D. F. Strobel

Johns Hopkins University, Baltimore, Maryland, United States

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Publications (291)1146.73 Total impact

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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.
    Icarus 07/2015; 254. DOI:10.1016/j.icarus.2015.03.021 · 2.84 Impact Factor
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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.
    Journal of Geophysical Research: Space Physics 02/2015; DOI:10.1002/2014JA020778 · 3.44 Impact Factor
  • 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.
    Icarus 12/2014; 246. DOI:10.1016/j.icarus.2014.04.011 · 2.84 Impact Factor
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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.
    Proceedings of the National Academy of Sciences 11/2014; DOI:10.1073/pnas.1416671111 · 9.81 Impact Factor
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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 04/2014; 119(4). DOI:10.1002/2013JA019440 · 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.
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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.
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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.
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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. DOI:10.1016/j.icarus.2013.10.009 · 2.84 Impact Factor
  • 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. DOI:10.1016/j.icarus.2013.10.011 · 2.84 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; 343(6167). DOI:10.1126/science.1247051 · 31.48 Impact Factor
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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.
  • 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.
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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.
    Journal of Geophysical Research Atmospheres 05/2013; 118(5):07-. DOI:10.1002/jgra.50122 · 3.44 Impact Factor
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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. DOI:10.1016/j.icarus.2012.12.010 · 2.84 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 Atmospheres 11/2012; 117(E11):E11006. DOI:10.1029/2012JE004222 · 3.44 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.
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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-. DOI:10.1029/2012GL053230 · 4.46 Impact Factor
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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.
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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. · 0.93 Impact Factor

Publication Stats

7k Citations
1,146.73 Total Impact Points


  • 1986–2015
    • Johns Hopkins University
      • • Department of Earth and Planetary Sciences
      • • Applied Physics Laboratory
      • • Department of Physics and Astronomy
      Baltimore, Maryland, United States
    • NASA
      Вашингтон, West Virginia, United States
  • 2011
    • University of Cologne
      • Institute of Geophysics and Meteorology
      Köln, North Rhine-Westphalia, Germany
  • 2003
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 1987
    • Pasadena City College
      Pasadena, Texas, United States
  • 1980
    • University of Michigan
      Ann Arbor, Michigan, United States
  • 1979
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States