1 / 46

J.-E. Wahlund, A. I. Eriksson, M. W. Morooka, G. Gustafsson, R. Boström, R. Modolo

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

harvey
Download Presentation

J.-E. Wahlund, A. I. Eriksson, M. W. Morooka, G. Gustafsson, R. Boström, R. Modolo

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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. 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

  2. Water in the Saturn System • Inside Saturn • Rings • Surfaces of icy moons • Enceladus plumes • Magnetosphere • Water products (O, H2, OH, …)

  3. Dust detector (CDA) E-ring

  4. SOI • Wahlund et al., GRL, 2005 • Ring plasma torus • 10 - 150 cm-3 < 6Rs • Te fr. 0.5 eV (2.2 RS) • to 7 eV (10 RS) • Dust Particle Charging • Dust-Plasma Interaction?

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

  6. SOI Hot ions near Dione/Rhea Ti ~ 1-3 keV could explain Magnetospheric Co-rotation Ions at 3-5 RS do not co-rotate Instead < VSC Water product ions in E-ring plasma torus

  7. Estimating Vi from a LP sweep Ubias < 0: I = Ii0 (1 - Ubias/Ti,eff) + Iph Ti,eff = Ti + mivi2/2e [eV] Ii0 nivi , √Ti 1) Slope Must assume mi 2) DC Level Need ne, Iph

  8. 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) Ti < few eV CAPS Ram energies near 100-200 eV Agreement with co-rotation vB & pick-up of locally produced ions Deflection near Enceladus Co-rotation Plasma Dilemma!?3-5 Rs Inconsistent measurements with regard to ion speed (vi) of the inner magnetosphere!?

  9. 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!?

  10. 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]

  11. Ex: LP Sweep Analysis (4.7-5 RS) • Inner Magnetosphere example • SOI results • [Wahlund et al., GRL, 2005] • Dust charging & USC • [Kempf et al., PSS, 2006] Ne≈ 60-70 cm-3 agree with fUH Te≈ 2 eV Usc≈ -5 V Vi Vi≈ 15-20 km/s ≈ Vsc Ne Te <mi> ~ 20 amu Jacobsen & Pedersen

  12. Ne from fuh • fuh≈ 70-75 kHz  61-70 cm-3

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

  14. 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

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

  16. n/n E-field filter Chorus Phase Frequency [Hz] 0 Hz 1 kHz 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!

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

  18. Waveform (1024 points snapshot) 3.8 cossd km/s LP/E- LP/E+ 14.5 +13.9 cossd ≈ 28.3 km/s E-/E+ -0.3 ms +0.9 ms Cross-correlation in time

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

  20. Waveform (1024 points snapshot) LP/E- LP/E+ 14.5 -3.5 cossd ≈ 11 km/s E-/E+ Cross-correlation in time

  21. S/C-Frame Rest-Frame Keplerian 12km/s E- S/C 14-15 km/s LP Co-Rotation 45-50 km/s E+ • CAPS prediction: • Look in anti-co-rotation direction • Look for ion signatures < 2-3 eV

  22. Dust-Plasma Coupling? ~ 2D ≈ 2 m - - - + - - - + - - + - • Dust Charge from Usc : -2 to -5 V • qdust ~ 700e/Volt  2000-4000 e/dust • CDA, qdust ~ 0.5 - 5 fC = 3000 - 30000 e/dust • [Kempf et al., PSS, 2006] • Cold ions (Ti < 5 eV) will be trapped close to dust particles • CDA & RPWS detects 0.1 m-3 for rdust > 2 m (rd-2.8 distribution) • Dust drag? • ndmd(GMS/r2) ~7.10-14 CDA observed > 2 m dust ~3.10-12 assuming nd ~ 20 m-3, 0.7 m dust • enivco-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

  23. Momentum Transfer nimidvi/dt = eni(E+viB) + nimig + niinin(vn-vi) + pi + mass load Near Enceladus: enivco-rot B ~2.10-13 nimig ~2.10-18 << niinin(vi-vn) ~2.10-17 << (using INMS nn ~105 cm-3) pi ~10-19 << mass loading ndmddvd/dt = qdnd(E+vdB) + ndmdg + … Magnetospheric conductances [Saur et al., JGR, 2004]

  24. (H+) W+ • ~ 5-100 m k ~ 0.06-1.3 m-1 Dust induced inhom. IA inhom/waves following co-rotation

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

  26. Suporting Results near Enceladus

  27. C. C. Porco et al., Science 311, 1303 - 1401 (2006)

  28. 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 • vi < 15 km/s • 1-2 cs • Dust-plasma interactions

  29. Enceladus, July 14 C/A • “Smooth” undisturbed profiles • No wake signatures • No shock signatures 40 000 km Ne = 60-80 cm-3, SOI: 40 cm-3 Te = 0.9 - 1.2 eV Usc = -1.9 to -2.5 V VH2O+ < 8 km/s rel. S/C TH2O+ < 6 eV Richardson, 1995: ~10 eV <mi>, water group ions

  30. Enceladus,March 9 C/A Undisturbed cold plasma < 100 000 km fr. C/A Ne = 60-70 cm-3 (peak at 90 cm-3) Te = 0.8 - 1.0 eV Usc = -1.8 to -2.0 V VH2O+ < 10 km/s rel. S/C TH2O+ < 9 eV <mi>, water group ions

  31. Enceladus, Feb. 17 C/A Undisturbed cold plasma < 80 000 km fr. C/A Ne = 100 -130 cm-3 Te = 1.0 - 1.4 eV Usc = -2.8 to -3.2 V VH2O+ < 5 km/s rel. S/C TH2O+ < 3 eV <mi>, water group ions

  32. C/A LP in S/C wake? 2000 km (z ≈ 800 km) 17000 km (z ≈ 6000 km) Charged dust signatures?

  33. CDA

  34. fUH Dust impacts

  35. Dust Charge Level? • Is difference in ne and ni due to dust charge? • (ni - ne) ≈ 20 - 40 cm-3 • Requires ndust ~ 0.001 cm-3 - Possible? • CDA detects in same region 0.1 m-3 for rdust > 2 m • RPWS dust counts are similar • Extrapolating dust distribution to smaller sizes • Dust down to 0.07 m (700 Å) necessary for rd-2.8 distribution • Do we have dust-plasma interaction?

  36. Charged dust? 0.7 RS C/A

  37. fUH Dust hits

  38. Conclusions • Cold Dense Plasma Torus (near Enceladus) • ne = 60-130 cm-3, Te = 0.8-1.4 eV, Ti < 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 40-50 km/s (EB-motion) • Dust-plasma interactions • (ni - ne)/ni • Mostly below 1% • Near Enceladus E-ring crossing = 30 - 50% • 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

  39. END

More Related