Ionosphere and Neutral Atmosphere• Temperature and density structure

• Hydrogen escape

• Thermospheric variations and satellite drag

• Mean wind structure

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

Strato(Latin: stratum);Layered

Meso(Greek: messos);Middle

Thermo(Greek: thermes);Heat

Exo(greek: exo);outside

Variation of the density in anatmosphere with constanttemperature (750 K).

• The radiation from the Sun at short wave lengths causes photo ionization of the atmosphere resulting in a partially ionized region called the ionosphere.

• Guglielmo Marconi’s demonstration of long distance radio communication in 1901 started studies of the ionosphere.

• Arthur Kennelly and Oliver Heaviside independently in 1902 postulated an ionized atmosphere to account for radio transmissions. (Kennelly-Heavyside layer is now called the E-layer).

• Larmor (1924) developed a theory of reflection of radio waves from an ionized region.

• Breit and Tuve in 1926 developed a method for probing the ionosphere by measuring the round-trip for reflected radio waves.

• The ionosphere vertical density pattern shows a strong diurnal variation and a solar cycle variation.

• Identification of ionospheric layers is related to inflection points in the vertical density profile.

Primary Ionospheric Regions

Region Altitude Peak Density

D 60-90 km 90 km 108 –1010 m-3

E 90-140 km 110 km Several x 1011 m-3

F1 140-200 km 200 km Several 1011-1012 m-3

F2 200-500 km 300 km Several x 1012 m-3

Topside above F2

• Diurnal and solar cycle variation in the ionospheric density profile.– In general densities are

larger during solar maximum than during solar minimum.

– The D and F1 regions disappear at night.

– The E and F2 regions become much weaker.

– The topside ionosphere is basically an extension of the magnetosphere.

• Composition of the dayside ionosphere under solar minimum conditions.

– At low altitudes the major ions are O2+ and

NO+

– Near the F2 peak it changes to O+

– The topside ionosphere becomes H+

dominant.

• For practical purposes the ionosphere can be thought of as quasi-neutral (the net charge is practically zero in each volume element with enough particles).

• The ionosphere is formed by ionization of the three main atmospheric constituents N2, O2, and O.

• The primary ionization mechanism is photoionization by extreme ultraviolet (EUV) and X-ray radiation.

– In some areas ionization by particle precipitation is also important.

• The ionization process is followed by a series of chemical reactions– Recombination removes free

charges and transforms the ions to neutral particles.

•Neutral density exceeds the ion density below about 500 km.

At 80-100 km, the time constant for mixing is more efficient than recombination, so mixing due to turbulence and other dynamical processes must be taken into account (i.e., photochemical equilibrium does not hold).

Mixing transports O down to lower (denser) levels where recomb-ination proceeds rapidly (the "sink" for O).

After the O recombines to produce O2, the O2 is transported upward by turbulent diffusion to be photodissociated once again (the "source" for O).

O Concentration

• Let the photon flux per unit frequency be– The change in the flux due to absorption by

the neutral gas in a distance ds is where n(z) is the neutral gas concentration,

is the frequency dependent photo absorption cross section, and ds is the path length element in the direction of the optical radiation. (Assuming there are no local sources or sinks of ionizing radiation.)

– (where is the zenith angle of the incoming solar radiation.

– The altitude dependence of the solar radiation flux becomes

where is the incident photon intensity per unit frequency.

– is called the

optical depth.

– There is usually more than one

atmospheric constituent attenuating the

photons each of which has its own cross

section.

dsnd

dzds sec

z

dzznz ')'(secexp)(

')'(sec dzznz

')'(sec dzznt z

t

)exp()( z

• The density (ns) of the neutral upper atmosphere usually obeys a hydrostatic equation

where m is the molecular or atomic mass, g is the acceleration due to gravity, z is the altitude and p=nkT is the thermal pressure.

• If the temperature T is assumed independent of z, this equation has the exponential solution

where is the scale height of the gas, and n0 is the density at the reference altitude z0.

• For this case

• For multiple species

• The optical depth increases exponentially with decreasing altitude.

