THE NEUTRAL ATMOSPHERE
• Temperature and density structure
• Hydrogen escape
• Thermospheric
variations and
satellite drag
• Mean wind structure
• Standard
atmospheres
• (Numerical simulation models to be
discussed after the ionosphere)
ASEN 5335 Aerospace Environment -- Upper Atmospheres
• Neutral chemistry
1
Tropo
(Greek: tropos);
“change”
Lots of weather
Strato
(Latin: stratum);
Layered
Meso
(Greek: messos);
Middle
Thermo
(Greek: thermes);
Heat
Exo
(greek: exo);
outside
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Noctilucent Clouds
 Clouds at extremely high altitude, about 85 km, that literally (as the name suggests)
shine at night. They form in the cold, summer polar mesopause and are believed to be
ice crystals. Because of their high altitude, in a very dry part of the atmosphere,
noctilucent clouds are rather an enigma and are being studied by a number of people
around the world.
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Temperature Decrease or
Water Vapor Increase ?
Number of nights per year, N, on which
noctilucent clouds were reported from northwest Europe, with the effect of solar activity
removed (Gadsden, 1990)
Evidence for decreases in mesospheric temperature
(Aikin et al., 1991). (a) NOAA TIROS data and (b)
Haute Provence lidar data.
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Tropo
(Greek: tropos);
“change”
Lots of weather
Strato
(Latin: stratum);
Layered
Meso
(Greek: messos);
Middle
Thermo
(Greek: thermes);
Heat
Exo
(greek: exo);
outside
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HEAT SOURCES AND SINKS
Thermosphere - Sources
• absorption of EUV (200-1000 Å; photo-ionization of O, N2, O2)
and UV (1200-2000 Å; photo-dissociation of O2) radiation;
photo-ionization and photo-dissociation lead to chemical
reactions and collisions that liberate heat.
• dissipation of upward propagating gravity waves
(weather systems; flow over topography) and tides
(periodic heating).
• joule heating of electric currents
(mostly auroral / polar regions)
• particle precipitation (mostly auroral / polar regions)
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Absorption of Solar Radiation vs. Height and Species
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Thermosphere - Sinks
Thermal conduction (molecular and turbulent) removes
heat from thermosphere to mesosphere (here collision
frequencies are high enough that polyatomic molecules CO2, O3,
H2O can radiate energy away in infrared).
Let
 = heat flux due to conduction =
dT
k
dh
As a first approximation, heat input is balanced by loss due to
conduction:
d
Q
dh
    Qdh
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Therefore
dT
dh
1
dT
  Qdh
dh z k z
must always be sufficiently large to conduct away
heat deposited at higher levels. Therefore
•
dT
0
dh
• also
dT
dh
above 200 km since
Q0
is maximum around 120 - 150 km
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Tropo
(Greek: tropos);
“change”
Lots of weather
Strato
(Latin: stratum);
Layered
Meso
(Greek: messos);
Middle
Thermo
(Greek: thermes);
Heat
Exo
(greek: exo);
outside
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Mesosphere - Sources
some UV absorption by O3 in lower region
heat carried downward from thermosphere
(minor contribution)
Mesosphere - Sinks
infrared radiation by CO2, O3, H2O, OH
Stratosphere - Sources
strong absorption of UV (2,000 - 3000 Å) by O3 (produces
maximum in temperature at stratopause)
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Troposphere - Sources
absorption by planetary surface of infrared and
visible radiation, and conduction to atmosphere
atmospheric absorption of terrestrial and solar IR.
latent heat release by water
Troposphere - Sinks (and Sources)
infrared radiation by surface, atmosphere (absorption)
evaporation of water
thermal convection important in transporting
heat between different levels
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Aerospace Environment
ASEN-5335
• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)
• Contact info: e-mail: lix@lasp.colorado.edu (preferred)
phone: 2-3514, or 5-0523, fax: 2-6444,
website: http://lasp.colorado.edu/~lix
• Instructor’s office hours: 9-11 am at ECOT 534
• TA’s office hours: 3:15-5:15 pm Wed at ECAE 166. E-mail:
Christopher.Hood@colorado.edu
•
•
•
•
READ classnotes and Chapter 6
HW6 due today
Quiz-7 on 4/29
HW7 due 5/1
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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> 700 keV ions and > 500 keV electrons
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> 20 MeV Ions (most protons)
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> 20 MeV Ions (most protons)
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> 20 MeV Ions (most protons)
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> 20 MeV Ions (most protons)
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> 20 MeV Ions (most protons)
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> 20 MeV Ions (most protons)
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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SEE HAZARD REGISTER & ANOMALIES
FOG anomalies (white stars) and
SEE register values (color scale)
Latitude
Latitude
Detected EDAC Errors
Longitude
FOG - Fiber Optic Gyroscope
Longitude
EDAC - Error Detection And Correction
Spacecraft altitude region in graphs is 1450-1710 km.
