D. F. Strobel

Johns Hopkins University, Baltimore, Maryland, United States

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Publications (298)1087.36 Total impact

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    ABSTRACT: The interaction of the Enceladean plume with its magnetospheric environment provides a unique natural laboratory for studying plasma-neutral-dust interaction processes. The goal of this study is to analyze the magnetic signatures of dust in order to constrain the dust plume. For the first time, the mutual feedback between the charged nanograins and their plasma environment is investigated. Our model of these interactions combines plasma simulations by means of the hybrid code A.I.K.E.F. (Adaptive Ion-Kinetic Electron-Fluid) with Monte-Carlo simulations of the 3D profiles of the gas and dust plumes. Data from several instruments of Cassini are considered: the applied neutral plume model is in good agreement with INMS data, whereas theoretical predictions of the peak ion density are compared against CAPS and RPWS data, and properties of the dust plume are obtained by comparing our results with Cassini MAG data from various Enceladus flybys including the recent E14– E19 encounters. Our main results are: (1) due to the ion-neutral chemistry, H3O + is the predominant ion species within the plume; (2) the high nanograin densities observed by CAPS require an effective ionization frequency larger than the sum of photoionization and electron impacts to fulfill quasi-neutrality; (3) the nanograin pick-up current makes only a minor contribution to the current systems,i. e. the major contribution of the dust to the current systems arises from electron absorption; (4) the pick-up of charged nanograins is clearly visible in the magnetic field signatures, even including the distant encounter E15; (5) MAG data indicates a southward extension of the charged dust plume of at least four Enceladus radii; (6) the modification of the current system by the nanograins is responsible for the surprising fact that Cassini did not detect a region with a reduced magnetic field strength.
    Journal of Geophysical Research: Space Physics 03/2014; · 3.44 Impact Factor
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    ABSTRACT: We report our discovery of water vapor plumes near the south pole of Jupiter's moon Europa with HST/STIS and present new STIS observations from 2014.
    02/2014;
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    ABSTRACT: Far-UV auroral imaging and stellar occultation techniques are able to identify whether water vapor plumes exist on Europa. Detailed observation plans for the JUICE Ultraviolet Spectrograph (UVS) are reported along with recent HST auroral imaging.
    01/2014;
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    ABSTRACT: We present a technique to search for plumes on Europa using new STIS images of the UV aurora morphology obtained during two HST visits in November and December 2012.
    01/2014;
  • Xun Zhu, Darrell F. Strobel, Justin T. Erwin
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    ABSTRACT: The original Strobel et al. (Strobel, D.F., Zhu, X., Summers, M.E., Stevens, M.E. [1996]. Icarus 120, 266–289) model for Pluto’s stratospheric density and thermal structure is augmented to include a radial momentum equation with radial velocity associated with atmospheric escape of N2 and in the energy equation to also include the solar far ultraviolet and extreme ultraviolet (FUV–EUV) heating in the upper atmosphere and adiabatic cooling due to hydrodynamic expansion. The inclusion of radial velocity introduces important negative feedback processes such as increased solar heating leading to enhanced escape rate and higher radial velocity with stronger adiabatic cooling in the upper atmosphere accompanied by reduced temperature. The coupled set of equations for mass, momentum, and energy are solved subject to two types of upper boundary conditions that represent two different descriptions of atmospheric escape: Jeans escape and hydrodynamic escape. For the former which is physically correct, an enhanced Jeans escape rate is prescribed at the exobase and parameterized according to the direct simulation Monte Carlo kinetic model results. For the latter, the atmosphere is assumed to remain a fluid to infinity with the escape rate determined by the temperature and density at the transonic point subject to vanishing temperature and pressure at infinity. For Pluto, the two escape descriptions approach the same limit when the exobase coincides with the transonic level and merge to a common escape rate ∼1028 N2 s−1 under elevated energy input. For Pluto’s current atmosphere, the hydrodynamic approach underestimates the escape rate by about 13%. In all cases, the escape rate is limited by the solar FUV–EUV power input. Specific results for the New Horizons Pluto flyby July 2015 are escape rate ∼3.5 × 1027 N2 s−1, exobase at 8r0 ∼ 9600 km, with Jeans λ ∼ 5 for a reference Pluto atmosphere model. With Pluto’s highly elliptic orbit and variable solar activity affecting its atmosphere, Pluto’s escape rates’ range is (1–10) × 1027 N2 s−1, exobase radius is bounded by ∼(5–13)r0, and at the exobase Pluto is locked in the enhanced Jeans regime with λ ∼ (6–4). Finally, a systematic review of previous approximate hydrodynamic escape models is presented to compare the constraints which determine the escape rate in each model.
