Tucson, Arizona, United States
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Garnier P.,Roche Holding AG | Garnier P.,French National Center for Scientific Research | Holmberg M.K.G.,Swedish Institute of Space Physics | Wahlund J.-E.,Swedish Institute of Space Physics | And 10 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

The Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in 2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250-450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current-voltage (I-V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I-V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I-V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I-V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov-Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere. Key Points The impact of energetic electrons on the Langmuir probe is shown and modeled We show the existence of critical/anticritical temperatures The secondary yield of the probe surface is estimated from onboard measurements ©2013. American Geophysical Union. All Rights Reserved.

Winslow R.M.,University of British Columbia | Johnson C.L.,University of British Columbia | Johnson C.L.,Planetary Science Institute Arizona | Anderson B.J.,Johns Hopkins University | And 10 more authors.
Geophysical Research Letters | Year: 2014

Solar wind protons observed by the MESSENGER spacecraft in orbit about Mercury exhibit signatures of precipitation loss to Mercury's surface. We apply proton-reflection magnetometry to sense Mercury's surface magnetic field intensity in the planet's northern and southern hemispheres. The results are consistent with a dipole field offset to the north and show that the technique may be used to resolve regional-scale fields at the surface. The proton loss cones indicate persistent ion precipitation to the surface in the northern magnetospheric cusp region and in the southern hemisphere at low nightside latitudes. The latter observation implies that most of the surface in Mercury's southern hemisphere is continuously bombarded by plasma, in contrast with the premise that the global magnetic field largely protects the planetary surface from the solar wind. Key Points Mercury's offset dipole is confirmed at the surface using proton reflectometry Loss cones are oberved in northern cusp and on low-southern-latitude nightside Mercury's southern hemisphere is bombarded by the solar wind and plasma sheet ©2014. American Geophysical Union. All Rights Reserved.

Thomsen M.F.,Los Alamos National Laboratory | Thomsen M.F.,Planetary Science Institute Arizona | Henderson M.G.,Los Alamos National Laboratory | Jordanova V.K.,Los Alamos National Laboratory
Space Weather | Year: 2013

Based on 13 years of data from the Magnetospheric Plasma Analyzers on six Los Alamos National Laboratory (LANL) geosynchronous satellites, the statistical behavior of environmental conditions that cause strong surface charging on the satellites is examined. Analysis of the Magnetospheric Plasma Analyzers data reveals the electron energy range (~5-10 keV) and threshold flux (1.4×103 cm-2 s-1 sr-1 eV -1 at 8 keV) that are most closely associated with satellite surface charging. We also find that the average ambient electron temperature in the plasma sheet correlates with the observed magnitude of the surface potential on the LANL satellites. Analysis of the statistical occurrence rates of (1) the observed surface potential on the LANL satellites, (2) 8 keV electron fluxes above the threshold, and (3) elevated values of the average electron temperature all reveal that an enhanced surface charging probability exists (a) during higher values of Kp, (b) in the local time range from premidnight through dawn, (c) during equinox seasons, and (d) during the declining phase of the solar cycle. With the exception of the solar cycle dependence, these are the same statistical dependences found by Choi et al. (2011) in the satellite anomaly database they examined. The local time dependence of those anomalies is a particularly strong diagnostic of surface charging as a probable cause of a significant number of them. Key Points Surface-charging is caused by enhanced electron fluxes with energies ~5-10 keVEnhanced surface-charging probability is highest from pre-midnight to dawnSurface charging is a probable cause of many reported satellite anomalies ©2013. American Geophysical Union. All Rights Reserved.

Travis B.J.,Los Alamos National Laboratory | Feldman W.C.,Planetary Science Institute Arizona | Maurice S.,Observatoire Midi Pyrenees
Journal of Geophysical Research E: Planets | Year: 2013

Recent discovery of transient ice deposits uncovered by five small craters between 40 and 55°N latitude, reinterpretation of MONS neutron data that indicate the wide-spread presence of ice within 1 m of the surface at midlatitudes (down to 30°N) of Mars, and evidence of recent periglacial activity within 10°N of the equator, all suggest ice may be or recently was present at latitudes where it is not expected and at unexplained abundance. As ice may be unstable under present Mars climatic conditions, a mechanism may be needed to explain the presence of ice in the near surface at these latitudes. Water release history, chemical composition, and heat fluxes are variable over the surface of Mars, and there could be more than one mechanism responsible for near-surface ice. The purpose of this study is to show that hydrothermal circulation of brines in the subsurface of Mars is a possible mechanism that can deposit ice and brine, close to, or even at, the surface of Mars. Furthermore, the action of brine convection can be related to some of the surface features associated with subsurface water during previous or even present epochs, such as polygonal ground and sorted stone circles. Key Points Subsurface brine convection can deposit ice and salt at the surface of Mars.Brine convection occurs over a range of parameter values.Variations in near-surface ice deposits can be related to patterned ground. ©2013. American Geophysical Union. All Rights Reserved.

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