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Inner Radiation Zone
and South Atlantic Anomaly
500 km alt.
Earth’s
Surface
SAA
• Region of enhanced Single Event Effects caused by the intense and very energetic proton (EP ≥ 10 MeV)
fluxes in the inner radiation belt.
• Possible problem region for satellites in low altitude orbit or in elliptical orbits that traverse low altitudes.
• In the South Atlantic region the energetic protons come closest to earth because of asymmetries in the
26
ASENfield.
5335 Aerospace Environment -- Upper Atmospheres
magnetic
Magnetospheric Substorm
• Major contributor of
space-weather effects
inside the magnetosphere
cavity.
• Heats plasma at large antisun distances and
“drives” it inward toward
the Earth.
• Generates auroral displays
in the high latitude
regions.
• Causes surface charging
of satellites in the premidnight to local morning
local time regions.
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Injection of Particles During a Substorm
(The Aerospace Corp.)
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Basic Radiation Physics
 As an example of the interaction between radiation and matter, an energetic electron passing
through air will leave a trail of ionized particles.
 If the electron is moving too
slowly, it lacks the energy
necessary to create ionizations.
 If the electron moves too fast, it
“passes through” without
effectively interaction with the
ambient atoms and molecules.
 Consequently, radiation damage
to materials is dependent not only
on the nature of the radiation, but
on the energy of the radiation and
the nature of the material itself
(see figure on next page).
 The official SI unit of radiation is the Gray, 1 Gray=radiation which deposits 1 J/Kg of materials.
 You may be more familiar with Rad or Roentgen: 1 Rad=radiation which deposits 0.01 J/Kg of materials. 1
Roentgen=amount of x-rays or gamma radiation that produces a given amount of ionization in air.
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 Two major factors in the determination of radiation damages: (1) total dose
over the life of a material, (2) dose rate, the rate at which energy is deposited.
 Different materials have different susceptibilities to damage:
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Relative Biological Equivalent (RBE) factors
 In biological applications, the terms REM and RBE are used.
 RBE (Relative Biological Effectiveness) is the number of rads of x-ray or gamma radiation that
produces the same biological damages as 1 Rad of the radiation being used.
 REM (Roentgen Equivalent in Man) is the product of the dose in Rad and the RBE factor.
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PARTICLE ENERGIES OF CONCERN
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Biological Risks
•
•
•
•
Primary biological risk from
space radiation exposure is
cancer
When radiation is absorbed in
biological material, the energy is
deposited along the tracks of
radiation.
Heavy ions produce much
denser pattern of ionization
more biological effects per unit
of absorbed radiation dose.
Secondary concerns such as
cataracts are beginning to
receive more administrative
attention
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Another example of satellite fragmentation in elliptical orbit

Nearly same apogee height; therefore,
fragmentation occurred near apogee
fragmentation
height = 2088 km
+
1976-126A
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Greenhouse Effect
atmosphere transparent to visible, but opaque to infrared
H2O, CO2
visible
infrared
289 k (288 k 10yr ago),
otherwise, 253 k.
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Sources of
Carbon Dioxide
CO2 build up due to fossil fuel
consumption. Currently the average
concentration is about 365 ppmv and is
increasing at a rate of about 0.4% (3
giga-tons) per year.
 On the other hand, without the
greenhouse effect, the average
temperature of the Earth would be 253
K but now the average temperature is
289 K (288 K 10 years ago).
 Fossil-fuel combustion is the principal
global source of carbon dioxide (~6070%), with deforestation estimated to be
the second major source (~30-40%). The
production of cement, which involves
crushing and baking calcium carbonate,
contributes the rest.
 USA (16%), EU (<16%), The nations of
eastern Europe (19%).
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Long-Term CO2
Measurements
 The long-term trend in carbon
dioxide concentrations, as
determined by measurements of the
composition of air in glacial ice
bubbles, is illustrated. This record
can be extended back for tens of
thousands of years. However, the
comparison of atmospheric CO2
levels before and after the Industrial
Revolution (beginning in the late 18th
century; the steam engine was
invented by James Watt in Scotland in
1751) is instructive.
