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Title: The Upper Ionosphere An introduction


1
The Upper IonosphereAn introduction
  • XIAO ZUO
  • Department of Geophysics,
  • Peking University ,
  • Beijing,100871, China
  • Email zxiao_at_pku.edu.cn

2
(No Transcript)
3
Space EnvironmentThe Earths Upper
AtmosphereThe Ionosphere and ThermosphereThe
Magnetosphere and magneto-pauseSolar Wind and
Interplanetary MediumSolar Corona
4
Upper Atmosphere and the nomenclature system
5
Main Contents
  • Production of ionization and formation of
    ionospheric sub-regions (layers)
  • Dynamics of upper atmosphere and TIDs
  • Electrodynamics of the ionosphere
  • Ionospheric measurements
  • SID and ionospheric storms

6
The Ionosphere
  • The upper atmosphere, beginning at about 50 km
    altitude, is partially ionized by ultraviolet and
    x-ray radiation from the sun. This region of
    partially ionized gas extends upwards to about
    1000 km altitude. The region is termed the
    ionosphere. The ionosphere is important as a
    source of plasma for the magnetosphere, and as a
    medium which reflects radio waves at frequencies
    from a few Hz up to several Megahertz. Like the
    rest of the earth-sun system we have explored it
    is a dynamic region with an amazing variety of
    features.

7
A brief history of study
  • Daily variations of geomagnetic field found
  • Trans-Atlantic telecommunication
  • Vertical sounding
  • Rocket and in situ measurements
  • Effects of electro-magnetic waves
  • Topside, Time delay, Refraction and
    reflection, Faraday rotation, Doppler shift,
    Absorption, .

8
How to define the term ionosphere?
  • Part of the upper atmosphere, where sufficient
    number of electrons exist to influence the radio
    wave propagation.
  • Partially ionized medium, motions of ionization
    still controlled (or, affected) by neutral
    components of the air.
  • Thus distinguished from magnetosphere

9
Regions in the ionosphere
  • D-region 70-95km
  • E-region 95-120km
  • F1-region 120-160km
  • F2-region above 160km, peak at 200-300km
  • During the night, in general, only F2 layer is
    there
  • How the ionosphere is produced and sub-layers
    formed? Equilibrium of Production, Loss and
    Movements of ionization

10
The profile of electron number density in the
ionosphere
IRI-display
11
Ionizing potential (1ev1.6x10-19 J)
  • Constituent Ionizing potential ?max (A)
  • NO 9.25
    1340
  • O2 10.08
    1027
  • H2O 12.60
    985
  • O3 12.80
    970
  • H 13.59
    912
  • O 13.61
    911
  • CO2 13.79
    899
  • N 14.54
    853
  • H2 15.41
    804
  • N2 15.58
    796
  • A 15.75
    787
  • Ne 21.56
    575
  • He 24.58
    504
  • An atmospheric component can only be ionized by
    radiation with wavelength shorter than ?max
  • So the real air is ionized by X-ray and extreme
    ultra-violet radiation (1-170A) and (170-1750A)

12
Production of ionization
  • Molecules and atoms are ionized by absorbing
    certain amount of energy from the solar radiation
  • The cross-section of absorption for each
    composition of air is needed (s)
  • The electron numbers released for unit energy
    absorbed by one molecule/atom is defined as the
    efficiency of ionization for that component (? )

13
A schematic diagram
14
Some simplifying assumptions
  • The radiation is monochromatic with photon flux
    I(h)
  • The atmosphere consists of a single absorbing
    gas, its concentration being n(h)
  • The atmosphere is plane and horizontally
    stratified
  • The scale height H is either independent of
    height or varies linearly with height

15
Equation set for production rate
Define dI/Idtsnds
16
Chapmans Formulae
17
Normalized Chapman production rate versus
reduced height, z, parametric in solar zenith
angle ?, at the equator
18
Production rate at noon-time from a simple
Chapman model Seasonal and solar cycle variations
19
Continuity equation of Ne
  • Rate of change of electron concentration
  • Gain by production
  • -Loss by destruction
  • -Change due to transport

20
E and F region photo-chemistry
  • Photo-ionization Transfer reaction
    Recombination
  • Two types of direct recombination(Electronion)
  • Atomic ions very slow
  • molecular ions much fast
  • Need some transfer reactions

