Twilight phenomena of Venus

Transcription

1 Twilight phenomena of Venus P. Tanga Laboratoire Lagrange Observatoire de la Côte d Azur Th. Widemann, B. Sicardy LESIA Observatoire de Paris Sakura/Europlanet workshop Paris, March

2 The «ring of light» P. Tanga, June 9, :05 UT Solar elongation 1.5 Phase angle 177 Mallama et al

3 el, Sydney

4 Past Venus transits 1631 Dec 7 Kepler s prediction 1639 Dec 4 first observation of Venus transits 1761 June 6 Solar parallax (Halley) - aureole 1769 June Dec 9 first photographic observations 1882 Dec June 8 digital imaging 2012 June Dec 11 4

5 The aureole on June 8, 2004 Interest: Confirm historical observations ( features? variability?) Quantitative data: Brightness Duration Link to amospheric properties? 5

6 Data sets Dutch Open Telescope (DOT - La Palma, Spain), 45 cm Themis (Tenerife, Spain), 90 cm Tourelle telescope (Pic du Midi, France), 50 cm Amateur instruments: L. Comolli (Milan, Italy) Coronagraph by A. and S. Rondi (Tarbes, France) Several visual reports with instruments of all sizes (> 8 cm) 6

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8 From visual data: Magnitude V~ -6 / -7 arcsec -1 Visibility sensitive to Seeing Contrast Instrument Sky transparency Filter density Magnification Intrinsic variability (relative to the past) unclear 8

9 DOT images 3 relevant wavelengths: 430, 431(B cont.), 655 (R cont.) nm CaII H band (396 nm) not used for the analysis 9

10 Flux measurements Brightness as a function of position angle (centered on Venus) Normalization relative to a patch on the photosphere (1x1 as 2 ) at 1 R v from the solar limb self consistency in photometric calibration 10

11 Photometric profiles S 11

12 model 12

13 Refraction angle ω : Refraction model where ν is the gas refractivity. At reference closest approach distance r 1/2 (Φ = 0.5): ν(r 1/2 ) = n 1/2 = gas number density at r 1/2 Geometric attenuation factor: 13

14 Deviation = attenuation Sun r cut 14 Baum and Code (1953) Surface brightness is preserved (transparent atmosphere) The image of the photospheric element is flattened by a factor Φ

15 Lensing 15 Unresolved atmospheric thickness = sum of lensed photospheric elements

16 Integration of all contribution Two paramenters only: Total thickness Δr : a value r = r cut exists, for which the light path is blocked by an opaque cloud layer Δr = r 1/2 -r cut. Scale heigth H 16

17 Results POLE Tanga et al. Icarus 218, (=pole) 17

18 June 8, 2004: summary A simple refraction model is capable of reproducing the aureole s brigthness Observed variation in time close to ~100 X (noise limited) Scale height, and altitude of refraction relative to the opaque cloud layer determined Meridional variation revealed by the arc anisotropy: all latitudes accessible at the same time Typ. brightness: V = -5.9 / arcsec ~100 X fainter than the reference photospheric element 18

19 projects for 2012 June our last opportunity (until 2117 ) 19

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21 Aims Improve the contrast of the aureole Multi-wavelength observations Better model: Absorption, scattering Realistic transition toward the opaque layer Sunset/sunrise asymmetries? Variability of the aureole in the mesosphere? Completing the phase curve of Venus? 21

22 June 8, 2004 Coronagraph by A. and S. Rondi 9 cm aperture 22

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24 The Cytherograph Project by A. and S. Rondi Aims: Best possible contrast on the aureole Image scale <0.5 Solar limb at the center of the FOV suitable filters (not for prominences!) 24

25 Prototype To be distributed in the visibility region ~8 to be built (~1200 Euros each) 25

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27 From Nice: transit egress: 6h37 6h55 27

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29 Open issues Which filters for the 2012 event? 1 to 10 nm pass band Preferred nm Optimum choice for Venus science? for exoplanet context? Technical aspects Testing Optimal exposure times 29

30 The real phase curve solar limb transit aureole? scattering ring 30

31 31 Link, 1969