Enhanced Chemistry and Transport in the Polar Mesosphere of Jupiter

Enhanced Transport in the Polar Mesosphere of Jupiter: Evidence from Cassini UVIS Helium 584 Å Airglow

C. D. Parkinson Division of Geological and Planetary Sciences, Caltech 150-21, Pasadena, CA 91125, USA Jet Propulsion Laboratory and the NASA Astrobiology Institute e-mail: [email protected]

A. S. Wong Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143, USA email: [email protected]

A. I. F. Stewart LASP, University of Colorado, 1234 Innovation Drive, Boulder, CO 80303, USA

Y. L. Yung Division of Geological and Planetary Sciences, Caltech 150-21, Pasadena, CA 91125, USA

J. L. Ajello MS 183-601, Jet Propulsion Laboratory, Pasadena, CA 91109, USA

Proposed Running Head: Enhanced Transport in Polar Mesosphere of Jupiter Editorial correspondence and proofs to: Chris Parkinson Key words: Jupiter, atmosphere, radiative transfer, planets Abstract

The eddy diffusion profile (Kz) in the auroral regions of Jupiter is not known. However, due to the intense auroral energy input, eddy mixing is expected to be much more effective and may be responsible for the enhancement of heavy hydrocarbon production in the polar region. In this paper, we search for evidence of increased eddy mixing in the Jovian auroral regions by comparing the Cassini Ultraviolet Imaging Spectrograph (UVIS) observations during the 2000 Jupiter flyby with new radiative transfer calculations of the He 584 Å airglow intensity. We derive the range for the eddy diffusion coefficients at the homopause

6 2 -1 7 (Kh) in the auroral regions to be at least 9  10 cm s , and possibly greater than 4  10 cm2 s-1.

1. 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 profile, Kz. At the homopause, the altitude where the molecular diffusion coefficient of methane equals the eddy diffusion coefficient, we define Kz = Kh. Values of

Kh for Jupiter have been obtained from analyses of (1) the H Lyman-alpha albedo [Wallace and Hunten, 1973; Yung and Strobel, 1980; Gladstone et al., 1996], (2) the fall-off in hydrocarbon profiles against H2 background, using the Voyager Ultraviolet Spectrometer

(UVS) occultation results for the CH4 and other hydrocarbon distributions [Festou et al., 1981; Yelle et al., 1996], and (3) the He 584 Å airglow [McConnell et al. 1981; Vervack et al., 1995]. Analysis of the equatorial He 584 Å emission data suggests values of Kh in the range 106–107 cm2 s-1 [McConnell et al., 1981] and 2 (+2, -1)  106 cm2 s-1 [Vervack et al., 1995], in reasonable agreement with those values obtained by methods (1) and (2). The eddy diffusion profile in the auroral regions of Jupiter is poorly known. However, due to the intense auroral energy input, eddy mixing is expected to be much more effective, and hence may be responsible for the enhancement of heavy hydrocarbon production in the polar regions [Wong et al., 2003]. Based on tidal theory, Lindzen [1971] finds that Kz ~ vL, where v = velocity of two air parcels, e.g. H2 and CH4, that are thoroughly mixed over distance L. In auroral regions, with additional energy input from the magnetosphere, both v and L increase relative to equatorial regions [see p. 210 of Atreya, 1986 for complete reference]. Hence Kz is expected to be greater in the polar regions. For the first time, recent observations by the Cassini Ultraviolet Imaging Spectrograph (UVIS) of the Jovian He 584 Å emission allow a derivation of the range for the eddy diffusion profile in the polar regions. In this paper, we investigate evidence for increased eddy mixing in the auroral regions of Jupiter by calculating the range of Kh from the UVIS data and employing new model atmospheres and radiative transfer models.

2. Observations The Cassini UVIS [Esposito et al., 1998; Esposito et al., 2003] observed the Jovian system during the Jupiter flyby from October 1, 2000 through March 22, 2001 during a period of near solar maximum activity. For 45 days, October 1 through November 14, 2000, observations were made in a mode that imaged the Jovian system on UVIS’s 2-D CODACON detectors, with spatial resolution of 1 mrad and spectral resolution of about 3

