ESS 200C Lecture 13 The Earth

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ESS 200CLecture 13The Earths Ionosphere
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• Ionospheric studies

• The radiation from the Sun at short wave lengths
  causes photo ionization of the atmosphere
  resulting in a partially ionized region called
  the ionosphere.
• Guglielmo Marconis demonstration of long
  distance radio communication in 1901 started
  studies of the ionosphere.
• Arthur Kennelly and Oliver Heaviside
  independently in 1902 postulated an ionized
  atmosphere to account for radio transmissions.
  (Kennelly-Heavyside layer is now called the
  E-layer).
• Larmor (1924) developed a theory of reflection of
  radio waves from an ionized region.
• Breit and Tuve in 1926 developed a method for
  probing the ionosphere by measuring the
  round-trip for reflected radio waves.

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The extent of the ionosphere

• There are ions and electrons at all altitudes in
  the atmosphere.
• Below about 60km the charged particles do not
  play an important part in determining the
  chemical or physical properties of the
  atmosphere.
• Identification of ionospheric layers is related
  to inflection points in the vertical density
  profile.

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Primary Ionospheric Regions Region Altitude Peak Density Primary Ionospheric Regions Region Altitude Peak Density Primary Ionospheric Regions Region Altitude Peak Density Primary Ionospheric Regions Region Altitude Peak Density
D 60-90 km 90 km 108 1010 m-3
E 90-140 km 110 km Several x 1011 m-3
F1 140-200 km 200 km Several 1011-1012 m-3
F2 200-500 km 300 km Several x 1012 m-3
Topside above F2
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• Diurnal and solar cycle variation in the
  ionospheric density profile.
• In general densities are larger during solar
  maximum than during solar minimum.
• The D and F1 regions disappear at night.
• The E and F2 regions become much weaker.
• The topside ionosphere is basically an extension
  of the magnetosphere.

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• Composition of the dayside ionosphere under solar
  minimum conditions.
• At low altitudes the major ions are O2 and NO
• Near the F2 peak it changes to O
• The topside ionosphere becomes H dominant.

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• For practical purposes the ionosphere can be
  thought of as quasi-neutral (the net charge is
  practically zero in each volume element with
  enough particles).
• The ionosphere is formed by ionization of the
  three main atmospheric constituents N2, O2, and
  O.
• The primary ionization mechanism is
  photoionization by extreme ultraviolet (EUV) and
  X-ray radiation.
• In some areas ionization by particle
  precipitation is also important.
• The ionization process is followed by a series of
  chemical reactions which produce other ions.
• Recombination removes free charges and transforms
  the ions to neutral particles.

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• Neutral density exceeds the ion density below
  about 500 km.

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Ionization profile

• Let the photon flux per unit frequency be
• The change in the flux due to absorption by the
  neutral gas in a distance ds is
• where n(z) is the neutral gas density, is
  the frequency dependent photo absorption cross
  section (m2), and ds is the path length element
  in the direction of the optical radiation.
  (Assuming there are no local sources or sinks of
  ionizing radiation.)
• (where is the zenith
  angle of the incoming solar radiation.
• The altitude dependence of the solar radiation
  flux becomes
• where is the incident photon
• intensity per unit frequency.

• is
  called the optical depth.
• There is usually more than one atmospheric
  constituent attenuating the photons each of which
  has its own cross section.

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• The density (ns) of the neutral upper atmosphere
  usually obeys a hydrostatic equation
• where m is the molecular or atomic mass, g
  is the acceleration due to gravity, z is the
  altitude and pnkT is the thermal pressure.
• If the temperature T is assumed independent of z,
  this equation has the exponential solution
• where is the scale
  height of the gas, and n0 is the density at the
  reference altitude z0.
• For this case

• For multiple species
• The optical depth increases exponentially with
  decreasing altitude.
• In the thermosphere solar radiation is absorbed
  mainly via ionization processes. Let us assume
  that
• Each absorbed photon creates a new electron-ion
  pair therefore the electron production is
• where Si is the total electron
• production rate
• (particles cm-3s -1).

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• Substituting for n and gives
• The altitude of maximum ionization can be
  obtained by looking for extremes in this equation
  by calculating
• This gives
• Choose z0 as the altitude of maximum ionization
  for perpendicular solar radiation
• This gives

• where
• This is the Chapman ionization function.
• The maximum rate of ionization is given by
• If we further assume that the main loss process
  is ion-electron recombination with a coefficient
  a and assume that the recombination rate is
• Finally if we assume local equilibrium between
  production and loss we get

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• The vertical profile in a simple Chapman layer is
• The E and F1 regions are essentially Chapman
  layers
• Additional production, transport and loss
  processes are necessary to understand the D and
  F2 regions.

