Figure 6.2: Visible light is just one part of the electromagnetic spectrum. Different types of EM radiation are distinguished by wavelength, from short wavelength gamma rays to long wavelength radio waves. The wavelength is about the size of the items pictured at the bottom of the diagram (the object that looks a bit like a peanut -about the size of the visible light wavelength - is supposed to be a small bacterium).

waves. The wavelength of each type of radiation is given in meters, using a shorthand notation used by scientists. For example, 10_1 mmeans 0.1 m, and 10"6 m means 0.000 001 m. Note that the number after the minus sign indicates how many digits there are after the decimal point. For example, gamma radiation has a tiny wavelength of 10"12 m = 0.000 000 000 001 m, about the size of an atom's nucleus.

The visible part of the spectrum—light—has a wavelength ranging from about 0.4 ^m (violet) to 0.8 ^m (red), where ^m stands for micrometer (sometimes called a micron), which is a millionth of a meter = 10"6 m = 0.000 001 m. Slightly shorter wavelengths take us into the ultraviolet region (which can cause sunburn on a sunny day), and the slightly longer wavelengths take us into the infrared region (which is basically heat radiation, such as that which you feel on your face in front of a glowing open fire).

The Sun emits EM radiation across the entire spectrum, but the peak of its output is at a wavelength of about 0.5 ^m, which is in the yellow part of the visible light spectrum. Evolutionary theory says that this is why our eyes are most sensitive in this part of the spectrum, the eye having evolved in an environment dominated by sunlight. The harmful emissions of gamma and X-rays from the Sun are fortunately (relatively) small. This short wavelength radiation is particularly hazardous to humans. It is, for example, one cause of radiation sickness after exposure to a nuclear bomb detonation. There is also a significant intensity of ultraviolet radiation, but fortunately we are protected from most of this at ground level by the famous ozone layer. We are now aware, however, that dangerous holes are being punched in this protective shield by the inadvisable use of certain artificial chemicals. Above the atmosphere, spacecraft are of course exposed to the full range of the EM radiation spectrum from the Sun.

The other form of radiation from the Sun, which can damage orbiting spacecraft and people, comes as a stream of energetic (high speed) subatomic particles called the solar wind. The source of this is the violent eruptions that take place on the Sun's searingly hot surface (at a temperature of around 6000°C), and in its atmosphere. Material is flung into space mostly in the form of protons and electrons, but also in the form of the nuclei of atoms stripped of their electrons—called ions. By the time it reaches Earth, this steady stream of solar wind has a density of a few tens of particles per cubic centimeter and is traveling at a speed typically between 300 and 1000 km per second (670,000 to 2,240,000 miles per hour). Despite the relatively low density of this stream of ions, it does have a significant effect on our planet, and in particular on the Earth's magnetic field. The Earth has its own magnetic field, which looks a bit like that of a bar magnet. Science teachers often demonstrate magnetic fields by sprinkling iron filings onto a sheet of paper, which in turn is placed over a bar magnet. By jiggling the paper, the iron filings outline the shape of the bar's magnetic field, to reveal a pattern like that shown in Figure 6.3a. This classic shape of Earth's magnetic field is referred to as a magnetic dipole. However, the solar wind also carries a magnetic field, and when this encounters Earth's field, the classic dipole shape is disturbed considerably.

Shielded region \

Bow shock

Solar wind

Figure 6.3: (a) The shape of the magnetic field of the Earth resembles that of a bar magnet, (b) Earth's magnetic field is changed by its interaction with the solar wind. Generally the solar wind particle radiation is deflected by Earth's protective magnetic field, although some charged particles are trapped in the magnetosphere, and some reach Earth over the polar regions causing auroral displays.

Shielded region \

Bow shock

Solar wind

Figure 6.3: (a) The shape of the magnetic field of the Earth resembles that of a bar magnet, (b) Earth's magnetic field is changed by its interaction with the solar wind. Generally the solar wind particle radiation is deflected by Earth's protective magnetic field, although some charged particles are trapped in the magnetosphere, and some reach Earth over the polar regions causing auroral displays.

This solar-terrestrial interaction between the solar wind and the Earth's magnetic field is complex, and acquiring an understanding of it has stretched the intellect and imagination of many talented scientists over a number of decades of research. To gain some insight ourselves, and to appreciate why it is important, we need to think about some basic aspects of electricity and magnetism. The first thing to note is that an electric current in a wire produces a magnetic field. This idea has been around for a long time, being first demonstrated in 1820. That this is true can be easily seen by placing a compass needle near a wire carrying electricity. Normally the needle would align itself along Earth's magnetic field and point north, but the electrical current produces its own magnetic field that disturbs the needle so that it no longer does so. It is a simple job to repeat this historic experiment with a compass, a length of insulated wire with the insulation stripped off the ends, and a battery. To make it work, we need direct current (DC)—a flow of electrons in the wire in one direction only, which is provided by the battery. A nearly expired AA battery from a portable CD player or similar device would do fine, as opposed to a fresh one. (We are going to effectively short the battery with the wire, so the battery will probably be no good afterward!) The setup is illustrated in Figure 6.4. The wire is positioned as close as possible above the compass needle, such that the wire is parallel to the north-pointing compass needle. It is sometimes easier to tape one end of the wire onto one of the battery terminals to ensure good electrical contact. If we gently stroke the other end of the wire on the second battery terminal, completing the circuit, we can easily see the current's magnetic field kicking the compass needle, tending to cause it to point in a direction at right angles to the wire.

Figure 6.4: A simple setup to demonstrate that a current produces a magnetic field.

