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  • 1
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    Measurements of Galactic Cosmic Ray Shielding with the CRaTER Instrument (2013)
    Wiley
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    Publication Date: 2013-03-28
    Description: [1]  The CRaTER instrument aboard the Lunar Reconnaissance Orbiter has been measuring energetic charged particles from the Galactic Cosmic Rays (GCRs) and Solar Particle Events in lunar orbit since 2009. CRaTER includes three pairs of silicon detectors, separated by pieces of tissue-equivalent plastic that shield two of the three pairs from particles incident at the zenith-facing end of the telescope. Heavy-ion beams studied in previous ground-based work have been shown to be reasonable proxies for the GCRs when their energies are sufficiently high. That work, which included GCR simulations, led to predictions for the amount of dose reduction that would be observed by CRaTER. Those predictions are compared to flight data obtained by CRaTER in 2010-2011.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Analysis of the potential radiation hazard of the 23 July 2012 SEP event observed by STEREO A using the EMMREM model and LRO/CRaTER (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: We present a study of the potential radiation hazard of the powerful, superfast interplanetary coronal mass ejection (ICME) observed by STEREO A on 23 July 2012. Using energetic proton flux data from the HET and LET instruments aboard STEREO A together with the EMMREM radiation model, we compute dose rates and accumulated doses during the event for both skin/eye and blood forming organs using four physically relevant levels of shielding. For spacesuit equivalent shielding, we compute a peak skin/eye dose rate of 1970 cGy-Eq/day, a value far greater than those of the 2003 Halloween storms or the January and March SEP events of 2012. However, due to the relative brevity of the event, the resulting accumulated dose was just 383 cGy-Eq, which is more aligned with the total doses of the 2003 Halloween and 2012 January/March events. Additionally, we use dose rates at STEREO B and LRO/CRaTER during the event to show how the radiation impact is affected by the position of the ICME relative to the observer. Specifically, we find that the energetic particle event associated with the local shock and ICME passage at STEREO A caused greatly enhanced dose rates when compared to STEREO B and LRO/CRaTER, which were longitudinally distant from the ICME. The STEREO A/B dose rates used here will soon be made available to the community as a tool for studying the energetic particle radiation of solar events from different longitudes as a part of NASA's Heliophysics Virtual Observatories and on the PREDICCS and CRaTER websites.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 3
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    Dielectric breakdown weathering of the Moon's polar Regolith (2015)
    Wiley
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    Publication Date: 2015-01-23
    Description: Galactic cosmic rays and solar energetic particles (SEPs) can charge the Moon's subsurface, a process expected to be particularly important in the polar regions. Experiments have shown that sufficient fluences (i.e., time-integrated fluxes) of energetic charged particles can cause dielectric breakdown, in which the electric field rapidly vaporizes small, filamentary channels within a dielectric. Lunar regolith has both the characteristics and, in some polar locations, the environment needed to make breakdown likely. We combine the JPL proton fluence model with temperature measurements from the Lunar Reconnaissance Orbiter's (LRO's) Diviner instrument and related temperature modeling to estimate how often breakdown occurs in the polar regions. We find that all gardened regolith within permanently shadowed regions (PSRs) has likely experienced up to 2 × 10 6 SEP events capable of causing breakdown, while the warmest polar regions have experienced about two orders of magnitude fewer events. We also use measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on LRO to show that at least two breakdown-inducing events may have occurred since LRO arrived at the Moon in 2009. Finally, we discuss how such “breakdown weathering” may increase the percentage of fine and monomineralic grains within PSRs; explain the presence of so-called “fairy castle” regolith structures; and contribute to other low-albedo features detected by LRO's Lyman Alpha Mapping Project (LAMP), possibly establishing a correlation between these features and the average temperatures within craters that are only partly in permanent shadow.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep-space exploration? (2014)
    Wiley
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    Publication Date: 2014-10-05
    Description: The Sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity between cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age, and relatively small solar energetic particle events. We use observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) to examine the implications of these highly unusual solar conditions for human space exploration. We show that while these conditions are not a show-stopper for long duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. While solar energetic particle events in cycle 24 present some hazard, the accumulated doses for astronauts behind 10 g/cm 2 shielding are well below current dose limits. Galactic cosmic radiation presents a more significant challenge: the time to 3% Risk of Exposure Induced Death (REID) in interplanetary space was less than 400 days for a 30 year old male and less than 300 days for a 30 year old female in the last cycle 23–24 minimum. The time to 3% REID is estimated to be ~20% lower in the coming cycle 24–25 minimum. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we estimate exposures in extreme solar minimum conditions and the corresponding effects on allowable durations.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 5
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    A Rapid Decrease of the Hydrogen Corona of Mars (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: Mars is believed to have lost much of its surface water 3.5 billion years ago, but the amounts that escaped into space and remain frozen in the crust today are not well known. Hydrogen atoms in the extended martian atmosphere, some of which escape the planet's gravity, can be imaged through scattered solar UV radiation. Hubble Space Telescope (HST) images of the ultraviolet H Ly α emission now indicate that the coronal H density steadily decreased by a factor of roughly 40 % over 4 weeks, a far greater variation than had been expected. The leading candidate cause is a decrease in the source rate of water molecules from the lower atmosphere, consistent with seasonal changes and a recent global dust storm. This implies that the rate of escape of martian hydrogen (and thereby water) into space is strongly dependent on the lower atmospheric water content and distribution.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Molecular Origin of Io's Fast Sodium (1991)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 1991-09-20
