ilkka sillanpää, d. young, f. crary (southwest research institute, usa) m. thomsen (los alamos...

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)

Titan Flyby T15 – Hybrid Model and Cassini

Multi-instrument Comparison

Magnetospheres of Outer Planets, Boston 11-15 July 2011

OverviewA 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

1. Titan and its plasma environment2. Plasma conditions during flyby T153. HYB-Titan model4. Simulation results5. Data comparisons6. Conclusions

Sillanpää Magnetospheres of Outer Planets, Boston 11-15 July 2011

Titan in Saturn’s Magnetosphere

Schematic of Saturn’s magnetosphere

Titan

Sillanpää UHA/ARK seminar, Helsinki, 1 April 2011 3/

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

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

Magnetic Field ObservationsDuring T15 Titan was mostly in the southern magnetospheric lobe.

Blue area marks Titan’s interaction area.Pink areas are current sheet crossings.

Upstream field:B = [ 1.32 ± 0.7,

3.88 ± 0.34, -1.46 ± 0.7] nT

|B| = 4.47 ± 0.23 nT

Against corot.

To Saturn

Northward

Abs.


Numerical Moments for IonsSpacecraft 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/sFlow is 17º outward from Saturnand 15º southward

n

vth

vφ

vr

|v|,vθ ,

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 boundariesTitan or the obstacle (internal)ionization, neutral exosphere, structure and composition of ionosphere, boundary conditionsPlasma flow (external) magnetic field, composition, density and velocities for ions

Hybrid Simulation Model

Hybrid Simulations

Run n(O+) pdyn (O+ fraction)

total ion density (O+ fraction)

Mass density (O + fraction)

1 0.003 cm−3 10.8 eV/cm3 (27%)

0.103 cm−3

(2.9%)0.179 amu/cm3 (27%)

2 0.008 cm−3 15.7 eV/cm3 (49%)

0.108 cm−3 (7.4%)

0.259 amu/cm3 (49%)

3 0.014 cm−3 21.5 eV/cm3 (63%)

0.114 cm−3 (12%) 0.355 amu/cm3 (63%)

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.


SimulationsMost obvious differences between the three runs are found in

the extent of the ionotail. CH4+ densities shown.

XY plane

XZ plane

Run 2 Run 3Run 1

North

Flow

Flow

Saturn

Run 3

Comparisons – Plasma Densities

Run 2

Run 1

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.

Comparison of Langmuir Probe electron density, plasma densities from simulation runs 2 and 3 and the total of the CAPS

numerical density moments (INUM).

Comparisons – Ion Energies

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+ and H2

+ (at 50 – 500 eV).

Slightly elevated energies in Titan’s shadow


Run 3

Comparisons – Ion EnergiesRun 2Run 1

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.


Why no upstream ions were seen in the wake?

O+ streamlines in the wake projected onto the XZ plane.(sim. coordinates – X against the used flow direction)

Run 2Run 1 Run 3


CAPS field of view


There was bending in the orbital plane!

O+ streamlines in the wake projected onto the XY plane.(sim. coordinates – X against the used flow direction)

Run 2Run 1 Run 3


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


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


Upstream Magnetic Field

B = [1.32 ± 0.7, 3.88 ± 0.34, -1.46 ± 0.7] nT|B| = 4.47 ± 0.23 nT

Run 2Run 1 Run 3

Comparisons extra – Magnetic Fields

Fundamental Equations:Position and velocities of particles

MovementMovement (1)(1)

Lorentz’s forceLorentz’s force (2)(2)

Maxwell’s equationsAmpère’s circuital lawAmpère’s circuital law (3)(3)

Faraday’s law of inductionFaraday’s law of induction (4)(4)

OthersOthersOhm’s law Ohm’s law (5)(5)

Definition of electric current Definition of electric current jj (6)(6)

Quasi-neutralityQuasi-neutrality (7)(7)

dtvxd

Bj 10

eeiii UneUnqj .

iie nqen 1

)(e

ee en

pBUEj

31 )( rrdtGMBvEqdtmvd P

EdtBd

Hybrid Plasma Model

Input Parameter SpaceFinding the optimal parameters is a tedious task; the discrete parameter space gets soon very daunting.

(number of simulations: ~(3 or 4) #parameters 38 =6561)

5 cases15 cases60 cases

Solutions: 1. 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)

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.

ilkka sillanpää, d. young, f. crary (southwest research institute, usa) m. thomsen (los alamos...

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