 The seasonal rise and fall of carbon
dioxide are in perfect rhythm with
life’s basic activity, photosynthesis. In
the summer, when the air is warm and
sunlight is plentiful, vegetation takes
up large amounts of carbon dioxide
through excess photosynthesis.
Recent variations in carbon dioxide, monthly values
in ppmv. Seasonal variation associated with
photosynthesis/respiration is several ppmv.
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Very Long-term CO2 and T
 The maximum variation in CO2
concentration ranges from about 170 to
280 ppmv, or a change of about 110
ppmv. This is only slightly greater than
the change in CO2 concentration that
has occurred during the past century
(about 80 ppmv) and is much smaller
than the change that will have occurred
by the time the CO2 concentration
doubles to about 600 ppmv in the
twenty-first century (according to some
projections).
 The variations in CO2 may not lead the
variations in temperature; rather, they
may follow the temperature changes.
This can be seen only at a finer time
resolution.
 Potential impacts of greenhouse
warming: surface temperature, a rise in
sea level (12 cm/1000yr, rate doubled in
the past 50 yr).
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HYDROSTATIC EQUILIBRIUM
If …..
n = # molecules per unit volume
P + dP
m = mass of each particle
dP
nm dh = total mass contained in
a cylinder of air (of unit
cross-sectional area)
Then, the force due to gravity on
the cylindrical mass = nmg dh
nmgdh
P
and the difference in pressure
between the lower and upper
faces of the cylinder balances
the above force in an equilibrium
situation:
P dP  P  nmgdh
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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
dP
 nmg
dh
Assuming the ideal gas law holds,
P  nkT  RT
R*
R
m
Then the previous expression may be written:
where H is called the scale height
and
kT
RT
H

mg
g
1 dP
1

P dh
H
R2
E
g  g(0)
RE  h2
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This is the so-called hydrostatic law or barometric law.
Integrating,
P  P0e
z
z dh
z
0H
where
and z is referred to as the "reduced height" and the subscript zero
refers to a reference height at h=0.

To z
n  no
e
T
Similarly,
TYPOS
For an isothermal atmosphere, then,
n  noe
h
PPe
o
h
H
ASEN 5335 Aerospace Environment -- Upper Atmospheres
H
  oe
h
H
41
Strictly speaking, since m varies from constituent to
constituent (i.e., H, He, O, O2, N2, ....), the above relations
apply to individual constituents, i.e.,
h H
i
P  Pioe
n  nioe
 hH
where Pi is the partial pressure and
i
  ioe
 hH
i
kT
Hi 
mi g
Thus, each individual constituent has the tendency to distribute
vertically according to its own individual scale height. The process
which makes this possible is molecular diffusion (see following
figure).
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For average dayside conditions
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43
Variation of the density in an
atmosphere with constant
temperature (750 K).
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44
Vertical distribution of density and temperature for high solar activity (F10.7 = 250) at noon
(1) and midnight (2), and for low solar activity (F10.7 = 75) at noon (3) and midnight (4)
according to the COSPAR International Reference Atmosphere (CIRA) 1965.
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Now, the efficiency of molecular diffusion increases
according to the mean free path of atmospheric particles,
and hence inversely with atmospheric density. At sufficiently low
altitudes in the atmosphere, molecular diffusion is not able to
compete with the various mixing processes in the atmosphere
(turbulent diffusion, wave and general dynamical transport, etc.).
The atmosphere, in fact, remains well-mixed below about 100 km.
This regime is called the homosphere and is characterized by a
constant mean molecular weight as a function of height:
 mi ni
-1
m
= 28 .9 7 g mo le
 ni
A mean scale height is thus defined:
kT
H
mg
78.1% N2 20.9% O2
.9% Ar
.03% CO2
.002% Ne .0005% He
Variable H2O
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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and all constituents possess the same scale height and
number density (and pressure) distributions with height:
T 
ni  n  no
e
T o 
z
H
 It is not until about 100 km (the
exact height is species dependent,
due to the dependence of molecular
diffusion velocity on mean
molecular weight) that molecular
diffusion begins to take over, and
each species separates according to
its individual scale height.
 This separation occurs at the
homopause, sometimes called the
turbopause. Above the homopause
is the heterosphere; homosphere
below.
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48
What is the meaning of temperature at high altitude?
Can we measure it with a thermometer ?
