The ionosphere is a layer of the upper atmosphere, between the thermosphere and the exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because it influences radio propagation of LF, MF, HF and low VHF (30-54 MHz, but sometimes higher into mid-band VHF) frequencies to distant places on the Earth.(1)
What is the Ionosphere?
Earth's atmosphere varies in density and composition as the altitude increases above the surface. The lowest part of the atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The gases in this region are predominantly molecular Oxygen (O2) and molecular Nitrogen (N2). All weather is confined to this lower region, which contains 90% of the Earth's atmosphere and 99% of the water vapor. The highest mountains are still within the troposphere and all of our normal day-to-day flying activities occur here. The high altitude jet stream is found near the tropopause at the the upper end of this region.
The atmosphere above 10 km is called the stratosphere. The gas is still dense enough that hot air balloons can ascend to altitudes of 15 - 20 km and Helium balloons to nearly 35 km, but the air thins rapidly and the gas composition changes slightly as the altitude increases. Within the stratosphere, incoming solar radiation at wavelengths below 240 nm is able to break up (or dissociate) molecular Oxygen (O2) into individual Oxygen atoms, each of which, in turn, may combine with an Oxygen molecule (O2), to form ozone, a molecule of Oxygen consisting of three Oxygen atoms (O3). This gas reaches a peak density of a few parts per million (ppm) at an altitude of about 25 km (16 miles). The ozone layer is shown by the yellow shaded region in the figure to the left.
The atmosphere becomes increasingly rarefied at higher altitudes. At heights of 80 km (50 miles) the gas is so thin that free electrons can exist for short periods of time before they are captured by nearby positive ions. The existence of charged particles at this altitude and above signals the beginning of the ionosphere, a region having the properties of a gas and of a plasma.
At the outer reaches of the Earth's environment, solar radiation strikes the atmosphere with a power density of 1370 W per m2 or 0.137 W per cm2, a value known as the "solar constant." This intense level of radiation is spread over a broad spectrum ranging from radio frequencies through infrared (IR) radiation and visible light to X-rays. Solar radiation at ultraviolet (UV) and shorter wavelengths is considered to be "ionizing" since photons of energy at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision.
When incoming solar radiation interacts with a gas molecule or free radical, part of this radiation is absorbed by the atom and a free electron and a positively charged ion are produced. Cosmic rays and solar wind particles also play a role in this process but their effect is minor compared with that due to the sun's ionizing electromagnetic radiation.
At the highest levels of the Earth's outer atmosphere, solar radiation is very strong but there are few molecules to interact with, so ionization is small. As the altitude decreases, more gas molecules are present so the ionization process increases. At the same time, however, an opposing process called recombination begins to take place in which a free electron is "captured" by a positive ion if it moves close enough to it. As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules, free electons and ions are closer together. The point of balance between these two processes determines the degree of "ionization" present at any given time.
At still lower altitudes, the number of gas atoms and molecules increases further and there is more opportunity for absorption of energy from a photon of UV solar radiation. However, the intensity of this radiation is smaller at these lower altitudes because some of it was absorbed at the higher levels. A point is reached, therefore, where lower radiation, greater gas density and greater recombination rates balance out and the ionization rate begins to decrease with decreasing altitude. This leads to the formation of ionization peaks or layers (also called "Heaviside" layers after the scientist who first proposed their existence).
Because the composition of the atmosphere changes with height, the ion production rate also changes and this leads to the formation of several distinct ionization peaks, the "D," "E," "F1," and "F2" layers.
The Value and Importance of Ionospheric Research
In 1864, a Scottish mathematician named James Clerk Maxwell published a remarkable paper describing the means by which a wave consisting of electric and magnetic fields could propagate (or travel) from one place to another. Maxwell's theory of electromagnetic (EM) radiation was eventually proven correct by the German physicist, Heinrich Hertz in the late 1880's in a series of careful laboratory experiments.
It was not until the last decade of the 19th century that an Italian scientist named Guglielmo Marconi converted these theories and laboratory experiments into the first practical wireless telegraph system for which he was granted a British patent. In 1899, Marconi demonstrated his wireless communication technique across the English Channel.
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. Because of his pioneering work in the use of electromagnetic radiation for radio communications, Marconi was awarded the Nobel Prize in physics in 1909.
