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This module serves as a background on the atmosphere. The atmosphere is important for Radio, Radio Astronomy, and Electricity.

Brief History of the Universe[edit]

brief overview of cosmology

talk about origins of CBR

talk about stars as blackbodies.

The material for the earth came from the fusion reactors of stars that exploded in supernova. So stars had to go from infancy to supernova before the earth could exist. Fusion is responsible for the elements up to iron and supernova's are responsible for the heavier elements.

link to cosmology

The Solar System[edit]

Relationship between earth creation and sun

talk about solar system and its electrical properties

Evolution of the Earth's Atmosphere[edit]

The current earth is 4.5 billion years old. It was created when a mars like object hit the old earth and created the moon. The moon condensed from a ball of gas and stabilized the spinning and hence the climate of the earth. If the moon were to blow up it due to its gravity it would reform. After the collision earth was a smoldering liquid with a layer of gases (hydrogen and helium) surrounding it trapped by Earth's gravity. This "hydrogen-helium" atmosphere burned off. About 3.5 billion years ago the earth cooled enough to form a solid crust. This crust was heavily populated with volcanoes which released steam (H_2O), carbon dioxide (CO_2), and ammonia (NH_3). That "carbon" atmosphere had ~100 times as much gas as the current atmosphere. The high levels of CO_2 in the atmosphere kept the earth from freezing as part of the greenhouse effect. The water vapor condensed to form oceans, which dissolved (absorbed) 50% of the CO_2. With the abundance of CO_2 came the rise of photosyntesysing plants which processed CO_2 by absorbing the carbon and releasing the oxygen. Over time the plants decayed and turned into fossil fuels. Carbon also became locked in limestone and other sedimentary rocks as well as animal shells. The oxygen and bacteria reacted with the ammonia to create nitrogen. At first the oxygen oxidized (reacted with) various elements, but eventually accumulated, with mass extinctions and evolution. With the rise of oxygen came the "oxygen-nitrogen" atmosphere and an ozone layer which protects life from UV radiation. At present the atmosphere consists of 78% nitrogen, 21% oxygen, 1% argon, ~0.035% carbon dioxide, water vapor, and other trace gases.

As the atmosphere has no abrupt cut-off, but rather thins gradually with increasing altitude, there is no definite boundary between the atmosphere and outer space. 75% of the atmosphere exists within 11 km of the planetary surface. In the United States, persons who travel above an altitude of 50 miles (80 kilometres) are designated as astronauts. 400,000 feet (75 miles or 120 kilometres) marks the boundary where atmospheric effects become noticeable during re-entry. The altitude of 100 kilometres or 62 miles is also frequently used as the boundary between atmosphere and space.

Issues related to the Atmosphere[edit]

  • Earthquakes and natural disasters
  • lightning
  • Global Dimming
  • Visible Stars
  • Ozone Hole

Use of CFCs destroys the ozone layer and allows UV rays to nuke planet.

ban on CFCs

  • Water Usage
  • Dams and power plants
  • Emissions

size of particles

cleaness of sky emissions control

    • dust clouds
  • Global Warming

The CO_2 trapped in the atmosphere will cook it.

has huge effects on energy usage


  • effects of people versus nature
  • carbon sinks
  • settle in ice areas
  • ice age
  • ocean holds lot of carbon
  • increased plant growth
  • type 3 versus type 4 plants
  • melting of ice caps
  • climate data
  • climate change
  • flooding of coastal areas
  • society lives on margin so small changes hurt
  • fossil fuels versus renewable fuels
  • Economics: money involved in
  • Kyoto: political unrest
  • laws regarding emissions
  • sustainability of earth
  • Alaska wildlife perserve,

has large implications for energy usage and international treaties.

