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Molecular Nitrogen and Methane Density Retrievals from Cassini UVIS
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Dayglow Observations of Titan’s Upper Atmosphere
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Michael H. Stevensa,*, J. Scott Evansb, Jerry Lumpec, Joseph H. Westlaked, Joseph M. Ajelloe,
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E. Todd Bradleyf, Larry W. Espositog
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a
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DC 20375
Space Science Division, Naval Research Laboratory, 4555 Overlook Ave., SW, Washington,
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b
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c
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d
Applied Physics Laboratory, Laurel, MD 20723
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e
Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 169-223, Pasadena, CA 91109
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f
Department of Physics, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL
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32816
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g
Computational Physics, Inc., 8001 Braddock Road, Suite 210, Springfield, VA 22151
Computational Physics, Inc., 1650 38th Street, Boulder, CO 80301
Laboratory for Atmospheric and Space Physics, 3665 Discovery Drive, Boulder, CO 80303
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*Corresponding author.
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Submitted to Icarus
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7 April 2014
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Abstract. We retrieve number densities of molecular nitrogen (N2) and methane (CH4) from
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Titan’s upper atmosphere using the UV dayglow. We use Cassini Ultraviolet Imaging
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Spectrograph (UVIS) limb observations from 800-1300 km of the N I 1493 Å and N II 1085 Å
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multiplets, both produced directly from photofragmentation of N2. UVIS N2 and CH4 densities
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are in agreement with measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS)
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from the same flyby if INMS densities are scaled up by a factor of 3.0 as reported in previous
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studies. We find that 1) the CH4 homopause on Titan is from 900-1050 km, 2) upper atmospheric
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temperatures vary by less than 10 K over 6 h at the same geographic location and 3)
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temperatures drop by 10-15 K from late morning to mid-afternoon. The capability of retrieving
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the global-scale composition from these data complements existing techniques and significantly
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advances the study of upper atmospheric variability at Titan and for any other atmosphere with a
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detectable UV dayglow.
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1. Introduction
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Since Cassini arrived at the Saturnian system, Titan’s upper atmospheric composition and
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temperature have shown remarkable variability from one flyby to the next [e.g. de La Haye et al.,
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2007; Cui et al., 2012]. Results of ~30 flybys from 2005 to 2010 revealed that upper
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atmospheric temperatures can vary between about 100-200 K and molecular nitrogen (N2)
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densities can vary by nearly an order of magnitude at 1100 km [e.g. Westlake et al., 2011;
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Snowden et al., 2013]. Identifying the mechanism for variations in Titan’s upper atmosphere is
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important in characterizing its evolution [e.g. Bell et al., 2010; Cui et al., 2012] and has therefore
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been a topic of considerable interest. Some suggest that the observed variability is driven by
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wave activity from below [Müller-Wodarg et al., 2006; Koskinen et al., 2011; Cui et al., 2013;
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Cui et al., 2014; Snowden and Yelle, 2014] while others argue that it is driven by precipitating
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particles from Saturn’s magnetosphere [Westlake et al., 2011; Bell et al., 2011a].
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Most of the analyses of Titan’s upper atmospheric density and temperature to date are based
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on observations by Cassini’s Ion Neutral Mass Spectrometer (INMS), which measures
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composition in situ from about 950 km to Titan’s exobase at about 1500 km and above. INMS
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samples Titan’s atmosphere near one geographic region for each flyby so that global scale
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variations around Titan are inferred over many flybys. The INMS analyses typically assemble
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several years of observations under a variety of geophysical conditions so that uncoupling spatial
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and temporal variations presents a challenge. New remote methods to synoptically retrieve upper
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atmospheric composition around Titan are needed to complement existing observations, help
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quantify the compositional variations and address whether the prevailing source is internal or
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external to Titan’s atmosphere.
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There have been several studies that focus on retrieving composition from stellar and solar
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occultations of Titan by the Ultraviolet Spectrometer (UVS) on Voyager [Broadfoot et al., 1981;
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Vervack et al., 2004] and by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini
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[Shemansky et al., 1995; Koskinen et al., 2011; Capalbo et al., 2013; Kammer et al., 2013].
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These studies reveal a wealth of information about Titan’s upper atmosphere but are also
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geographically and temporally limited making them difficult to use for interpreting global scale
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variability.
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Our understanding of Titan’s EUV and FUV dayglow has advanced dramatically since the
first observations by the Voyager 1 UVS in 1980 [Broadfoot et al., 1981]. Although the primary
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production was identified as N2 photoelectron excitation and photofragmentation decades ago
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[e.g. Strobel et al., 1992] the complex spectral content remained uncertain and disputed [Stevens,
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2001], so that the amount of solar production remained ambiguous. Cassini UVIS disk
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observations have since shown that that the Titan dayglow is produced primarily by solar
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processes and that its production from precipitating particles is either undetectable or weak
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(~10%) [Ajello et al., 2007; West et al., 2012; Ajello et al., 2012]. Using UVIS limb
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observations Stevens et al. [2011] found that the brightest features in the EUV and FUV are
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within about 10% of model radiance predictions using only solar processes to produce the
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dayglow. Good agreement of UVIS Titan dayglow data with solar driven model results was also
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found by Lavvas et al. [2011].
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Motivated by these dramatic improvements to our understanding of Titan’s EUV and FUV
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dayglow and the high quality of the UVIS observations, we herein present a new method for
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retrieving composition and temperature from Titan’s upper atmosphere using dayglow
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observations. The direct retrieval of constituent density profiles retrieved using the dayglow has
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never been attempted before from any planetary atmosphere to our knowledge, but represents a
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significant advance since it allows for global-scale compositional analysis over time scales of
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hours.
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We present details of the retrieval in Section 2. In Section 3 we discuss the observations and
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the spectral analysis of the relevant dayglow features. In Section 4 we present results for N2 and
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methane (CH4) densities as well as temperature and compare them directly to INMS observations
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from the same flyby. We also present retrievals from one flyby during which UVIS stares at a
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point on the dayside disk for several hours and another flyby during which UVIS scans over the
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dayside at a fixed latitude to isolate diurnal variations. In Section 5 we discuss the results and in
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Section 6 we summarize the most significant conclusions of this study.
