A Tale of Two Planets: Cassini UVIS He 584Å Airglow at Jupiter and Saturn Chris Parkinson, Caltech Planetary Science Seminar January 10, 2006

Overview of the talk - I  Enhanced Transport in the Polar Mesosphere of Jupiter: Evidence from Cassini UVIS He 584Å Airglow  Introduction  Observations  Results and Discussion  Other authors: Y.L. Yung, A.I.F. Stewart, J.M. Ajello, A-S. Wong  Enhanced Transport in the Polar Mesosphere of Jupiter: Evidence from Cassini UVIS He 584Å Airglow  Introduction  Observations  Results and Discussion  Other authors: Y.L. Yung, A.I.F. Stewart, J.M. Ajello, A-S. Wong

I. Introduction  One of the fundamental properties of a planetary atmosphere is the amount of mechanical mixing forced by the large scale circulation, planetary and gravity waves, and other processes.  In a one-dimensional model, this mixing is often characterized by the eddy diffusion coefficient, K  One of the fundamental properties of a planetary atmosphere is the amount of mechanical mixing forced by the large scale circulation, planetary and gravity waves, and other processes.  In a one-dimensional model, this mixing is often characterized by the eddy diffusion coefficient, K

 Values of K h for Jupiter have been obtained from analyses of 1.H Lyman-alpha albedo, 2.fall-off in hydrocarbon profiles against H 2 background, using the Voyager Ultraviolet Spectrometer (UVS) occultation results for the CH 4 and other hydrocarbon distributions, and 3.He 584 Å airglow.  Analysis of the equatorial He 584 Å emission data suggests values of K h in the range 10 6 – 10 7 cm 2 s -1 [McConnell et al., 1981] and 2 (+2, -1)  10 6 cm 2 s -1 [Vervack et al., 1995], in reasonable agreement with those values obtained by methods 1. and 2.  Values of K h for Jupiter have been obtained from analyses of 1.H Lyman-alpha albedo, 2.fall-off in hydrocarbon profiles against H 2 background, using the Voyager Ultraviolet Spectrometer (UVS) occultation results for the CH 4 and other hydrocarbon distributions, and 3.He 584 Å airglow.  Analysis of the equatorial He 584 Å emission data suggests values of K h in the range 10 6 – 10 7 cm 2 s -1 [McConnell et al., 1981] and 2 (+2, -1)  10 6 cm 2 s -1 [Vervack et al., 1995], in reasonable agreement with those values obtained by methods 1. and 2.

 Since for an ionospheric source the column amount of H above the absorbing layer of CH 4 decreases monotonically with increasing K the measured Lyman-  brightness could yield an estimate of the K.  Determining K h using this method is strictly valid only if 1.resonance scattering alone is responsible for the observed Lyman-  emission, 2.H is produced only from the photochemical processes involving H 2, CH 4, etc, and 3.that the solar Lyman-  flux is known accurately [Atreya, 1986].  Since for an ionospheric source the column amount of H above the absorbing layer of CH 4 decreases monotonically with increasing K the measured Lyman-  brightness could yield an estimate of the K.  Determining K h using this method is strictly valid only if 1.resonance scattering alone is responsible for the observed Lyman-  emission, 2.H is produced only from the photochemical processes involving H 2, CH 4, etc, and 3.that the solar Lyman-  flux is known accurately [Atreya, 1986].

 However, the status of our knowledge of the Lyman-  budget for Jupiter is uncertain  Other sources of excitation may play a role [McConnell et al., 1989].  Moreover, issues regarding the solar Lyman-  flux has always been a problem [Parkinson et al., 1998].  Hence, the Cassini UVIS He 584 Å airglow may be the best way to determine a “ reasonably ” secure value for K h.  However, the status of our knowledge of the Lyman-  budget for Jupiter is uncertain  Other sources of excitation may play a role [McConnell et al., 1989].  Moreover, issues regarding the solar Lyman-  flux has always been a problem [Parkinson et al., 1998].  Hence, the Cassini UVIS He 584 Å airglow may be the best way to determine a “ reasonably ” secure value for K h.

