Chapter 13 Titan’s Magnetospheric and Plasma Environment

J-E. Wahlund, C. Bertucci, A. Coates & R. Modolo

St. Jacut, June 20-23, 2011

Outline of Chapter 13

! Forms ! Induced magnetosphere ! Boundaries ! Wake structure

!  ! Cause ionization ! Affects ionosphere structure ! Triggers ionospheric chemistry ! Cause ionosphere dynamics ! Provides O+ to atmosphere (from Enceladus) ! Generates current systems ! Cause heating (Ti, Te, Tn) ! Forms acceleration structures ! Generates Alfvén & plasma waves near boundaries, energy ! Cause bulk plasma escape ! Cause ion pick-up escape ! …

Cold dense fluid-like plasma

13.1 Introduction

13.2 Upstream conditions !  Saturn magnetospheric regions/plasma

regimes encountered by Titan –  Magnetospheric variations

!  Magnetosheath, Lobe-like, Plasma sheet, Bimodal !  Magnetosphere quasi-periodicity (10.7h) !  Upstream waves !  Sub-sonic, super-Alfvénic, high beta flow !  No bow shock, No Venus-like ionopause

–  Solar variations (EUV, X, c.r.)

–  Interaction will depend on orbit position (SLT), SZA and magnetospheric ram angle & magnetospheric & solar conditions

N2+ ProductionParabolic, Magnetospheric Electrons only

700

1200

1700

2200

2700

0,0001 0,001 0,01 0,1 1 10Production Rates [cm-3 s-1]

Alti

tude

[km

]

25 eV 50 eV 100 eV

200 eV

0

1000

2000

3000

4000

5000

-400 -300 -200 -100 0 100 200 300

Ele

ctro

n de

nsity

(cm

-3)

58.9o

1841 65.8o

1562 73.6o

1350 82.2o

1218 91.1o

1174 100.1o

1222 108.6o

1357 116.4o

1572

TimeSZAAltitude

RPWS

Modelsolar only

ModelBoth

Solar Photons Only

750

950

1150

1350

1550

1750

1950

100 1000 10000Density [cm-3]

Alt

itud

e [k

m]

t = -400t = -200t = -100t = 0t = +70t = +130t = +200

t = -400

Wahlund et al., 2005 Cravens et al., 2005

See more in ionosphere chapter

13.2 Ionization by Solar EUV/X & Magnetospheric particle impacts

RPWS/LP Ion Densities [cm-3]

SZA

Alti

tude

[km

]

2000

800 0 180

0

4000

Dayside Solar EUV 2500 - 3500 cm-3

Nightside Magnetospheric conditions 400 - 1000 cm-3

1000

1200

Confirms results by Ågren et al., 2009 for Ne

Electron (when negative ions present) Wahlund et al., in prep. 2011

Complex ionospheric structure Magnetospheric variation in particle precipitation Induced dynamics

Characterizing the magnetospheric upstream conditions is important!

Chemistry – Transport control boundary

Upstream Plasma density (Ne)

Rev20

Equatorial plane (|Z|<0.5 RS) Morooka et al., 2009 Map of magnetospheric regions

2 orders of magnitude variations in ne SKR-periodic ~ planetary rotation (10.7 h)

Titan position

Magnetosheath

Plasma sheet

Lobes (low Ne)

Upstream Electron distributions Rymer et al., 2009

Plasma sheet, T13 Lobe-like, T8

Magnetosheat T32

Bi-modal, T31

Upstream Ion distributions Nemeth et al., 2011

Plasma sheet

Lobe-like

Magnetosheath

Bi-modal (heavy ions)

Kliore et al., 2008

Magnetic field strength/orientation Stretch angle Str = arctan(By/Bz)

Sweepback angle Swe = arctan(-Bx/By)

Bertucci et al., 2009 Simon et al., 2010

B most often distorted from dipole configuration.

Plasma sheet variations

After Simon et al., 2010

Convection Electric field & Ion Composition

Arridge et al., Space Sci Rev., 2011

Garnier et al., 2010 (for ENA/energetic e-)

Heavy & energetic ions Enhance the pressure.

Upstream classification

After Nemeth et al., 2011 After Simon et al., 2010

Upstream conditions is rather well determined for each Titan flyby.

13.3 Titan’s Induced Magnetosphere

after M. Blanc

Cravens et al., 1998

Bertucci et al., 2009

Magnetic pileup boundary

Bertucci et al., 2009

T5

MLB! MLB!

H = 320±50 km! Ionosphere!

Exo-Ionosphere![Ågrén et al., 2007]

Exo-ionosphere of cold ionospheric plasma

Ågren et al., 2007

Compared to Mars w. a bow shock/MP far from ionosphere Titan magnetospheric interaction occurs close to the ionosphere itself!

MHD modeling of pressure balance Ulusen et al., 2010 Magnetospheric forcing > thermal pressure at Times => ion-neutral collision drag important

Pressure balance Cravens et al., 2010

Pressure balance depend on magnetospheric conditions & SZA-ram angle

Time constants Cravens et al. 2010

Transport-chemistry boundary near 1200 km Transport time scales < magnetic diffusion time scales

Titan has an extended cold plasma region

•  Dense cold plasma –  Extend several Rtitan –  Not hot pick-up ions –  Transport dominated

•  Major region for plasma escape –  ~ few 1025 ions/s

Night heating

Edberg et al., 2010

Wake/night density variations (T15)

Simulations put the observations in a global context

simulation

CAPS-ions Modolo et al., in prep. 2011

Stable draping configuration > 1800 km?

