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The Ionosphere

t is surprising to most people that the iono- sphere makes up less than one percent of the mass of the above 100 km (see figure 1). Even though the iono- sphere only contains a small fraction of atmo- spheric material, it is very important because of its influence on the passage of waves. Most of the ionosphere is electrically neutral, but when solar radiation strikes the chemical constituents of the atmosphere are dislodged from and to pro- duce the ionospheric . This occurs on the sunlit side of the , and only the shorter of solar radiation, (the extreme ul- traviolet and X-ray part of the ), are energetic enough to produce this . The presence of these charged particles makes the upper atmosphere an electrical conductor, which supports electric currents and affects ra- dio waves.

Ionospheric Characteristics

The ionosphere has historically been divided Figure 1. The ionosphere lies well above where air- into regions (D, E, and F), with the term ªlayerº craft fly, but is located at altitudes where many sat- referring to the ionization within a region. The ellites and the Space Shuttle orbit. 2

customers is the F2 layer, where con- 1000 centrations reach their highest values. At high latitudes there is another source of - ization called the . The aurora is a display of lights caused by electrons and protons strik- ing the atmosphere at high speed. The particles Topside O+ H+ come from the and spiral down the magnetic field lines of the Earth. These par- ticles produce a spectacular array of light, and when they strike the atmosphere they also pro- 600 duce ionization. The auroral oval is named be- Nmax cause of its shape, and exists in both the North- ern and Southern hemispheres above about 60 Hmax F2 Layer degrees (see ªAuroraº, Topics SE±12). F Region At low latitudes the largest electron densities Altitude in km O+ are found in peaks on either side of the magnet- F1 Layer ic ; this is a feature known as the equato- rial anomaly. The peak ionospheric concentra- tions do not occur on the equator, as might be 150 + + E Region O2 NO expected from the maximum in solar ionizing 90 radiation, but instead are displaced on either D Region side. This peculiarity is due to the geometry of the magnetic field and the presence of electric Density: 104 105 106 fields. The electric fields that transport the plas- Electrons/cm3 ma are caused by a polarizing effect of the ther- Figure 2. Various layers of the ionosphere and mospheric winds. their predominant ion populations are listed at their respective heights above ground. The density in the ionosphere varies considerably, as shown. lowest is the D-region, covering altitudes be- Ionospheric Variability tween about 50 and 90 km. The E-region lies The ionosphere varies greatly because of between 90 and 150 km, and the F-region is the changes in the two sources of ionization and be- ionosphere above the E-region. Within the E- cause it responds to changes in the neutral part region is the normal E layer, produced by solar of the upper atmosphere in which it is em- radiation, and sporadic layers, designated Es. bedded. This region of the atmosphere is Within the F-region are the F1 and F2 layers. known as the . Since it responds The top of the ionosphere is at about 1000 km, to solar EUV radiation, the ionosphere varies but there is no real boundary between the plas- over the 24-hour period between daytime and ma in the ionosphere and the outer reaches of nighttime and over the 11-year cycle of solar the Earth's magnetic field, the activity. This variability is captured in Table 1, and magnetosphere. Figure 2 shows the iono- which illustrates changes in the F-region maxi- spheric layers and the principle that com- mum electron density (Nmax) for the diurnal pose each region. One important layer from the variation and over the . On shorter standpoint of Navigation and Communication time-scales, solar X-ray radiation can increase 3 dramatically when a occurs; this in- creases the D- and E-region ionization. During 15 Maximum a the auroral source of ion- Usable ization becomes much more intense and vari- able, and expands to lower latitudes. In extreme Short-Wave Fade cases auroral displays can be seen as far south 10 Usable as Mexico. Frequency The other main source of variability in the iono- Window sphere comes from charged particles respond- ing to the neutral atmosphere in the thermo- 5 sphere. The ionosphere responds to the thermospheric winds; they can push the iono- Frequency (MHz) Lowest Usable sphere along the inclined magnetic field lines to Frequency a different altitude. The ionosphere also re- 0 sponds to the composition of the thermosphere, 0 6 12 18 24 which affects the rate that ions and electrons re- Time (Hours) combine. During a geomagnetic storm, the en- X-Ray Event ergy input at high latitudes produces waves and changes in thermospheric winds and composi- Figure 3. The usage frequency window for radio tion. This produces both increases (ªpositive propagation lies between the lowest and maxi- mum usable . When the window phasesº) and decreases (ªnegative phasesº) in closes, as shown here, a shortwave fade occurs. the electron concentration. Table1 presents the diurnal and solar cycle vari- bances. The LUF is controlled by the amount of ability of three important ionospheric parame- absorption of the in the D- and E-re- ters: Nmax, MUF and TEC. For HF commu- gion, and is severely affected by solar flares nication the radio waves propagate from one (figure 3). Total Electron Content (TEC) is an location to another by bouncing off the iono- important ionospheric parameter for naviga- sphere. The critical parameters are the maxi- tion customers. All single-frequency GPS re- mum and lowest usable frequencies (MUF and ceivers (over a million users) must correct for LUF) that the ionosphere can ªsupport.º The the delay of the GPS signal as it propagates MUF depends on the peak electron density in through the ionosphere from GPS the F-region and the angle of incidence of the (22,000 km altitude). TEC is a measure of this radio wave. This changes through the , over delay, and it is required if an accurate position the solar cycle, and during geomagnetic distur- location is to be provided by the GPS receiver.

