The Cassini-Huygens Analysis and Results of the Mission (CHARM) November 30, 2004

1. The Cassini-Huygens Analysis and Results of the Mission (CHARM) November 30, 2004 Saturn's Magnetic Bubble Frank Crary

2. Overview • Planetary Magnetospheres • The Earth’s Magnetosphere • Comparison between Earth, Jupiter and Saturn • The CAssini Plasma Spectrometer (CAPS) • The Solar Wind and Saturn’s Aurora • CAPS’ First Look at Saturn’s Magnetosphere • Plasmasphere • Acceleration/Injection Events and Plasma Transport • Oxygen and Nitrogen • The “Ionosphere” of Saturn’s Rings • The Ionosphere of Titan

3. The Earth’s Magnetosphere • Solar wind: A flow of ions and electrons from the Sun • Typical speeds are ~400-750 km/s (supersonic) • The solar wind also carries a magnetic field • The Earth’s magnetic field deflects the solar wind • A bow shock forms upstream of the planet • The shocked flow moves around in the “magnetosheath” • The Earth’s field is compressed on near noon • The Earth’s field is stretched out on the night side into a “magnetotail” • The magnetosphere is the protected region of space • Inside, there are trapped radiation belts & the “plasmasphere” • “Cold”, “dense” plasma from the Earth’s ionosphere • ~1 eV = 11,605 K, 1000 cm-3 ~ 2x10-26 g/cm3 • This plasma is coupled to Earth’s magnetic field and circulates West-to-East at the Earth’s rotation rate (“co-rotates”) • The interaction w/ the solar wind may drive aurora, enhance strength of the radiation belts & cause magnetic “storms”

4. Solar Wind-Magnetosphere Interaction

5. Regions of the Earth’s Magnetosphere

6. Comparison of Planetary Magnetospheres

7. The Cassini Orbital Tour

8. The CAssini Plasma Spectrometer (CAPS) • Designed to study ion and electron flux, as a function of energy, angle & composition • 1eV and 30-50 keV • Roughly hemispheric field of view • Determine density, flow velocity, temperature, etc. • Three sensors: Electron (ELS), ion composition (IMS) and ion beam (IBS) spectrometers • 17% (ELS and IMS) and 1.6% (IBS) energy resolution • ~20o (ELS and IMS) and 1.5o (IBS) angular resolution • Measures ion composition with M/M > 10 • Measures energy spectra in 2-4 sec. • 3-D distributions (energy, elevation & azimuth) in ~200 sec. • Voyager PLS covered 1 eV to 5 keV, 1-D energy spectra, in 96 sec., with no direct composition data

9. The CAssini Plasma Spectrometer (CAPS)

10. The Solar Wind & Saturn’s Aurora • Based on Voyager, Saturn’s magnetosphere was believed to be intermediate between Earth and Jupiter • Earth: Solar wind driven, no internal plasma sources • Jupiter: Mostly driven by internal processes, strong internal source, strong magnetic field, rapid rotation • Saturn: Solar wind driven (?), Earth-like magnetic field, moderate internal plasma sources, rapid rotation • Cassini and Hubble Space Telescope coordinated measurements to test this in January, 2004 • Cassini made dedicated measurements of solar wind • HST took UV images of Saturn every other day

11. Cassini and HST measurements • Cassini was 560 RS upstream of Saturn • Observations covered most of a solar rotation • January 8 to 30, 2004 • 81% duty cycle for ion measurements • Density, flow speed and temperature measured by CAPS, magnetic field by MAG • Also energetic particle plasma wave, auroral radio obs. • HST observed with STIS instrument • Eight orbits over 1 rotation of Saturn on January 8 • One orbit roughly every two days, Jan. 10 to 30

12. Cassini Solar Wind Observations

13. Response of Saturn’s Aurora to a Shock

14. Results of Cassini/HST Observations • Saturn’s aurora ( magnetospheric dynamics) is strongly controlled by solar wind (Earth-like) • Auroral brightness correlated well with solar wind dynamic pressure or electric field (Jupiter-like?) • Direction of solar wind magnetic field does not seem be important (not Earth-like) • Response of the aurora to a solar wind shock • Aurora brighten and latitude of oval shift • Filling of dawn side of oval, shift to higher latutudes • Filling of night side and shift to lower latitudes at Earth • Saturn’s magnetosphere seems to have Earth-like and Jupiter-like features, but not simply an intermediate case

15. Cassini’s 1st look at Saturn’s magnetosphere • 1st bow shock crossing: June 27, at 49 RS • 1st magnetopause crossing: June 28, at 33 RS • Five distinct regions within Saturn’s magnetosphere • Very low density region w/ hot electrons, ~14 RS (between orbits of Titan and Rhea) to the magnetopause • Low density, but patchy region between ~10 and ~14 RS • High density region with many, transient bursts of energetic electrons, between ~5 and ~10 RS • Calm, high density region without transient events or high energy electrons, inside ~5 RS • Ring “ionosphere” over and around the main rings

