R. E. Johnson

University of Virginia, Charlottesville, Virginia, United States

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Publications (520)1113.36 Total impact

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    R. E. Johnson
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    ABSTRACT: Atmospheric loss induced by an incident plasma, often called atmospheric sputtering, can significantly alter the volatile inventories of solar system bodies. Based on the present atmospheric sputtering rate, the net loss of nitrogen from Titan in the last 4 Gyr was small, consistent with Titan retaining a component of its primordial atmosphere. However, atmospheric sputtering by the magnetospheric plasma ions and by pickup ions, even at present levels, would have caused the loss of a large, residual Titan-like atmosphere from Io and Europa and a significant fraction of such an atmosphere from Ganymede. At Callisto, higher magnetospheric plasma densities would have been required for the loss of such an atmosphere. Since higher plasma densities were probable in earlier epochs, the evolution of the volatile inventories of each of the Galilean satellites has been profoundly affected by the interaction of their atmospheres with the Jovian magnetospheric plasma.
    Preview · Article · Jan 2016 · The Astrophysical Journal
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    O. J. Tucker · R. E. Johnson · L. A. Young
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    ABSTRACT: Recent hybrid fluid/molecular kinetic models demonstrated that Pluto's upper atmosphere is warmer and more extended than previously thought (Erwin, J.T., Tucker, O.J., Johnson, R.E. [2013]. Icarus 226, 375-384; Tucker, O.J., Erwin, J.T., Deighan, J.I., Volkov, A.N., Johnson, R.E. [2012]. Icarus 217, 408-415). Here we examine the effect of Charon on the molecular escape rate from Pluto's extended atmosphere, simulate the spatial distribution of N2 in this binary system, and describe the resulting accumulation of N2 on Charon. These Monte Carlo simulations are carried out for approximate solar medium conditions at ∼33 AU. Including Charon's gravity and orbital motion, the atmosphere on the Pluto's Charon-facing hemisphere is more strongly bound to the system and is more extended than the atmosphere on Pluto's anti-Charon hemisphere. Accounting for Charon's gravity the net escape from the system is reduced by ∼5%. Most of the loss is direct from Pluto's exobase region with ∼1-2% due to scattering by Charon. About ∼0.2% of the flux from Pluto's exobase impinges on Charon: ∼5.7 × 1025 N2/s at nominal solar medium conditions. This is a source of condensed N2 for Charon's night side and forms a tenuous atmosphere. For the approximate range of surface temperatures, the residence time of N2 on the surface can range from a fraction of a second to 10s of years with the near surface line of sight column densities varying from ∼3 × 1018 N2/m2 up to >6 × 1019 N2/m2. Such an atmosphere could be detectable during the solar occultation that will occur during the New Horizon encounter providing a measure of the transfer of gas between bodies in this binary system.
    Full-text · Article · Dec 2014 · Icarus
  • A. K. Woodson · H. T. Smith · F. J. Crary · R. E. Johnson
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    ABSTRACT: We investigate the complex interaction between Saturn's magnetosphere and Titan's upper ionosphere using ion data acquired by the Cassini Plasma Spectrometer (CAPS) during the T40 encounter. Bounds on ion-group abundances at altitudes between ~2733 and ~12,541 km are determined by fitting mass spectra with model functions derived from instrument calibration data. The spectra are dominated by H+, H2+, H3+, and two hydrocarbon groups with mass ranges 12–19 and 24–32 amu, respectively. Notably, this constitutes the first reported observation of H3+ in Titan's exosphere. These measurements are discussed in the context of data from the CAPS Electron Spectrometer (ELS) and Cassini's Ion and Neutral Mass Spectrometer (INMS), which fortuitously sampled the ionospheric outflow during the T40 encounter at altitudes between ~2225 and ~3034 km [Westlake et al., 2012]. The CAPS data reveal a composition that is constitutively similar to that sampled by INMS, with hydrocarbon ions first observed as far as ~11,000 km from Titan and increasing by more than an order of magnitude along Cassini's inbound trajectory. In addition, we juxtapose the CAPS ion data with numerical results from three different interaction models and show that it is consistent with the location of the field-draping boundary described by Ulusen et al. [2012] and the Saturnward ion tail predicted by Sillanpää et al. [2006].
