D. F. Strobel

Johns Hopkins University, Baltimore, Maryland, United States

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Publications (323)1379.61 Total impact

  • No preview · Article · Apr 2016 · Journal of Geophysical Research: Space Physics
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    J. M. Moore · W. B. McKinnon · J. R. Spencer · A. D. Howard · P. M. Schenk · R. A. Beyer · F. Nimmo · K. N. Singer · O. M. Umurhan · O. L. White · [...] · M. Vincent · R. Webbert · S. Weidner · G. E. Weigle · K. Whittenburg · B. G. Williams · K. Williams · S. Williams · W. W. Woods · E. Zirnstein ·
    Full-text · Dataset · Mar 2016
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    F. Bagenal · M. Horanyi · D. J. McComas · R. L. McNutt · H. A. Elliott · M. E. Hill · L. E. Brown · P. A. Delamere · P. Kollmann · S. M. Krimigis · [...] · M. Vincent · R. Webbert · G. E. Weigle · O. L. White · K. Whittenburg · B. G. Williams · K. Williams · S. Williams · W. W. Woods · A. M. Zangari ·
    [Show abstract] [Hide abstract] ABSTRACT: The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto (SWAP) instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The region’s surprisingly small size is consistent with a reduced atmospheric escape rate, as well as a particularly high solar wind flux. Observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument suggest that ions are accelerated and/or deflected around Pluto. In the wake of the interaction region, PEPSSI observed suprathermal particle fluxes equal to about 1/10 of the flux in the interplanetary medium and increasing with distance downstream. The Venetia Burney Student Dust Counter, which measures grains with radii larger than 1.4 micrometers, detected one candidate impact in ±5 days around New Horizons’ closest approach, indicating an upper limit of <4.6 kilometers–3 for the dust density in the Pluto system.
    Full-text · Article · Mar 2016 · Science
  • G. R. Gladstone · S. A. Stern · K. Ennico · C. B. Olkin · H. A. Weaver · L. A. Young · M. E. Summers · D. F. Strobel · D. P. Hinson · J. A. Kammer · [...] · R. Webbert · S. Weidner · G. E. Weigle · O. L. White · K. Whittenburg · B. G. Williams · K. Williams · S. Williams · A. M. Zangari · E. Zirnstein ·
    [Show abstract] [Hide abstract] ABSTRACT: Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto’s atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto’s atmosphere to space. It is unclear whether the current state of Pluto’s atmosphere is representative of its average state—over seasonal or geologic time scales.
    No preview · Article · Mar 2016 · Science
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    J. M. Moore · W. B. McKinnon · J. R. Spencer · A. D. Howard · P. M. Schenk · R. A. Beyer · F. Nimmo · K. N. Singer · O. M. Umurhan · O. L. White · [...] · M. Vincent · R. Webbert · S. Weidner · G. E. Weigle · K. Whittenburg · B. G. Williams · K. Williams · S. Williams · W. W. Woods · E. Zirnstein ·
    [Show abstract] [Hide abstract] ABSTRACT: NASA’s New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Pluto’s encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than ~10 million years. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, the latter likely caused by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest impactor size-frequency distributions proposed for the Kuiper belt.
    Full-text · Article · Mar 2016 · Science
  • [Show abstract] [Hide abstract] ABSTRACT: We report far-ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived brightnesses at the oxygen lines at 1304 Å and 1356 Å are consistent with zero signals within the 1-σ propagated statistical uncertainties. The OI 1304 Å brightness of 0.12 ± 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 ± 13.4) × 1010 cm−2. Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 × 1026 molecules s−1 was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of ∼1% in the 1300 Å to 1400 Å range.
    No preview · Article · Mar 2016 · Geophysical Research Letters
  • [Show abstract] [Hide abstract] ABSTRACT: We analyze a large set of far-ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time-variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawn side with only few exceptions, which can not be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near-surface OI] 1356 Å to OI 1304 Å emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O2 prevails at least up to altitudes of ∼900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the surface-near O2 atmosphere of ∼5% (upstream) and ≲1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.
    No preview · Article · Feb 2016 · Journal of Geophysical Research: Space Physics
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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Nov 2015
  • [Show abstract] [Hide abstract] 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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    [Show abstract] [Hide abstract] 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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    [Show abstract] [Hide abstract] 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).
    Preview · Article · Sep 2015 · The Journal of Geophysical Research Planets
  • [Show abstract] [Hide abstract] 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
  • [Show abstract] [Hide abstract] 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
  • [Show abstract] [Hide abstract] 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
    [Show abstract] [Hide abstract] 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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    [Show abstract] [Hide abstract] 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
  • [Show abstract] [Hide abstract] 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
  • [Show abstract] [Hide abstract] 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
  • [Show abstract] [Hide abstract] 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

Publication Stats

9k Citations
1,379.61 Total Impact Points

Institutions

  • 1975-2014
    • Johns Hopkins University
      • • Department of Earth and Planetary Sciences
      • • Department of Physics and Astronomy
      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
  • 1984
    • United States Naval Research Laboratory
      Washington, Washington, D.C., United States
  • 1982
    • Sierra Tucson
      Tucson, Arizona, United States
    • York University
      Toronto, Ontario, Canada
  • 1980
    • University of Michigan
      Ann Arbor, Michigan, United States
  • 1979
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States