Download Physics: Particles from Space - Advice for Practitioners (Revised

NATIONAL QUALIFICATIONS CURRICULUM SUPPORT
Physics
Particles from Space
Advice for Practitioners
Nathan Benson
[REVISED ADVANCED HIGHER]
The Scottish Qualifications Authority regularly reviews
the arrangements for National Qualifications. Users of
all NQ support materials, whether published by
Education Scotland or others, are reminded that it is
their responsibility to check that the support materials
correspond to the requirements of the current
arrangements.
Acknowledgement
The author gratefully acknowledges Prof Martin A. Hendry FRSE, University of Glasgow, for
his constructive comments and contributions.
The publisher gratefully acknowledges permission to use the following sources: Figures 2, 3, 4,
7, 8 and 12 courtesy of NASA; Figure 10 courtesy of SSERC.
© Crown copyright 2012. You may re-use this information (excluding logos) free of charge in
any format or medium, under the terms of the Open Government Licence. To view this licence,
visit http://www.nationalarchives.gov.uk/doc/open-government-licence/ or e-mail:
[email protected].
Where we have identified any third party copyright information you will need to obtain
permission from the copyright holders concerned.
Any enquiries regarding this document/publication should be sent to us at
[email protected].
This document is also available from our website at www.educationscotland.gov.uk.
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Introduction
Contents
Introduction
4
Section 1: Cosmic rays
Possible introduction to learners
Origin and composition
Interaction with the Earth’s atmosphere
6
6
7
9
Section 2: The solar wind and magnetosphere
Structure of the Sun
Interaction with the Earth’s magnetic field
Charged particles in a magnetic field
11
11
19
22
Appendix 1: Key stages in the early development of cosmic ray
theories
27
Appendix 2: Using the Teltron dual-beam tube to demonstrate helical
motion in a magnetic field
29
Glossary
33
Recommended reading
34
Bibliography
36
Useful websites
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INTRODUCTION
Introduction
These notes have been written primarily to support practitioners with the
introduction of the properties of stars and stellar evolution topics as specified
in the Quanta and Waves unit of the new Advanced Higher Physics course.
The sections in this document correspond to the headings in the Content
column of the SQA Arrangements document. The sub-headings correspond to
the first sentence or key words in the relevant paragraph in the Notes or
Contexts columns.
As this is an area in which many practitioners may not have extensive
experience, this document provides a background from which to present the
topics in an informed way and confident manner. Consequently , information
is given beyond the bare requirements of the Arrangements document. Fuller
information can be obtained from the recommended texts and bibliography.
With the widespread use of the internet, it is important to anticipate
alternative terminology and units of measurement that learners may come
across.
Anecdotes and interesting facts are included to add curiosity and intrigue.
These can be used as appropriate to enrich lessons.
The very nature of this subject matter means practical work is somewhat
limited, but where possible suggestions have been included for quick
illustrations of principles and sometimes further investigations.
Some teaching points are included, especially in areas of possible common
misconception.
Suggested reading as well as the normal bibliography is included. Stellar
physics is a constantly changing subject, but the Arrangements contain some
of the most up-to-date topics ever included in an Advanced Higher Physics
course.
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These notes are not prescriptive, neither do they imply that every suggested
activity should be undertaken or every anecdote used. Hopefully practitioners
will find them a useful resource in gaining a background in the topics and
providing avenues for further inquiry if and when required.
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COSMIC RAYS
Section 1: Cosmic rays
Possible introduction to learners
In the early 1900s, radiation was detected using an electroscope. However,
radiation was still detected in the absence of known sources. This was
background radiation.
Austrian physicist Victor Hess made measurements of radiation at high
altitudes from a balloon, to try and get away from possible sources on Earth.
He was surprised to find the measurements actually increased with altitude.
At an altitude of 5000 m the intensity of radiation was found to be five times
that at ground level.
Hess named this phenomenon cosmic radiation (later to be known as cosmic
rays).
It was thought this radiation was coming from the Sun, but Hess obtained the
same results after repeating his experiments during a nearly complete solar
eclipse (12 April 1912), thus ruling out the Sun as the (main) source of
radiation.
In 1936 Hess was awarded the Noble Prize fro Physics for the discovery of
cosmic rays.
For his Nobel lecture see:
http://www.nobelprize.org/nobel_prizes/physics/laureates/1936/hes slecture.html
Demonstrate, and/or discuss the operation of the Taylor or Wilson cloud
chamber.
Interesting fact
Charles T R Wilson is the only Scot ever to be awarded the Nobel Prize for
Physics. He was awarded it in 1927 for the invention of the cloud chamber.
For a recent (2004) paper on ‘C T R Wilson and the discovery of cosmic rays’
see http://www.astro.gla.ac.uk/users/alec/wilson/cosmic1.pdf .
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Robert Millikan coined the phrase ‘cosmic rays’, believing them to be
electromagnetic in nature.