For an ideal gas consisting of perfectly elastic spheres in
random thermal motion, and under equilibrium conditions, the
number of molecules dN out of a total N having a speed between c
and (c+dc) is given by the Maxwell-Boltzmann distribution:
 
2
mc
3/2

dN |cc dc
m
2
2kT
 4
e
c dc
N
2kT
The above provides a definition of kinetic temperature valid
whenever a gas is in thermal equilibrium.
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In practice, at very high altitudes in the earth's atmosphere, the
gas temperature can only be determined from a measurement of the
particle velocities since any heat sensing instrument would radiate
away any energy it received faster than collisions could raise it to the
gas temperature.
Above a certain level the mean free path of the particles
exceeds the atmospheric scale height:
The region where
is called the exosphere,
and the level where l  H
is called the exobase.
Sometimes the level
where l  H is called
the barosphere. In the
exosphere particles
are in ballistic orbits
around the earth.
lH
lH
Above the exobase there exists a substantial
fraction of the particles with velocities greater than
the escape velocity (~ 11 km/sec):
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Particles above escape velocity
can leave the atmosphere. This effect
is only important for atomic Hydrogen
in the earth's atmosphere.
Escape
sink
vertical
flux
H2O, CH 4
H
source
The escape flux increases with temperature, causing the
hydrogen density to decrease with increasing solar activity since
molecular diffusion is too slow to keep up.
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Atmospheric Compositions Compared
The atmospheres
of Earth, Venus and
Mars contain many
of the same gases,
but in very different
absolute and
relative abundances.
Some values are
lower limits only,
reflecting the past
escape of gas to
space and other
factors.
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Average Temperature Profiles
for Earth, Mars & Venus
Mars
Venus
night
day
Venus
Earth
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Altitude Profiles of Neutral Gas Densities on Venus
Venus
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Altitude Profiles of Neutral Gas Densities on Mars
Mars
Note: Although the surface pressures on Earth (1 bar), Venus (90 bars) and Mars (6 mb) vary
widely, the atmospheric density near 100-120 km is comparable on all 3 planets due to a fortuitous
combination of temperature structure and gravitational acceleration.
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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ATOMIC OXYGEN
Atomic oxygen is an important atmospheric constituent:
• necessary for the formation of ozone, O3
• accounts for most of the satellite drag from 180 - 500 km
• highly corrosive to aerospace materials
In addition, O plays an important role in earth's ionosphere which
distinguishes it from Mars and Venus:
• EUV radiation photoionizes O to produce O+ and e-, but these
do not readily combine together again. Recombination is only
fast for molecular ions (i.e., O2 ,NO  , etc. )
• Instead, conversion to molecular ions is required:
O

 N 2  NO

O
O


 O2  O2  O
However, these reactions are much slower than recombination, and
ionization levels can persist into the night (i.e., in the absence of
photoionizing radiation). Important for ham radio operators !
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SATELLITE DRAG AND THE EVOLUTION
OF MODEL ATMOSPHERES
Through Kepler's laws, one can derive the rate of change of
orbital period ( T ) in terms of the atmospheric density:
dT/dt = -(3/2)Bp (/p)ds
where B = B-factor (ballistic coefficient) = CDA/m
P = density at perigee
CD =drag coefficient
 = density
A = cross-sectional area of the
s/c presented to the flow
m = mass of the s/c
s =satellite path
From radar tracking, one can derive the atmospheric
density (the more accurate the tracking, the shorter time required to
determine density). Typical resolution is about 1 day below 200 km
and 10 days at 500 km.
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The above procedure requires some knowledge about the
variation of density with height.
In the isothermal part of the
atmosphere (above about 200 km) this reduces to a simple
exponential dependence ( unless we are near the level where [O] = [H]
) with a nearly constant scale height; in any case, if a reasonable
initial first guess of the vertical structure is provided, a robust
interative procedure usually leads to an accurate determination that is
independent of the initial guess.
Now, according to the equation
ns Dns0exp[- (z-z0)/Hs], if the relative
composition (i.e., from some
average of rocket mass
spectrometer data), and
temperature at a convenient
lower boundary are specified
with some shape parameters
allowing the temperature to
asymptotically approach a constant value, satellite drag data can be used
to infer these “exospheric temperatures”.
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This leads to the so-called "static diffusion models"
developed by Jacchia and which form the basis of many operational
drag models.
The derived exospheric temperatures (and the
densities) reveal many of the variations typical of the thermosphere:
annual, semiannual, solar activity, magnetic activity, diurnal, etc.
(see following figures).
