EXTENSION 14. Chapter 5 The Cosmic Engine Unit 5.5 Our Sun: An Active Star. The Sun s Influence on the Earth s Environment

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1 EXTENSION 14 Chapter 5 The Cosmic Engine Unit 5.5 Our Sun: An Active Star The Sun s Influence on the Earth s Environment Exercise The Sun influences life on the Earth in many ways. The most obvious is as the source of all electromagnetic radiation, which supports life. Our food chain depends on plants and they, in turn, on photosynthesis. The production of organic compounds (e.g. glucose) from inorganic ones (e.g. carbon dioxide and water), and the release of oxygen into our atmosphere rely on visible light. Photosynthesis is optimised to the Sun s spectrum. All of the energy sources we use today result from solar radiation falling on our planet. Hydroelectric power results from rainfall on high ground. This in turn needs the Sun to produce clouds through evaporation. Coal needed the Sun to provide energy for photosynthesis in early plants. Wind power needs solar heating to provide convection currents to produce the wind. Only recently have we been able to utilise the Sun s power directly by using photoelectric solar cells and solar hot water systems. Only nuclear power generation could be thought to be independent of the Sun. The Sun can also produce damaging effects. As well as producing visible light, the Sun also bathes the Earth in ultraviolet and X-rays, and radio waves. While the effect of radio waves is minimal, the high energy radiation in the ultraviolet and X-ray range can severely damage life on Earth. Ultraviolet and X-rays can form free radicals in cells, which can cause cancers and mutations. Fortunately we are well shielded from these radiations and the most damaging radiations do not reach the Earth s surface. Other less obvious influences are associated with the stream of energetic particles that form the solar wind. They too could cause cancers and cell mutations. Fortunately we also possess a shield against harmful effects that might result from exposure to these particles: our magnetic field diverts a large part of the solar wind around the planet. 1 List the types of solar emissions that reach the Earth. The Earth s magnetic field Most planets possess a measurable magnetic field. The shape of the Earth s magnetic field is best understood as being similar to that of a simple bar magnet, although its origin is less well understood.

2 Figure 1 The Earth s magnetic field is like that of a simple bar magnet. N S magnetic N S N The poles on a simple magnetic compass will align with the Earth s magnetic field. By our definition magnetic north is the direction in which the N-seeking end of the compass will point. The south-seeking end of the compass will point toward magnetic south. We notice that as we near the geographic poles, the direction of magnetic N deviates from geographic north or true north. The magnetic axis of the Earth s field is not precisely aligned with the rotation axis about which it spins. At present, the north magnetic pole is at longitude 101º W and latitude 75 N. We also notice that as we move away from the equator toward the poles our simple compass no longer floats in a level or horizontal position. The needle is pulled downward. The field lines near the poles of the Earth are not horizontal, but seem to penetrate the Earth s surface. We can measure the angle of this dip with a dip circle or compass that is free to move only in the vertical plane. The angle of dip is the angle between the magnetic field and the horizontal. Figure 2 The Earth s magnetic field is tilted from the vertical. angle of dip The Earth s magnetic field can be compared to that of a huge bar magnet. To produce the observed field, the bar magnet would need to be buried within the Earth. Its poles would be well below the surface to account for the observed angle of dip. The axis of the bar magnet would need to be offset from the rotation axis of the Earth to account for the observed declination. Since the N-seeking end of the compass is attracted by south magnetic poles, this means that the south pole of the bar magnet is located somewhere under the geographic north pole (and the north pole of the bar magnet located under the geographic south pole).

3 A possible source The analogy of the magnetic field being like that of a bar magnet is only a representation of the field. The position of the magnetic poles is not fixed relative to the Earth s axis of rotation. The magnetic poles wander, drifting about 1 degree of longitude every 5 years. The magnetic field also seems to vary in strength, and can even reverse its direction so that the N magnetic pole becomes the S magnetic pole. Any model for the source of the magnetic field must also account for the polar wandering and the magnetic reversals that occur every few million years. The true situation is that the Earth s magnetic field is associated with the rotation of the Earth and convection in the molten outer core. Gary Glaztmaier (Los Alamos National Laboratory) has only recently been able to model these convection cells by using a supercomputer and demonstrated that they can produce a stable field like that of the Earth. In modelling years of currents in several months of computer time, Glaztmaier also demonstrated that the field direction could flip so that the north pole became the south pole. A vortex of molten material has recently been found circulating around the Earth s axis, approximately halfway toward the Earth s centre. It is thought that all planets with magnetic fields must also have molten cores. Figure 3 One possible source of the Earth s magnetic field. magnetic field lines current in molten material Figure 4 The Earth s magnetic field acts as a shield against the solar wind. Magnetospheres Our magnetic field cannot be viewed in isolation. It interacts with the magnetic field embedded in the solar wind and with the charged particles that spiral along the field lines. We live inside a magnetosphere that serves as a shield against the solar wind and keeps the majority of the highenergy particles from reaching the Earth. The few that do penetrate interact with the Earth s magnetic field and produce the effect in the upper atmosphere known as the Aurora. shockwave magnetopause Van Allen belts solar wind