    No preview · Article · Nov 2014 · Journal of Geophysical Research: Space Physics
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    ABSTRACT: We have modeled an electron precipitation pattern expected on Mimas, Tethys, and Dione, using two different approaches. In the first approach, we adapt a previously developed model to compute an integrated energy flux into the surfaces of Mimas, Tethys, and Dione. This is a guiding-center, bounce-averaged model. In the second approach, we track individual particles in an electromagnetic field for an inert or slightly magnetized satellite. This second approach allows us to include the effects of electron pitch angle and gyrophase on the weathering pattern. Both methods converge on an enhanced dose pattern on each satellite’s leading hemisphere that is lens-shaped. We also present mission-averaged electron energy spectra obtained near these satellites by Cassini’s Magnetosphere Imaging Instrument (MIMI). These data are interpreted using our current understanding of both the environment and the instrument’s response. Fits to the data are integrated to find an energy flux into each satellite’s surface, as a function of longitude and latitude. Using positions on the moon accessible to energetic electrons from the modeling and the integrated energy flux based on data, we find lens patterns that fall off with increasing moon latitude. The predicted patterns are qualitatively consistent with some but not all of the optical observations made of these hemispheres.
    No preview · Article · May 2014 · Icarus
  • 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.
    No preview · Article · Oct 2013
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    ABSTRACT: Since the MAVEN mission will obtain information on the pick-up ion densities and speeds, atmospheric neutrals and ions, and UV emission profiles, the atmospheric sputtering by the re-impact of the heavy pick-up ions incident on Mars can be evaluated and compared with photo-induced loss processes under a variety of solar conditions occurring during the mission. Based on the simulation results from a 3D Monte Carlo model coupled to a molecular dynamic calculation, the atmospheric sputtering efficiencies due to pick-up O+ for a number of energy and angle spectra are studied statistically. The sputtered hot neutrals populating the extended corona and the escape components are estimated when various solar wind conditions and solar cycle variations are considered. We find that the sputtering efficiencies can be characterized by the total incident fluxes as weighted with different incident energies. The dependencies between the incident pick-up ions and the sputtered hot neutrals lead to certain “response relations”. These can be utilized to predict the sputtering rate when incident pick-up O+ fluxes become available. A sputtered hot neutral corona can then be constructed as a reference before modeling results are generated.
    No preview · Article · Oct 2013
  • A. N. Volkov · R. E. Johnson · O. J. Tucker
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    ABSTRACT: Steady monatomic and diatomic gas flows from a spherical source into a vacuum in a gravitational field are studied using direct statistical simulation. The qualitative gravitation effect on the flow is shown to be independent of the intermolecular collision model. Three characteristic Jeans parameter ranges can always be distinguished, namely, the subcritical range, on which the flow in a weak gravitational field is similar with the outflow in the absence of gravitation, the supercritical range, on which the outflow velocity remains small even at large distances from the source, and a narrow transitional range between the two former ranges. The presence of internal degrees of freedom of gas molecules displaces the transitional range toward the greater values of the Jeans parameter and leads to an increase in the outflow velocity and the gas temperature; however, in the initial region the latter effect is expressed only slightly. The normalized escape flow is a nonmonotonic function of both the Jeans parameter and the Knudsen number and is different for monatomic and diatomic gases within 50% on the parameter range considered.
    No preview · Article · Mar 2013 · Fluid Dynamics
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    ABSTRACT: We present the first calculation of Europa's sputtering (ion erosion) rate as a function of position on Europa's surface. We find a global sputtering rate of 2×1027 H2O s-1, some of which leaves the surface in the form of O2 and H2. The calculated O2 production rate is 1×1026 O2 s-1, H2 production is twice that value. The total sputtering rate (including all species) peaks at the trailing hemisphere apex and decreases to about 1/3rd of the peak value at the leading hemisphere apex. O2 and H2 sputtering, by contrast, is confined almost entirely to the trailing hemisphere. Most sputtering is done by energetic sulfur ions (100s of keV to MeV), but most of the O2 and H2 production is done by cold oxygen ions (temperature ˜ 100 eV, total energy ˜ 500 eV). As a part of the sputtering rate calculation we compared experimental sputtering yields with analytic estimates. We found that the experimental data are well approximated by the expressions of Famá et al. for ions with energies less than 100 keV (Famá, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156-161), while the expressions from Johnson et al. fit the data best at higher energies (Johnson, R.E., Burger, M.H., Cassidy, T.A., Leblanc, F., Marconi, M., Smyth, W.H., 2009. Composition and Detection of Europa's Sputter-Induced Atmosphere, in: Pappalardo, R.T., McKinnon, W.B., Khurana, K.K. (Eds.), Europa. University of Arizona Press, Tucson.). We compare the calculated sputtering rate with estimates of water ice regolith grain size as estimated from Galileo Near-Infrared Mapping Spectrometer (NIMS) data, and find that they are strongly correlated as previously suggested by Clark et al. (Clark, R.N., Fanale, F.P., Zent, A.P., 1983. Frost grain size metamorphism: Implications for remote sensing of planetary surfaces. Icarus 56, 233-245.). The mechanism responsible for the sputtering rate/grain size link is uncertain. We also report a surface composition estimate using NIMS data from an area on the trailing hemisphere apex. We find a high abundance of sulfuric acid hydrate and radiation-resistant hydrated salts along with large water ice regolith grains, all of which are consistent with the high levels of magnetospheric bombardment at the trailing apex.