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2. Retrievals of Composition and Temperature from Titan Dayglow
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2.1 Generalized Retrieval and ANalysis Tool: GRANT
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GRANT is a generalized retrieval algorithm and analysis tool that infers composition from
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terrestrial and extraterrestrial dayglow observations. This tool merges two different software
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packages, the Atmospheric Ultraviolet Radiance Integrated Code (AURIC) [Strickland et al.,
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1999] and the OPTimal estimation (hereafter OPT) retrieval algorithms [Lumpe et al., 1997,
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2002, 2007]. The GRANT framework uses AURIC as a forward model driver for the optimal
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estimation routines in OPT, deriving an optimal atmospheric state solution by minimizing the
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difference between forward model calculations and measurements. For the Titan UVIS
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observations used herein, GRANT finds an optimal fit to the N2 and CH4 number density profiles
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given observed limb radiances of selected emission features.
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AURIC was originally designed for the interpretation of terrestrial dayglow emissions [e.g.
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Bishop and Feldman, 2003]. The model calculates volume excitation rates of nearly all known
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terrestrial UV emissions. N2 UV dayglow production is typically driven by processes directly
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related to energetic solar irradiance: photoelectron excitation, photodissociative excitation (PDE)
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and photodissociative ionization (PDI) [Meier et al., 1991; Samson et al., 1991; Bishop and
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Feldman, 2003]. PDE and PDI are sometimes together called photofragmentation. AURIC has
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been used to study the N2 EUV dayglow from the Earth [Siskind et al., 1995; Strickland et al.,
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1997; Majeed and Strickland, 1997; Bishop and Feldman, 2003; Bishop et al., 2007]. Results
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from earlier terrestrial versions of the model were used to study the N2 EUV dayglow of Titan
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[Strobel et al., 1992; Stevens, 2001] and Neptune’s moon Triton [Strobel et al., 1991; Stevens,
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2002], which were observed by the UVS on Voyagers 1 and 2.
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The OPT retrieval algorithms provide a generic and flexible implementation of the optimal
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estimation constrained linear inversion technique. This technique is quite general and has been
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widely applied to retrieval problems in atmospheric and geophysical remote sensing [Rodgers,
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2000]. To apply the algorithms to a particular retrieval problem an appropriate forward function
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must be defined, as well as the necessary interface subroutines to define the data and retrieval
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space vectors. The forward model function defines the mathematical and physical mapping
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between the model and data space, and therefore completely defines the particular problem to be
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solved.
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Deposition and attenuation of the solar EUV and soft X-ray flux are critical elements for
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calculating the associated photo-excitation sources. Care must be taken in the implementation
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and selection of the solar irradiance itself. Older modeling studies of the Titan dayglow [e.g.
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Strobel et al., 1991] used the spectrum of Hinteregger [1976; 1981], based on the principal solar
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observations available at the time. More recent solar work [e.g. Richards et al., 1994; Warren et
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al., 1998; Lean et al., 2003] point out the limitations and uncertainties of the Hinteregger solar
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flux model, particularly for the X-ray ultraviolet [XUV, 1-300 Å, Woods et al., 2005]
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wavelengths most responsible for EUV and FUV dayglow emissions. The Hinteregger XUV flux
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is now considered to be a factor of two or more lower than modern values for wavelengths below
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200 Å. We use the current solar spectral irradiance from the Solar Extreme ultraviolet
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Experiment (SEE) on the Thermosphere, Ionosphere, Mesosphere, Energetic and Dynamics
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(TIMED) satellite as reported by Lean and Woods [2010], which has been shown to agree with
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multiple satellite datasets to within 20% [Lean et al., 2011]. This irradiance is binned at 0.5 Å
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resolution for use in our simulations and scaled as the square of the distance to Titan (9.57 A.U.).
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The brightest systems in Titan’s UV dayglow produced from processes involving N2 are the
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N2 Lyman-Birge-Hopfield bands and N2 Vegard-Kaplan bands as well as a variety of N I and N
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II multiplets excited primarily through photofragmentation of N2. Because photofragmentation is
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a relatively simple process that facilitates our analysis, we choose two relatively bright and
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isolated photofragmented features for our N2 retrievals: The N I 1493 Å and N II 1085 Å
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multiplets. Although these are atomic features, they are produced directly from processes on N2
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(photofragmentation and a small amount of photoelectron excitation) and are therefore good
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diagnostics for the N2 density in Titan’s upper atmosphere. While we retain direct photoelectron
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excitation of atomic nitrogen in the forward model calculations, the contribution from this source
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is negligible on Titan. Consequently, the 1493 Å and 1085 Å features do not provide any
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information on atomic nitrogen concentration.
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Previous terrestrial work using AURIC showed that modeled N I 1493 Å and N II 1085 Å
features are in agreement with observations to within 10% [e.g. Bishop and Feldman, 2003].
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Analyses of Titan dayglow data using forward models have also fit these emissions to within
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10% [Lavvas et al., 2011; Stevens et al., 2011], underscoring the reliability of using them for N2
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density retrievals. The selection of these two features on Titan is also driven by their opacity to
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CH4: N I 1493 Å is optically thin whereas N II 1085 Å is significantly attenuated by CH4 near
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the altitude of peak production [e.g. Strobel et al., 1992]. These two multiplets lie on either side
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of a CH4 absorption ledge and therefore can be used to constrain the CH4 density along the line-
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of-sight (LOS).
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The algorithms used in this analysis simultaneously retrieve the N2 and CH4 density profiles
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by fitting limb scan data. The retrieval uses a total of 28 retrieval parameters for each limb scan:
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constituent densities on a common altitude grid (12 grid points at 100-km vertical binning), cross
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section scale factors (two for 1493 Å and one for 1085 Å) and a solar EUV scale factor.
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Occasionally Cassini is close enough to Titan so that the UVIS data can be binned in 50 km
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vertical intervals, thereby doubling the number of grid points for the constituent densities. The
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retrieval altitude grid is primarily dictated by the vertical distance subtended by a UVIS pixel.
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The retrieval altitude grid extends from 400 to 1500 km with equally spaced increments but
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UVIS is only sensitive to N2 and CH4 densities from 800-1300 km.
