Ionospheric Tomography (Physics of Earth and Space Environments)


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Physics of Earth and Space Environments: Ionospheric Tomography by Viacheslav E. Kunitsyn and Evgeni D. Ionospheric Tomography by V. The detailed derivations and explanations make this book an excellent starting point for non-specialists. The monograph is devoted to a new branch of remote sounding of the iono sphere - ionospheric tomography. Adoption of tomographic methods seems to be an inevitable stage of the evolution of almost all diagnostic systems.

Ad vanced techniques for remote sensing and progressive means for data process ing open the possibility of reconstructing the spatial structure of the medium on the base of tomography.

PROPAGATION OF ELECTROMAGNETIC WAVES _ PART 02

In this book, mainly the problems of satellite radio tomography of the ionosphere are discussed, and only one subsection is alloted to optical ionospheric tomography. Modern radio sounding techniques make it possible by means of satellite facilities to probe the ionosphere within a wide range of varying positions of transmitting-receiving systems and to apply tomographic methods.

The most basic result of electromagnetic wave passage is a general heating of the plasma. The heating is caused by collisions among the accelerated charged particles, which randomizes the particle velocities. In addition to heating, a variety of plasma instabilities can be excited, generating a plethora of plasma wave modes. Studying these effects has led to the construction of facilities, colloquially known as ionospheric heaters, that transmit high-power high-frequency waves into the ionosphere and diagnose the impacts using remote sensing instruments.

Space weather is a relatively new discipline that encompasses the other disciplines of space physics but focuses on societal impacts and on prediction of events with potentially adverse effects. Examples of such effects include increased level of ionospheric scintillation causing outages of GPS signals, induced currents in power. Large space weather events can have severe societal impacts. One of the most cited examples is a March geomagnetic storm in which the Quebec power grid was disrupted, leaving over , customers without power for nearly 10 hours.

The goal of space weather research is to predict the occurrence and intensity of such space phenomena, with the expectation being that the availability of such predictions would allow system operators to take steps to mitigate the potentially adverse effects. Making predictions with useable accuracy, however, is a rather daunting task.

Predicting space weather is difficult for several reasons. First, the volume of space that contributes to the impacts is large. As a result, conditions at one location are influenced by conditions in regions that may be many tens of Earth-radii away. Secondly, the evolution of the magnetosphere-ionosphere system in time is a strong function of the drivers, but also depends on the state of the system at the time the drivers become active.

Changes of the main drivers, the solar wind and IMF, are communicated rapidly to the entire system and can induce effects within minutes after reaching the magnetopause.

The precise nature of the effects depends critically on the state of the system. For example, if an IMF transition that would increase the level of driving arrives at the magnetopause at a time when the system is quiescent, it would likely induce increased convection and a general expansion of the auroral oval toward lower latitudes. If, however, the magnetosphere is in an excited state when the transition arrives, the effects will be quite different.

Within a short time, the increased convection could result in a magnetospheric substorm, which is the rapid release of energy stored in the magnetotail, resulting in active auroral displays, large ionospheric currents, and highly structured ionospheric plasma. The sheer volume of the magnetosphere makes characterization using in situ sensing impractical. Hence, observations of the magnetosphere-ionosphere state can be obtained only through remote sensing. As discussed above, the high conductivity of the magnetospheric magnetic field lines makes these observations possible.

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The same convection and particle precipitation measurements used in basic magnetospheric and ionospheric research apply to space weather as well. Remote sensing observations can be used for a variety of space weather measurements, including the strength of convection, the latitude of auroral precipitation, the location of auroral currents, and the location of potentially scintillation-causing irregularities.

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National Science Foundation supports the operation of six incoherent-scatter radar facilities, three of which are in the United States. Following the scientific background, spectrum usage is discussed beginning with the lowest active remote sensing frequencies. Secondly, the evolution of the magnetosphere-ionosphere system in time is a strong function of the drivers, but also depends on the state of the system at the time the drivers become active. In each section, a general background is provided along with some pertinent issues to which active remote sensing is applied. GIRO is a distributed network of ionosondes that provide specification of the ionospheric electron density below the F-region peak altitude over much of the globe.

Ionospheric active sensing observations use radio transmitters that operate at frequencies ranging from kilohertz up to a few gigahertz, with certain ranges being more actively used than others. The observations come primarily from ground-based radar systems but include satellite-borne radio beacons. This section provides a summary divided into ground-based and space-based systems, starting with the lowest frequencies and working upward. In each case, the choice of frequency is directly related to the type of information sought from the observations.

In almost every case, the frequency is determined by the properties of the target medium. In order to sense the plasma for the desired measurement, it is necessary to use a signal that interacts with it in a specific way. Using a different frequency would not provide the same information. For example, an ISR operating in the UHF band provides information on the density and temperature of the plasma, while a coherent-scatter radar operating in the HF or VHF bands provides information on the spectrum of irregularities present in the plasma.

While both instruments provide information on the same plasma, the measurements are distinct. The lowest transmitter frequencies used in space physics research are in the VLF band. The Siple Station, Antarctica, antenna was a 40 km dipole transmitting signals at frequencies around 3 kHz, which were received at the conjugate point in the northern hemisphere at Roberval, Canada.

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When these transmissions reached the magnetosphere, they interacted with the trapped energetic particle populations, resulting in amplification of the signals and the triggering of other emissions. From the observed signatures it was possible to draw conclusions about the particle populations, and about the wave-particle interactions. No dedicated space physics transmitters operating at the VLF band exist today.

