Saturn Upper Atmosphere
UVIS team meeting (06/17/2014)
Cassini UVIS: Esposito et al. (2004)
T. T. Koskinen
(Cassini participating scientist, LPL, University of Arizona)
Collaborators: B. R. Sandel, R. V. Yelle, I. Müller-Wodarg, F. J. Capalbo, G. M. Holsclaw,
W. E. McClintock
CDAPS Project Summary
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Saturn exobase temperatures and altitudes
(see Koskinen et al. 2013)
Use density and temperature profiles
from Cassini/UVIS occultations to
study the deposition and
redistribution of energy in Saturn’s
thermosphere
Use hydrocarbon density profiles from
Cassini/UVIS occultations to study
photochemistry and dynamical trends
in Saturn’s upper stratosphere
Atmospheric densities for Proximal
orbits (with D. Strobel)
Use UVIS observations to study
photochemistry and aerosols in
Titan’s upper atmosphere
Complementary to CIRS and VIMS
observations
UVIS Saturn occultations
See also:
Shemansky and Liu (2012)
Cassini UVIS occultations
UVIS stellar (green) and solar (red) occultations. The vertical dashed
line shows the Equinox of August 11, 2009 (DOY 223).
Retrieving the density of H2
UVIS EUV spectrum of
the Sun
Density of H2 and constraints on H
UVIS EUV spectrum of
the Sun
Dashed line based on
best-fit H2 profile
Best fit H2 profile from
63 nm: no room for H
above 2000 km
Solar occultation geometry
1 mrad
Apparent solar diameter
200-500 km
Cassini
Thermosphere
Saturn 1 bar level
Modeling the sunset
230 km
Solar occultations: Exospheric temperatures
Thermospheric temperature from Cassini UVIS (Koskinen et al. 2013), Voyager UVS
(Smith et al. 1983) vs. planetocentric latitude
Solar occultations: Shape
Below: Altitude of the 0.01 nbar level above the
100 mbar reference
Above: Radius of the 0.01 nbar level from 15
occultations (this work), the 100 mbar reference
level (dotted, Anderson and Schubert 2007)
Koskinen et al. (2013)
Stellar occultations: EUV channel
1300 km (50 km
bin)
96.4 – 97.0 nm
Exospheric temperatures
Results from 33 occultations, including Cassini UVIS solar occultations (purple
diamonds, Koskinen et al. 2013), stellar occultations (black triangles, this work) and
Voyager UVS solar and stellar occultation (green squares, Smith et al. 1983).
H2 density profiles: Equator
Black (3.5S), green (3.4S), red (1.7N) diamonds: Cassini occs Dec 2008 – Jan 2009
(this work). Black solid line: V2 egress occ (Vervack and Moses, 3.5 – 3.9N).
H2 densities
Dec 2008-Jan 2009
Altitudes above 1 bar level
Thermosphere shape I
Radial distance to the 0.01 nbar level from solar occultations (purple diamonds) and stellar occultations
(black diamonds). The solid line is the 100 mbar reference level (Anderson and Schubert 2007) and
the dashed line is the expected shape of the 0.01 nbar level.
Thermosphere shape II
Altitude of the 0.01 nbar level from solar occultations (purple, Koskinen
et al. 2013) and stellar occultations (black, in preparation).
Thermosphere shape III
Temperature profiles
Cassini UVIS
(this work)
Cassini CIRS
(Guerlet et al. 2009)
Saturn’s atmospheric structure at 20-35N in March-May, 2006
(black). Results from 4 UVIS occultations are shown. The red
points show the equatorial T-P profile from 2009.
T-P profiles and expansion
UVIS
CIRS
Black: Northern low latitude 2006, Red: Equator
2009.
Temperature at 1 nbar at low northern latitudes.
Model atmosphere at 26N, 2006
UVIS
T(1 nbar) = 290 K
CIR
S
Kzz = 6 x 107 cm2 s-1
Diamonds:
UVIS data
UVIS
CIR
S
Model atmosphere in 2009
The red crosses (right) are predicted altitudes based on the observed temperature changes (left).
UVIS: Warming
in the lower
thermosphere?
CIRS: 6-10 K warming
mid-northern latitude
(Fletcher et al.2010)
Energy deposition
ST25, 3.8S, 2009_003
Required heating rate at 0.1 – 1
nbar near the equator 30 – 40
times the solar EUV heating rate.
In addition to the time-dependent trend, occultations
from similar time periods indicate that temperature
increases with latitude from the equator to the poles.
What do the models say about this?
High latitude electrodynamics
Auroral Joule heating provides ~10 TW
of energy at the poles that can solve
the temperature problem (Cowley et al.
2004a,b).
Rotation
Westward Coriolis force ( -2W x u ), aided by
ion drag ( j x B ), turns meridional flow from
the poles into zonal flow and traps the energy
at the poles (e.g., Smith et al. 2007).
Meridional transport?
Above left: Observed/model temperatures (revised GCM of Müller-Wodarg et al. 2012). Above right: Zonal mean
temperatures from the model. Below: Zonal winds from the model.
Summary I
• Exospheric temperature 370 – 540 K (agrees with
Voyager UVS and H3+ emission measurements):
increases by 100 – 150 K from equator to pole
• Temperature structure qualitatively consistent with
high latitude heating – quantitatively existing models
disagree with the data
• Exobase altitude 2700 – 3000 km
• Mixing ratio of H typically less than 5 %
Summary II
• Pressure level altitudes are generally inconsistent
with potential theory: differences in T-P profiles
• Enhanced heating of the thermosphere leads to the
expansion of the pressure levels by 300-400 km
between 2005 and 2011 at mid-to low latitudes (N)
• Homopause at the 0.1 – 1 microbar level with Kzz = 6
x 107 cm2 s-1 at mid-northern latitudes: compare with
Kzz = 5 x 106 cm2 s-1 from Smith et al.(1983) at the
same location
BACKUP
Reference kronoid
The Anderson and Schubert (2007)
model is based on a rotation period of
(10h32m35s) that minimizes the zonalwind driven altitude perturbations while
matching the Pioneer/Voyager/Cassini
data on Saturn’s shape.
H2 densities
Density profiles ranging from the pole (bottom black, 71N) to the equator (top
purple) in both hemispheres.
T-P comparison
ST25, 3.8S, 2009_003
To absorb or not to absorb…?
C III line not absorbed by H2…
Stellar occultations allow for the
absorption bands to be properly
resolved.
1300 km (50 km
bin)