Transcript Ionospheric Morphology An Introduction

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?
 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
2
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
3
Solar Activity Variations
4
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
5
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
6
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
Chapman Geometry
7
Ionization Rate
 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





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=pσnI
8
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
60
30
c = 0
9
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
10
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
11
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
12
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
13
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+)
14
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
15
Critical Frequency
Magnetosphere  Height at which
Microwave
LF
Waves
radio waves reflect
is dependent on
maximum electron
density of a layer
Ionosphere
 Critical frequency
defined as highest
MF-HF
frequency reflected
Waves
for normal incidence
 Maximum electron
density related to
Atmosphere
critical frequency by
 ne = 1.24 * 104 * f2
 ne in cm-3
Earth
16
 f in MHz
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/
17
Altitude (km)
Rockets and the Ionosphere
 Launch rocket
with instrument
 Record ascent and
descent data
 Advantage: good
height resolution
 Disadvantage:
one-shot deal
18
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)
19
Ionospheric Mapping With GPS
20
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
21