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148 Cards in this Set
- Front
- Back
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Van Allen Radiation Belts
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discovered by rocket probe in 1958, donut shape regions of high energy
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charged particles move around
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magnetic field B
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Satelight and land based radio communications are influenced by earth's electronic atsmosphere
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known as ionosphere
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Newton's first law of motion
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will keep moving unless another force acts upon it
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Newton's second law of motion
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F=ma
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Newton's third law of motion
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Equal and opposite reaction
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in low earth atmosphere
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satilight will experience atmospheric drag and orbit will decay
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If shuttle stopped orbiting
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anstraunouts would not be weightless
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Polor orbits
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go over poles, usually low orbits, earth spins east to west, over time, get a full picture of earth
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Geostationary orbits
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take 24 hours for each orbit
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Kepler's 2nd Law
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areas swept out are equal
P^2=Ka^3 |
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Apopsis
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furthest point
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Periosis
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closest pont
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LEO
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height below 2000 km
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MEO
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height below 10,000 km
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Geostationary Orbit
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36,000 km.
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minumum height for a 5 year lifetime
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450 km
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passive sensing
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detection of what is naturally emitted
•main source of emission is blackbody radiation (from ground or Sun) •on planets/moons without atmosphere γ-rays can be detected in orbit from radioactivity in rocks |
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active sensing
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involves generation of signal by satellite (or plane) and observation of reflected signal
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spectrum divided into ‘bands’ by convention
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•unfortunately, not everyone agrees on band boundaries
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Radar
•Radio detection and ranging |
•Developed for military purposes Watson-Watt from
Father of Radar: Sir Robert Watson-Watt (1892 –1973) •All early work was aimed at how far away objects were •Radar is an example of activeremote sensing |
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If the object is moving with speed saway from the radar, the return frequency, f ',is less than the sending frequency, f
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•f ' = f (1-s/v), droppler effect
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Signal radiated with intensity Ioat unit distance
•Signal reaching a particular object I = I0/d2 |
•Signal re-radiated = f I0/d2, f is the fraction reflected
•Signal received back at transmitter = (fIo/d2)/d2= fIo/d4 |
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Electromagnetic radiation is produced by an oscillating electric charge
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•the radiation is at the same frequency as the oscillation•Since currents are moving charges, to generate a radio wave, you need to generate an oscillating electric current at the desired frequency
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An aerial is a device that converts electrical power to EM radiation and vice-versa
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•with a good aerial, there is no reflection of power at the aerial
•a given aerial is as good at receiving as it is at transmitting |
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Efficient aerials are at least half-a-wavelength (λ/2) long
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Consider a ‘half-wave dipole’
•In plane containing the aerial •no radiation parallel to aerial •maximum radiation ⊥aerial •In plane ⊥to aerial |
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Making the aerial more directional
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•Adding more elements side by side gives a vertical aerial directionality in a horizontal plane
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Receiving aerials like this are called yagis
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•they usually have one signal element and the rest are parasiticelements that act in sympathy
•the parasitic elements re-radiate so that they re-inforcethe signal at the active element •this does not happen for reception of angled waves |
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SOHO -SOlar& Heliospheric Observatory
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•Launched 1996•ESA/NASA•12 instruments to explore inner and outer Sun
•Huge success •Orbits around first Lagrangian point •Permanent view of Sun |
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Ulysses
•Joint ESA/NASA •launched 1990 |
•Explores Sun out of the ecliptic, almost looking over the Sun’s poles
•Has found magnetic flux leaving the Sun is almost independent of latitude |
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Cluster II
4 identical satellites flying through the magnetosphere |
•ESA mission launched 2000
•collect 3D data on magnetic fields, electric fields and particles •orbits intersects bow shock and magnetopause •observing waves in the plasma |
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Cosmic Rays
•Discovered by Victor Franz Hess |
•balloon experiments in 1912