Table 1. Ionospheric Variability Ionospheric Parameter Diurnal (Mid-Latitude) Solar Cycle (Daytime) 5 6 3 5 6 3 Nmax 1 10 to 1 10 electrons/cm 4 10 to 2 10 electrons/cm 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 50 1016 electrons/m2 10 to 50 1016 electrons/m2 Factor of 10 Factor of 5 4

The variability in TEC over the diurnal and so- 10 lar cycle periods is also shown in Table 1.

0 Ionospheric Scintillation 40 sec. There are certain regions of the ionosphere ±10 (mainly the high latitude and low latitude F-re- Figure 5. A signal is disrupted by the onset of scin- gion,) and certain local times (principally after tillation in the ionosphere. sunset) when the ionosphere may become high- ly turbulent. ªTurbulenceº is defined here as cy) radio wave propagating through a ªclearº the presence of small-scale (from centimeters environment that abruptly becomes filled with to meters) structures or irregularities imbedded small-scale irregularities. The term ªiono- in the large-scale (tens of kilometers) ambient spheric scintillationº has been used to describe ionosphere. Under favorable conditions, plas- this effect on the radio signals. The bigger the ma density irregularities are generated just after amplitude fluctuations of the scintillated signal sunset in the equatorial region and may last for the greater the impact on Com- several hours. At high latitudes, these irregula- munication and Navigation sys- rities may be generated during either the day- tems. time or at night. For both low and high latitude regions these small-scale irregularities occur ______most frequently during the solar cycle maxi- mum period (see figure 4). References

80 The references listed below provide much more 60 information on ionospheric phenomena for the Local time interested reader. 40 S Davies, K., ªIonospheric Radio,º IEE 20 Electromagnetic Waves series 31, Peter Peregrinus Ltd., London, UK, 1989. Sunset Sunrise S Hargreaves, J.K., ªThe solar-terrestrial environment,º Cambridge Atmospheric 20 And Space Sciences series, Cambridge Univ. Press, Cambridge, UK, 1992. 40 S Jursa, Adolph S., ed., Handbook of Geo- 60 physics and the Space Environment, Air 80 Force Lab, Hanscom AFB, Figure 4. Global depth of scintillation during MA, 1985. low and moderate solar activity. S Kelley, Michael C., ªThe Earth's Iono- sphere: Plasma Physics and Electrody- namics,º Academic Press, 1989. The existence of these small-scale structures can seriously affect the nature of radio waves as they propagate through the ionosphere where S Visit our Website at they are imbedded. Figure 5 shows the effect http://sec.noaa.gov on the amplitude of a UHF (~250 Mhz frequen- Written by Dave Anderson and Tim Fuller-Rowell, 1999