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  • 1
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    The Mushroom: a Half-Sky Energetic Ion and Electron Detector (2016)
    Wiley
    Details
    Publication Date: 2016-12-10
    Description: We present a time-of-flight mass spectrometer design for the measurement of ions in the ~30 keV to 10 MeV range for protons (up to ~40 MeV and ~150 MeV for He and heavy ions, respectively) and ~30 keV to 1 MeV range for electrons, covering half of the sky with 80 apertures. The instrument, known as the “Mushroom”, owing to its shape, solves the field of view problem for magnetospheric and heliospheric missions that employ three-axis stabilized spacecraft, yet still require extended angular coverage; the Mushroom is also compatible with a spinning spacecraft. The most important new feature of the Mushroom is the method through which uncomplicated electrostatic optics and clean position sensing combine to permit many apertures to fit into a compact, low-mass sensor head (or wedge), several of which (ideally eight) compose a full instrument. Most of the sensor head's volume is an empty, equipotential region, resulting in the modest 250 g mass of each ten-aperture wedge. The Mushroom is capable of separating ion species across most of its energy range and angular field of view. For example, separation of the neighboring 3 He and 4 He isotopes is excellent; the full-width/half-maximum mass resolution has been measured to be 0.24 amu to 0.32 amu, respectively. Converting this to a Gaussian width σ m in mass m this represents a σ m / m mass resolution better than 0.04. This separation is highly desirable for the flight program for which the first Mushroom was built, the Solar Probe Plus mission. More generally we estimate the mass resolution to be σ m / m  ≈ 0.1, but this is energy, mass, and angularly dependent. We also discuss the solid-state detector stack capability, which extends the energy range of protons and helium, with composition, to ~100 MeV.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Long- and short-term variability of Saturn's ionic radiation belts (2011)
    Wiley
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    Publication Date: 2011-02-24
    Description: Earlier studies of Saturn's inner ionic radiation belts revealed that their content was surprisingly constant while their evolution appeared decoupled from dynamics of the Saturnian magnetosphere. Saturn's icy moons in combination with the neutral gas and dust that surround the planet seem to effectively restrict radial transport of energetic ions and are responsible for all these unusual characteristics. A possible process through which MeV ions may be populating the regions between the icy moons is cosmic ray albedo neutron decay (CRAND). While some circumstantial evidence suggests that this process actually occurs, the concept of CRAND has only been applied to the proton energy spectrum above ∼10 MeV; the source of ions below 10 MeV is not yet obvious. Additional hints about the nature of this source are now becoming evident by monitoring Saturn's radiation belts about half a solar cycle (from the declining phase of the solar maximum to solar minimum). Using Cassini's magnetosphere imaging instrument and low-energy magnetospheric measurement system (MIMI/LEMMS) data from June 2004 to June 2010, we detect a weak intensification of the trapped proton component that probably originates from CRAND (〉10 MeV). This anticipated enhancement, due to the solar cycle modulation of the galactic cosmic ray influx at Saturn, is closely followed by ions in the 1–10 MeV range. This observation sets constraints on the nature of those ions' source: this source should be connected (directly or indirectly) to the access of galactic cosmic rays in the Saturnian system. We also find evidence indicating that the ionic belts experience short-term variability following the occurrence of solar energetic particle events at Saturn's distance, probably associated with coronal mass ejections that propagate in the heliosphere. LEMMS data contain clear evidence of Earth-like Forbush decreases following such events. These decreases may explain the lack of an (expected) ionic belt intensification between 2004 and 2006.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    The importance of plasma β conditions for magnetic reconnection at Saturn's magnetopause (2012)
    Wiley
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    Publication Date: 2012-04-20
    Description: Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma β conditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti-parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheath β conditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind-magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth-like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 4
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    Pitch angle distributions of energetic electrons at Saturn (2011)
    Wiley
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    Publication Date: 2011-01-29
    Description: Pitch angle distributions of energetic electrons (110–365 keV) at Saturn are statistically analyzed for 2005–2009. Using a nondipolar model magnetic field, pitch angle distributions are mapped to the magnetospheric equator and sorted by equatorial crossing distance. The results are quantified using a standard function for the pitch angle distribution, f(α) = AsinKα (where α is the pitch angle and K is the power). Inside of ∼10 RS, the distributions are mostly peaked at 90° (K 〈 0), signifying a trapping distribution. Outside of this distance, the distributions are mostly field aligned (K 〉 0) with maxima near 0° and 180°. The 10 RS boundary maps to Saturn's ionosphere at latitudes equatorward of the aurora. Very few “flat” distributions are observed (K ≈ 0). The pitch angle distributions are not as well organized in local time as they are in radial distance, but over the 5 year survey between 10 and 20 RS field-aligned distributions appear most often near midnight, while trapping distributions are found elsewhere.
