Ground-based instrument coverage in Africa before 2007 (left panel) and in 2015 (right panel)
Although humanity has learned to use the properties of the ionosphere in many beneficial ways over the last century, there is still a great deal to understand about the physical and chemical processes controlling its behaviour, particularly with respect to its dynamic response to solar influences. The ionosphere affects modern technologies such as civilian and military communications, navigation systems and surveillance systems, etc. For many communication and navigation systems, this arises because the systems use signals transmitted to and from satellites, which must therefore pass through the ionosphere. For the most reliable communication and navigation it is necessary to correct the signals for effects imposed by the ionosphere. To do that the properties of the ionosphere, such as its variability with respect to magnetospheric disturbance, time of day, season of the year, and solar cycle variability must be well understood and for more than 70 years scientists around the globe have been observing ionospheric variability using different observing techniques. Radio wave propagation is modified in a number of ways by the effects of the integrated electron density along the ionospheric ray path between the satellite and the receiver, the so-called total electron content (TEC). In consequence, TEC is a key parameter in the description of the impact of the ionized atmosphere on the propagation of radio signals. Therefore, understanding the variablity of TEC is crucial for the operation of many applications, including navigation satellite systems like Global Positioning System (GPS), Global Navigation Satellite System (GLONASS),and the future Galileo system. TEC can be measured by a number of essentially standard techniques, including Faraday rotation, group delay, and dispersive carrier phase. The TEC estimation technique from GPS satellite signal group delay and dispersive carrier phase advance is the popularly used method. Therefore, using data from both ground-based GPS receivers and the GPS receivers on Low Earth Orbit (LEO) satellites, GPS TEC measurments become one of the best for observing the entire vertical extent of the ionosphere (i.e. both the bottomside and topside ionosphere), especially for correcting satellite navigation systems. The Incoherent scatter RADAR, supperdan radar, and ionosondes measurements are also important ionospheric observations, manily the bottom side ionosphere.
In a landmark experiment on December 12, 1901, Marconi, who is often called the "Father of Wireless," demonstrated transatlantic communication by receiving a signal in St. John's Newfoundland that had been sent from Cornwall, England. Marconi's famous experiment showed the way toward worldwide communication, but it also raised a serious scientific dilemma. Up to this point, it had been assumed that electromagnetic radiation transmitted into the atmosphere travelled in straight lines in a manner similar to light waves. If this were true, the maximum possible communication distance would be determined by the geometry of the path. The radio signal would be heard up to the point where some intervening object blocked it. If there were no objects in the path, the maximum distance would be determined by the transmitter and receiver antenna heights and by the curvature of the Earth. Drawing from light as an analogy, this distance is often called the Line-of-Sight (LOS) distance. In Marconi's transatlantic demonstration, something different was happening to cause the radio waves to apparently bend around the Earth's curvature so that the communication signals could be heard over such an unprecedented distance. In 1902, Oliver Heaviside and Arthur Kennelly each independently proposed that a conducting layer existed in the upper atmosphere that would allow a transmitted electromagnetic (EM) signal to be reflected back toward the Earth. In 1902, Lodge was probably the first to suggest that the reflecting properties of the conducting layer are due to free electrons produced by the action of solar radiation. Until 1926 this conducting layer of the atmosphere was named the Heaviside layer or Appleton layer. In 1926 the British National Committee for radiotelegraphy suggested a non-personal name for the conducting layer of the atmosphere and the name ionosphere was coined by Watson-Watt and came into common use about 1932. The ionosphere is that part of the upper atmosphere comprising free electrons (ionization) that occur with a sufficient density to have an appreciable influence on the propagation of radio frequency electromagnetic waves. This ionization depends primarily on the Sun and its activity. It was, however, not until mid-1920’s that the existence of an ionized conducting layer in the upper atmosphere was fully proven. During the mid-1920’s, specifically in 1924, Edward Appleton and others demonstrated the existence of the ionosphere by studying the reflections of radio waves from the atmosphere, which is the basic idea used in the ionosonde invention. This then made possible the scientific communities’ dream to be able to observe the Earth’s ionosphere from the ground. Widespread use of the ionosonde permitted systematic scientific studies to be carried out to determine the ionsophere’s characteristics and variability and its effect on radio waves. Since then the ionosphere has been extensively studied and most of its principal features, though not all, are now fairly well understood in terms of the physical and chemical process of the upper atmosphere. An ionosonde is a radar that emits short bursts of radio waves at increasingly higher frequencies in the MF and HF bands. Each frequency is reflected at a particular value of electron density. By measuring the round trip time (ground to ionosphere and back) of each burst, it is possible to estimate the vertical profile of electron density immediately above the instrument, up to the height of the peak electron density occurring in the ionosphere. Typically, this is from about 100 km altitude to 250 or 450 km. The ionosphere extends through several distinct regions of the neutral atmosphere (such as mesosphere, thermosphere, and exosphere), forming layers which may be identified by their interaction with MF and HF radio waves. These layers are known as the D, E, and F layers, and their locations are shown in Figure for both night and day at mid-latitudes. The existence of such a conductive layer in the Earth's atmosphere, provided people around the globe with the ability to dramatically overcome the expected line-of-Sight limitation on radio communication.
