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  • GEOPHYSICS  (12)
  • Geochemistry, Geophysics, Online Only, Planetary Science  (3)
  • 1
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    The low-frequency source of Saturns kilometric radiation (2018)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publikationsdatum: 2018-10-05
    Beschreibung: Understanding how auroral radio emissions are produced by magnetized bodies requires in situ measurements within their source region. Saturn’s kilometric radiation (SKR) has been widely used as a remote proxy of Saturn’s magnetosphere. We present wave and plasma measurements from the Cassini spacecraft during its ring-grazing high-inclination orbits, which passed three times through the high-altitude SKR emission region. Northern dawn-side, narrow-banded radio sources were encountered at frequencies of 10 to 20 kilohertz, within regions of upward currents mapping to the ultraviolet auroral oval. The kilometric waves were produced on the extraordinary mode by the cyclotron maser instability from 6– to 12–kilo–electron volt electron beams and radiated quasi-perpendicularly to the auroral magnetic field lines. The SKR low-frequency sources appear to be strongly controlled by time-variable magnetospheric electron densities.
    Schlagwort(e): Geochemistry, Geophysics, Online Only, Planetary Science
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Geologie und Paläontologie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
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    In situ collection of dust grains falling from Saturns rings into its atmosphere (2018)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publikationsdatum: 2018-10-05
    Beschreibung: Saturn’s main rings are composed of 〉95% water ice, and the nature of the remaining few percent has remained unclear. The Cassini spacecraft’s traversals between Saturn and its innermost D ring allowed its cosmic dust analyzer (CDA) to collect material released from the main rings and to characterize the ring material infall into Saturn. We report the direct in situ detection of material from Saturn’s dense rings by the CDA impact mass spectrometer. Most detected grains are a few tens of nanometers in size and dynamically associated with the previously inferred "ring rain." Silicate and water-ice grains were identified, in proportions that vary with latitude. Silicate grains constitute up to 30% of infalling grains, a higher percentage than the bulk silicate content of the rings.
    Schlagwort(e): Geochemistry, Geophysics, Online Only, Planetary Science
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Geologie und Paläontologie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
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    Dust grains fall from Saturns D-ring into its equatorial upper atmosphere (2018)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publikationsdatum: 2018-10-05
    Beschreibung: The sizes of Saturn’s ring particles range from meters (boulders) to nanometers (dust). Determination of the rings’ ages depends on loss processes, including the transport of dust into Saturn’s atmosphere. During the Grand Finale orbits of the Cassini spacecraft, its instruments measured tiny dust grains that compose the innermost D-ring of Saturn. The nanometer-sized dust experiences collisions with exospheric (upper atmosphere) hydrogen and molecular hydrogen, which forces it to fall from the ring into the ionosphere and lower atmosphere. We used the Magnetospheric Imaging Instrument to detect and characterize this dust transport and also found that diffusion dominates above and near the altitude of peak ionospheric density. This mechanism results in a mass deposition into the equatorial atmosphere of ~5 kilograms per second, constraining the age of the D-ring.
    Schlagwort(e): Geochemistry, Geophysics, Online Only, Planetary Science
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Geologie und Paläontologie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
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    Observations of a free-energy source for intense electrostatic waves (1980)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: Significant progress has been made in understanding intense electrostatic waves near the upper hybrid resonance frequency in terms of the theory of multiharmonic cyclotron emission using a classical loss-cone distribution function as a model. Recent observations by Hawkeye 1 and GEOS 1 have verified the existence of loss-cone distributions in association with the intense electrostatic wave events, however, other observations by Hawkeye and ISEE have indicated that loss cones are not always observable during the wave events, and in fact other forms of free energy may also be responsible for the instability. Now, for the first time, a positively sloped feature in the perpendicular distribution function has been uniquely identified with intense electrostatic wave activity. Correspondingly, we suggest that the theory is flexible under substantial modifications of the model distribution function.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters; 7; May 1980
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
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    Escaping nonthermal continuum radiation (1981)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: Gurnett (1975) provided details of the continuum radiation spectrum, separating the lower frequency trapped component from an escaping component at frequencies high enough to propagate freely out of the magnetosphere and into the solar wind. The primary motivation for the present investigation is related to the new ISEE observations which show that the escaping continuum radiation differs from the trapped component in several temporal and spectral characteristics. The observations of escaping nonthermal continuum radiation are considered, taking into account spectral and temporal characteristics of escaping continuum radiation and the source region of escaping continuum radiation. Attention is given to the relationship between escaping and trapped continuum radiation, and the relationship between terrestrial escaping continuum radiation and narrowband Jovian kilometric radiation
    Schlagwort(e): GEOPHYSICS
    Materialart: AD-A104719 , Journal of Geophysical Research; 86; July 1
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 6
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    Detailed observations of the source of terrestrial narrowband electromagnetic radiation (1982)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: Detailed observations are presented of a region near the terrestrial plasmapause where narrowband electromagnetic radiation (previously called escaping nonthermal continuum radiation) is being generated. These observations show a direct correspondence between the narrowband radio emissions and electron cyclotron harmonic waves near the upper hybrid resonance frequency. In addition, electromagnetic radiation propagating in the Z-mode is observed in the source region which provides an extremely accurate determination of the electron plasma frequency and, hence, density profile of the source region. The data strongly suggest that electrostatic waves and not Cerenkov radiation are the source of the banded radio emissions and define the coupling which must be described by any viable theory.
