The ionosphere is considered as a plasma consisting of positively charged ions and electrons. Ions and electrons are oscillating in close proximity, with a frequency termed ion and electron plasma frequency respectively. Due to the Earth's magnetic field, a Lorentz force is exerted which acts as a centripetal causing the ions and electrons to perform a circular motion around the magnetic field lines with a frequency termed ion and electron cyclotron frequency respectively. Due to their thermal motion, the ions and electrons are continuously spinning or performing a helical motion around the Earth's magnetic field lines. A more complex situation occurs upon the natural presence of an electric field (e.g. equator) or upon the effect of an electric field from an electromagnetic beam used with the purpose of ionosphere modification.
When an electromagnetic beam transverses the ionospheric plasma it accelerates its particles i.e. ions, electrons and neutrals, but most of all the electrons due to their small mass. Electrons collide with neutrals and ions but they do not transfer a significant amount of energy to them during collisions, because the mass of the electron is much smaller. Due to this fact, energy acquired by the wave, corresponding to ordered motion is transformed into random thermal energy of the electron. The final result is the extraction of energy from the wave and the increase of the thermal energy or temperature of the electrons in a process called electron heating. The electromagnetic wave is attenuated in this process.
Strong absorption of a radio wave occurs in a region where the plasma frequency is close to the radiofrequency because the wave is slowed near this natural resonance and the electrons moving under the influence of the wave are more likely to collide with ions.
As electron temperature increases, plasma pressure increases and starts streaming outside the heated region along the magnetic field lines until pressure is lowered. Following decrease of electron density, electromagnetic energy is focused on the region of reduced density leading to further heating and expansion. This results in large-scale irregularities in electron density, aligned along the geomagnetic field. These can be used to scatter electromagnetic waves with the purpose of transmissions/communications.
Parallely, due to non-linear wave interaction processes, upon modulation of a certain plasma parameter, it is possible that generation of “parametric wave-plasma instabilities” is induced. In this case, a high power HF electromagnetic wave provides the initial driving or pump field whose energy cascades (decays) into a lower frequency electrostatic electron plasma wave and a lower frequency ion-acoustic wave.
The electron density irregularities create an effective reflector of large radar cross section (≈ 105 to 109 m2) in the ionosphere. This has been used to transmit voice, teletype, facsimile and pulsed transmissions between ground terminals separated by thousands of kilometres.
The ionosphere is a layer of the atmosphere at a distance of 60 to 1000 Km from the Earth's surface, where solar and cosmic radiation act on molecules such as N2 and O2 leading to the release of electrons in the process of ionization. This results in the generation of a population of positive ions and free electrons in a gaseous phase called plasma. Plasma can be artificially generated by heating a neutral gas or by applying a strong electromagnetic field under the effect of which the gas becomes electrically conductive. The resulting positive ions and electrons can be influenced by long-range electromagnetic fields (Wikipedia). The plasma characteristics are defined by certain parameters which will be described below.
The ionosphere is usually considered as a cold weakly ionized gas, that is a plasma, with the Earth's magnetic field superimposed and is therefore thought to constitute a magnetoplasma. The electrons and the ions in a plasma are oscillating (moving back and forth naturally) or in other words, they are performing a simple harmonic motion with a specific angular frequency ω, corresponding to a frequency f, which for the electron is called electron plasma frequency and for the ion, ion plasma frequency. This frequency is proportional to the square root of the local ion/electron concentration. It is considered that if the electrons in a plasma are displaced from their equilibrium position, an electric field arises because of charge separation. This electric field produces a restoring force similar to that of a spring and is given by Hooke's law. Since the electrons have inertia, the system behaves as a harmonic oscillator. The resulting oscillations are called electron plasma oscillations or Langmuir oscillations and the oscillation frequency is called plasma frequency (ref.).
When a charged particle is injected into a magnetic field with a velocity v, we consider that its velocity is analyzed in two components, one that is parallel to the magnetic field lines and one that is perpendicular to these. Due to the perpendicular velocity component, a Lorentz force will be exerted by the magnetic field which will act as a centripetal force that will cause the particle to perform a circular motion with an angular velocity ω which corresponds to an angular frequency termed cyclotron frequency or gyromagnetic frequency. (It is noted that this velocity is given by the equation ω(c)= q B / m, where B is the magnetic field strength). We distinguish the electron cyclotron frequency and ion cyclotron frequency for electrons and ions respectively. Due to their thermal motions, the electrons and ions are continuously spinning or performing a helical motion in the Earth's magnetic field around its magnetic field lines (Figure 1). It is noted that the sense of their rotation is opposite and that the radius, being dependent on mass, is greater for the positive ion than for the electron.
Figure 1: The electrons and ions are continuously spinning or performing a helical motion in the Earth's magnetic field around its magnetic field lines with opposite sense of rotation and different radius (Figure 1.9 from https://books.google.com/books?id=qdWUKSj5PCcC)
In conclusion, in a plasma, ions and electrons oscillate in the proximity of each other and also spiral around the magnetic field lines.
