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  • 1
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    An Artificial Neural Network for Inferring Solar Wind Proxies at Mars (2018)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We present a novel method to determine solar wind proxies from sheath measurements at Mars. Specifically, we develop an artificial neural network (ANN) to simultaneously infer seven solar wind proxies: ion density, ion speed, ion temperature, and interplanetary magnetic field magnitude and its vector components, using spacecraft measurements of ion moments, magnetic field magnitude, magnetic field components in the sheath, and the solar extreme ultraviolet flux. The ANN was trained and tested using3 years of data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. When compared with MAVEN spacecraft's in situ measured values of the solar wind parameters, we find that the ANN proxies for the solar wind ion density, ion speed, ion temperature, and interplanetary magnetic field magnitude havepercentage differences of 50% or less for 84.4%, 99.9%, 86.8%, and 79.8% of the instances, respectively. Forthe cone angle and clock angle proxies, 69.1% and 53.3% of instances, respectively, have angle differences of 30* or less.
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC-E-DAA-TN62945 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 45; 20
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  • 2
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    Magnetic Reconnection on Dayside Crustal Magnetic Fields at Mars: MAVEN Observations (2018)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The identification of magnetic reconnection on the dayside of Mars has been elusive owing to the lack of comprehensive plasma and field measurements. Here we present direct measurements of dayside in situ reconnection signatures by the comprehensive particles and fields package on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft over strong crustal magnetic fields in the southern hemisphere of Mars. During a crossing of a bifurcated current sheet consisting of northward and southward magnetic fields, MAVEN recorded (i) ionospheric photoelectrons trapped on closed magneticfield lines, (ii) Hall magnetic fields and a nonzero normal field with polarity consistent with a crossing northward of the X line, and (iii) northward Alfvenic ion jets. Dayside magnetic reconnection on crustal magnetic fields could control the global configuration and topology of the Martian magnetosphere and alter the ion escape pattern from the dayside ionosphere.
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC-E-DAA-TN60521 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 45; 10; 4550-4558
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  • 3
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    One-Hertz Waves at Mars: MAVEN Observations (2018)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We perform a survey of 1-Hz waves at Mars utilizing Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observations for a Martian year. We find that the 1-Hz wave occurrence rate shows an apparent variation caused by masking of the waves by background turbulence during the times when the background turbulence levels are high. To correct for this turbulence masking, we select waves that occur in time intervals where the background turbulence levels are low. We find that the extreme ultraviolet flux does not affect the wave occurrence rate significantly, suggesting that the newly born pickup ions originating in the Mars's exosphere contribute minimally to the 1-Hz wave generation. We find that the wave occurrence rates are higher for low Mach numbers and low beta values than for high Mach numbers and high beta values. Further, we find that a high percentage of 1-Hz waves satisfy the group-standing condition, which suggests that a high percentage of the waves seen as monochromatic waves in the spacecraft frame can be broadband waves in the solar wind frame that have group velocities nearly equal and opposite to the solar wind velocity. We infer that the wave occurrence rate trends with the Mach number and proton beta are a consequence of how the Mach numbers and beta values influence the wave generation and damping or how those parameters affect the group-standing condition. Finally, we find that the 1-Hz waves are equally likely to be found in both the quasi-parallel and the quasi-perpendicular foreshock regions.
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC-E-DAA-TN60517 , Journal of Geophysical Research: Space Physics (ISSN 2169-9402) (e-ISSN 2169-9380); 123; 5; 3460-3476
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  • 4
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    Lunar Gravity Field Determination Using SELENE Same-Beam Differential VLBI Tracking Data (2010)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A lunar gravity field model up to degree and order 100 in spherical harmonics, named SGM 100i, has been determined from SELENE and historical tracking data, with an emphasis on using same-beam S-band differential VLBI data obtained in the SELENE mission between January 2008 and February 2009. Orbit consistency throughout the entire mission period of SELENE as determined from orbit overlaps for the two sub-satellites of SELENE involved in the VLBI tracking improved consistently from several hundreds of metres to several tens of metres by including differential VLBI data. Through orbits that are better determined, the gravity field model is also improved by including these data. Orbit determination performance for the new model shows improvements over earlier 100th degree and order models, especially for edge-on orbits over the deep far side. Lunar Prospector orbit determination shows an improvement of orbit consistency from I-day predictions for 2-day arcs of 6 m in a total sense, with most improvement in the along and cross-track directions. Data fit for the types and satellites involved is also improved. Formal errors for the lower degrees are smaller, and the new model also shows increased correlations with topography over the far side. The estimated value for the lunar GM for this model equals 4902.80080 +/- 0.0009 cu km/sq s (10 sigma). The lunar degree 2 potential Love number k2 was also estimated, and has a value of 0.0255 +/- 0.0016 (10 sigma as well).
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC.JA.5447.2011 , Journal of Geodesy; 85; 4; 205-228
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