RPWS Cold Plasma Results from the Inner Magnetosphere of Saturn – dust-plasma interaction near the E-ring? J.-E. Wahlund, A. I. Eriksson, M. W. Morooka, G. Gustafsson, R. Boström, R. Modolo Swedish Institute of Space Physics, Uppsala T. F. Averkamp, G. B. Hospodarsky, W. S. Kurth University of Iowa, Iowa City K. S. Jacobsen, A. Pedersen Oslo University, Oslo S. Kempf, R. Srama Max-Planck-Institut für Kernphysik, Heidelberg, Germany

Inside Saturn Rings Surfaces of icy moons Enceladus plumes Magnetosphere Water products (O, H 2, OH, …) Water in the Saturn System

Dust detector (CDA) E-ring

Wahlund et al., GRL, 2005 Ring plasma torus cm -3 < 6Rs T e fr. 0.5 eV (2.2 R S ) to 7 eV (10 R S ) Dust Particle Charging Dust-Plasma Interaction? SOI

CDA Dust Particle Charging Kempf et al., PSS, 2006

Hot ions near Dione/Rhea T i ~ 1-3 keV could explain Water product ions in E-ring plasma torus Magnetospheric Co-rotation Ions at 3-5 R S do not co-rotate Instead < V SC SOI

Estimating V i from a LP sweep 2) DC Level Need n e, I ph 1) Slope Must assume m i U bias < 0: I = I i0 (1 - U bias /T i,eff ) + I ph T i,eff = T i + m i v i 2 /2e [eV] I i0  n i v i, √T i

Co-rotation Plasma Dilemma!? 3-5 R s LP Ram energies < 20 eV Agreement with Keplerian motion of ions together with dust/neutrals Effects of dust-plasma coupling found near Enceladus/Ring plane (RPWS-CDA) T i < few eV CAPS Ram energies near eV Agreement with co-rotation v  B & pick-up of locally produced ions Deflection near Enceladus Inconsistent measurements with regard to ion speed (v i ) of the inner magnetosphere!?

Possible LP sweep error sources Error in used LP theory (2 independent checks) W. Hoegy, NASA, Goddard, Maryland –[Hoegy & Brace, Rev. Sci. Instruments, 1999] K. S. Jacobsen & A. Pedersen, University of Oslo Energetic particle impacts Ion composition Small addition of H + Dirt on probe Negative water-dust Leak current Shock in front of S/C Ion ring distribution Probe in wake Etc … LP seems ok!? so does CAPS!?

 n/n Interferometry : :00 UT Rs from Saturn (outbound, inner magnetosphere) km from Equatorial plane Use two 10 m RPWS antenna elements + LP in current sampling mode up to 7 ksamples/s [  n/n component]

Jacobsen & Pedersen ViVi NeNe TeTe N e ≈ cm -3 agree with f UH T e ≈ 2 eV U sc ≈ -5 V V i ≈ km/s ≈ V sc ~ 20 amu Inner Magnetosphere example SOI results [Wahlund et al., GRL, 2005] Dust charging & U SC [Kempf et al., PSS, 2006] Ex: LP Sweep Analysis (4.7-5 R S )

N e from f uh f uh ≈ kHz  cm -3

Plasma Speed from Interferometry Phase: Phase Dispersion: Doppler: Equating: Plasma inhom.: d k, vsk, vs f 

PSD Few emissions Chorus ~ kHz Broadband emissions < 200 Hz Ion acoustic? Long antenna measurements depend on 1/RC coupling to plasma   n/n below 500 Hz LP noisy, best coherence between antenna elements Reaction wheel interference

Co-Rotation Flow S/C LP E- E+ LP E-/+

E- vs E+ phase 512 fft, 32 averages All 13 such averages 2  n/n-signature slopes! km/s (  sd = 0 assumed) – Co-rot: km/s – CAPS happy! km/s (  sd = 0 assumed) – Keplerian: 11.5 km/s – LP happy!  E-field  n/n  filter Chorus Phase Frequency [Hz] 0 Hz1 kHz

LP vs E-/E+ Plasma inhom. exist in whole frequency range One slope only (slow Keplerian) (V SC -V plasma ) = 3-5. cos  sd km/s No fast component detectable!? Coherence length effect on antenna? LP vs E+ signal mostly incoherent  Antenna measures E-field LP measures  n/n  n/n  LP/E- LP/E+

cos  sd ≈ 28.3 km/s -0.3 ms 3.8 cos  sd km/s +0.9 ms Waveform (1024 points snapshot) Cross-correlation in time LP/E- LP/E+ E-/E+