 K in the auroral regions of Jupiter is poorly known, cf. Vervack et al., 1995 reanalysis of Voyager N-S scan data.  In auroral regions, with additional energy input from the magnetosphere, we speculate that it is not unreasonable to expect K to increase relative to equatorial regions  Since eddy mixing might be expected to be much more effective, this mechanism may be responsible for the enhancement of heavy hydrocarbon production in the polar regions [Wong et al., 2003].  K in the auroral regions of Jupiter is poorly known, cf. Vervack et al., 1995 reanalysis of Voyager N-S scan data.  In auroral regions, with additional energy input from the magnetosphere, we speculate that it is not unreasonable to expect K to increase relative to equatorial regions  Since eddy mixing might be expected to be much more effective, this mechanism may be responsible for the enhancement of heavy hydrocarbon production in the polar regions [Wong et al., 2003]. Motivation

 Fig. 1 Disc-averaged HeI 584A brightnesses compared to the auroral signal (normalized) for the period 01 Oct - 14 Nov 2000 representing 42 complete Jovian rotations. The error bars on 584A points are the 1-sigma statistical uncertainties; the equivalent uncertainties in the auroral points are smaller than the symbols. The average of the 584A points is 8.8R ± 1.8R, and they correlate with the auroral signal with a coefficient of The two high auroral points correspond to the passage of large solar wind shocks.

Fig. 2. The EUV spectrum ( A) of Jupiter accumulated over sec on December 2000, at a distance of 246 R j (17.6 million km) from the planet. Note the change in scale near 870A. Several emissions from Io's plasma torus are indicated, specifically S II, S III, and S IV.

Fig. 3. (Upper) The N-S profile of the HeI 584A emission, measured on December 2000 at a spatial resolution of 0.25 Rj. Increasing detector row corresponds to a S to N direction. The background noise level is shown, as is an empirical model of the 584A airglow signal, which varies as the cosine of latitude and peaks at 9.2R. (Lower) The deduced excess signal which can be attributed to auroral processes. Also shown is the auroral signal normalized to the excess 584A signal.

Model Description(s)  Apply resonance scattering model using Feautrier technique to solve radiative transfer equation assuming partial frequency redistribution [Gladstone, 1982; Parkinson et al., 1998].  The H 2 and He densities used in the resonance scattering model calculated by a 1-D photochemical model for the Jovian polar atmosphere [Wong et al., 2003].  We consider the solar zenith angle = viewing angle = latitude.  Temperature profile and auroral ion production rates are taken from the Jovian auroral thermal model of Grodent et al. [2001] in which the energy flux is 30.5 ergs cm -2 s -  Apply resonance scattering model using Feautrier technique to solve radiative transfer equation assuming partial frequency redistribution [Gladstone, 1982; Parkinson et al., 1998].  The H 2 and He densities used in the resonance scattering model calculated by a 1-D photochemical model for the Jovian polar atmosphere [Wong et al., 2003].  We consider the solar zenith angle = viewing angle = latitude.  Temperature profile and auroral ion production rates are taken from the Jovian auroral thermal model of Grodent et al. [2001] in which the energy flux is 30.5 ergs cm -2 s -

 Principal parameters involved in determining the He 584 Å emission are f He, the He volume mixing ratio well below the homopause, the solar He 584 Å flux and line shape, the atmospheric temperature profile, and K h,.  We use f He = [Neimann, 1996; von Zahn and Hunten, 1996], line integrated solar flux at 1 A.U. is 4  10 9 cm 2 s -1, corresponding to solar maximum conditions.  A Gaussian line shape with a 1/e half-width of 73 m Å (or 122 m Å FWHM) [Maloy et al., 1978; McConnell et al., 1981]  The molecular diffusion coefficients, D, used in our calculations are taken from Mason and Marerro (1970),  Principal parameters involved in determining the He 584 Å emission are f He, the He volume mixing ratio well below the homopause, the solar He 584 Å flux and line shape, the atmospheric temperature profile, and K h,.  We use f He = [Neimann, 1996; von Zahn and Hunten, 1996], line integrated solar flux at 1 A.U. is 4  10 9 cm 2 s -1, corresponding to solar maximum conditions.  A Gaussian line shape with a 1/e half-width of 73 m Å (or 122 m Å FWHM) [Maloy et al., 1978; McConnell et al., 1981]  The molecular diffusion coefficients, D, used in our calculations are taken from Mason and Marerro (1970),