21
Some special notes on ionospheric photo-chemistry
  • O can not recombine directly.(to conserve both
    momentum and energy need to emit photon(very
    slow) or 3-body collision(too rare in E, F
    region)
  • N2 is virtually absent need reaction with O as
    well as recombination in order to explain
  • Recombination may leave (mainly) O atoms in
    excited state, giving airglow
  • If all species are molecular, recombination is
    direct
  • When atomic species exist, situation is
    complicated

22
A much simplified scheme
  • Only O,N2 are considered, O is ionized as O and
    e with rate q
  • O transfer its charge to N2 to produce NO
  • NO is recombined directly with e
  • O gt O gt NO gt N, O

23
Simplified scheme-continue
  • Under photo-chemical equilibrium
  • qkON2aNOe

24
qkON2aNOeThere are two special cases
  • At lower heights
  • Plenty of molecules
  • O converted to NO as soon as it is formed
  • Nearly all ions are NO
  • Neutrality requires
  • qaNOe ae2
  • At higher altitudes
  • Molecules are scarce
  • O converted to NO very slowly,but NO recombine
    quickly
  • Nearly all ions are O
  • qkON2kN2e

25
F1-layer A transition
  • At intermediate heights
  • Solution of this quadratic equation
  • For Ggtgt 1, a type and
  • if G ltlt1, we have type

26
A comparison of q and N
27
F1 layer (ledge)
28
Transition height of atype toßtype F1 layer

29

Photo-chemical equilibrium no longer valid


Above the last peak of q,q will decrease for
ever,positively proportional to
n(O), Meanwhile,ß(h) decreases upwards
proportional to n(N2) Since the rate of
decreasing ofß(h) upwards is faster than
production rate q,electron density will increase
with height following
This is because

30
Photo-chemical equilibrium E and F region
  • Red curve is q with 2 peaks (relative values)
  • At 105km, peak of q coincide with N
  • F1 not obviously appears in this example
  • Above about 220km, N increases forever,
    considering only P-H equilibrium
  • F2 peak needs an explanation

31
Effect of motion of ionization on equilibrium
profile of electrons
  • Above F1 region, photo-chemical equilibrium is no
    longer valid,motion of ionization is important
  • the last term will take important role if
  • a), N varies significantly within a distance V/
    or
  • b), The spatial variation of V is sufficiently
    rapid

32
Plasma Ambipolar diffusion
  • Motions of Charged particles
  • Neutrality NiNe, AmbipolarViVe,Sum of the
    above two
  • Under equilibrium and iso-thermal condition

33
Effect of Ambipolar diffusion
34
Morphology of the Background Ionosphere
  • Daily variation
  • Day-to-day variation
  • Seasonal Variation
  • Annual (half annual) variation
  • Solar cycle
  • Latitudinal and longitudinal, regional

35
Ionospheric D-region
  • This is the lowest region of the ionosphere and
    is thus produced by the penetrating component of
    the incident radiation, namely short wavelength
    ultraviolet (Lyman a with ? 121.6 nm) and
    x-rays.
  • The D region is formed primarily by the
    ionization of the trace atmospheric constituent
    NO ( NO 107 cm-3 as compared with N2
    1014 cm-3 at 85 km) by the relatively intense
    Lyman-a radiation (I8 3.3 1011 photons/cm2
    sec) from the sun. Ionization of N2 and O2 by
    solar x-rays is a secondary process. The
    contribution of this latter process is small
    except during a solar flare. The dominant loss
    process is the dissociative recombination of
    electrons with various molecular positive ions.
  • Electron Density
  • The electron density increases from about 100
    electrons/cm3 at 60 km to about 104 electrons/cm3
    at 90 km, around noon. It is greater in the
    summer than in the winter, and greater at sunspot
    maximum. At night, when there is no incident
    radiation, the electrons quickly recombine with
    the molecular positive ions, so that the D-region
    disappears, except at latitudes greater than
    about 65o, where particle bombardment sustains
    the ionization.

36
Anomalies of D-region
  • Sudden Ionospheric Disturbance (SID)
  • During a solar flare, the electron density in the
    D-region increases by a large factor as a result
    of the considerable increase in the solar hard
    x-rays of wavelength less than 10 Å. This
    increase is a factor of 100 to 1000 depending on
    the severity of the flare. Since the increased
    electron
  • density occurs at altitudes where the electron
    collision frequency is high, radio waves
    propagating in the ionosphere are almost
    completely absorbed, and high-frequency (HF)
    communications are disrupted over the sun-lit
    hemisphere. This is sometimes called a radio
    blackout. A related phenomenon is the PCA event.
  • Polar Cap Absorption (PCA)
  • Polar Cap Absorption is the name given to the
    severe attenuation suffered by a HF radio wave
    propagating in the ionosphere at a high latitude
    near the polar caps, in the daytime or at night,
    soon after a solar flare. It is caused by the
    high flux of solar protons emitted during a large
    flare, and deflected to the polar regions by the
    geomagnetic field.