•Nobel prize in physics 1936 •Cosmic rays are a natural radiation background •Primary ‘rays’ are very energetic ions •Secondary rays are produced in showers as primary rays interact in atmosphere•Origin outside solar system |
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Wilson cloud chamber
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•C T R Wilson invented the cloud chamber to study condensation phenomena
•Chamber proved ideal for showing the tracks of ionising particles •Only Scot to win the Nobel Prize for Physics (1927) •Other Nobel prizes were generated using the cloud chamber |
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Radiation measured by the energy imparted
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• SI unit is Gray (Gy) ≡ 1 J kg-1
• Biological effectiveness measured in Sieverts (Sv) • 1 Sv ≡ Radiation weighting factor × Gy |
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Satellite and land-based radio communications are influenced by the Earth’s electronic atmosphere, known as the ionosphere
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•term coined by Robert Watson-Watt, Brechinborn inventor of radar
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The Earth is surrounded by layers of ionised matter that reflect radio waves
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yep
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Several atmospheric layers exist with ‘free electrons’ and +veions, the ingredients of a plasma
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•these layers slow and can reflect low frequency radio waves originating on Earth•they transmit high frequencies. Layers labelled:•D ~ 70 km•E ~ 100 km•F1~ 200 km; F2~ 350 km•60 km to 600 km is called the ionosphere•lower layers appear mainly during daylight•communication with satellites needs to use high frequencies
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The magnetosphere•The Earth’s magnetic field inside and close to the Earth is approximately like that of a bar magnet
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•Outside the Earth it is distorted and more complex because of the interaction with the solar wind•The magnetopause is the boundary between the solar wind plasma and the ionospheric plasma
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The Earth’s outer environment is complex:
•plasma •deformed and stretched out magnetic field lines •electrical currents of millions of amps |
•This environment is subject to big changes
•Satellites, the ISS and near-Earth space business operate here •understanding it is an essential challenge |
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Van Allen radiation belts
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•The Van Allen belts were discovered by rocket probe (Explorers 1 and 4) in 1958
•They are doughnut shaped regions of high energy electrons and protons•They extend from about 2 to 5 Earth radii beyond the surface |
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The inner belt is around 1.5 RE, mainly protons
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•The outer belt at 3 –9 REis trapped magnetospheric plasma•contributes a ring current that can induce Earth currents
•highly fluctuating •lower energy protons contribute mostly |
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Harmful effects of the belts
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•Degradation of satellite components, notably semiconductor and optical devices
•Generation of spurious background noise in detectors •Cause of errors in digital circuits •Production of electrostatic charging within insulators •A health threat to astronauts |
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The physics of motion in magnetic and electric fields
•charged particles have 3 motions |
•gyration•takes << ms•motion along magnetic field lines•takes ~0.1 s•drift around the Earth•takes minutes
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•Charged particles circulate around magnetic field lines, B•The magnetic force on them is at right angles to their velocity and to B
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•this force maintains their motion in a circle•this force doesn’t change the energy of the particle•gyration frequency fis•for electrons with B= 10-5T, f= 2.8×105Hz•Circle radius ∝(perpendicular speed)/f
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If the particles also have a motion parallel to the magnetic field lines, they spiral up or down the field lines
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•the motion parallel to the line is unaffected by B
•again, there is no change in the energy of the particle |
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The lines of the Earth’s field converge at the magnetic poles•Particles spiralling polewardsdo so in faster, tighter circles•Their progress towards the poles becomes slower (because their kinetic energy is conserved)
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•They are eventually reflected back up the field line
•The magnetic poles create magnetic mirrors for charged particles |
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Add an electric field Eperpendicular to the magnetic field B
•Charged particles drift at right angles to both fields |
•drift doesn’t depend on particle charge or mass•+veand negative particles drift in the same direction•speed of drift is E/B•This drift causes a ring current around the Earth E B
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The transformer principle
•Induction was discovered by Michael Faraday in the 1830s |
•A changing currentin one coil (the primary) sets up (induces) a changing current in a parallel neighbouring coil (the secondary)
•Magnetic flux links the two coils |
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Motion of electrons in the magnetosphere
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•strong electric currents
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Permanent auroral oval around both poles
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•Electrons spiral down field lines towards poles to height of 300 –80 km•incident energy ~6 KV•aurora is emission from excited upper-atmosphere moleculesHST
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Auroral colours
•Emission lines from N2and O |
•Lowest emission < 100 km
•blue & red from N2and N2+ •Medium height 100 –200 km •Green (557.7 nm) from O •High > 200 km •red (630 nm) from O •Colour mixing of blue, green and red can produce a huge range of colours |
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Solar wind and space weather is crucial to the origin of the aurora
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•Solar wind originates outside the visible Sun
•it represents a streaming away of coronal material |
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Why is the Corona a few million degrees?