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    Topics: Geosciences , Physics
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  • 5
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    Dynamics and seasonal variations in Saturn's magnetospheric plasma sheet, as measured by Cassini (2011)
    Wiley
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    Publication Date: 2011-04-05
    Description: We analyze electron plasma, energetic ion, and magnetic field data from four almost vertical Cassini passes through the nightside plasma sheet of Saturn (segments of the high-latitude orbits of the spacecraft) separated in two subsets: two passes of identical geometry from January 2007 with Cassini crossing the equatorial plane in the postmidnight sector at a distance of ∼21 Saturn radii (RS) and two passes from April 2009, also of identical geometry, with Cassini crossing the equatorial plane in the premidnight sector again at a distance of ∼21 RS. The vertical structure and variability of the plasma sheet is described for each individual pass, and its basic properties (scale height, vertical displacement, tilt angle, hinging distance) are computed. The plasma sheet presents an energy-dependent vertical structure, being thicker by a factor of ∼2 in the energetic particle range than in the electron plasma. It further exhibits intense dynamical behavior, evident in the energetic neutral atom emission. In two of the four passes, we observe a clear north-south asymmetry, presumably a combined result of vertical plasma sheet motion and short time scale dynamics. Comparison between the 2007 and 2009 passes reveals a clear change in the tilt and vertical offset of the planetary nightside plasma sheet, which progressively becomes aligned to the solar wind direction as we approach Saturnian equinox (August 2009). Temperature, pressure, and number density in the center of the sheet remain relatively stable and essentially unaffected by the seasonal change.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Energetic electron spectra in Saturn's plasma sheet (2011)
    Wiley
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    Publication Date: 2011-07-12
    Description: The differential spectra of energetic electrons (27–400 keV) in Saturn's plasma sheet can be characterized by power law or kappa distributions. Using all available fluxes from 2005 to 2010, fits to these distributions reveal a striking and consistent pattern of radial dependence in Saturn's plasma sheet (∣z∣ 〈 1 RS = 60,268 km). The electron spectral indices show harder spectra at large radial distances (20–30 RS), softer spectra at middle radial distances (10–20 RS), and very steep spectra inside the orbit of Rhea (∼8.5 RS). The dayside spectra are somewhat harder than the nightside spectra outside the orbit of Titan (∼20 RS), although there is no local time dependence inside ∼10 RS. This spectral behavior exhibited essentially no dependence on pitch angle and remained remarkably constant throughout the Cassini mission. Inward of about 10 RS, the presence of the electron radiation belts and losses of lower-energy electrons to the gas and grain environment give rise to the very hard spectra in the inner magnetosphere, while the hard spectra in the outer magnetosphere may derive from auroral acceleration at high latitudes. The gradual softening of the spectra from 20 to 10 RS is explained by inward radial diffusion.
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    Topics: Geosciences , Physics
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  • 7
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    Long term time variations of the suprathermal ions in Saturn's magnetosphere (2011)
    Wiley
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    Publication Date: 2011-09-20
    Description: We use the Charge-Energy-Mass Spectrometer (CHEMS) on Cassini to study long-term time variations of the suprathermal (E/Q = 27–220 keV/e) ions in Saturn's equatorial ring current (Latitude = −10° to 10°, dipole L = 7–16) from the end of 2004 to the end of 2010. We identify five equatorial time periods, which vary in length from 124 to 282 days, and determine the average ion composition of the ring current for each period. The species examined are the water group W+ (O+, OH+, H2O+, and H3O+), H+, H2+, He++, He+, and O++. We find that the combined partial number density over this energy range varies less than a factor of two from a minimum of 1.3 × 10−3 cm−3 to a maximum of 2.3 × 10−3 cm−3, implying that long-term time variations in neutral source strength must be modest. The most abundant species (63% W+, 30% H+, 5% H2+) show no long-term trends. The H+/W+ and H2+/W+ ratios vary less than half as much as individual densities. The smaller variations in the ratios could be understood if all three species mostly originate from the Enceladus plumes either directly or via dissociation, but our results can accommodate differing neutral sources (currently thought to be Enceladus for W, Saturn's atmosphere for H, and Titan for H2) if the suprathermal ion density variations are largely caused by changes in the overall acceleration rate of the thermal plasma. Both He++, originating from the solar wind, and He+, interplanetary pickup ions of interstellar origin, show a substantial decrease in 2009–2010 near solar minimum.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 8
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    Post-equinox periodicities in Saturn's energetic electrons (2011)
    Wiley
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    Publication Date: 2011-12-24
    Description: Since Saturn's vernal equinox in August 2009 (day 223), energetic electrons (110–365 keV) have exhibited a variety of periodic and aperiodic behavior within a spectral window of 5–15 hours. From late 2009 through the end of 2010, when the observed at dusk, a single period near 10.7 hours dominated the Lomb spectra of these particles. Near the end of 2010, however, the energetic electrons displayed multiple periods, with the strongest at 10.65 hours. The periodicity observed after equinox has a mean value of 10.69 ± 0.06 hours and agreed closely with that of Saturn kilometric radio (south) emissions. By early 2011, when the observer had moved to the dayside, the periodicities abruptly disappeared and the Lomb spectra show no periodicity. This behavior may suggest changes in Saturn's ionosphere as a result of seasonal change, or may alternately imply a local time dependence of periodicity caused by magnetodisk thickness asymmetry.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 9
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    ENA periodicities and their phase relations to SKR emissions at Saturn (2011)
    Wiley
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    Publication Date: 2011-08-24
    Description: Cassini orbits during days 200–366 in 2004 afforded the opportunity to continuously observe energetic neutral atom (ENA) emissions from long range (〉50 RS, 1 RS = 60268 km) on Saturn's dawn side. Images of energetic neutral hydrogen (25–55 keV) and oxygen (90–160 keV) were projected onto the noon-midnight plane, corrected for travel time from Saturn, averaged into half hour time bins and finally averaged into a 60 × 40 RS spatial bin. The time profiles of these bin averages were then subjected to a Lomb periodogram analysis. The H periodogram exhibits a weak periodicity (SNR = 9.1) with a major peak at 10.78 hours and several minor peaks. The O periodogram displays strong periodicities (SNR = 36.2) with a major peak at 10.78 hours and a various secondary peaks. A cross correlation of the SKR signal with the ENA signals reveals that the H signal leads the SKR by 1.46 ± 0.08 hours, while the O signal leads the SKR by 2.21 ± 0.14 hours.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 10
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    NASA's Living with a Star Initiative (2007)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2007-09-05
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Karpen, J T -- Krimigis, S M -- Appleby, J F -- Lawler, A -- New York, N.Y. -- Science. 2000 Aug 25;289(5483):1296.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17772991" 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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