Aerospace EnvironmentASEN-5335

Read Chapter 7 and class notes. Quiz # 7 (the last one) on 4/29, close book. Homework # 7 (the last one) due 5/1. Final on 5/7, close book.

About HW7.

Data for this Assignment

The data for this assignment consists of dayside (1030 LT) and nightside (2230

LT) densities measured at 61.5? N and 1.5? N during July 20-27, 1983, and the

3-hourly Kp values during this interval. You can obtain the data from here.

Note: Please read "read.me" file that explains the observed values, adjusted

values, and absolute values and use values from "1983.adj" file for this

assignment. Note that numbers listed here are all multiplied by ten.

Ionospheres

1. What is an ionosphere ?

2. How is an ionosphere formed ?

3. What determines the peak density and height of an ionosphere ?

4. How is aurora related to ionospheric formation and decay ?

5. What are the differences and similarities between the ionospheres of Earth, Mars and Venus ?

The Ionosphere is a Weakly Ionized Plasma

broad definition: “the ionosphere is that region of the atmosphere (or gaseous envelope) surrounding a solar system body where significant numbers of low-energy free electrons and ions are present”

Neutral density far exceeds the electron or ion density

Existence of ionosphere suggested -- by Gauss, Lord Kelvin and Stewart Balfour in the 19th century

First direct verification of its existence -- Marconi succeeded in sending radio signals across the Atlantic

‘ionosphere’ coined by R.A. Watson in 1926

First direct evidence of an ionosphere on a planet other than earth -- radio occultation measurements by Mariner 5 as it flew by Venus on October 19, 1967

1874: born in Italy

1895: 1.5 mile wireless

1899: cross English Channel

1901: cross Atlantic ocean

1909: Nobel Prize in Physics

Formation of Ionospheres

Free electrons and positiveions can be formed by

a) photoionization of neutrals

b) energetic particles knocking electrons off neutrals

There are ions and electrons at all altitudes of the terrestrial atmosphere. Below about 60 km thermal charged particles (which have comparable energies to the neutral gas constituents) do not play any significant role in determining the chemical or physical properties of the atmosphere.

Above 60 km, the presence of electrons and ions becomes increasingly important. This region of the upper atmosphere is called the ionosphere.

The Ionosphere

The typical vertical structure of the ionosphere is shown on top-right: strong diurnal variation and solar cycle variation.

The identification of the atmospheric layers is usually reflected to inflection points in the vertical density profile: The main regions are local minimums.

Figure on the lower-right shows the typical composition of the dayside ionosphere under solar minimum condition.

At low altitudes the major ions are O2+ and NO+, near

the F2 peak it changes to O+, and in the topside ionosphere eventually H+ becomes dominant.

The ionosphere is formed by the ionization of three primary atmospheric constituents: N2, O2, and O. The primary ionization mechanism is photoionization by extreme ultraviolet (EUV) and X-ray radiation. In some regions ionization by precipitating magnetospheric particles or solar particles and cosmic rays may also be important.

The primary ionization process is usually followed by a series of chemical reactions, which produce other ions while preserving the total number of electrons. Finally, recombination removes free charges and transforms the ions to neutral particles.

Dayside-Ionospheric composition

Dayside-Ionospheric composition, compared with neutral particles

Consider electromagnetic radiation entering an atmosphere and being gradually weakened by various atomic and molecular interactions (such as photoionization). Let us denote the photon flux per unit frequency by . The change of flux, d, due to absorption by the neutral gas in an infinitesimal distance, ds, is:

d= - n ds, (1)

where n(z) is the altitude-dependent neutral gas concentration, is the frequency-dependent photoabsorption cross section, and ds is the path length element in the direction of the optical radiation (see figure below).

Ionization Profile

Equation (1) assumes that there are no local sources or sinks of radiation. This condition is usually satisfied in the dayside upper atmosphere for the ionizing UV radiation.