  • Asteroid Impact

fear of asteroids destroying cities

where asteroids come from

many movies portray people dealing with asteroids

planetary collisions


  • Earth stops spinning
  • Magnetic Field Turning

Every so often the Earth's magnetic flips.

fear related to Van Allen belts and bleeding of atmosphere

  • Gamma Ray Bursts

Fear that the earth will be sterilized by gamma ray burst

  • Degenerate matter
  • Objects in orbit

congested orbit

  • Radiation

Going outside atmosphere exposes to lots of radiation.

  • Historical temperature record

IPCC and Anthropogenic Climate Change.

Atmospheric Phenomenon[edit]


In optics, a prism is a device used to reflect light or to break it up (to disperse it) into its constituent spectral colors (colors of the rainbow), traditionally built in the shape of a right prism with triangular base.

As light moves from one medium (e.g. air) to another denser medium (the glass of the prism), it is slowed down and as a result either bent (refracted) or reflected. The angle that the beam of light makes with the interface as well as the refractive indices of the two media determine whether it is reflected or refracted, and by how much (see refraction, total internal reflection).

Reflective prisms are used to reflect light, for instance in binoculars, since they are easier to manufacture than mirrors. Dispersive prisms are used to break up light into its constituent spectral colors because the refractive index depends on frequency (see dispersion); the white light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Blue light is slowed down more than red light and will therefore be bent more than red light. There are also polarizing prisms which can split a beam of light into components of varying polarization.

Until Isaac Newton, it was thought that prisms added colors to white light. Newton placed a second prism such that a separated color would pass through it and found the color unchanged. He concluded that prisms separate colors. He also used a lens and a second prism to recompose the rainbow into white light.

Types of prisms

Reflective prisms:

   * Pentaprism
   * Porro prism
   * Porro-Abbe prism
   * dove prism
   * dichroic prism 

Dispersive prisms:

   * Amici prism
   * Abbe prism
   * Pellin-Broca prism
   * triangular prism 

Polarizing prisms:

   * Wollaston prism 


Common optical phenomenon are often due to the interaction of light from the sun or moon with the atmosphere, clouds, water, or dust and other particulates. One common example would be the rainbow, when light from the sun is reflected off water droplets in rain as it falls to the ground. Others, such as the green ray, are so rare that many consider them to be mythical. Some, such as instances of fata Morgana, are commonplace only in certain locations.

A rainbow is an optical or meteorological phenomenon that causes a (nearly) continuous spectrum of light to appear in the sky when the sun shines onto falling rain. It is a colored arc with red on the outside and violet on the inside: see color for the full sequence.

The rainbow effect can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude or angle. The most spectacular rainbow displays when half of the sky is still dark with draining clouds and the observer is at a spot with clear sky overhead. Another common place to see the rainbow effect is near waterfalls. Rainbow fringes can sometimes be seen at the edges of backlit clouds and as vertical bands in distant rain or virga. The effect can also be artificially created by dispersing water-vapour into the air during a sunny day.

Sylvanshine is an optical phenomenon in which dew-covered trees of species whose leaves are wax-covered retroreflects beams of light, as from a vehicle's headlights, sometimes causing trees to appear to be snow-covered at night during the summer. The phenomenon was named and explained in 1994 by Professor Alistair Fraser of Pennsylvania State University, an expert in meteorological optics. According to his explanation, the wax on the leaves causes water to form beads, which become, in effect, lenses.

Gegenschein (German for counterglow) is a faint brightening of the night sky in the region of the zodiac directly opposite the Sun caused by reflection of sunlight by small dust particles that lie in the plane of the Solar system. It appears as a softly glowing oval region a few degrees wide and 10-15° in length, oriented along the plane of the ecliptic. It is so faint that it cannot be seen if there is any moonlight or if it falls in the vicinity of the Milky Way.

The zodiacal light is a faint glow which appears in a band along the ecliptic or zodiac from the vicinity of the Sun. It may be best observed in the western sky in the spring after the sunset twilight has completely disappeared, or in the eastern sky in the autumn just before the morning twilight appears. It is so faint that it is completely masked by moonlight. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for 60% of the total skylight on a moonless night. There is a slightly increased illumination of the zodiacal light directly opposite the Sun known as gegenschein.

