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chapter 20 the sun affects the earth in many ways

The Sun is a typical star, emitting electromagnetic radiation and particles that influence the Earth

The Sun affects the Earth in many ways

CHAPTER 20

introduction

The Sun is a source of almost every type of electromagnetic radiation known, with the exception of some types of gamma rays. It also emits matter in a stream known as the solar wind. Together, this means the Sun has enormous influence on the Earth. Its stability over thousands of millions of years has allowed life to evolve. However, our Sun is not as stable as some may think. When solar storms erupt, the Earth can be affected in a number of spectacular ways.

figure 20.1 A solar prominence, or flare, photographed in ultraviolet light by the Solar and Heliospheric Observatory (SOHO)

20.1 Energy release from nuclein Identify that energy may be released from the nuclei of atomsRadioactivity was discovered by accident in 1896 when Henri Becquerel (18521908) decided to investigate whether there was any connection between naturally occurring

phosphorescence and X-rays, newly discovered in 1895. For his experiment, Becquerel chose to use uranium salts, which phosphoresce on exposure to light. He began to prepare the experiment, in the darkness of his laboratory, but he found that the salts exposed his photographic plates, even though they didnt have sufficient light to phosphoresce. What Becquerel had detected was radioactivity, that is, energy being emitted spontaneously from the atoms of unstable nuclei.

figure 20.2 Cherenkov radiation is seen as a blue glow in the water, a result of particles with very high speeds being emitted by nuclear radiation

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Most nuclei are stable. Consisting of protons and neutrons, they are unaffected by chemical reactions and have remained the same since they were formed, either in the Big Bang or in the core of a star or supernova. However, some nuclei are not stable and spontaneously release energy in the form of a helium nucleus ( particle), an electron ( particle) or a gamma ray. These nuclei are called radioactive. It is not possible to predict when a particular radioactive nucleus will emit this energy but a large number of radioactive nuclei are said to have a half-life. The half-life is the time it takes for half of the nuclei to emit their radiation.

The release of energy from the nucleus has been used in a variety of ways. Nuclear medicineincluding the diagnosis and treatment of canceras well as nuclear power and nuclear weapons are examples. It was Albert Einstein who first predicted that energy from the nucleus could be put to use.

figure 20.3 A 15 kiloton atomic bomb set off in 1953 in the US as part of the extensive testing program for nuclear weapons: these weapons obtain their energy by the fission (splitting) of large nuclei into smaller ones

20.2Alpha (), beta () and gamma () raysn Describe the nature of emissions from the nuclei of atoms

as radiation of alpha and beta particles and gamma rays in terms of:

ionising power penetrating power effect of magnetic field effect of electric field

alpha particles

Alpha particles consist of two protons and two neutrons. It is the same as a helium nucleus and is very stable. Many large nuclei, such as americium-241 (see Fig. 20.4) are alpha particle emitters. When a nuclei emits an alpha particle its mass decreases as well as its atomic number, so it changes, or transmutates, into a different element.

Alpha particles are good ionisers due to their mass and size. To cause ionisation, an electron must be knocked out of its orbit in an atom. This property makes them useful in ionising smoke detectors. An alpha particle will only travel for several centimetres through air before it captures electrons and becomes a helium atom. Helium is

figure 20.4 The alpha particle source americium-241 in an ionising smoke detector

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a harmless inert gas. This is why smoke detectors pose no danger to the occupants of houses; however, their disposal in landfill presents a problem for the environment.

Alpha particles possess the charge of two protons. When they move through an electric or magnetic field a force is exerted on them. This force causes them to accelerate and deflect from their original path.

Beta particles

Beta particles are high-speed electrons that come from the nucleus. Electrons are not found in the nucleus. A neutron-rich nucleus may be a source of a beta particle when a neutron spontaneously decays into a proton and an electron. The electron is immediately ejected from the nucleus at high speed (i.e. a beta particle) and the nucleus from which it originated has one extra protonso it changes its atomic number and thus element. Beta particles are much smaller and have only about 1/7200th the mass of an alpha particle. They cannot cause ionisation as well as alpha particles, but they can penetrate matter further. Where alpha particles can be stopped by a sheet of paper, beta particles will be stopped by a sheet of metal.

Beta particles have the charge of an electron and also experience a force when they move through electric or magnetic fields. At the same speed, a beta particle will experience half the force that an alpha particle does. The very small mass of beta particles compared to alpha particles means that the force they experience causes a much greater deflection, and their paths will be observed to bend to a much greater extent than alpha particles.

gamma rays

Gamma rays are very high-frequency, short-wavelength electromagnetic radiation. They are emitted from the nucleus and are also associated with fission (the splitting of nuclei) and fusion (the joining together of nuclei) and can accompany or decays. Gamma rays have a very high ability to penetrate matter while having a low ability to cause ionisation. They can be used to treat cancerous tumours and in specialised industrial applications.