By measuring the intensity of cosmic rays at different latitudes (they were
found to be more intense in Panama than in California), Compton showed that
they were being deflected by the Earth’s magnetic field and so must consist
of electrically charged particles, ie electrons or protons rather than
‘corpuscles of light’ (ie photons in the form of gamma radiation).
See Time Magazine, 13 February 1933:
http://www.time.com/time/magazine/article/0,9171,745165,00.html
Demonstrate the deflection of some types of radiation by a magnetic field
using a combined alpha, beta and gamma source and a strong magnet.
Origin and composition
The term cosmic rays is not precisely defined, but a generally accepted
description is ‘high energy particles arriving at the Earth which have
originated elsewhere’.
Composition
Cosmic rays come in a whole variety of types, but the most commo n are
protons, followed by helium nuclei. There is also a range of other nuclei as
well as individual electrons and gamma radiation (see Table 1).
Table 1 Composition of cosmic rays
Nature
Protons
Alpha particles
Carbon, nitrogen and oxygen nuclei
Electrons
Gamma radiation
Approximate percentage of all
cosmic rays
89
9
1
less than 1
less than 0.1
The energies of cosmic rays cover an enormous range, with the most
energetic having energies much greater than those capable of being produced
in current particle accelerators.
The highest energies produced in particle accelerators are of the order of
1 teraelectronvolt (10 12 eV).
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COSMIC RAYS
Cosmic rays have been observed with energies ranging from 10 9 to 10 20 eV.
Those with energies above 10 18 eV are referred to as ultra-high-energy
cosmic rays (UHECRs).
Interesting facts
A cosmic ray with energy exceeding 10 20 eV was recorded in 1962.
The ‘Oh-my-God’ (OMG) particle with energy of 3 × 10 20 eV was recorded in
Utah in 1991.
Converting to joules (J), 3 × 10 20 eV = 3 × 10 20 × 1.6 × 10 –19 J = 48 J, ie ~50 J.
That is enough energy to throw a throw a 25-kg mass (eg a bag of cement)
2 m vertically upwards. It is also approximately equal to the kinetic energy of
a tennis ball returned at about 100 mph by, say, Andy Murray.
Order of magnitude open-ended question opportunity here:
mass = 60 g = 0.06 kg, speed = 45 m s –1 ,
kinetic energy = 0.5 × 0.06 × 45 × 45 = 60 J
The OMG particle was probably a proton and as such had about 40 million
times the energy of the most energetic protons ever produced in an Earthbased particle accelerator!
See: http://en.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray
Such UHECRs are thought to originate from fairly l ocal (in cosmological
terms) distances, ie within a few hundred million light years. This is because
were they to originate from further away it would be hard to understand how
they get all the way here at all, since the chances are they would have
interacted with Cosmic Microwave Background Radiation (CMBR) photons
along the way, producing pions – cf. next section on atmospheric interactions.
(This also provides a good link with the revised Higher Physics Arrangements
on the Big Bang.)
Origin
The lowest energy cosmic rays come from the Sun and the intermediate
energy ones are presumed to be created within our g alaxy, often in connection
with supernovae. The main astrophysics (rather than particle physics) to come
from the study of cosmic rays concerns supernovae since they are believed to
be the main source of cosmic rays.
However, the origin of the highest energy cosmic rays is still uncertain.
Active galactic nuclei (AGN) are thought to be the most likely origin for
UHECRs. A group of cosmologists (including Martin Hendry from the
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COSMIC RAYS
University of Glasgow) are working on the statistical analysis of apparent
associations between the incoming direction of the highest energy cosmic
rays and active galaxies.
Interaction with the Earth’s atmosphere
When cosmic rays reach the Earth, they interact with the Earth’s atmosphere,
producing a chain of reactions resulting in the production of a large number
of particles known as a cosmic air shower (Figure 1). Air showers were first
discovered by the French scientist Pierre Auger in 1938. Analysing these
showers allows the initial composition and energies of the original (primary)
cosmic rays to be deduced.
When cosmic rays from space (primary cosmic rays) strike particles in the
atmosphere they produce secondary particles, which go on to produce more
collisions and particles, resulting in a shower of particles that is detected at
ground level. The primary cosmic rays can usually only be detected directly
in space, for example by detectors on satellites, although very high energy
cosmic rays, which occur on rare occasions, can penetrate directly to ground
level.
Primary cosmic ray
p




n




e+
e–
e+

e–
p, proton; e–, electron; e+ positron; neutrino;
pions; muons; gamma
Figure 1 Air shower.
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COSMIC RAYS
Detection
Consequently there are two forms of detector : those that detect the air
showers at ground level and those located above the atmosphere that detect
primary cosmic rays.
Cherenkov radiation
Air shower particles can travel at relativistic speeds. Although relativity
requires that nothing can travel faster than the speed of light in a vacuum,
particles may exceed the speed of light in a particular medium, for example
water. Such particles then emit a beam of Cherenkov radiation – the radiation
that causes the characteristic blue colour in nuclear reactors. (This is a bit
like the optical equivalent of a sonic boom.)
See http://imagine.gsfc.nasa.gov/docs/science/how_l2/cerenkov.html .