    Full-text · Article · Mar 2013 · Planetary and Space Science
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    W. -L. Tseng · R. E. Johnson · W. -H. Ip
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    ABSTRACT: The Voyager flyby observations revealed that a very broad doughnut shaped distribution of the hydrogen atoms existed in the Saturnian magnetosphere. Recent Cassini observations confirmed the local-time asymmetry but also showed the hydrogen cloud density increases with decreasing distance to Saturn. The origin of the atomic hydrogen cloud has been debated ever since. Therefore, we have carried out a global investigation of the atomic hydrogen cloud taking into account all possible sources: 1) the Saturnian atmosphere, 2) the H2 atmosphere of main rings, 3) Enceladus H2O and OH torus, 4) Titan H2 torus and 5) the atomic hydrogen directly escaping from Titan. We show that the H ejection velocity and angle distribution are modified by collisions of the hot H, produced by electron-impact dissociation of H2, with the ambient atmospheric H2 and H. This in turn affects the morphology of the escaping hydrogen as does the morphology of the ionospheric electron distribution. That Saturn atmosphere is an important source is suggested by the fact that the H cloud peaks well below the ring plane, a feature that, so far, we can not reproduce by the dissociation of the ring H2 atmosphere or other proposed sources. Our simulations show that H directly escaping from Titan is a major contribution in the outer magnetosphere. The morphology of Titan H torus, shaped by the solar radiation pressure and the Saturnian oblateness, can account for the local time asymmetry near Titan orbit. Dissociation of H2O and OH in the Enceladus torus contributes inside ~5 RS, but dissociation of Titan H2 torus does not due to the significant energy released. The total number of H observed by Cassini inside 5 RS: our modeling results suggest ~20% from dissociation in the Enceladus torus, ~10% from dissociation of ring H2 atmosphere, and ~50% from Titan H torus implying that ~20% comes from the Saturnian atmosphere.
    Full-text · Article · Feb 2013 · Planetary and Space Science
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    ABSTRACT: The solar wind continuously flows out from the Sun and directly interacts with the surfaces of dust and airless planetary bodies throughout the solar system. A significant fraction of solar wind ions reflect from an object's surface as energetic neutral atoms (ENAs). ENA emission from the Moon was first observed during commissioning of the Interstellar Boundary Explorer (IBEX) mission on 3 December 2008. We present the analysis of 10 additional IBEX observations of the Moon while it was illuminated by the solar wind. For the viewing geometry and energy range (> 250 eV) of the IBEX-Hi ENA imager, we find that the spectral shape of the ENA emission from the Moon is well-represented by a linearly decreasing flux with increasing energy. The fraction of the incident solar wind ions reflected as ENAs, which is the ENA albedo and defined quantitatively as the ENA reflection coefficient RN, depends on the incident solar wind speed, ranging from ~0.2 for slow solar wind to ~0.08 for fast solar wind. The average energy per incident solar wind ion that is reflected to space is 30 eV for slow solar wind and 45 eV for fast solar wind. Once ionized, these ENAs can become pickup ions in the solar wind with a unique spectral signature that reaches 3vSW. These results apply beyond the solar system; the reflection process heats plasmas that have significant bulk flow relative to interstellar dust and cools plasmas having no net bulk flow relative to the dust.