To overcome this lack of dedicated transmitters, the research continues using other means. Magnetospheric research using the VLF band has been continued by using HF band heating facilities to generate the signals by modulating electrical currents flowing in the ionosphere. Ionospheric research using VLF signals currently relies on radio navigation and time signals transmitted by various government agencies in the kHz band.

Small phase and amplitude. The next most widely used band for space science is the HF band.

When a wave propagates into a region where the plasma frequency is comparable to but below the wave frequency, the propagating wave experiences a decrease in velocity and a change in direction. As the wave approaches an altitude where the plasma frequency is equal to the wave frequency, the phase index of refraction approaches zero and the wave reflects. Usually an ionosonde begins its transmissions at a frequency of around 1 MHz or lower and steps to some upper frequency determined by the highest expected plasma frequency, which may be 20 MHz or higher.

At each frequency step, short pulses are transmitted, then reflected by the plasma and received by the ionosonde receiver. The time between transmission and reception is used to estimate the altitude of the reflection boundary. Successively higher frequencies reflect from higher altitudes until a frequency is reached that is above the plasma frequency at the density peak altitude.

Beyond this frequency, no more reflections are observed. There are a few different ionosonde designs in use today.

Ionospheric Tomography

The most common model is the Digisonde, manufactured by Lowell Digisonde International. The system can transmit between kHz and 30 MHz, with a peak power of W. GIRO is a distributed network of ionosondes that provide specification of the ionospheric electron density below the F-region peak altitude over much of the globe.

Ionospheric heating uses high-power transmitters to study the interaction of radio-frequency electromagnetic waves with plasmas, with waves of high enough amplitude to cause measurable effects. In heating experiments, energy is transferred from the electromagnetic waves to the plasma. The mechanism for the energy transfer depends on the properties of the plasma and the frequency and polarization of the waves.

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If the frequency of the wave is close to one of the resonant frequencies of the plasma, energy coupling is highly efficient, and the transfer can lead to explosive growth of plasma instabilities. Away from a resonant frequency, the energy transfer is less efficient, but because of collisions among the plasma constituents or between the plasma and the neutral gas, the energy transfer leads to a general heating of the plasma, modifying the properties of the plasma, such as its electrical conductivity. In addition, the heating can lead to motion of the plasma through diffusion and thermal upwelling.

Because of energy coupling mechanisms,. In the process of radio-frequency ionospheric heating, interesting and useful effects can be produced. For example, under suitable ionospheric conditions, operating the transmitter in a mode that deposits energy into the D-region and lower E-region can alter the ionospheric conductivity and modulate the naturally occurring electrojet currents, which as a result radiate electromagnetic waves at the modulation frequency.

Other effects include the generation of field-aligned irregularities in the plasma, artificial auroras, large-scale modification of the plasma density, and stimulated electromagnetic emissions. In the United States, there are two HF band heating facilities. The facility has a large phased-array antenna consisting of crossed dipoles covering about 33 acres. It is the highest-power, broadest-frequency, and most flexible facility of its kind.

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The facility will use an array of dipoles below a wire subreflector suspended over the m diameter dish. While the facility will not cover the same range of frequencies as HAARP or achieve the same power level, it will have the advantage of being colocated with the Arecibo Incoherent-Scatter Radar, which is the most sensitive ISR in the world. Having such a powerful diagnostic instrument is a significant advantage for detailed studies of the plasma processes under investigation.

SuperDARN is an international network of coherent-scatter radars used for observing plasma flows in the ionosphere. At present, the network consists of 33 radars distributed around the globe, with 22 in the Northern Hemisphere and 11 in the Southern Hemisphere. The observation mechanism used by coherent-scatter radars differs from that of ionosondes.

While it is still necessary to use a frequency that interacts with the plasma, the scattering mechanism is Bragg scatter from field-aligned irregularities, which arises in regions where there are spatially periodic fluctuations of the plasma density with a wavelength equal to half of the probing wavelength. Regions of plasma turbulence have a broad spectrum of wavelengths, so usually if turbulence is present there will be a component with the appropriate spatial scale.

Scattering occurs whenever the wavelength-matching criterion is met, but the scattered signal returns to the radar location only when it is directed along the line from which it came. SuperDARN uses the HF band because refraction of the signals bends them toward the horizontal, resulting in perpendicularity to the magnetic field over large regions of space and particularly in the F-region at. The lack of significant refraction at the VHF band, however, greatly impacts the focus of the research. Without refraction, however, perpendicularity to the field at F-region altitudes cannot be achieved at high latitudes.

The result is that research investigations using VHF coherent-scatter radars focus primarily on observations of E-region plasma irregularities. In the United States, there are two currently operational fixed-location VHF radars for ionospheric remote sensing: The radars are operated by Cornell University on a campaign basis at a frequency of Additional portable radars are operated during research campaigns at a variety of locations.

These radars operate at National Science Foundation supports the operation of six incoherent-scatter radar facilities, three of which are in the United States. Moreover, MIRTO is a valid tool of electromagnetic monitoring of the Mediterranean area useful also to verify the existence of ionospheric events due to seismic occurrences.

MIDAS has been recently applied to produce, for the first time, a tomographic imaging of the ionosphere over Antarctica Fig. Main content Navigation bar Inside menu Footer Search: Geomagnetic Phenomena Sun-Earth Relations: Units Ionospheric Observatories and Electromagnetic Detection. Facilities eSWua Data Bases.