• In the thermosphere solar radiation is absorbed mainly via ionization processes. Let us assume that

• Each absorbed photon creates a new electron-ion pair therefore the electron production is

where Si is the total electron production rate (particles cm-3s-1).

dz

nkTd

dz

dpnmg

)(

H

zznn

)(exp 0

0

mgkTH

HzndzH

zzn

z

)(sec''

expsec 00

ttt

t Hzn )(sec

i

dszndsS vii )(

• Substituting for n and gives

• The altitude of maximum ionization

can be obtained by looking for

extremes in this equation by calculating

• This gives

• Choose z0 as the altitude of maximum ionization for perpendicular solar radiation

• This gives

where

• This is the Chapman ionization

function.

• The maximum rate of ionization is

given by

• If we further assume that the main loss

process is ion-electron recombination

with a coefficient and assume that the

recombination rate is

• Finally if we assume local equilibrium

between production and loss we get

)(z

H

zzHn

H

zznS iii

00

00 expsecexp

0dz

dSi

1)()(sec maxmax zzHni

0max0

zz

H

z

H

zSSi expsec1exp0

10

e

HS

cos0max SS

2en

2ei nS

• The vertical profile in a simple Chapman layer is

• The E and F1 regions are essentially Chapman layers while additional production, transport and loss processes are necessary to understand the D and F2 regions.

H

z

H

zSne exp

2

sec

22

1exp0

• The D Region– The most complex and least understood layer in the ionosphere.– The primary source of ionization in the D region is ionization by solar X-rays and

Lyman- ionization of the NO molecule.– Precipitating magnetospheric electrons may also be important.– The primary positive ions are O2

+ and NO+

– The most common negative ion is NO3-

• The E Region– Essentially a Chapman layer formed by EUV ionization.– The main ions are O2

+ and NO+

– Although nitrogen (N2) molecules are the most common in the atmosphere N2+ is not

common because it is unstable to charge exchange. For example

– Oxygen ions are removed by the following reactions

2222 NOON

22

2

NOON

NNOON

OOOO

NNONO

22

2

• The F1 Region

– Essentially a Chapman layer.

– The ionizing radiation is EUV at <91nm.

– It is basically absorbed in this region and does not penetrate into the E region.

– The principal initial ion is O+.

– O+ recombines in a two step process.• First atom ion interchange takes place

• This is followed by dissociative recombination of O2+ and NO+

OOOO

NNONO

22

2

OOeO 2

ONeNO

• The F2 Region– The major ion is O+.– This region cannot be a Chapman layer since the atmosphere above the F1 region optically thin

to most ionizing radiation.– This region is formed by an interplay between ion sources, sinks and ambipolar diffusion.– The dominant ionization source is photoionization of atomic oxygen

– The oxygen ions are lost by a two step process• First atom-ion interchange

• Dissociative recombination

– The peak forms because the loss rate falls off more rapidly than the production rate.– The density falls off at higher altitudes because of diffusion- no longer in local photochemical

equilibrium.

eOhO

NNONO

OOOO

2

22

ONeNO

OOeO

2

Radio Sounding Principles • Waves in a plasma experience reflection and refraction

• Radio waves are reflected at wave cutoffs (n = 0)

• Echoes are received if the gradient at the reflection point is normal to the incident signals.

• The echo frequency gives the plasma conditions of the reflection point, and the time delay gives the distance of reflection point.

• From a series of sounding frequencies, a density profile can be obtained

SOUNDEREcho

Refracted rays

Refracted rays

n=0n>0

n<0

Reflected ray

Iono

sond

e F

ield

of

Vie

w a

nd I

SR

Rad

ar

Pen

cil B

eam

Ionosonde

Specular Reflection and Scatter

Radio Sounding: specular reflectionwide beamScatter Radar: Scatter, pencil beam

Modern Ionosonde and Transmit Antenna

Transmit antennaDigisonde DPS

Quiet Daytime Mid Latitude Ionogram

Electron density profile

The Digisonde Profilogram

midnight

Cachimbo 16 October 2002

noon

Ionospheric Drift Measurements

• Fourier analysis for Doppler spectra

• Interferometry with spaced receive antennas

Digisonde Skymaps

Real Time Digisonde F Region Drift

Digisonde Network

• The dense regions of the ionosphere (the D, E and F regions) contain concentrations of free electrons and ions. These mobile charges make the ionosphere highly conducting.