    Full-text · Article · Feb 2013 · Journal of Geophysical Research Atmospheres
  • Justin Deighan · A. D. Howard · R. E. Johnson
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    ABSTRACT: Modern Mars is a cold, dry world where weathering is slow and volcanic activity is dormant. This allows for the accumulation of 0.13% O2 in the thin CO2 dominated atmosphere, making it relatively oxidizing. In contrast, during the Noachian and Hesperian eras > 3 Gya, Mars is thought to have had extended warm periods with persistent liquid water and intense volcanic eruptions. The atmosphere was likely much thicker, though still CO2 dominated, and oxidizing atmospheric species were either weathered out or reduced by volcanic gases. We examine here the photochemical transition of the atmosphere between these two extremes--oxidizing and reducing--as driven by the oxidation sinks of weathering and volcanic activity. A series of illustrative case studies in atmosphere-surface coupling are explored to quantify dominant processes and plausible O loss rates. These are compared with the rates of photochemical H2 production and escape to space to determine the redox state of the atmosphere. Our atmospheric photochemistry model is coupled to a 1D radiative-convective model to capture temperature profile feedback due to the production of O3 in oxidizing atmospheres. Studying the transition between an oxidizing and reducing atmosphere on Mars is of interest for understanding weathering mineralogy, loss of atmosphere, and habitability for life. This work was supported by a grant from the NASA Planetary Atmosphere program.
    No preview · Article · Oct 2012
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    ABSTRACT: Prior to Cassini's arrival at Saturn, Titan-generated nitrogen ions were thought to dominate heavy plasma in Saturn's magnetosphere. Therefore, the presence of a Titan nitrogen torus was anticipated. However, it is now known water-group ions dominate Saturn's heavy ion magnetospheric plasma. While nitrogen ions have been detected beyond the orbit of Rhea, these ions appear to be primarily originating from the Enceladus plumes with little nitrogen plasma detected in the magnetosphere near Titan's orbit. In fact, pick-up oxygen ions from Enceladus are much more abundant than nitrogen in Titan’s orbit. These results appear inconsistent with the expectation that Titan's dense relatively unprotected atmosphere should provide a significant source of heavy particles to Saturn's magnetosphere. This inconsistency suggests that the plasma environment at Titan's orbit is much more complex than originally anticipated. In this talk, we expand on our previous research that categorizes the plasma environments near Titan to include all locations along Titan's orbit. Using these categories, we develop characteristic plasma spectra of each type of environment, update ionization lifetimes for each region and apply these results in a 3D Monte Carlo model to more accurately examine the fate of nitrogen and methane escaping Titan's atmosphere to support the possible presence of a Titan torus despite the lack of observations. We also present preliminary Cassini data analysis that is focused on regions where such a torus could be detected. This work is supported by the NASA Cassini Data Analysis Program and NASA JPL contract 1243218 for Cassini MIMI and CAPS investigation.
    No preview · Article · Oct 2012
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    ABSTRACT: Rapid thermospheric flows can significantly enhance the atmospheric loss rates and structures of atmospheric coronae of planetary bodies. Using descriptions of atmospheric escape based on molecular kinetic models, we show that such flows at the exobase of Titan could significantly increase the calculated constituent thermal and nonthermal escape rates. In particular, we show here that the effect of thermospheric winds at the exobase cannot be ignored when calculating the escape of methane from Titan. Such enhancements are likely also relevant to Pluto and exoplanet atmospheres.
    No preview · Article · Aug 2012 · Geophysical Research Letters
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    ABSTRACT: There have been three Cassini encounters with the south-pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by CAPS (similar to 10(4) amu/e). These particles are interpreted as singly charged nanometer-sized water-ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to similar to 2600 e/cm(3) for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E-ring material.
    No preview · Article · May 2012 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: Tethys' leading hemisphere has a newly discovered thermal anomaly. Data from Cassini's Composite Infrared Spectrometer (CIRS) instrument taken in June 2007 and September 2011 confirms that a lens-shaped region, centered on Tethys' leading hemisphere at equatorial latitudes is anomalously warmer at night and cooler during the day than its surroundings. The local time coverage now provided by the CIRS data set is sufficient to constrain the thermal surface property variation across this region. The thermal inertia inside of the anomalous region is observed to be three times higher than that outside of the anomaly, whereas the albedo remains consistent across the region. The mapped portion of the thermally anomalous region coincides in shape and location to a region of high-energy electron deposition from Saturn's magnetosphere, which also has unusually high near-UV reflectance and low near-IR reflectance (Schenk et al., 2011). A similar thermal anomaly was previously detected on Mimas' leading hemisphere, in a region that also undergoes high-energy electron bombardment and has high near-UV reflectance (Howett et al., 2011; Schenk et al., 2011). Therefore, high-energy electrons, which penetrate both Mimas' and Tethys' surface to the centimeter depths probed by diurnal temperature variations, also likely alter the surface texture and dramatically increase its thermal inertia.