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Cross section scale factors are used to compensate for systematic biases between observed
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brightnesses and values calculated by AURIC (e.g., due to errors in cross sections and instrument
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calibration). The solar EUV scale factor allows for adjustments in solar EUV input into the
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atmosphere relative to an a priori TIMED/SEE spectrum (scaled to Titan’s distance from the
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Earth) from the day of the flyby. The solar EUV scale factor is applied uniformly for solar fluxes
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at wavelengths below 45 nm and accounts for uncertainties in the absolute measurement as well
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as short term variations in the EUV flux. It is worth mentioning that scale factors for cross
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sections and solar EUV produce different effects in forward model brightnesses, and therefore
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can be retrieved without concern for cross coupling. The cross section scale factor uniformly
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shifts the magnitude of the entire limb profile, whereas the solar EUV scale factor affects the
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shape of the limb profile (i.e. the effect of scaling the solar EUV is greatest near the peak of
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emission).
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Accurate retrieval of species concentrations requires forward model radiances to be
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consistent with observations, meaning the forward model must include any instrument effects
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that are not accounted for by instrument calibration. This can be achieved with a suitable
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instrument model that is coupled with the forward model within the retrieval loop. In the spectral
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dimension, we do not apply any smoothing, but extract atomic emission brightnesses directly
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from model spectra corresponding to each tangent altitude. To ensure consistency with observed
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limb profiles, we sum over all wavelengths associated with a given atomic feature to capture the
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full intensity. For spatial smoothing we use a box-car average with a width of 9 km, which yields
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an effective vertical resolution of ~90 km. Our one-dimensional model calculations assume
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isotropy along the LOS.
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The forward model problem is stated in general form as
, where y represents the
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measurement vector of limb radiance profiles, x is the atmospheric state vector to be retrieved,
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and the full forward model
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model. This nonlinear equation is solved using the optimal estimation technique [Rodgers,
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2000], with the solution vector
is defined by the combination of AURIC and the UVIS instrument
obtained by iterating from the a priori vector
according to
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(1)
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In Equation (1)
,
and
are the data and a priori covariance matrices,
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respectively, and the kernel matrix
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state parameters:
is the derivative of the forward model with respect to the
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(2)
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The covariance of the solution is a weighted sum of errors from the a priori contribution and
direct inversion of the data:
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(3)
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The 1-σ retrieval errors reported here are the diagonal elements of the retrieval covariance
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matrix: σ i ~
Ŝii . Since this approach is strictly formal error propagation, it may underestimate
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the true errors, which must be derived through a detailed error analysis using simulations to
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quantify individual error components.
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The a priori matrix
provides the smoothing constraints in the retrieval. In practice it is
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adjusted to optimize the tradeoff between noise suppression and information content from the
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data. In our case the a priori covariance for each number density profile varies with altitude in
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the following manner:
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(4)
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where i and j are altitude indices, zi is the ith altitude, and H is a length scale that determines the
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degree of vertical coupling along the rows of the covariance matrix. We fix H at 80 km for N2
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and 139 km for CH4. The expression for the diagonal elements in Equation (4) weights the data
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with altitude in Titan’s atmosphere to facilitate convergence, where the weighting is determined
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from simulations of the airglow data between 800 and 1300 km. We use a priori values of 1.0
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with a variance of 0.8 for the cross section and solar EUV scale factors. N2 and CH4 a priori
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density profiles are the same profiles used in Stevens et al. [2011]. We emphasize that there is no
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correlation assumed between the scale factors or between the N2 and CH4 densities.
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2.2 Temperature Retrievals
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We retrieve the upper atmospheric temperature at Titan by inferring a scale height to the
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retrieved N2 density profile from 800-1300 km where the dayglow signal peaks. Following the
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approach of Westlake et al. [2011], we assume that the temperature is isothermal and fit a scale
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height to the density profile after replacing the geometric altitude with geopotential altitude. In
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this way we account for the change in gravitational acceleration over the altitudes of interest. The
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assumption of an isothermal atmosphere may not be valid for Titan’s upper atmosphere as
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discussed by Snowden et al. [2013] and Kammer et al. [2013] but can be used as an indication of
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large scale variability which is the motivation herein. The uncertainty to each temperature fit is
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calculated using simulations of the retrieved N2 density profile, where each grid point for the
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entire profile is randomly varied within the density uncertainty and a temperature is fit at each
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iteration. We take the average temperature from 150 iterations and the derived temperature
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uncertainty is the standard deviation of all of the solutions.
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3. The UVIS Dayglow Observations
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UVIS is an imaging spectrograph that consists of an EUV channel (561–1182 Å) and a FUV
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channel (1115–1913 Å). The detector has 1024 pixels in the spectral dimension and 64 pixels in
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the spatial dimension and each spatial pixel typically contains a UV spectrum from each channel.
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The low-resolution slit was used for both channels in our obserations, yielding a spectral
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resolution in the EUV and FUV of 4–6 Å. The rectangular field of view of the slit is 2 × 59 mrad
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in the EUV and 1.5 × 60 mrad in the FUV. Further details of the UVIS instrument can be found
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in Esposito et al. [2004].
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We require limb observations for which the tangent point is illuminated by the sun because
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the dayglow observed by UVIS is at least an order of magnitude brighter than the nighttime
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airglow [Ajello et al., 2007; Ajello et al., 2012; West et al., 2012] and because the XUV solar
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flux driving the UV dayglow is well understood as discussed in Section 2.1. The features
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considered herein are excited from 800-1300 km altitude [Stevens et al., 2011], so that limb
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observations must include these altitudes. The emission is strongly dependent on the solar zenith
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angle, which is explicitly included when inverting each radiance profile. We use the average
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solar zenith angle at 1000 km for our retrievals, which can vary by up to ~5° for other altitudes
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depending on the geometry of the observation. It is furthermore desirable for Cassini to be as
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close as possible to Titan to maximize the vertical resolution of the density profiles. This
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typically means using data for which Cassini was less than ~100,000 km from Titan. This
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distance generally translates to a vertical resolution of 50-200 km for each pixel projected to the
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tangent point, depending on the distance from Titan, the channel used and the orientation of the
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slit. Averaging spectra over a wide range of tangent altitudes increases the signal-to-noise ratio at
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the expense of vertical resolution. For the results presented herein, we find that co-averaging the
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spectra into altitude bins of 50 km or 100 km provides sufficiently high signal-to-noise ratio for
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our retrievals while still retaining adequate vertical resolution.
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For this work we consider three different Cassini flybys of Titan:
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1) T55 on 21 May 2009. This flyby also comes with INMS N2, CH4 observations against
which we can directly compare our UVIS retrievals.