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• The D Region
• The most complex and least understood layer in
  the ionosphere.
• The primary source of ionization in the D region
  is ionization by solar X-rays which ionize both
  N2 and O2
• Lyman-a ionization of the NO molecule.
• Precipitating magnetospheric electrons may also
  be important.
• Initial positive ions are N2, O2 and NO
• The primary positive ions are O2 and NO
• The most common negative ion is NO3-
• The first step in making a negative ion is

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• The E Region
• Essentially a Chapman layer formed by EUV
  ionization
• The main ions are O2 and NO
• Although nitrogen (N2) molecules are the most
  common in the atmosphere N2 is not common
  because it is unstable to charge exchange. For
  example
• Oxygen ions are removed by the following
  reactions

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• The F1 Region
• Essentially a Chapman layer.
• The ionizing radiation is EUV at lt91nm.
• It is basically absorbed in this region and does
  not penetrate into the E region.
• The principal initial ion is O.
• O recombines in a two step process because
  recombination of oxygen is slow.
• First atom ion interchange takes place
• This is followed by dissociative recombination of
  O2 and NO

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• The F2 Region
• The major ion is O.
• This region cannot be a Chapman layer since the
  atmosphere above the F1 region is optically thin
  to most ionizing radiation.
• This region is formed by an interplay between ion
  sources, sinks and ambipolar diffusion.
• The dominant ionization source is photoionization
  of atomic oxygen
• The oxygen ions are lost by a two step process
• First atom-ion interchange
• Dissociative recombination
• The peak forms because the loss rate falls off
  more rapidly than the production rate.
• The density falls off at higher altitudes because
  of diffusion- no longer in local photochemical
  equilibrium.

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• Ionospheric conductance
• The dense regions of the ionosphere (the D, E and
  F regions) contain concentrations of free
  electrons and ions. These mobile charges make the
  ionosphere highly conducting.
• Electrical currents can be generated in the
  ionosphere.
• The ionosphere is collisional. Assume that it has
  an electric field but for now no magnetic field.
  The ion and electron equations of motion will be
• where is the ion neutral collision
  frequency and is the electron neutral
  collision frequency.
• For this simple case the current will be related
  to the electric field by
• where is a scalar conductivity.
• If there is a magnetic field there are magnetic
  field terms in the momentum equation. In a
  coordinate system with along the z-axis the
  conductivity becomes a tensor.

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• Specific conductivity along the magnetic field
• Pedersen conductivity in the direction of the
  applied electric field
• Hall conductivity in the direction
  perpendicular to the applied field
• where and are the total electron and
  ion momentum transfer collision frequencies and
  and are the electron and ion
  gyrofrequencies.
• The Hall conductivity is important only in the D
  and E regions.
• The specific conductivity is very important for
  magnetosphere and ionosphere physics. If
  all field lines would be equipotentials.
• Electric fields generated in the ionosphere
  (magnetosphere) would map along magnetic field
  lines into the magnetosphere (ionosphere)

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• Assume a generalized Ohms law of the form
  and that
• The total current density in the ionosphere is
• where and refer to perpendicular
  and parallel to the magnetic field.
• Space plasmas are quasi-neutral so
• where
• The current continuity equation can be written
  where is along
  the magnetic field.
• Integrate along the magnetic field line from the
  bottom of the ionosphere to infinity. Since the
  field lines are nearly equipotentials we can
  write
  where the perpendicular height integrated
  conductivity tensor is

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• Ionospheric Pedersen conductance viewed from
  dusk.
• Note the large day-night asymmetry. This results
  from of ionization by solar EUV radiation.

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• Ionospheric Hall and Pedersen conductance from a
  simulation of the magnetosphere during a
  prolonged period with southward IMF.
• The white lines show the ionospheric convection
  pattern.
• Precipitation from the magnetosphere enhances
  both the Hall and Pedersen conductances at night.

Pedersen Conductance
Hall Conductance
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• Field aligned currents from the simulation in the
  previous calculation.
• Cold colors indicate currents away from the Earth
  and hot colors indicate currents toward the
  earth.
• The high latitude currents are caused by the
  vorticity of polar convection cells.

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• Within the high latitude magnetosphere (auroral
  zone and polar cap) plasmas undergo a circulation
  cycle.
• At the highest latitudes the geomagnetic field
  lines are open in that only one end is
  connected to the Earth.
• Ionospheric plasma expands freely in the flux
  tube as if the outer boundary condition was zero
  pressure.
• For H and He plasma enters the flux tube at a
  rate limited by the source.
• The net result is a flux of low density
  supersonic cold light ions into the lobes.
• The surprising part is that comparable O fluxes
  also are observed.