D. F. Strobel

Johns Hopkins University, Baltimore, Maryland, United States

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Publications (307)1242.21 Total impact

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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: We retrieved the density and temperature profiles in Saturn's thermosphere from 26 stellar occultations observed by the Cassini/UVIS instrument. These results expand upon and complement the previous analysis of 15 Cassini/UVIS solar occultations by Saturn's upper thermosphere. We find that the exospheric temperatures based on the stellar occultations agree with the solar occultations and range from 380 K to 590 K. These temperatures are also consistent with the recent re-analysis of the Voyager/UVS occultations. The retrieved density profiles support our earlier inference that the shape of the atmosphere at low pressures is consistent with a meridional trend of increasing temperatures with absolute latitude. This implies a high-latitude heat source, such as auroral heating, although the existing circulation models that include auroral heating still underestimate the equatorial temperatures by overestimating the meridional temperature gradient. This suggests either that the circulation models are somehow incomplete or there is some other heat source at low to mid latitudes that is relatively less efficient than high-latitude heating. We also find evidence for the expansion of the exobase by about 500 km between 2006 and 2011 near the equator, followed by possible contraction after 2011. The expansion appears to be caused by significant warming of the lower thermosphere that anti-correlates with solar activity and may be connected to changes in global circulation. Lastly, we note that our density profiles are in good general agreement with the Voyager/UVS data. In particular, the Voyager density profiles are most consistent with the Cassini/UVIS stellar occultations from late 2008 and early 2009 that roughly coincide in season with the Voyager flybys.
    No preview · Article · Oct 2015 · Icarus
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    ABSTRACT: The Pluto system was recently explored by NASA’s New Horizons spacecraft, making closest approach on 14 July 2015. Pluto’s surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto’s atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto’s diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto’s large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.
    Full-text · Article · Oct 2015 · Science
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    ABSTRACT: NASA's New Horizons spacecraft flies past Pluto on July 14, 2015, carrying two instruments that detect charged particles. Pluto has a tenuous, extended atmosphere that is escaping the planet's weak gravity. The interaction of the solar wind with Pluto's escaping atmosphere depends on solar wind conditions as well as the vertical structure of Pluto's atmosphere. We have analyzed Voyager 2 particles and fields measurements between 25 and 39 AU and present their statistical variations. We have adjusted these predictions to allow for the Sun's declining activity and solar wind output. We summarize the range of SW conditions that can be expected at 33 AU and survey the range of scales of interaction that New Horizons might experience. Model estimates for the solar wind stand-off distance vary from ~7 to ~1000 RP with our best estimate being around 40 RP (where we take Pluto's radius to be RP = 1184 km).
    No preview · Article · Sep 2015 · The Journal of Geophysical Research Planets
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    ABSTRACT: We report the result of a search for evidence of an O2-dominated atmosphere on Callisto, using the high far-ultraviolet sensitivity of the Hubble Space Telescope Cosmic Origins Spectrograph (COS). Observations of Callisto’s leading/Jupiter-facing hemisphere show, for the first time, variable-strength atomic oxygen (O I) emissions with brightness up to 4.7 ± 0.7 Rayleighs for the O I 1304 Å triplet and 1.9 ± 0.4 Rayleighs for the O I 1356 Å doublet, averaged over the 2.5 arcsec. diameter COS aperture. Because the observations were made in Earth’s shadow, and are brighter than expected emission from nighttime geocoronal airglow or other plausible sources, we are confident that they originate from Callisto or its immediate vicinity. In addition, COS’s limited (∼1 arcsec) spatial resolution implies a 2σ detection of excess 1356 Å emission concentrated on the disk of Callisto itself, with brightness 3.2 ± 1.6 Rayleighs. The (O I 1356 Å)/(O I 1304 Å) emission ratio from Callisto’s disk favors dissociative excitation of O2, suggesting that O2 is the dominant atmospheric component rather than other possible oxygen-bearing alternatives. Photoelectrons, rather than magnetospheric electrons, are the most likely source of the dissociative excitation. This detection yields an O2 column density of ∼4 × 1015 cm−2 on the leading/Jupiter facing hemisphere, which implies that Callisto’s atmosphere is collisional and is the fourth-densest satellite atmosphere in the Solar System, in addition to being the second-densest O2-rich collisional atmosphere in the Solar System, after Earth. Longitudinal variations in published densities of ionospheric electrons suggest that O2 densities in Callisto’s trailing hemisphere, which we did not observe, may be an order of magnitude greater. The aperture-filling emissions imply that there is also an extended corona of predominantly O I 1304 Å emission around Callisto, with observed strength of 1–4 Rayleighs, likely due to solar resonance scattering from sputtered atomic O, with a density of up to 104 cm−3 at the exobase.
    No preview · Article · Jul 2015 · Icarus
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    ABSTRACT: We have used Chandra to observe Pluto and search for X-ray emission due to solar wind ions charge exchanging with gas molecules escaping from its atmosphere.
    No preview · Article · Feb 2015
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    ABSTRACT: We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiter's time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0° ± 1.3°. Our model calculations predict a significantly stronger oscillation by 5.8° ± 1.3° without ocean compared to 2.2°±1.3° if an ocean is present. Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte-Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymede's dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.
    No preview · Article · Feb 2015 · Journal of Geophysical Research: Space Physics
  • T. E. Cravens · D. F. Strobel