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•Coronal material is heated from below by the effects of unstable and constantly changing magnetic fields
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We live in the Sun
•Solar wind always blows |
•It has a slow and a fast component
•fast component (~750 km s-1) is steadier and comes from coronal holes nearer the solar poles •reaches full speed in <10 solar radii •slow component (~300 km s-1) is less steady and has a more equatorial origin •reaches full speed at ~25 solar radii •Solar wind varies with the 11 year sunspot cycle |
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Magnetic fields and plasma
•Close to the Sun, the strong magnetic field controls the motion of the plasma |
•as the field lines loop round, so the plasma in the inner corona follows the field lines
•Far from the main body of the Sun, the magnetic field is comparatively weak and is controlled by the plasma •this is the case in the solar wind |
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‘Frozen’ or ‘trapped’ magnetic field
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•In a plasma like the solar wind there is virtually no electrical resistance to the motion of charge
•Maxwell’s equations of E&M predict that magnetic field in such a plasma is trapped and carried along unchanged by the plasma in motion •Hence magnetic field generated on the Sun reaches the Earth |
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The Sun’s spiral magnetic field
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•As the hot solar wind is ejected, it drags out its accompanying magnetic field•The resulting field lines spiral out from the Sun
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When fields collide
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•The Earth’s field acts like an umbrella
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Within the magnetopause
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•Sun side•Earth’s field is compressed to within about 10 RE (Earth radii)•note the cusps
•Night side •tail stretches > 200 RE•plasma sheet •Van Allen belts |
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Coronal mass ejections (CMEs)
•Coronal mass ejections are now considered the source of major geomagnetic storms |
•not solar flares
•frequency ~1 per day •total mass in one ejection could be ~1010tonnes •energy ~1024 J (>> 108megatons) •speeds of leading edge at the Sun ~ 1000 km s-1 •Earth’s magnetosphere is hit hard |
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Sunspot 9393
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• First pass from late March to early April, 2001
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Out there
• ‘Out there’ • EM radiation • solar wind |
• cosmic rays
• micrometeorites • Closer to Earth • ‘radiation’ belts • Earth’s atmosphere provides enough protection for life • can we exist outside it? |
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EM radiation
from the Sun |
• At Earth ~1366 W m-2
• Photosphere of Sun appears ~‘blackbody’ at 5780K • Total energy o/p ~1026 W • Distribution ~ follows Planck radiation law • Peak wavelength is in the visible • 7% UV, 44 % visible • 37% near IR, 11% far IR • 1% radio spectrum |
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Digression on
space suits • Space suits need to: • suggestions from class • They operate at reduced pressure, with an internal atmosphere of pure O2 |
• several hours of acclimatisation are necessary
to remove N2 from the blood • what happens to the breathed out CO2? • how is temperature control achieved? |
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Radiant energy
emitted by a hot body • Total radiant energy (E) emitted per m2 of surface per second for a black body at temp T |
• , where σ is 5.67×10-8 W m-2 K-4
• Stefan-Boltzmann Law; σ is Stefan’s constant |
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Radiation received = radiation re-emitted
• Consider the body at distance Earth is from Sun • incoming energy spread over a disk of area πr2 |
• re-radiated energy comes from area of a sphere 4πr2
• R(1 - a)πr2 = σ T4×4πr2 • T4 = R(1-a)/4σ |
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As a formula: Rd = R1/d2 ; where Rd is the rate
energy is received at distance d |
• E.g., the Earth at 1 AU distant from the Sun
receives solar radiation at a rate of 1366 W m-2 |
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Different parts of the spectrum have different historical names
• Diagram shows approx wavelengths of the boundaries • wavelengths determine the equipment used to transmit & receive |
• Energy, E, comes in packets (‘photons’) that depend on the
wavelength (λ) through Planck’s constant h • packets are measured in eV (‘electron volts’) • > 2 eV will break some chemical bonds • much of the UV and beyond is chemically damaging |
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Shorter than 200
nm there is much more radiation than a blackbody emits • Where does this come from? |
• the outer
atmosphere of the Sun |
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Solar wind is a flux of plasma
coming from the Sun • plasma is an electrical neutral ‘gas’ of positively and negatively charged ‘particles’ |
• solar wind:
• +ve particles are mainly protons (H+), He nuclei (He2+) and heavier element ions • -ve particles are electrons • ‘trapped’ magnetic field, the IMF (‘interplanetary magnetic field’) • The solar wind has a significant impact on everyone’s use of space |
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ACE
• Advanced Composition Explorer |
• ACE sits permanently between the Earth and Sun,
about 1.5×106 km from the Earth • ACE orbits around the first Lagrangian point • ACE has six instruments that monitor particle content, speed, density, etc. and the interplanetary magnetic field • also monitors galactic cosmic rays |
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Variability of
the solar wind |