Historically the ionosphere has been described as consisting of several layers. The earliest detected was the E layer, a name chosen by Appleton. The reason for his choice is not clear, but one suggestion is that in describing the reflected radio waves, he used the letter E to represent the electric field of the waves. In alphabetical order the D and F layers, below and above the E layer, respectively, have been named thereafter. The F region is usually further divided into F1 and F2 layers, because a second ledge sometimes appears in the profile below the main (F2) peak.
The D region is the lowest region of the ionosphere, between 50 and 90 km altitude. D region ionization is produced mainly by solar radiation of wavelength 102 to 122 nm (e.g. Lyman radiation). The primary positive ions formed in this region are and . Negative ion formation, due to electron attachment is also possible in this region. The free electron density in general is the lowest of all in this region and it varies widely depending on the time of day. It is greatest shortly after noon and is extremely small at night. There are also large seasonal changes - free electron densities are highest in summer and lowest in winter. This behaviour is basically that expected due to the diurnal and seasonal variations in the amount of solar radiation impinging on the D region caused by the rotation of the Earth on its axis about the Sun. The D region is the chief absorber of high-frequency radio waves because it has a relatively high atmospheric density, which results in high electron-neutral collision rates that cause energy to be lost from radio waves which propagate as oscillations within the electron medium. The amount of absorption decreases as the radio wave frequency increases and the D layer can cause absorption of radio signals at frequencies up to the low VHF band, however, there is no measurable effect on GPS signals.
Above the D region, from 90 to 130 km, is the E region. The principal ionization source of E region is the solar EUV radiation and the main resulting ions in this region are and. The maximum free electron density in the E region occurs at an altitude of about 110 km and is between 100 and 1,000 times larger than the D-region densities. Because of these larger densities and the reduced absorption of radio waves, the E region is much more important than the D region as a reflector of radio waves and it can allow communications beyond the horizon up to 2,000 km away. Layers in the E region are classified into normal and sporadic E layers. The normal E layer is similar in most respects to the D region, fluctuating on a daily and seasonal basis in almost exactly the same way under solar control. Sporadic E layers (highly variable thin layers) occur at different times of the day and during different seasons, depending on latitude. They occur most often during the day at mid-latitudes and in summer, while at higher latitudes they occur most often at night and in the winter. Their principal cause at mid-latitudes is a variation of wind velocity with height, in the form of a wind shear, which in the presence of the geomagnetic field, acts to compress the ionization into a layer. A sporadic E layer can greatly increase the radio wave frequencies that the E layer is able to reflect, however, it can also be a problem, as it may prevent such frequencies from reaching the F layer which supports propagation to a greater range in a single hop than does the E region.
The F region is the uppermost of the ionospheric regions and stretches from 130 km upwards, although current convention is to use the term “ionosphere” to refer to the region up to 1000 km altitude and the term “plasmasphere” to refer to the region of ionisation above 1000 km. Solar EUV radiation in the interval of wavelength 10-80 nm is the prime cause of F region ionization and the dominant ions are , , and. The F1 and F2 layers are the sub-component layers of the F layer. The F1 layer is a daytime phenomenon and forms in the altitude range 130 to 210 km. The free electron density is about 10 times that of the normal E region. Most radio waves propagating at oblique incidence that can penetrate the E region can also penetrate the F1 layer, so it has little effect on radio communications. The F2 layer is the highest layer in the ionosphere and also, normally, has the highest free electron density. This enhanced free electron density in spite of the fact that the extremely low density of the atmosphere at these heights means that the recombination rate is reduced significantly. However, even though the rate of ionisation production is very low in the F region and is decreasing with height, the loss due to recombination decreases even faster with height. The end result is that the electron density increases with height until diffusion becomes dominant, causing the electron density to then decrease with height, and hence the F2 peak is formed. At night, the lower recombination rate and a night time flux of ionisation from the plasmasphere above maintain the F2 layer so that it does not exhibit the same dominant diurnal solar control evident in the D and E layers. It is, however, subject to seasonal variations and changes in solar activity. Aspects of the auroral phenomenon at high latitudes occur in the F2 region and the influx of charged particles associated with these events can have important effects on radio wave propagation. The F2 region above the maximum ionization density peak is called the topside F2 layer of the ionosphere. It extends upward with decreasing density to a transition height where O+ ions become less numerous than either H+ or He+. The transition height varies but seldom drops below 500km at night or 800km in the daytime, although it may lie as high as 1100 km. Above the transition height, the smaller amount of ionization has relatively little influence on radio signals propagating through it. However, the effects can still be significant, such as for distant GPS satellites, when the amount of ionisation above the O+ - H+ transition height is a significant fraction of the total amount of ionisation between the satellite and a ground station.