    Icarus 01/2014; 228:301–314. · 3.16 Impact Factor
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    ABSTRACT: We have carried out a comprehensive analysis of a large set of spatially resolved observations of Io’s OI 1304 Å, OI] 1356 Å, SI 1479 Å and SI] 1900 Å aurora taken by the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) between 1997 and 2001. We find that the variability of the observed morphologies can be solely explained by the changes of the plasma and magnetic field environment of the Io torus and by the viewing perspective. The variations in brightness are strongly correlated with the periodic variations of the ambient electron density. Based on these findings we develop a phenomenological model for the spatial distribution of the oxygen and sulfur emissions in Io’s vicinity. Taking into account Io’s position with respect to the plasma torus, the orientation of Jupiter’s magnetic field and the viewing perspective of the observation, the model calculates the auroral morphology and brightness. By fitting the model parameters to the observations we find that the model is able to reproduce the main features in all images obtained over a period of five years with one parameter set for each emission multiplet. The spatial distribution of the OI] 1356 Å, OI 1304 Å, SI 1479 Å, and SI] 1900 Å multiplets are shown to be very similar. In contrast to previous investigations, the model results reveal that the majority of the radiation from the bound atmosphere is emitted within 100 km above the surface. The equatorial aurora spots extend far into the wake region explaining observed features in the downstream region. The relative brightness of two the equatorial spots is best explained by our model if the emission on the day-side flank of Io is higher by a factor of ∼1.5 with respect to the nightside flank. The measured brightness during an observation in eclipse is significantly lower than expected from the fitted model. The day–night asymmetry and the brightness decrease in eclipse support the idea of a wide collapse of Io’s atmosphere in shadow. Since our phenomenological aurora model is able to reproduce the main features of the observed morphology by taking into account the variations of the magnetospheric parameters, it can be applied to predict the emission for future UV aurora observations for a given time and position of the observer.
    Icarus 01/2014; 228:386–406. · 3.16 Impact Factor
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    ABSTRACT: In November and December 2012 the Hubble Space Telescope (HST) imaged Europa's ultraviolet emissions in the search for vapor plume activity. We report statistically significant coincident surpluses of hydrogen Lyman-α and oxygen OI130.4 nm emissions above the southern hemisphere in December 2012. These emissions are persistently found in the same area over ~7 hours, suggesting atmospheric inhomogeneity; they are consistent with two 200-km-high plumes of water vapor with line-of-sight column densities of about 10(20) m(-2). Nondetection in November and in previous HST images from 1999 suggests varying plume activity that might depend on changing surface stresses based on Europa's orbital phases. The plume was present when Europa was near apocenter and not detected close to its pericenter, in agreement with tidal modeling predictions.