37
Ionospheric E-region
  • E layer 90km-140 km
  • This is the best understood region of the
    ionosphere, and the first layer identified in
    ionospheric research (It was the electric layer
    - hence the e-layer)
  • Formation
  • The E-region is formed primarily by the
    ionization of O2. The solar radiations primarily
    responsible for the ionization are Lyman-ß of
    wavelength 1025.7 Å (I8 3.6 109 photons/cm2
    sec), and the CIII line of wavelength 977 Å . An
    additional production process is the ionization
    of N2 (and O2) by X-rays of wavelength in the
    range 10-100 Å.
  • The N2 ions are converted to O2 and NO ions by
    rapid charge exchange. The net charge production
    rate is about 104 to 105 electrons/cm3 sec at 105
    km for ? 10o. At high latitudes, particle
    radiation makes a significant contribution to the
    ionization at all hours. The dominant ions in the
    E-region are O2 and NO, so that the dominant
    loss process is the dissociative recombination of
    the electrons with these ions.

38
E-region-2
  • The peak electron density around noon, at equinox
    at the equator (i.e., ? 0o) is ?2 105 el/cm3.
    The diurnal, seasonal and latitudinal variation
    is in approximate agreement with the Chapman
    theory. The height of the peak varies with ? in
    agreement with the theory, with h0? 105 km and H?
    8 km. Chapman theory shows that the production
    rate, defined by qmax, is linearly proportional
    to cos?. If the dominant loss process is
    dissociative recombination, i.e., q aNe2 , then
    Ne max should vary as cos(0.5?). The maximum
    electron density is found by experiment to vary
    with ? as cos(0.6?). The slight difference in the
    exponent (0.6 versus 0.5) can be accounted for by
    the height variation of the scale height and of
    the recombination coefficient. Note that the
    functional dependence implied by ? can be either
    time of day or latitude. Based on a large number
    of measurements, the solar-cycle variation of the
    electron density may be expressed by
  • (Ne)maa(10.004Rz)
  • where Rz is the sunspot number and a 1.3x105
    el/cm3 at ? 0 ('a' varies slightly from month
    to month)
  • The E-region persists even during the night, with
    electron densities in the range 500-10,000
    electrons/cm3. The nighttime E-region is thought
    to be maintained by solar extreme ultraviolet
    (EUV) radiation, primarily Lyman-a and Lyman-ß
    which has been scattered from the exospheric
  • hydrogen - e.g. the geocoronal glow.

39
Disturbances (Anomalies) of E-region
  • Within the E-region, local enhancements in
    electron density are frequently observed. These
    are known as sporadic-E, or Es. Ground-based
    observations (global network of ionosondes) show
    that Es is more prevalent in summer than in
    winter, at mid-latitudes. At the geomagnetic
    equator (actually the magnetic dip equator), Es
    is observed mainly in the daytime, throughout the
    year. Rocket-borne experiments have shown that,
    at mid-latitudes, Es is a thin layer, of
    thickness in the order of a few hundred meters,
    with Ne greater than (Ne)max of the E-layer. The
    processes involved in the production of Es are
    rather complicated.

40
The ionospheric F-region
  • 140km-1000km
  • This is the region that is primarily responsible
    for the reflection of radio waves in
    high-frequency communication, broadcasting, and
    OTHR (over-the-horizon radar) - hence the most
    important of the ionospheric regions.
  • Formation
  • The primary production process in the F-region is
    the ionization of atomic oxygen, O, by solar
    radiation of ? lt 911 A
  • . The spectral bands responsible for the
    ionization are the Lyman
  • continuum 800 - 910 Å (I81.01010 ph/cm2 sec)
    the wavelength range 200-350 Å , including the
    strong He II line at 304 Å (I81.5 1010 ph/cm2
    sec) and the wavelength range 500-700 Å . A
    secondary production process is the ionization of
    molecular nitrogen and oxygen by solar radiation
    of ? lt 796 Å . Peak electron production occurs in
    the height range of 160-180km, but the peak Ne
    occurs at a greater height (see below).
  • The primary positive ions produced by the above
    radiations are O, N2 and O2. Various chemical
    process then convert these ions into different
    ions, so that the observed dominant ions are O,
    NO and O2 in the height range 140-200 km (F1),
    and O in the height range 200-400 km (F2).
  • The F-region divides into two layers, called F1
    and F2, particularly in the summer in the
    daytime. The F1-layer, forms a ledge in the
    electron density profile at the bottom of the
    F2-layer.