• Electrical currents can be generated in the ionosphere.• The ionosphere is collisional. Assume that it has an electric field but for

now no magnetic field. The ion and electron equations of motion will be

where is the ion neutral collision frequency and is the electron neutral collision frequency.

– For this simple case the current will be related to electric field by where is a scalar conductivity.

• If there is a magnetic field there are magnetic field terms in the momentum equation. In a coordinate system with along the z-axis the conductivity becomes a tensor.

eene

iini

umEe

umEq

Ej

0

in en

0

000

0

0

PH

HP

B

• Specific conductivity – along the magnetic field

• Pedersen conductivity – in the direction of the applied electric field

• Hall conductivity – in the direction perpendicular to the applied field

where and are the total electron and ion momentum transfer collision frequencies and and are the electron and ion gyrofrequencies.

• The Hall conductivity is important only in the D and E regions.• The specific conductivity is very important for magnetosphere and

ionosphere physics. If all field lines would be equipotentials.• The total current density in the ionosphere is

iieee mm

ne

1120

iii

i

eee

eeP mm

ne11

22222

iii

i

eee

eeH mm

ne11

22222

e ie i

0

B

EBEEj HP

0

• Within the high latitude magnetosphere (auroral zone and polar cap) plasmas undergo a circulation cycle.– At the highest latitudes the geomagnetic field lines are “open” in that only

one end is connected to the Earth.

– Ionospheric plasma expands freely in the flux tube as if the outer boundary condition was zero pressure.

• For H+ and He+ plasma enters the flux tube at a rate limited by the source.

• The net result is a flux of low density supersonic cold light ions into the lobes.

• The surprising part is that comparable O+ fluxes also are observed.

Vertical distribution of density and temperature for high solar activity (F10.7 = 250) at noon (1) and midnight (2), and for low solar activity (F10.7 = 75) at noon (3) and midnight (4) according to the COSPAR International Reference Atmosphere (CIRA) 1965.

Atmospheric Compositions Compared

The atmospheresof Earth, Venus andMars contain manyof the same gases,but in very differentabsolute andrelative abundances.Some values arelower limits only, reflecting the pastescape of gas tospace and otherfactors.

MarsVenus

Earth

night

day

VenusAverage Temperature Profiles for Earth, Mars & Venus

Formation of Ionospheres

2 2

2 2

Photo ionization: If ( 10 to 20 )

In the terrestrial ionosphere:

M M

photoelectron

photoelectron

photoelectron

photoelectron

h I I eV

h M M e E

h N N e E

h O O e E

h O O e E

HYDROSTATIC EQUILIBRIUM If …..n = # molecules per unit volume

m = mass of each particle

nm dh = total mass contained in a cylinder of air (of unit cross-sectional area)

Then, the force due to gravity on the cylindrical mass = g nmdh

and the difference in pressure between the lower and upper faces of the cylinder balances the above force in an equilibrium situation:

dP

nmgdh

P + dP

P

PdP Pnmgdh

Assuming the ideal gas law holds,

Then the previous expression may be written:

where H is called the scale height and

dP

dh nmg

P nkT RT

R R*

m

1

P

dP

dh

1

H

H kT

mg

RT

g

g g(0)RE

2

RE h 2

This is the so-called hydrostatic law or barometric law.

Integrating, where

and z is referred to as the "reduced height" and the subscript zero refers to a reference height at h=0.

Similarly,

For an isothermal atmosphere, then,

P P0e z

z dh

H0

z

T zon n eo T

hH PPeo

h

Hn n eo

hHeo

17 2 13 2

At top of atmosphere .

sec

The photoabsorption cross section 10 10

exp sec ,

where sec

n n

n

z

n

F F

dF n F ds F n dz

cm m

F z F n dz F e

z n

Exponential decrease of photon flux

z

z dz

2

The production rate at height z is

( ) .

In an isothermal atmosphere, if we assume equilibrium ,

i.e. , and :

1exp 1 sec ,

2

zn n

e d e

e e m

P z n F z F n z e

L k n

n z n z e

e eproduction P = loss L

is the height of maxi

Chapman 1932

mum production ( 1).

m

n

m

z z

H

z

Absorption of Solar Radiation vs. Height and Species