Zodiacal light is produced by sunlight reflecting off particles of dust present throughout much of the solar system. The amount of material needed to produce the observed zodiacal light is amazingly small; if it were in the form of 1mm particles each with the same albedo (reflecting power) as Earth's moon each particle would be 5 miles from its neighbours. The material producing the zodiacal light is located in a lens-shaped volume of space centered on the sun and extending well out beyond the orbit of Earth. The gegenschein may be due to the fact that particles directly opposite Earth from the sun would be in full phase.

A rainbow

A sun dog is a natural but uncommon optical phenomenon observable in the atmosphere, typically when a low sun shines through loose cirrus clouds, e.g. in a milky-white winter afternoon sky. The sunlight is reflected and refracted by ice crystals and splits up into colors because of dispersion, similarly to the rainbow. The orientation of the ice crystals involved in this process is important. The crystals are hexagonal cylinders, and they have to be oriented vertically.

A double rainbow: Note reversed colors in outer (secondary) bow (left)

In a very few cases, a moonbow, or night-time rainbow, can be seen on strongly-moonlit nights. As human visual perception for color in low light is poor, moonbows are perceived to be white.

Table of contents [showhide] 1 Physics of rainbows 2 Rainbow in legends 3 Rainbow in the Bible 4 Rainbow in literature 5 See also 6 External link

Physics of rainbows

Isaac Newton was the first to demonstrate that white light was composed of the light of all the colors of the rainbow, which one glass prism could split into the full spectrum of colors, and another could recombine into a beam of white light.

The rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops. The light is first refracted as it enters the surface of the raindrop, undergoes total internal reflection from the back of the drop, and is again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back at an angle of about 40-42°, regardless of the size of the drop. Since the water of the raindrops is dispersive, the amount that the sunlight is bent depends upon the wavelength (color) of the light's constituent parts. Blue light is refracted at a greater angle than red light, but because of the reflection from the back of the raindrop, the red light appears higher in the sky, and forms the outer color of the rainbow.

The rainbow does not actually exist as a location in the sky, but is an optical illusion whose apparent position depends on the observer's location. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. These raindrops are perceived to constitute the rainbow by that observer. Its position is always in the opposite direction of the sun with respect to the observer, and the interior is actually a magnified image of the sun, which can be seen to be slightly brighter than the exterior. The bow is centered on the shadow of the observer's head, appearing at an angle of approximately 40-42° to the line between the observer's head and its shadow (hence there is no rainbow if the sun is at a higher altitude than 42°: the rainbow would be below the horizon). Sometimes, a second, dimmer rainbow is seen outside the primary bow, caused by a double reflection of the sunlight inside the raindrops, and appears at an angle of 50-53°. Because of the extra reflection, the colors of the bow are inverted compared to the primary bow, with blue on the outside and red on the inside. From an aeroplane one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre.

Light being refracted to cause a rainbow-effect

Path of light rays in the formation of a primary rainbow

Path of light rays in the formation of a secondary rainbow

Rainbow in legends

The rainbow has a place in legend due to its beauty and the difficulty in explaining the phenomenon before Galileo's treatise on the properties of light. In Greek mythology, it is a path made by a messenger (Iris) between Earth and Heaven. The Irish leprechaun's secret hiding place for his crock of gold is usually said to be at the end of the rainbow. In Chinese mythology, the rainbow was a slit in the sky sealed by Goddess Nüwa using stones of seven different colors. In Hindu mythology, the rainbow is called Indradhanush- meaning the bow of Indra, the God of lightning and thunder. In Norse Mythology, a rainbow called the Bifröst Bridge connects the realms of Asgård and Midgård, homes of the gods and humans, respectively.

Rainbow in the Bible It is mentioned in the Genesis 9 [1] , as a sign of God's covenant with mankind. After Noah survives the flooding of the earth in the story of Noah's Ark God sent the rainbow to promise that he would never again send such a flood to destroy the world.