 K is adjustable in this study.  K (cm 2 s -1 ) =(1.46  10 6 )  (1.4  / n t (z)) 0.65 above 100 mbar level, and  10 3 below 100 mbar,  We consider four cases of enhanced eddy diffusion profiles, given by  K, where  = 1, 3, 10 and 30.  The homopause pressure level and K h for each  are: (1.2  mb, 9  10 5 cm 2 s -1 ), (3.0  mb, 9  10 6 cm 2 s -1 ), (2.6  mb, 2  10 8 cm 2 s -1 ), and (2.7  mb, 3  10 9 cm 2 s -1 ), respectively.  In each case, we generate the corresponding density profiles for background gases for each of the polar latitudes of 60°, 65°, and 70°.  K is adjustable in this study.  K (cm 2 s -1 ) =(1.46  10 6 )  (1.4  / n t (z)) 0.65 above 100 mbar level, and  10 3 below 100 mbar,  We consider four cases of enhanced eddy diffusion profiles, given by  K, where  = 1, 3, 10 and 30.  The homopause pressure level and K h for each  are: (1.2  mb, 9  10 5 cm 2 s -1 ), (3.0  mb, 9  10 6 cm 2 s -1 ), (2.6  mb, 2  10 8 cm 2 s -1 ), and (2.7  mb, 3  10 9 cm 2 s -1 ), respectively.  In each case, we generate the corresponding density profiles for background gases for each of the polar latitudes of 60°, 65°, and 70°.

Fig. 4: For eddy diffusion enhancement factor  of 1, 3, 10 and 30, the four plots show the mixing ratios of He (dashed line), CH 4 (solid line), C 2 H 2 (dot-dashed line), C 2 H 4 (dash-dot-dot dotted line) and C 2 H 6 (dotted line), versus pressure, calculated using a Jovian polar chemical model at latitudes of 65°.

 From Figures 2 and 3, we see that there is some evidence that high-latitude regions are brighter in He 584 Å emission than low latitudes.  These very interesting observations suggest that (a) electron impact is important, or (b) the atmosphere is horizontally/vertically inhomogeneous, or (c) both.  From Figures 2 and 3, we see that there is some evidence that high-latitude regions are brighter in He 584 Å emission than low latitudes.  These very interesting observations suggest that (a) electron impact is important, or (b) the atmosphere is horizontally/vertically inhomogeneous, or (c) both.

 The peak intensity of emission layer occurs at 1400 km with a layer width of about 400 km.  The electron flux is 3 x 10 9 cm -2 s -1 at 1400 km  He 584 Å emission cross section at 100 eV is 8 x cm -2 (Shemansky et al. [1985]) and He number density is 10 4 cm -3 at 1400 km with a corresponding column density of cm -2.  The helium is excited with the primary electron flux which is composed of a superposition of three maxwellians: a hard flux at 15 keV with peak deposition at 250 km; a soft flux at 3 keV with peak depostion at 600 km; and a weak flux at 100 eV with peak deposition at about 1500 km  The peak intensity of emission layer occurs at 1400 km with a layer width of about 400 km.  The electron flux is 3 x 10 9 cm -2 s -1 at 1400 km  He 584 Å emission cross section at 100 eV is 8 x cm -2 (Shemansky et al. [1985]) and He number density is 10 4 cm -3 at 1400 km with a corresponding column density of cm -2.  The helium is excited with the primary electron flux which is composed of a superposition of three maxwellians: a hard flux at 15 keV with peak deposition at 250 km; a soft flux at 3 keV with peak depostion at 600 km; and a weak flux at 100 eV with peak deposition at about 1500 km

Fig. 5: He 584 Å auroral volume emission rate as a function of altitude.

