Space Environment Center

325 Broadway, Boulder, CO 80303-3326 SE-14

The Ionosphere

t is surprising to most people that the ionosphere makes up less than one percent of

the mass of the atmosphere above 100 km

(see figure 1). Even though the ionosphere only contains a small fraction of atmospheric material, it is very important because of

its influence on the passage of radio waves.

Most of the ionosphere is electrically neutral,

but when solar radiation strikes the chemical

constituents of the atmosphere electrons are

dislodged from atoms and molecules to produce the ionospheric plasma. This occurs on the

sunlit side of the Earth, and only the shorter

wavelengths of solar radiation, (the extreme ultraviolet and X-ray part of the spectrum), are

energetic enough to produce this ionization.

The presence of these charged particles makes

the upper atmosphere an electrical conductor,

which supports electric currents and affects radio waves.

Ionospheric Characteristics

The ionosphere has historically been divided

into regions (D, E, and F), with the term “layer”

referring to the ionization within a region. The

Figure 1. The ionosphere lies well above where aircraft fly, but is located at altitudes where many satellites and the Space Shuttle orbit.
2 

90

150

600

1000

E Region

F1 Layer

F Region

Topside O+ H+

O+

O2

+ NO+

104 105 10 Density: 6

Altitude in km

Hmax

Nmax

Figure 2. Various layers of the ionosphere and

their predominant ion populations are listed at

their respective heights above ground. The density

in the ionosphere varies considerably, as shown.

F2 Layer

D Region

Electrons/cm3

lowest is the D-region, covering altitudes between about 50 and 90 km. The E-region lies

between 90 and 150 km, and the F-region is the

ionosphere above the E-region. Within the Eregion is the normal E layer, produced by solar

radiation, and sporadic layers, designated Es.

Within the F-region are the F1 and F2 layers.

The top of the ionosphere is at about 1000 km,

but there is no real boundary between the plasma in the ionosphere and the outer reaches of

the Earth’s magnetic field, the plasmasphere

and magnetosphere. Figure 2 shows the ionospheric layers and the principle ions that compose each region. One important layer from the

standpoint of Navigation and Communication

customers is the F2 layer, where electron concentrations reach their highest values.

At high latitudes there is another source of ionization called the aurora. The aurora is a display

of lights caused by electrons and protons striking the atmosphere at high speed. The particles

come from the magnetosphere and spiral down

the magnetic field lines of the Earth. These particles produce a spectacular array of light, and

when they strike the atmosphere they also produce ionization. The auroral oval is named because of its shape, and exists in both the Northern and Southern hemispheres above about 60

degrees geomagnetic latitude (see “Aurora”,

Space Environment Topics SE–12).

At low latitudes the largest electron densities

are found in peaks on either side of the magnetic equator; this is a feature known as the equatorial anomaly. The peak ionospheric concentrations do not occur on the equator, as might be

expected from the maximum in solar ionizing

radiation, but instead are displaced on either

side. This peculiarity is due to the geometry of

the magnetic field and the presence of electric

fields. The electric fields that transport the plasma are caused by a polarizing effect of the thermospheric winds.

Ionospheric Variability

The ionosphere varies greatly because of

changes in the two sources of ionization and because it responds to changes in the neutral part

of the upper atmosphere in which it is embedded. This region of the atmosphere is

known as the thermosphere. Since it responds

to solar EUV radiation, the ionosphere varies

over the 24-hour period between daytime and

nighttime and over the 11-year cycle of solar

activity. This variability is captured in Table 1,

which illustrates changes in the F-region maximum electron density (Nmax) for the diurnal

variation and over the solar cycle. On shorter

time-scales, solar X-ray radiation can increase
3 

dramatically when a solar flare occurs; this increases the D- and E-region ionization. During

a geomagnetic storm the auroral source of ionization becomes much more intense and variable, and expands to lower latitudes. In extreme

cases auroral displays can be seen as far south

as Mexico.

The other main source of variability in the ionosphere comes from charged particles responding to the neutral atmosphere in the thermosphere. The ionosphere responds to the

thermospheric winds; they can push the ionosphere along the inclined magnetic field lines to

a different altitude. The ionosphere also responds to the composition of the thermosphere,

which affects the rate that ions and electrons recombine. During a geomagnetic storm, the energy input at high latitudes produces waves and

changes in thermospheric winds and composition. This produces both increases (“positive

phases”) and decreases (“negative phases”) in

the electron concentration.