Rainbow in literature

The rainbow has also been used in more contemporary settings, such as the song Over the Rainbow in the musical film of The Wizard of Oz, and in selling Lucky Charms by alluding heavily to leprechaun mythology.

One of the poems of William Wordsworth goes...

   My heart leaps up when I behold
       A rainbow in the sky: 
   So was it when my life began; 
   So is it now I am a man; 
   So be it when I shall grow old,
       Or let me die!... 

Why the sky is blue Rayleigh scattering

This means that blue light is scattered much more than red light. In the atmosphere, this results in blue photons being scattered across the sky to a greater extent than photons of a longer wavelength, and so one sees blue light coming from all regions of the sky whereas the rest is still mainly coming directly from the Sun.

A notable exception occurs during sunrise and sunset, when the Sun's light must pass through a much greater thickness of the atmosphere to reach an observer on the ground. This extra distance causes multiple scatterings of blue light, but relatively little scattering of red light; this is seen as a pronounced red-hued sky in the direction towards the sun.


The phenomenon is due to very fine particles of dust suspended in the high regions of the atmosphere that produce a scattering effect upon the component parts of white light.

Alpenglow (German: Alpenglühen) is an optical phenomenon. When the sun sets in the west, a horizontal red glowing band can sometimes be observed in the east. In mountainous areas such as the Alps, this can be caused by snow, moisture, and ice on mountain sides which receive the red scattered light from the setting sun.

In the absence of mountains, the aerosols in the eastern part of the sky themselves can still be illuminated in the same way by the remaining red scattered light straddling the border of the Earth's own shadow. This back-scattered light produces a red band above the darkness rising in the east. The difference to Gegenschein, which is also found in the east, is that Alpenglow is caused inside the Earth's atmosphere.

green ray

The green ray or green flash occurs shortly after sunset or even at dawn. Just as the sun sets/rises, a green ray shoots up. This is a rare optical phenomenon.

Its explanation lies in refraction of light in the atmosphere and is enhanced by atmospheric layering. It is usually observed from a low altitude where there is an unobstructed view of the horizon, such as on the ocean. Whilst we would expect to see a blue light, the blue is dispersed (this is why the sky is blue) and the green flash is visible from a fraction of a second to a couple of seconds duration.

With slight magnification the effect can be seen on the top limb of the solar disk on most clear-day sunsets. However the flash effect requires a stronger layering of the atmosphere.

Anticrepuscular rays a term used in atmospheric optics for when light beams scattered by clouds in front of the setting sun appear to re-converge on the opposite horizon.

Crepuscular rays a term used in atmospheric optics for when light beams scattered by clouds in front of the setting sun appear to diverge. The rays are parallel, but appear to diverge because of parallax.

An elve (originally elf) is an optical and electromagnetic phenomenon observed from space. Elves are visible as red flashes of electricity that shoot upward from thunderclouds.

A glory is an optical phenomenon produced by light scattered back toward its source by a cloud of uniformly-sized water droplets. A glory can have multiple colored rings. The angular size is much smaller than a rainbow, about 5° to 20°, depending on the size of the droplets. Since it is seen in the direction opposite the sun it is most commonly observed while airborne, with the glory surrounding the airplane's shadow on clouds.

Halos are optical phenomena that appear in the sky near or around the Sun or Moon, for example "sun dogs" or the parahelia, or a paraselene or "mock-moon." There are many other types of halos, but they are all caused by ice crystals high in the atmosphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Optical phenomena such as halos were used as an empirical means of weather forecasting before meteorology was developed.

Halos can also have unusual shapes, for example a cross. Emperor Constantine I of the Roman Empire is said to have seen such a halo in 313 near Trier. This sign is supposed to have prompted him to become a Christian.

Various Atmospheric Regions[edit]

The boundaries between these regions are named the tropopause, stratopause and mesopause.