2) T81 on 30 Jan 2012. We select a portion of the data from this flyby during which UVIS
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obtained five different profiles over six hours at one geographic region to quantify short-
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term variability.
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3) T94 on 12 Sep 2013. This flyby contains a sequence during which UVIS scans the
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dayside disk near the equator to quantify any longitudinal or local solar time (LST)
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dependence.
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In Sections 3.1 and 3.2 we describe the spectral fitting that we use to extract N I 1493 Å and N II
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1085 Å radiances and show forward model results for the extracted radiances. Then in Section 4
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we present compositional retrievals from the three flybys listed above.
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3.1 Spectral Fitting: Dayglow Limb Observations from T55
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The UVIS observation sequences from each Cassini flyby of Titan is different so it is
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instructive to see how the projected slit is oriented with respect to Titan for the flybys we
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consider herein. The projection of the FUV slit during the inbound leg of the T55 flyby is shown
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in Figure 1, where the sub-spacecraft point is in the center of the image. For this sequence the slit
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was oriented tangentially to Titan’s limb and each of the five projections in Figure 1 represents
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the FUV field-of-view (FOV) for one record of observations. There were 33 records obtained but
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to reduce clutter in the Figure we only show the FOV for every eighth record. Each pixel
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represents one FUV spectrum and for this image the total radiance observed between 1485-1500
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Å is shown, which is dominated by the N I 1493 Å multiplet. For reference we indicate the
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tangent altitudes of 900 and 1200 km, where the UV emissions are primarily produced. The
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image serves to indicate the variability of the emission from one pixel to the next and that they
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generally peak near 900 km.
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Figure 1. UVIS viewing geometry for the T55 dayglow observations of Titan. Radiances
imaged are totals from 1485-1500 Å, which is dominated by the N I 1493 Å multiplet. The data
are unsmoothed and the radiance for each pixel as it is projected onto Titan’s limb is shown. The
five records (out of 33) represent the projected UVIS field of view on the outbound leg of the
flyby in equal time increments over the period indicated. The 900 km and 1200 km tangent
altitudes are indicated for reference and only the daytime data are included in the density
retrievals (SZA<80°).
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The shaded area of the disk in Figure 1 indicates the night side of Titan and in our analysis
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we only use pixels for which the solar zenith angle at the tangent point is less than 80º to avoid
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rapidly changing lighting conditions near the terminator, which complicate the one-dimensional
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isotropic analysis employed here. On this inbound leg of the flyby Cassini was on average
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~75,000 km from Titan so that when the slit is oriented tangentially as in Figure 1 each FUV
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pixel subtends about 112 km vertically. We co-average all the dayside spectra into 11 different
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50 km altitude bins from 800-1300 km, where the center of a bin is at each 50 km increment (800
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km, 850 km, 900 km, etc.). After the N I 1493 Å emission is determined within each altitude bin
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as described below, the T55 radiances are interpolated onto a regularly spaced 50 km profile.
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The relevant conditions at 1000 km are given in Table 1.
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Table 1. T55 Observations: 21 May 2009
a
Instrument
UT
Lat (ºN)
Lon (ºW)
SZA (º)
LST (h)
UVISa
16:37 - 18:47
35
260
69
16.2
INMS
22:42
-22
178
141
21.9
Conditions for 1000 km tangent altitude.
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The N I 1493 Å feature is slightly blended with the N2 Lyman-Birge-Hopfield (LBH) band
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system as shown in Figure 2a for a 900 km tangent altitude. We first synthesize the N I 1493
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multiplet and the N2 LBH bands at high spectral resolution and smooth them with the UVIS
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instrument line shape [Stevens et al., 2011]. We limit our FUV spectral analysis to wavelengths
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from 1480-1520 Å and fit to the spectra with the two features using a multiple linear regression
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algorithm. This expanded spectral region allows for a more reliable fit to the relatively weak N2
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LBH bands. We remove N2 LBH and the resultant N I 1493 Å spectrum is shown in Figure 2b
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with the retrieved radiance indicated. The radiance uncertainty is calculated by taking the
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standard deviation of the residual (data – composite fit) in Figure 2a and multiplying it by the
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square root of the number of spectral pixels in Figure 2b. Outliers that are in excess of 4σ
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occasionally appear in the co-averaged spectra and these pixels are given the value on the red
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curve in Figure 2a. The fit is also optimized in wavelength as there is occasionally a small drift
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of up to two pixels, which we correct by comparing against the composite fit in Figure 2a and
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shifting the data in one pixel increments until the residual is minimized.
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Figure 2. FUV N I 1493 Å multiplet at 900±25 km for flyby T55. Top panel: The multiplet is
blended with N2 LBH bands (red dashed curve), which are both fit to the data (black histogram)
in the spectral region shown. Bottom panel: The LBH contribution is removed from the data and
the residual (black histogram) is shown with the N I 1493 Å component of the fit (red curve).
The N I 1493 Å radiance and uncertainty are indicated in the panel.
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Similarly, the N II 1085 Å feature is slightly blended with the N2 BH I (Birge-Hopfield I)
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band system as shown in Figure 3a. We limit our EUV analysis to wavelengths from 1050-1092
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Å, do a regression fit to the spectra and remove N2 BH I. The wavelength region extends to the
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shorter wavelengths to more reliably infer the weak N2 BH I bands. The resultant N II 1085 Å
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spectrum is shown in Figure 3b with the retrieved radiance and uncertainty indicated. The
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spectral analyses in Figures 2 and 3 are repeated for each altitude increment and the radiance
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profiles are shown in Figure 4.
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Figure 3. N II 1085 Å multiplet at 900±25 km for flyby T55. Top panel: The N2 BH I(1,v″)
bands (red dashed curve) and the N II 1085 Å multiplet are both fit to the data (black histogram)
in the spectral region shown. Bottom panel: The BH I contribution is removed from the data and
the residual (black histogram) is shown along with the fit (red curve). The N II 1085 Å radiance
and uncertainty are indicated in the panel.
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Figure 4 Observed N I 1493 Å (solid red) and N II 1085 Å (solid black) radiance profiles from
T55. Retrieved radiance profiles are overplotted as dashed curves. Initial a priori guesses to the
profiles are also included as dotted lines. Although the two features are produced from the same
processes on N2, the N II 1085 Å profile shape is modified by CH4 absorption.