Atmospheric fluorescence
When charged particles pass close to atoms in the atmosphere, they may
temporarily excite electrons to higher energy levels. The photons emitted
when the electrons return to their previous energy levels can then be detected.
The Pierre Auger observatory in Argentina was set up to study high-energy
cosmic rays. It began operating in 2003 and at that time was the largest
physics experiment in the world. It is spread over several thousand square
miles and uses two basic types of detectors:
 1600 water tanks to detect the Cherenkov effect
 four detectors of atmospheric fluorescence.
See the main Auger observatory home page:
http://www.auger.org/
For information about the detector see:
http://www.auger.org/cosmic_rays/detector.html
and for an animation of an air shower see:
http://www.auger.org/features/shower_simulations.html
The next proposed ‘big thing’ in ground -based cosmic ray detection is the
Cherenkov Telescope Array (CTA), which should be operational within the
next decade. See: http://www.cta-observatory.org/.
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Section 2: The solar wind and magnetosphere
Structure of the Sun
The interior of the Sun consists of three main regions:
1.
2.
3.
the core, within which nuclear fusion takes place
the radiative zone, through which energy is transported by photons
the convective zone, where energy is transported by convection.
The extended and complex solar atmosphere be gins at the top of the
convective zone, with the photosphere.
The photosphere is the visible surface of the Sun and appears smooth and
featureless, marked by occasional relatively dark spots, called sunspots.
Moving outwards, next is the chromosphere. Sharp spicules and prominences
emerge from the top of the chromosphere.
The corona (from the Greek for crown) extends from the top of the
chromosphere. The corona is not visible from Earth during the day because of
the glare of scattered light from the brill iant photosphere, but its outermost
parts are visible during, for example, a total solar eclipse.
The depth of each layer relative to the radius of the Sun ( R s ) is shown in
Figure 5. The photosphere is about 330 km deep (0.0005R s ) and the
chromosphere is about 2000 km (0.003R s ) deep.
Coronagraphs are special telescopes that block out the light from the
photosphere to allow the corona to be studied. These are generally used from
mountain tops (where the air is thin) and from satellites. They have been able
to detect the corona out beyond 20R s , which is more than 10% of the way to
Earth.
The corona is permeated by magnetic fields. In particular there are visible
loops along which glowing ionised gaseous material can be seen to travel.
They have the shape of magnetic field lines and begin and end on the
photosphere.
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SOLAR WIND AND MAGNETOSPHERE
Figure 2 Coronal hole.
Information about magnetic fields in the corona has come from the study of
emitted X-rays, obtained from satellites and space stations. The corona is the
source of most of the Sun’s X-rays because of its high temperature, which
means it radiates strongly at X-ray wavelengths. The corona’s X-ray emission
is not even, however, with bright patches and dark patches . The dark areas
hardly emit any X-rays at all and are called coronal holes.
The Solar and Heliospheric Observatory (SOHO) is a project of international
collaboration between the European Space Agency (ESA) and the National
Aeronautics and Space Administration (NASA) to study the Sun from its deep
core to the outer corona and the solar wind.
The SOHO website
(http://sohowww.nascom.nasa.gov/classroom/sun101.html ) has excellent
teaching resources and the 4-minute video ‘Sun overview’ would make an
excellent introduction to this topic. (Click on the Classroom tab followed by
Students & teachers, Activities).
On 25 October 2006 a Delta II rocket was launched from Cape Canaveral
carrying two nearly identical spacecraft. Each satellite was one half of a
mission entitled Solar TErrestrial RElations Observatory (STEREO) and they
were destined to do something never done before – see the whole of the Sun
simultaneously. With this new pair of viewpoints, scientists will be able to
see the structure and evolution of solar storms as they blast from the Sun and
move out through space.
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The STEREO mission has excellent educational resources – see
http://stereo.gsfc.nasa.gov/.
The solar wind
There is a continual flow of charged particles emanating from the Sun
because of the high temperature of the corona. This gives some particles
sufficient kinetic energy to escape from the Sun’s gravity. This flow is called
the solar wind and is plasma composed of approximately equal numbers of
protons and electrons (ionised hydrogen). It can be thought of as an extension
of the corona itself and as such reflects its composition. The solar wind also
contains about 8% alpha particles (helium nuclei) and trace amounts of heavy
ions and nuclei (C, N, O, Ne, Mg, Si, S and Fe) .
The solar wind travels at speeds of between 300 and 800 kms –1 , with gusts
recorded as high as 1000 km –1 (2.2 million miles per hour).
Comets
Although it was known that solar eruptions ejected material that could reach
the Earth, no-one suspected that the Sun was continually losing material
regardless of its apparent activity. It had been known for a long time that
comet tails always pointed away from the Sun, although the reason was
unknown. Ludwig Biermann (of the Max Planck Institute for Physics in
Göttingen) made a close study of the comet Whipple-Fetke, which appeared
in 1942. It had been noted that comet tails did not point directly away from
the Sun. Biermann realised this could be explained if the comet was m oving
in flow of gas streaming away from the Sun. The comet’s tail was acting like
a wind-sock, In the early 1950s Biermann concluded that even when the Sun
was quiet, with no eruptions or sunspots, there was still a continuous flow of
gas from it.
