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  • 1
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Studies and models of trace-gas flux in the Arctic consider temperature and moisture to be the dominant controls over land–atmosphere exchange,, with little attention having been paid to the effects of different substrates. Likewise, current Arctic vegetation maps for models of vegetation ...
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2011-08-18
    Description: Two events are reported in which 2-10 keV electrons of solar energy have undergone significant adiabatic mirroring and pitch angle scattering in large scale magnetic structures in the interplanetary medium within a distance of about 0.5 AU from the earth. Electrons of 3 keV, typical of the energies measured, have a speed of about one-tenth of the speed of light, so that their travel time from the sun at 0 deg pitch angle would be about 100 minutes. Their cyclotron radius is about 20 km for a pitch angle of 30 deg, and a field of magnitude of 5 nT, and the cyclotron period is about 7.1 milliseconds. The electrons are scattered by spatial variations in the interplanetary magnetic field. When the spatial variations are convected past a stationary spacecraft by a 500 km/sec solar wind, they are seen as temporal fluctuations at a frequency of about 3 Hz.
    Keywords: SOLAR PHYSICS
    Type: Geophysical Research Letters; 8; July 198
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  • 3
    Publication Date: 2011-08-19
    Description: NASA's Fast Auroral Snapshot, or 'FAST' satellite, scheduled for launch in 1993, will investigate the plasma physics of the low altitude auroral zone from a 3500-km apogee polar orbit. FAST will give attention to wave, double-layer, and soliton production processes due to electrons and ions, as well as to wave-wave interactions, and the acceleration of electrons and ions by waves and electric fields. FAST will employ an intelligent data-handling system capacle of data acquisition at rates of up to 1 Mb/sec, in addition to a 1-Gbit solid-state memory. The data need be gathered for only a few minutes during passes through the auroral zone, since the most interesting auroral phenomena occur in such narrow regions as auroral arcs, electrostatic shocks, and superthermal electron bursts.
    Keywords: GEOPHYSICS
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  • 4
    Publication Date: 2019-07-13
    Description: We report on narrowband electromagnetic waves at frequencies between the local electron cyclotron and lower hybrid frequencies observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft in the Martian induced magnetosphere. The peaked electric field wave spectra below the electron cyclotron frequency were first observed by Phobos-2 in the Martian magnetosphere, but the lack of magnetic field wave data prevented definitive identification of the wave mode and their generation mechanisms remain unclear. Analysis of electric and magnetic field wave spectra obtained by MAVEN demonstrates that the observed narrowband waves have properties consistent with the whistler mode. Linear growth rates computed from the measured electron velocity distributions suggest that these whistler mode waves can be generated by cyclotron resonance with anisotropic electrons. Large electron anisotropy in the Martian magnetosphere is caused by absorption of parallel electrons by the collisional atmosphere. The narrowband whistler mode waves and anisotropic electrons are observed on both open and closed field lines and have similar spatial distributions in MSO and planetary coordinates. Some of the waves on closed field lines exhibit complex frequency-time structures such as discrete elements of rising tones and two bands above and below half the electron cyclotron frequency. These MAVEN observations indicate that whistler mode waves driven by anisotropic electrons, which are commonly observed in intrinsic magnetospheres and at unmagnetized airless bodies, are also present at Mars. The wave-induced electron precipitation into the Martian atmosphere should be evaluated in future studies.
    Keywords: Lunar and Planetary Science and Exploration; Geophysics
    Type: GSFC-E-DAA-TN40200 , Journal of Geophysical Research Space Physics (e-ISSN 2169-9402); 121; 10; 9717-9731
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  • 5
    Publication Date: 2019-07-13
    Description: As an airless body in space with no global magnetic field, the Moon is exposed to both solar ultraviolet radiation and ambient plasmas. Photoemission from solar UV radiation and collection of ambient plasma are typically opposing charging currents and simple charging current balance predicts that the lunar dayside surface should charge positively; however, the two ARTEMIS probes have observed energydependent loss cones and high-energy, surface-originating electron beams above the dayside lunar surface for extended periods in the magnetosphere, which are indicative of negative surface potentials. In this paper, we compare observations by the ARTEMIS P1 spacecraft with a one dimensional particle-in-cell simulation and show that the energy-dependent loss cones and electron beams are due to the presence of stable, non-monotonic, negative potentials above the lunar surface. The simulations also show that while the magnitude of the non-monotonic potential is mainly driven by the incoming electron temperature, the incoming ion temperature can alter this magnitude, especially for periods in the plasma sheet when the ion temperature is more than twenty times the electron temperature. Finally, we note several other plasma phenomena associated with these non-monotonic potentials, such as broadband electrostatic noise and electron cyclotron harmonic emissions, and offer possible generation mechanisms for these phenomena.