Ulusen et al., submitted 2011 MHD modeling

Transient events Wei et al., 2011

MP crossing 3h before Enhanced SW dynamic pressure Compressed magnetosphere effect

Strong fossil field left!? Shielding currents in conductive ionosphere?

B = 37 nT

Wave induced Plasma Escape [Dobe & Szego, JGR, 2005]

•  Solar wind driven ion acoustic (lower hybrid) wave generation cause “enhanced drag” of cold ionospheric ions

T9 Plasma waves (ion acoustic like)

Canu et al., in prep

T11 •  Possible ion acoustic like

emissions below 100 Hz •  Cs~ 7 km/s

–  Beam driven/ion-ion instab?

f [Hz]!"

[de

g]!

E- vs E+!

Wahlund et al., in prep

13.4 Modeling of Titan’s interaction with the external plasma environment ! MHD codes

–  Keller et al., 1994; Kabin et al., 1999; Ledvina & Cravens, 1998; Ma et al., 2004; 2006; 2007; 2009; Ulusen et al., 2010; submitted 2011;

!  Hybrid codes –  Backes et al., 2005; Sillanpää et al., 2006; Simon et al., 2006; Kallio et al., 2007;

Modolo et al., 2007; Modolo & Chanteur, 2008; Ledvina et al., submitted 2011;

!  Wish-list for future –  Include ionosphere (altitude profiles of species)

–  Include electro-dynamic to ionosphere (conductive)

–  Include transient magnetospheric variations

!  See talks by Ulusen, Ledvina, Modolo tomorrow!

Inclusion of ionosphere Altitude distribution of ion species – affects ion escape

Conductive region – affects electro-dynamic coupling

C2H5+ Ne HCNH+

From Ledvina et al., 2011

|E| |B|

13.5 Electro-dynamic coupling !  Induced magnetosphere electro-dynamics

!  Voyager & Cassini results Ness et al., 1982

Backes et al., 2005

Lobe structure & tail currents in neutral sheet

Neubauer et al., 2006

Titan Dynamo Region Rosenqvist et al.,

–  Dynamo region: 1000-1450 km (near exobase)

•  #Pedersen ! 0.002-0.05 S/m, Double peaked!

•  #Hall ! 0.01-0.3 S/m

–  Depend strongly on B(z, draping config.)

–  $Pedersen ! 1300 - 22000 S ! 4-100%$Alfvén

Mars conductivity profiles (MARSIS)

Opgenoorth et al., 2010

Ionospheric currents & electric fields Ågren et al., 2011

Cause of current system?

A cross tail electric field set up in the wake of Titan could lead to a cross tail current that would close over the outer surface of the tail, and interact with the ionosphere via field aligned currents

A current system generated by shielding currents may set up magnetic fields that are oppositely directed to the upstream field around Titan, which is what is observed

Alfvén wave induced currents

Neutral winds in the ionosphere

Diffusion of fossil fields lines

13.6 Plasma escape from Titan !  Voyager

–  Gurnett et al., 1982

–  Simple pressure balance in tail => few 1024 particles/s (large error) Cassini

Topside ionosphere escape 2.1025 s-1 Cui et al., 2010

Ionospheric Plasma Escape from Titan (T9) !  Differential outflow processes

–  Light ions

!  Magn. ionization side !  charge-exchange

–  Heavies

!  Solar ionization side !  Convection E-fields !  Alfvén wings?

!  Occurs during enhanced magnetospheric plasma outflow event

!  Plasma erosion:

–  Heavies: ~4%1025 ions/s

–  Light ions: ~2%1025 ions/s

[Modolo et al., GRL, 2007a & b]

Edberg et al., 2011

Stationary ionospheric fluxtube outflow

Possible sources: Enhanced ambipolar diffusion by atm-photo-e- Magnetic moment pumping Alfvén wave induced transport

T9 cold plasma outflows

Bertucci et al., Coates et al., Wei et al., Modolo et al., 2007

Energy/nucleon for N2+

x= -2, 0,+2,+4,+6 RT energy range 0-60eV

Modolo & Chanteur, 2008

Ion pick-up region beside the cold plasma outflow region

13.7 Future outlook !  Upstream plasma reasonably known by now

!  Titan interaction with its space environment is highly dynamic –  Need more focus on electro-dynamics –  Need include the conductive ionosphere in this interaction

!  Titan magnetospheric interaction has effects in upper atmosphere –  Need better understanding of atmospheric effects (e.g., organics & haze

formation)

Wahlund et al., 2010

•  Cold plasma structures in Titan’s wake/exo-ionosphere? (T15) –  Venus like? –  Ne drop of an order of magnitude

Density depletions in wake/nightside

Modolo et al., in prep. 2011

Ex: Conductivity profiles •  MAG ! B •  Neutral Atmosphere model

[Müller-Wodarg et al., JGR, 2008]

$p ! 1300-2200 S ! 4-100x $A

Using RSS ne below 950 km [Kliore et al., 2008]

Double Pedersen Conductivity profles!

Conductivies •  [E.g., Boström et al., 1964] •  Collisions:

–  &in = 2.6.10-9 nn ('0/µA) •  '0 is atomic polarizability (in 10-24 cm-3) •  1.76 for N2 •  2.59 for CH4

•  [Banks & Kockarts, 1973] –  &en = 5.4.10-10 nn"Te[K]

•  [Kelley, 1989] •  Using Te = 0.1 eV

Titan Conductivity statistics

50% of observations

All 34 observations