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  • Weitere Quellen  (118)
  • Lunar and Planetary Science and Exploration  (92)
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  • 1
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    Magnetospheric Multiscale Satellites Observations of Parallel Electric Fields Associated with Magnetic Reconnection (2016)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: We report observations from the Magnetospheric Multiscale satellites of parallel electric fields (E (sub parallel)) associated with magnetic reconnection in the subsolar region of the Earth's magnetopause. E (sub parallel) events near the electron diffusion region have amplitudes on the order of 100 millivolts per meter, which are significantly larger than those predicted for an antiparallel reconnection electric field. This Letter addresses specific types of E (sub parallel) events, which appear as large-amplitude, near unipolar spikes that are associated with tangled, reconnected magnetic fields. These E (sub parallel) events are primarily in or near a current layer near the separatrix and are interpreted to be double layers that may be responsible for secondary reconnection in tangled magnetic fields or flux ropes. These results are telling of the three-dimensional nature of magnetopause reconnection and indicate that magnetopause reconnection may be often patchy and/or drive turbulence along the separatrix that results in flux ropes and/or tangled magnetic fields.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN41303 , Physical Review Letters (ISSN 0031-9007) (e-ISSN 1079-7114); 116; 23; 235102
    Format: text
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  • 2
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    Transient, Small-Scale Field-Aligned Currents in the Plasma Sheet Boundary Layer During Storm Time Substorms (2016)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: We report on field-aligned current observations by the four Magnetospheric Multiscale (MMS) spacecraft near the plasma sheet boundary layer (PSBL) during two major substorms on 23 June 2015. Small-scale field-aligned currents were found embedded in fluctuating PSBL flux tubes near the Separatrix region. We resolve, for the first time, short-lived earthward (downward) intense field-aligned current sheets with thicknesses of a few tens of kilometers, which are well below the ion scale, on flux tubes moving equatorward earth ward during outward plasma sheet expansion. They coincide with upward field-aligned electron beams with energies of a few hundred eV. These electrons are most likely due to acceleration associated with a reconnection jet or high-energy ion beam-produced disturbances. The observations highlight coupling of multiscale processes in PSBL as a consequence of magnetotail reconnection.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN41203 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 43; 10; 4841–4849
    Format: text
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  • 3
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    The Substructure of a Flux Transfer Event Observed by the MMS Spacecraft (2016)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: On 15 August 2015, MMS (Magnetospheric Multiscale mission), skimming the dusk magnetopause, detected an isolated region of an increased magnetic strength and bipolar Bn, indicating a flux transfer event (FTE). The four spacecraft in a tetrahedron allowed for investigations of the shape and motion of the FTE. In particular, high-resolution particle data facilitated our exploration of FTE substructures and their magnetic connectivity inside and surrounding the FTE. Combined field and plasma observations suggest that the core fields are open, magnetically connected to the northern magnetosphere from which high-energy particles leak; ion "D" distributions characterize the axis of flux ropes that carry old-opened field lines; counter streaming electrons superposed by parallel-heated components populate the periphery surrounding the FTE; and the interface between the core and draped regions contains a separatrix of newlyopened magnetic field lines that emanate from the X line above the FTE.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN41132 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 43; 18; 9434-9443
    Format: text
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  • 4