Interesting facts
In 1959, the Russian space probe Luna 1 made the first direct observation and
measurement of the solar wind. The probe carried different sets of scientific
devices for studying interplanetary space, including a magnetometer, Geiger
counter, scintillation counter and micrometeorite detector.
 It was the first manmade object to reach escape velocity (good link with
the gravity topic).
 It was supposed to hit the Moon but a failure of the launch vehicle's
control system caused it to miss by about 6000 km.
 The probe released a cloud of sodium gas, which allowed astronomers to
track it visually using the glowing orange trail.
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SOLAR WIND AND MAGNETOSPHERE
 It finally went into orbit around the Sun between the orbits of Earth and
Mars.
Coronal holes
The magnetic field lines from coronal holes don’t loop back onto the surface
of the corona. Instead they project out into space like broken rubber bands,
allowing charged particles to spiral along them and escape from the Sun.
There is a marked increase in the solar wind when a coronal hole faces the
Earth.
A pdf document (just over one page) for learners on the solar wind and the
Genesis project is available from NASA here:
http://genesismission.jpl.nasa.gov/educate/scimodu le/SSWPrOptPDFs/4Who
HasMass/SolarWind-ST-PO.pdf
Brief details of the solar wind composition (SWC) experiment carried out by
Apollo 11 are given here (see Figures 3 and 4):
http://ares.jsc.nasa.gov/humanexplore/exploration/exlibrary/docs/apollocat/pa
rt1/swc.htm
Figure 3 Apollo 11 SWC.
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Figure 4 SWC foil experiment.
SOLAR WIND AND MAGNETOSPHERE
Solar flares
Solar flares are explosive releases of energy that radiate energy ov er virtually
the entire electromagnetic spectrum, from gamma rays to long wavelength
radio waves. They also emit high-energy particles called solar cosmic rays.
These are composed of protons, electrons and atomic nuclei that have been
accelerated to high energies in the flares. Protons (hydrogen nuclei) are the
most abundant particles followed by alpha particles ( helium nuclei). The
electrons lose much of their energy in exciting radio bursts in the corona.
These generally occur near sunspots, which leads to the suggestion they are
magnetic phenomena. It is thought that magnetic field lines become so
distorted and twisted that they suddenly snap like rubber bands. This releases
a huge amount of energy, which can heat nearby plasma to 100 million kelvin
in a few minutes or hours. This generates X -rays and can accelerate some
charged particles in the vicinity to almost the speed of light .
Interesting facts
An unusually bright flare can temporarily increase the Sun’s brightness by
1%.
Solar flares were first observed on the Sun by English astronomer Richard
Carrington in 1859.
It happened at 11:18 AM on the cloudless morning of Thursday,
September 1st, 1859. Just as usual on every sunny day, the 33-year-old
solar astronomer was busy in his private observatory, projecting an
image of the sun onto a screen and sketching what he saw. On that
particular morning, he traced the outlines of an enormous group of
sunspots. Suddenly, before his eyes, two brilliant beads of white light
appeared over the sunspots; they were so bright he could barely stand
to look at the screen.
Carrington cried out, but by the time a witness arrived minutes later,
the first solar flare anyone had ever seen was fading away.
(Courtesy NASA: http://science.nasa.gov/science-news/science-atnasa/2011/19sep_secretlives/)
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SOLAR WIND AND MAGNETOSPHERE
Corona
Convective zone
0.3Rs
Radiative zone
0.5Rs
Core
0.2Rs
Chromosphere
Photosphere
Solar wind
Figure 5 Structure of the Sun.
The energies of solar cosmic ray particles range from milli -electronvolts (10 –3
eV) to tens of giga-electronvolts (10 10 eV).
The highest energy particles arrive at the Earth within half an hour of the
flare maximum, followed by the peak number of particles 1 hour later.
Particles streaming from the Sun after solar flares or other major solar events
can disrupt communications and power deliver y on Earth.
Interesting facts
A major solar flare in 1989:
 caused the US Air Force to temporarily lose communication with over
2000 satellites
 induced currents in underground circuits of the Quebec hydroelectric
system, causing it to be shut down for more than 8 hours.
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For the biggest solar flare on record, see:
http://sohowww.nascom.nasa.gov/hotshots/X17/
The solar cycle
All solar activities show a cyclic variation with a period of about 11 years.
Sunspots
The Sun can be viewed using a telescope or binoculars (with one side blocked
off) and projecting the image onto white card or paper (see Figure 6).
Learners should be warned about the dangers of viewing the Sun directly,
with or without the use of an optical instrument.
Figure 6 Using a telescope to view the Sun.
What is seen is light emitted by the gases of the photosphere. Dark spots are
visible on this and are called sunspots. By observing over a number of days
they will be seen to move (due to the rotation of the Sun) and also change in
size, growing or shrinking. The sunspots look dark because they are cooler
than the surrounding photosphere. A large group of sunspots is called an
active region and may contain up to 100 sunspots.
