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In the highest reaches of the atmosphere, above about eighty kilometers, molecular and atomic constituents are energized and ionized by solar ultraviolet radiation and, at high latitudes, by energetic electrons of solar and magnetospheric origin. In this transition region between the Earth's atmosphere and near-Earth space (the ionosphere), free electrons abound. The resulting plasma responds to the geomagnetic field and to electric fields imposed by dynamos operating in the Earth's atmosphere and through interaction of its magnetosphere with the solar wind. In addition to the forces that control the neutral thermosphere, therefore, quite different influences also control the dynamics of the ionized gas making up the ionosphere.
With little doubt, the most widely recognized and beautiful of ionospheric phenomena is the aurora (borealis and australis). Produced by energetic electrons streaming along geomagnetic field lines, the auroral light results from the excitation of atoms, molecules, and ions at ionospheric altitudes above about 100 km. Controlled by electromagnetic forces far out in the Earth's magnetosphere, the aurora dances in a repertoire that ranges from "Swan Lake" to the "Ride of the Valkyries." The photo below was taken in central Alaska by Dr. David C. Fritts, Vice President and Sr. Research Scientist at NWRA's CoRA Division.
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Aurora
borealis photograph taken by Dr. David Fritts, NWRA, Boulder, CO. Click thumbnail for full-size version. |
Traditionally, the ionosphere has served humankind as an effective mirror from which to reflect shortwave radio signals, providing a capability for long-distance communications in essentially all parts of the world. From this perspective, the ionosphere is a benign entity, and knowledge about it and especially about its diurnal, seasonal and solar-produced variations is important for prediction of the frequencies and powers needed for such surface-to-surface communications.
In this age of satellite communications, the ionosphere takes on quite a different personality. Such communication systems operate at frequencies sufficiently high that their signals propagate through the ionosphere as light does through a window, rather than reflecting from it. Just as a structured window can spoil a view seen through it, an irregular ionosphere can disrupt a radio signal propagated through it. Viewed at radio wavelengths, a satellite moving behind plasma-density irregularities in the ionosphere, or in front of which such irregularities are drifting, scintillates for the same reason that a star viewed through the irregular neutral atmosphere scintillates, or "twinkles."
While a scintillating star may have its charm, a scintillating satellite does not, and the ionosphere is the culprit that causes the unsteadiness of signal received therefrom. Scintillation consists of random fluctuations in the intensity, the angle of arrival, and, for coherent sources, the phase of a radio signal. It is of interest both because of the communication degradation that it represents and because of the information it contains about the ionospheric structures that cause it.
These structures are strongest and most disruptive under conditions of magnetospheric/ionospheric disturbance, particularly near the geomagnetic equator and at high latitudes. Scientists at NWRA have studied this phenomenon extensively, including involvement in three satellites dedicated to ionospheric scintillation research (Wideband, HiLat, and Polar BEAR) and development of a state-of-the-art global model of the ionospheric plasma-density irregularities that cause scintillation and their effects on transionospheric propagation (the WBMOD model). The WBMOD model has been the primary tool used by the US Air Force since 1984 for assessing scintillation effects on operational systems. WBMOD has been distributed to over 30 individuals and organizations worldwide.
NWRA is expanding the capabilities of the research-oriented WBMOD program in the SCINTMOD program, an NWRA product that will provide more options to the user in both run modes and in types of output products. This new program is the core of the NWRA scintillation prediction page. The image below was generated using this code and the Generic Mapping Tool (GMT) graphics package.
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Sample Output from WBMOD Click thumbnail for full-size version. |
Coherent receivers also provide diagnosis of larger-scale ionospheric structures by measuring the integral of plasma density (the so-called total electron content, or TEC) along the radio line of sight. A chain of such receivers can provide a set of TEC records, including measurements at different view angles through a common volume of ionospheric plasma as a satellite orbits over it. Such multiple path-integral records can be inverted tomographically to produce two-dimensional images of plasma structures larger than the diffraction limit (typically a few km). Practicalities of satellite orbits, as well as radio-propagation effects such as refraction, usually limit the resolution to a few tens of km. NWRA has been a major development center of "ionospheric tomography."
Following up on simulations of "ionospheric tomography," NWRA engineers developed receivers (the NWRA ITS10) to be deployed in such a chain. In addition, NWRA scientists have collaborated with a physical oceanographer from the University of Washington to adapt inversion software, developed for ocean acoustic tomography, to the task. The ITS10 design also has been augmented (as the ITS10S and subsequently the ITS20S) to permit measurement of intensity and dispersive-phase scintillation. Recently a three-frequency version (the ITS30S) has been developed for use with forthcoming research satellites. Four ITS10S units, along with two CIDR receivers developed by the University of Texas at Austin's Applied Research Laboratory, now operate in a chain spanning Alaska from the Gulf to Arctic coasts. Tomographic images are available on the HAARP Web site. An example produced by four of the six receivers appears below.
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Tomographic image of electron density Click thumbnail for full-size version. |
In this era of dependence on the Global Positioning System (GPS), differences between TEC and its values predicted by ionospheric models contribute significantly to errors in the determination of range between the GPS satellites and single-frequency receivers. In addition, both receiver and satellite biases affect removal of TEC effects on two-frequency receivers. NWRA personnel have developed an algorithm for self-calibration of range errors (SCORE) to address the bias-determination problem for two-frequency receivers, permitting comparisons of accurate TEC measurements against the broadcast GPS TEC model. Examples of such comparisons are displayed on the HAARP Web site. An example appears below.
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TEC measurements compared to GPS model Click thumbnail for full-sized version |
Other NWRA projects have involved diagnosis of the ionospheric plasma by means of satellite-borne optical and insitu measurements, toward the end of refining scintillation and other operational codes.
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