Ionospheric Morphology
Prepared by Jeremie Papon, Morris Cohen,Benjamin Cotts, and Naoshin HaqueStanford University, Stanford, CA
IHY Workshop on Advancing VLF through the Global AWESOME
Network
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What is the Ionosphere?
The atmosphere above ~70km that is partially ionized by ultraviolet radiation from the sun This region of partially ionized gas
extends upwards to high altitudes where it merges with the magnetosphere
Discovered in the early 1900s in connection with long distance radio transmissions Scientists postulated, and later
proved, that long distance radio communication was possible due to reflection off of an ionized region in the atmosphere
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Overview of the Ionosphere
Structure of ionosphere continuously changing Varies with day/night, seasons,
latitude and solar activity Essential features are usually identifiable Ionosphere divided into layers, according
to electron density and altitude D Layer (or D Region) E Layer F Layer
Several reasons for distinct layers Solar spectrum energy deposited at
various altitudes depending on absorption of atmosphere
Physics of recombination depends on density of atmosphere (which changes with altitude)
Composition of atmosphere changes with height
DayNight
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Solar Activity Variations
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Atmospheric Composition Profiles
These charts show density of ions and neutral molecules with respect to altitude
Numbers vary slightly due to seasonal/daily variation of atmosphere
Notice that even where electron/ion density peaks, it is still well below the density of neutral molecules That’s why ionosphere is referred to as
weakly ionized plasma
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Ionization of the Atmosphere
Formation of layers can be understood by considering ionization of any molecule (or atom) B in the atmosphere B + hf → B+ + e-
Rate of this reaction will depend on concentration of molecules B and photons hf
At high altitudes there are many photons, but few particles
At low altitudes there are many particles but few photons of sufficient energy to cause ionization
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Chapman Geometry
Chapman Layers
Sydney Chapman used several assumptions to develop a simplified theoretical model Atmosphere consists of only one gas Radiation from the sun is
monochromatic Atmospheric density decreases
exponentially with height Solar radiation is attenuated
exponentially Earth is flat (In order to simplify
geometry)
Each atmospheric species has its own ionization potential and reaction rate Ionosphere can be modeled as
superposition of simple Chapman layers
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dI = σ n I ds differential energy absorption
I is intensity of radiation from sun σ is energy absorption per unit
volume n is particle density
Ionization Rate
Consider cylinder of length ds, end area dA
Suppose p electrons produced by each unit of energy absorbed by molecules
Rate of electrons per unit volume (ionization rate) q
q dA ds = dI p dA
= σ n I ds dA p q = p σ n I
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Production Layers
As sun drops in sky, peak of production layer higher than at midday and overall production is less
Steeper gradient of production vs. height on lower side of layer than upper side
Shape of curve independent of absorption cross section σ
= 0
3060
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Electron Density
• To derive electron density of a layer:
• Combine electron losses with production
•Rate of loss of electrons per unit volume is proportional to ne
2
• In equilibrium q = α ne2
• ne = (ne)max exp {0.5 (1 – y – exp(-y))}
• y = h – hm
H
• H is scale height: vertical distance over which pressure of atmosphere decreases by factor of e
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Limitations of Chapman Law
Effect of magnetic field Collisions Scale height is not constant Assumes steady state
No other ionization sources Constant solar intensity
Gives only qualitative description Severely underestimates nighttime d-region
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Ionospheric Layers
D region (50-90 km) Lowest region, produced by Lyman series alpha radiation
(λ = 121.6 nm) ionizing Nitric Oxide (NO) Very weakly ionized
Electron densities of 108 – 1010 e-/m3 during the day
At night, when there is little incident radiation (except for cosmic rays), the D layer mostly disappears except at very high latitudes
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Ionospheric Layers
E Region (90-140 km) Produced by X-ray and far ultraviolet radiation ionizing
molecular oxygen (O2)
Daylight maximum electron density of about 1011 e-/m3
Occurs at ~100km
At night the E layer begins to disappear due to lack of incident radiation
This results in the height of maximum density increasing
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Ionospheric Layers
F1 Layer (140-200km) Electron density ~3*1011 e-/m3
Caused by ionization of atomic Oxygen (O) by extreme ultraviolet radiation (10-100nm)
F2 Layer (>200km) Usually has highest electron density (~2*1012 e-/m3) Consists primarily of ionized atomic Oxygen (O+) and Nitrogen (N+)
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Why is Study of the Ionosphere Important?
It affects all aspects of radio wave propagation on earth, and any planet with an atmosphere
Knowledge of how radio waves propagate in plasmas is essential for understanding what’s being received on an AWESOME setup
It is an important tool in understanding how the sun affects the earth’s environment
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Critical Frequency
Microwave
MF-HF Waves
LF Waves
Earth
Ionosphere
Atmosphere
Magnetosphere Height at which radio waves reflect is dependent on maximum electron density of a layer
Critical frequency defined as highest frequency reflected for normal incidence
Maximum electron density related to critical frequency by ne = 1.24 * 104 * f2
ne in cm-3
f in MHz
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Ionograms
Ionograms are a plot of the virtual height of the ionosphere vs. frequency (shown here in km vs. Mhz) Show altitude and critical
frequency at which electromagnetic waves at normal incidence reflect
Produced by ionosondes, which sweep from ~ 0.1 – 30 Mhz, transmitting vertically up into the atmosphere
Get real time ionograms online http://137.229.36.56/
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Rockets and the Ionosphere
Launch rocket with instrument
Record ascent and descent data
Advantage: good height resolution
Disadvantage: one-shot deal
Alt
itud
e (k
m)
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GPS and the Ionosphere
GPS signals through ionosphere Linear polarized wave two circularly polarized
waves Angle of rotation proportional to electron density
integrated along path
Network of GPS receivers can map ionosphere by measuring Total Electron Content (TEC)
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Ionospheric Mapping With GPS
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References
Tascione, T., Introduction to the Space Environment, Krieger Pub. Co., 1994.
Ratcliffe, J.A., An Introduction to the Ionosphere and Magnetosphere, Cambridge University Press, 1972.
Fraser-Smith, A., Introduction to the Space Environment: The Ionosphere
Kelley, M. C, and Heelis, R. A., The Earth's Ionosphere: Plasma Physics and Electrodynamics, Academic Press, 1989.
NGDC/STP Real Time Ionograms, available online http://www.ngdc.noaa.gov/stp/IONO/grams.html