Outer Layers (Thermosphere Ionosphere):
Energy Input to the Upper Atmosphere:
• We saw earlier that the energy supplied to the Troposphere
comes from heat radiating from the ground.
• Above the troposphere the sources of energy are different and
less uniform.
•
For the most part the source is always the same.
LIGHT
• Different parts of the Sun’s
‘spectrum’ of emitted light are
absorbed at different regions
of the atmosphere.
Atmospheric Layers and Solar Input: Summary
Energy Deposition:
We’ve already covered how the surface of the Earth absorbs sunlight
and warms to provide the source of heat in the troposphere.
How is energy absorbed into the atmosphere?
•
Photochemistry: Sunlight breaks apart a molecule.
OH + light ⇒ O + H
• Photoionization: Sunlight breaks a neutral molecule or atom in
to charged particles.
O2 + light ⇒ O2+ + e-
So How Does This Heat the Atmosphere?
The amount of energy it takes to break apart an atom or molecule (or
to ionize it) is a fixed amount, a threshold.
The amount of energy in light depends on frequency and wavelength
(color).
All light with a shorter wavelength (bluer) or a higher frequency (bluer
again!) than the light with the threshold energy can trigger the
breakup.
It is the products of the breakup that carry any extra energy away.
XV + light ⇒ YV1 + ZV2
From this perspective we are thinking of temperature and velocity (V,
V1, and V2) being equivalent, which they are! (note that wind is
something differente from what we are discussing here! ).
Where is the energy deposited?
Incoming energy is absorbed only by those atoms and molecules that
it can break apart.
Each allowed interaction has a chance of happening when a photon
of sufficient energy passes by.
Convention has us call this chance a ‘cross-section’, which is
equivalent to treating each atom or molecule as a billiard ball.
Using the cross-section, the chance
of an interaction is equal to the ‘size’
of the atom or molecule.
A typical cross-section is 10-18 cm
or one interaction for every
1000000000000000000 photons that
pass by an atom!!!!
Many Targets Fill the Hole:
As the number of particles in a given area increases, the chances of
an interaction do as well.
Eventually there may be
enough particles that they
‘block’ the light.
We call the amount of blocking the ‘optical depth’ of the target.
If most light passes through ‘column’ of the atmosphere, it is referred
to as ‘optically thin’. If most is blocked, then it is ‘optically thick’
Hitting the Wall:
If the atmosphere was completely uniform in density, then energy
would be deposited over a wide range of altitudes.
However, the atmosphere is not
uniform, but becomes less dense as
we go higher.
This means that there is much more
of a target below the level where the
atmosphere becomes optically thick
than above.
In cases like this, the rate of the reaction becomes sharply peaked on
the point where optical depth approaches 100%, with little happening
above and almost nothing below.
Energy Deposition Regions:
In the atmosphere then, this means that the rate and location of
energy input depend on:
• The density of the target.
• The energy of the reaction.
• The composition of the
atmosphere.
• The solar spectrum.
Thus, the rate and altitude of energy input to the upper
atmosphere is stratified into regions that are unique to the
conditions on the Earth.
Escape of Atmospheric Energy:
If the absorption of photons increases the temperature of the
atmosphere, then the escape of photons should cool it. How does
this happen?
Greenhouse Gasses
Solar
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Earth
Energy Deposition/Loss and the Structure of the
Upper Atmosphere:
Above the troposphere, many of the
thermal properties of the upper
atmosphere are determined by where
and how much energy is deposited.
It is this process that controls the
extent of the atmosphere layers.
The Stratosphere:
The stratosphere is the atmospheric layer directly above the
tropopause. It is the first region characterized by atmospheric
energy deposition.
The primary source of energy in
the stratosphere is photochemical.
Incoming ultraviolet light is
absorbed first by O2 and then by
Ozone (O3).
Much of the O3 that is destroyed
is at the top of the stratosphere.