 These weak and soft primary electrons will produce the high altitude He aurora that would emerge from the planet to be observed by Cassini if it is there.  The cross section and the energy flux allow the estimation of the column emission intensity of He 584 Å due to electron precipitation  We find to be negligible at less than 0.1 R, thus eliminating point (a) as a possible emission source.  These weak and soft primary electrons will produce the high altitude He aurora that would emerge from the planet to be observed by Cassini if it is there.  The cross section and the energy flux allow the estimation of the column emission intensity of He 584 Å due to electron precipitation  We find to be negligible at less than 0.1 R, thus eliminating point (a) as a possible emission source.

Fig 6. Calculated He 584 Å emission intensity from Jupiter as a function of  for different latitudes.

Conclusions  We estimate the increased eddy mixing in the Jovian auroral regions by comparing the Cassini (UVIS) observations during the 2000 Jupiter flyby with radiative transfer calculations of the He 584 Å airglow intensity.  Equatorial K h is on the order of 2 x 10 6 cm 2 s -1  The calculated range for K h in the auroral regions to be at least 8 x 10 6 cm 2 s -1, and possibly greater than 4 x 10 7 cm 2 s -1, 5 to more than 10 times of the nominal eddy mixing profile K  We estimate the increased eddy mixing in the Jovian auroral regions by comparing the Cassini (UVIS) observations during the 2000 Jupiter flyby with radiative transfer calculations of the He 584 Å airglow intensity.  Equatorial K h is on the order of 2 x 10 6 cm 2 s -1  The calculated range for K h in the auroral regions to be at least 8 x 10 6 cm 2 s -1, and possibly greater than 4 x 10 7 cm 2 s -1, 5 to more than 10 times of the nominal eddy mixing profile K

Overview of the Talk - II Cassini UVIS Update: He 584Å  Dayglow at Saturn  Introduction  Observations  Results to date and Discussion  Other contributors: A.I.F. Stewart, Y.L. Yung  Introduction  Observations  Results to date and Discussion  Other contributors: A.I.F. Stewart, Y.L. Yung

Before  Voyager 1 and 2 epoch UVS He 584 Å measurements showed brightness at disk center to be 3.1 +/- 0.4 and 4.2 +/- 0.5 R, respectively (Sandel et al., 1982)  Previous Voyager determinations of eddy diffusion at the homopause (K h ) differ:  4x10 7 < K h < 1.2x10 8 cm 2 s -1 (Sandel et al, 1982; Atreya, 1982)  K h < 5x10 6 cm 2 s -1 (Smith et al, 1983)  Voyager 1 and 2 epoch UVS He 584 Å measurements showed brightness at disk center to be 3.1 +/- 0.4 and 4.2 +/- 0.5 R, respectively (Sandel et al., 1982)  Previous Voyager determinations of eddy diffusion at the homopause (K h ) differ:  4x10 7 < K h < 1.2x10 8 cm 2 s -1 (Sandel et al, 1982; Atreya, 1982)  K h < 5x10 6 cm 2 s -1 (Smith et al, 1983)

 A reassessment by Parkinson et al, 1998 show that K h is likely > 10 8 cm 2 s -1 during Voyager encounters  Main uncertainties are the  integrated He 584 Å solar line flux  planetary mixing ratio of He in the deep atmosphere  unknown uncertainties in the airglow measurement due to calibration  A reassessment by Parkinson et al, 1998 show that K h is likely > 10 8 cm 2 s -1 during Voyager encounters  Main uncertainties are the  integrated He 584 Å solar line flux  planetary mixing ratio of He in the deep atmosphere  unknown uncertainties in the airglow measurement due to calibration

Now  More precise, recent Cassini measurements give us the ability to use helium as a effective diagnostic tool to say something more conclusive about the dynamics in the Saturnian atmosphere, ameliorating previous difficulties

UVIS Data collection  XUV summary structure file data 1024x64  Rows are spatial, columns spectral  XUV summary structure file data 1024x64  Rows are spatial, columns spectral

Spectral representation Angstroms

Spatial representation: edge of planet clearly at row 16 and 46 for both EUV and FUV S N Ring shadow effects In the northern hemisphere?

