Slide 1

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. Bostrm, 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 fr Kernphysik, Heidelberg, Germany Slide 2 Inside Saturn Rings Surfaces of icy moons Enceladus plumes Magnetosphere Water products (O, H 2, OH, ) Water in the Saturn System Slide 3 Dust detector (CDA) E-ring Slide 4 Slide 5 Wahlund et al., GRL, 2005 Ring plasma torus 10 - 150 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 Slide 6 CDA Dust Particle Charging Kempf et al., PSS, 2006 Slide 7 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 Slide 8 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 Slide 9 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 100-200 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!? Slide 10 Slide 11 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!? Slide 12 n/n Interferometry 2005.10.30 00:40 - 02:00 UT 4.7 - 5.0 Rs from Saturn (outbound, inner magnetosphere) 1000 - 1500 km from Equatorial plane Use two 10 m RPWS antenna elements + LP in current sampling mode up to 7 ksamples/s [ n/n component] Slide 13 Jacobsen & Pedersen ViVi NeNe TeTe N e 60-70 cm -3 agree with f UH T e 2 eV U sc -5 V V i 15-20 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 ) Slide 14 N e from f uh f uh 70-75 kHz 61-70 cm -3 Slide 15 Plasma Speed from Interferometry Phase: Phase Dispersion: Doppler: Equating: Plasma inhom.: d k, vsk, vs f Slide 16 PSD Few emissions Chorus ~ 1-1.5 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 Slide 17 Co-Rotation Flow S/C LP E- E+ LP E-/+ Slide 18 Slide 19 E- vs E+ phase 512 fft, 32 averages All 13 such averages 2 n/n-signature slopes! 42-55 km/s ( sd = 0 assumed) Co-rot: 46-48 km/s CAPS happy! 12-14 km/s ( sd = 0 assumed) Keplerian: 11.5 km/s LP happy! E-field n/n filter Chorus Phase Frequency [Hz] 0 Hz1 kHz Slide 20 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+ Slide 21 14.5 +13.9 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+ Slide 22 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+ Slide 23 14.5 -3.5 cos sd 11 km/s Waveform (1024 points snapshot) Cross-correlation in time LP/E- LP/E+ E-/E+ Slide 24 Co-Rotation 45-50 km/s S/C 14-15 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 Slide 25 Slide 26 Dust-Plasma Coupling? Dust Charge from U sc : -2 to -5 V q dust ~ 700 e/Volt 2000-4000 e/dust CDA, q dust ~ 0.5 - 5 fC = 3000 - 30000 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 ) ~7. 10 -14 CDA observed > 2 m dust ~3. 10 -12 assuming n d ~ 20 m -3, 0.7 m dust en i v co-rot B ~2. 10 -13 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) + -10000e + + + + + + + ++ + + + + + + + + + + + + ~ 2 D 2 m - - - - - - - - - - - - Slide 27 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~2. 10 -13 n i m i g~2. 10 -18