Å. The range to Jupiter decreased from 1170 to 618 Jupiter radii (Rj); Jupiter’s phase decreased from 20° to 18°; the sub-spacecraft latitude remained between 3°N and 4°N. The spectral range of UVIS’s EUV spectrometer is 569–1190 Å, and Jupiter’s image at 584 Å was well separated both from its auroral and airglow H2 emissions (850 Å and higher) and from the emissions from Io’s plasma torus at 642 Å, 659 Å, and many longer wavelengths. Over the 45 days, 2000 images, each with an integration period of 1000 s, were obtained covering 7 out of every 12 Jovian rotations. This is equivalent to 23 days of continuous observation. Because of the considerable range to Jupiter in this dataset, the planet's disc was not resolved. The following analysis deals only with the total photon flux from the entire disc. The images analysed here were added together in groups of 29–36 (i.e., covering approximately 1 Jovian rotation) to improve the signal-to-noise. After subtracting a considerable contribution due to instrument noise, and after scaling to a standard range of

1000 Rj, the 62 such groups yielded on average about 460 counts, with an rms deviation of about 135 counts. Using the instrument sensitivity at 584 Å, the count rate was converted to the equivalent brightness of a uniformly emitting flat disk having the same diameter as the planet. The brightnesses thus obtained averaged 7.9 Rayleighs (R), with an rms deviation of 2.6 R and a range of 3–15 R. Typical disc averaged brightnesses for Voyager and EUVE (Extreme Ultraviolet Explorer) were ~4 R and 1-4 R, respectively. The image count rates at 584 Å and at 865–1090 Å are shown in the upper panel of Figure 1. The two prominent spikes in the longer-wavelength signal coincide with the arrival at Jupiter of strong solar-wind shocks [Gurnett et al., 2002]. The lower panel of the same figure is the correlation diagram of the two signals. The correlation coefficient is small, indicating that the auroral-zone contribution to the 584 Å signal is not large. (It is nevertheless interesting that the two “spike” points are consistent with the trend of the main body of data points.) The scatter in the 584 Å signal is twice what would be expected on statistical grounds alone, and may arise from dynamical changes in the Jovian thermosphere.

On 14 December 2000, Jupiter was at zero phase, at a range of 245 Rj. UVIS observed Jupiter for a total of about 5.4 hours. The line-of-sight was fixed on the planet's center, and the slit was parallel to its rotation axis. The EUV channel placed 8 pixels along the sub- spacecraft meridian. For this dataset, the north and south auroral zones and the non-auroral airglow are all clearly separable. Since the planet was viewed through the intervening io plasma torus, the spectra obtained are contaminated with the torus's ionic emissions. This is more evident at wavelengths near 584 Å, where the planet itself is black (except for the helium feature), than at wavelengths longward of 850 Å, where the planet emits copiously from its aurorae and airglow.

Figure 2 shows parts of the spectra obtained by summing all the good data received, and subtracting the appropriate background. Figure 2a covers 570-770 Å, with the location of HeI 584 Å indicated. All of the other emissions belong to ions in the torus. In this particular data set, the southern aurora showed more 584 Å than the northern aurora, while the equatorial spectrum (from about 5°N to 9°S) lies between them. Figure 2b covers 925- 1125Å, with the position of HI Lyman-beta indicated. In this range the spectra are very similar, as is to be expected since they all derive mostly from electron impact on the dominant constituent H2. (In this wavelength region, the emission in in the Lyman bands of

H2.) Here, the aurorae are considerably stronger than the airglow. It should be remembered that the auroral zones themselves are not resolved; rather the auroral spectrum represents all the photons emitted at high latitudes, whether from the narrow auroral zones or from the airglow beyond about 60°N or 45°S.

Figure 3 shows all the good images summed over wavelength in two zones - all wavelengths (upper) and 581-587 Å (lower panel). This summing shows the signal dependence on north-south distance from the planet's center. The upper panel is dominated by H2 Lyman-bands emissions; the auroral signals are larger than the airglow signal at moderate latitudes. It is likely that the southern auroral arc fell close to the division between two pixels, hence the signal is split between them. (Jupiter underwent about one complete rotation during the collection of this data, so the auroral arc would have undergone its full latitudinal motion.) The HeI 584 Å signal is much fainter and some of its variation is due to noise. However, the auroral zones are still brighter than the airglow. The absence of a signal in the pixel at 1.0 radii South is significant, and is likely due to the large air-mass factor (and therefore increased absorption by H2) near the limb. Several more data-sets taken close to the planet were obtained; we defer their analysis to a later paper.

3. Model Description Resonance scattering of sunlight by He atoms is the principal source of the planetary emission at 584 Å [e.g., Carlson and Judge, 1974; 1976]. Since He is heavier than the background H2 atmosphere, its mixing ratio falls off rapidly above the homopause. This results in the He being immersed in and overlaid by an absorbing atmosphere of H2. The scattering region, i.e., the region where the absorption optical depth in H2 at 584 Å is less than one, generally lies well above the homopause. As Kh increases, more He is mixed into the scattering region and thus the reflected He 584 Å intensity increases.