Total count rate over 1 “movie” segment (450 min)

Data analysis  A series of Saturnian observations totalling 60 hours has been looked at  He 584 Å line IS present with sunlit disk averaged brightness of / R obtained (5-sigma detection)  Hard work needed to get more than a long term disk average out of data  A series of Saturnian observations totalling 60 hours has been looked at  He 584 Å line IS present with sunlit disk averaged brightness of / R obtained (5-sigma detection)  Hard work needed to get more than a long term disk average out of data

 It would appear that the Cassini He 584 Å brightness is lower now than during the Voyager epoch with the corresponding effects on K h  Possible causes could be  shadowing of the northern hemisphere of the planet by the rings, which wasn't the case during the Voyager measurements,  seasonal variations, viz., the data examined from the Cassini mission falls approximately 24 and 23 years following the Voyager encounters.  differences in the solar flux  It would appear that the Cassini He 584 Å brightness is lower now than during the Voyager epoch with the corresponding effects on K h  Possible causes could be  shadowing of the northern hemisphere of the planet by the rings, which wasn't the case during the Voyager measurements,  seasonal variations, viz., the data examined from the Cassini mission falls approximately 24 and 23 years following the Voyager encounters.  differences in the solar flux

Shadowing by the rings  As seen from Earth, the rings change orientation with a period of 29.5 years after being twice edge on at 15.5 and 13.5 years.  Voyager epoch measurements the rings we edge on, whereas the northern polar region is shadowed now  As seen from Earth, the rings change orientation with a period of 29.5 years after being twice edge on at 15.5 and 13.5 years.  Voyager epoch measurements the rings we edge on, whereas the northern polar region is shadowed now

Seasonal variation  A Saturnian year is 29.5 earth years and Voyager measurements were 24.5 and 23.5 years ago indicating seasonal differences  Comparing to the earth, if the Cassini measurements were corresponding to Jan 01, the Voyager encounters would have been occurring sometime in March, which is an appreciable difference.  A Saturnian year is 29.5 earth years and Voyager measurements were 24.5 and 23.5 years ago indicating seasonal differences  Comparing to the earth, if the Cassini measurements were corresponding to Jan 01, the Voyager encounters would have been occurring sometime in March, which is an appreciable difference.

Solar flux  Voyager encounter occurred during a period of solar maximum  Cassini measurements were taken approximately 2 years following solar maximum, so the solar flux is somewhat reduced from solar max  Voyager encounter occurred during a period of solar maximum  Cassini measurements were taken approximately 2 years following solar maximum, so the solar flux is somewhat reduced from solar max

Preliminary results  Compare ~ 1 R for Cassini to the Voyager values shown in Figures 4 and 5 in Parkinson et al., 1998  For their standard parameters, K h would be ~3x10 7 cm 2 s -1, and using the EUVT94 model solar flux, ~3x10 6 cm 2 s -1 for Cassini measurements.  Compare ~ 1 R for Cassini to the Voyager values shown in Figures 4 and 5 in Parkinson et al., 1998  For their standard parameters, K h would be ~3x10 7 cm 2 s -1, and using the EUVT94 model solar flux, ~3x10 6 cm 2 s -1 for Cassini measurements.

C C

Issues and Ongoing work  Current "best estimate" for the He mixing ratio needs to be quantified, but Figure 5 in that paper explores a parameter space of mixing ratios, from which we can obtain a preliminary estimate if it deviates from the standard value quoted there.  Need to properly quantify solar flux and effects of ring shadowing in calculations  unknown uncertainties in the airglow measurement due to calibration need estimation  Current "best estimate" for the He mixing ratio needs to be quantified, but Figure 5 in that paper explores a parameter space of mixing ratios, from which we can obtain a preliminary estimate if it deviates from the standard value quoted there.  Need to properly quantify solar flux and effects of ring shadowing in calculations  unknown uncertainties in the airglow measurement due to calibration need estimation