. But although this is often presented as a one-way process in which the Earths good magnetism fights off dastardly invading radiation, things are not quite so black and white. Were it not for the pressure of the incoming radiation, the Earths magnetism would spread far deeper into space. The place where the incoming solar wind and the Earths magnetic field are matched in strength is caused the magnetopause. Here, a hot layer of charged particles called the inagnetosheath forms.

The upshot is that most of the solar wind particles stream about the Earth and head off into the outer solar system. The overall effect looks rather like air streaming round one of those odd helmets which cyclists wear to break time-trial records. The long tail at the back is called the magnetotail and is many times larger than the Earth. It is detectable at distances far beyond any at which the Earths gravitation could be discerned.

THE SOLAR WIND INTERACTING WITH THE EARTH

Some particles do, however, find their way towards the Earths surface. They can do this because the magnetosphere (the bubble of magnetism that surrounds the Earth) is in fact far from spherical. Indeed, because the Earths magnetic field is roughly aligned with the Earths rotation (far more on this in Chapter 4), the lines of force that we use to visualize it can be thought of as springing out of the Earths surface in the Arctic and Antarctic and looping round in space to join up. And charged particles like nothing better than a line of magnetic force to run along. When they do this, they end up close to the Earths surface at regions called the polar cusps. Here, charged particles are fed into the ionosphere, a region of ions (charged atoms) and electrons in the upper atmosphere.

Many of these particles get spread out into two layers, the Van Allen belts, named after the US scientist James Van Allen (1914-2006). He discovered them in one of the first experiments carried out with satellites, in 1958. Because the Earths magnetic field is not symmetrical, there is a spot in space above the South Atlantic (yes, they call it the South Atlantic Anomaly) where the inner Van Allen belt gets especially close to the Earths surface and forms a hazard to satellites. The Van Allen belts are also a danger to people travelling in space, so space missions are designed to minimize the time that astronauts spend travelling through them. On the ground, these effects are felt less often. But especially at high  latitudes, tangible electric currents can be delivered to the Earths surface, where they can pose hazards to electronic equipment. In 1989, the Canadian province of Quebec had its entire electricity system fail when such a current blew out the transformers. The more active the Sun is, the livelier all these effects become.

Lights in the sky

These charged particles from the Sun are also responsible for one of natures most amazing sights, the aurora, which is also seen at its most dramatic during periods of maximum solar activity. It comes in two types, the Aurora Borealis in the northern hemisphere and the Aurora Australis in the southern, but that is a distinction without a difference as the two words apply to the same thing in different places. Both are manifestations of interplanetary particle physics visible to the naked eye.

The basic working of an aurora is simple enough. As we have seen, the charged particles which penetrate the Earths magnetosphere generally end up in the Van Allen belts. But their confinement is finely balanced. If too many arrive at once, they overflow and hurtle inwards towards the north or south magnetic pole along the lines of force of the Earths magnetic field. As they get lower, they meet the upper atmosphere and interact with the electrons in the air molecules far above our heads. The result is visible light. Aurorae (or auroras — take your pick) are common in the solar system. In 2005 they were detected on Mars, and they are also known to occur on Neptune, Saturn, Jupiter and Ganymede, one of Jupiter’s satellites.

It is only in the past century that we have realized what an aurora is.

It has long been known that major aurorae are associated with magnetic disturbances, shifts of the compass needle away from its normal orientation. So something electromagnetic is clearly going on. However, the idea that anything happening on the Sun could have such a direct effect on the Earth was regarded as science fiction until the twentieth century. In 1859 the British astronomer Sir Richard Carrington observed a bright flare on the Sun, but made no connection with the aurorae that followed, seen deep into equatorial latitudes, or the chaotic interruptions of telegraph traffic that ensued. Later on Lord Kelvin, a distinguished scientist whose word was more or less scientific law in Victorian Britain, published an article saying that any such connection was no more than coincidence.

(Kelvin also “proved” that the Earth was 40 million years old, as we saw on p. 17.) Key among the scientists who proved him wrong was Kristian Birkeland, a Norwegian whose struggles are described in Lucy Jagos The Northern Lights. Nowadays we accept easily that the number and intensity of aurorae we observe vary with the sunspot cycle.

THE AURORA BOREALIS

The aurora is best observed along a belt (the annulus) of the Earths surface that loops around the magnetic pole about 1500km from it. Any time you see an aurora, it is probably about 100km above your head. Its most characteristic colour is green, a wavelength of light created by charged oxygen atoms. Second favourite is red, also due to oxygen, and a blue colour from nitrogen is third. These colours are very precisely defined. If you look at the spectrum of auroral light, they appear as sharp lines. Each corresponds exactly to the amount of energy emitted when an electron around an oxygen or nitrogen atom that has been pushed into a higher-level orbit after absorbing energy from incoming solar particles falls back again to where it started. The lower orbit needs less energy and the difference is emitted as light. As well as emitting light, the aurora can be detected with radar and can be used, like the ionosphere, for bouncing radio signals, a popular sport with the ham radio crowd.

Within the aurora there can often be seen shapes, described by the experts as arcs, bands, surfaces (which can be diffuse or pulsating) and coronas. These shapes can change rapidly (at up to 100km a second), with a rippling or pulsating effect, especially in response to major electrical storms. The resulting displays have been thought of as representing gods in battle and other mysterious likenesses. Their interpretation in Norse myth is especially gloomy, with suicide and violent death featuring prominently. Birkeland and others proved that inhabitants of the polar regions were wrong to think that the aurora could touch the ground. They are always far in the upper atmosphere. This is just as well, since being touched by the aurora was about the worst luck anyone could have.

The aurora still has its mysteries. There have been many reports of noises associated with the aurora which have been hard to dismiss despite being equally hard to explain. These accounts, over 300 of them, are highly consistent, and often refer to a hissing or rustling sound. The idea refuses to die despite there being no obvious way for light 100km up to make sounds down here. One possible explanation is that strong electrical fields make objects at the Earths surface emit noise. Another idea is that the same fields might act directly on the parts of the brain that feed our sense of hearing. We know that the currents involved in an aurora are in the millions of amps (think of several million domestic lightbulbs at once) so effects on the ground cannot be ruled out. Oddly, the many reports of aurora sound seem not to contain any account of a successful recording of it.