4 The effect of the Earth s magnetic field is to deflect the charged particles in the solar wind, forming an elongated cavity. Within the cavity, the magnetosphere, the magnetic field is dominated by the Earth s magnetic field. Outside the cavity, the solar wind s magnetic field dominates. When the solar wind encounters the Earth s magnetic field, it abruptly slows from supersonic speeds. This rapid drop in speed results from a shock wave. Immediately behind this shock wave lies the magnetopause, where the magnetic forces of the Earth s field exactly balance those of the solar wind. The ionosphere, Van Allen belts and the aurora Most of the protons and electrons in the solar wind are deflected around the magnetosphere and never get close to the Earth. However, some leak past the magnetopause and become trapped in the Earth s magnetic field in two regions called the Van Allen radiation belts. The Earth s magnetic field acts as a bottle. Imagine charged particles entering the field at the equator and spiralling down a field line towards the poles. As they spiral down, they encounter more and more charged particles, until the electrostatic force between charges reverses their direction. They continuously oscillate up and down between poles. Discovered in 1958, the inner and outer Van Allen belts lie at heights above the Earth s surface of between 2000 km and 5000 km (the inner belt), and km and km (the outer belt). The energetic particles in these belts pose no direct threat to the surface of the Earth, but they can damage electronics on spacecraft that pass through them. Some of the charged particles enter the upper part of the Earth s atmosphere. At heights of km the fast moving particles collide with atoms of nitrogen and oxygen and excite them so that they emit light at red and green wavelengths. The result is the Northern and Southern Lights or the aurora. Variations over time The strength of the solar wind varies, depending on the solar cycle and the coronal activity. When the Sun is most active, the wind is strongest and exerts greatest pressure on the Earth s magnetosphere. The front of the magnetopause gets pushed back closer to the Earth. Figure 5 Aurora around the north magnetic pole. This image of the Northern Lights was made at ultraviolet wavelengths. The Aurora Borealis is a circle, 4500 km in diameter, centred on the north magnetic pole. Satellites in high orbits can then be exposed to the increased flux of high energy particles in the solar wind. The increased strength of the solar wind also produces higher flows of charged particles along the field lines in the magnetosphere, and we experience an enhanced aurora as a result. The increase in charged particles entering the upper atmosphere also disrupts the ionosphere. This

5 layer of atmosphere, which begins above about 60 km, acts as a reflector for short-wave radio communications. It is this reflection that allows long-distance communication. Any changes in this layer therefore disrupt the transmission of radio signals. One interesting result of an active Sun is that when the solar wind is strongest we see fewer cosmic ray particles coming from outside the solar system. It is rather similar to a yacht trying to sail upstream against a strong wind. Sudden events in the solar corona eject massive flares of particles. These flares can have major effects both at high altitudes and on the Earth s surface. The rapidly moving charges can induce high currents or power surges to flow in the power grids that carry power across Canada and the northern USA. Circuit breakers have in the past operated to cut the power off over large areas. Exercises Activity 2 Draw representation of the path of the solar wind as it flows around the Earth. 3 What particles make up the solar wind? 4 Explain how the aurora works. 1 Sunspot activity a Identify data sources and gather information to assess the impact of sunspot activity on the Earth s power grids and communications. b How do solar activity maxima affect us? Figure 6 The spectrum of the Sun s radiation. The major absorption bands in the visible and infrared spectra occur because of molecules in the troposphere. Short wavelength ultraviolet radiation is absorbed by ozone in the mesosphere. Solar radiation and the Earth s atmosphere The most obvious effect the Sun has on us is due to its light and heat. Could we survive if it suddenly stopped shining, or decreased its power output by 10%? What would be the effect of a 2% increase in power? To understand how solar radiation interacts with the Earth s surface and atmosphere we need to study the atmospheric structure and the way in which solar energy filters through the atmosphere. Intensity O K black body representing solar spectrum above atmosphere O 3 H 2 O O 2 H 2 O H 2 O H 2 O CO 2 H 2 O CO 2 H 2 O CO nm 500 nm 1 µm 1.5 µm 2 µm 2.5 µm Near Infrared Wavelength Ultraviolet Blue Red The solar spectrum at the top of the atmosphere The spectrum of the light arriving from the Sun at the top of our atmosphere is comparatively smooth. To all intents and purposes the Sun behaves as a black body. The spectrum peaks at a wavelength of about 500 nm in the centre of the visible waveband. Superimposed on this smooth continuum are groups of absorption lines, which are caused by cool gases in the Sun s

