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

2. CHARM telecon November 30, 2004 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. CHARM telecon November 30, 2004 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/cm 3 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. CHARM telecon November 30, 2004 Solar Wind-Magnetosphere Interaction

5. CHARM telecon November 30, 2004 Regions of the Earth’s Magnetosphere

6. CHARM telecon November 30, 2004 Comparison of Planetary Magnetospheres EarthJupiterSaturn Radius [km]6,37371,40060,330 Spin Period [hrs]249.910.7 Surface Magnetic Field [Gauss] 0.314.280.22 Dipole Tilt11.3 o 9.6 o 0.0 o Distance from Sun [AU]1.05.29.5 Solar Wind Density [cm -3 ]~10~0.5~0.1 Size of Magnetosphere~10 R E 50-100 R J 15-25 R S Internal plasma sources [kg/s]510002 (?)

7. CHARM telecon November 30, 2004 The Cassini Orbital Tour

8. CHARM telecon November 30, 2004 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 –~20 o (ELS and IMS) and 1.5 o (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. CHARM telecon November 30, 2004 The CAssini Plasma Spectrometer (CAPS)

10. CHARM telecon November 30, 2004 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. CHARM telecon November 30, 2004 Cassini and HST measurements Cassini was 560 R S 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. CHARM telecon November 30, 2004 Cassini Solar Wind Observations

13. CHARM telecon November 30, 2004 Response of Saturn’s Aurora to a Shock

14. CHARM telecon November 30, 2004 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. CHARM telecon November 30, 2004 Cassini’s 1 st look at Saturn’s magnetosphere 1 st bow shock crossing: June 27, at 49 R S 1 st magnetopause crossing: June 28, at 33 R S Five distinct regions within Saturn’s magnetosphere –Very low density region w/ hot electrons, ~14 R S (between orbits of Titan and Rhea) to the magnetopause –Low density, but patchy region between ~10 and ~14 R S –High density region with many, transient bursts of energetic electrons, between ~5 and ~10 R S –Calm, high density region without transient events or high energy electrons, inside ~5 R S –Ring “ionosphere” over and around the main rings

16. CHARM telecon November 30, 2004 CAPS Energy Spectra June 30-July 1 Ions Electrons

17. CHARM telecon November 30, 2004 Electron Densitie and Temperature

18. CHARM telecon November 30, 2004 Outer, very low density region Extends from magnetopause to ~14 S –Sharp boundary with denser region, ~0.05 R S thick Resembles the Earth’s plasmapause –Boundary between circulating and open streamlines Simple scaling from the Earth  plasmapause near magnetopause, not at ~14 R S –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. CHARM telecon November 30, 2004 Circulation in Saturn’s Magnetosphere Adapted from Cowley et al., 2004

20. CHARM telecon November 30, 2004 Inner Regions, inside 10 R S 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. CHARM telecon November 30, 2004 Examples of acceleration/injection events Electrons Ions

22. CHARM telecon November 30, 2004 Oxygen and Nitrogen N+N+ O+O+

23. CHARM telecon November 30, 2004 Oxygen and Nitrogen Ions were mostly protons, oxygen and water group –H +, O +, OH +, H 2 O +, H 3 O + –Produced by ionizing H 2 O + –Likely sources are the E ring & icy satellites Some H 2 + 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 O 2 + observed over the main rings

24. CHARM telecon November 30, 2004 CAPS Rev A Periapsis

25. CHARM telecon November 30, 2004 Cassini/CAPS detection of O 2 + 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 R S (~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 O 2 + ) –Observed over B, Cassini Division and A ring –Relative abundance varies –Also detected on Cassini/INMS instrument

26. CHARM telecon November 30, 2004 Cassini’s trajectory over the rings

27. CHARM telecon November 30, 2004 Energy spectrograms over the rings Electrons Ions Corotating 32 AMU 16 AMU 2 AMU 1 AMU

28. CHARM telecon November 30, 2004 Where does the O 2 + 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 O 2 + is a neutral O 2 atmosphere O 2 is produced when energetic particles or UV photons hit ice –Observed/inferred (from O 3 obs.) at moons of Jupiter & Saturn Both processes also produce comparable amounts of H 2 –No evidence of H + or H 2 + in CAPS data Preferential loss of H may help explain an O 2 atmosphere

29. CHARM telecon November 30, 2004 Titan in Saturn’s Magnetosphere

30. CHARM telecon November 30, 2004 Overview of Magnetospheric Interaction Ions Electrons H + at corotation (197 km/s)

31. CHARM telecon November 30, 2004 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 R T across track –Consistent with recently ionized particles & heavy mass loading Sub-Saturn flank –Rapid increase in flow speed 1.5 R T 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. CHARM telecon November 30, 2004 Closest Approach  30 minutes Electrons Ions

33. CHARM telecon November 30, 2004 Closest Approach  10 minutes Ions Electrons

34. CHARM telecon November 30, 2004 IBS Ion Spectra 15:29:15 UTC 1220 km altitude 84 o solar phase angle Fit with 7 Ion species