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SOLAR WIND AND MAGNETOSPHERE
The earliest recording of the observation of sunspots is by the Chinese
astronomer Gan De in 364 BC. Observations using telescopes were first made
in the early 17th century by Galileo, by Thomas Harriot (English) in 1610 and
independently by the German theologian David Fabricius along with his
eldest son Johannes in 1611.
See http://mintaka.sdsu.edu/GF/vision/others.html for details of the eye
injuries sustained by early solar observers.
The general pattern of the movement of sunspots (individual sunspots may
appear and disappear – short-lived ones only lasting a few hours whereas
others may last for several months) shows that the Sun is rotating with an
average period of about 27 days with its axis of rotation tilted slightly to the
plane of the Earth’s orbit.
Unlike the Earth, the Sun does not have a single rotation period. The period is
25 days at the Sun’s equator and lengthens to 36 days near the poles. Sections
at different latitudes rotate at different rates and so this is called differential
rotation.
The three main features of the solar cycle are:
1.
2.
3.
the number of sunspots
the mean latitude of sunspots
the magnetic polarity pattern of sunspot groups.
The number of sunspots increases and decrease s with an 11-year cycle, the
mean solar latitude at which the sunspots appear progresses towards the solar
equator as the cycle advances and the magnetic polarity patter n of sunspot
groups reverses around the end of each 11-year cycle (making the full cycle
in effect 22 years).
The magnetosphere
The magnetosphere is the part of the Earth’s atmosphere dominated by the
Earth’s magnetic field. This region also contains a diffuse plasma of protons
and electrons. The magnetic field resembles that of a bar magnet, tipped at
about 11° to the Earth’s rotational axis. However , the magnetic field is
believed to be generated by electric currents in conducting material inside the
Earth, like a giant dynamo. Geological evidence shows that the direction of
the Earth’s magnetic field has reversed on several occasions, the most recent
being about 30,000 years ago. This lends evidence for the ‘dynamo’ model as
the reversal can be explained in terms of changes in the flow of conducting
fluids inside the Earth.
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Interaction with the Earth’s magnetic field
The solar wind interacts with the magnetosphere and distorts its pattern from
the simple model outlined above. The Earth’s magnetic field also protects it
from the solar wind, deflecting it a bit like a rock deflecting the flow of water
in a river. The boundary where the solar wind is first deflected is called the
bow shock. The cavity dominated by the Earth’s magnetic field is the
magnetosphere (see Figures 7 and 8).
Figure 7 The magnetosphere as visualised in 1962.
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SOLAR WIND AND MAGNETOSPHERE
Figure 8 Current perception of the Earth’s magnetosphere .
For more information and diagrams of the magnetospheres of some planets
visit http://ccmc.gsfc.nasa.gov/educational/MagnetosphereWebPage.php .
High-energy particles from the solar wind that leak into the magnetosphere
and become trapped form the Van Allen belts of radiation. These are toroidal
in shape and concentric with the Earth’s magnetic axis. There are two such
belts: the inner and the outer.
The inner Van Allen belt lies between one and two Earth radii from its axis,
(R E < inner belt < 2R E ) then there is a distinct gap followed by the outer belt
lying between three and four Earth radii (3R E < outer belt < 4R E ). The inner
belt traps protons with energies of between 10 and 50 MeV and electrons with
energies greater than 30 MeV. The outer belt contains fewer energetic protons
and electrons.
The charged particles trapped in the belts spiral along magnetic field lines
and oscillate back and forth between the northern and southern mirror points
with periods between 0.1 and 3 seconds (see Figure 9).
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Mirror point
Particle helical trajectory
Van
Allen
belt
section
Earth
Magnetic
field line
Mirror point
Figure 9 Van Allen belt.
Particles in the inner belt may interact with the thin upper atmosphere to
produce the aurorae. These result from the excitation of different atoms in the
atmosphere, each of which produces light with a characteristic colour due to
the different energy associated with that atomic transition .
Carbon dating
The most common isotope used for radiometric dating is carbon -14, which
decays to nitrogen-14 with a half-life of 5730 years.
When primary cosmic rays collide with gas molecules in the atmosphere, they
may knock out nucleons (protons or neutrons). A resulting high energy
neutron may then collide with a nitrogen-14 nucleus. By the process of
charge exchange one of the nitrogen’s protons turns into a neutron while the
incoming neutron changes into a proton, thus producing a carbon -14 nucleus.
n+
14
N→
14
C+p
Plants (and the animals that eat them) take in atmospheric carbon through
carbon dioxide. When the plant (or animal) dies, this uptake ceases and the
fraction of radioactive carbon-14 within the plant starts to decrease as the
carbon-14 decays, emitting a beta particle and an electron anti -neutrino.
14
C→
14
N + e– + νe
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Cosmic ray exposure age
The length of time that has passed since a meteorite broke off its parent
meteor can be estimated using its cosmic ray exposure age. Cosmic rays can
only penetrate to a depth of about 1 m into a meteor or meteorite and
consequently none of the products of cosmic ray induced nuclear reactions
can build up.