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3.2 GRANT Retrievals: Limb Radiance Profiles
The GRANT retrievals are compared against observed NI 1493 Å and NII 1085 Å limb
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radiance profiles in Figure 4. Calculated limb profiles use N2 and CH4 densities that are modified
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from an a priori guess until a best fit solution is found, as discussed in Section 2.1. The
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calculated a priori limb radiance profiles are included in Figure 4 and are from the N2 and CH4
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density profiles used in Stevens et al. [2011]. The agreement between retrieved limb radiance
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profiles and the data is excellent at nearly all altitudes for both features. Note that the retrieved
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radiance profiles are significantly different from the a priori profiles indicating that the data are
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driving the fit rather than the a priori guess.
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We can further test our understanding by calculating the radiances along the UVIS LOS for
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other prominent features in Titan’s UV dayglow and comparing against observations. For this
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comparison we choose the N2 Vegard-Kaplan (VK) bands (1540-1900 Å), the N I 1200 Å
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multiplet (1190-1205 Å) and the N2 LBH bands (1540-1900 Å). For the N2 VK bands we remove
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the blended photofragmented N I multiplets as well as the N2 LBH bands. For the N I 1200 Å
15
multiplet we remove the neighboring wing of the bright H I Lyman-α feature. For the N2 LBH
16
bands we remove the N I multiplets and the VK bands.
17
Comparisons of the observed N2 VK, N I 1200 A and N2 LBH radiance profiles with our
18
model results are shown in Figure 5. The blending of the N I 1200 Å with the bright H Lyman-α
19
feature at 1216 Å is significant but the spectral resolution of UVIS is high enough to separate the
20
two emission features. The Lyman-α line shape is slightly broader than that of N I 1200 Å and
21
Stevens et al. [2011] discuss the spectral de-convolution of these two features in greater detail.
22
Near the peak of the N I 1200 Å emission profile between 875-1025 km the contribution from
23
the wing of the Lyman-α feature is ~10 R between 1190-1205 Å, which has been removed from
19
1
the N I 1200 Å radiance profile shown in Figure 5. The uncertainty for this is relatively small for
2
N I 1200 Å because the broad wing is discernible from the discrete multiplet.
3
4
5
6
7
8
9
Figure 5. T55 radiance profiles of three bright FUV emissions on Titan: N2 VK, N I 1200 Å and
N2 LBH (solid curves). Using the retrieval of N2 and CH4 constrained by the radiance profiles in
Figure 4, calculated radiances for these three emissions are shown (dashed curves). The
agreement of the N2 VK and LBH bands is good, where the LBH calculation uses two different
sets of emission cross sections (see text).
We emphasize the calculated radiances in Figure 5 are determined entirely from the best fit
10
solution to the N I 1493 Å and N II 1085 Å radiance profiles and so the model fits are
11
unconstrained by the data shown in those panels. The calculated VK radiances include the effects
12
of cascade [Strickland et al., 1999] but not quenching by atomic oxygen, which is not significant
13
in Titan’s upper atmosphere. The modeled N2 VK radiances are in excellent agreement with the
14
observed VK radiances at nearly all altitudes.
15
The N I 1200 Å multiplet is systematically over-predicted near peak production when
16
compared against the data. This underscores a longstanding problem in the terrestrial dayglow
17
whereby the N I 1200 Å multiplet appears to show optical thickness effects and is systematically
20
1
over-predicted, even though the N* fragments are expected to be kinetically energetic and
2
therefore optically thin. Meier [1991] discusses this problem in detail and we refer the interested
3
reader to that work for additional details. This known over-prediction is the primary reason that
4
the N I 1200 Å multiplet was not included as part of the compositional retrievals in this work.
5
The N2 LBH bands are in very good agreement with the observations. We have used the
6
LBH emission cross sections of Young et al. [2010] following the approach of Stevens et al.
7
[2011]. However, we also include a solution using the older cross-sections of Ajello and
8
Shemansky [1985] for comparison and shaded the region between the two. In this case our data
9
is not of high enough quality to distinguish between the two solutions.
10
4. GRANT Retrievals: Composition
11
4.1 T55: Comparison with INMS
12
Retrieved N2 and CH4 number densities corresponding to the T55 radiance profiles in
13
Figures 4 are shown in Figure 6. The a priori density profiles are also shown for reference. Over-
14
plotted with these densities are ingress and egress profiles from INMS from the same T55 flyby,
15
where the INMS densities are scaled upward by a factor of 3.0 due to a calibration correction
16
[Bell et al. 2010; Westlake et al., 2011; Bell et al., 2011b; Snowden et al., 2013]. The agreement
17
between the UVIS remote sensing result and the INMS in situ results is good from 1000-1150
18
km and serves to show that the retrieval of composition from the UV dayglow is reliable. From
19
1150-1300 km the UVIS densities are systematically higher than the INMS densities, which may
20
be due to the geophysical differences between the two sets of observations summarized in Table
21
1 such as latitude and/or LST. Figure 6 also shows the resultant CH4 mixing ratio profile extends
22
to low enough altitudes to vertically resolve the CH4 homopause, above which molecular
23
diffusion controls its vertical distribution and the atmosphere is no longer well-mixed. From the
21
1
change in slope of the CH4 mixing ratio profile in Figure 6 we infer the CH4 homopause to be at
2
1000±25 km for T55, consistent with modeling studies using INMS data to constrain the CH4
3
profile above 1000 km [e.g. Bell et al., 2011b].
4
5
6
7
8
9
10
11
12
Figure 6. Compositional retrievals from the UVIS dayglow. Left panel: N2 and CH4 number
densities retrieved from the UVIS radiance profiles in Figure 4 (solid black). Over-plotted are the
INMS number densities from the same flyby (red symbols), where the uncertainties are smaller
than the symbol size. Initial a priori guesses of the N2 and CH4 densities for the UVIS retrievals
are over-plotted as the dotted lines. Temperatures inferred from the N2 data of UVIS and INMS
are indicated. Right panel: Corresponding UVIS and INMS CH4 mixing ratios, indicating that
the CH4 homopause is near 1000±25 km.