DESPITE THE IMMENSE SUCCESS OF THESE MODELS AT THE
TIME, THEY SUFFER FROM SOME FUNDAMENTAL LIMITATIONS:
• The derived temperature is more of a 'virtual' temperature than
'real' (kinetic) temperature
• Rocket measurements of O, O2 at the lower boundary are difficult
to interpret (i.e., O recombines into O2 against walls of measuring device,
meaning that O can be underestimated and O2 overestimated).
• Wind-induced diffusion is also important, i.e., for [O]
• The 'static diffusion' or 'hydrostatic' restriction is not amenable to
addressing vertical transport (i.e., upwelling) or horizontal transport.
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Correlation of
density and
temperature
with longterm changes
in solar
activity.
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SNOE/SAMPEX Measurements/Quiet Time
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SNOE/SAMPEX Measurements/Disturbed Time
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“Kp effect”
at 338 and
427 km
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“Kp effect”
at 709 and
1001 km
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Aerospace Environment
ASEN-5335
• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)
• Contact info: e-mail: lix@lasp.colorado.edu (preferred)
phone: 2-3514, or 5-0523, fax: 2-6444,
website: http://lasp.colorado.edu/~lix
• Instructor’s office hours: 9-11 am at ECOT 534
• TA’s office hours: 3:15-5:15 pm Wed at ECAE 166. E-mail:
Christopher.Hood@colorado.edu
• READ classnotes and Chapter 6
• Quiz-7 on 4/29
• HW7 due 5/1
ASEN 5335 Aerospace Environment -- Upper Atmospheres
65
 RESOLVING THE MYSTERY OF WHERE, WHEN AND HOW AURORAL ERUPTIONS START,
THE INITIATION OF SUBSTORMS
 Determining the sources of the storm-time “killer” MeV electrons.
 What controls efficiency of solar wind – magnetosphere coupling?
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NASA/MIDEX: $173M. Launch year: 2006.
Science Team  A True International Collaboration
Five identical s/c (65kg + 35kg fuel)/ea.
Probe instruments:
ESA: Thermal plasma
SST: Super-thermal plasma
FGM: Low frequency magnetic field
SCM: High frequency magnetic field
EFI: Electric field
Principal Investigator
Vassilis Angelopoulos, UCB
EPO Lead: Nahide Craig, UCB
Program Manager: Peter Harvey, UCB
Industrial Partner: SWALES Aerospace
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Events occurring during a substorm
Auroral
Eruption
ASEN 5335 Aerospace Environment -- Upper Atmospheres
Current
Disruption
Reconnection
68
Mission overview: Fault-tolerant design has
constellation and instrument redundancy
D2925-10 @ CCAS
EFIs
SCM
ESA
BGS
SST
Operations
UCB
FGM
Mission I&T
Swales
Tspin=3s
Instrument I&T
UCB
Ground
ASEN 5335 Aerospace Environment -- Upper Atmospheres
Probe instruments:
ESA: Thermal plasma
SST: Super-thermal plasma
FGM: Low frequency magnetic field
SCM: High frequency magnetic field
EFI: Electric field
69
First bonus: What produces storm-time “killer”
MeV electrons and source of these MeV electrons
Affect satellites and
humans in space
ANIK telecommunication satellites lost
for months, likely due to MeV
electrons deep dielectric discharge
ASEN 5335 Aerospace Environment -- Upper Atmospheres
(Li et al., 2003)
70
Source:
– Inward radial diffusion?
– Wave acceleration at
radiation belt region?
THEMIS:
–Tracks radial motion of
electrons
• Measures source and
diffusion
• Frequent crossings
–Measures E, B waves locally
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Second bonus: What controls efficiency
of solar wind – magnetosphere coupling?
Important for solar wind energy transfer in Geospace
Need to determine how:
– Localized pristine solar wind features…
– …interact with magnetosphere
THEMIS:
– Alignments track evolution of solar wind
– Inner probes determine entry type/size
ASEN 5335 Aerospace Environment -- Upper Atmospheres
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Flux
Enhance
factor
Orbit Inclination
25
0.9
62
1.09
26
0.905
88
1.39
27
0.91
89
1.38
28
0.912
90
1.37
28.5
0.9135
91
1.38
29
0.915
92
1.4
30
0.92
93
1.44
31
0.922
94
1.5
32
0.927
95
1.55
56
1.06
120
1.18
57
1.065
121
1.165
58
1.075
122
1.155
59
1.08
123
1.14
60
1.09
124
1.125
61
1.1
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Tropo
(Greek: tropos);
“change”
Lots of weather
Strato
(Latin: stratum);
Layered
Meso
(Greek: messos);
Middle
Thermo
(Greek: thermes);
Heat
Exo
(greek: exo);
outside
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For average dayside conditions
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SATELLITE DRAG AND THE EVOLUTION
OF MODEL ATMOSPHERES
Through Kepler's laws, one can derive the rate of change of
orbital period ( T ) in terms of the atmospheric density:
dT/dt = -(3/2)Bp (/p)ds
where B = B-factor (ballistic coefficient) = CDA/m
P = density at perigee
CD =drag coefficient
 = density
A = cross-sectional area of the
s/c presented to the flow
m = mass of the s/c
s =satellite path
From radar tracking, one can derive the atmospheric
density (the more accurate the tracking, the shorter time required to
determine density). Typical resolution is about 1 day below 200 km
and 10 days at 500 km.
