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  • 1
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    Flat Proton Spectra in Large Solar Energetic Particle Events (2018)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: We present solar energetic particle events observed at 1 AU from the Sun for which the proton energy spectra at energies between ~50 keV to ~1 MeV flatten during a period of at least ~12 hours prior to the passage of the associated interplanetary shock. The flattening of the proton energy spectra occurs when the source of the particles (presumably the traveling interplanetary shock) is still downwind from the spacecraft and particle intensities are still continuously increasing. The arrival of the shock at the spacecraft is then characterized by a steepening of the spectra, where low-energy proton intensities show a more pronounced enhancement than the high-energy proton intensities. We discuss the mechanisms that may result in this flattening of the spectra in terms of current models presented in the literature.
    Keywords: Solar Physics
    Type: GSFC-E-DAA-TN65592 , Journal of Physics: Conference Series (ISSN 1742-6588) (e-ISSN 1742-6596); 1100; 1; 012014|Annual International Astrophysics Conference; Mar 05, 2018 - Mar 09, 2018; Sante Fe, NM; United States
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  • 2
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    Evolution of the Suprathermal Proton Population at Interplanetary Shocks (2019)
    In:  CASI
    Details
    Publication Date: 2019-08-09
    Description: We investigate the evolution of the suprathermal (ST) proton population as interplanetary shocks cross 1 au. The variability of the ST proton intensities and energy spectra upstream of the shocks is analyzed in terms of the shock parameters, upstream magnetic field configurations, and preexisting upstream populations. Propitious conditions for the observation of ST particles at distances far upstream from the shock occur in parallel shock configurations when particles can easily escape from the shock vicinity. In this situation, ST intensity enhancements show onsets characterized by velocity dispersion effects and energy spectra that develop into a hump profile peaking around 10 keV just before the arrival of the shock. The observation of field-aligned proton beams at low energies (510 keV) is possible under conditions that facilitate the scatter-free propagation of the particles streaming out of the shock. Upstream of perpendicular shocks, ST intensity enhancements are only observed in close proximity to the shock. Power-law proton spectra develop downstream of the shocks. The functional form for the downstream phase-space density proportional to v(exp -5) is observed only over a limited range of ST energies. The absence of ST populations observed far upstream of interplanetary shocks raises questions about whether ST protons contribute as a seed particle population in the processes of particle acceleration at shocks.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN70043 , Astronomical Journal (ISSN 0004-6256) (e-ISSN 1538-3881); 158; 1; 12
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  • 3
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    In Situ Radiometric and Exposure Age Dating of the Martian Surface (2014)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of Gale Crater. A K-Ar age of 4.21 +/- 0.35 billion years represents a mixture of detrital and authigenic components and confirms the expected antiquity of rocks comprising the crater rim. Cosmic-ray-produced 3He, 21Ne, and 36Ar yield concordant surface exposure ages of 78 T 30 million years. Surface exposure occurred mainly in the present geomorphic setting rather than during primary erosion and transport. Our observations are consistent with mudstone deposition shortly after the Gale impact or possibly in a later event of rapid erosion and deposition. The mudstone remained buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks exposed by this mechanism may thus offer the best potential for organic biomarker preservation against destruction by cosmic radiation.
    Keywords: Lunar and Planetary Science and Exploration; Geosciences (General)
    Type: GSFC-E-DAA-TN21533 , Science; 343; 6169
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  • 4
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    Interplanetary Coronal Mass Ejection Observed at STEREO-A, Mars, Comet 67P/Churyumov-Gerasimenko, Saturn,and New Horizons En Route to Pluto: Comparison of Its Forbush Decreases at 1.4, 3.1, and 9.9 AU (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-08-14
    Description: We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, Mars Atmosphere and Volatile EvolutioN Mission (MAVEN), Mars Odyssey, and Mars Science Laboratory (MSL) missions, 44 h before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a nonambiguous signature at New Horizons. A potential detection of this ICME by Voyager 2 at 110-111 AU is also discussed. The multispacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116deg, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P, and Saturn.
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC-E-DAA-TN51107 , Journal of Geophysical Research: Space Physics (ISSN 2169-9402) (e-ISSN 2169-9402); 122; 8; 7865-7890
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