Gamma rays are not particles and do not possess any electric charge. They are unaffected by electric and magnetic fields. Unlike alpha and beta particles, they will move through these fields undeflected.

This information is summarised in Table 20.1. See also Figure 20.5.

table 20.1 Radiation types and their properties

radiation what it is ionising ability penetrating power

effect of electric or magnetic field

(alpha) particle He nucleusi.e. two protons and two neutrons

High Low Small deflection

(beta) particle An electron Moderate Moderate Great deflection

(gamma) ray Electromagnetic radiation

Low High No deflection

radioactivesource

beta particle paths ()

alpha particle paths

cloudchamber

paper

metal (e.g. aluminium sheet)

lead or concrete

figure 20.5 The different penetrating powers of radiation: alpha particles are blocked by a sheet of paper; beta particles can be blocked by a metal sheet; gamma rays can penetrate lead and concrete to significant depths

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first-hand investigation

Pfa

P4

Physics skills

11.2 AE11.3 AD12.1 AD12.2 A, B12.4 A, C, D, E14.1 A, E

Comparing the penetrating power of alpha, beta and gamma radiation

n Perform a first-hand investigation to gather information to compare the penetrating power of alpha, beta and gamma radiation in a range of materials

Warning: This investigation involves the use of radioactive materials. These should be stored carefully in lead-lined containers whenever they are not being used. They should be handled with care and accounted for at the conclusion of the lesson.

apparatus

Alpha, beta and gamma sources, as available commercially in sealed containers (Po-210 as an alpha source, Sr-90 as a beta source, Co-60 as a gamma source); GeigerMller tube and counter; various materials including paper, metal sheets (aluminium and lead), glass, etc.

method

n Hold each source at the end of the GeigerMller tube and take a count over a 20-second period.

n For each source, place the paper, then metal sheets and glass between the source and the GeigerMller tube and take a reading of the count over 20-second periods.

n Record the results in a table and compare the different sources penetrating power through the materials used.

alternative method using a wilson cloud chamber

n A Wilson cloud chamber (see Fig. 20.6) needs to be prepared beforehand, if one is available.

n Use different radioactive sources in the cloud chamber (see Fig. 20.7) and compare the length and strength of the observed paths.

n A magnet held above the cloud chamber will cause alpha particles to deflect in one direction and beta particles to deflect more in the opposite direction. Any gamma ray paths will be undeflected.

radioactivesource

beta particle paths ()

alpha particle paths

cloudchamber

paper

metal (e.g. aluminium sheet)

lead or concrete

figure 20.6 The paths of alpha and beta particles seen in a Wilson cloud chamber

figure 20.7 Radioactive sources used in this investigation

TR

Risk assessment matrix

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20.3 The solar windn Identify the nature of emissions reaching the Earth from

the Sun

n Describe the particulate nature of the solar windThe solar wind is a stream of particles ejected from the Sun. It consists mostly of protons travelling on average 400 km s1. This speed can vary from between 300800 km s1. Electrons and ions make up a small percentage of the solar wind. The Suns extremely hot corona (see Fig. 20.8), a region surrounding the Sun that extends as far as several million kilometres above the photosphere (the surface of the Sun), causes particles to move so fast that gravity cannot keep them around the Sun. At about one million degrees Celsius, the corona is the source of the solar wind. It is also the source of coronal mass ejectionslarge amounts of solar wind material flung out into space in times of magnetic storms on the Sun.

The density of the solar wind is measured in the number of protons per cubic centimetre (p cm3). Although usually between 110 p cm3, the density can be many times this when a solar flare, or coronal mass ejection, is directed towards and strikes the Earth.

USEFUL WEBSITE:Monitor the Suns activity:

http://www.spaceweather.com/

The solar wind is being constantly measured and recorded by a space observatory known as SOHOSolar and Heliospheric Observatory, which orbits the Sun in a position where the Earths gravitational field is exactly balanced by the Suns. SOHO is about 1.5 million kilometres away from the Earth in a line directly between the Earth and the Sun.

figure 20.8 The Suns corona taken in the X-ray waveband

deflected solar wind particlesmagnetotail

incoming solar wind particles

solar cusp

Earthsatmosphere(0-100 km)

plasma sheet

neutral sheet

bow shock magnetosheath

figure 20.9 The solar wind travels through space and is deflected by the Earths magnetosphere

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n Describe sunspots as representing regions of strong magnetic activity and lower temperature

n Outline the cyclic nature of sunspot activity and its impact on Earth through solar winds

The Suns surface, or photosphere, is the region around the Sun above which the hot gases do not emit significant amounts of light. It is not a real surface, as on rocky planets and moons. The Suns size is measured to this point.

