Particle Precipitation into the Jovian and Saturnian Ionospheres Particle Precipitation into the Jovian and Saturnian Ionospheres #P11B-1216#P11B-1216

Christopher D. Parkinson1, Michael Liemohn1, Xiaohua Fang2, Stephen Bougher1, and Yuk Yung31AOSS Dept., University of Michigan, Ann Arbor, MI; 2LASP, University of Colorado, Boulder, CO, 3GPS, Caltech, Pasadena, CA

I. ABSTRACTHigh energy H/H+ ion precipitation into Jupiter and Saturn’s upper atmosphere is modeled and discussed. Such particle transport has previously been modeled for Earth and we have extended this work for the Jovian and Saturnian ionospheres using different cross sections for relevant "background" species. Atmospheric effects of these precipitating hot ions in these atmosphere are studied and reported on. Solar wind protons as well as pick-up ions from the planetary exosphere routinely enter and alter the upper atmosphere. Atmospheric effects of these precipitating hot ions in the Jovian and Saturnian atmospheres is described. A study of the ionization, excitation, and energy deposition is conducted. The result is a robust examination of the influence of energetic ion transport on the upper atmospheres of Jupiter and Saturn..

II. MODEL DESCRIPTION

The 3-D Monte Carlo test particle precipitation model [Parkinson et al., 2008; Fang et al., 2004]:

IV. RESULTS

V. SUMMARY AND CONCLUSION

Giant planetary model atmospheres:

III. DISCUSSION

(1) Higher energy particles have less energy deposition at high altitudes, but more energy deposition at lower altitudes.

(2) The dip angle of the assumed magnetic field lines and the gyration radius have only a small influence on the fluxes and energy deposition rates, with the lower dip angles (more horizontal field lines) should in slightly more deposition at high altitudes and a bit less at low altitudes, but we do not see this in this study.

(3) There are 4 distinct layers for the precipitating particles, as evidenced in the integrated flux plots. There is a high-altitude region of few collisions, a transition layer below this where the ion fluxes are depleted and the fast neutral fluxes increase, a quasi-equilibrium zone with relatively constant fluxes, and finally a low-altitude depletion region where the fluxes rapidly drop to zero.

(4) The peak of ionization and excitation from precipitating solar wind ions is found to be typically around 250-375 km which is lower than the typical EUV ionization peak.

(5) Jupiter and Saturn have a similar scenario for energetic particle precipitation to the Earth because of the dipole nature of the magnetic fields. O+ needs to be considered for the Saturnian system.

Reference for the hydrogen hot ion precipitation model:Parkinson, C. D., M. W. Liemohn, X. Fang, Hydrogen Hot Ion Precipitation in the Martian

Ionosphere, J. Geophys. Res., SWIM Special Issue, in press, 2008.

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Figure 1: Jovian model atmosphere.Figure 2: Ray diagram showing slant path geometries for a dipole magnetic field vs a point source crustal field anomaly

Our approach uses a three-dimensional Monte Carlo test particle trajectory calculation to simulate the motion of precipitating ions through giant planeary upper atmospheres. It follows the guiding center motion, their conversion to and from an energetic neutral hydrogen atom, and their eventual energy deposition to the ionosphere and thermosphere. To handle the 3-D scattering of precipitating particles, the Monte Carlo model used for this work has been previously described for an Earth-specific model by Fang et al. [2004, 2005]. This code was written independently in 3-D, but has been extensively compared with the 1-D model of Solomon [2001]. The Monte Carlo model monitors the trajectories of incident energetic particles in a collision-by-collision manner down to an assigned low-energy cutoff limit. A variety of effects due to inelastic and elastic collisions are accumulated over the course of the particle traveling in a planetary upper atmosphere. Note that this is the initial application of this previously Earth-specific model (Fang et al., 2004) to the Jovian and Saturnian space environments. The code launches particles from a single location on the upper boundary, and then calculates the 3-D spread in the atmosphere. Multiple point source results over a latitude-longitude grid and the results are summed to create a 3-D total precipitation result. This is what was done for the data-model comparisons presented in Fang et al. [2004] and for the auroral arc spreading simulations of Fang et al. [2005]. For the Jovian and Saturnian precipitation calculations, the precipitation is calculated on a coarse grid in longitude and latitude over the day thermosphere with sensitivity studies of key parameters performed, viz., dip angle, gyration radius, and injection energy conditions.