Line of Sight
Marconi's famous experiment showed the way toward world wide communication, but it also raised a serious scientific dilemma. Up to this point, it had been assumed that electromagnetic radiation traveled 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 as shown in Figure 1 to the left. 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 tranmitter and receiver antenna heights and by the bulge (or 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 from England could be heard over such an unprecedented distance. Skywave
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 signal to be reflected back toward the Earth. Up to this time, there was no direct evidence of such a region and little was known about the physical or electrical properties of the Earth's upper atmosphere. If such a conductive layer existed, it would permit a dramatic extension of the "Line-of-Sight" limitation to radio communication as shown in Figure 2 to the left. During the mid-1920's, the invention of the ionosonde allowed direct observation of the ionosphere and permitted the first scientific study of its characteristics and variability and its effect on radio waves.
The excitement of Marconi's transatlantic demonstration inspired numerous private and commercial experiments to determine the ultimate capabilities of this newly discovered resource, the ionosphere. Among the most important early experiments were those conducted by radio amateurs who showed the value of the so-called high frequencies above 2 MHz for long distance propagation using the ionosphere.
The Importance of Ionospheric Research
Although our society has learned to use the properties of the ionosphere in many beneficial ways over the last century, there is still a great deal to learn about its physics, its chemical makeup and its dynamic response to solar influence. The upper portions of the ionosphere can be studied to some extent with satellites but the lower levels are below orbital altitudes while still too high to be studied using instruments carried by balloons or high flying aircraft. Much of the current theory is inferred by observing the ionosphere's effect on communication systems. In addition, some very useful information has been obtained using sounding rockets (for example, from the Poker Flat Research Range near Fairbanks, AK). Active ionospheric research facilities, like HAARP, have provided detailed information that could not be obtained in any other way, about the dynamics and responses of the plasma making up the ionosphere. Incoherent Scatter Radars (ISRs), such as the one that will be built at the HAARP observatory, can study from the ground, small scale structures in the ionosphere to nearly the degree that an instrument in the layer could provide.
The ionosphere affects our modern society in many ways. International broadcasters such as the Voice of America (VOA) and the British Broadcasting Corporation (BBC) still use the ionosphere to reflect radio signals back toward the Earth so that their entertainment and information programs can be heard around the world. The ionosphere provides long range capabilities for commercial ship-to-shore communications, for trans-oceanic aircraft links, and for military communication and surveillance systems. The sun has a dominant effect on the ionosphere and solar events such as flares or coronal mass ejections can lead to worldwide communication "blackouts" on the short wave bands.
Signals transmitted to and from satellites for communication and navigation purposes must pass through the ionosphere. Ionospheric irregularities, most common at equatorial latitudes (although they can occur anywhere), can have a major impact on system performance and reliability, and commercial satellite designers need to account for their effects.
In the Auroral latitudes, the ionosphere carries a current that may reach magnitudes up to or beyond a million amperes. This current, which is called the auroral electrojet, can change in dramatic ways under solar influence, and, when it does, currents can be induced in long terrestrial conductors like power lines and pipe lines. While such effects found in nature cannot be reproduced by active ionospheric research, the sensitive instruments at observatories like HAARP can follow the progress of natural magnetic storms and provide insight into the physical mechanisms at work in the ionosphere.
To varying degrees, the ionosphere is a plasma, the most common form of matter in the universe, often called the fourth state of matter. Plasmas do not exist naturally on the Earth's surface, and they are difficult to contain for laboratory study. Plasmas appear as a result of electrical discharges inside low pressure gas electronic tubes, like fluorescent lamps. Many current active ionospheric research programs are efforts to improve our understanding of this type of matter by studying the ionosphere, the closest naturally occurring plasma.
Recently, it has become possible to produce computer simulations of ionospheric processes. The development of computer visualizations have allowed us to see and appreciate the enormous variability and turbulence that occurs in the ionosphere during a major solar geomagnetic storm and the resultant effects that can impact radio communication and navigation systems.
Active ionospheric research facilities like HAARP attempt to produce small temporary changes in a limited region directly over the facility which, in no way, compare to the worldwide events frequently caused by the sun. But the extraordinary suite of sensitive observational instruments installed at observatories like HAARP permit a detailed and comprehensive correlation with the induced effects, resulting in new insights into the ways the ionosphere responds to a much wider variety of natural conditions.
Protection provided by the ionosphere
Earth's atmosphere is a mixture of gases, mostly Nitrogen and Oxygen. At the surface, nearly all of these gases are in molecular form (ie., two atoms of Oxygen, O2 or two atoms of Nitrogen, N2 ). As the altitude above the earth increases, the density of the gases decreases rapidly and the makeup of the gases also changes as some of the molecules are broken into individual atoms by incoming solar radiation. The figure to the left shows how the concentration of atomic and molecular gases changes as the altitude above the earth's surface increases.