This apparent surface of the Sun has a temperature of 5500oC. It is in violent motion, subject to the strong convection currents welling up from beneath. The motion of charged particles produces intense, dynamic magnetic fields. When these fields warp and twist around each other, magnetic storms can distort and disrupt the motion of the convection currents near the surface and sunspots are formed (see Fig. 20.11). These regions, often larger than several Earths, are cooler than the surrounding photosphere by several hundred degrees, making them appear dark next to the very bright normal areas.

Sunspots were first observed by Galileo in 1610. An image of the Sun can be produced on a screen when sunlight is passed through a very small hole made in a piece of cardboard. (This is easily done as a class exercise.) The number of sunspots present goes through a cycle that is, on average, 11 years long (see Fig. 20.12). The time between peak sunspot activity has varied in the past from 9 to 13 years. In

20.4Sunspots

figure 20.10 An aurora photographed over Finland in October 2008: this aurora was not predicted and occurred when a change in the interplanetary magnetic field allowed the solar wind to pour into the Earths polar regions, interacting with the upper atmosphere and causing light to be emitted

figure 20.11 (a) Sunspots, when they occur, are clearly visible on the surface of the Sun

figure 20.11 (b) A close-up showing the features of sunspots: note the granular appearance of the Suns surface

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between these peak times, sunspot activity declines. In October 2008, there were no sunspots at all. A period of peak sunspot activity is next expected around 2012 or 2013, but its arrival cannot be predicted accurately.

In periods of peak sunspot activity, the total energy radiated by the Sun increases slightly and the solar wind is most intense. During these times, solar flares and coronal mass ejections are more likely to

occur as the Suns surface becomes more turbulent and violent. If the Earth happens to lie in the path of a coronal mass ejection or a particularly intense solar wind, our magnetic field can be disturbed and the flow of charged particles in the upper atmosphere can disrupt power supplies and satellite communications.

Deflected solar wind particlesMagnetotail

Incoming solar wind particles

Solar cusp

Earthsatmosphere(0-100km)

Plasma sheet

Neutral sheet

Bow shock Magnetosheath

No.

of s

unsp

ots

Year1760 1780 1800 1820 1840 1660 1880

300

200

100

0

Year1880 1900 1920 1940 1960 1980 2000

No.

of s

unsp

ots 300

200

100

0

figure 20.12 Sunspot numbers recorded over the past 250 years: these records show the sunspot cycle, along with the irregular nature of the cycle

PFA

P5Describes the scientific principles employed in particular areas of research in physics

Remote sensing to make observations and gather data; hypothesising to make predictions of future events; constructing models to explain observations and modifying models when observations do not fitthese are some of the scientific principles used in the study of sunspots and associated solar phenomena. These applied scientific principles are outlined here.

remote sensing

It is not possible to make direct measurements of conditions on the Suns surface. Temperatures, magnetic fields and other conditions must be monitored and observed from a distance by remote sensing using instruments on Earth, in orbit around the Earth and the Sun, and on board space probes sent flying past the Sun. In the past few decades the improvements in technology and the investments made in space probes have resulted in a vast leap in observations by remote sensing techniques.

hypothesising

The conditions on the Suns surface are not found anywhere on Earth. We cannot replicate them accurately, and therefore they cant be investigated closely. To explain the observations made, theories and hypothesise need to be made based on known

Explaining sunspots using scientific principles

TR

Mapping the PFAsPFA scaffold P5

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secondary source investigation

Pfa

P3, 4, 5

Physics skills

11.1 A, B, D, E12.3 AE12.4 A, C, D, E, F14.1 A, B, F, G, H14.2 A, C, D

Assessing the effects of sunspot activity

n Identify data sources, gather and process information and use available evidence to assess the effects of sunspot activity on the Earths power grid and satellite communications

When the charged particles that make up the solar wind enter the region of space influenced by the Earths magnetic field, they are deflected towards the polar regions. Figure 20.9 shows their spiral paths towards the north and south magnetic poles. This flow changes the magnetic field of the Earth on the surface, which in turn can induce voltages in long wires such as those used to transmit electricity over large distances. Automatic safeguards that protect electricity transmission grids from being overloaded may be triggered, cutting off power supplies to whole regions. Such an event occurred with little or no warning in the north-east of the United States and south-eastern region of Canada in 1989 during a solar flare event. The Hydro-Quebec power transmission system was shut down, causing over six million people to have their electricity supply cut.