It is obvious from the figure that ds=sec dz. Equation (1) can be readily solved to obtain the altitude dependence of the solar radiation flux:

(z) = (2)

The altitude of maximum ionization can be obtained by solving the following equation:

dSi/dz=0 sec i H n(zmax) = (zmax)=1,

where zmax is the altitude of maximum ionization. It can be seen that ionization is maximum at the altitude where the optical depth is unity. We can choose z0=zmax|=0 =0 (the altitude of maximum ionization for perpendicular solar radiation), and obtain:

S=S0exp(1-z/H-exp[-z/H]) , where S0= (/H)e-1, this is the classical Chapman formula. It predicts the ionization profile due to the absorption of solar radiation in an isothermal atmosphere.

Ionization Profile

In the thermosphere solar radiation is absorbed mainly via ionization processes. Let us assume that ~ i and calculate the electron production rate profile. In this approximation each absorbed photon creates a new electron-ion pair, and therefore the electron production in a ray-path element, ds, is: Si ds =n i (z) ds, where Si is the total electron production rate. Substituting the altitude dependence of the concentration and the photon flux (Equation (5) and nn0exp[- (z-z0)/H]), we obtain:

Si =

Altitude Profile of Electron Density

since an electron and an ion must come together in order to recombine, and we assume that the ion and electron concentrations are equal. Neglecting transport processes, one can assume local equilibrium between production and loss:

Si= ne2 ne=(S0/)1/2exp{1/2 [1 – z/H – exp(-z/H)]}

This is the vertical profile of the electron density in a simple Chapman layer.

Although the Chapman formulas provide a convenient analytical tool to describe ionospheric altitude profiles, the method is overly simplified. Usually one has to use complicated numerical techniques to obtain realistic altitude profiles taking into account various production, loss, and transport processes.

The Chapman formalism provides a good first step in understanding ionospheric profiles, the simple theory gives a good general understanding of the D, E, and F regions of the ionosphere.

Using the Chapman formula for ionization, it is possible to determine a simple approximation for the altitude profile of electron density. To do so one has to assume that the main loss process is ion-electron recombination with a recombination coefficient of . In this very simple approximation we assume that the total recombination rate is ne

2,

Aerospace EnvironmentASEN-5335

Read Chapter 7, 8, 9, and class notes. Quiz # 7 today, close book, starts at 3:00pm,

after FCQ. Homework # 7 (the last one) due 5/1. Final on 5/7, close book, 1:30-4:00pm. Instructor’s office hours: 9-11 am at ECOT 534 TA’s office hours: 3:15-5:15 pm Wed at ECAE

166. E-mail: Christopher.Hood@colorado.edu

Flux

Enhance factor

25 0.9 62 1.09

26 0.905 88 1.39

27 0.91 89 1.38

28 0.912 90 1.37

28.5 0.9135 91 1.38

29 0.915 92 1.4

30 0.92 93 1.44

31 0.922 94 1.5

32 0.927 95 1.55

56 1.06 120 1.18

57 1.065 121 1.165

58 1.075 122 1.155

59 1.08 123 1.14

60 1.09 124 1.125

61 1.1

Orbit Inclination

?

The altitude of maximum ionization can be obtained by solving the following equation:

dSi/dz=0 sec i H n(zmax) = (zmax)=1,

where zmax is the altitude of maximum ionization. It can be seen that ionization is maximum at the altitude where the optical depth is unity. We can choose z0=zmax|=0 =0 (the altitude of maximum ionization for perpendicular solar radiation), and obtain:

S=S0exp(1-z/H-exp[-z/H]) , where S0= (/H)e-1, this is the classical Chapman formula. It predicts the ionization profile due to the absorption of solar radiation in an isothermal atmosphere.

Ionization Profile

In the thermosphere solar radiation is absorbed mainly via ionization processes. Let us assume that ~ i and calculate the electron production rate profile. In this approximation each absorbed photon creates a new electron-ion pair, and therefore the electron production in a ray-path element, ds, is: Si ds =n i (z) ds, where Si is the total electron production rate. Substituting the altitude dependence of the concentration and the photon flux (Equation (5) and nn0exp[- (z-z0)/H]), we obtain:

Si =

Altitude Profile of Electron Density

since an electron and an ion must come together in order to recombine, and we assume that the ion and electron concentrations are equal. Neglecting transport processes, one can assume local equilibrium between production and loss:

Si= ne2 ne=(S0/)1/2exp{1/2 [1 – z/H – exp(-z/H)]}

This is the vertical profile of the electron density in a simple Chapman layer.