    Science 12/2013; · 31.20 Impact Factor
  • Justin Erwin, R. E. Johnson, D. F. Strobel, X. Zhu
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    ABSTRACT: We developed a one-dimensional model of Pluto’s atmosphere from the surface to above the exobase by connecting a fluid solution of the lower and middle atmosphere to a kinetic solution of the upper atmosphere. In this way we consistently model the transition from the collisional lower atmosphere where solar heating occurs to the near-collisionless, escaping, upper atmosphere. IR heating and cooling are included using a detailed non-LTE model for methane and carbon monoxide previously used for the lower atmosphere of Pluto. UV heating of methane and nitrogen is included in the middle atmosphere. Direct-Simulated Monte-Carlo (DSMC) is used to model the transition from the fluid to rarified flow. Jeans escape can also be used to approximate the upper boundary conditions for the fluid model, but does not yield the same description of the upper atmosphere as the DSMC. The resulting atmosphere is highly extended, with the exobase varying between 5 and 10 planetary radii depending on the solar activity, and the total molecular escape rate does not exceed 1028 s-1. The upper atmospheric structure and the escape rate are highly variable due to the solar UV heating. While the adiabatic cooling due to escape is found to be non-negligible in the lower atmosphere, the density below 500km in altitude does not vary more than 5%. Results are presented for various solar UV heating rates, and sensitivity to methane and carbon-monoxide mixing ratio as well as orbital radius will be discussed.
    10/2013;
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    ABSTRACT: We report on the detection of O I 1304 Å and 1356 Å emission from Callisto, using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. An O2-dominated atmosphere on Callisto has been suspected for many years, but the only previously detected atmospheric components have been CO2 and ionospheric electrons, both found by the Galileo orbiter in 1997-1999. The new, faint O I detections 4 Rayleighs at 1356 Å, assuming uniform emission from Callisto's disk) include a component centered on or close to Callisto's disk that has a 1304/1356 Å ratio consistent with electron-impact dissociation of O2. In addition, there is apparently a more extended component dominated by 1304 Å emission and apparently derived from atomic oxygen. The observed emission is consistent with upper limits from previous, less sensitive, observations. We present our observations, analysis to separate Callisto emission from geocoronal and reflected solar O I signals, and the implications for Callisto's atmosphere: that it is collisionally thick, as inferred from the Galileo radio occultation measurements of ionospheric electrons, and its column density of O2 is probably comparable in magnitude to Io's SO2 column density. This puts Callisto in competition with Io for the third-most-massive satellite atmosphere in the Solar System, after Triton and Titan.
    10/2013;
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    ABSTRACT: a b s t r a c t The Cassini mission has investigated Titan's upper atmosphere in detail and found that, under solar irra-diation, it has a well-developed ionosphere, which peaks between 1000 and 1200 km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N 2 and CH 4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy depo-sition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions' dis-sociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan's upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan's upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38 nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200 km (where we have made estimates of the effec-tive recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan's upper atmosphere.
    Icarus 03/2013; 223(1):234-251. · 3.16 Impact Factor
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    ABSTRACT: 1] In this study, we reanalyze the CH 4 structure in Titan's upper atmosphere combining the Cassini Ion Neutral Mass Spectrometer (INMS) data from 32 flybys and incorporating several updates in the data reduction algorithms. We argue that based on our current knowledge of eddy mixing and neutral temperature, strong CH 4 escape must occur on Titan. Ignoring ionospheric chemistry, the optimal CH 4 loss rate is $3 Â 10 27 s À1 or 80 kg s À1 in a globally averaged sense, consistent with the early result of Yelle et al. (2008). The considerable variability in CH 4 structure among different flybys implies that CH 4 escape on Titan is more likely a sporadic rather than a steady process, with the CH 4 profiles from about half of the flybys showing evidence for strong escape and most of the other flybys consistent with diffusive equilibrium. CH 4 inflow is also occasionally required to interpret the data. Our analysis further reveals that strong CH 4 escape preferentially occurs on the nightside of Titan, in conflict with the expectations of any solar-driven model. In addition, there is an apparent tendency of elevated CH 4 escape with enhanced electron precipitation from the ambient plasma, but this is likely to be a coincidence as the time response of the CH 4 structure may not be fast enough to leave an observable effect during a Titan encounter.
