Transcript ESS 154 Solar Terrestrial Physics

Ionosphere and Neutral Atmosphere
• Temperature and density structure
• Hydrogen escape
• Thermospheric
variations and
satellite drag
• Mean wind structure
Tropo
(Greek: tropos);
“change”
Lots of weather
Strato
(Latin: stratum);
Layered
Meso
(Greek: messos);
Middle
Thermo
(Greek: thermes);
Heat
Exo
(greek: exo);
outside
Variation of the density in an
atmosphere with constant
temperature (750 K).
• 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 Marconi’s 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.
• The ionosphere vertical density pattern shows a strong
diurnal variation and a solar cycle variation.
• Identification of ionospheric layers is related to inflection
points in the vertical density profile.
Region
Primary Ionospheric Regions
Altitude
Peak
Density
D
60-90 km
E
90-140 km
F1
140-200 km
F2
200-500 km
Topside
above F2
90 km 108 –1010 m-3
110 km Several x 1011 m-3
200 km Several 1011-1012 m-3
12 -3
300 km Several x 10 m
• 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.
•
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.
•
•
•
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
– Recombination removes free
charges and transforms the
ions to neutral particles.
•Neutral density exceeds the ion density
below about 500 km.
At 80-100 km, the time constant for mixing is more efficient than
recombination, so mixing due to turbulence and other dynamical
processes must be taken into account (i.e., photochemical
equilibrium does not hold).
Mixing transports
O down to lower
(denser) levels
where recombination proceeds
rapidly (the "sink"
for O).
O Concentration
After the O recombines to produce O2, the O2 is transported upward
by turbulent diffusion to be photodissociated once again (the
"source" for O).

•
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 d  n   ds
where n(z) is the neutral gas
concentration,  is the frequency
dependent photo absorption cross
section, 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.)
– ds  sec  dz (where  is the
zenith angle of the incoming solar
radiation.
– The altitude dependence of the
solar radiation flux becomes



  ( z )    exp  sec     n( z ' )dz' 
z


where   is the incident photon
intensity per unit frequency.
–
  sec    n( z' )dz'
z
is called the
optical depth.
–
There is usually more than one
atmospheric constituent attenuating the
photons each of which has its own cross
section.

  sec     t n( z' )dz'
t
z
 ( z)    exp( )
•
The density (ns) of the neutral
upper atmosphere usually obeys a
hydrostatic equation
dp
d (nkT )
nmg  

dz
dz
•
where m is the molecular or atomic
mass, g is the acceleration due to
gravity, z is the altitude and p=nkT
is the thermal pressure.
If the temperature T is assumed
independent of z, this equation has
the exponential solution
n  n0 exp
•
 ( z  z0 )
H
where H  kT mg is the scale
height of the gas, and n0 is the
density at the reference altitude z0.
For this case

For multiple species
  sec    t nt ( z ) H t
•
The optical depth increases
exponentially with decreasing
altitude.
In the thermosphere solar radiation
is absorbed mainly via ionization
processes. Let us assume that   i
Each absorbed photon creates a
new electron-ion pair therefore the
electron production is
Si ds  n vi ( z)ds
where Si is the total electron
production rate (particles cm-3s-1).
•
•
 z' z0 
 dz'  sec   n( z ) H
H


   sec     n0 exp
z
•
t
•
Substituting for n and  (z) gives
 z  z0
 z  z0  
Si  n0  i  exp 
 sec   i Hn0 exp
  •
H
H



•
The altitude of maximum
ionization can be obtained by
•
•
•
Choose z0 as the altitude of
maximum ionization for
perpendicular solar radiation
z0  z max  0
This gives

z
 z 
Si  S0 exp1   sec  exp  
 H 
 H
 
e 1
H
This is the Chapman ionization
function.
•
looking for extremes in this
equation by calculating
dS i
0
dz
This gives
sec i Hn( zmax )   ( zmax )  1
where S0 
The maximum rate of ionization is
given by
Smax  S0 cos 
•
If we further assume that the main
loss process is ion-electron
recombination with a coefficient 
and assume that the
recombination rate is  ne2
•
Finally if we assume local
equilibrium between production
and loss we get S   n 2
i
e
•
The vertical profile in a simple
Chapman layer is
ne 
•
1
z sec 
 z 
exp 