Although the Chapman formulas provide a convenient analytical tool to describe ionospheric altitude profiles, the method is overly simplified. Usually one has to use complicated numerical techniques to obtain realistic altitude profiles taking into account various production, loss, and transport processes.

The Chapman formalism provides a good first step in understanding ionospheric profiles, the simple theory gives a good general understanding of the D, E, and F regions of the ionosphere.

Using the Chapman formula for ionization, it is possible to determine a simple approximation for the altitude profile of electron density. To do so one has to assume that the main loss process is ion-electron recombination with a recombination coefficient of . In this very simple approximation we assume that the total recombination rate is ne

2,

Ion Composition and Chemistry: D region

extreme complex, and it involves negative ions, hydrated ions, and electrons. The primary source of ionization is by solar X-ray (in the 0.2 to 0.8 nm range) and Lyman- (121.6 nm).

The X-rays ionize all ions, and consequently the most important primary ions are formed from the dominant constituents of the neutral atmosphere, N2 and O2. In addition, the Lyman- line ionizes the minor neutral constituent, NO.

The initial positive ions are N2+, O2

+, NO+, and to a lesser extent O+. The unstable N2

+ ion is rapidly converted to O2+ by

the charge exchange reaction: N2+ + O2 O2

+ + N2. This process leaves O2

+ and NO+ as the major positive ions, and most of them are combined with H2O to become hydrated ions: O2

+.xH2O, NO+.xH2O; x=1,2,3…

To be ionized a neutral atom or molecular must absorb a photon with energy above the ionization potential. Inspection of the table above reveals that the X-ray (0.1 to 17 nm) and EUV (17 to 175 nm) parts of the solar spectrum are major sources of ionization in the ionosphere.

The D region (60-90 km) is the most complex and least understood region of the terrestrial ionosphere. The chemistry is

h=6.626x10^(-34)J s

The E region (90--140 km) is essentially a Chapman layer that is formed by the 80 to 102 nm part of the EUV spectrum. The main initial ion in this region is O2

+, with some production of N2

+ and O+. The N2+ ions are rapidly transformed

to other ions.

The end results is that the major ions in the dayside E regions are O2

+ and NO+ (under average condition the O2+

concentration is somewhat larger). The total ion density profile in the E layer is basically consistent with a Chapman layer.

The upper-right figure shows the daytime positive ion composition measured in situ by a rocket-borne ion mass spectrometer. Inspection of the figure shows that during the day the dominant ion in the lower F1 region is O2

+ and the upper F region is NO+.

The F1 region (140-200 km) is also essentially a

Chapman layer. The principal initial ion in F1 region is O+, with some contribution from N2

+.

In lower F1 region the lifetime of the O+ is quite short ( a few seconds), and therefore the NO+ molecular ion become dominant. Above 170 km, O+ rapidly takes over and becomes the major ion.

Ion Composition and Chemistry: E, F1 region

Attracting Outgassed Contaminants

Outgassed contaminants from a satellite are ionized by solar UV.

This creates a positively charged large molecule.

The ionized contaminant can be pulled back to the satellite if it is negatively charged.

The re-attracted contaminants can stick to satellite surfaces where they modify the optical and thermal properties of the surfaces.

CHARGEDCONTAMINANTSON SATELLITESURFACE

UV

e-

+

| | | |

| | | |

| | | |

| | | |

OUTGASSINGSATELLITE

(The Aerospace Corp.)

MOLECULES LEAVING SATELLITE

Ion Composition and Chemistry: F2 region

The F2 region (200-500 km) cannot be in local

photonchemical equilibrium since the atmosphere above F1 region is optically thin to most ionizing radiation. The formation of the F2 peak is caused by an interplay between ion sources, sinks, and ambipolar diffusion.

Transport phenomena must be taken into account.

The Major ion is O+ with a density peak in the 200 to 400 km range.