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The above procedure requires some knowledge about the
variation of density with height.
In the isothermal part of the
atmosphere (above about 200 km) this reduces to a simple
exponential dependence ( unless we are near the level where [O] = [H]
) with a nearly constant scale height (?); in any case, if a reasonable
initial first guess of the vertical structure is provided, a robust
interative procedure usually leads to an accurate determination that is
independent of the initial guess.
Now, according to the equation
ns Dns0exp[- (z-z0)/Hs], if the relative
composition (i.e., from some
average of rocket mass
spectrometer data), and
temperature at a convenient
lower boundary are specified
with some shape parameters
allowing the temperature to
asymptotically approach a constant value, satellite drag data can be used
to infer these “exospheric temperatures”.
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This leads to the so-called "static diffusion models"
developed by Jacchia and which form the basis of many operational
drag models.
The derived exospheric temperatures (and the
densities) reveal many of the variations typical of the thermosphere:
annual, semiannual, solar activity, magnetic activity, diurnal, etc.
DESPITE THE IMMENSE SUCCESS OF THESE MODELS AT THE
TIME, THEY SUFFER FROM SOME FUNDAMENTAL LIMITATIONS:
• The derived temperature is more of a 'virtual' temperature than
'real' (kinetic) temperature
• Rocket measurements of O, O2 at the lower boundary are difficult
to interpret (i.e., O recombines into O2 against walls of measuring device,
meaning that O can be underestimated and O2 overestimated).
• Wind-induced diffusion is also important, i.e., for [O]
• The 'static diffusion' or 'hydrostatic' restriction is not amenable to
addressing vertical transport (i.e., upwelling) or horizontal transport.
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IN THE 1970'S, TWO DATA SETS BECAME AVAILABLE THAT
ADDRESS THE PREVIOUSLY-MENTIONED LIMITATIONS:
• determinations of Tex from incoherent scatter radar
measurements.
• satellite mass spectrometer measurements of O, O2, N2,
He, H, etc., and also measurements of Tex. (satellites like OGO-6, AE,
and many others).
HENCE LEADING TO THE
Mass Spectrometer Incoherent Scatter (MSIS) models of A.
Hedin (NASA/GSFC).
However, the MSIS models are not optimized with respect to satellite
drag, and so have not been widely adopted for ephemeris
computations in lieu of the Jacchia models. In principle, though,
getting closer to the correct physics should lead to improved orbital
predictions.
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Some of the other models developed
during the past 30 years
EMPIRICAL
U.S. Standard 1962
U.S. Standard Supplements, 1966
MSIS86, MSIS90, MSISE90
Jacchia 1964
CIRA-1961, 1965
Jacchia-1971, 1977
CIRA-1986
NUMERICAL/THEORETICAL MODELS
University College London Thermosphere-Ionosphere Model
(now Coupled Thermosphere-Ionosphere Model (CTIM)
at CU/CIRES)
NCAR Thermosphere-Ionosphere GCM
TGCM
TIGCM
TIE-GCM
TIME-GCM
We will return to a discussion and demonstration of these models
shortly.
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Making best use of the air drag: MGS Aerobraking
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11 September 1997
Mars Orbit Insertion
This date marked the arrival of Mars Global Surveyor at the red planet. A 22-minute firing of Surveyor's
main rocket engine placed the spacecraft into a highly elliptical orbit taking 44 hours, 59 minutes, and 34
seconds to complete. The initial orbit had a low point of 262 km above the northern hemisphere, and a
high point of 54,026 km above the southern hemisphere.
17 September 1997
Start of Aerobraking
On September 17th, the flight team started performing a series of orbit changes to lower the low point of
its orbit into the upper fringes of the Martian atmosphere. On every pass through the atmosphere, the
spacecraft slowed down by a slight amount because of air resistance, and the high point of the orbit to
begin to drop. The original plan was to use this "aerobraking" scheme to lower the high point of the orbit
from 56,026 km down to 450 km by repeatedly flying through the atmosphere for four months.