6 photosphere. The temperatures at the surface of the Sun are sufficiently high that its gases exist as individual atoms and ions. Our atmosphere changes this spectrum dramatically. The Earth s lower atmosphere is sufficiently cool for gases to exist as molecules. The main constituents of our atmosphere are simple diatomic molecules such as nitrogen (N 2 ) and oxygen (O 2 ), as well as more complex molecules such as water (H 2 O), carbon dioxide (CO 2 ) and ozone (O 3 ). These molecules all absorb light strongly in the visible and infrared spectrum. In addition, the ozone molecules absorb high-energy ultraviolet radiation. The molecular absorption spectra are not sharp like atomic absorption, but show broad bands of absorption. The structure of the atmosphere As we go up through the atmosphere we find that its density and pressure drop rapidly with height. Eighty per cent of the total mass of the atmosphere lies below 10 km. The Earth s atmosphere, like other planetary atmospheres, is divided into four main layers. Each layer receives heat from the Sun as well as heat radiated upward from the Earth s surface. The divisions between layers are based on the way in which the temperature changes with altitude and, therefore, on the mechanism that heats each layer. From the bottom upwards the layers are: the troposphere the stratosphere the mesosphere the thermosphere. The troposphere The troposphere extends from the Earth s surface to an altitude of 12 km. Most of the atmosphere is within this layer and our weather systems are confined to it. Figure 7 The temperature profile with altitude. Altitude (km) oxygen and nitrogen atoms absorb UV Thermosphere Mesosphere ozone absorbs UV Stratosphere molecules absorb IR Troposphere Temperature ( C) The temperature at the base of the troposphere is the same as that of the Earth s surface, about 20 C. As we rise higher we observe that the temperature of the air around us drops slowly with height. You will find many airlines show the altitude and outside temperature on their flight information channel so you can observe these changes. By the time we reach the edge of the troposphere, the temperature has dropped to 60 C. Heat radiated from the ground, provides the main source of heating in this layer. The Earth s surface radiates as a black body with a temperature of 20 C. Most of the energy is emitted at infrared wavelengths (λ = 2900/T or at approximately 10 microns). This comparatively low-energy radiation is mainly absorbed by molecules, such as H 2 O, CO 2, NH 3, which gain vibrational and rotational energy. Convection is an important method of heat exchange between the surface and the troposphere. Convection currents rising over hotter regions near the equator suck air in across

7 the Earth s surface and drive the major wind and weather systems. This global circulation system is made more complex by the Earth s rotation. If there were no rotation, then all winds would flow from the poles towards the equator. However, as air flows from the poles it moves further away from the Earth s rotation axis and slows down relative to the surface. It moves westwards. The Earth s comparatively fast rotation breaks the convection pattern into three separate cells in each hemisphere. The stratosphere From 12 km to 50 km, in the region of the stratosphere, the temperature begins to rise. This means that there must be some new energy-absorbing process operating in the stratosphere. It is here that ozone, which is very efficient at absorbing ultraviolet radiation, is found. Ultraviolet radiation has sufficient energy to break the O 3 molecules into O 2. This effectively shields us from extreme ultraviolet radiation from the Sun, which can cause cell damage. Convection currents cannot exist in a region where higher layers have higher temperatures. It is partly for this reason that modern aircraft fly in the lower stratosphere, where the flight is less bumpy. The mezosphere At altitudes above 50 km very little ozone exists. There are few other atoms or molecules that can absorb the solar radiation penetrating to this depth. It is far enough from the ground that the region gains little energy from below. At altitudes above 50 km, the temperature of the atmosphere continues to drop slowly until a height of 80 km when it reaches a minimum of 75 C. The thermosphere At the top of the atmosphere, oxygen and nitrogen cannot exist as molecules, only as atoms. This low-density gas of oxygen and nitrogen atoms absorbs very high energy ultraviolet radiation arriving from the Sun. The temperature in these upper layers therefore rises with altitude. There is no definite limit to the height of the thermosphere. The density of the atmosphere continues to drop as altitude increases, and the thermosphere eventually merges with the solar wind. Exercises 5 What are the layers of the atmosphere and what kinds of electromagnetic radiation does each absorb? 6 Mars and Venus have no ozone layer. how would you expect the atmospheric profiles to differ from that of the Earth? 7 Why does the Earth s surface radiate mainly at 10 microns? The Earth s surface temperature Radiant energy budgets The Earth has an average surface temperature of 20 C. This temperature depends not only on the amount of power we receive from the Sun, but also on how fast we can radiate energy away from the Earth. If the Earth is to maintain a stable surface temperature, the power it receives from the Sun (and other sources) must exactly equal the power it radiates away into space. In other words it has to balance its energy budget. It is fairly simple to imagine what would happen if we suddenly turned off the Sun. The temperature would drop. The Earth would continue to radiate heat into space and so cool. What would happen if you could cover the Earth with a blanket that would let the Sun s energy through but prevent the heat the Earth was trying to radiate away from escaping? The Earth as a black body We have discussed briefly that the Earth s surface radiates energy like a black body with a temperature of 20 C (288 K). The power radiated increases as: temperature 4 (T 4 ) the surface area radiating energy.