Nonetheless, a reasonably good approximation of a real electron density profile can be obtained by a judicious mix of different Chapman layers.

In summary, the major ion species in D, E, F1, F2 regions are hydrated ions (O2

+.xH2O, NO+.xH2O;

x=1,2,3…), O2+, NO+, O+, respectively.

Ionosphere vs. plasmasphere

Mars and Venus

N2+, O2

+ and O+ are the most abundantly produced ions in the terrestrial ionosphere because N2, O2

and O are the most abundant neutral species in the lower part of thermosphere. (However, the most abundant ions in existence below 300 km are NO+, O2

+ and O+ .)

Similarly, on Mars and Venus the most abundantly produced ions are CO2

+ and O+, and the most abundant ions in existence are O2+ and O+

below about 200 km. As on earth, chemistry determines the final outcome.

The lower ionospheres (z < 160 km) of Mars and Venus are analogous to the E-region of earth. The key reactions are:

CO2+ is a minor ion and the major ion is O2

+ , although there is almost no neutral O2 on Venus or Mars. Note that some of the O2

+ is converted to NO+

The peak densities on Venus and Mars are at ~140 km in the E -region.

CO2 h CO2 e

CO2O O2

CO

O2N NO O

The Dayside Ionosphere of Venus

Model simulations (above) and in-situ measurements (right) madeon Pioneer Venus.

The Dayside Ionosphere of Mars

Model simulations (solid lines) and in-situ measurements from Viking-Ifor the dayside ionosphere of Mars.

Ionospheric Variability

External Sources:

(1) Solar Radiation (X-ray and UV): solar cycle, seasonal, solar rotation, solar flares.

(2) Solar Wind (velocity, density, IMF, energetic particles): solar cycle, seasonal, solar rotation, CMEs and solar flares.

(3) Solar and Lunar Tides

Internal Sources:

(1) Earth’s rotations Circulation, Convection, Turbulences.

(2) Earth’s magnetic field interaction with solar wind, charged particle’s motions.

(3) Earth’s Ion compositions productions and recombination.

Electron Precipitation & the Aurora

Ionization Rate Profiles for Various Electron Energies

Energetic electrons, precipitating into the upper atmosphere along high-latitude magnetic field lines, ionize the neutral atmospheric species. The more energetic electrons penetrate deeper into the atmosphere. Note that the ionization rate profiles resemble the Chapman profile for photoionization.

Aerospace EnvironmentASEN-5335

Read Chapter 7, 8, 9, and class notes. Final on 5/7, close book, 1:30-4:00pm. A calculator may

be needed.

Instructor’s office hours: 9-11 am Wed. at ECOT 534 TA’s office hours: 9-11 am Tue. at ECAE 166. E-mail:

Christopher.Hood@colorado.edu\ HW # 7 can be picked up during the office hours or before

the final.

Tropo (Greek: tropos); “change”Lots of weather

Strato(Latin: stratum);Layered

Meso(Greek: messos);Middle

Thermo(Greek: thermes);Heat

Exo(greek: exo);outside

For average dayside conditions

Absorption of Solar Radiation vs. Height and Species

The Ionosphere is a Weakly Ionized Plasma

broad definition: “the ionosphere is that region of the atmosphere (or gaseous envelope) surrounding a solar system body where significant numbers of low-energy free electrons and ions are present”

Neutral density far exceeds the electron or ion density

Altitude Profile of Electron Density

since an electron and an ion must come together in order to recombine, and we assume that the ion and electron concentrations are equal. Neglecting transport processes, one can assume local equilibrium between production and loss:

Si= ne2 ne=(S0/)1/2exp{1/2 [1 – z/H – exp(-z/H)]}

This is the vertical profile of the electron density in a simple Chapman layer.

Although the Chapman formulas provide a convenient analytical tool to describe ionospheric altitude profiles, the method is overly simplified. Usually one has to use complicated numerical techniques to obtain realistic altitude profiles taking into account various production, loss, and transport processes.

The Chapman formalism provides a good first step in understanding ionospheric profiles, the simple theory gives a good general understanding of the D, E, and F regions of the ionosphere.