11 October 1997
Pause in Aerobraking
The flight team performed a maneuver to raise the low point of the orbit out of the atmosphere. This
suspension of aerobraking was performed because air pressure from the atmosphere caused one of
Surveyor's two solar panels to bend backward by a slight amount. The panel in question was slightly
damaged shortly after launch in November 1996.
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7 November 1997: Resumption of Aerobraking
The decision to lower the low point of the orbit back into the atmosphere and resume aerobraking came after several
weeks of analysis. Flight team members concluded that aerobraking is safe, provided that it occurs at a more gentle pace
than proposed by the original mission plan.
November 1997 to May 1998: Aerobraking Phase 1
Under the new mission plan, aerobraking will occur with the low point of the orbit at an average altitude of 120 km, as
opposed to the original altitude of 110 km. This slightly higher altitude results in a decrease of 66% in terms of air
resistance pressure experienced by the spacecraft. During these six months, aerobraking will reduce the orbit period
down to between 12 to 6 hours.
May 1998 to November 1998: Science Phasing
Sometime slightly before May, aerobraking will be temporarily suspended to allow the orbit to drift into the proper
position with respect to the Sun. Without this hiatus, Surveyor would complete aerobraking with its orbit in the wrong
solar orientation.
In order to maximize the efficiency of the mission, these six months will be devoted to collecting as much science data as
possible. Data will be collected between two to four times per day, at the low point of each orbit.
November 1998 to March 1999: Aerobraking Phase 2
During these six months, aerobraking will continue and will shrink the high point of the orbit down to 450 km. At this
altitude, Surveyor will circle Mars once every two hours.
March 1999: Start of Mapping
Aerobraking is scheduled to terminate at the same time the orbit drifts into its proper position with respect to the Sun. In
the desired orientation for mapping operations, the spacecraft will always cross the day-side equator at 2:00 p.m.(local
Mars time) moving from south to north. This geometry was selected to enhance the total quality of the science return
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ATMOSPHERIC DENSITY PERTURBATIONS
Important atmospheric properties as far as aerospace applications are
concerned, are atmospheric density and density variability.
Here we will concentrate on providing some sense of the origins and
characteristics of density variability in
---
the reentry regime, ca. 60-100 km
low earth orbit, i.e., 150-500 km
The reentry regime lies within the so-called "transition
region" or "MLT region" (for mesosphere/lower thermosphere).
Most density variability in the reentry regime is due to
"meteorological influences" originating at much lower altitudes (see
accompanying figure)
• gravity waves excited by convective
systems, instabilities, flow over
topography, etc.
• atmospheric tides driven by the
periodic absorption of solar
radiation by ozone and water vapor
• planetary waves with periods of 2-20 days which appear to originate as
quasi-resonances of the atmosphere, or as a result of instabilities.
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The ITM System
H
Escape
Magnetospheric
Coupling
B
400 km
E
Energetic
Particles
Ion Outflow
E
Wind Dynamo
Polar/Auroral
Dynamics
Mass Transport
Joule Heating
Wave
Generation
60 km
B
ITM System
Solar Heating
CO2 Cooling
Turbulence
Topographic
Generation
of Gravity
Waves
O3
solar-driven tides
Planetary Waves
0 km
H2O
Pole
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CO2
CH4
Convective
Generation
of Gravity
Waves
Equator
86
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The most variable parts of the solar spectrum are
absorbed above about 100 km
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The wavelengths
most significant
for the space
environment are
X-rays, EUV and
radio waves.
Although these
wavelengths
contribute
only about 1%
of the total energy
radiated, energy at
these wavelengths
is most
variable
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Gravity waves and tides are common sources of density
variability in the MLT region, and both of these are reflected
in accelerometer measurements made during Shuttle reentry
Density relative to MSIS86 model for seven Shuttle flights
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Ground tracks for several STS missions
Typical spatial coverage
during Shuttle re-entry
Density ratio to model
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Gravity waves
can also be
observed in
emission
intensities from
the MLT region,
i.e., in the nearUV emission
from OH
originating from
a thin (~10 km)
layer near 85 km
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The fluctuations that gravity waves produce in the natural IR
background also interfere with various surveillance systems.
Due to the exponential decrease of density, amplitudes of
gravity waves grow exponentially with height --- in the "reentry"
regime they become so large that they go unstable, generate
turbulence, and deposit heat and momentum into the atmosphere.