• The solar wind and
related particle flux from the Sun is the most variable component of space weather • The solar wind can be a hazard to man and instrumentation |
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What exactly are you measuring?
• There are various useful coordinate systems • ACE data reports results in GSE coordinates |
• “Geocentric solar ecliptic”
• X-direction is Earth – Sun line • Z direction is ecliptic north pole • A magnetic field B in diagram • Bx, By, Bz • or B, θ, ϕ |
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Flux is quoted in particles cm-2 s-1
• in 1 second all the particles in a cylinder of length v pass through unit area • if v = 500 km s-1 ≡ 5×107 cm s-1 • 4 particles cm-3 ≡ flux of 2 ×108 particles cm-2 s-1 • ≡ flux of 2 ×1012 particles m-2 s-1 |
• Fluence is quoted in particles cm-2
• radiation damage depends on fluence |
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If particles are in ‘thermal equilibrium’ with their
surroundings, their average KE = thermal energy |
• Temperature T is given by
• E.g. coronal proton, m = 1.67×10-27 kg, v = 2.5×105 ms-1 • T = 2.52×106 K • Calculation fails if particles aren’t in thermal equilibrium • temperature is a concept that applies to systems in equilibrium |
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Temperature of a
stream of particles |
• Normally gas particles are
spread around an average speed of zero → • Temperature is related to the spread of their speed, the average of v2 • If the same particles are all given a speed (say 10) then their temperature stays the same • The spread of v2 about the average is the same |
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Solar wind temperatures
• Just moving a box of particles at speed v0 doesn’t change its temperature • What counts is the speed of the wind particles once the average motion has been subtracted • ACE’s measurements show T ~ 105 K |
• the spread of particle velocities of the protons is
~50 km s-1 • electron temperatures are comparable to proton temperatures |
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Additional solar wind indicators
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• Dials and the auroral
oval give a quick overview of the solar wind on the Earth • dials show ‘real-time’ display and history loop |
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Geostationary satellite environment
• Example of fluctuating environment at height of geostationary satellite |
• Kp is the ‘planetary K index’, a measure of the
fluctuations in the Earth’s magnetic field in range 0 to 9 |
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Background
• Solar system contains asteroids and comets, some of which cross the Earths path |
• Impact craters observable on
terrestrial planets and their moons • Telescope search techniques have discovered dozens of near earth asteroids and short period comets. • Did an asteroid impact cause extinction of the dinosaurs? • Do we need to be worried |
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Was it caused by
Asteroid/Comet impact? • 65 million years ago half the species on earth became extinct • 10 km diameter asteroid/comet impact at Chicxulub - Mexico |
• Crater 200km in diameter, equivalent to 5 billion Hiroshima
bombs • Estimate 100 trillion tons of dust moved into atmosphere • This could have changed climate |
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Impact craters identified on Earth
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• More than 130 have been identified - these range
up to 2 billion years old • Range between 140-200 km in diameter |
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Comet Definition
• A “small” object with a visible transient atmosphere - the coma or tail • Comets have a solid nucleus - dirty snowball |
• ion tail - most prominent
formed by the solar wind • dust tail - formed by solar radiation pressure |
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Asteroids
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• Small objects (up to a few
tens of km diameter) in orbits similar to those of the major planets • They have no atmosphere • Primarily rocky objects with little or no volatile material • Most asteroids in belt between Mars and Jupiter and in plane of the solar system |
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Meteoroid
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• A fragment of
interplanetary debris in space • Occasionally a meteoroid survives its flight through the atmosphere and lands on the ground as a meteorite |
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Near Earth Objects (NEOs)
• Have orbits that closely approach or intersect that of the earth. • 90% of NEOs are asteroids or short period comets • 10% are longer period comets (period >20years) • It is estimated that NEOs >1km are of greatest concern- approx 2000 of them |
• Current estimate of risk of major
strike by NEO 1:10000 • NEOs can be detected by reflected sunlight, hence possibility of carrying out surveys |
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Impactor Size
• 10-100 m diameter strike several times per millennium - if made of rock tend to disrupt/explode in atmosphere and flatten trees e.g. over 20km radius |
• 100 m - 1km on average strike
earth once every 5000 years, produce craters 3km in diameter • 1-5km impact will cause severe global consequences, 100km diameter crater, occur approx. once every 300-500 thousand yrs • > 2 km - global catastrophe |
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Risk Analysis
• 1:500,000 year global catastrophe from NEO impact • Annual probability of impact 1/500,000 • Assumed fatalities from impact, 1/4 earth population • Annual probability of individual death: 1/2,000,000 |
• Equivalent annual deaths 125 USA (26 UK)
• Compare for USA food poisoning by botulism (<10 per year), tornadoes (100per year, car accidents (40,000 per year) |
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Searches for NEOs
