cassini/huygens at saturn mkd_6dec.pdf · 2018-12-11 · our knowledge about saturn’s internal...
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Cassini/Huygens at Saturn
Michele K. Dougherty
Imperial College London 1
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Cassini Science TargetsSaturn and Rings Titan
Icy Satellites Magnetosphere
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• Fractures/ Tiger Stripes near south pole
• Warm Spot near south pole
• Internal heat leaking out?
• Warmest temperature over one of fractures
• ISS & CIRS data (Porco et al., Spencer et
al, 2006)
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5Porco et al., 2006
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INMS
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Our knowledge about Saturn’s internal magnetic
field prior to the Cassini Grand Finale
– Axisymmetric magnetic moments up to degree 3 well resolved[Dougherty et al., 2005; Burton et al., 2009; Cao et al., 2011]
– Hint for axisymmetric degree 4 & 5 moments ~ 100 nT from Cassini
Saturn Orbital Insertion [Cao et al., 2012]
– Highly axisymmetry field with dipole tilt < 0.06 degrees [Cao et al., 2011],
less stringent constraints on non-axisymmetry of degree 2 and beyond
– Very slow secular variation, -1.2 +/- 1.6 nT/yr [Cao et al., 2011],
at least an order of magnitude smaller than that of the geomagnetic field
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Current Theoretical Explanation for the
Extreme Axisymmetry
• Proposed by Stevenson [1980, 1982]
• A stably stratified and electrically conducting
fluid layer on top of Saturn’s dynamo region
electromagnetically filtered out the non-
axisymmetric part of dynamo-generated
magnetic field
• Differential rotation in the stable layer is a key
ingredient
• Qualitatively supported by numerical
simulations [Christensen & Wicht 2008; Stanley
2010]
- dipole tilt can be as small as 0.6 degrees in
the simulations, still an order of magnitude
larger than observed upper limits
9
[Stanley 2010]
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End of Cassini mission – 15th September 2017
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Calibrated MAG data from 9 Grand Finale orbits
High-latitude auroral FACs
Dougherty et al., 2018, Science
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Calibrated MAG data from 9 Grand Finale orbits
Low latitude FACs confined to
within inner-edge of D-ring
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A new field-aligned current system connecting
Saturn and inner-edge of D-ring
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Saturn’s magnetic equator northward offset
directly measured along Grand Finale Orbits
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Longitudinal variation of Saturn’s magnetic equator
measured along Grand Finale Orbits < 18 𝑘𝑚
1° tilt ~ 1900 km Δ𝑍 Measured 𝚫𝒁 < 𝟏𝟖 𝒌𝒎0.1° tilt ~ 190 km Δ𝑍 Dipole tilt < 0.0095°
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Saturn’s magnetic dipole tilt < 35 𝑎𝑟𝑐𝑠𝑒𝑐𝑜𝑛𝑑𝑠
Dipole tilt of Saturn:
< 1° (Smith et al. 1981)
< 0.06° (Cao et al. 2011)
< 35 arcseconds
(this study)
1000 times smaller than
that of Earth and Jupiter
Cannot claim detection of purely axisymmetric field yet, some/all of variations could
be due to external effects, such as PPOs and FACs
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What we knew before Cassini Grand Finale
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What we know now
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Inferred small-scale magnetic structures at 0.75Rs
• Field beyond degree 3
• Magnetic evidence for
deep differential
rotation [Cao & Stevenson
2017; Glatzmaier 2018]
• Consistent with gravity
measurements[Iess et al. 2018, under review]
𝐵𝑟
+/- 10% of deep dynamo field
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Summary and outlook
• Cassini Grand Finale MAG measurements revealed that Saturn’s
magnetic field is unique and full of surprises:
- low-latitude field-aligned current system inside the D-ring
- Saturn’s magnetic dipole tilt < 35 arcseconds- axisymmetric small-scale magnetic structures beyond degree 9
• Magnetic evidence for deep differential rotation inside Saturn
• Continue to:
- understand and quantify magnetospheric/ionospheric signatures
- investigate the effects of the rings