    Journal of Geophysical Research 11/2012; 117(E11):E11006. · 3.17 Impact Factor
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    ABSTRACT: We connect a fluid model with a molecular-kinetic model of escape to simulate the atmosphere of Pluto and to obtain an accurate description of its escaping atmosphere. The atmosphere extends out to several Pluto radii, with adiabatic cooling being the dominant process in the upper atmosphere. The escape rates found are consistent with previous fluid models, but the structure of the upper atmospheric is significantly affected by our description of the escape process. Direct-Simulated Monte-Carlo (DSMC) is used to model the transition from the fluid to rarified flow. Jeans escape can also be used to approximate the upper boundary conditions for the fluid model, but does not yield the same description of the upper atmosphere as the DSMC. We include a detailed radiative heating model down to the surface, including both IR and UV sources, arriving at a description of the full atmosphere. With Pluto’s extended atmosphere, the effect of Charon on the escape process must be considered. After finding a consistent solution between heating, gravity and the escape process, we then estimate the influence of solar wind on the extended atmosphere.
    10/2012;
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    ABSTRACT: On the dayside, Titan's main ionospheric region (with observed electron densities often exceeding 3000 cm-3) is located at altitudes between 1000 and 1200 km. The production of free electrons occurs mainly through photoionization of N2 and CH4 and the loss of the electrons happens primarily through dissociative recombination with positively charged molecular ions yielding neutral products. Knowledge of the effective recombination coefficient, k(z), at different altitudes, z, in Titan's ionosphere is important in order to get a better understanding of the ionospheric structure. Neglecting electron transport and assuming 1) that number densities of negative ions are minute and 2) that steady state conditions applies, ne(z), is related to k(z) and the electron production rate, Pe(z), according to Pe (z) = k(z)!(ne (z))2 (1) We use an energy deposition model combined with Cassini data from four Titan encounters to assess the effective recombination coefficient in Titan's sunlit upper atmosphere via Eq. (1). We use N2 and CH4 density profiles derived by the Ion Neutral Mass Spectrometer (INMS) in order to determine Pe(z). The XUV/EUV solar spectrum impinging on the top of the atmosphere is obtained from measurements with the TIMED/SEE instrument. We use thermal electron number densities and electron temperatures, Te, derived from measurements with the Langmuir Probe, a subsystem of the Cassini Radio and Plasma Wave Science (RPWS) experiment. The Te data is needed to transfer the obtained k(z) values to their corresponding values, k300(z), at a reference electron temperature of 300 K assuming a standard electron temperature dependence of the effective recombination coefficient. We find a good agreement between the altitudes where the calculated electron production rates are peaking and the altitudes where the observed electron number densities are peaking. We find that the effective recombination coefficient at a reference electron temperature of 300 K, k300, increases with decreasing altitudes, which we attribute to the increased complexity of the ion population towards lower altitudes and laboratory results from dissociative recombination reactions of individual ion species. At low altitudes the derived values of k300 are less than the ones derived by Galand et al. (2010, J. Geophys. Res. 115, A07312), primarily as a result of the revised INMS neutral densities. We obtain, k300(z) values, which appear to be too high (by about a factor of 2-3) judging from laboratory measurements and comparisons with the (number density weighted) average rate coefficient among the major ion species in Titan's upper atmosphere (e.g., HCNH+, C2H5+, CH3CNH+, HCCCNH+). We will discuss potential reasons for the discrepancy found.
    09/2012;
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    ABSTRACT: Solar XUV photons can provide enough energy to account for the observed nitrogen UV dayglow emissions above 800 km, but a small or sporadic contribution from energetic particles cannot be ruled out. Furthermore, ion production at altitudes deeper than 800 km as inferred from radio occultation cannot be produced by solar XUV stimulation and implies energy deposition from protons and oxygen ions. Here we examine UV spectra and visible-wavelength images of Titan in Saturn's shadow, when XUV stimulation is absent. UV emissions are observed in one of the three sets of spectra, and the intensity of these emissions is about a factor of 10 less than the peak intensity reported on the dayside. We observe visible-wavelength emissions for the first time. No horizontally resolved auroral structures are seen in the visible images. At visible wavelengths Titan has a global emission at the haze-top level that is not understood, although cosmic ray ionization and chemiluminescence are candidates needing further investigation.