We retrieve temperature by deriving the scale height from the N2 density profile as
13
discussed in Section 2.2. The UVIS temperature result of 182±12 K is indicated in Figure 6 as
14
well as the INMS result of 143 K, which is derived from the inbound and outbound leg using
15
observations between 1000 and 1300 km. The INMS temperature is close to that inferred for T55
22
1
by Westlake et al. (147.2±6.4 K). The UVIS temperature is higher than that inferred by INMS,
2
as one may expect from the larger scale height of the UVIS data in Figure 6. If the UVIS
3
retrieval is confined to altitudes from 1000-1300 km, the UVIS temperatures are slightly lower at
4
173±10 K, but still higher than the INMS result. We will discuss the temperature differences in
5
the context of the observational conditions for each instrument further in Section 5.
6
4.2 T81: Targeting Temporal Variability
7
Given the good agreement of the UVIS dayglow retrievals with the INMS measurements
8
of N2 and CH4 composition in Figure 6, we now explore the compositional variability in both
9
time and space. To target temporal variability, we choose an outbound sequence from flyby T81
10
in January 2012 during which the UVIS field-of-view was oriented radially in the northern
11
hemisphere as shown in Figure 7. During this outbound leg of the flyby, UVIS stared at a point
12
on the limb near 50° N for about 2 h and then returned to the same point about 4 h later to stare
13
for about 40 min. We divided the first 2 h portion of the sequence into 4 subsets of data and then
14
averaged the final 40 min staring sequence for the last subset. In this way we assembled five
15
independent sets of EUV and FUV radiance profiles from the same geographic point on the limb
16
over 6 ½ h of elapsed time. All the observations from T81 shown herein were made in the
17
morning from 6.6-8.7 LST and the conditions for these six sequential subsets of observations are
18
summarized in Table 2.
19
We vertically averaged the data from each pixel into 50 km altitude bins except for the
20
last subset, which was averaged in 100 km bins because of the larger distance from Titan. We
21
show two images of the 1493 Å radiance on the limb of Titan representing the first and last
22
records within this sequence in Figure 7. The apparent size of the UVIS field of view becomes
23
1
larger between the first and last record as the spacecraft recedes from Titan on the outbound leg
2
of the flyby.
3
4
5
6
7
8
Figure 7. UVIS viewing geometry from the outbound leg of the T81 flyby. Radiances imaged
are from the same N I 1493 Å spectral region as used in Figure 1 and are shown for each pixel.
The two records shown represent the projected UVIS field of view near the beginning (16:37
UT) and the end (23:13 UT) of the indicated time period. The 900 km and 1200 km tangent
altitudes are indicated for reference. The sub-solar point is the open circle.
9
10
For each of the subsets of data summarized in Table 2 we spectrally fit the N I 1493 Å
11
and N II 1085 Å multiplets in the same way as described in Section 3.1. We run invert each T81
12
radiance profile to obtain a 6 ½ h time series of N2 and CH4 density profiles. The a priori profile
24
1
is the same as that used in the T55 retrieval and the profiles are retrieved in chronological order
2
as listed in Table 2.
3
4
5
Table 2. T81 UVIS Observations: 30 Jan 2012
a
UT
Lat (ºN)
Lon (ºW)
SZA (º)
LST (h)
LOS Distancea
(103 km)
16:33-16:45
46
237
53
8.7
67
17:05-17:29
49
243
57
8.3
78
17:33-18:09
51
248
60
8.1
88
18:05-18:45
52
253
64
7.8
99
22:52-23:32
56
275
74
6.6
190
Distance from Cassini UVIS to the tangent point.
6
7
N2 and CH4 density profiles from the five time steps are assembled together in Figure 8
8
along with the corresponding CH4 mixing ratio profiles. The data indicate a CH4 homopause at
9
950±25 km for T81, which is about 50 km lower than for T55. The tendency for both N2 and
10
CH4 profiles to approach the same value at the top and the bottom of the altitude region shown is
11
a reflection of the lack of information in the dayglow such that the retrieval reverts to the a priori
12
value in these regions. Also shown in Figure 8 is the average of all five profiles as well as the
13
average uncertainty.
25
1
2
3
4
5
6
Figure 8. Compositional retrievals for the T81 data used herein. Left panel: Retrieved N2 and
CH4 number density profiles for each profile listed in Table 2 with uncertainties (black). Also
shown is the average of the five profiles assembled for this flyby with uncertainties (red). Right
panel: Corresponding CH4 mixing ratio profiles (black) along with the average of the five orbits
(red).
7
8
9
Figure 9 shows the temperature retrieved from each of the N2 density profiles in Figure 8
as a function of the elapsed time. The average temperature of the six profiles is 170 K and
10
generally consistent with the range of temperatures reported from INMS observations [Westlake
11
et al., 2011; Snowden et al., 2013]. More importantly, there is not a significant variation in the
12
upper atmospheric temperature from one profile to the next, even though the last N2 profile in the
13
time series is sampled in altitude increments that are twice as large as the others. This implies
14
that the temperature retrievals are not sensitive to the vertical resolution of the observations. We
26
1
will explore the variation of temperature over the diurnal cycle with the sequence of data from
2
T94 in the next section.
3
4
5
6
7
8
9
Figure 9. Temperatures inferred from UVIS N2 number density profiles for T81 in Figure 8 as a
function of elapsed time. The altitudes over which the temperatures are inferred are indicated in
the upper left of the plot. The temperatures do not vary significantly from one profile to the next
in the time series.
Figure 10 shows the variation of N2 density during the observations at three different
altitudes: 1100 km, 1000 km and 900 km. Here we have taken the average density for each
10
altitude from the five profiles and calculated the relative variation about that average
11
(∂[N2]/[N2]). We find that at 900 km, 1000 km and 1100 km the N2 density variations are
12
between 10-30%. However, these variations are not significantly greater than the uncertainties of
13
the UVIS measurements.
27
1
CH4 densities are also not clearly distinguishable above the uncertainties of the
2
observations from 900-1100 km. Above 1100 km the CH4 opacity is weak so that the retrieved
3
mixing ratios are relatively unconstrained, which is reflected in the relatively large error bars
4
shown at these altitudes in Figure 8. We will focus next on the spatial variation of N2 and CH4 by
5
scanning the limb at a wide variety of longitudes on the dayside at near the equator. In this way
6
we will diagnose the variations of these constituents with LST and/or longitude.