It is easy to see a similarity between the solar wind and the current in the wire: both are rapidly moving streams of charged particles. In the wire, the current is made up of a flow of charged electrons, whereas in the solar wind the current is generated by a stream of ionized (charged) particles emanating from the Sun. However, the point is that the solar wind carries its own magnetic field, and when this hits Earth's magnetic field, the classic dipole shape is squashed on the Sunward side, and stretched in the down-Sun direction into a shape something like that shown in Figure 6.3b. This region filled by Earth's magnetic field is referred to as the magnetosphere.

The fact that Earth has a magnetic field is a bit of a saving grace in itself, as it prevents the damaging effects of the solar wind from reaching Earth's surface directly. Instead, the solar wind's stream of particle radiation passes through the "bow shock,'' which slows down the flow, before it is deflected around the magnetosphere. The explanation of this shock front is a bit technical, but basically it is similar to the shock wave in front of a supersonic aircraft. Although the major part of the radiation is diverted, some particles are trapped in Earth's field, and some penetrate Earth's defenses and are funneled down onto the north and south magnetic poles, producing the spectacular manifestation of the Northern and Southern Lights. Otherwise know as aurora, these subtle, colorful, and dynamic displays of glowing lights seen in the night sky at high latitudes can arouse a sense of awe in even the most jaded and uninterested of individuals. The glow in the air is caused by exactly the same mechanism as the light coming from a neon strip light; when we switch it on, a flow of charged particles (electrons in this case) is passed through the neon gas, causing the atoms of neon to glow with a characteristic "white'' color. Similarly, when the charged particles from the solar wind race down through the atmosphere over the polar regions, the air glows with colors characteristic of the different gases, mainly oxygen and nitrogen, thus producing the auroral display. This intense flux of charged particles also dumps vast amounts of energy into the atmosphere, resulting in an increase in atmospheric temperature of the order of hundreds of degrees Celsius at high altitudes.

The intensity of the Sun's output, both EM radiation and solar wind, varies over an 11-year period called the solar cycle. Roughly every 11 years the output goes from a maximum, through a minimum, and back to a maximum again, with the last peak (at the time of this writing) occurring approximately in the year 2001.

At times of solar maximum, the disturbing effects of our nearest star become even more vigorous. The frequency and violence of outbursts on the Sun's surface, referred to as solar flares, increase. These hurl billions of metric tonnes of ionized material into interplanetary space. As a consequence, energetic "wodges" of solar wind move outward from the Sun at high speed, and if Earth just happens to be in the wrong place at the wrong time, it will be enveloped in this cloud of energetic charged particles—an event sometimes referred to as a solar storm. A picture of such a wodge— more correctly referred to as a coronal mass ejection (CME)—is shown in Figure 6.5. This lovely image was acquired by one of the Stereo spacecraft in January 2007. An intense pulse of solar wind is shown on the right-hand side of the picture, with Venus at the bottom left and Mercury at the bottom right. During solar maximum, the frequency and intensity of auroral displays increase, the upper atmosphere heats up and expands, and orbiting satellites can receive damaging and sometimes fatal doses of particle radiation. Electrical power grids on the ground can also come under attack, due to the interaction between the Earth's and the solar wind's magnetic fields. During a solar storm, the magnetic field of the solar wind is relatively intense and time-varying, and this buffets the Earth's field, squashing and stretching it in response to the solar bombardment. At ground level the resulting movement of the magnetic field can induce surges of electrical current in long

Figure 6.5: A coronal mass ejection propagating across the inner solar system, with Venus and Mercury clearly seen in the lower part of the image. The Sun is just off the right-hand side of the picture. (Image courtesy of the National Aeronautics and Space Administration [NASA].)

conductors, such as power lines and pipelines, which can cause terrestrial power systems to overload and blackout. Notably, such an occurrence caused a massive blackout in the province of Quebec, Canada, during a solar maximum storm in 1989.

The fact that the movement of a magnetic field relative to a wire induces an electric current in the wire has also long been known. This principle of electromagnetic induction was first discovered by Michael Faraday in 1831, and was greeted at the time as an interesting curiosity. However, in the years following, this discovery led to the vast industrial application of electrical power generation that has transformed every aspect of our technological world. Put simply, modern power generation is achieved by huge generators that are rotated rapidly by some means, such as heating a fluid to drive a turbine, which in turn drives the generator. The heat source can be through the burning of coal or oil, or by the harnessing of nuclear power. But the point is that the generator rotates a huge harness of wire in a magnetic field, which induces an electrical current in the wire, thus producing electrical power to supply the national grid. Consequently, we have seen that, on the one hand, a moving stream of charged particles (an electrical current) can produce a magnetic field, and, on the other, the relative movement between a wire and a magnetic field can induce an electrical current in the wire. During the latter half of the 19th century, electromagnetic theory was developed, principally by the Scottish physicist James Clerk Maxwell, and we have come to understand these two effects as opposite sides of the same coin. The development of Maxwell's equations, which lay down the theoretical framework for electromagnetism, ranks as one of the greatest achievements of 19th century physics.

Returning to the nature of the Sun's output, and its total dominance over the solar system environment, we are rather fortunate to have a planet that provides an ozone shield to protect us from electromagnetic ultraviolet radiation, and a magnetic field to protect us from high energy particle radiation. Today, as I write, just happens to be one of those cold, clear, bright November mornings that seem so rare in British winters. The Sun, despite being low on the horizon, is absolutely brilliant, seemingly dominating the whole of the eastern hemisphere of the sky, in keeping with its awesome power and its influence over life on Earth and over the solar system in general. After reading this, my hope is that when you're on your way to work tomorrow, you too might look skyward and contemplate how remarkable is our companion star—in fact, if you reflect on it too much, it can be daunting that all that stuff is happening just a short distance away (in cosmic terms) from where we live!

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