A more complicated situation is presented when an electric field is imposed on the magnetoplasma. Electron and ions will drift in a direction perpendicular to both fields, in a process called electromagnetic drift, which constitutes the main mechanism that causes plasma to move across magnetic field lines. Electric fields are significant near the dip equator where vertical motions are important and in the auroral zones and polar caps where horizontal motions are important (ref.- section 18.104.22.168). Also, electric fields can be provided by electromagnetic beams with the purpose of ionosphere modification.
When a wave (with electric field E and angular frequency ω) transverses the ionospheric plasma, it sets in motion or accelerates its particles i.e. the ions, the electrons and the neutrals but mostly the electrons due to their small mass. Electrons collide with neutrals and ions but they do not transfer a significant amount of energy to them during collisions because the mass of the electron is much smaller. Due to this fact, energy acquired by the wave, corresponding to ordered motion is transformed into random thermal energy of the electron. The final result is the extraction of energy from the wave and the increase of the thermal energy or temperature of the electrons in a process called electron heating. The electromagnetic wave is attenuated in this process. The ionosphere consists of different regions, the D region present during the day closest to the Earth, the adjacent E region and the uppermost F region. Electron heating is easier in the F-region (compared for instance with the E-region) because of the low frequency of collision of electrons with ions and neutrals.
Strong absorption of a radio wave occurs in a region where the plasma frequency is close to the radiofrequency because the wave is slowed near this natural resonance and the electrons moving under the influence of the wave are more likely to collide with ions. As a result, energy is extracted from the wave and is added to the random electron thermal energy.
As electron temperature increases, plasma pressure increases and starts streaming outside the heated region. Since the electrons are constrained by the geomagnetic field to spiral around its magnetic field lines, the plasma expands along these lines until the pressure decreases to that of the surrounding medium. It is noted that electrons cannot move independently of the positive ions but are dragging these along, in a way that conserves the neutrality of the medium. Due to the heavy mass of the ions, a prolonged duration of heating is required before plasma starts expanding. Following decrease of electron density, electromagnetic energy is focused on the region of decreased density leading to further heating and expansion. This results in large-scale irregularities in electron density, aligned along the geomagnetic field and with transverse dimensions of approximately 1 km. It is noted that this occurs mainly in the F-region of the ionosphere. Therefore, heating (ohmic heating) produces thermal self-focusing leading to the formation of electron density irregularities that could scatter electromagnetic waves. One of the unexpected effects of the early HF ionospheric modification experiments was that except for the generation of large scale irregularities there was also generation of small scale (approximately 1 m) field-aligned irregularities at a certain distance from the above which caused backscatter to VHF and UHF waves.
Additionally, modification of the upper ionosphere by high power HF waves, may generate ELF and VLF waves in the lower ionosphere via modulation of the auroral electrojet which then emits these waves.
We consider that ionospheric modification consists of two mechanisms. The first is ohmic heating and the second is generation of “parametric wave-plasma instabilities”. The first is a non-linear but classical process and the second involves non-linear wave interaction processes. Specifically, in the second case, the term “parametric” refers to the periodic modulation of a certain parameter of an oscillating system with sufficient amplitude at a certain frequency to cause the oscillation to become unstable. An HF electromagnetic wave excites parametrically an instability, which decays (or in other words the EM wave decays) into a Langmuir wave and an ion acoustic wave. In other terms, a high power HF electromagnetic wave provides the initial driving or pump field whose energy cascades into a lower frequency electrostatic electron plasma wave and a lower frequency ion-acoustic wave.
Using a transmitter with an effective radiated power (ERP) of 0.5 MW or greater, large scale and small scale irregularities of electron density aligned with the Earth’s magnetic field, termed field-aligned irregularities develop within seconds due to ohmic heating and development of parametric instabilities and plasma waves. This necessitates that the heating transmitter frequency is below the critical frequency of the F-region i.e. the frequency that allows for reflection instead of penetration (≤ 12 MHz) and a frequency which matches the plasma frequency at some height in the ionosphere.
Upon these condition the electron density irregularities create an effective reflector of large radar cross section (≈ 105 to 109 m2) at altitudes of 250 to 300 km in the ionosphere.
The scattering properties of the field-aligned irregularities have been used to transmit voice, teletype, facsimile and pulsed transmissions between ground terminals separated by thousands of kilometres and using frequencies, ranging from HF to UHF, which would not otherwise have been useful for these paths.
F-region scattering is characterized by high degree sensitivity concerning reception location. The locations on the Earth at which signals are received by this scattering mechanism depend in part upon the geomagnetic position and the altitude of the modified ionospheric region.
In general, the signals can be received in an area on the equatorial side of the modified region which has a large East-West extent, ranging up to about 4 000 km, but only 200 to 500 km in North-South extent.