exp  

2
 H 
 2 2H
S0
The E and F1 regions are essentially
Chapman layers while additional
production, transport and loss
processes are necessary to
understand the D and F2 regions.
• 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 and Lyman-  ionization of the NO molecule.
– Precipitating magnetospheric electrons may also be important.
– The primary positive ions are O2+ and NO+
– The most common negative ion is NO3• 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
N2  O2  O2  N2
N 2  O  NO   N
N 2  O  O   N 2
– Oxygen ions are removed by the following reactions
O   N 2  NO   N
O   O2  O2  O
• The F1 Region
– Essentially a Chapman layer.
– The ionizing radiation is EUV at <91nm.
– 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.
• First atom ion interchange takes place
O   N 2  NO   N
O   O2  O2  O
• This is followed by dissociative recombination of O2+ and NO+
O2  e  O  O
NO   e  N  O
•
The F2 Region
– The major ion is O+.
– This region cannot be a Chapman layer since the atmosphere above the F1
region 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
O  h  O   e
– The oxygen ions are lost by a two step process
• First atom-ion interchange
O   O2  O2  O
O   N 2  NO   N
• Dissociative recombination
O2  e  O  O
NO   e  N  O
– 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.
Radio Sounding Principles
• Waves in a plasma experience reflection and refraction
• Radio waves are reflected at wave cutoffs (n = 0)
• Echoes are received if the gradient at the reflection point is
normal to the incident signals.
• The echo frequency gives the plasma conditions of the reflection
point, and the time delay gives the distance of reflection point.
• From a seriesn>0
of sounding
frequencies,
a density profile can be
Refracted
rays
n=0
obtained n<0
SOUNDER
Echo
Reflected ray
Refracted rays
Ionosonde Field of View and ISR Radar
Pencil Beam
Specular Reflection and Scatter
Radio Sounding:
specular reflection
wide beam
Scatter Radar:
Scatter, pencil beam
Ionosonde
Modern Ionosonde and Transmit Antenna
Digisonde DPS
Transmit antenna
Quiet Daytime Mid Latitude
Ionogram
Electron density profile
The Digisonde Cachimbo
Profilogram
16 October 2002
midnight
noon
Ionospheric Drift Measurements
• Fourier analysis for Doppler spectra
• Interferometry with spaced receive antennas
Digisonde Skymaps
Real Time Digisonde F Region Drift
Digisonde Network
• 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
qE  mi inui


 eE  me enue
where  in is the ion neutral collision frequency and  en is the electron
neutral collision frequency.


– For this simple case the current will be related to electric field by j   0 E
where  0 is a scalar conductivity.
• If there is a magnetic field there are magnetic field terms
in the

momentum equation. In a coordinate system with B along the z-axis
the conductivity becomes a tensor.
 P

   H
 0

 H
P
0
0

0
 0 
• Specific conductivity – along the magnetic field
 1
1 


  e me  i mi 
 0  e 2 ne 
• Pedersen conductivity – in the direction of the applied electric field

i
1
1 


2
2
2
2
  e  e me  i  i mi 
e
 P  e 2 ne 
• Hall conductivity – in the direction perpendicular to the applied field

e
i
1
1 


2
2
2
2



m



m
e
e
i
i
i 
 e
 H  e 2 ne 
where e and  i are the total electron and ion momentum transfer
collision frequencies and  e and i 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  0   all field lines would be equipotentials.
• The total current density in the ionosphere
 is




B E
j   0E   PE   H
B
• 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.
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.
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.
Average Temperature Profiles
for Earth, Mars & Venus
Mars
Venus
night
day
Venus
Earth
Formation of Ionospheres
Photo ionization: If h  I M
h  M  M  e  E photoelectron

In the terrestrial ionosphere:
h  N 2  N 2  e  E photoelectron
h  O2  O  e  E photoelectron

2
h  O  O   e  E photoelectron
( I M  10 to 20 eV )
HYDROSTATIC EQUILIBRIUM
If …..
n = # molecules per unit volume
P + dP
m = mass of each particle
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 = g nmdh
and the difference in pressure
between the lower and upper
faces of the cylinder balances
the above force in an equilibrium
situation:
dP
nmgdh
P dP  P  nmgdh
P

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
This is the so-called hydrostatic law or barometric law.
Integrating,
PPe
0
z
where
z dh
z
0H
and z is referred to as the "reduced height" and the subscript zero
refers to a reference height at h=0.
 
To  z
nn
e
o T
Similarly,
For an isothermal atmosphere, then,
nn e
o
h
H
  o e
h
H
PPe
o
h
H
Exponential decrease of photon flux
At top of atmosphere F  F .
dF    nn F ds    F nn dz sec 
The photoabsorption cross section   10 17 cm 2  10 13 m 2



F  z   F exp  sec     nn dz   F e   ,
z



where    z   sec     nn  z  dz
z
The production rate at height z is
P ( z )    nn F  z   F  nn  z  e
   z 
.
In an isothermal atmosphere, if we assume equilibrium ,
i.e. production Pe = loss Le , and Le  kd ne2 :
1

ne  z   ne  zm  exp  1    sec  e    ,
2

z  zm
 
Hn
Chapman 1932
zm is the height of maximum production (  1).
Absorption of Solar Radiation vs. Height and Species