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Ionospheric Morphology

Ionospheric Morphology. Prepared by Jeremie Papon, Morris Cohen, Benjamin Cotts, and Naoshin Haque Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network. What is the Ionosphere?.

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Ionospheric Morphology

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  1. Ionospheric Morphology Prepared by Jeremie Papon, Morris Cohen, Benjamin Cotts, and Naoshin Haque Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

  2. 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

  3. Overview of the Ionosphere Day Night • 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

  4. Solar Activity Variations

  5. 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

  6. 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

  7. Chapman Layers Chapman Geometry • 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

  8. dI = σ n I ds • differential energy absorption • Iis 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

  9. Production Layers 60 30 c = 0 • 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

  10. 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 ne2 • 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

  11. 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

  12. 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

  13. 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

  14. 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+)

  15. 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

  16. Critical Frequency Magnetosphere Microwave Ionosphere MF-HF Waves LF Waves Atmosphere Earth • 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

  17. 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/

  18. Rockets and the Ionosphere Altitude (km) • Launch rocket with instrument • Record ascent and descent data • Advantage: good height resolution • Disadvantage: one-shot deal

  19. 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)

  20. Ionospheric Mapping With GPS

  21. 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

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