The Role of Ozone:
Remember from our earlier lecture the effect of Chlorofluorocarbons
on Ozone.
Cl + O3 ⇒ ClO + O2
ClO + O ⇒ Cl + O2
The above is called a catalytic reaction.
The O3 is destroyed, but not Cl!
Energy isn’t delivered in this cycle, BUT…
Without O3 to absorb UV radiation the
heating balance of the stratosphere and is
affected!
The Mesosphere:
Above the Stratopause is another, thin atmospheric layer called the
Mesosphere or “Middle Sphere”.
Energy input to the Mesosphere comes from the formation of Ozone,
which occurs near the stratopause (the bottom).
• Like the troposphere, temps in
the mesosphere drop rapidly with
altitude. And we now understand
that this is due to cooling at the top
and heating at the bottom!
• The cooling of the mesophere is
due mainly to CO2.
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Temperature (F)
• The rapid drop in temperature
makes this the most turbulent part of
the atmosphere.
The Thermosphere & Ionosphere
Above the Mesopause the atmosphere begins to heat rapidly as the
atmosphere absorbs solar UV and X-Ray radiation.
The thermosphere is the hottest and thinnest region of the
atmosphere (effectively space).
The thermosphere is characterized
by four features.
• Mixing of atmospheric gasses
begins to break down.
• It derives much of its energy from
photoionization.
• The charged atmosphere
connects directly to the near space
environment.
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(750)
Temperature (F)
• It is the most thermally variable.
Shaken, Not Stirred:
The atmosphere consists of several gasses that are all mixed
together, including O2, N2, CO2, Ar, He, CH4, etc. The altitude
distribution of the atmosphere (depends on average mass,
temperature, and, gravity).
In the Troposphere and Mesosphere convection keeps these
gasses well mixed.
In the Stratosphere and Thermosphere convection is less important.
Two processes affect how the atmosphere is mixed.
• Eddy Mixing: Eddy mixing is the process of using small
random motions in a gas to mix constituents of different
masses.
• Molecular Diffusion: Diffusive mixing is the process of
“settling out”, where each gas behaves as though it were
alone, with its own altitude distribution.
Transitions:
At each level in the atmosphere there is ”characteristic time scale”
over which eddy mixing or diffusive mixing occur. Which ever
occurs faster determines the structure of the atmosphere.
• In the Stratosphere eddy mixing
is much more efficient at organizing
the atmosphere than diffusion.
• In the Thermosphere eddy
mixing is less effective. Above
~110 km, the pace of diffusion
begins to dominate.
• The altitude where this occurs is called the “Turbopause” (end of
turbulence).
Thermosphere Variability:
Because the thermosphere is so thin, small changes in energy input
have a large effect on temperature.
• Day time temperatures can reach 1500C.
• At night temperatures drop as much as 1000C.
• Heating of the Thermosphere comes form the most variable part
of the Sun’s energy output.
• When the Sun is “Active” the Thermosphere gets hotter and
extends out into space farther. This can affect the orbits of
satellites.
• High activity periods occur predictably every 11 years. This is
called the Solar Cycle.
The Ionosphere:
The “Ionosphere” isn’t really an atmospheric region, but rather a
series of layers where photoionization rates are greatest.
The ionosphere is a manifestation of the optical depth effects
described earlier. Different reactions with sunlight set up different
regions.
While ions are produced
throughout the atmosphere, there
are three maximum areas.
• The D-Region
• The E-Region
• The F-Region
We can “see” the ionosphere
layers by reflecting radio waves off
them.
Solar Input:
The ionosphere is generated by different parts of the solar spectrum.
The ion populations are different in each region.
Ion Populations:
• The D region is located in the
Mesosphere from 60-90 km.
• Its ion population is
dominated by NO+ and O2+
• The E region is located in the
Thermosphere from 90-150 km.
• Its ion population is also
dominated by NO+ and O2+
• The F region is located in the
Thermosphere from 150-800 km.
• Its ion population is
dominated by O+