The principal parameters involved in determining the He 584 Å emission are fHe, the He volume mixing ratio well below the homopause, the solar He 584 Å flux and line shape, the atmospheric temperature profile, and Kh,. In our model we use fHe = 0.136 [Gautier and Owen, 1989]. The line integrated solar flux at 1 A.U. is 2  109 cm2 s-1, corresponding to solar maximum conditions. A Gaussian line shape with a 1/e half-width of 73 mÅ (or 122 mÅ FWHM) [Maloy et al., 1978; McConnell et al., 1981] is used, and is compatible with the analyses of Chassefiere et al. [1988], Bush and Chakrabarti [1995] and Krasnopolsky and Gladstone [1996]. The reader is referred to Parkinson et al., [1998] for a comprehensive discussion regarding He 584 Å solar flux and line shape. We apply a resonance scattering model that uses the Feautrier technique to solve the equation of radiative transfer assuming partial frequency redistribution [Gladstone, 1982].

The H2 and He densities used in the resonance scattering model are calculated by a one- dimensional photochemical model for the polar atmosphere of Jupiter [see Wong et al., 2003]. Both neutral and ion reactions are included in this model. The temperature profile and auroral ion production rates are taken from the Jovian auroral thermal model of

-2 -1 Grodent et al. [2001] in which the energy flux is 30.5 ergs cm s . Kz is adjustable in this study. For a nominal Kz, we use the expression similar to one suggested by Atreya et al. [1981] and in Gladstone et al. [1996]:

2 -1 6 13 0.65 Kz (cm s ) = (1.46  10 )  (1.4  10 / nt(z)) above 100 mbar level, and 103 below 100 mbar, where nt(z) is the total number density at altitude z. We consider four cases of enhanced eddy diffusion profiles, given by Kz, where  = 1, 3, 10 and 30. The homopause pressure

-3 5 2 -1 -4 6 2 -1 level and Kh for each  are: (1.2  10 mb, 9  10 cm s ), (3.0  10 mb, 9  10 cm s ), (2.6  10-5 mb, 2  108 cm2 s-1), and (2.7  10-6 mb, 3  109 cm2 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°. Figure 4 shows the calculated density profiles of key species at 65° with  1, 3, 10, and 30. As expected, for models with higher , the mixing ratios of He and all hydrocarbons increase significantly above the homopause. In the upper region of the model atmosphere, the increase in He abundance is five orders of magnitude between the cases of 1Kz and 30Kz! Indeed, Wong et al. [2003] have found that a  of ~15 is necessary to match the production of benzene and hydrocarbon aerosols with relevant observations (see Wong et al., 2003 and references therein).

4. Results and Discussion

Figure 5 shows a plot of auroral volume emission rate versus altitude for He 584 Å. The peak intensity of the emission layer occurs at 1400 km with a layer width of about 400 km. The density of helium is 104 cm-3 at 1400 km with a column density of 1011 cm-2. Integrated over the secondary electron distribution, the electron flux is 3 x 109 cm-2 s-1 at 1400 km and the He 584 Å emission cross section at 100 eV is 8 x 10-18 cm-2. Using the Grodent et al. [2001] two stream model atmosphere, we estimate the column emission intensity of He 584 Å due to electron precipitation to be negligible at less than 0.1 R. We compare calculated He 584 Å emission intensities of various values of  with measured intensity, and the results are shown in Figures 6. In Figure 6(a), the calculated He 584 Å intensity is plotted as a function of latitude for each value of . The intensity at 60° is much greater than those at higher latitudes, showing a He 584 Å emission oval zone. Figure 6(b) displays the same result from a different perspective. The calculated He 584 Å intensity is plotted as a function of  for latitudes 60°, 65°, and 70°. The UVIS He 584 Å measurement range is shown in each figure. When comparing the model results with the measurement using Figure 6(b), one can determine the possible range for . For example,  at 60° is between 6 and greater than 30 in order to reproduce the observed He 584 Å emission intensity in the range of 3 and 15 R. Similarly, (65°) is in the range of 7.5 to more than 30 and (75°) is in the range of 15 to more than 30. These results derived from the present study for He 584 Å airglow are consistent with that of the previous chemical study for aerosol production [Wong et al., 2003]. The major uncertainty in our analysis is in the Cassini/UVIS airglow measurement. Differences in the solar angle and gravity can also result in uncertainties, but to a lesser extent. Despite this, our investigation clearly shows that the mechanical mixing in the polar mesosphere of Jupiter is in the range of 6 to more than 30 times of the nominal eddy mixing profile Kz. More observations are needed to confirm this conjecture. More realistic modeling than a simple 1-D model with eddy diffusion is required. We hope this work provides motivation for the development of 2-D and 3-D models for the upper atmosphere of Jupiter and other giant planets.