Other planets
All planets that have magnetic fields have a similar pattern of bow shocks,
magnetospheres and radiation belts due to the interaction of the solar wind
with the planet’s intrinsic magnetic field.
The magnetic fields of Earth, Jupiter and Saturn are closely aligned with their
rotational axes and centred near the centre of the planets. Ho wever, the
magnetic fields of Uranus and Neptune are tilted relative to their rotation
axes and centred far away from the respective planet’s centre.
Charged particles in a magnetic field
The force acting on a charge q, moving with velocity v through a magnetic
field B is given by:
F = qvB
where F, v and B are all mutually at right angles to each other.
Circular motion
As F is always at right angles to v, the particle will move with uniform
motion in a circle, where F is the central force (assuming any other forces are
negligible), so:
Equating the magnetic force to the central force we get:
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so
This can be illustrated and verified using the axial gun of a Teltron Dual Beam tube and
Helmholtz coils. Teltron tubes are manufactured by 3B Scientific
(http://www.3bscientific.co.uk/Teltron/Teltron -Dual-Beam-Tube-SU18570,p_83_675_1313.html).
The instruction document in pdf format can be downloaded from the 3B
website. The English version of the instructions starts on page 7.
See example experiment 5.1 on page 8.
The velocity of the electron can be obtained using:
qV = ½mv 2
where q and m are the charge and mass of an electron, respectively, and V is
the anode potential.
B can be obtained from the current and Helmholtz coil data (section 5.1.1 in
the instruction document) . The radius or diameter of the circular motion can
be measured by placing a calibrated grid behind the tube and compared to the
value obtained above.
Helical motion
Note: Make a clear distinction between helical – constant radius, and spiral –
increasing radius.
If the charged particle crosses the magnetic field lines at an angle, then its
velocity can be resolved into two orthogonal components: one perpendicular
to the field and the other parallel to it.
The perpendicular component provides the central force, which produces
uniform circular motion as shown above. The component parallel to the
magnetic field does not cause the charge to experience a magnetic force so it
continues to move with constant velocity in that direction, resulting in a
helical path.
This can be illustrated using the dual-beam tube, with a Helmhotz coil
positioned at the front of the tube so it produces an axial magnetic field
(Figure 10). The electron beam from the axial gun is now undeflected.
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SOLAR WIND AND MAGNETOSPHERE
However, when a deflector plate voltage is applied so the electron beam has a
velocity component perpendicular to the axial magne tic field, it now travels
in a helical path (Figure 11).
For detailed instructions see Appendix 2.
Figure 10 Helmholz coil at front of dual beam tube.
Figure 11 Helical motion of electron beam.
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SOLAR WIND AND MAGNETOSPHERE
Aurorae
The aurora (aurora borealis in the Northern hemisphere – the northern lights;
aurora australis in the southern hemisphere – southern lights) are caused by
solar wind particles which penetrate the Earth’s upper atmosphere , usually
within 20° of the north or south poles (Figure 13). Between 80 and 300 km
above the Earth’s surface (aeroplanes fly at around 10 km altitude) these
particles strike nitrogen molecules and oxygen atoms, causing them to
become excited and subsequently emit light in the same way as happens in
electric discharge lamps. The most common colours, red and green, come
from atomic oxygen, and the violets (seen at lower altitudes) come from
molecular nitrogen.
Figure 12 Aurora borealis.
Interesting facts
The earliest depiction of an aurora may be a Cro -Magnan cave painting (c.
30,000 BC).
The earliest written record of aurora is in China (2600 BC).
For email alerts that give early warning of potential auroral displays in the
UK see http://aurorawatch.lancs.ac.uk/.
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SOLAR WIND AND MAGNETOSPHERE
See this link for a nice video of the aurora from the International Space
Station:
http://www.nasa.gov/topics/shuttle_station/features/20110917 -aurora.html
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APPENDIX I
Appendix 1: Key stages in the early development of
cosmic ray theories
Courtesy of NASA. See http://helios.gsfc.nasa.gov/hist_1900.html and
http://helios.gsfc.nasa.gov/hist_1950.html for full information.
Year
Event
1911
Charles Wilson invented a cloud chamber, which was used to
detect alpha and beta particles and electrons .
1912
Cosmic ray research began in 1912 when Victor Hess, of the
Vienna University, and two assistants flew in a balloon to an
altitude of about 16,000 feet. They discovered evidence of a very
penetrating radiation (cosmic rays) coming from outside our
atmosphere. In 1936, Hess was awarded the Nobel Prize for this
discovery.
1925
Robert Millikan introduced the term ‘cosmic rays’.
1929
Using a newly invented cloud chamber, Dimitry Skobelzyn
observed the first ghostly tracks left by cosmic rays .
1929
Walter Bothe and Werner Kolhorster verified that the Skobelzyn ’s
cloud chamber tracks are curved, showing that cosmic rays are
charged particles.