The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies between the different atmospheric layers:

The average temperature of the atmosphere at the surface of earth is 14 °C.

the atmosphere weighs 5.1 × 1018 kg

Height equator/poles Layer Temperature Absorbs
Earth decreases
Ocean increases
16-18 km/10 km Troposphere decreases heat
17-50 km/50 km Stratosphere increases 270 K UV
50-80 km/85 km Mesosphere decreases 200 K meteors
80-640+ km Thermosphere increases 2,000-2,500°C high energy
500-(1000-10000) km Exosphere decrease vacuum


The bottom layer of the atmosphere is the earth. It is a solid (compress gas) that conducts sound well, but absorbs all radiation. Earthquake detectors.


If earth were a different size it might have ended up being a gas. The ocean is dense and filled with conductive salt water and nonconductive fresh water. This makes radio communication hard and is much better suited to sound based communication.

Troposphere (mixing layer): 16-18 km/10 km[edit]

Contains 80% of atmosphere and is responsible for clouds and most other weather. Since based on air temperature decrease with altitude. responsible for wind due to circulation of heat in atmosphere.

Aircraft are trapped in it because they need air to fly.

Tropospheric scatter (synonym troposcatter): reflect TV and FM off troposphere. Range 100 miles. used in billboard antennas.

Ends at Tropopause.


270 k at stratopause. warms up due to ozone layer that absorbs UV

horizontal mixing of gases in radiative, dynamical, and chemical processes

has circulation called quasi-Biennial Oscillation (QBO) in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere. The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers such as ozone or water vapor. In northern hemispheric winter, sudden stratospheric warmings can often be observed which are caused by the absorption of Rossby waves in the stratosphere.

Ozone Layer: 10-50 km[edit]

Where the ozone layer is found. Ozone is a minor constituent by volume.

Ionosphere: 550 km[edit]

Contains ions, approximately the mesosphere and thermosphere.


Layer in which millions of meteors daily collide with billions of gas particles and burn up. Responsible for meteor showers. If the meteor is big enough it hits the ground and leaves a crater. In addition to the mesosphere the earth also has the moon, Jupiter, and the Sun protecting it from meteors.

Thermosphere (heat layer)[edit]

Within this layer, ultraviolet radiation causes ionization.

At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures can rise to 2,000°C. Radiation causes the scattered air particles in this layer to become charged electrically (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000 km above the Earth's surface, the atmosphere blends into space. The few particles of gas here can reach 2,500°C (4500°F) during the day.


Where atmosphere thins out into space.


  1. magnetosphere - the region where the Earth's magnetic field interacts with the solar wind from the Sun. It extends for tens of thousands of kilometers, with a long tail away from the Sun.
  1. Van Allen radiation belts - regions where particles from the Sun become concentrated.

A magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. The Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune. Mercury is magnetized, but too weakly to trap plasma. Mars has patchy surface magnetization.

Table of contents [showhide] 1 Earth's magnetosphere 2 History of magnetospheric physics 3 Related topics 4 References

Earth's magnetosphere

Earth's magnetic field originates in its liquid core, where electric currents are excited by fluid flows (by a so-called dynamo process). The field's intensity is about 6×10-5 tesla at the magnetic poles [1], located about 10 degrees off the geographic poles. At the surface, the field resembles a dipole with irregular components added, and the field is largely dipole like to distances of 5-8 radii of the Earth (RE).

The distant field of the Earth is greatly modified by the solar wind, a hot outflow from the sun, consisting of solar ions (mainly hydrogen) moving at about 400 km/sec (proton energy 1 kilo-electron-volt) with typical density at the Earth's orbit of 6 ions/cc. The Earth's field forms an obstacle to the solar wind, which confines its field lines and plasmas into an elongated cavity, known as the Earth's magnetosphere. The boundary between the two is called the magnetopause.