At ionospheric heights, atmospheric gases have thinned out dramatically. Moreover, at ionospheric altitudes, atomic Oxygen, O, dominates molecular Oxygen, O2. In ionospheric physics, these non-ionized particles are called "neutrals."
The gases at all heights provide protection from the sun's ultraviolet (UV) radiation. At the highest levels of the ionosphere where the F2 layer is found (above 250 km or 150 miles), the gases interact with Extreme Ultraviolet (EUV) radiation. At lower altitudes (less than 30 km or 20 miles), far below the height where HAARP has any effect, the gases interact with lower energy UV and create and are absorbed by the ozone layer. Again, HAARP has no affect on the gases at these lower altitudes.
In the ionosphere, protection is obtained when a neutral atom absorbs incoming radiation from the sun (a photon) and becomes an ion when one of its electrons is liberated. (Please also see the About the Ionosphere page where this process is discussed in greater detail.)
Prior to the absorption of the incoming EUV radiation, we have:
- One high energy (EUV) photon
- One Oxygen atom (a "neutral")
The photon gives up its energy in the collision and causes one of the electrons of the oxygen atom to be dislodged. The result is:
- No EUV photon (it has been consumed in the collision)
- One Oxygen ion (positively charged)
- One electron (negatively charged)
The result has been that a neutral (an oxygen atom) has been ionized and an incoming photon has been blocked. This is the process by which ionization occurs. Referring to the chart, at the height of the F2 layer where the peak of ionization occurs, the density of ionized atoms (almost entirely Oxygen at this altitude) is around 700,000 to 1,000,000 per cubic centimeter (cm3). Electrons have the same density, in order to ensure macroscopic electrical charge ballance. The density of non-ionized, or neutral Oxygen atoms is around 500,000,000 per cm3 or about 500 times as many in any given volume. The density of Nitrogen (molecular at this altitude) is equal to that of Oxygen (again 500 times as great as the ions).
We have used the heading image on this page to illustrate this point. The blue dots could represent the number of Oxygen neutrals in a given volume at 250 km (150 miles) the height of the peak ion density in the F2 layer. The green dots would then represent the number of Nitrogen neutrals present in the same volume. There are 1000 of each. The ions in this volume would then be represented by the two yellow dots, a ratio of 500 to one.
While it is certainly possible that an incoming EUV photon may collide with an already-ionized Oxygen atom, it is clear that the neutral Oxygen atoms greatly outnumber (by 500:1) the ionized Oxygen atoms. Clearly, the neutrals are the primary protection - not the ionized atoms. (Electrons, because of their very small cross section, do not afford any protection from UV radiation). Another way of looking at this is that the ionization in this part of the earth's atmosphere is the manifestation of the protection being afforded by the neutrals. The ionization does not, in itself, provide any meaningful protection and the fact that ionization disappears at night is further evidence that the protecting action of the neutrals has ceased temporarily, until the sun rises.
HAARP creates an external electric field at the F2 layer height. Particles interact with an electric field only if they are charged (ionized). As a result, HAARP only affects the 0.2% of the ionospheric volume directly over the facility that has already been ionized by the sun (the yellow dots in the image). The remaining 99.8% of the gas in this limited volume is in the neutral state and remains unaffected by HAARP and ready to intercept incoming UV radiation. That portion of the ionosphere that is not directly over the facility is not affected in any way by HAARP. As a result, there will be no impact produced by HAARP on the protective qualities of the earth's atmosphere. This was the conclusion of the environmental impact process, and the question was thoroughly studied by experts in the field prior to granting permission to proceed with the project.
It is very important to realize that the bulk composition of the gas in the volume that is being studied changes imperceptibly. The protective qualities of the atmosphere over HAARP do not change. It takes very sensitive instruments to observe the effects, and some of the best instruments currently available for this purpose are installed at the HAARP facility.
- K. Rawer. "Wave Propagation in the Ionosphere", Kluwer Acad.Publ., Dordrecht 1993. ISBN 0-7923-0775-5
- Kelley, M. C., "The Earth's Ionosphere", Academic Press, Inc, San Diego, 1989.
- Davies, Kenneth, "Ionospheric Radio", Peter Peregrinus Ltd., London, 1990.
- F.C. Judd, G2BCX: "Radio Wave Propagation (HF Bands)", Heinemann, London, ISBN 0-434-90926-2, 1987.
- G. Jacobs, W3ASK, T.J. Cohen, N4XX and R.B. Rose, K6GKU: "The new Shortwave Propagation Handbook", CQ Communications, Inc., ISBN 0-943016-11-8, 1995.
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