Acknowledgements This work was supported by NASA Grant NAG5-6263 and the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement No. CAN-00-OSS-01 and issued through the Office of Space Science. A.I.F.S. acknowledges support from the National Aeronautics and Space Administration through JPL Contract 961196, and also the great and successful efforts of the UVIS team. ASW thanks Sushil Atreya for discussion and valuable comments. The authors thank editor Julie Moses and reviewers Jack McConnell and Mike Summers for their throughout and thoughtful review and comments. References

Atreya, S. K., T. M. Donahue, and M. C. Festou, Jupiter: structure and composition of the upper atmosphere (theory), Astrophys. J. 247, L43–L47, 1981. Broadfoot, A. L., B. R. Sandel, D. E. Shemansky, J. C. McConnell, G. R. Smith, J. B. Holberg, S. K. Atreya, T. M. Donahue, D. F. Strobel, and J. L. Bertaux, Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter, J. Geophys. Res., 86, 8259, 1981. Bush, B, C., and S. Chakrabarti, Analysis of Lyman  and He I 584-Å airglow measurements using a spherical radiative transfer model, J. Geophys. Res., 100, 19,609-19,625, 1995. Carlson, R. W., and D. L. Judge, Pioneer 10 ultraviolet photometer observations at Jupiter encounter, J. Geophys. Res., 79, 3623-3633, 1974. Carlson, R. W., and D. L. Judge, Pioneer 10 ultraviolet photometer observations at Jupiter: The helium to hydrogen ratio, In Jupiter, edited by T. Gehrels, pp. 418-440, University of Arizona Press, Tucson, 1976. Chassefieere, E., F. Dalaudier, and J.-L. Bertaux, Estimate of interstellar helium parameters from Prognoz 6 and Voyager 1/2. EUV resonance glow measurements taking into account a possible redshift in the solar line profile, Astron. Astrophys., 210, 113-- 122, 1988. Esposito, L. W., J. E. Colwell, and W. E. McClintock, Cassini UVIS observations of Saturn’s rings, Planet. Space Sci., 46, 1221-1235, 1998. Esposito, L. W., C. A. Barth, J. E. Colwell, G. M. Lawrence, W. E. McClintock, A. I. F. Stewart, H. U. Keller, A. Korth, H. Lauche, M. C. Festou, A. L. Lane, C. J. Hansen, J. N. Maki, H. Jahn, R. Reulke, K. Warlich, D. E. Shemansky, and Y. L. Yung, The Cassini Ultraviolet Imaging Investigation, Space Science Reviews, in press, 2003 Festou, M. C., S. K. Atreya, T. M. Donahue, B. R. Sandel, D. E. Shemansky, and A. L. Broadfoot, Composition and thermal profiles of the Jovian upper atmosphere determined by the Voyager Ultraviolet Stellar Occultation Experiment, J. Geophys. Res., 86, 5715, 1981. Gladstone, G. R., Radiative transfer with partial frequency redistribution in inhomogeneous atmospheres: application to the Jovian aurora, J. Quant. Spect. Rad. Trans., 27, 545- 556, 1982. Gladstone, G. R., M. Allen, and Y. L. Yung, Hydrocarbon photochemistry in the upper atmosphere of Jupiter, Icarus, 119, 1–52, 1996. Grodent, D., J. H. Waite Jr., and J.-C. Gerard, A self-consistent model of the Jovian auroral thermal structure, J. Geophys. Res. 106, 12933–12952, 2001. Gautier, D., and T. Owen, The composition of outer planets atmospheres, in Origin and Evolution of Planetary and Satellite Atmospheres. ed. J. Pollack and M. S. Matthews, Tuscon: University of Arizona Press, p. 487, 1989. Gurnett, D. A., W. S. Kurth, G. B. Hospodarsky, A. M. Persoon, P. Zarka, A Lecacheux, S. J. Bolton, M. D. Desch, W. M. Farrell, M. L. Kaiser, H.-P. Ladreiter, H. O. Rucker, P. Galopeau, P. Louarn, D. T. Young, W. R. Pryor, and M. K. Dougherty, Control of Jupiter’s radio emission and aurorae by the solar wind, Nature, 415, 985-987, 2002. Krasnopolsky, V. A., and G. R. Gladstone, Helium on Mars: EUVE and PHOBOS data and implications for Mars’ evolution, J. Geophys. Res., 101, 15765-15772, 1996. Lindzen, R. S., Tides and gravity in the upper atmosphere, In Mesospheric models and related experiments, edited by G. Fiocco, Reidel, Dordrecht, Holland, p. 122-130, 1971. Maloy, J. O., R. W. Carlson, U. G. Hartmann, and D. L. Judge, Measurement of the profile and intensity of the solar He I 584 Å resonance line, J. Geophys. Res., 83, 5685- 5690, 1978. McConnell, J. C., B. R. Sandel, and A. L. Broadfoot, Voyager UV Spectrometer observations of the He 584 Å dayglow at Jupiter, Planet. Space Sci., 29, 283-292, 1981. Niemann, H. B., S.K. Atreya, G. R. Carignan, T. M. Donahue, J. A. Haberman, D. N. Harpold, R. E. Hartle, D. M. Hunten, W. T. Kasprzak, P. R. Mahaffy, T. C. Owen, N. W. Spencer, S. H. Way, The Galileo Probe Mass Spectrometer: composition of Jupiter's Atmosphere, Science, 272, 849--851, 1996. Parkinson, C.D., E. Griffioen, J.C. McConnell, G.R. Gladstone, and B.R. Sandel, He 584 Å Dayglow at Saturn: A Reassessment, Icarus, 133, 210--220, 1998 Vervack, R. J., B. R. Sandel, G. R. Gladstone, J. C. McConnell, and C. D. Parkinson, A re- evaluation of the Jovian He 584 Å emissions, Icarus, 114, 163-173, 1995. Wallace, L., and D. M. Hunten, The Lyman-alpha albedo of Jupiter, Astrophys. J., 182, 1013-1031, 1973. Wong, A. S., Y. L. Yung, and A. J. Friedson, Benzene and haze formation in the polar atmosphere of Jupiter, Geophys. Res. Lett., 30, 1447, 2003. Yelle, R., L. A. Young, R. J. Vervack Jr., R. Young, L. Pfister, and B. R. Sandel, Structure of Jupiter’s upper atmosphere: predictions for Galileo, J. Geophys. Res., 101, 2149, 1996. Yung, Y. L., and D. F. Strobel, Hydrocarbon photochemistry and Lyman Alpha albedo of Jupiter, Astrophys. J., 239, 395-402, 1980.