Table1 presents the diurnal and solar cycle variability of three important ionospheric parameters: Nmax, MUF and TEC. For HF communication the radio waves propagate from one

location to another by bouncing off the ionosphere. The critical parameters are the maximum and lowest usable frequencies (MUF and

LUF) that the ionosphere can “support.” The

MUF depends on the peak electron density in

the F-region and the angle of incidence of the

radio wave. This changes through the day, over

the solar cycle, and during geomagnetic disturMaximum

Frequency

Lowest Usable

Frequency

Usable

Window

Frequency

Time (Hours)

Usable

X-Ray Event

Short-Wave Fade

Frequency (MHz)

0

5

10

15

0 6 12 18 24

Figure 3. The usage frequency window for radio

propagation lies between the lowest and maximum usable frequencies. When the window

closes, as shown here, a shortwave fade occurs.

bances. The LUF is controlled by the amount of

absorption of the radio wave in the D- and E-region, and is severely affected by solar flares

(figure 3). Total Electron Content (TEC) is an

important ionospheric parameter for navigation customers. All single-frequency GPS receivers (over a million users) must correct for

the delay of the GPS signal as it propagates

through the ionosphere from GPS satellites

(22,000 km altitude). TEC is a measure of this

delay, and it is required if an accurate position

location is to be provided by the GPS receiver.

Table 1. Ionospheric Variability

Nmax 1105 to 1106 electrons/cm3 4105 to 2106 electrons/cm3

Factor of 10 Factor of 5

Maximum Usuable Frequency 12 to 36 MHz 21 to 42 MHz

Factor of 3 Factor of 2

Total Electron Content 5 to 501016 electrons/m2 10 to 501016 electrons/m2

Factor of 10 Factor of 5

Ionospheric Parameter Diurnal (Mid-Latitude) Solar Cycle (Daytime)
4 

The variability in TEC over the diurnal and solar cycle periods is also shown in Table 1.

Ionospheric Scintillation

There are certain regions of the ionosphere

(mainly the high latitude and low latitude F-region,) and certain local times (principally after

sunset) when the ionosphere may become highly turbulent. “Turbulence” is defined here as

the presence of small-scale (from centimeters

to meters) structures or irregularities imbedded

in the large-scale (tens of kilometers) ambient

ionosphere. Under favorable conditions, plasma density irregularities are generated just after

sunset in the equatorial region and may last for

several hours. At high latitudes, these irregularities may be generated during either the daytime or at night. For both low and high latitude

regions these small-scale irregularities occur

most frequently during the solar cycle maximum period (see figure 4).

Sunset Sunrise

20

40

60

80

20

40

60

80

Local time

Figure 4. Global depth of scintillation fading during

low and moderate solar activity.

The existence of these small-scale structures

can seriously affect the nature of radio waves as

they propagate through the ionosphere where

they are imbedded. Figure 5 shows the effect

on the amplitude of a UHF (~250 Mhz frequen10

0

–10

Figure 5. A signal is disrupted by the onset of scintillation in the ionosphere.

40 sec.

cy) radio wave propagating through a “clear”

environment that abruptly becomes filled with

small-scale irregularities. The term “ionospheric scintillation” has been used to describe

this effect on the radio signals. The bigger the

amplitude fluctuations of the scintillated signal

the greater the impact on Communication and

Navigation systems.

_____________________________________

References

The references listed below provide much more

information on ionospheric phenomena for the

interested reader.

 Davies, K., “Ionospheric Radio,” IEE

Electromagnetic Waves series 31, Peter

Peregrinus Ltd., London, UK, 1989.

 Hargreaves, J.K., “The solar-terrestrial

environment,” Cambridge Atmospheric

And Space Sciences series, Cambridge

Univ. Press, Cambridge, UK, 1992.

 Jursa, Adolph S., ed., Handbook of Geophysics and the Space Environment, Air

Force Geophysics Lab, Hanscom AFB,

MA, 1985.

 Kelley, Michael C., “The Earth’s Ionosphere: Plasma Physics and Electrodynamics,” Academic Press, 1989.

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Written by Dave Anderson and Tim Fuller-Rowell, 1999