1932
Dr Robert A. Millikan of Caltech, winner of the 1923 Nobel Prize
for Physics, completed a series of tests on the intensity of cosmic
rays at various altitudes in a Condor bomber from March Field,
California.
1925
Explorer II, a 113,000 cubic foot rubberized helium balloon
ascended to the official record of 22,066 m while collecting
atmospheric, cosmic ray and other data.
1937
Seth Neddermeyer and Carl Anderson discovered the muon
(shortened version of mu meson) in cosmic rays.
1938
Pierre Auger discovered ‘extensive air showers’, which are
showers of secondary subatomic particles caused by the collision
of high energy cosmic rays with air molecules .
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APPENDIX 1
1949
Enrico Fermi proposed that cosmic ray protons are accelerated by
bouncing off moving magnetic clouds in space, as in the shock
waves around a supernova.
1951
Ludwig Biermann found evidence for the solar wind during his
study of comet tails.
1954
First measurements of high-energy cosmic rays via sampling of
extended air showers done at the Harvard College Observatory.
1958
James Van Allen discovered belts of radiation around the Earth .
1959
Luna 1 – USSR Lunar Flyby, the first lunar flyby. Luna 1 first
measured the solar wind.
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APPENDIX II
Appendix 2: Using the Teltron dual-beam tube to
demonstrate helical motion in a magnetic field
(Courtesy SSERC: www.sserc.org.uk)
The dual-beam tube – using the axial gun to produce a helical
electron path
The dual-beam tube is a partly evacuated electron tube filled with helium at
low pressure and equipped with both axial and perpendicular electron guns.
The tube can be used to determine the specific charge of the electron (e/m)
using either of the two guns. The tube can also be us ed to demonstrate the
helical path of an electron beam when passing through a magnetic field. For
this demonstration only the axial electron gun is used.
The anode is set at 60 V and applied to this axial gun. This provides the
forward velocity to the electron beam. The electron beam traverses
horizontally through the tube (Figure 14).
Axial electron
beam
Deflector plate
Axial gun
Perpendicular gun
Figure 13 Dual beam tube
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APPENDIX II
A single Helmholtz coil is placed into the groove in
the holder so that it encircles the front end of the
tube (Figure 15). When a current is passed through
the Helmholtz coil, the electron beam is subjected to
a magnetic field (B) axially. Since this magnetic
field and the forward velocity of the electrons are in
the same direction, the beam is not deflected at this
stage.
Figure 14
However, when a voltage is applied to a small deflector plate built inside the
tube, a component of the deflection will be in the direction of travel and there
will also be a component perpendicular to it (Figure 16).
Deflector plate
Perpendicular
velocity, vp
Horizontal
velocity, va
Axial gun
B
Figure 15 Beam velocities
This application of both the anode voltage and t he deflector plate voltage
causes the electrons to emerge from the axial gun with two components of
velocity (one horizontal and one perpendicular). Since the perpendicular
component is at right angles to the magnetic field, the electron beam now
travels in a helix (Figure 17).
Coil
Deflector plate
Helical electron
path
B
Axial gun
Figure 16 Helical path inside tube
The perpendicular component, v p , being at right angles to the magnetic field
B, will cause the electrons to experience a centripetal force and so they will
tend to travel in a circular path. The combination of this circular motion and
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APPENDIX II
the forward axial velocity, v a , makes the electron beam travel in its helical
path. The beam no longer goes round the axis of the field but returns to a
different position along the axis after every loop (Figure 18).
Axi
s
va = forward axial component of velocity
vp = perpendicular component of velocity
velocity
of velocity
vp
va
B
Electrons
looping out of
the paper
Figure 17 Helical path – explanation
If the direction of the magnetic field B is reversed , by reversing the direction
of the current in the coil, the helical path rotates around the other side o f the
axis of travel (Figure 19). In Figure 18 the electrons would then change to be
looping into the paper.
B1
B
R
Figure 18 View of electron beam end-on, moving towards observer (in this case the
deflector plate has provided the electrons with an upward velocity component ).
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APPENDIX II
Reversing the current in the coil reverses the direction of the magnetic field
and causes the helical path to rotate around the other side of the axis (Figure
19).
Figure 19 Effect of reversing the direction of the magnetic field; helical path viewed
from above.
Increasing the anode voltage increases the forward axial velocit y of the
electron beam. If the voltage applied to the deflector plate remains
unchanged, then the helical path will have the same radius but become more
stretched out (Figure 20).
Figure 20 The effect of increasing the anode voltage; helical path viewed from side.
If the forward axial velocity remains constant but the perpendicular
component of velocity is increased (by increasing the plate voltage) then the
radius of the loops of the helix increases (Figure 21).
Figure 21 The effect of increasing the deflector plate voltage; helical path viewed
from above.
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GLOSSARY
Glossary
Air shower
The spray of fundamental particles produced when
primary cosmic rays interact with particles in the
atmosphere.
Bow shock
The boundary between the undisturbed solar wind and the
region being deflected around a planet.