A strong scattering region near 110 km altitude in the E-region can also be produced when the heating transmitter is operating on frequencies below the E-region critical frequency. It appears that the scattering in this case is less aspect sensitive than that from the F-region and, thus, signals may be received on the ground in areas having a greater North-South extent than that found for the F-region.
It is noted that there could be interference between earth terminals and satellites, since scattering occurs in all directions defined by the scattering cone. Therefore, an earth transmitter may have energy scattered into space, and vice versa, due to the irregularities in the modified region.
1. “Ionospheric effects and operational considerations associated with artificial modification of the ionosphere and the radio-wave channel” (International Telecommunication Union) https://www.itu.int/rec/R-REC-P.532-1-199203-I/en
2. Ionospheric radio by Kenneth Davies https://books.google.com/books?id=qdWUKSj5PCcC
3. New Scientist - "Modifying the ionosphere with radio waves” http://bit.ly/2UyD5zW
Amateur radio operators (radioamateurs) may similarly use clouds to bounce off their transmissions or alternatively utilize miscallenous structures including buildings. For istance, Paris radioamateurs may use buildings at La Défense district.
Radio waves below 40 MHz are reflected by the ionosphere which waves beyond 40 MHs tend to penetrate the ionosphere. The ionosphere is considered as an ionized gas or plasma which has a dielectric constant that is a function of various parameters including electron concentration and frequency of operation. An angular plasma frequency ω can be defined which is purely a function of electron density.
• For frequencies > ω, the wave will be refracted by the plasma
• For frequencies < ω, the wave would be totally reflected
• For frequencies >> ω (VHF frequencies and above) “the waves simply pass through the plasma without significant refraction, but there can be other effects, especially if the plasma is magnetized by the Earth’s magnetic field and the medium becomes anisotropic. Waves at these frequencies undergo Faraday rotation by the ionosphere, whereby there polarization vector is rotated as the wave passes through the atmosphere.”
“Hence, there exists a minimum set of conditions on the electron density, frequency, and angle of incidence, for the wave to be returned to Earth.”
From this, it follows that for a given angle of incidence θ and frequency fob (where ob stands for oblique incidence), the minimum electron density required to achieve total internal reflection is Nmin (...).
“This value of fob is called the maximum usable frequency, and is less than 40 MHz, and can be as low as 25-30 MHz in period of low solar activity.”
“The maximum skip distance D = dmax can be achieved by aiming the radiation from the antenna so that the radiation leaves the antenna parallel to the Earth’s surface. This situation is shown in Figure 2. Factoring in atmospheric refraction, the maximum skip distance is given by dmax” “which shows that very large propagation distances are possible, especially when the upper ionospheric layers are used. For example, if the F layer is used, using an effective Earth radius of 8497 km gives a skip distance of 4516 km. Multiple skips are possible by using reflections of the earth to establish a multi-reflection process.”
Figure 2: A single skip of a radio wave using the ionosphere (Reference).
New Scientist - "Modifying the ionosphere with radio waves” http://bit.ly/2UyD5zW
The Platteville Atmospheric Observatory near Boulder, Colorado was one of the first major ionospheric heaters in the world. Linked to the Institute of Telecommunication Sciences (ITS) and the U.S. Department of Commerce and Telecommunications, it operated from 1968 to 1984 on ionospheric processes. It is still operational performing wind profile studies. The transmitting aerial array consisted of 10 elements forming a ring of 110 cm in diameter. Using an effective radiating power of 100 MW, the upgoing power would be distributed over a circular area in the ionosphere of 100 Km in diameter with an approximate power flux density of less than 50μW/m2. The installation was designed to perform ionospheric modification with frequencies from 5 MHz to 10 MHz using right or left circular polarization. It was found that depending on the polarization, there were different profiles of velocity and paths transversed during propagation. Right polarization was associated to “ordinary waves” and O-mode, while left polarization to “extraordinary waves” and X-mode.
When modification was performed with X-mode excitation, electron temperature would increase by 35% in the F-region area attained by the beam. Detection of electron heating was quantified by the attendant effect on the rate of dissociation-recombination of electrons and molecular oxygen ions which leads to the 630 nm emission of oxygen (red line) and the generation of air glow. For this process, the reaction rate is inversely proportional to electron temperature so that the emission intensity decreases compared to background when the temperature is rapidly increased, and increases after the heater is off. Experiments showed that the increase and decrease of electron temperature occurs within tens of seconds.
When modification was performed with O-mode excitation, there was an unexpected and nearly opposite result; there was an increase in the 630 nm (red) oxygen line after power-on and decrease after power-off. The generation of airglow (enhancement of natural airglow) implied that electrons excited oxygen by collisions. For this process, significant numbers of electrons with energies greater than 2 electron volts (eV) are required, when the ambient level is approximately 0.5 eV. Enhancement of other emission lines indicated that some electrons obtained energies equal to perhaps 10 eV. These processes appear to require generation of plasma "parametric" instabilities. The term "parametric" refers to the periodic modulation of a certain parameter of an oscillating system with sufficient amplitude at a certain frequency to cause the oscillation to become unstable.