41
The loss process of F-region
  • The main loss process in the F1-layer (
    140-200km) is the dissociative recombination of
    the electrons with the molecular positive ions.
    In the F2-layer ( 200km to 400km) the main
    chemical loss process is a two-stage process in
    which ion-molecule reactions first convert O to
    the molecular ions NO and O2 by the reactions
  • O N2 ? NO N and O O2 ? O2 O,
  • and the electrons then recombine dissociatively
    with these ions. This loss process obeys a linear
  • law, i.e., L ß(h) Ne, with the loss coefficient
    ß(h) decreasing with height faster than the
    production q(h) decreases with height, so that
    the electron density increases with height above
    the level of peak production (as noted earlier).
    The F2-peak is formed by the combined action of
    this linear loss process and the transport of
    electrons by plasma diffusion. In fact, the
    transport of electrons by various processes plays
    an important role in the morphology of the
    F2-layer.
  • Above 500 km, significant amounts of the hydrogen
    ion H are observed and, at heights greater than
    about 1200 km, H is the dominant ion, especially
    at night.

42
Electron Density of F-region
  • The maximum electron density of the
    F1-layer is about 3105 el/cm3, around noon. The
    layer disappears at night. The peak of the
    F2-layer is about an order of magnitude higher in
    density. The diurnal, seasonal and latitudinal
    variation of the electron density and height of
    the F2 layer are NOT in accordance with the
    ?-variation. For instance, Ne max is
  • (a) greater in winter (2.5106 at Rz 200) than
    in summer (7 105 at Rz 200). This is called
    the 'winter anomaly',
  • (b) greater on either side of the equator than at
    the equator - the Appleton (equatorial) anomaly,
    and
  • (c) greater before noon, or afternoon, than at
    noon in the summer.
  • Points a and c are illustrated in Figure 7.6,
    which shows how winter densities are higher than
    summer, and shows the increase in electron
    density with increasing solar activity.

43
Winter Anomaly of F2-region
Typical electron density profiles at three
(Zurich) sunspot epochs for winter and summer
noon conditions at Washington DC (Belvoir).
Magnetically quiet days were chosen for the above
profiles. J. W. Wright, Dependence of the
Ionospheric F Region on the Solar Cycle, Nature,
page 461, May 5, 1962.
44
From IRI-90
45
Disturbances (Anomalies) Of F-region
  • The departures from a simple solar control of the
    electron density in the F2-region are referred to
    as anomalies. These anomalies are the result of
    several factors
  • a) marked seasonal and solar-cycle variation of
    the temperature and consequently the scale height
    H,
  • b) seasonal variation of the O/O2 and O/N2
    ratios,
  • c) transport of electrons by diffusion, winds
    (global atmospheric circulation),
  • electromagnetic forces ('fountain effect')
    etc.
  • d) transport of electrons between geomagnetic
    conjugate points.
  • The F2-region is also disturbed by solar flares.
    About 24-36 hrs after a solar flare, particle
    radiation from the sun causes (Ne)max in the
    F2-layer to increase for a few hours at high
    latitudes. This is followed by a decrease which
    lasts for several hours. Such ionospheric
    disturbances could seriously affect
    high-frequency communication and broadcasting, is
    called as ionospheric storms

46
Ionosphere morphology - Latitude Structure
  • The ionospheric structure described above is
    descriptive of the mid-latitude ionosphere. Near
    the magnetic equator, and at high latitudes,
    there are substantial deviations from the simple
    forms described by Chapman theory. These are
    largely due to the influence of the magnetosphere
    on the earths upper atmosphere. The most
    prominent feature of the ionosphere is the aurora
  • There is also a substantial structure at lower
    latitudes. curving from 20 latitude to near
    the equator at the right side. This structure is
    due to the solar driven upwelling of the neutral
    atmosphere at the equator, which in turn forces
    an upward flow of the plasma there. This has been
    termed the equatorial fountain. The plasma drifts
    back downward 5-10 degrees away from the equator.

