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Titan Flyby T15 – Hybrid Model and Cassini Multi-instrument Comparison

Titan Flyby T15 – Hybrid Model and Cassini Multi-instrument Comparison. Ilkka Sillanp ää , D. Young, F. Crary (Southwest Research Institute, USA) M. Thomsen (Los Alamos National Laboratory, USA) D. Reisenfeld (University of Montana, USA)

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Titan Flyby T15 – Hybrid Model and Cassini Multi-instrument Comparison

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  1. Titan Flyby T15 – Hybrid Model and Cassini Multi-instrument Comparison Ilkka Sillanpää, D. Young, F. Crary (Southwest Research Institute, USA) M. Thomsen(Los Alamos National Laboratory, USA) D. Reisenfeld(University of Montana, USA) J-E.Wahlund (Institutet för Rymdfysik, Uppsala, Sweden) C. Bertucci(Instituto de Astronomía y Física del Espacio, Argentina) E. Kallio, R. Jarvinen, P. Janhunen (Ilmatieteen laitos, Helsinki, Finland) Magnetospheres of Outer Planets, Boston 11-15 July 2011

  2. Overview A case study on a Titan flyby by Cassini spacecraft: • Multi-instrument measurements • Plasma simulations of the flyby that use upstream flow conditions from measurements before encounter with Titan • Comparisons of measurements and simulations • Titan and its plasma environment • Plasma conditions during flyby T15 • HYB-Titan model • Simulation results • Data comparisons • Conclusions Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011

  3. Schematic of Saturn’s magnetosphere Titan Titan in Saturn’s Magnetosphere Titan’s orbit is imbedded in Saturn’s magnetospheric plasma flow. This flow consists of hydrogen ions and a varying amount of oxygen ions. T15 Sillanpää UHA/ARK seminar, Helsinki, 1 April 2011 3/

  4. Flyby Geometry • Cassini’s Titan Flyby T15 on July 2, 2006 closest approach at 09:21 UTC • Flyby was through Titan’s wake along orbital plane • Part of the trajectory was in Titan’s shadow shadow Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 4/16

  5. Magnetic Field Observations During T15 Titan was mostly in the southern magnetospheric lobe. Against corot. To Saturn Northward Abs. Upstream field: B = [ 1.32 ± 0.7, 3.88 ± 0.34, -1.46 ± 0.7] nT |B| = 4.47 ± 0.23 nT Blue area marks Titan’s interaction area. Pink areas are current sheet crossings. Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 5/16

  6. Numerical Moments for Ions n vth vφ vr |v|,vθ, Spacecraft rolls affect the moments Marked with rose n(H+)=0.07 cm-3 n(H2+)=0.03 cm-3 n(O+)~ 0.008 cm-3 Ui=[100, -30 -28] km/s |U| = 108 km/s Flow is 17º outward from Saturn and 15º southward

  7. Hybrid Simulation Model Hybrid: kinetic treatment for ions, electrons treated as fluid Quasi-neutrality: ne = e-1qi ni Ions moved by the Lorentz force  drifts, gyroeffects Self-consistent propagation of particle motion and fields Inputs needed for simulation run: General (geography): SLT, subsolar latitude, outer boundaries Titan or the obstacle (internal) ionization, neutral exosphere, structure and composition of ionosphere, boundary conditions Plasma flow(external) magnetic field, composition, density and velocities for ions

  8. Hybrid Simulations Three simulation runs were made with the HYB-Titan model using different O+ densities for the upstream flow. The total ion density varied very little, but the dynamic pressure almost doubled. Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 8/16

  9. Simulations Most obvious differences between the three runs are found in the extent of the ionotail. CH4+ densities shown. Run 1 Run 3 Run 2 XY plane Saturn Flow XZ plane North Flow

  10. Comparisons – Plasma Densities Comparisons show the extend of the interaction region is much more extended along Cassin trajectory with low oxygen density (and lower dynamic pressure) of the magnetospheric flow. Run 1 Run 2 Run 3 Comparison of Langmuir Probe electron density, plasma densities from simulation runs 2 and 3 and the total of the CAPS numerical density moments (INUM).