Cherenkov
(Cerenkov)
radiation
Light produced by charged particles when they pass
through a substance faster than the speed of light in that
substance.
Chromosphere
The part of the Sun’s atmosphere just above the
photosphere.
Corona
The outermost region of the Sun’s atmosphere consisting
of thin ionised gasses at a temperature of about 10 million
kelvin.
Magnetosphere
The region around a body in which its own magnetic field
is dominant over any external fields.
Solar flare
A violent release of energy, including electromagnetic
radiation and sub-atomic particles, from the surface of the
Sun.
Solar wind
The continual outward flow of charged particles (mainly
protons and electrons) from the Sun.
Sunspot
A small area on the photosphere that appears dark becuse
it is cooler than its surroundings.
Supernova
(supernovae)
A supernova is new star which appears due to the violent
explosion of a star increasing its brightness by up to one
billion times. Supernovae may only last about a year
before fading.
Van Allen belts
Two toroidal zones, concentric with the Earth’s axis , in
which charged particles (protons and electrons) are
trapped.
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RECOMMENDED READING
Recommended reading
Fix J D, Astronomy: Journey to the Cosmic Frontier
This book is highly recommended. The most up-to-date edition is the 6th
edition, published in 2011. It is pitched at a suitable level with good clear
explanations and illustrations. There are also additional online resources
available to support the book. See also http://highered.mcgrawhill.com/sites/0073512184/student_view0/index.html .
Of particular interest for this unit is Chapter 17 The Sun.
Seeds, M A, Foundations of Astronomy
This is another highly recommended book.
In addition to the conventional layout of chapter summaries etc it has some
novel ideas in its layout, including:
1.
2.
3.
4.
occasional double-page poster-like spreads, eg On magnetic solar
phenomena
windows on science, eg Avoiding hasty judgements: scientific faith
parameters of science, eg Temperature and heat
review: critical enquiry panels, eg ‘How deeply into the Sun can we
see?’
Part 2: The Stars is the most relevant section and includes:
 Chapter 6-5 Space Astronomy
 Chapter 8 The Sun – our star
Bennett J, Donahue M, Schneider N, Voit M, The Cosmic Perspective
An introduction to astronomy for non -science undergraduates. Very clear and
useful summaries at the end of each chapter.
This book also has the following novel features:
1.
common misconceptions panels, eg The sun is not on fire
2.
mathematical insight sections where equations and numerical
calculations are dealt with, eg Mass–energy conversion in the Sun
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RECOMMENDED READING
The most relevant part is Part V Stellar Alchemy, which includes Chapter 15
Our star.
Nicolson I, Unfolding Our Universe
Good, clear colour diagrams throughout the book explain the concepts well to
a ‘physics’ audience. Inset boxes give additional information, often including
equations and calculations. Good glossary inc luded.
Unlike other books in this list, this one is targeted at the general reader rather
than undergraduates, although unlike many astronomy books aimed at a
general readership it does use panels to deal with equations and mathematical
aspects. It also has good clear diagrams and graphs. Unfortunately there only
appears to be the original edition published in 1999.
The most relevant chapter is Chapter 8 The Sun: Our neighbourhood star.
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BIBLIOGRAPHY
Bibliography
BBC Sky at Night magazine, November 2011
(www.skyatnightmagazine.com).
Bennett J, Donahue M, Schneider N, Voit M, The Cosmic Perspective, 3rd
edition, Addison-Wesley, 2004, ISBN 0-8053-8762-5.
Fix J D, Astronomy: Journey to the Cosmic Frontier, 2nd edition, McGrawHill, 1999, ISBN 0-07-289854-2.
Kippenhahn R, Discovering the Secrets of the Sun, Wiley, 1994, ISBN0-471941-630.
Nicolson I, Unfolding Our Universe, Cambridge University Press, 1999,
ISBN 0-521-59270-4.
Seeds M A, Foundations of Astronomy, 7th edition, Thomson, 2003, ISBN 053439204-0.
Shaviv G, The Life of Stars, Springer, 2009, ISBN 978-3-642-02088-9.
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USEFUL WEBSITES
Useful websites
NASA’s Imagine the Universe educational pages
The one on cosmic rays is quite elementary, with links to a simple quiz and
cool fact(s).
http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html
Stanford Linear Accelerator virtual visitor site
There is an excellent section on high energy cosmic rays, suitable for
learners. There is also a brief section ‘Introduction to cosmic rays.’
http://www2.slac.stanford.edu/vvc/cosmicrays/
Space Weather Research Explorer
This is part of the Exploratorium (the museum of science, art and human
perception in San Francisco), with information, animations and live data and
images on the following:
 coronal holes
 solar flares
 coronal mass ejections
 solar wind
 magnetosphere
 auroras
 space weather and you.
http://www.exploratorium.edu/spaceweather/holes.html
Fix J D, Astronomy: Journey to the Cosmic Frontier
This website offers additional resources to support the 4th edition of th is
book, including:
 multiple choice quizzes
 animations
 flashcards
 glossary.
http://highered.mcgraw-hill.com/sites/007299181x/student_view0/
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