47
Aurora and Equatorial anomaly
Auroral imagery from DE-1. This image was taken
on 8 November 1981 by the Dynamics Explorer 1
Spin-Scan Auroral Imager. The filter used here
passed the OI wavelengths of 130.4 and 135.6 nm
(far UV). A coastline map was superimposed on the
image, which was taken at an altitude of several
earth-radii above the northern polar cap. The
illuminated hemisphere is to the left - the polar
region is largely in darkness. These images were
the first to show an uninterrupted look at the
entire auroral oval. Figure is from Plate 1 of,
Images of the Earths Aurora and Geocorona from
the Dynamics Explorer Mission, L. A. Frank, J. D.
Craven, and R. L. Rairden, Advances in Space
Research, vol. 5, No. 4, pp. 53-68, 1985.
Equatorial density profiles. The profiles shown
here were obtained from the IRI-95 model, run for
22 September 1995. The resulting latitude
profiles were plotted beginning at the bottom
with a profile at 160 km altitude. Subsequent
plots were offset upward by 0.25106 (up to
320km) and then by 0.5106. The zero points are
indicated by the short horizontal lines leading
from the plot to the altitude labels
48
Upper atmosphere Dynamics
49
Forces acted on an air cell
50
Thermospheric wind
51
Continue
52
Buoyancy frequency
53
Fundamental equations
54
Linearization
55
Continue
56
Dispersion relation
57
Continue
58
Group velocity
59
Electro-dynamics in the ionosphere
  • Mid- and low latitudes

60
Some notes
  • There is electrical field and neutral wind which
    make the charged particles to move, this motion
    is influenced by geomagnetic field and collision
    with neutrals
  • At quasi-equilibrium, inertia of electrons and
    ions ignored, gravity ignored, too.

61
Motions of electrons and ions
62
Mobility
63
Special cases
64
Electric conductivity
65
Electric current
66
Cowling conductivity
67
Wind effects
68
Wind effects-2
69
Wind effects-3
70
Ionospheric propagation of radio waves and
ionospheric measurements
71
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72
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73
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74
Reflection of radio wave
75
Reflection-2
  • To summarize For vertical incidence reflection
    will occur at a level in the ionosphere where
  • (a) n 0 Index of refraction vanishes
  • (b) f fp radio frequency plasma frequency
  • (c) vp 8 Phase velocity becomes infinite
  • (d) vg 0 Group velocity velocity of energy
    transport becomes 0.

76
Ionospheric effects on
telecommunications
  • BackgroundGlobal SW,daily?seasonal?annual and
    solar cycle
  • AnomalyTrans-equatorial,absorption anomaly
  • Irregularities Scattering and scintillation
  • Sudden disturbancesBlack-out,atmospherics, phase
    and frequency shift (time-keeping)
  • StormsUseable frequency changes
  • Artificial modulation and non-ionospheric
    effectsThunder-storm,volcano, severe
    weather,Nuclear exploit,very powerful radar

77
Non-linear effects
  • Abnormal absorption
  • Heating to modulate the ionosphere
  • Focus and defocus

78
Ionospheric measurements
  • Langmuir probes and ionmass spectrometes
  • Ionosondes
  • GPS signals TEC and radio occultation
  • Incoherent backscatter radar

79
Ionograms
80
Ionospheric Irregularities
  • E and F Region

81
Equatorial E region
82
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83
contunue
84
Discussion on the growth rate
Criteria for a positive growth rate
85
F region
86
Continue
87
Continue
88
Continue
89
Continue
90
Computer Simulation
91
Observations of equatorial electro-jet
irregularities
92
Spread-F
  • First identified by Booker and Wells (1938)
  • Spread F (ESF) originally referred to spread in
    the ranges of the F layer trace in a nighttime
    equatorial ionogram
  • Different observational systems ionosondes, IS
    radars, etc.
  • Scale sizes from l0s of centimeters to 100s of
    kilometers

93
ESF and Bubbles
94
Computer Simulation of Bubbles
95
Scintillation distribution
96
SID and Ionospheric Storms
97
Some Frontiers in recent ionospheric research
  • Couplings between magnetosphere and ionosphere
  • Couplings between ionosphere and lower
    atmosphere-lithosphere
  • Mechanism of anomalies and irregularities
  • Global and local features of the ionosphere
  • Modeling and predictions

98
References
  • Introduction of ionospheric physics, Henry
    Rishbeth and Owen K. garriott, Academic press,
    1969
  • The upper atmosphere and solar-terrestrial
    relations, J.K.hargreaves, 1979
  • The earths ionospherePlasma physics and
    electrodynamics, Michael C. Kelley, 1989
  • IonospheresPhysics, plasma physics, and
    chemistry, Robert W. Schunk and Andrew F Nagy,
    2000
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