The generated turbulence accounts for the "turbulent
mixing" and the turbopause that we talked about before.
The deposited momentum produces a net meridional
circulation, and associated rising motions (cooling) at high latitudes
during summer, and sinking motions
(heating) during winter, causing the
so-called "mesopause anomaly" in
temperature.
u ........ fv   1  u w 



t
z
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Atmospheric tides also make major contributions to
atmospheric variability at the same altitudes -- basically,
atmospheric tides are just gravity waves whose horizontal scales
are so large that the rotation of the earth (Coriolis effect) must be
taken into account. They also have different forcing mechanisms.
It is quite common to see large diurnal (24-hour period) and
semidiurnal (12-hour period) tidal oscillations in the MLT region.
However, it is difficult to obtain direct, continuous, density
measurements.
Instead, various radar methods are used to
measure tidal wind fields, as to be shown in the following pages,
and some relatively basic theory can be used to estimate the
accompanying density variations.
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Ratio of density to
initial reference value
The effects of joule heating on atmospheric density near 170 km at high
latitudes, is made readily apparent by the correlation between the AE and Kp
indices, and the densities derived from accurate tracking of a Navy doppler
beacon satellite (density averages over 3-5 revs). Remember that AE is a
measure of the current flowing in the auroral electrojet.
99
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International Space Station As Flown Altitude Profile
420.0
417.5
415.0
412.5
410.0
407.5
405.0
402.5
400.0
397.5
395.0
392.5
390.0
387.5
385.0
382.5
380.0
377.5
375.0
372.5
370.0
367.5
365.0
362.5
360.0
357.5
355.0
352.5
350.0
347.5
345.0
342.5
340.0
337.5
335.0
332.5
330.0
327.5
325.0
322.5
320.0
11/17/98
ISS-3A (STS-92)
M ated Reboost
ISS-1R (SM )
- Rend Burns
ISS-2A2B
DAM
ISS-2A (STS-088)
- M ated Reboost
ISS-2A Test Burns
ISS-2A1 (STS-096)
M ated Reboost
ISS-2A1
DAM
ISS-2A.1
Reboost
ISS-1AR
Insertion
Burns
ISS-2A2B (STS-106)
M ated Reboost
ISS-1P (P 251)
- Test Burn
Reboosts
Mag Storm
ISS-2A2A (STS-101)
M ated Reboost
01/16/99
03/17/99
05/16/99
07/15/99
226.8
225.4
224.1
ISS-5A.1
(STS-102)
222.7
M 221.4
ated Reboost
220.0
218.7
217.3
216.0
214.6
213.3
211.9
210.6
209.2
207.9
206.5
205.2
203.8
202.5
201.1
199.8
198.4
197.1
195.7
194.4 ISS-5A.1
193.0 3P Test
3P Reboost
191.7
3P P hase
190.3
189.0
187.6
186.3
184.9
183.6
ISS-5A
182.2
(STS-098)
DAM
180.9
M ated Reboost
179.5
178.2
176.8
175.5
174.1
172.8
Invariant Alt. (nm)
Invariant Alt. (km) - Ha, Hp, Have
(Based on MCC-M/USSP Tracked SV Data)
09/13/99
11/12/99
01/11/00
03/11/00
05/10/00
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07/09/00
09/07/00
11/06/00
01/05/01
03/06/01
05/05/01
100
The SETA (Satellite Electrostatic Triaxial Accelerometer) experiment
also accurately measured density at low satellite altitudes
(170-240 km).
The
accompanying
figure illustrates the
response of (SETAmeasured) density at
200 km to highlatitude heating (see
Kp scale at bottom).
The
equatorward
penetration of the
"density
bulge" is
assisted
by
the
summer-to-winter
solar-radiation-driven
circulation in the
summer
(northern)
hemisphere,
and
retarded by the solardriven circulation in
the winter (southern)
hemisphere.
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The rapid variations in heating characteristic of the high-latitude region
represent an in-situ source of gravity waves for the thermosphere.
A variety of wave modes are generated, some of which propagate directly to lower
latitudes, some are ducted by the temperature structure of the lower thermosphere, and
some are thought to reflect off the surface of the earth and reenter the thermosphere
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A typical pass of the SETA satellite
reveals a range of wave scales
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Average periodograms for magnetically quiet vs. active conditions reveals a
considerable enhancement of power for ~1000-km scale waves during active
conditions -- these are probably the "direct" waves generated in the auroral
region mentioned previously.
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