7
8
9
10
Figure 10. The relative variability of T81 N2 number densities from at 1100 km (top panel),
1000 km (middle panel) and 900 km (bottom panel). Retrievals are for five subsets of
observations from nearly the same geographic location (see Table 2).
11
4.3 T94: Daytime Constituent Variability
12
On 12 September 2013 UVIS obtained limb spectra on the dayside of Titan as Cassini
13
flew under its south pole on the outbound leg of the T94 flyby. These observations were made on
14
the dayside from 11º N to 14º S latitude and the viewing geometry is illustrated in Figure 11. For
28
1
this study we divided the observations into nine different EUV and FUV radiance profiles and
2
each of them contain two records of data. The nine profiles cover about 7.3 h LST and relevant
3
conditions for these profiles are summarized in Table 3. The variation of solar zenith angle is
4
also indicated in Figure 11 and shows that the observations include LST near local noon.
5
6
7
8
Table 3. T94 UVIS Observations: 12 Sep 2013
UT
Lat (ºN)
Lon (ºW)
SZA (º)
LST (h)
LOS Distance
(103 km)
10:36
11
199
14
11.2
57
10:58-11:21
11
198
14
11.3
68
11:43-12:05
10
196
13
11.4
82
12:49
6
185
14
12.3
100
13:11-13:34
3
176
21
12.9
111
13:56
0
166
31
13.6
129
14:40-15:02
-6
149
49
14.8
140
15:24-15:47
-10
137
62
15.6
154
16:31
-14
125
74
16.5
172
29
1
2
3
4
5
6
7
8
9
Figure 11. Projected field of view for the T94 observations. UVIS is looking toward the
southern hemisphere. Left panel: The N I 1493 Å radiances for each pixel over the same spectral
region shown in Figures 1 and 7. The 900 km and 1200 km tangent altitudes are indicated for
reference. The peak production is clearly seen near 900 km tangent altitude. Right panel: Solar
zenith angles corresponding to the limb observations in the left panel.
We extracted N I 1493 Å and N II 1085 Å radiance profiles in the same way as described in
Section 3a and use these profiles to retrieve N2 and CH4 densities. We used the same a priori
10
profile from the analysis of the T55 and T81 flybys and we bin each profile in 100 km
11
increments. Attributes for the profiles are listed in Table 3.
30
1
2
3
4
5
6
Figure 12. Compositional retrievals for the T94 data used herein. Left panel: Retrieved N2 and
CH4 number density profiles with uncertainties for each profile listed in Table 3 (black) along
with the average for each (red). Right panel: Corresponding CH4 mixing ratio profiles (black)
along with the average (red).
7
We assemble all nine profiles from T94 in Figure 12 and also include the average over all
8
of the profiles. The homopause altitude is near 950±25 km and not inconsistent with that inferred
9
from the T81 flyby. The temperatures inferred from each N2 density profile in T94 are shown in
10
Figure 13 as a function of LST. The average temperature from these nine profiles is 183 K and
11
systematically about 13 K higher than those inferred from T81. This indicates that there is a
12
mechanism acting globally to elevate upper atmospheric temperatures. Furthermore, the data also
13
suggest that there is a LST dependence to the observed temperatures with a minimum of 178 K
14
near 15 LT and increasing by 10-15 K in the late morning and late afternoon. The temperature
31
1
uncertainties (about 10-15 K) are large relative to the LST variations inferred and we are also
2
unable to distinguish between a LST dependence and a longitudinal dependence of the
3
temperatures. Nonetheless, the nine profiles assembled together in Figure 13 coherently indicate
4
that the temperature is lower by about 10-15 K in the mid-afternoon compared to noon or late
5
afternoon in the upper atmosphere of Titan. We emphasize that this result is dependent on which
6
altitudes are considered when inferring temperature. Also included in Figure 13 are temperature
7
retrievals whereby we have limited our analysis to 800-1100 km. In this case the temperature
8
drops by ~40 K in the late afternoon compared to observations in the middle of the day,
9
suggesting more solar control for these altitudes.
10
11
12
13
14
Figure 13. Local solar time (LST) dependence of temperatures inferred from the T94 N2 number
density profiles in Figure 12. Temperatures are retrieved from two different altitude regions and
the LST dependence depends on which region is chosen.
32
1
Figure 14 shows N2 density variations as a function of LST for T94 at 1100 km, 1000 km
2
and 900 km. The variations are calculated in the same way as in Figure 10, where deviations
3
around the average for all nine profiles are shown for each altitude. Except for the last profile
4
near 16.5 LST, the variation at 900 km, 1000 km and 1100 km is not clearly distinguishable
5
above the uncertainty. The drop in density at the end of the time series is also reflected in the
6
lower temperature for the 800-1100 km result in Figure 14. Variations in CH4 density for T94 are
7
not clearly distinguishable above the measurement uncertainty at any given altitude.
8
9
10
11
12
Figure 14. The relative variability of T94 N2 number densities at 1100 km (top panel), 1000 km
(middle panel) and 900 km (bottom panel). Except for the profile near 16.5 LST, the data
indicate no coherent variation of densities from 900-1100 km over the LST sampled for T94.
33
1
5. Discussion
2
The UVIS number density retrievals from the dayglow probe the 800-1300 km region on
3
Titan and extend low enough to resolve the CH4 homopause, thereby providing new constraints
4
to global-scale dynamical models. These remote sensing observations offer the following
5
benefits to the study of upper atmospheric variability of Titan:
6
1) Comparisons with the in situ INMS results from the same Titan orbit not only help to
7
validate the UVIS retrievals but also provide insight to global-scale variability.
8
2) UVIS can stare at a single location for hours at a time to help quantify the temporal
9
10
11
variability of density and temperature.
3) The observations allow for synoptic description of N2 and CH4 density and temperature
variability for each flyby at a wide variety of latitudes and LST.
12
The variability in composition and temperature that we show herein for three Cassini flybys
13
of Titan is similar to that previously reported and is summarized in Figure 15. Figure 15 shows
14
the average N2 density profiles for T55, T81 and T94 as well as the fits to each profile. The
15
temperature variation between flybys is 10-20 K and we also note that the temperature for T94 is
16
higher than for almost all other flybys analyzed to date [e.g. Snowden et al., 2013]. It is unlikely
17
that variations in the solar flux are controlling the differences between T81 and T94 since the
18
F10.7 solar flux for T81 was actually larger (114 solar flux units, s.f.u.) than that for T94 (93
19
s.f.u.). This conclusion was also reached by Westlake et al. [2011] and Snowden et al. [2013] in
20
their analysis of the INMS data. We have analyzed data from Cassini’s Magnetospheric Imaging
21
Instrument (MIMI) and found that the precipitating particle flux was weak during these flybys,
34
1
although we emphasize that time scales for the atmospheric response to magnetospheric input are
2
considerably longer than can be obtained from one Cassini flyby [Bell et al., 2011b].