8 The peak in the spectrum of the radiated energy occurs at a wavelength of λ microns = 2900/T = 2900/296 = 10 microns. This means that most of the energy is radiated in the infrared range and very little at visible wavelengths. The greenhouse effect The incoming energy from the Sun is in the visible spectrum. The processes in our atmosphere that absorb solar radiation at wavelengths below 1 micron are mainly due to ozone, oxygen and water molecules. The Earth s atmosphere is therefore semi-transparent to incoming radiation allowing about 50% of the energy to heat its surface. The Earth s surface radiates in the near infrared at a wavelength of 10 microns. The absorbing molecules at these wavelengths include water as well as carbon dioxide, sulfur dioxides, nitrous oxides and methane. Figure 8 The transmission of the Earth s atmosphere from the visible to far infrared. Transmission CO 2 CO 2 CO 2 CO 2 CO 2 O 2 H 2 O H 2 O H 2 O near infrared mid infrared far infrared ultraviolet visible Wavelength (µm) By increasing the amount of carbon dioxide, sulfur dioxides, nitrous oxides and methane in the atmosphere we can affect the absorption in the infrared without affecting the power coming into our atmosphere from the Sun. We have a blanket! Increasing the amounts of these blanketing molecules would cause the Earth to radiate less power. The amount of power from the Sun would remain unaffected and our energy budget would become unbalanced. The temperature would begin to rise. However, there is a safety valve to the greenhouse effect. As the temperature of the Earth rises, it radiates at shorter wavelengths. The effectiveness of the absorbing molecules decreases at shorter wavelengths, so eventually we would re-establish an equilibrium situation in which the power arriving from the Sun exactly matched that radiating from the surface. Climate change The Earth s climate is known to change on time scales of millions of years (the ice ages) to hundreds of years. Evidence for the long-period changes in temperature can be found in ice cores in the Antarctic, in tree rings, and layers in clays (varves). Historical records relate that the 1600s were unusually cold and that the River Thames regularly froze over. The Vikings colonised Greenland during an unusually warm period before being caught by a cold snap. Climate changes can be due to a number of causes: 1 The changes in the Earth s tilt and distance from the Sun. Slow variations in tilt occur with a period of about years. The eccentricity of our orbit changes over a period of years. Both can affect the climate. 2 Changes in the Sun s power output. The Sun is remarkably constant in its power output. Although 2 billion years ago it may have been radiating at 85% of its present value, recent variations appear to be less than 1% of the total output.

9 Exercise 3 Changes in the Earth s atmosphere. It is clear that since the industrial revolution humanity has been adding greenhouse gases to the atmosphere. The concentration of carbon dioxide has been increasing rapidly since the 1950s. In addition methane and other greenhouse gases have also been increasing. Since we do not fully understand the ways in which carbon dioxide can be absorbed in the ecosystem, it is extremely difficult to model the effects of greenhouse gases on global temperatures. However, an international body, the IPCC (Intergovernmental Panel on Climate Change), has attempted to estimate the best and worst case scenarios. If we continue business as usual, the concentration of carbon dioxide is expected to double by The temperature rise will be between 2 C and 5 C. Sea level will rise by between 10 cm and 30 cm, and in the worst case 1 m, by 2100 AD. 8 Research the role of the ozone layer in protecting life on Earth. Illustrate trends and patterns in the information you collect. a Is there evidence for rapid changes to the ozone layer? b What is causing these changes? Activities 2 Perform an investigation to demonstrate the absorption of thermal energy and other forms of radiation such as light by: a a solid object b a liquid. You should present a plan and identify equipment necessary for the set of experiments. Discuss the procedures with your teacher before beginning to gather data. The report must demonstrate: how your procedures allow valid and reliable data and information to be collected the technology you used and its suitability and effectiveness the physical concepts behind the experiment. You should also compare your observations and measurements with those already published in data tables and other publications. Hint: A temperature rise in an object can be used to measure the rate of heating. Which heats faster, a black or silver object? Which cools faster, a silver or black object? 3 Design and undertake an experiment to demonstrate that a hole in the side of an otherwise opaque, hollow container is the most perfect black body that can be constructed. Before you start, submit a proposed plan to your teacher. Describe your equipment or combination of equipment and procedure. Explain why your equipment, your procedure, or the repetition of your procedure is appropriate. Trial your investigation, record your results and carry out repeat trials as necessary.