    Geophysical Research Letters 09/2012; 39(18):18204-. · 3.98 Impact Factor
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    Darrell F. Strobel
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    ABSTRACT: One of Professor Donald M. Hunten’s lasting contributions to the field of planetary atmospheres was the principle of the (Hunten) limiting flux, where the escape of light species is limited by the rate at which they can diffuse through the atmosphere. While his limiting flux expression has been well tested for hydrogen’s escape from the Earth’s atmosphere (e.g., Hunten and Strobel (J. Atmos. Sci. 31, 305 (1974)); Hunten and Donahue (Ann. Rev. Earth Planet Sci. 4, 265 (1976))), it has not been tested for Titan’s atmosphere, which was the original motivation for the principle. The Cassini–Huygens mission has provided sufficient data on the variation of the H2 mole fraction with altitude to test its applicability and validity. Only in the vicinity of the homopause does the limiting flux expression yield the actual H2 escape flux, because the mole fraction varies with altitude. This paper deals also with our current understanding of the three major constituents of Titan’s atmosphere (N2, CH4, and H2) from the various measurements by instruments on the Cassini orbiter and the Huygens probe. Specific problems addressed are additional required sources of H2, the CH4 escape rate, and the possible role of energetic electron and ion precipitation from Saturn’s magnetosphere.
    Canadian Journal of Physics 08/2012; 90(8):795-805. · 0.90 Impact Factor
  • Darrell F. Strobel
    Canadian Journal of Physics 08/2012; 90(8):795-805. · 0.90 Impact Factor
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    ABSTRACT: We analyze a set of Hubble Space Telescope (HST) observations of the auroral UV emission from Jupiter's satellite Io and find a remarkably stable emission pattern over a period of 5 years. Io's auroral radiation is generated by collisions of impinging magnetospheric electrons and the atmospheric gas particles. The radiation is often used as a tool to infer properties of both the satellite's plasma environment and the atmosphere. In our study, we investigate 40 images of the spatially resolved OI1356 Å emission on Io's dayside atmosphere taken by the HST Space Telescope Imaging Spectrograph (STIS) between 1997 and 2001. We construct a phenomenological model for the three dimensional distribution of the local UV emission in Io's vicinity, which only depends on the properties of the ambient plasma. Model images generated by integrating the local emission along the respective line-of-sight show very good agreement with the 40 STIS observations for all major auroral features. We find that the sunlit hemisphere appears to be brighter than the nightside hemisphere. Furthermore, a comparison with a STIS observation taken in eclipse indicates a collapse of the lower atmosphere, when Io moves through Jupiter's shadow. These findings imply a primarily sublimation driven atmosphere.
    04/2012;
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    ABSTRACT: Jupiter's largest satellite Ganymede is unique because it possesses an internal magnetic field strong enough to create a small magnetosphere around the satellite.The interaction between Ganymede's magentosphere and the Jovian magnetic field and magnetospheric plasma generates classic polar auroral emissions from Ganymede which have been captured in a series of stunning ultraviolet images using the Hubble Space Telescope on four different dates from 1998 to 2007. Only data from the first set of obserations in 1998 has been published (Feldman et al. 2000; Eviatar et al. 2001). We have used a common data reduction process on all four sets of images, and combined them to produce a near global map of the locatoin of Ganymede's auroral oval, which appears to be relatively stable in the seven years spanning the observations. We compare the location of this auroral oval with several model predictions of the boundary between open and closed magnetic field lines (Koop and Ip 2002; Khurana et al. 2007; Jia et al. 2009), a region where strong field aligned currents are thought to produce the auroral emissions. We also compare the location of the auroral emission with the polar cap boundary on Ganymede as dlineated by color ratio images acquired by the Galileo mission (Khurana et al. 2007). [The figure below shows Ganymede's auroral oval emission from the trailing hemisphere of the satellite, which is the downstream hemisphere relative to the plasma flow.] Eviatar, A. , D. F. Strobel, B. C. Wolven, P. D. Feldman, M. A. McGrath, and D. J. Williams (2001), Astrophys. J, 555, 1013-1019. Feldman, P. D., M. A. McGrath, D. F. Strobel, H. W. Moos, K. D. Retherford, and B. C. Wolven (2000), Astrophys. J, 535, 1085-1090. Jia, X., R. J. Walker, M. G. Kivelson, K. K. Khurana, and J. A. Linker (2009), J. Geophys. Res., 114, A09209, doi:10.1029/2009JA014375. Khurana, K. K., R. T. Pappalardo, N. Murphy, and T. Denk (2007), Icarus, 191, 193-202, doi:10.1016/j.icarus.2007.04.022. Kopp, A. and W.-H. Ip (2002), J. Geophys. Res, 107(A12), 1490, doi:10.1029/2001JA005071.