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    Terrestrial Analogues for Lunar Impact Melt Flows (2016)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: Lunar impact melt deposits have unique physical properties. They have among the highest observed radar returns at S-Band (12.6 cm wavelength), implying that they are rough at the decimeter scale. However, they are also observed in high-resolution optical imagery to be quite smooth at the meter scale. These characteristics distinguish them from well-studied terrestrial analogues, such as Hawaiian pahoehoe and a a lava flows. The morphology of impact melt deposits can be related to their emplacement conditions, so understanding the origin of these unique surface properties will help to inform us as to the circumstances under which they were formed. In this work, we seek to find a terrestrial analogue for well-preserved lunar impact melt flows by examining fresh lava flows on Earth. We compare the radar return and high-resolution topographic variations of impact melt flows to terrestrial lava flows with a range of surface textures. The lava flows examined in this work range from smooth Hawaiian pahoehoe to transitional basaltic flows at Craters of the Moon (COTM) National Monument and Preserve in Idaho to rubbly and spiny pahoehoe-like flows at the recent eruption at Holuhraun in Iceland. The physical properties of lunar impact melt flows appear to differ from those of all the terrestrial lava flows studied in this work. This may be due to (a) differences in post-emplacement modification processes or (b) fundamental differences in the surface texture of the melt flows due to the melts' unique emplacement and/or cooling environment. Information about the surface properties of lunar impact melt deposits will be critical for future landed missions that wish to sample these materials.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: GSFC-E-DAA-TN36349 , Icarus (ISSN 0019-1035); 281; 73-89
    Format: text
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  • 5
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    Magnetospheric Multiscale Observations of Magnetic Reconnection Associated with Kelvin-Helmholtz Waves (2016)
    In:  Other Sources
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    Publikationsdatum: 2019-08-26
    Beschreibung: The four Magnetospheric Multiscale (MMS) spacecraft recorded the first direct evidence of reconnection exhausts associated with Kelvln-Helmholtz (KH) waves at the duskside magnetopause on 8 September 2015 which allows for local mass and energy transport across the flank magnetopause. Pressure anisotropy-weighted Walen analyses confirmed in-plane exhausts across 22 of 42 KH-related trailing magnetopause current sheets (CSs). Twenty-one jets were observed by all spacecraft, with small variations in ion velocity, along the same sunward or antisunward direction with nearly equal probability. One exhaust was only observed by the MMS-1,2 pair, while MMS-3,4 traversed a narrow CS (1.5 ion inertial length) in the vicinity of an electron diffusion region. The exhausts were locally 2-D planar in nature as MMS-1, 2 observed almost identical signatures separated along the guide-field. Asymmetric magnetic and electric Hall fields are reported in agreement with a strong guide-field and a weak plasma density asymmetry across the magnetopause CS.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN41246 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 43; 11; 5606-5615
    Format: text
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  • 6
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    Electron Jet of Asymmetric Reconnection (2016)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: We present Magnetospheric Multiscale observations of an electron-scale current sheet and electron outflow jet for asymmetric reconnection with guide field at the subsolar magnetopause. The electron jet observed within the reconnection region has an electron Mach number of 0.35 and is associated with electron agyrotropy. The jet is unstable to an electrostatic instability which generates intense waves with E(sub parallel lines) amplitudes reaching up to 300 mV/m and potentials up to 20% of the electron thermal energy. We see evidence of interaction between the waves and the electron beam, leading to quick thermalization of the beam and stabilization of the instability. The wave phase speed is comparable to the ion thermal speed, suggesting that the instability is of Buneman type, and therefore introduces electron-ion drag and leads to braking of the electron flow. Our observations demonstrate that electrostatic turbulence plays an important role in the electron-scale physics of asymmetric reconnection.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN41316 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 43; 11; 5571-5580
    Format: text
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  • 7
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    Steepening of Waves at the Duskside Magnetopause (2016)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: Surface waves at the magnetopause flanks typically feature steeper, i.e., more inclined leading (antisunward facing) than trailing (sunward facing) edges. This is expected for Kelvin-Helmholtz instability (KHI) amplified waves. Very rarely, during northward interplanetary magnetic field (IMF) conditions, anomalous inverse steepening has been observed. The small-scale tetrahedral configuration of the Magnetospheric Multiscale spacecraft and their high time resolution measurements enable us to routinely ascertain magnetopause boundary inclinations during surface wave passage with high accuracy by four-spacecraft timing analysis. At the dusk flank magnetopause, 77%/23% of the analyzed wave intervals exhibit regular inverse steepening. Inverse steepening happens during northward IMF conditions, as previously reported and, in addition, during intervals of dominant equatorial IMF. Inverse steepening observed under the latter conditions may be due to the absence of KHI or due to instabilities arising from the alignment of flow and magnetic fields in the magnetosheath.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN40963 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 43; 14; 7373–7380