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Photographic searches
with wide field terrestrial telescopes • Spacewatch CCD scanning programme |
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Stardust Mission
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• Primary goal to collect comet dust and volatile
samples from the comet Wild 2 in January 2004 • It will also bring back samples of interstellar dust • Low cost mission |
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Trajectory
• Stardust will make three orbits around the sun (at end of 1st orbit will get gravity assist from earth) • On 3rd orbit it will encounter Comet Wild 2 |
• Launched 7 February 1999
• Encounter January 2004 • Return to Earth January 2006 |
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Dust Collector
• Particle samples collected by exposing blocks of aerogel to the sample flux during spaceflight |
• One side of cell for
collecting comet samples, the other side for interstellar samples - area of each side 1000cm2 • Once the gel is returned to earth, the particles composition can be determined in the laboratory |
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Sample Return to Earth
• Planned landing site Utah Test and Training range • Sample return canister will be released from spacecraft approx. 3 hours before entry |
• Free-fall to 3km height
where parachute will be deployed • Landing footprint estimated to be 30km x 84 km • Collection of capsule by helicopter |
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Some Initial Findings
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• Minerals found in Aerogel – it is thought
that these minerals (eg olivine) which would be formed in the vicinity of the sun have somehow migrated into the outer reaches of the solar system) • Many of the comet particles are built like loose dirt-clods composed both of "large strong rocks" as well as very fine powdery materials |
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Controlled Re-entry
• Deboosting a satellite can be used to control the re-entry |
• Aim for large ocean
areas • Uncertainties in down range and cross range yield an impact footprint • Simulation software used to model re-entry scenarios |
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Spacecraft Re-entry
• Orbiting satellites eventually return to earth - atmospheric drag • As satellite re-enters friction with the atmosphere generates a great deal of heat - burn up? |
• Satellites can survive reentry,
since larger and move more slowly than meteors. Random re-entry of a rockets fuel tank |
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How do you clean up the Space
Debris? |
• Perhaps some day,
orbiting garbage collectors will be used to clean these trash filled low orbits. • Old satellites and upper stages would have to be removed from orbit also |
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65 million years ago
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1/2 spieces became extinct
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ion tail
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most prominent formed by solar wind
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dust tail
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formed by solar radiation pressure
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Cosmic Rays
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natural radiation, primary rays are very enteregetic ions, secondary are produced in showers
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solar system magnetic waves fields deviate cosmic rays
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average dose to pollution: Costmic rays contribute 13.5 percent
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Passive sensing
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what is naturally emitted, mainly black body radiation
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a perfect black body is a black hole
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but a black hole will emot black body radition
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Radar
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spins in 360 degrees
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Em wave caused by radiation of charged object
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accelerated object will cause EM radiation
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If aerial vertical
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intensity of signal horizontal
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12 element antiphase array
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called phased array
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Space weather
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a major part is thea ctivity of the sun
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EM radiation
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screened by Earths atmosphere, no living creature could love natrually in space
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Solar wind
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from sun, severe during solar storm, northern lights
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Cosmic Rays
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source is partially a mystery
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Microelectrosatist
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dust, distegrated comets, can cause physical damage
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Axis of ratation defines geographic poles, close to magnetic poles
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when charged particles approaches earth, it tends to spiral around this magnetic field line
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Field lines focus on poles, guiding charged particles there