16. CAPS Energy Spectra June 30-July 1 Electrons Ions

17. Electron Densitie and Temperature

18. Outer, very low density region • Extends from magnetopause to ~14S • Sharp boundary with denser region, ~0.05 RS thick • Resembles the Earth’s plasmapause • Boundary between circulating and open streamlines • Simple scaling from the Earth  plasmapause near magnetopause, not at ~14 RS • Scaling based on northward solar wind magnetic field • Cassini approach measurements  mostly east/west field near Saturn • Sharpness of boundary  solar wind activity? • At Earth, sharp magnetopause  recent compression of the magnetosphere

19. Circulation in Saturn’s Magnetosphere Adapted from Cowley et al., 2004

20. Inner Regions, inside 10 RS • Source of plasma from E ring and/or icy satellites • Neutral gas sputtered off surfaces and is later ionized • O and H clouds seen by Cassini/UVIS, OH by HST • Plasma rotates with the planet • Equatorial plasma is coupled to Saturn’s ionosphere by magnetic field lines and field-aligned currents •  Outward centrifugal force • Outward force can cause instabilities • A dense fluid on “top” of a less dense one is unstable • Drives turbulence & radial transport outward of source • No similar instability inside the source region • Similar mechanism observed at Jupiter

21. Examples of acceleration/injection events Ions Electrons

22. Oxygen and Nitrogen N+ O+

23. Oxygen and Nitrogen • Ions were mostly protons, oxygen and water group • H+, O+, OH+, H2O+, H3O+ • Produced by ionizing H2O+ • Likely sources are the E ring & icy satellites • Some H2+ observed • Possibly from Saturn’s ionosphere • Some N+ (~5% abundance) measured • Not directly from Titan • Ions gain energy when they diffuse into a stronger magnetic field • Ions from Titan would have >1 keV, observed N+ was <100 eV • N from Titan implanted into icy surfaces & locally ionized? • N from ammonia on Enceladus (suggested by geologists) • O+ and O2+ observed over the main rings

24. CAPS Rev A Periapsis

25. Cassini/CAPS detection of O2+ over the rings • During orbital insertion, Cassini was over main rings • 17,000 above ring plane when crossing the B ring • Closest approach at 1.3 RS (~18,000 km above the clouds…) • Main engine burn inbound, ring science outbound • CAPS turned down high voltages during burn • No data until ~30 min after end of burn • Ion mass spectra taken w/ voltages in uncalibrated state • Spacecraft was in eclipse behind Saturn & over dark side of the rings • Electrons w/ few to few dozen eV energy • Over Cassini Division and A ring, mostly where rings are thin • Ions w/ a probable mass of 16 and 32 AMU (O+ and O2+) • Observed over B, Cassini Division and A ring • Relative abundance varies • Also detected on Cassini/INMS instrument

26. Cassini’s trajectory over the rings

27. Energy spectrograms over the rings Corotating 32 AMU 16 AMU 2 AMU 1 AMU Ions Electrons

28. Where does the O2+ come from? • Short ion lifetimes over the rings • B ring is optically (and collisionally) thick • Collisions with ring particles every ½ bounce period (~12 hours) • No time for any exospheric/physical chemistry • Most likely source of O2+ is a neutral O2 atmosphere • O2 is produced when energetic particles or UV photons hit ice • Observed/inferred (from O3 obs.) at moons of Jupiter & Saturn • Both processes also produce comparable amounts of H2 • No evidence of H+ or H2+ in CAPS data • Preferential loss of H may help explain an O2 atmosphere

29. Titan in Saturn’s Magnetosphere

30. Overview of Magnetospheric Interaction Ions H+ at corotation (197 km/s) Electrons

31. Titan’s interaction with Saturn’s Magnetosphere • Highly variable electron population above 100 eV • Anti-Saturn flank • Gradual slowing of flow from ~4 to 1.5 RT across track • Consistent with recently ionized particles & heavy mass loading • Sub-Saturn flank • Rapid increase in flow speed 1.5 RT past closest approach • Increases to ~ 2x pre-encounter flow speed • Gradually decreases to pre-encounter values • Consistent with flow past an satellite without mass loading • Ionization and atmospheric loss concentrated on one side? • Theoretically predicted: Plasma flow induces an electric field across Titan, pointed away from Saturn

32. Closest Approach 30 minutes Ions Electrons

33. Closest Approach 10 minutes Ions Electrons

34. IBS Ion Spectra 15:29:15 UTC 1220 km altitude 84o solar phase angle Fit with 7 Ion species