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Gjerloev J.W.,Johns Hopkins University | Hoffman R.A.,15107 Interlachen Drive | Ohtani S.,Johns Hopkins University | Weygand J.,Institute of Geophysics and Planetary Physics | Barnes R.,Johns Hopkins University
Annales Geophysicae | Year: 2010

We present results from a study of the behavior of the auroral electrojet indices following abrupt southward turnings of the IMF Bz. The auroral electrojet indices are calculated from observations made by more than 100 ground based stations provided by the SuperMAG collaborators. Based on three simple criteria we selected 73 events. In each event the interval of analysis started at the time of the IMF Bz southward turning and ended 45 minutes later or at the onset of any abrupt energy unloading event in the magnetosphere, regardless of size. We refer to this period as the "pre-unloading phase". To isolate the dependence of the auroral electrojets on the solar induced ionospheric conductivity during this phase we separated the standard AU/AL indices into two new sets of indices defined by the upper and lower envelope of the north-south component for all sunlit stations (AUs/ALs) and for all stations in darkness (AUd/ALd). Based on events and statistical analyses we can conclude that following a southward turning of the IMF Bz the AUd/ALd indices show no measurable response while the AUs/ALs indices clearly intensify. The intensifications of AUs/ALs are dependent on the intensity of the solar wind driver (as measured by IMF Bz or the Akasofu μ parameter). The lack of AUd/ALd response does not depend on the intensity of any subsequent substorm. We find that during these isolated events the ionospheric current system is primarily confined to the sunlit ionosphere. This truncated version of the classical global DP-2 current system suggests that auroral electrojet continuity is not maintained across the terminator. Because of its conductivity dependence on the solar zenith angle, this truncated global current pattern is expected to be highly dependent on UT and season and thus can be asymmetric between hemispheres. Thus we argue that the global two-cell DP-2 current system is not a consequence only of a southward turning of the IMF but requires also the reduction of the conductivity gradient at the terminator. © Author(s) 2010. Source


Mitri G.,CNRS Nantes Laboratory of Planetology and Geodynamics | Coustenis A.,University Paris Diderot | Fanchini G.,Smart Structures Solutions S.r.l | Hayes A.G.,Cornell University | And 13 more authors.
Planetary and Space Science | Year: 2014

Fundamental questions involving the origin, evolution, and history of both Titan and the broader Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe. Observations from an orbit-platform can improve our understanding of Titan's geological processes, surface composition and atmospheric properties. Further, combined measurements of the gravity field, rotational dynamics and electromagnetic field can expand our understanding of the interior and evolution of Titan. An in situ exploration of Titan's lakes provides an unprecedented opportunity to understand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry and habitability potential, and study meteorological and marine processes in an exotic environment. We briefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and a lake probe. © 2014 Elsevier Ltd. Source


Roussos E.,Max Planck Institute for Solar System Research | Kollmann P.,Max Planck Institute for Solar System Research | Kollmann P.,Johns Hopkins University | Krupp N.,Max Planck Institute for Solar System Research | And 12 more authors.
Icarus | Year: 2012

Saturn's moon Rhea is thought to be a simple plasma absorber, however, energetic particle observations in its vicinity show a variety of unexpected and complex interaction features that do not conform with our current understanding about plasma absorbing interactions. Energetic electron data are especially interesting, as they contain a series of broad and narrow flux depletions on either side of the moon's wake. The association of these dropouts with absorption by dust and boulders orbiting within Rhea's Hill sphere was suggested but subsequently not confirmed, so in this study we review data from all four Cassini flybys of Rhea to date seeking evidence for alternative processes operating within the moon's interaction region. We focus on energetic electron observations, which we put in context with magnetometer, cold plasma density and energetic ion data. All flybys have unique features, but here we only focus on several structures that are consistently observed. The most interesting common feature is that of narrow dropouts in energetic electron fluxes, visible near the wake flanks. These are typically seen together with narrow flux enhancements inside the wake. A phase-space-density analysis for these structures from the first Rhea flyby (R1) shows that Liouville's theorem holds, suggesting that they may be forming due to rapid transport of energetic electrons from the magnetosphere to the wake, through narrow channels. A series of possibilities are considered to explain this transport process. We examined whether complex energetic electron drifts in the interaction region of a plasma absorbing moon (modeled through a hybrid simulation code) may allow such a transport. With the exception of several features (e.g. broadening of the central wake with increasing electron energy), most of the commonly observed interaction signatures in energetic electrons (including the narrow structures) were not reproduced. Additional dynamical processes, not simulated by the hybrid code, should be considered in order to explain the data. For the small scale features, the possibility that a flute (interchange) instability acts on the electrons is discussed. This instability is probably driven by strong gradients in the plasma pressure and the magnetic field magnitude: magnetometer observations show clearly signatures consistent with the (expected) plasma pressure loss due to ion absorption at Rhea. Another potential driver of the instability could have been gradients in the cold plasma density, which are, however, surprisingly absent from most crossings of Rhea's plasma wake. The lack of a density depletion in Rhea's wake suggests the presence of a local cold plasma source region. Hybrid plasma simulations show that this source cannot be the ionized component of Rhea's weak exosphere. It is probably related to accelerated photoelectrons from the moon's negatively charged surface, indicating that surface charging may play a very important role in shaping Rhea's magnetospheric interaction region. © 2012 Elsevier Inc. Source


Bunch N.L.,Stanford University | Spasojevic M.,Stanford University | Shprits Y.Y.,Institute of Geophysics and Planetary Physics
2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 | Year: 2011

Determining the global distribution of chorus wave power in the off-equatorial region is a crucial component in understanding the contribution of chorus to radiation belt acceleration and loss. In this study we employ a database of chorus observations from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) onboard the Polar spacecraft, which are used to generate probability statistics, and approximate typical magnetic wave power as a function of space and geomagnetic activity. Previous studies concerning chorus wave power statistics have focused on a band-integrated and time-averaged data product which is typically used as the fundamental chorus input to radiation belt diffusion models. We estimate this quantity by which mean magnetic wave power and occurrence probability are incorporated to determine what we call "composite" wave power. These data products are a crucial step forward in understanding the radiation belt wave environment and evaluating essential inputs for radiation belt models. © 2011 IEEE. Source


Garg A.,Institute of Geophysics and Planetary Physics | Cook K.H.,Institute of Geophysics and Planetary Physics | NikoLaev S.,Institute of Geophysics and Planetary Physics | Huber M.E.,Johns Hopkins University | And 13 more authors.
Astronomical Journal | Year: 2010

We present 2323 high-amplitude σ-Scuti (HADS) candidates discovered in the Large Magellanic Cloud by the SuperMACHO survey (Rest et al.). Frequency analyses of these candidates reveal that several are multimode pulsators, including 119 whose largest amplitude of pulsation is in the fundamental (F) mode and 19 whose largest amplitude of pulsation is in the first overtone (FO) mode. Using Fourier decomposition of the HADS light curves, we find that the period-luminosity (PL) relation defined by the FO pulsators does not show a clear separation from the PL relation defined by the F pulsators. This differs from other instability strip pulsators such as type c RR Lyrae. We also present evidence for a larger amplitude, subluminous population of HADS similar to that observed in Fornax. © THE America Astronomical society. All rights reserved. Source

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