    Description: Neutral sodium emissions encircling Jupiter exhibit an intricate and variable structure that is well matched by a simple loss process from Io's atmosphere. These observations imply that fast neutral sodium is created locally in the Io plasma torus, both near Io and as much as 8 hours downstream. Sodium-bearing molecules may be present in Io's upper atmosphere, where they are ionized by the plasma torus and swept downstream. The molecular ions dissociate and dissociatively recombine on a short time scale, releasing neutral fragments into escape trajectories from Jupiter. This theory explains a diverse set of sodium observations, and it implies that molecular reactions (particularly electron impact ionization and dissociation) are important at the top of Io's atmosphere.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Schneider, N M -- Trauger, J T -- Wilson, J K -- Brown, D I -- Evans, R W -- Shemansky, D E -- New York, N.Y. -- Science. 1991 Sep 20;253(5026):1394-7.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17793479" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 7
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    The formation of molecular hydrogen from water ice 1 in the lunar regolith by energetic charged particles (2013)
    Wiley
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    Publication Date: 2013-05-21
    Description: [1]  On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket into the permanently shadowed region (PSR) within Cabeus crater and detected water vapor and ice, as well as other volatiles, in the ejecta plume. The Lyman Alpha Mapping Project (LAMP), a far ultraviolet (FUV) imaging spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), observed this plume as FUV emissions from the fluorescence of sunlight by molecular hydrogen (H 2 ) and other constituents. Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can dissociate the molecules in water ice to form H 2 . We examine how much H 2 can be formed by these types of particle radiation interacting with water ice sequestered in the regolith within PSRs, and we assess whether it can account for the H 2 observed by LAMP. To estimate H 2 formation, we use the GCR and SEP radiation dose rates measured by the LRO Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The exposure time of the ice is calculated by considering meteoritic gardening and the penetration depth of the energetic particles. We find that GCRs and SEPs could convert at least 1-7% of the original water molecules into H 2 . Therefore, given the amount of water detected by LCROSS, such particle radiation-induced dissociation of water ice likely could likely account for a significant percentage (10-100%) of the H 2 measured by LAMP.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Atmospheric radiation modeling of galactic cosmic rays using LRO/CRaTER and the EMMREM model with comparisons to balloon and airline based measurements (2016)
    Wiley
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    Publication Date: 2016-08-19
    Description: We provide an analysis of the galactic cosmic ray (GCR) radiation environment of Earth's atmosphere using measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) aboard the Lunar Reconnaissance Orbiter (LRO) together with the Badhwar-O'Neil model and dose lookup tables generated by the Earth-Moon-Mars Radiation Environment Module (EMMREM). This study demonstrates an updated atmospheric radiation model that uses new dose tables to improve the accuracy of the modeled dose rates. Additionally, a method for computing geomagnetic cutoffs is incorporated into the model in order to account for location-dependent effects of the magnetosphere. Newly available measurements of atmospheric dose rates from instruments aboard commercial aircraft and high-altitude balloons enable us to evaluate the accuracy of the model in computing atmospheric dose rates. When compared to the available observations, the model seems to be reasonably accurate in modeling atmospheric radiation levels, overestimating airline dose rates by an average of 20%, which falls within the uncertainty limit recommended by the ICRU. Additionally, measurements made aboard high-altitude balloons during simultaneous launches from New Hampshire and California provide an additional comparison to the model. We also find that the newly incorporated geomagnetic cutoff method enables the model to represent radiation variability as a function of location with sufficient accuracy.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 9
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    Deep dielectric charging of regolith within the Moon's permanently shadowed regions (2014)
    Wiley
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    Publication Date: 2014-07-05
    Description: Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can penetrate deep within the lunar surface, resulting in deep dielectric charging. This charging process depends on the GCR and SEP currents, as well as on the regolith's electrical conductivity and permittivity. In permanently shadowed regions (PSRs) near the lunar poles, the discharging timescales are on the order of a lunation (~20 days). We present the first predictions for deep dielectric charging of lunar regolith. To estimate the resulting subsurface electric fields, we develop a data-driven, one-dimensional, time-dependent model. For model inputs, we use GCR data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on board the Lunar Reconnaissance Orbiter (LRO) and SEP data from the Electron, Proton, and Alpha Monitor (EPAM) on the Advanced Composition Explorer (ACE). We find that, during the recent solar minimum, GCRs create persistent electric fields up to sim 700 V/m. We also find that large SEP events create transient but strong electric fields (≥10 6  V/m) that may induce dielectric breakdown. Such breakdown would likely result in significant modifications to the physical and chemical properties of the lunar regolith within PSRs.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Validation of PREDICCS using LRO/CRaTER observations during three major solar events in 2012 (2013)
    Wiley
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    Publication Date: 2013-05-12
    Description: [1]  PREDICCS (Predictions of radiation from REleASE, EMMREM, and Data Incorporating the CRaTER, COSTEP and other SEP measurements, http://prediccs.sr.unh.edu ) is an online system designed to provide a near-real-time characterization of the radiation environment of the inner heliosphere. PREDICCS utilizes data from various satellites in conjunction with numerical models such as the Earth-Moon-Mars Radiation Environment Module (EMMREM) to produce dose rate and particle flux data at the Earth, Moon and Mars. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument, launched aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft in 2009 and designed to measure energetic particle radiation, offers an opportunity to test the capability of PREDICCS to accurately describe the lunar radiation environment. We provide comparisons between dose rates produced by PREDICCS with those measured by CRaTER during three major SEP events occurring in 2012. In addition, using EMMREM data products together with our archive of measured CRaTER dose rates, we compute the modulation potential at the Moon throughout the LRO mission and using this, compute the background GCR dose rate during each event. We demonstrate reasonable agreement between PREDICCS and CRaTER dose rates and come to the conclusion that PREDICCS provides credible characterization of the lunar radiation environment. This study represents the first multi-event validation, via in situ measurement, of radiation models such as EMMREM, which should prove to be valuable in future efforts in risk assessment and in the study of radiation in the inner heliosphere.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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