1.4 cos  sd km/s 2.1 cos  sd km/s +2 ms Waveform (1024 points snapshot) Cross-correlation in time LP/E- LP/E+ E-/E+

cos  sd ≈ 11 km/s Waveform (1024 points snapshot) Cross-correlation in time LP/E- LP/E+ E-/E+

Co-Rotation km/s S/C km/s LP E- E+ Keplerian 12km/s Rest-Frame S/C-Frame CAPS prediction: Look in anti-co-rotation direction Look for ion signatures < 2-3 eV

Dust-Plasma Coupling? Dust Charge from U sc : -2 to -5 V q dust ~ 700 e/Volt  e/dust CDA, q dust ~ fC = e/dust –[Kempf et al., PSS, 2006] Cold ions (T i < 5 eV) will be trapped close to dust particles CDA & RPWS detects 0.1 m -3 for r dust > 2  m (r d -2.8 distribution) Dust drag? n d m d (GM S /r 2 ) ~ CDA observed > 2  m dust ~ assuming n d ~ 20 m -3, 0.7  m dust en i v co-rot  B ~ Conclusions (preliminary): Interferometer results suggests that two ion populations exist –One Co-rotating with magnetic field (45-50 km/s) –One rotating with close to Keplerian speed (11-14 km/s) e ~ 2 D ≈ 2 m

Momentum Transfer n i m i dv i /dt = en i (E+v i  B) + n i m i g + n i  in in (v n -v i ) +  p i + mass load Near Enceladus: en i v co-rot  B~ n i m i g~ << n i  in in (v i -v n )~ << (using INMS n n ~10 5 cm -3 )  p i ~ << mass loading n d m d dv d /dt = q d n d (E+v d  B) + n d m d g + … Magnetospheric conductances [Saur et al., JGR, 2004]

(H + ) W+W+ ~ m k ~ m -1 Dust induced inhom.IA inhom/waves following co-rotation

T11 Possible ion acoustic like emissions below 100 Hz C s ~ 7 km/s Dobe & Szego, JGR, 2005 Beam driven/ion-ion instab? f [Hz]  [deg] E- vs E+

C. C. Porco et al., Science 311, (2006)

RPWS Langmuir Probe results J-E. Wahlund, et al. Results from flybys: February 17 March 9 July 14 Cold dense plasma torus associated with E-ring No Enceladus Ionosphere, but overall enhencement No co-rotation v i < 15 km/s 1-2 c s Dust-plasma interactions

C/A T e = eV U sc = -1.9 to -2.5 V V H2O+ < 8 km/s rel. S/C T H2O+ < 6 eV Richardson, 1995: ~10 eV, water group ions N e = cm -3, SOI: 40 cm -3 Enceladus, July km “Smooth” undisturbed profiles No wake signatures No shock signatures

N e = cm -3 (peak at 90 cm -3 ) Enceladus,March 9 T e = eV U sc = -1.8 to -2.0 V V H2O+ < 10 km/s rel. S/C T H2O+ < 9 eV, water group ions C/A Undisturbed cold plasma < km fr. C/A

Enceladus, Feb. 17 T e = eV U sc = -2.8 to -3.2 V V H2O+ < 5 km/s rel. S/C T H2O+ < 3 eV, water group ions N e = cm -3 C/A Undisturbed cold plasma < km fr. C/A

C/A 2000 km (  z ≈ 800 km) km (  z ≈ 6000 km) Charged dust signatures? LP in S/C wake?

CDA

Dust impacts f UH

Dust Charge Level? Is difference in n e and n i due to dust charge? (n i - n e ) ≈ cm -3 Requires n dust ~ cm -3 - Possible? –CDA detects in same region 0.1 m -3 for r dust > 2  m –RPWS dust counts are similar –Extrapolating dust distribution to smaller sizes –Dust down to 0.07  m (700 Å) necessary for r d -2.8 distribution Do we have dust-plasma interaction?

Charged dust? 0.7 R S C/A

Dust hits f UH

Conclusions Cold Dense Plasma Torus (near Enceladus) n e = cm -3, T e = eV, T i < 3-10 eV Near Keplerian rotation. Ion flow speed < 15 km/s relative S/C Water group ions No (or small) Enceladus ionosphere signature Hotter Co-rotating ions (detected by CAPS) Near km/s (E  B-motion) Dust-plasma interactions (n i - n e )/n i –Mostly below 1% –Near Enceladus E-ring crossing = % In regions where CDA & RPWS detects significant amounts of dust Resolve (sub) co-rotation More dn/n Interferometer measurements More comparisons with CDA dust measurements in E-ring crossings Theory application