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  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 Day Night

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 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

8  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

9 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  30  60 

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 n e 2 In equilibrium q = α n e 2 n e = (n e ) max exp {0.5 (1 – y – exp(-y))} y = h – h m 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 = nm) ionizing Nitric Oxide (NO)  Very weakly ionized  Electron densities of 10 8 – e - /m 3 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 ( km)  Produced by X-ray and far ultraviolet radiation ionizing molecular oxygen (O 2 )  Daylight maximum electron density of about e - /m 3  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 ( km)  Electron density ~3*10 11 e - /m 3  Caused by ionization of atomic Oxygen (O) by extreme ultraviolet radiation (10-100nm)  F2 Layer (>200km)  Usually has highest electron density (~2*10 12 e - /m 3 )  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 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  n e = 1.24 * 10 4 * f 2  n e 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 

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

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.,  Ratcliffe, J.A., An Introduction to the Ionosphere and Magnetosphere, Cambridge University Press,  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,  NGDC/STP Real Time Ionograms, available online