Using the Chapman formula for ionization, it is possible to determine a simple approximation for the altitude profile of electron density. To do so one has to assume that the main loss process is ion-electron recombination with a recombination coefficient of . In this very simple approximation we assume that the total recombination rate is ne

2,

Artificial Depletion of the Ionosphere -- ‘Ionospheric Holes’

Launches of opportunity (i.e., skylab); space shuttle; chemical releases

Plasma neutralization:

H2 O OH H

OH e O* H

H2O O H2O O

H2O e OH* H

Exhaust products are typically

H2 and H2O

numerical simulation

6300Å

Natural Ionospheric ‘Holes’ or Depletions as Revealed by Ground-Based All-Sky Images of the 6300A Nightglow Near the Magnetic Equator

LASP seminar, 4 pm this afternoon, 10 years of SAMPEX mission

by Dan Baker

Radio Wave Propagation and Effects of Plasmas

The ionosphere can affect the transmission of radio waves in at least two ways. First, the charged particles can remove energy from an electromagnetic wave and thus attenuate the signal. Second, a wave traveling a path along which the electron density is not constant undergoes a change in its direction.

A plasma is a gaseous mixture of electrons, ions, and neutral particles. If, by some mechanism, electrons are displaced from ions in a plasmas: the resulting separation of charge sets up an electric field which attempts to restore equilibrium. Due to their momenturm, the electrons will overshoot the equilibrium point, and accelerate back. This sets up an oscillation.

The frequency of this oscillation is called the plasma frequency, = 2f = (4Nee2/me)1/2, which depends upon theproperties of the particular plasma under study. This situation is modified if we take into account the movement of the ions, the thermal motion of the electrons, and neutral particles.

A radio wave consists of oscillating electric and magnetic fields. When a low-frequency radio wave (i.e., frequency less than the plasma frequency) impinges upon a plasma, the local charged particles have sufficient time to rearrange themselves so as to “cancel out” the oscillating electric field and thereby “screen” the rest of the plasma from the oscillating E-field.

Interactions with a radio wave

This radio wave (low frequency) cannot penetrate the plasma,

and is reflected.

For a high frequency wave (i.e., frequency greater than the plasma frequency), the particles do not have time adjust themselves to produce this screening effect, and the wave passes through.

The critical frequency of the ionosphere

(foF2) represents the minimum radio frequency capable of passing completely through the ionosphere.

N(cm-3)=1.24x104 f2 (MHz)

When collisions between oscillating electrons and ions and neutral particles becomes sufficiently frequent (as in the D-region, 60 – 90 km), these collisions “absorb” energy from the radio wave leading to what is called radio wave absorption.

Neutral Particle Density, Temperature, Scale Height

One of the most important features of the atmosphere is the vertical profile of density and pressure. The altitude variation of these quantities is described by assumption of hydrostatic equilibrium.

From the momentum equation: dp/dz=-mng or dp/dz=-(mg/kT)p= -(1/H)p, where the scale height, H=kT/mg, is introduced. H is a slowly varying function of location and it can be considered locally constant. In this case, we have the well-known barometric altitude formula: p(z)=p0exp[-(z-z0)/H].

H is a linear function of the gas temperature, T, supposing that T is also slowly varying, we obtain a similar barometric altitude formula for the gas density : (z)=0exp [-(z-z0)/H].

For average dayside conditions

Ionospheric Disturbances

Ionospheric disturbances occur when a region of the Earth’s ionosphere (50 – 500 km altitude) experiences a temporary fluctuation in degree of ionization.

This variation can result from geomagnetic activity (and the influences of the neutral atmosphere), or it can be the direct result of X-rays and EUV produced by a solar flare.

Sudden Ionospheric Distrurbances (SIDs) are a category of disturbance which occurs almost simultaneously with a flare’s X-ray emission (so SIDs are generally constrained in dayside).

Ionospheric Disturbances

Short Wave Fade (SWF) is a particular type of SID that can severely hamper HF radio users (up to 20 – 30 MHz) by causing increased signal absorption which may last for up to 1-2 hours.

