Remote Sensing of Planetary Auroral Emissions

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Remote Sensing of Planetary Auroral
Emissions (LAMS Workshop 2002)
The Role of Planetary Magnetic Field Models
Nick Achilleos, (Michele Dougherty, Giacomo
Giampieri) (Imperial College)

• The creation of field models ? Type of data
  needed, techniques of fitting
• The importance of field models in data analysis
  ? How they are used, examples of analyses
• Goals for future work ? Field model for Saturn /
  Cassini

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• FIELD MODELS
• Result of analysing magnetic field data from
  spacecraft (e.g. Voyager, Pioneer, Galileo,
  Cassini in future)
• Spacecraft trajectories sample a spatially
  small subset of global field structure ? Use
  sophisticated techniques, generalized inverse
  methods
• BTOTAL BINT BEXT
• curl BINT 0 (no source outside planet)
• BINT -grad ?, div BINT 0
• BINT -grad (? spherical harmonic fns)
• BEXT ? current sheet, magnetopause currents,
  boundary layers

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• Examples
• Jupiter
• O6 Model by Connerney et al (1992, in
  Planetary Radio Emission III) - up to 6th order
  polynomials used, based on Voyager/Pioneer data
  (magnetodisc component)
• VIP4 Model by Connerney et al (1998, JGR 103
  11929) - incorporates observations of IR-bright
  spot at magnetic footprint of Ios orbit
• Recent plasma disk models/analysis of Jupiter,
  summarised in Russell et al (1999 PSS 47 1101)
• Far-field model used by Prangé et al for
  analysing aurora from Comet SL9 (1995 Science 267
  1317)
• Saturn
• Z3 Model by Acuna et al from V1/V2 (1983 JGR 88
  8771)
• SPV Model by Davis and Smith (1990 JGR 95 15257)

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• Satellite footprints, auroral ovals,
  high-latitude features
• Magnetic field models essential for mapping
  auroral features to magnetospheric locations
• Distortion of field from simple dipole ?
  Affects location of magnetically conjugate point
  in magnetosphere
• Rotational versus solar wind auroral
  drivers (compare Jupiter and Saturn)
• N.B. External auroral drivers e.g. Comet
  Shoemaker-Levy 9 !

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Prangé et al -aurora from Comet SL9 (1995
Science 267 1317)
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From Waite et al (Nature 410, 787, 2001)
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From Satoh and Connerney (Icarus 141, 236, 1999)
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From Satoh and Connerney (Icarus 141, 236, 1999)
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• ADDITIONAL EXAMPLES
• Detection of field-aligned currents during
  Ulysses flyby of Jupiter (Dougherty et al, 1993
  PSS 41 291)
• Analysis of solar-wind auroral features inside
  main oval (HST FOC) - bright feature at constant
  magnetic local time (Prangé et al 1998 JGR 103 E9
  20195)
• Studies of Io aurora - Structure of Io
  spot-like emission, dissipation of energy in
  Io-Jupiter interaction (single flux-tube
  circuit or wave-like interaction with multiple
  reflections ?)

The accuracy of magnetic field models profoundly
affects our interpretation of magnetospheric
structure and physics. Analyses use field models
for many different science activities.
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• CURRENT AND FUTURE WORK
• European Members of Cassini Magnetometer Team
• M. K. Dougherty, G. Giampieri (Imperial College)
• Modelling of Saturns ring current ( 7 - 15 RS).
  Saturn internal field is highly axisymmetric
  (surprising?). Can use it to examine external
  field term. Inbound / outbound observations by
  spacecraft (V1/V2, Ulysses) ? Disk has local-time
  structure ? Can use data on different passes to
  probe compressed or extended magnetosphere,
  thick/thin current disk.
• P. Hanlon (Imperial College) - study of Galileo
  data ? current sheet structure.
• Modelling of Saturns internal field (up to 5th
  order) using Cassini MAG periapse data. Origin of
  field (fossil, dynamo, internal currents),
  production of an accurate field model for data
  analysis.
• J. Scuffham (Imperial College) - study of
  external field terms (scalable model)
• S. Cowley, E.Bunce (Leicester University)
• Field perturbations as a diagnostic of plasma
  dynamics in magnetosphere of Jupiter / Saturn
• Current systems associated with M-I coupling
  (field models used to infer location and
  magnitude)
• THE CASSINI MISSION
• An essential component of the most successful
  studies of Jovian aurorae and magnetosphere is a
  high-quality, three-dimensional field model. A
  combination of datasets from past missions as
  well as from Cassini will be required to produce
  the most accurate field model of this kind for
  Saturn.

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R. Prangé, L. Pallier, S. Cowley, E. Bunce, J.
Connerney HST / STIS observations, Dec 7 / 8
2000 South auroral oval asymmetric in local time
(brighter, higher latitude in morning sector).
Mapping shows that part of the auroral emission
related to corotation aurora, other parts to
solar wind aurora Auroral storm observed,
possibly related to event in solar wind measured
by Cassini at Jupiter. SPV Model used.