    Format: text
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  • 8
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    Observation of High-Frequency Electrostatic Waves in the Vicinity of the Reconnection Ion Diffusion Region by the Spacecraft of the Magnetospheric Multiscale (MMS) Mission (2016)
    In:  Other Sources
    Details
    Publikationsdatum: 2017-05-04
    Beschreibung: We report Magnetospheric Multiscale observations of high-frequency electrostatic waves in the vicinity of the reconnection ion diffusion region on the dayside magnetopause. The ion diffusion region is identified during two magnetopause crossings by the Hall electromagnetic fields, the slippage of ions with respect to the magnetic field, and magnetic energy dissipation. In addition to electron beam modes that have been previously detected at the separatrix on the magnetospheric side of the magnetopause, we report, for the first time, the existence of electron cyclotron harmonic waves at the magnetosheath separatrix. Broadband waves between the electron cyclotron and electron plasma frequencies, which were probably generated by electron beams, were found within the magnetopause current sheet. Contributions by these high-frequency waves to the magnetic energy dissipation were negligible in the diffusion regions as compared to those of lower-frequency waves.
    Schlagwort(e): Geophysics
    Materialart: GSFC-E-DAA-TN41309 , Geophysical Research Letters (ISSN 0094-8276; e-ISSN 1944-8007); Volume 43; Issue 10; 4808–4815
    Format: text
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  • 9
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    Opportunity, Geologic and Structural Context of Aqueous Alteration in Noachian Outcrops, Marathon Valley and Rim and Endeavour Crater (2016)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: In its 12th year of exploration and 1600 sols since arrival at the rim of the 22 km-diameter Noachian Endeavour impact crater, Mars Exploration Rover Opportunity traversed from the summit of the western rim segment "Cape Tribulation" to "Marathon Valley", a shallow trough dissecting the rim and the site of strong orbital detection of smectites. In situ analysis of the exposures within Marathon Valley is establishing some of the geologic and geochemical controls on the aqueous alteration responsible for smectite detection known to occur in crater rims throughout Noachian terrains of Mars.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: JSC-CN-35230 , Lunar and Planetary Science Confernece; Mar 21, 2016 - Mar 25, 2016; The Woodlands, TX; United States
    Format: application/pdf
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  • 10
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    A Probabilistic Asteroid Impact Risk Model (2016)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: Asteroid threat assessment requires the quantification of both the impact likelihood and resulting consequence across the range of possible events. This paper presents a probabilistic asteroid impact risk (PAIR) assessment model developed for this purpose. The model incorporates published impact frequency rates with state-of-the-art consequence assessment tools, applied within a Monte Carlo framework that generates sets of impact scenarios from uncertain parameter distributions. Explicit treatment of atmospheric entry is included to produce energy deposition rates that account for the effects of thermal ablation and object fragmentation. These energy deposition rates are used to model the resulting ground damage, and affected populations are computed for the sampled impact locations. The results for each scenario are aggregated into a distribution of potential outcomes that reflect the range of uncertain impact parameters, population densities, and strike probabilities. As an illustration of the utility of the PAIR model, the results are used to address the question of what minimum size asteroid constitutes a threat to the population. To answer this question, complete distributions of results are combined with a hypothetical risk tolerance posture to provide the minimum size, given sets of initial assumptions. Model outputs demonstrate how such questions can be answered and provide a means for interpreting the effect that input assumptions and uncertainty can have on final risk-based decisions. Model results can be used to prioritize investments to gain knowledge in critical areas or, conversely, to identify areas where additional data has little effect on the metrics of interest.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: ARC-E-DAA-TN37003
    Format: application/pdf
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