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC-E-DAA-TN9069 , Geophysical Research Letters; 39; 1; L01102
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  • 6
    Publication Date: 2019-07-12
    Description: The auroral ionosphere Langmuir-wave electric field amplitude modulation observations presented may be accounted for by a transverse modulational instability that is resonant with lower hybrid of ion Bernstein waves. In independent observations, the modulation frequencies were near the characteristic frequencies of perpendicular modes. These data strongly suggest the need for two- or three-dimensional model descriptions of nonlinear Langmuir wave evolution in the low-beta plasma.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 1177-118
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  • 7
    Publication Date: 2019-07-12
    Description: Two sounding rockets were used to simultaneously place two payloads on nearly the same magnetic field lines in the late-evening auroral zone. The dc electric field measurements obtained by the payloads imply a parallel potential of several kV below the higher payload, probably occurring between the two payloads. No possible resistive mechanism in observed in the 100 Hz-10 kKz band which could support the parallel field; this implies the existence of resistance-generating turbulence which are either at other frequencies, or of a type not measured by the probes, or localized parallel potentials not coincident with the payload trajectories.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 12173-12
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  • 8
    Publication Date: 2019-07-12
    Description: Shear Alfven waves with amplitudes greater than 100 mV/m were observed on two recent sounding rocket flights. The largest waveforms are best described as a series of step functions, rather than as broadband noise or as single frequency waves. Complete two-dimensional E and B measurements at 4-ms time resolution were made, showing a downward propagation direction and implying insignificant reflection from the ionosphere at frequencies greater than 1 Hz. Intense, field-aligned, low-energy electron fluxes accompany the waves. Acceleration of these electrons by the Alfven waves is shown to be feasible. The waves in at least one case have a sufficently large ponderomotive potential to generate the observed density fluctuations of order one.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 12157-12
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  • 9
    Publication Date: 2019-07-12
    Description: Particle distributions, waves, dc electric fields, and magnetic fields were measured by two sounding rockets at altitudes of 950 and 430 km through an energetic (greater than 5 keV) narrow (about 10 km) stable discrete arc. Although the payloads' magnetic footprints were separated by only 50 km, differences in the arc's structure were observed including the spatial width, peak energy, and characteristic spectra. The energetic electron precipitation included both slowly varying isotropic fluxes that formed an inverted-V energy-time signature and rapidly varying field-aligned fluxes at or below the isotropic spectral peak. The isotropic precipitation had a flux discontinuity inside the arc indicating the arc was present on a boundary between two different magnetospheric plasmas. Dispersive and nondispersive bursts of field-aligned electrons were measured throughout the arc, appearing over broad energy ranges or as monoenergetic beams. Dispersive bursts gave variable source distances less than 8000 km. Plateauing of some of the most intense bursts suggests that waves stabilized these electrons. During the lower altitude arc crossing, the field-aligned component formed a separate inverted-V energy-time signature whose peak energy was half the isotropic peak energy.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 6533-654
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  • 10
    Publication Date: 2019-07-10
    Description: We describe the basic principles, instrumentation, and feasibility of a multi-satellite mission that combines in situ observations of plasma and electromagnetic fields with radio tomography imaging. We show that a 16-satellite radio tomography experiment can produce two-dimensional images of plasma density in the earth's magnetosphere at sufficient spatial (1/2 R(sub E)) and temporal (approximately 10s) resolution to address key problems of magnetospheric physics. The same mission can incorporate electron and ion analyzers, magnetometers, and electric field instruments on the same spacecraft. We suggest that the large-scale images are more valuable when combined with in situ observations, supporting an unambiguous interpretation of the in situ data and an investigation of the interdependence of small- and large-scale plasma processes.
    Keywords: Geophysics
    Type: Science Closure and Enabling Technologies for Constellation Class Missions; 29-35
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