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Chapter 13 Titan’s Magnetospheric and Plasma Environment J-E. Wahlund, C. Bertucci, A. Coates & R. Modolo St. Jacut, June 20-23, 2011
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