    No preview · Article · Apr 2012
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    ABSTRACT: We present observations of CAPS electron and ion spectra during Titan distant tail crossings by the Cassini spacecraft. In common with closer tail encounters, we identify ionospheric plasma in the tail. Some of the electron spectra indicate a direct magnetic connection to Titan's dayside ionosphere due to the presence of ionospheric photoelectrons. Ion observations reveal heavy and light ion populations streaming into the tail. Using the distant tail encounters T9, T75 and T63, we estimate total plasma loss rates from Titan via this process.
    Full-text · Article · Apr 2012 · Journal of Geophysical Research Atmospheres
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    M. K. Elrod · W.-L Tseng · R. J. Wilson · R. E. Johnson
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    ABSTRACT: 1] With the discovery by the Cassini spacecraft of an oxygen atmosphere over Saturn's main rings, and a strong source of water products from the plumes of Saturn's moon Enceladus, our picture of the physics of Saturn's magnetosphere from the main rings to inside the orbit of Enceladus has changed dramatically. This region contains oxygen ions from the ring atmosphere and water-group ions from the Enceladus torus. The purpose of this study is to examine ion densities, temperatures, and composition from several equatorial periapsis passes from 2004 to 2010 for the region from 2.4 to 3.5 Saturn radii ($60,300 km) in addition to Voyager 2 in order to separate contributions from Saturn's ring atmosphere from the water products in the Enceladus torus and to describe the temporal variations in the plasma. Because of the high background due to so-called penetrating radiation in this region, only six orbits are used in this study. Our analysis indicates that large variations in ion density, temperature, and composition occurred between the Voyager 2 flyby, 2004, and 2010. Although the Enceladus plumes may be variable, we propose that the large change in the ion density from 2004 to equinox near 2010 is due to the seasonal variation in the ring atmosphere. Our interpretation of the plasma data is supported by a simple photochemical model, combining the water products from Enceladus and the seasonal variations in the ring atmosphere.
    Preview · Article · Mar 2012 · Journal of Geophysical Research Atmospheres
  • W.-L. Tseng · M. K. Elrod · R. E. Johnson
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    ABSTRACT: We predicted would-be seasonal variations in the ring atmosphere due to the orientation of the ring plane to the Sun. Therefore, it would exhibit seasonal variations in the magnetospheric O_2^+ ion density. We also confirmed the result by CAPS data.
    No preview · Article · Mar 2012
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    ABSTRACT: We characterize the relative importance of energetic electrons and protons to the weathering of five of the inner satellites of Saturn. To do this, we present data from the Magnetospheric Imaging Instrument on the Cassini spacecraft, some of which is averaged over the whole mission to date. We also compute averaged proton and electron energy spectra relevant to the distances of these inner satellites. Where data are available, we estimate the power per unit area into a satellite's surface. For electron energy deposition into satellite leading hemispheres, we find the power per unit area is greatest at Mimas and falls off with distance from Saturn. Using fluxes of 1–50MeV protons detected within the sweeping corridors of Mimas and Enceladus, we find the corresponding deposition would be about 2×108 and 3.7×107eV/cm2s.
    Full-text · Article · Feb 2012 · Planetary and Space Science
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    ABSTRACT: During a close pass of Cassini through the plasma wake of Saturn's moon Dione on April 7, 2010 the Cassini Plasma Spectrometer (CAPS) detected molecular oxygen ions (O2+) on pick up ring velocity distributions, thus providing the first in situ detection of a neutral exosphere surrounding the icy moon. The density of O2+ determined from the CAPS data ranges from 0.01 to 0.09 /cm3 and is used to estimate the exosphere O2 radial column density, obtaining the range 0.9 to 7 × 1011/cm2 . CAPS was unable to directly detect pick up H2O+ from the exosphere but the observations can be used to set an upper limit to their density of ˜10 times the O2+ density.
    Full-text · Article · Feb 2012 · Geophysical Research Letters

Publication Stats

11k Citations
1,113.36 Total Impact Points


  • 1029-2016
    • University of Virginia
      • • Department of Materials Science and Engineering
      • • School of Engineering and Applied Science
      • • Laboratory for Atomic and Surface Physics (LASP)
      Charlottesville, Virginia, United States
  • 2011
    • CUNY Graduate Center
      New York, New York, United States
  • 2006-2009
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, MD, United States
  • 2008
    • Georgia Institute of Technology
      Atlanta, Georgia, United States
  • 2007
    • NASA
      Washington, West Virginia, United States
  • 2005
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 1983-2004
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2002
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1999
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, California, United States
  • 1989-1991
    • Uppsala University
      Uppsala, Uppsala, Sweden