Fig. 1. (Upper) Brightnesses derived from the instrument count rates averaged over 62 complete or almost complete Jovian rotations. Points marked with squares refer to the He

584 Å signal; the histogram represents the H2 EUV band signal (airglow and aurora). The

H2 signal has been divided by 100. The two prominent spikes in the H2 band signal coincide with solar wind shocks (see text). (Lower) Correlation diagram for the two signals. The correlation coefficients are too small to infer a strong connection, although coincidentally the best linear fit to all 62 points is essentially identical to the fit with the two “spike” points omitted.

Fig. 2. (Upper) - The EUV spectrum (570-770A) spectrum of Jupiter for three zones: south and north aurorae and equatorial airglow. Most of the emissions arise from ions in the Io plasma torus, specifically S II, S III, and S IV. The HeI 584 Å feature is discussed in the text. (Lower) - The same spectra (925-1125 Å). Apart from HI Lyman-beta at 1026 Å, the emissions are the Lyman bands of H2. The equatorial airglow is noticably weaker than the auroral-zone emissions.

Fig. 3. The Jovian EUV spectrum observed from 245 Rj, with spatial resolution of about

0.25 Rj. (Upper panel) - signal summed between 560 and 1170 Å. The dominant emissions are the Lyman bands of H2, excited by dayglow photoelectrons in the airglow and by precipitated electrons in the auroral zones. (Lower panel) - signal summed over 6 Å centered at 584 Å and scaled upward by a factor of 1000. See text for a discussion of the latitudinal variations.

Fig. 4: For eddy diffusion enhancement factor  of 1, 3, 10 and 30, the four plots show the mixing ratios of He (dashed line), CH4 (solid line), C2H2 (dot-dashed line), C2H4 (dash-dot- dot dotted line) and C2H6 (dotted line), versus pressure, calculated using a Jovian polar chemical model at latitudes of 65°.

Fig. 5: He 584 Å auroral volume emission rate as a function of altitude. Fig. 6: (a) Calculated He 584 auroral volume emission rate versus altitude for He 584 Å emission intensity on Jupiter as a function of latitude for each value of eddy diffusion enhancement factor . (b) Calculated He 584 Å emission intensity on Jupiter as a function of  for different latitudes. The calculations are for a subsolar source using the standard temperature profile, standard value for fHe and the model atmosphere generated using the standard eddy diffusion profile. On the right hand side of each diagram, we show the range of emergent He 584 Å intensities taken from the Cassini/UVIS experiment observations.