    Schlagwort(e): GEOPHYSICS
    Materialart: AD-A125949 , Geophysical Research Letters; 9; Dec. 198
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
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    Direction-finding measurements of auroral kilometric radiation (1975)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: Direction-finding measurements with plasma wave experiments on the Hawkeye 1 and Imp 8 satellites are used to locate the source region of auroral kilometric radiation. This radiation has peak intensities between about 100 and 300 kHz and is emitted in intense sporadic bursts lasting for from half an hour to several hours. At peak intensity the total power emitted in this frequency range exceeds 1 billion W. The occurrence of this radiation is known to be closely associated with bright auroral arcs which occur in the local evening auroral regions. Hawkeye 1 provides direction-finding measurements of kilometric radiation from observations at high latitudes over the northern polar regions, and Imp 8 provides similar observations at large radial distances near the equatorial plane. Results from both satellites place the source of the intense auroral kilometric rdiation in the late local evening at about 22.0 hours LT and at a distance of about 0.75 earth radii from the polar axis of the earth.
    Schlagwort(e): GEOPHYSICS
    Materialart: Journal of Geophysical Research; 80; July 1
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 8
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    Electron distribution functions associated with electrostatic emissions in the dayside magnetosphere (1979)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: The analysis deals with electron distribution functions constructed from ISEE-1 dayside observations of low-energy magnetospheric electrons at low latitudes. The discussion covers some examples corresponding to three distinct types of odd half harmonic electrostatic emissions, observed in the plasma wave experiment on ISEE-1, which occur between the electron cyclotron frequency and the upper hybrid frequency.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters; 6; Oct. 197
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
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    A comparison of intense electrostatic waves near f-UHR with linear instability theory (1979)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: Intense electrostatic waves near the upper hybrid resonance frequency (f-UHR) have recently been detected near but beyond the plasmapause between + or -50 deg magnetic latitude at all local times. The paper shows that the electrostatic waves observed with the ISEE 1 plasma wave receiver near f-UHR are in qualitative agreement with linear theory of multicyclotron harmonic emissions. The peak amplitudes and bandwidths observed are similar to those calculated for maximum spatial growth rates. Measurements on Hawkeye of similar waves suggest that the electric field polarization is also consistent with theory. For a wide range of plasma parameters, nonconvective instability or large spatial growth rates occur within the cyclotron band encompassing the cold upper hybrid frequency.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters; 6; June 197
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 10
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    Intense electrostatic waves near the upper hybrid resonance frequency (1979)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: Plasma wave measurements using instruments on the Imp 6 and Hawkeye satellites are utilized in a study of very intense electrostatic waves near the upper hybrid resonance frequency in the region just outside the plasmapause. Studies of these electrostatic disturbances show that the events occur at local times and at magnetic latitudes varying from the equator to 50 deg, and the polarization of these waves is such that the wave electric field vector is oriented perpendicular to the geomagnetic field. In most cases the center frequency of the intense waves corresponds to an (n + 1/2) fg(-) harmonic near the upper hybrid resonance frequency. The hot distribution on function is described for a few events showing temperature anisotropy and a loss cone distribution. A possible mechanism for producing intense waves near the upper hybrid resonance frequency is suggested, and evidence which indicates that the intense electrostatic waves may be a source of nonthermal continuum radiation is given.
    Schlagwort(e): GEOPHYSICS
    Materialart: AD-A075294 , Journal of Geophysical Research; 84; Aug. 1
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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