  11. Comparisons – Ion Energies Slightly elevated energies in Titan’s shadow CAPS time-of-flight data shows the low-energy ions as H+, H2+ and CH4+; also indication of ~29 amu (N2+) from 09:00 to 09:45 UTC. In the ingress there are two energy peaks: higher energies are O+ (~1000 eV) and the lower both H+ andH2+ (at 50 – 500 eV). Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 11/16

  12. Comparisons – Ion Energies Run 2 Run 1 Run 3 Low-energy region begins very early in run 1. The drop in energies is sharp in runs 2 and 3, as it is in the CAPS ion energy observations. The end of the low energies is abrupt for run 1; for runs 2 and 3 it is less cleary definable. Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 12/16

  13. Why no upstream ions were seen in the wake? Run 2 Run 1 Run 3 O+ streamlines in the wake projected onto the XZ plane. (sim. coordinates – X against the used flow direction) Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 13/16

  14. CAPS field of view Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 14/16

  15. There was bending in the orbital plane! Run 2 Run 1 Run 3 O+ streamlines in the wake projected onto the XY plane. (sim. coordinates – X against the used flow direction) Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 15/16

  16. Conclusions part 1 • Cassini measurements gave upstream conditions for the flyby • The most important uncertainties in conditions were spaned by three simulation runs • Simulation results corresponded to the data to a large degree. • The extent of Titan’s interaction region along the flyby trajectory varied significantly between the simulation runs • The best fit gave an estimate of the oxygen density in the upstream flow • The 3D structure of Titan’s wake was seen in the simulations and explained the flyby observations • Disappearance of the flow ions in the wake in the data gave a reason to investigate the trajectories of the ions in the wake: • CAPS field of view and simulation results explained the discrepancy between data and simulations Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 16/16

  17. Overall Conclusions • The extent of Titan’s wake and ionotail are greatly influenced by the oxygen content in the flow • Multi-instrument analysis yields a comprehensive and multi-faceted picture of the plasma dynamics otherwise unobtainable • Global hybrid model provides insights into the physics and processes at Titan that observations cannot directly address: • Tail’s 3d structure • Magnetic field around Titan • Specific information on all ion species (densities, energies, total fluxes) • Paper on this study is in press at The Journal of Geophysical Research Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011 17/16

  18. Upstream Magnetic Field B = [1.32 ± 0.7, 3.88 ± 0.34, -1.46 ± 0.7] nT |B| = 4.47 ± 0.23 nT

  19. Comparisons extra – Magnetic Fields Run 2 Run 1 Run 3

  20. Hybrid Plasma Model Fundamental Equations: Position and velocities of particles Movement (1) Lorentz’s force (2) Maxwell’s equations Ampère’s circuital law(3) Faraday’s law of induction (4) Others Ohm’s law (5) Definition of electric current j(6) Quasi-neutrality (7)

  21. Input Parameter Space Finding the optimal parameters is a tedious task; the discrete parameter space gets soon very daunting. (number of simulations: ~(3 or 4) #parameters 38 =6561) • Solutions: • Rely solely on the ‘Best Estimate’ of parameters or moments (number of simulation runs: 1) • 2. Change one parameter at a time to find new Best Fits (tree) • (# parameters x few) • 3.Focus on key parameters, reduce the number of parameters you are going to vary to 1 or 2 – and do a full study of them • (3, 5 or 9 runs) 5 cases 15 cases 60 cases

  22. Titan – Saturn’s Unique Moon Titan is optically the largest satellite in the Solar System. While it does not have intrinsic magnetic field it has a very dense nitrogen atmosphere (1.5 bar) and also very extensive exosphere. Also only other place in the universe that we know of that has lakes and rivers currently. They are methane, however; the surface temperature is 95 K.

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