Geomagnetic storms may profoundly alter ionospheric electron densities over large segments of the middle and high latitudes. Ionospheric storms may occur in the absence of a discernable geomagnetic storm, but they are generally similar in their effects on HF systems regardless of the origin and association. The origin of a purely ionospheric storm is uncertain.

For a given path, the workable frequency bands lies between the maximum usable frequency (MUF) capable of be reflected by the ionosphere and lowest usable frequency (LUF) still maintaining connectivity along the same path. In the Figure, ray path 1 is associated with the LUF and ray path 3 corresponds to the MUF path.

Comparisons MUF/LUF between Quiet and Disturbed Condition

Impact of ionospheric conditions on HF propagation. The range of useable frequencies, between the MUF and LUF, is reduced during disturbed periods.

Solar Effects on Radio Waves

Radio Noise Storms. Sometimes an active region on the Sun can produce increased noise levels, primarily at frequencies below 400 MHz. This noise may persist for days, occasionally interfering with communication systems using an affected frequency.

Solar Wind Recurrence. The Sun rotates once approximately every 27 days. There is a tendency for long lasting solar wind anomalies, such as those originating from coronal holes, to pass the Earth every 27 days. These solar wind anomalies can cause recurring geomagnetic disturbances. Solar Radio Bursts. Radio wavelength energy is constantly emitted from the Sun; however, the amount of radio energy may increase significantly during a solar flare. These bursts may interfere with radar, HF (3 – 30 MHz) and VHF (30 – 300 MHz) radio, or satellite communication systems. Radio burst data are also important in helping to predict whether we will experience of the delayed effects of solar particle emissions.

Solar Effects on Radio Waves

Systems in the VHF through SHF range (30 MHz to 30 GHz) are susceptible to interference from solar radio noise. If the Sun is in the reception field of the receiving antenna, solar radio bursts may cause Radio Frequency Interference (RFI) in the receiver, as depicted below.

Solar Particle Events and Polar Cap Absorption

Part of the energy released in solar flares are in the form of accelerating particles (mostly proton and electrons) to high energies and released into space.

The energetic charged particles emitted will be guided by the IMF (not to be confused with solar wind, which guides the IMF).

The intensity of the particle event will depend upon the size of the solar flare, the relative position of the Earth and the flare on the Sun, and the structure of the IMF.

The major impacts of a solar particle event are spacecraft anomalies (charging, single event upsets, optical sensor disorientation, etc.) and Polar Cap Absorption (PCA)

PCA events occur when high energy protons spiral along the Earth’s magnetic field lines towards the polar ionosphere’s D-region (50 – 90 km altitude).

These particles cause significant increased ionization levels, resulting in severe absorption of HF radio waves used for communication and some radar system.

This phenomenon, sometimes referred to as “polar cap blackout”, is often accompanied by widespread geomagnetic and ionospheric disturbances.

PCA

In addition, LF and VLF systems may experience phase advances when operating in or through the polar cap during a PCA event due to changes in the Earth-ionosphere waveguide.

Energetic Particle Effects on High Latitude HF Communications

Polar airline routes loose ground communications Alternate routes required Uses more fuel Flight delays

Sample of Flights Affected:

• 10/26/00: Lost of HF prior to 75N, re- route off Polar route with Tokyo fuel stop. 15:00 flight now 20:30

• 11/10/00: Due to poor HF, ORD to HKG flown non-polar at 47 minute penalty

• 3/30/01-4/21/01: 25 flights operated on less than optimum polar routes due to HF disturbances resulting in time penalties ranging from 6 to 48 minutes

• 11/25/00: Polar flight re-route at 75N due to Solar Radiation, needed Tokyo fuel stop

• 11/26/00: Operated non-polar at 37 minute penalty due to solar radiation

• 11/27/00: Operated non polar at 32 minute penalty due to solar radiation.

• 11/28/00: Operated non-polar at 35 minute penalty due to solar radiation

Polar 2

Polar 3

Polar 1

Polar 4

Polar Airline Routes

North Pole Chicago

Hong Kong

Alaska

Radio BlackoutDuring

Particle Events

Time Scale for Solar Flare Effects