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

CHARM teleconNovember 30, 2004

The Cassini-Huygens Analysis and Results of the Mission (CHARM)

November 30, 2004

Saturn's Magnetic Bubble Frank Crary


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


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”


Solar Wind-Magnetosphere Interaction


Regions of the Earth’s Magnetosphere


Comparison of Planetary Magnetospheres

Earth Jupiter Saturn

Radius [km] 6,373 71,400 60,330

Spin Period [hrs] 24 9.9 10.7

Surface Magnetic Field [Gauss]

0.31 4.28 0.22

Dipole Tilt 11.3o 9.6o 0.0o

Distance from Sun [AU] 1.0 5.2 9.5

Solar Wind Density [cm-3] ~10 ~0.5 ~0.1

Size of Magnetosphere ~10 RE 50-100 RJ 15-25 RS

Internal plasma sources [kg/s]

5 1000 2 (?)


The Cassini Orbital Tour


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


The CAssini Plasma Spectrometer (CAPS)


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


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


Cassini Solar Wind Observations


Response of Saturn’s Aurora to a Shock


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


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


CAPS Energy Spectra June 30-July 1

Ions

Electrons


Electron Densitie and Temperature


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


Circulation in Saturn’s Magnetosphere

Adapted fromCowley et al., 2004


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


Examples of acceleration/injection events

Electrons

Ions


Oxygen and Nitrogen

N+ O+


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


CAPS Rev A Periapsis


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


Cassini’s trajectory over the rings


Energy spectrograms over the rings

Electrons

Ions Corotating32 AMU16 AMU

2 AMU1 AMU


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


Titan in Saturn’s Magnetosphere


Overview of Magnetospheric Interaction

Ions

Electrons

H+ atcorotation(197 km/s)


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


Closest Approach 30 minutes

Electrons

Ions


Closest Approach 10 minutes

Ions

Electrons


IBS Ion Spectra

15:29:15 UTC

1220 km altitude84o solar phase angle

Fit with 7 Ion species

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

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