On the sun's side of Earth, the magnetopause distance is approximately 10 Earth-radii. Abreast of the earth the distance grows to about 15 earth radii (distances change with solar wind pressure and density; The magnetosphere is made to flap and compress by the solar wind) while on the night side it extends into a long cylindrical magnetotail at least several hundred radii long, gradually turning into a wake.

The magnetosphere contains magnetically trapped plasma (gas of free ions and electrons). One distinguishes the inner radiation belt, a by-product of cosmic radiation discovered in 1958 by James Van Allen using the Explorer 1 and 3 satellites, and the ring current, a large belt of lower energy particles deposited mainly by magnetic storms, source of a widespread magnetic field of its own. The trapped plasma interacts with the low-density conductive plasma of the ionosphere, the upper layer of the atmosphere.

The ionosphere is formed as sunlight, especially ultraviolet, hits the upper atmosphere. It is used to reflect radio waves for communications. Some scientists believe that without a magnetosphere, the Earth would have lost the majority of its water and atmosphere, and resemble Mars or Mercury. However, Venus retains a dense atmosphere even though it lacks any magnetic field.

History of magnetospheric physics

The Earth's magnetosphere was discovered in 1958 by Explorer I during the research performed for the International Geophysical Year. Before this, scientists knew electric currents did flow in space, because solar eruptions sometimes led to "magnetic storm" disturbances. No one knew however where those currents flowed and why, and the solar wind was also unknown.

When it overloads particles strike the upper atmosphere and fluoresce causing the aurora.

belts is in two parts.

capable of penetrating ~1g/cm2 of lead shielding.

  1. inner radiation belt: consists of protons
  1. outer radiation belt: consists of electrons

at most 7 RE radius earth

radiation belts surrounding

Sun does not support

The term Van Allen Belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The atmosphere limits the belts' particles to regions above 200-1000 km (1), while the belts do not extend past 7 RE (1). The belts are confined to an area which extends about 65° (1) from the celestial equator.

The Inner Van Allen Belt[edit]

The inner radiation belt extends over altitudes of 650-6,300 km (up to one RE). This ring is most concentrated in the Earth's equatorial plane. It consists mostly of protons on the order of 10-50 MeV, a by-product of collisions between cosmic ray ions and atoms of the atmosphere. The belt also contains electrons, low-energy protons, and oxygen atoms with energies of 1-100 keV (3). When these electrons strike the atmosphere they cause the polar aurora.

The intensity of the belt fluctuates, partly due to the influence of the solar cycle, and is strongest between 2-5,000 km. The inner radiation belt comes nearest to Earth's surface at the South Atlantic Anomaly.

The number of cosmic ray ions is relatively small and the inner belt therefore accumulates slowly, but because the trapped protons are very stable in this belt (with particle lifetimes of up to ten years), high intensities are reached as they build up over many years.

The belt was discovered by a Geiger counter on board the Explorer 1 satellite built by James van Allen and the University of Iowa and launched on January 31, 1958 as part of the IGY. The instrumentation on board Explorer 1 actually registered no radiation at the altitude of the radiation belts, an anomaly which was explained, by Explorer III's more sophisticated data recording capabilities, as being due to intense radiation having overwhelmed the earlier detector.

The Outer Van Allen Belt[edit]

The outer radiation belt extends from an altitude of about 10,000-65,000 km and has its greatest intensity between 14,500-19,000 km. The outer belt is thought to consist of plasma trapped by the Earth's magnetosphere. The USSR's Lunik I reported that there were very few particles of high energy within the outer belt. The electrons here have a high flux and along the outer edge and E > 40 Kev electrons can drop to normal interplanetary levels within about 100km (a decrease by a factor of 1000). This drop-off is a result of the solar wind.

The particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source.

The outer belt is larger and more diffuse than the inner, surrounded by a low-intensity region known as the ring current. Unlike the inner belt, the outer belt's particle population fluctuates widely and is generally weaker in intensity (less than 1 MeV), rising when magnetic storms inject fresh particles from the tail of the magnetosphere, and then falling off again.