    Journal of Geophysical Research 12/2011; · 3.17 Impact Factor
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    ABSTRACT: We analyze a set of HST/STIS observations of Io's auroral UV emission and find a remarkably stable emission pattern over a period of 5 years when we take into account the viewing geometry and Io's position with respect to Jupiter's magnetosphere. Io's auroral radiation is generated by collisions of impinging magnetospheric electrons and the atmospheric gas particles. The radiation is often used as a tool to infer properties of both the satellite's plasma environment and the atmosphere. We investigate the spatially resolved OI1356 Å emission extracted from 18 observations of Io's dayside atmosphere by the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST) taken between 1997 and 2001. We then construct a phenomenological model for the three dimensional distribution of the local UV emission in Io's vicinity, which only depends on the properties of the ambient plasma. Model images, generated by integrating the local emission along the respective line-of-sight, show good agreement with the 18 STIS observations for all major aurora features. Thus, we do not find strong variations of the oxygen abundance, although Io's atmosphere is assumed to be highly variable due to the changing volcanic activity. A comparison with a STIS observation taken in eclipse indicates a collapse of the lower atmosphere, when Io moves through Jupiter's shadow.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: We report on new Space Telescope Imaging Spectrograph (STIS) observations of Ganymede's auroral emissions obtained during two visits with the Hubble Space Telescope (HST) when Ganymede was located at eastern elongation. The observations of the first visit with five consecutive orbits were successfully obtained on November 19, 2010 and the second visit, also with five orbits, is scheduled during Jovian opposition in October/November 2011. We will present results of the full campaign in case of a successful execution of the second visit. Our observations cover more than half a cycle of system III longitudes of Ganymede's positions within Jupiter's magnetosphere for each visit. The observations clearly display northern and southern auroral ovals, which we analyze with respect to brightness and locations. Our goal is to set constraints on the interaction of Ganymede's mini-magnetosphere with Jupiter's magnetosphere, Ganymede's magnetic field and plasma environment, and, if possible, on Ganymede's neutral atmosphere.
    AGU Fall Meeting Abstracts. 12/2011;

Publication Stats

6k Citations
1,087.36 Total Impact Points

Institutions

  • 1986–2014
    • Johns Hopkins University
      • • Department of Earth and Planetary Sciences
      • • Department of Physics and Astronomy
      • • Applied Physics Laboratory
      Baltimore, Maryland, United States
  • 2011
    • University of Cologne
      • Institute of Geophysics and Meteorology
      Köln, North Rhine-Westphalia, Germany
  • 2009
    • Laboratoire d'Etudes en Géophysique et Óceanographie Spatiales
      Tolosa de Llenguadoc, Midi-Pyrénées, France
  • 2008
    • Pierre and Marie Curie University - Paris 6
      Lutetia Parisorum, Île-de-France, France
  • 2003
    • George Mason University
      • Department of Computational and Data Sciences
      Fairfax, VA, United States
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 1999
    • University of Wisconsin, Madison
      • Department of Physics
      Madison, MS, United States
  • 1979
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States