3
4
5
6
7
8
9
Figure 15. Average N2 number density and CH4 mixing ratio profiles for the three Cassini flybys
considered herein. Fits to the N2 densities assuming an isothermal atmosphere are included as the
dashed lines with the resultant temperatures indicated for each flyby.
In Figure 6 we show that the temperature retrieved from the UVIS T55 data is ~40 K
10
higher than that retrieved from the INMS data for the same flyby. The observations are made 6 h
11
apart in elapsed time so temporal variations are likely to be smaller than this, as indicated by our
12
analysis of the T81 data in Figure 9. Similarly latitudinal variations are unlikely to explain these
13
differences since the observations were made in the same latitude region on opposite sides of the
35
1
equator (see Table 1). One important difference between the UVIS and INMS observations of
2
T55 is that the UVIS observations are from the day side near 16 LST where as the INMS
3
observations are from the night side near 22 LST. Müller-Wodarg et al. [2000] estimated that
4
day to night temperature differences were ~10 K, whereas Bell et al. [2011a] modeled global
5
mean day to night temperature differences and found that they were about 20 K near 1000 km.
6
To reconcile the T55 UVIS temperature with the INMS temperature the day to night temperature
7
difference needs to be at least 40 K. Such a diurnal variation is within the scatter of the INMS
8
observations but a coherent variation between 16-22 LST is not evident when all of the INMS
9
data are assembled together [Snowden et al., 2013]. This lack of coherence indicates that
10
another process controls the INMS observed temperature variations from one flyby to the next
11
[cf. Westlake et al., 2011; Snowden and Yelle, 2014].
12
The idea that temperature variations on Titan depend on altitude was explored in detail by
13
Snowden et al. [2013], although they considered the INMS data above 1000 km. We provide
14
additional insight to the LST variation at Titan in Figure 13, where we have retrieved
15
temperatures from T94 using only the lower altitudes (800-1100 km) that are mostly inaccessible
16
to INMS. In this case the diurnal variability is more coherent, with a temperature drop of ~40 K
17
from midday to late afternoon. This suggests that diurnal temperature variations on Titan are
18
much larger than predicted below 1100 km. Additional analyses of UVIS dayglow data on the
19
dayside disk could further clarify these diurnal variations.
20
6. Summary
21
We have presented retrievals of N2 and CH4 number densities in Titan’s upper
22
atmosphere using the UV dayglow observed by UVIS on Cassini for three different flybys from
23
2009-2013. This new technique is enabled by improved forward models of N2 dayglow
36
1
production in recent years on both Titan and Earth We use two features for our retrievals: the N
2
I 1493 Å FUV multiplet and the N II 1085 Å EUV multiplet. Both of these are produced
3
directly from N2 photofragmentation and are therefore diagnostic of N2 density variations. Only
4
the EUV N II 1085 Å multiplet is significantly attenuated on the limb by CH4, allowing for the
5
direct retrieval of CH4 number densities.
6
The retrievals of N2 and CH4 density require that observations are made from 800-1300
7
km tangent altitude and that Cassini is close to Titan so that the UVIS vertical resolution is high
8
enough on the limb. The UVIS dayglow density retrievals are in good agreement with INMS
9
densities from 1000-1150 km for the T55 flyby, when the INMS densities are increased by a
10
factor of 3.0. Above 1150 km UVIS densities are systematically larger than the scaled INMS
11
densities, which is possibly due to temperature differences between the two measurements.
12
The altitude region of UVIS retrievals is broad enough to vertically resolve the CH4
13
homopause. For all three of the flybys the CH4 homopause is from 900-1150 km and consistent
14
with previous modeling studies constrained by INMS data [e.g. Bell et al., 2011b].
15
Upper atmospheric temperature variations inferred at altitudes between 800-1300 km
16
from one flyby to the next are large (10-20 K). The N2 density and temperature variations are
17
similar to those observed in the INMS data.
18
In addition to the similarities with previous observations described above, the remote
19
dayglow observations by UVIS provide additional insight to variations in Titan’s upper
20
atmosphere that complement the in situ measurements. We have used observations from two
21
different flybys as examples. The first uses observations from a January 2012 flyby (T81)
22
during which the UVIS obtained limb observations over a period of 6 h from nearly the same
23
geographical region. We find that the upper atmospheric temperature varied by less than 10 K
37
1
during this time period. The second uses observations from a September 2013 flyby (T94)
2
during which UVIS scanned the dayside limb near the equator over 7 h LST. We find that there
3
is a temperature drop of 10-15 K from late morning to mid-afternoon. We also find that the
4
diurnal variation depends on which altitudes are analyzed so that if we limit the UVIS
5
temperature retrieval to lower altitudes between 800-1100 km there is a drop in temperature of
6
~40 K from midday to late afternoon, which is much larger than previous estimates. However,
7
with this limited amount of data we emphasize that we cannot separate a longitudinal
8
dependence on temperature with LST dependence.
9
The retrieval of composition from the dayglow allows for synoptic global-scale
10
observations of upper atmospheric variability not possible from in situ measurements or
11
occultations. Analyses of Titan UVIS dayglow limb data from other flybys can clarify some of
12
the initial conclusions described above, particularly if complemented by INMS or occultation
13
observations from the night side. Furthermore, if UVIS results are assembled for a variety of
14
representative plasma environments they can help determine whether the primary source of the
15
observed upper atmospheric variability is internal or external to the upper atmosphere.
16
17
Acknowledgements
MHS, JSE and JMA gratefully acknowledge the NASA Cassini Data Analysis Program
18
for supporting this work (#NNH13AV62I). We thank Alain Jouchoux, Robert West and the
19
UVIS Operations Team for their help in arranging the observations presented herein and R.R.
20
Meier for assistance with the solar irradiance calculations for this work and for many useful
21
discussions.
22
38
1
2
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