There is debate as to whether the outer belt was discovered by the US Explorer IV or the USSR Sputnik II/III.

Radial Diffusion Induced by Magnetic Fluctuations[edit]

A sudden increase in solar wind pressure can cause the radiation belts to change shape. In such an instance, particles on the sunward side of the planet will be carried inward (toward the planet), while particles on the far side of the planet will be carried further from the planet. This can give the radiation belts somewhat of a tear-drop shape. After such an incident, the belts tend to return to a more spherical shape.

Without this sort of "mirroring," ions and electrons would not be trapped in the Earth's magnetosphere, but would instead follow their guiding field lines into the atmosphere, where they would be absorbed and become lost. What happens instead is that every time a trapped particle approaches Earth, it is reflected back. It is thus confined to the more distant section of the field line.

The Van Allen Belt's Impact on Space Travel[edit]

Solar cells, integrated circuits, and sensors can be damaged by radiation. In 1962, the Van Allen belts were temporarily amplified by a high-altitude nuclear explosion and several satellites ceased operation. Magnetic storms occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics and logic circuits have made satellites more vulnerable to radiation, as incoming ions may be as large as the circuit's charge. The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation.

A object satellite shielded by 3 mm of aluminum will receive about 2500 rem (3) (25 Sv) per year.

Belts of Other Planets[edit]

The gas giant planets Jupiter, Saturn, Uranus and Neptune, all have intense magnetic fields with radiation belts similar to the Earth's outer belt.

Jupiter's belt is the strongest, first detected via its radio emissions in 1955 though not understood at the time. Jupiter's belt is strongly affected by its large moon Io, which loads it with many ions of sulfur and sodium from the moon's volcanoes.

Saturn seems to have an "inner belt" similar to the Earth's, observed by Pioneer 11 during its 1979 fly-by and probably produced by cosmic rays which eject neutrons from Saturn's planetary rings.

The Van Allen Belts and Why They Exist[edit]

The Soviets once accused the US of creating the inner belt as a result of nuclear testing in Nevada. The US has, likewise, accused the USSR of creating the outer belt through nuclear testing. It is uncertain how particles from such testing could escape the atmosphere and reach the altitudes of the radiation belts. Tom Gold has argued that the outer belt is left over from the aurora while Alex Dessler has argued that the belt is a result of volcanic activity

It is generally understood that the Van Allen belts are a result of the collision of Earth's magnetic field with the solar wind. Radiation from the solar wind then becomes trapped within the magnetosphere. The trapped particles are repelled from regions of stronger magnetic field, where field lines converge. This causes the particle to bounce back or "mirror."

See also: Sherwood machine

The South Atlantic Anomaly is the region where Earth's inner van Allen radiation belt makes its closest approach to the planet's surface. Or, for a given altitude, the radiation intensity is higher over this region than elsewhere. It is produced by a "dip" in the Earth's magnetic field at that location, caused by the fact that the center of Earth's magnetic field is offset from its geographic center by 280 miles.

The South Atlantic Anomaly is of great significance to satellites and other spacecraft that orbit at several hundred miles' altitude and at orbital inclinations between 35° and 60°; these orbits take satellites through the Anomaly periodically, exposing them to several minutes of strong radiation each time. The International Space Station, orbiting with an inclination of 51.6°, required extra shielding to deal with this problem.

Van Allen Radiation Belt[edit]


In astronomy, the heliopause is the boundary where our Sun's solar wind is stopped by the interstellar medium.

The solar wind blows a "bubble" in the interstellar medium (the rarefied hydrogen and helium gas that permeates the galaxy). The outer border of this "bubble" is where the solar wind's strength is no longer great enough to push back the interstellar medium. This is known as the heliopause, and is often considered to be the outer border of the solar system.

Inside the heliopause is a boundary called the "termination shock" where supersonic solar wind particles are slowed to subsonic speeds by the interstellar medium. The layer between the termination shock and the heliopause is known as the heliosheath.

