To intercompare various techniques used in reconstructing tomographic images, and to benchmark those results with direct observations obtained by the incoherent scatter technique, an experimental campaign and subsequent analysis program-the Russian-American Tomography Experiment (RATE)-were implemented in late 1993. Russian experiment teams from the Polar Geophysical Institute in Murmansk and Moscow State University joined with American investigators from the Phillips Laboratory and the Massachusetts Institute of Technology, and an array of four receiving stations was set up in the northeastern United States and in eastern Canada to obtain data for the tomographic reconstructions. Phase-difference and total-phase tomographic reconstruction techniques have been employed and are intercompared. The spatial/altitude distribution of ionospheric electron content was observed by the MIT Millstone Hill incoherent scatter radar that scanned the ionosphere in a plane parallel to the satellite overflights. We present preliminary reconstructions of the ionospheric structure observed during a severe midlatitude ionospheric storm that took place during the campaign. The drastic large-scale changes in the ionospheric structure that accompanied the November 1993 storm were well observed by the two diagnostic techniques.
Data collection for the first ground‐based ionospheric tomography campaign in North America was conducted over a 48‐hour period in mid‐November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U. S. Navy Navigation Satellite System spacecraft over a chain of ground‐based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual‐frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two‐dimensional (2‐D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2‐D contour “maps” of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the experiment is discussed.
Abstract. Data from the charging hazards and wake studies (CHAWS) flight experiments on board space shuttle missions STS-60 and STS-69, during which a negatively biased, high-voltage (0-5 kV) probe was placed in a plasma wake in low Earth orbit, are presented. For these experiments the source of the wake was the 4-m-diameter Wake Shield Facility (WSF), which was operated both as a free-flying spacecraft and attached to the shuttle orbiter's robot arm. Current collection by the biased probe is investigated as a function of the density and temperature of the ambient plasma and the probe's location in the plasma wake. Current collection behavior is determined by the expansion of the highvoltage sheath into the ambient plasma stream. Consistent with preflight predictions, current collection on the probe is highly nonuniform, varying by more than 5 orders of magnitude across the surface of the probe. The onset of current collection, however, begins at voltages that are an order of magnitude lower than anticipated. This is likely due to the low-energy, turbulent plasma (typically 2-5% of the ambient density and up to 40% on occasion) observed in the ambient environment. This important minority constituent of the plasma was observed in the vicinity of the shuttle Orbiter and observed while the WSF was free-flying.
or combining multiple detectors. In contrast, thermal plasmas are more easily measured in bulk, with differential information being more difficult to obtain. Retarding Potential Analyzers (RPAs) and Drift Meters (DMs) are often used to measure thermal ions. However, their capability is limited by hardware factors such as a lower limit on measuring electric current, and by the sensitivity of low-energy particles to the structure of fields often used in such devices. These factors dictate a minimum detector size and limit low-density measurement capability. The Digital Ion Drift Meter (DIDM) is being developed as a replacement for analog spacecraft driftmeters. It combines the single-ion counting capability of a Micro-Channel Plate (MCP) with a position-determining anode. Careful electrostatic design is needed to create an instrument that can simultaneously measure bulk and differential plasma propert,ies at very low densities. A simple pinhole to image ions straight onto an MCP would be light-sensitive. Instead, the DIDM design turns ions around and accelerates them using the MCP's surface voltage. The imaging effect of the aperture is preserved by careful CAD modeling of the detector's geometry. Performance has been opt,imized by simulating the trajectories of ions entering the device. Computer programs are used to simulate trajectories over a comprehensive velocity spectrum. The models are evolved into manufacturing specifications and verified in a calibration facility.Comparisons between models and prototype performance are good. This approach speeds the design cycle and provides a capability to investigate effects beyond the reach of practical lab techniques, such as very-low-energy particles and finiteaperture effectsIn recent years, orbital and sub-orbital tethered satellite systems have seen their first flight demonstrations. These first flights have principally emphasized electrodynamic applications using tethers with conducting wires. For example, the First Tethered Satellite System (TSS-I) deployed an electrically conductive, 1.6 m diameter satellite to a distance of 267 m above the payload bay of the Space Shuttle on an insulated, conducting tether. The reflight of TSS-1, dubbed TSS-lR, should deploy the same satellite out to 20.7 km and capture useful data on how an electrodynamic tether system functions in the ionosphere. Future engineering and scientific applications of electrodynamically tethered systems have been proposed, including: power and thrust generation for orbiting systems [l], including possibly the International Space Station Alpha [2]; large orbiting ULF/ELF/VLF communication antennas 131; extremely-long-baseline double probes for measurement of ionospheric electric field structure [41; remote sensing of planetary geologies from orbit; and radio astronomy [5].Before such electrodynamic tether systems can be fully exploited, however, a complete understanding of their electrical response is needed. Understanding the plasmaiconductor system requires knowledge of its steady-state and its tr...
space such as AKAKS, CRRES, etc.. Since potential difference can be varied in a wide range both cold plasma parameter!; and high energy particles characteristics can be measured with the same space tethers used as a diagnostic probe. New possibilities related to the case of applied AC voltage in a frequency range wp, << w <( wpe are discussed. REFERENCES[l] Danilov V.V., Yu.V.Vasil'yev"Active experiment in space: man-made control of particle precipitation from the Earth's radiation belts using high-voltage string system", Ilokl.
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