For our calculations, we have adopted a standard reference 1-D profile based on the Caltech KINETICS and U-M JTGCM model atmosphere [S. Bougher, private communication] going from 250 km to approximately 2300 km on 50 km intervals. Key species include H and He in a background atmosphere of H2, as illustrated in Figures 1a and b, corresponding to dayside conditions.

Figure 4Figure 3

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Figures 9, 10, and 11 show the integrated proton (p = H+) and hydrogen fluxes for Jupiter over the range of injection energies, dip angles, and gyration radii considered, respectively. We see from the view of particle precipitation at 10keV that the atmosphere can be divided into 4 regions, viz.,

(1) >1100 km (collisionless): downward H+ fluxes are larger than downward H fluxes; (2) ~800-1100 km (transition layer): downward H+ and H fluxes are comparable; (3) ~350-800 km (quasi-equilibrium zone): the ratio of H+ and H fluxes stays almost invariant. (4) <350 km (depletion zone): the precipitating particle fluxes drop rapidly to zero.

Little change is seen for the last two parameters, but there is a strong dependence of these features with varying injection energies. Our code additionally allows us to determine trajectories of the particles along the magnetic field lines through our model atmospheres. Giant planets have a similar scenario for energetic particle precipitation as the Earth because of the global nature of the dipole field.. Our results for different dip angles (75 and 90°) were not significantly different. So, as for earth for Earth, such huge internal dipole fields that its cusp region is highly localized with nearly vertical fieldlines. What we have seen generally is that higher energy precipitating particles penetrate deeper into the atmosphere than lower energy precipitating particles, which is expected, and the rates due to ionization are much larger than those for excitation. Results for Saturn are similar to those of Jupiter, with variations occurring due to small differences in composition, difference in number density vs altitude, and differences magnetic field strength. As noted previously, this study presents the initial results from the modification of our precipitation code for Earth for use with the Jovian and Saturnian scenario. This work will be extended further to include O+/O precipitation with the view to keeping track of secondary hot ions/neutrals, which is important for sputtering in the Saturnian upper atmosphere.

Figure 2 shows the geometry of H and H+ in our slant path calculations through a planetary atmosphere with a dipole field versus one without. Each proton moves along the field line for a given dip angle (red), starting from a point at the top of the atmosphere. Neutral H follows a scattered ballistic path through the atmosphere (blue). In Figures 3 to 8 illustrate sensitivity studies of the ionization and excitation rates as a function of altitude about our standard reference parameters, viz., dip angle of 90° (vertical field lines), gyration radius of 0.03 km, injection energy of 1 keV, and the dayside model atmosphere. Our results for Jupiter are normalized at the top to a tenth of that for the Earth, viz., 0.1 erg/cm2 and 105 test particles. This is analogous to typical solar wind ions entering the atmosphere for a typical giant planet dipole magnetic source region. Figures 3 and 4 illustrate how the ionization and excitation rates respectively vary with altitude for the injection energies 1 keV, 10 keV, 100 keV, and 1000 keV for the species H2. We see that lower energy particles do not penetrate as deeply into the atmosphere and have much more of their ionization and excitation happening in the upper part of the atmosphere as compared to higher energy injection particles. Figure 5 and Figure 6 respectively show how the ionization and excitation rates vary with altitude as a function of changing dip angle. For the cases examined, the excitation/ionization rate show no dependency upon this parameter. More dip angles of lower value need to be considered to determine if a higher dip angle might imply that since the path length through the atmosphere is shorter, precipitating particles can penetrate and deposit more of their energy deeper in the atmosphere. Figures 7 and 8 show how the ionization and excitation rates vary with altitude for sample gyration radii. A comparison of each scenario shows no dependency on this parameter. Clearly, hot hydrogen ions are precipitating to approximately to 300 km or below where the excitation and ionization rates are falling rapidly to zero and to altitude levels lower than EUV photons can penetrate. Peak rate values are occurring at approximately 250 - 375 km. Hence, they (and their associated phenomena) should be more readily detectable below regions where EUV photons no longer dominate.

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