The distance to the heliopause is not precisely known. It is probably much smaller on the side of the solar system facing the orbital motion through the galaxy. It may also vary depending on the current velocity of the solar wind and the local density of the interstellar medium. It is known to lie far outside the orbit of Pluto. The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause.

When particles emitted by the sun bump into the interstellar ones, they slow down while releasing energy (warming up). Many particles accumulate in and around the heliopause, highly energised by their negative acceleration, creating a shockwave.

An alternative definition is that the heliopause is the magnetopause between the solar system's magnetosphere and the galaxy's plasma currents.

A solar flare is a violent eruption that explodes from a star's photosphere with energies equivalent to tens of millions of hydrogen bombs. Solar flares from the Sun send out streams of highly energetic solar wind that can present a radiation hazard to spacecraft outside of the planetary magnetospheres and can disrupt radio signals on Earth. Solar flares were first observed on the Sun in 1859 by English astronomer Richard Carrington. They have also been observed to varying degrees on other stars in modern times. The frequency of solar flares varies, from several per day when the Sun is particularly "active" to less than one each week when the Sun is "quiet." Solar flares may take several hours or even days to build up, but the actual flare takes only a matter of minutes to release its energy. The resulting shockwaves travel laterally through the photosphere and upward through the chromosphere and corona at speeds on the order of 5,000,000 kilometers per hour.

Solar activity is classified as A, B, C, D, M or X according to the brightness of its X-rays near earth, measured in W/m2. Each class is ten times more powerful than the preceding one, with X at 10-4 W/m2. Within a class there is a linear scale from 1 to 9, so an X2 flare is four times more powerful than an M5 flare. Solar activity is normally within the A to C range. Class D flares have little effect on earth, while the more powerful M and X flares can cause disruption and damage. Flares generally stay below X10, but infrequently X designations run 'off the charts'. X20 events that were recorded on 16 August 1989 and 2 April 2001 were outshone by a flare on November 4, 2003 that was the most powerful flare ever recorded in the history of astronomy, estimated at X45. Sunspot Region 486 (shown in the illustration) was the most turbulently active sunspot ever recorded.

Energetic particles emitted by solar flares are a primary contributor to the aurora borealis and aurora australis. See also Solar proton event.

The radiation risk posed by solar flares is one of the major concerns in discussions of manned missions to Mars. Some kind of physical or magnetic shielding will be required.

The heliosheath is the zone between the termination shock and the heliopause at the outer border of the solar system. It lies approximately 80 to 100 AU from the Sun. The current mission of the Voyager 1 and 2 spaceprobes includes studying the heliosheath.

Termination Shock[edit]

In astronomy, the termination shock is theorised to be a boundary marking one of the outer limits of the sun's influence. It is where the bubble of solar wind particles slows down to below supersonic speed and heats up due to collisions with the galactic interstellar medium. It is believed to be about 100 Astronomical Units from the Sun.

The termination shock boundary fluctuates in its distance from the sun as a result of fluctuations in solar flare activity i.e. changes in the ejections of gas and dust from the sun.

The Voyager I spacecraft is believed to have passed termination shock in February 2003.

Outer Space[edit]

Gamma ray bursts from black holes.

The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas that exists between the stars (or their immediate "circumstellar" environment) within a galaxy. This gas is usually extremely tenuous, with typical densities ranging from a few tens to a few hundredths of a particle per cubic centimeter. Generally the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals", in astronomical parlance) present in trace amounts.

The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of Kelvin), warm (thousands of Kelvin), and cold (tens of Kelvin). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases is still a matter of considerable contention in scientific circles.

Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.

Space is the relatively empty parts of the universe, outside the atmospheres of planets. It is sometimes called "outer space" to distinguish it from airspace and terrestrial locations. It may be considered to start at a height of ca. 100 km above the Earth's surface. For the term in general see Space.