Satellite communications may also be affected directly and indirectly by solar flare events. The stream of charged particles may induce voltages in satellite electronics that can overload and even destroy the delicate components that are used. Additionally, the interaction of the particles in the solar wind with the ionosphere produces radio wave energy that may swamp the weak signals being sent by satellites. The GPS operations are particularly vulnerable, as the satellite signals are very weak once they reach Earths surface.

physics. If these hypothesise do not fit subsequent observations they need to be modified or replaced. The scientific method is based on this procedureobserve, hypothesise, test and modify if needed.

models

Models used to explain the development of sunspots are developed that fit in with theories, hypotheses and nuclear physics knowledge applied to the whole of the Sun, not just its surface. The entanglement of magnetic field lines originating from beneath the Suns surface and extending well into space go a long way in explaining sunspots. There are still gaps in the model but as research in this fascinating field progresses, the models we have may be modified and refined to incorporate the missing pieces.

USEFUL WEBSITES:Sun science:

http://cse.ssl.berkeley.edu/hessi_epo/html/sun.htmlSunspot history:

http://cse.ssl.berkeley.edu/SEGwayed/lessons/sunspots/CSIRO Sunspot resources:

http://www.csiro.au/resources/ps2ac.htmlTodays space weather:

http://www.swpc.noaa.gov/today.html

WWW>

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Solar flare activity is now closely monitored by a number of satellites, including SOHO and RHESSI (see Fig. 20.13). Even a few hours notice of a solar storm will give the operators of satellites and power grids time to take protective action.

USEFUL WEBSITES:A solar flare event in October 2003:

http://www.space.com/scienceastronomy/solar_flare_031028.htmlHow GSP navigation satellites might be affected by solar flares:

http://www.news.cornell.edu/stories/Sept06/solar.flares.gps.TO.htmlThe RHESSI satellite observatory:

http://www.spaceflightnow.com/news/n0203/29rhessi/How solar flares may disrupt GPS in 2011:

http://technology.newscientist.com/article/dn10189-solar-flares-will-disrupt-gps-in-2011.htmlReports of the solar flare event in October 2003:

http://news.bbc.co.uk/2/hi/science/nature/3223739.stmNOAA magazine article:

http://www.magazine.noaa.gov/stories/mag131.htmResources useful for this activity:

http://www.solarstorms.org/

figure 20.13 The RHESSI solar flare observatory, launched in 2002 to photograph and monitor solar flare activity

CHAPTER REvision quEsTions

1. Explain the differences in alpha particles, beta particles and gamma rays in terms of their:(a) ionising ability(b) penetrating power(c) paths when moving through electric and magnetic fields

2. From where does the energy come when alpha particles or beta particles are emitted?

3. Describe what happens to the nucleus when it emits (a) an alpha particle and (b) a beta particle in terms of the number of protons and neutrons left behind.

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4. If electrons do not exist in the nucleus, how do beta particles come from here?

5. (a) What constitutes the solar wind?(b) How fast does the solar wind travel through space?(c) From where does the solar wind originate?

6. Why do sunspots appear dark?

7. Identify two phenomena that vary with the solar cycle.

8. A coronal mass ejection has just struck the Earths magnetic field. Describe the events that might be observed from the Earths surface over the next few hours.

THE CosmiC EnginE: REviEw quEsTions

1. For any one model of the Universe between the time of Aristotle and Newton:(a) Sketch the arrangement of the Universe that was proposed by this model.(b) State how the model proposed that the heavenly bodies were being held in place.(c) Outline any limitations of the model.(d) Compare the model to the accepted present-day model of the Universe.

2. Describe how the models of the Universe changed over time with advances in available technology.

3. (a) Describe the observations made by Hubble that led to the confirmation of the expanding nature of the Universe.

(b) Outline the role Friedmann played in the discovery of the expanding Universe. 4. Outline the processes immediately after the Big Bang that are believed to have led to the

formation of the first stars.

5. Describe the colour changes that may be observed as a very hot body becomes hotter.

6. Relate the colour of stars to how astronomers can determine their surface temperature.

7. Sketch the axes for a HertzsprungRussell diagram. Label both axes and the sketch the approximate positions of:(a) white dwarf stars(b) very cool but very large stars(c) a Main Sequence star like our Sun(d) a blue super giant star.

8. Explain why the term Main Sequence as applied to stars is not really a sequence.

9. Outline the key differences between white dwarf stars and a typical Main Sequence star.

10. Outline the mechanism that is believed to occur in the core of stars that provides them with their source of energy.

11. A star is observed to have a brightness of 16 units. How much larger would it need to become (with every other factor held constant) if its brightness were to increase to 64 units?

12. Describe the changes to the brightness of star when your spaceship moves from 1000 light years away to 10 light years distant.

CHAPTER 20 The Sun affects the Earth in many ways20.1 Energy release from nuclei20.2 Alpha (), beta () and gamma () raysFirst-hand investigation: Comparing the penetrating power of alpha, beta and gamma radiation20.3 The solar wind20.4 SunspotsExplaining sunspots using scientific principlesSecondary source investigation: Assessing the effects of sunspot activityChapter revision questions