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

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• 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