New results in polarimetry of planetary thermospheric emissions: Earth and Jovian cases. 

M. Barthelemy (1), J. Lilensten (1), C. Simon (2), H. Lamy(2), G Gronoff (3),

H. Menager (1), S. Miller (4), M. Lystrup (5), H. Rothkael (6), J. Moen (7).

 

(1)IPAG, France(2)BIRA-IASB, Belgium(3)NASA, Langley,VA, USA(4)UCL, UK(5)University of Colorado, USA(6)Polish Space Research Center, Poland(7)University in Oslo, Norway

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Polarization processes

• Impact polarizationPolarization rate

depends of:– Pitch angle

distribution

– Kind of particles

– Energy

– Depolarization processes as collisions.

• Emission in an anisotropic region: for ex Electric field.

Ex. Lyman alpha polarization rate by electron impact. Laboratory measurement (from James et al 1998).

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Earth case: POLARLIS, or the measurement of the POLarization of the Oxygen thermospheric Red Line In Svalbard.

• Photo-polarimeter: SPP at KHO in Longyearbyen (Svalbard) and at Hornsund .

Steerable Polarization Photometer (SPP) built at the Oslo University (UiO). It includes 2 channels and a pan-tilt unit. Aperture is 2°.

Channel 1: Photomultiplier.

Red filter centered on 6300 A with a FHWM of 1 nm.

Linear polarization analyser. One rotation of the analyser every 4.02 s.

Channel 2: Same, without the polarization filter.

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17 jan 2007 late afternoon. From Lilensten et al. 2008. GRL

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Reassessment

-Error in calibration angles. Correction around 45°

-New measurements at Hornsund (no light pollution)

-Confirmation of the detection-But rates around 1% after data processing-Direction close to the vertical (difference was due to the pollution)

-Compatible with Bommier et al. 2011-Raw polarisation before depolarizing collisions: ~18%-Vertical ie //B

- Barthelemy et al. submitted 2010.

Jovian case

• H3+ emissions in the IR.

• The emission process is different: No emission due to electronic impact because of the chemical process of formation, but…

• Possibility to get alignment due the electric fields in the auroral region.

2 half nights observation for August 2008 at the UKIRT with the instrument UIST-IRPOL in the Long L band (3.6-4.2 µm). Barthelemy et al. 2010, submitted

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Seventh data set:I at 3.95µm

q (Normalized Stokes parameter)

u (Normalized Stokes parameter)

p (%) (Debiaised)

Θ (°) (ref is the slit direction)

v (Normalized Stokes parameter)

The polarization direction are difficult to interpret…

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Interpretation:

Conclusion and Perspectives

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Jovian case.

•Auroral UV emissions.•Lyman , the most promising:

•Very intense (100 kR)

•Emission due to electrons and/or protons. Various polarization rate and direction with the energy.

•Radiative transfer in a presence of magnetic field for an allowed transition.

•Due to this, possible variation of the polarization rate along the line profile. Need for very high resolution.

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Needed in a dream world

• Spatial resolution around 100km2• Width of the oval ~200km.

• Spectral resolution• Lyman alpha width is 0.02 nm and it exists H2 lines in

coincidence with H-Lyman alpha.• Typical lyman alpha filter width (~5nm) Some lines of

H2 but faint compared to H-Lyman alpha

• Polarimetric accuracy under 1%• Only in the night side (dayside emission can

reach 10% of the auroral emission!)

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Feasability• Flux (at Ly alpha)

• Expected spatial resolution:100 km2

• 100 kR in the oval (i.e. 1e11 ph.cm-2.s-1 averaged in all directions).• Orbit : for example EJSM in resonance 2:1 with Europa i.e. around

106km for the centre of the planet

i.e. 7.9 10-1 ph.cm-2.s-1

Need an effective area of the instrument of 12.5 cm2 on a basis of 10ph.s-1. pix-1 (with an efficiency of the optics of 0.05, this correspond to an aperture of 250cm2)

Pb moving pieces!!!

• With a “better” orbit (more flux) : example of JUNO.Polar orbit.Closest point 4000km~12000km above the oval.

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Spectra/Lyman alpha filter

• Interest of spectra:• H2 lines

• No mixing between H2 and Lyman alpha lines if sufficient resolution.

– But not enough flux considering EJSM orbits to get H2 lines.

– And possible problem with the grating.

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Optical solution?

• Exemple: SMESE-Lyot like design (mirrors)

From Auchere et al. 1st SMESE workshop.

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Conclusions

• Difficult design due to the short wavelength– But

• Strong interest

• Best solution?• Mission to the planet (EJSM; too late for Juno)VS• Space telescope

• Rq: Saturn has Ly alpha aurora 10 times fainter.

Perspectives and needs.• Diversity of processes,diversity of emissions

→Specific information for each line and planet.• Others examples :

– O 8446 Å, O 1304 Å* (Earth, Venus, Mars)– Na I 5890 Å (Mercury)….in progress

– Needs for the jovian case– Ab initio calculation of the link between H3+ lines

polarization and the fields (E and/or B).– Lab experiment.

– In EJSM frame: Difficult to have a Ly alpha polarimeter. H3+ measurement as combined measurements

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Polarisation as a tool to study the Solar System and beyond

Hervé Lamy1 & Mathieu Barthélemy2

1 Belgian Institute for Space Aeronomy (BISA)2 Institut de Planétologie et d’Astrophysique de

Grenoble (IPAG)

COST proposal reference: oc-2010-2-8667