[Show abstract][Hide abstract] ABSTRACT: This investigation extends the work presented by Bell et al. (2010a,
2010b). Using the one-dimensional (1-D) configuration of the Titan
Global Ionosphere-Thermosphere Model (T-GITM), we quantify the relative
importance of the different dynamical and chemical mechanisms that
determine the CH4 escape rates calculated by T-GITM.
Moreover, we consider the implications of updated Huygens Gas
Chromatograph Mass Spectrometer (GCMS) determinations of both the
40Ar mixing ratios and 15N/14N isotopic
ratios in work by Niemann et al. (2010). Combining the GCMS constraints
in the lower atmosphere with the Ion Neutral Mass Spectrometer (INMS)
measurements in work by Magee et al. (2009), our simulation results
suggest that the optimal CH4 homopause altitude is located at
1000 km. Using this homopause altitude, we conclude that topside escape
rates of 1.0 × 1010 CH4 m-2
s-1 (referred to the surface) are sufficient to
reproduce the INMS methane measurements in work by Magee et al. (2009).
These escape rates of methane are consistent with the upper limits to
methane escape (1.11 × 1011 CH4
m-2 s-1) established by both the
Cassini Plasma Spectrometer (CAPS) and Magnetosphere Imaging Instrument
(MIMI) measurements of Carbon-group ions in the near Titan
Journal of Geophysical Research Atmospheres 11/2011; 116(E11-E11):E11002. DOI:10.1029/2010JE003639 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In Bell et al. (2010) (paper 1), we provide a series of benchmark simulations that validate a newly developed Titan Global Ionosphere-Thermosphere Model (T-GITM) and calibrate its estimates of topside escape rates with recent work by Cui et al. (2008), Strobel (2009), and Yelle et al. (2008). Presently, large uncertainties exist in our knowledge of the density and thermal structure of Titan's upper atmosphere between the altitudes of 500 km and 1000 km. In this manuscript, we explore a spectrum of possible model configurations of Titan's upper atmosphere that are consistent with observations made by the Cassini Ion-Neutral Mass Spectrometer (INMS), Composite Infrared Spectrometer, Cassini Plasma Spectrometer, Magnetospheric Imaging Instrument, and by the Huygens Gas Chromatograph Mass Spectrometer and Atmospheric Science Instrument. In particular, we explore the ramifications of multiplying the INMS densities of Magee et al. (2009) by a factor of 3.0, which significantly alters the overall density, thermal, and dynamical structures simulated by T-GITM between 500 km and 1500 km. Our results indicate that an entire range of topside CH4 escape fluxes can equivalently reproduce the INMS measurements, ranging from ∼108 − 1.86 × 1013 molecules m−2 s−1 (referred to the surface). The lowest topside methane escape rates are achieved by scaling the INMS densities by a factor of 3.0 and either (1) increasing the methane homopause altitude to ∼1000 km or (2) including a physicochemical loss referred to as aerosol trapping. Additionally, when scaling the INMS densities by a factor of 3.0, we find that only Jeans escape velocities are required to reproduce the H2 measurements of INMS.
Journal of Geophysical Research Atmospheres 12/2010; 115(E12-E12):E12018. DOI:10.1029/2010JE003638 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We employ a newly developed Navier-Stokes model, the Titan Global Ionosphere-Thermosphere Model (T-GITM) to address the one dimensional (1-D) coupled composition, dynamics, and energetics of Titan's upper atmosphere. Our main goals are to delineate the details of this new theoretical tool and to present benchmark calibration simulations compared against the Ion-Neutral Mass Spectrometer (INMS) neutral density measurements. First, we outline the key physical routines contained in T-GITM and their computational formulation. Then, we compare a series of model simulations against recent 1-D work by Cui et al. (2008), Strobel (2008, 2009), and Yelle et al. (2008) in order to provide a fiducial for calibrating this new model. In paper 2 and a future paper, we explore the uncertainties in our knowledge of Titan's atmosphere between ∼500 km and 1000 km in order to determine how the present measurements constrain our theoretical understanding of atmospheric structures and processes.
[Show abstract][Hide abstract] ABSTRACT: Rhea is Saturn’s second-largest moon, and orbits at 8.7 Saturn
radii from the planet. The moon is continuously bombarded by
magnetospheric plasma: the absorption of thermal plasma that overtakes
Rhea in its orbit results in the formation of an upstream plasma wake.
High energy electron dropouts - microsignatures, caused by the
absorption of more energetic particles by the moon, are also observed.
The unusually broad electron microsignatures observed near the moon are
suggested to be evidence for the existence of a debris disk orbiting the
moon (Jones et al. 2008). We present our current state of knowledge of
the Rhea-magnetosphere interaction, based on data obtained by the
Cassini CAPS and MIMI instruments during the spacecraft’s two
closest encounters with the moon to date, on November 26, 2005, and
August 30, 2007. We report on the detection of pickup ions at the moon
by the CAPS instrument. This detection agrees with the results of
Martens et al. (2008), who previously reported an enhancement in
molecular oxygen ion distributions at the L shell of Rhea. We also
summarize expectations for the upcoming close encounter on March 2,
[Show abstract][Hide abstract] ABSTRACT: Rhea is the second largest Saturnian satellite, with a radius of 765 km.
It orbits Saturn at a radial distance of about 8.74 Saturn radii, within
the magnetosphere's corotating thermal plasma. Rhea's orbital speed is
less than the corotation speed, and so the thermal plasma forms a wake
in the direction of Rhea's orbital motion. During November 26, 2005,
Cassini passed within 500km of Rhea and across this wake, observing the
thermal plasma with the Cassini Plasma Spectrometer (CAPS). The bulk
flow of this plasma is deflected around the moon and this change in
velocity is investigated here utilizing the Ion Mass Spectrometer, IMS,
of CAPS. Preliminary studies suggested the ion flow velocity deflection
is small and that ion densities for water group ions and H+ are
approximately 4.0 and 0.4 cm-3, with temperatures approximately 150 and
30 eV respectively. Improved ion moments are presented by employing
electron densities found from the upper hybrid emission as measured by
the Radio and Plasma Wave Science (RPWS) instrument to constrain the
total ion density within the calculations.