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these collide with atmophereic particles, causing emission of photons
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GPS
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makes sure fishermen are not fishing where they are not supposed to
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In space
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red blood cells were effect, causing anemia, sleep disorders, some irratable
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take 1 kg of water on mir
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cost 10000 dollars
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most orbital debries located
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in leo
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average debries
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is moving 10 km/s wrt orbiting sate
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I mm-1 cm
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cause damage, but not penetrate
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1 cm - 10 cm
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penetrated, and blow up?
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Kessler Effect
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domino damage spreading through satelights
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MIR has been in LEO for 13 years
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no damage yet
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1 piece of debris
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enters a day
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models have been devoloped
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to characterise debris,
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HIT facility, 3 guns shoot projectitiles up to 7 km/s
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in space for 5.7 years,
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orbital satllites
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return by drag, friction generates heat, burn up, but slower than metoroids, so less likely to brun up
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without solar wind
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Earths B filed is like that of a bar magnet
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Van Allen Belt
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mainly protons and heavy ions, electrons too light to be trapped by VAB
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Electrons are trapped by cusp of
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magnetosophere, cause of aurosis, ring shaped distribution where electrons conjugate, centered around magnetic pole/auraoral ring/oval
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electrons from solar wind guided by pole, come down
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cause Auroroal emmission?
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Solar wing originates outside the photosphere of sun
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streaming arry of coronal material
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solar wind hotter than Temperature of sun
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solar wind changes al the time, change magnetic fields
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North pole is at south pole
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until 2012, every flip causes sun spots and radiation burst,
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solar wind
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fast a poles, slower at equator, spirals out
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Magnetosphere
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Sunside- 10 Re
Night side 200 Re |
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magnetosiore
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dominant field transition between solar wind and earth
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magnetosphere
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is boundary between solar wind plastma and ionosophere plasma
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dust tail of comets
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less visable, cuased by impact of suns photons on dust
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most asteroids on are on a
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belt between mars and jupitor, on plane of solar system
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metoriod
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fragment of debris in space
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mars is cooler because
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farther from sun
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sun plasma
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electrical nuetral gas of posiitively and negatively charged paritcles, high energy, some say the 4th state of mater
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Solar Wind
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protons, He nuclei and heaiver element ions
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ACE
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Advanced Composition explorer, between forst largiagian point (pull of sun and Earths balance)
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Yagis antennas recieve and re emit signs to get constructive interference at actice antenna
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dependent on direction and frequency
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when wavelength is smaller, microwaves
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it is more efficient to reflect rays with dish type aerial
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