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    ABSTRACT: Exospheric neutral atoms and molecules (primarily N2, with trace amounts of CH4 and CO according to our current understanding of Pluto's atmosphere) escape from Pluto and travel into interplanetary space for millions of kilometers. Eventually, the neutrals are ionized by solar EUV photons and/or by collisions with solar wind electrons. The mass-loading associated with this ion pick-up is thought to produce a comet-like interaction of the solar wind with Pluto. Within a few thousand kilometers of Pluto the solar wind interaction should lead to a magnetic field pile-up and draping, as it does around other “non-magnetic” bodies such as Venus and comets. The structure of plasma regions and boundaries will be greatly affected by large gyroradii effects and the extensive exosphere. Energetic plasma should disappear from the flow within radial distances of a few thousand kilometers due to charge exchange collisions. An ionosphere should be present close to Pluto with a composition that is determined both by the primary ion production and ion-neutral chemistry. One question discussed in the paper is whether or not the ionosphere has a Venus-like sharply defined ionopause boundary or a diamagnetic cavity such as that found around comet Halley. Simple physical estimates of plasma processes and structures in the collision-dominated region are made in this paper and predictions are made for the New Horizons mission.
    No preview · Article · Dec 2014 · Icarus
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    ABSTRACT: We report far-ultraviolet observations of Jupiter's moon Europa taken by Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) in January and February 2014 to test the hypothesis that the discovery of a water vapor aurora in December 2012 by local hydrogen (H) and oxygen (O) emissions with the STIS originated from plume activity possibly correlated with Europa's distance from Jupiter through tidal stress variations. The 2014 observations were scheduled with Europa near the apocenter similar to the orbital position of its previous detection. Tensile stresses on south polar fractures are expected to be highest in this orbital phase, potentially maximizing the probability for plume activity. No local H and O emissions were detected in the new STIS images. In the south polar region where the emission surpluses were observed in 2012, the brightnesses are sufficiently low in the 2014 images to be consistent with any H2O abundance from (0-5)×10(15) cm(-2). Large high-latitude plumes should have been detectable by the STIS, independent of the observing conditions and geometry. Because electron excitation of water vapor remains the only viable explanation for the 2012 detection, the new observations indicate that although the same orbital position of Europa for plume activity may be a necessary condition, it is not a sufficient condition. However, the December 2012 detection of coincident HI Lyman-α and OI 1304-Å emission surpluses in an ∼200-km high region well separated above Europa's limb is a firm result and not invalidated by our 2014 STIS observations.
    Full-text · Article · Nov 2014 · Proceedings of the National Academy of Sciences
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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.
    No preview · Article · Apr 2014 · Journal of Geophysical Research: Space Physics
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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.
    No preview · Article · Feb 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.
    No preview · Article · Jan 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.
    Full-text · Article · Jan 2014
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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.
    No preview · Article · Jan 2014 · Icarus
  • 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.
    No preview · Article · Jan 2014 · Icarus
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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.
    Full-text · Article · Dec 2013 · Science
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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.
    No preview · Article · Oct 2013
  • 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: We present four sets of ultraviolet images of Ganymede acquired with the Hubble Space Telescope (HST) from 1998 to 2007, all of which show auroral emission from electron excited atomic oxygen. The three different hemispheres of Ganymede captured in the observations show strikingly different emission morphologies. Ultraviolet emission at 1356 angstrom is brightest at relatively high latitude on the orbital trailing (upstream plasma) hemisphere and in an auroral oval that extends to as low as similar to 10 degrees N latitude on the orbital leading (downstream plasma) hemisphere. Two sets of images of the Jupiter-facing hemisphere acquired at nearly the same sub-Earth longitude but separated by similar to 4 years show very similar emission morphology that is consistent with the pattern of emission seen in the upstream and downstream images: the emission is at high latitude in the upstream quadrant and at low latitude in the downstream quadrant. This implies that the large-scale, nominal auroral oval on Ganymede is apparently quite stable with time, despite significant brightness fluctuations within the overall stable pattern during the 10-30min time scale between individual images. The overall emission morphology appears to be driven primarily by the strong Jovian magnetospheric plasma interaction with Ganymede and does not appear to be strongly influenced by the orientation of the background Jovian magnetic field. The observed auroral oval pattern is reasonably well matched by a magnetohydrodymanic (MHD) model optimized to fit the Galileo magnetic field measurements near Ganymede. The location of the auroral oval from these data provides a reasonable match to the location of the well-defined visible boundary of the Ganymede polar cap except in the northern, leading hemisphere.
    Preview · Article · May 2013 · Journal of Geophysical Research Atmospheres

Publication Stats

9k Citations
1,242.21 Total Impact Points


  • 1975-2015
    • 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
  • 1987
    • Pasadena City College
      Pasadena, Texas, United States
  • 1986
    • NASA
      Вашингтон, West Virginia, United States
  • 1979-1984
    • United States Naval Research Laboratory
      Washington, Washington, D.C., United States
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States
  • 1982
    • Sierra Tucson
      Tucson, Arizona, United States
    • York University
      Toronto, Ontario, Canada
  • 1980
    • University of Michigan
      Ann Arbor, Michigan, United States