There are many causes of variability in the ionospheric plasma. For example, the ionosphere responds to:
(a) solar EUV radiation, which causes the ionospheric electron concentration to vary diurnally, latitudinally, seasonally and over the 11-year cycle of solar activity.
(b) thermospheric winds which, through collisions, transport the ionospheric plasma along the magnetic field lines, thus raising or lowering the ionosphere.
(c) the composition of the thermosphere, which affects the rate at which ions and electrons re-combine.
(d) geomagnetic storms, in which energy deposited at high latitudes produces atmospheric waves and changes in thermospheric winds and composition. Both increases (“positive phases”) and decreases (“negative phases”) in the electron concentration occur.
(e) the geometry of the magnetic field and the presence of electric fields, causing plasma drift across magnetic field lines that leads to, for example, the equatorial anomaly which consists of an ionization trough at the geomagnetic equator and ionization crests on either side of the equator.
The name geomagnetic storm is traditional, because the storm phenomena were first observed as changes in the geomagnetic field, (called geomagnetic activity) and until recently these storms have been monitored primarily by measuring the geomagnetic activity. Geomagnetic storms are initiated by two important interplanetary structures, viz., fast Coronal Mass Ejections (CMEs) of plasma and long durations of intense southward interplanetary magnetic field (IMF) Bz. The long duration southward IMF interconnects with the Earth's magnetic field allowing solar wind energy to enter the magnetosphere and consequently the ionosphere. The coupling mechanism between the solar wind and the magnetosphere is magnetic reconnection. The interconnection of IMF and magnetospheric dayside fields leads to enhanced reconnection of the fields on the nightside with the continuous deep injection of plasma sheet plasma into the nightside. The processes become intensified during severe magnetic storm periods. The nightside continuous plasma injection into the magnetosphere leads to the formation of the storm time ring current, which is one of the major current systems in the Earth's magnetosphere. It circles the Earth in the equatorial plane and is generated by the longitudinal drift of energetic (10 to 200 keV) charged particles trapped on field lines between L ~ 2 and 7 (distance from the Earth measured in terms of the number of Earth radii). The Figure at the right depicts the solar-interplanetary-magnetospheric coupling. Ground-based magnetometers provide the best indicator or estimator of the level of geomagnetic disturbance of the plasma environment of the Earth and a variety of indices are used to describe the magnetic activity. The most frequently used are the Dst, AE, and Kp indices. Magnetic storms have three distinct phases:
- Storm onset phase, identified by an increase in Dst which is called the Storm Sudden Commencement (SSC),
- Storm main phase, consisting of a decrease of the Dst value to a minimum value which marks the end of the main storm phase and the beginning of the recovery phase. It also characterises the build-up of the intensified ring current due to high energy particle injection.
- Storm recovery phase is the return of Dst to its prestorm value.
Ionospheric storms represent an extreme form of 'space weather', which can have significant effects on many sophisticated ground and space based technology systems which are very important to our way of life. These effects include electric power brownouts and blackouts due to damaging currents induced in electric power grids, damage to satellites caused by high energy particles, increased risk of radiation exposure by humans in space and in high-altitude aircraft, changes in atmospheric drag on satellites, errors in GPS and VLF navigation systems, loss of HF communications, and disruption of UHF satellite links due to scintillations. Under certain circumstances the economic losses from geomagnetic storms could run into hundreds of millions of dollars. The international network of geomagnetic observatories monitors the onset of solar-induced storms and gives warnings to military and commercial operations and other facilities that can then take action to minimize the likelihood of economic losses. However our ability to predict in detail ionospheric storm effects is incomplete and this is one of the motivations for conducting research aimed at understanding ionospheric storm processes.