arXiv:1011.2680v1 [astro-ph.EP] 11 Nov 2010 Venus wind map at cloud top level with the MTR/THEMIS visible spectrometer. I. Instrumental performance and first results. Patrick Gaulme a Fran¸ cois-Xavier Schmider b Catherine Grec b Arturo L´opez Ariste c Thomas Widemann a Bernard Gelly c a LESIA, Observatoire de Paris, 5 place J. Janssen, F-92195 Meudon cedex b Laboratoire Fizeau, Universit´ e de Nice Sophia-Antipolis, CNRS-Observatoire de la Cˆ ote d’Azur, F-06108 Nice cedex 2 c THEMIS Observatory, La Laguna, Tenerife, Spain Abstract Solar light gets scattered at cloud top level in Venus’ atmosphere, in the visible range, which corresponds to the altitude of 67 km. We present Doppler velocity measurements performed with the high resolution spectrometer MTR of the Solar telescope THEMIS (Teide Observatory, Canary Island) on the sodium D2 solar line (5890 ˚ A). Observations lasted only 49 min because of cloudy weather. However, we could assess the instrumental velocity sensitivity, 31 m s −1 per pixel of 1 arcsec, and give a value of the amplitude of zonal wind at equator at 151 ±16 m s −1 . Key words: Venus, Wind, Clouds, Visible, Spectrometry PACS: 1 Introduction 1 ESA’s Venus Express (VEx) space probe has been orbiting around Venus since 2 April 2005. The mission’s main goal is a better understanding of the atmo- 3 spheric circulation, in particular of the wind super-rotation. The key questions 4 regard the meridian circulation at top cloud level, the vertical extension of 5 Hadley cells and the latitudinal dependance of the zonal wind. VEx obtains 6 wind velocity map with cloud tracking and wind vertical profile from thermal 7 wind maps. VEx measurements are limited by three factors. First, because of 8 Email address: [email protected](Patrick Gaulme). Preprint submitted to Elsevier 12 November 2010
18
Embed
Venus wind map at cloud top level with the MTR/THEMIS visible spectrometer, I: Instrumental performance and first results
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
arX
iv:1
011.
2680
v1 [
astr
o-ph
.EP]
11
Nov
201
0
Venus wind map at cloud top level with the
MTR/THEMIS visible spectrometer.
I. Instrumental performance and first results.
Patrick Gaulme a Francois-Xavier Schmider b Catherine Grec b
Arturo Lopez Ariste c Thomas Widemann a Bernard Gelly c
aLESIA, Observatoire de Paris, 5 place J. Janssen, F-92195 Meudon cedex
bLaboratoire Fizeau, Universite de Nice Sophia-Antipolis, CNRS-Observatoire de
la Cote d’Azur, F-06108 Nice cedex 2
cTHEMIS Observatory, La Laguna, Tenerife, Spain
Abstract
Solar light gets scattered at cloud top level in Venus’ atmosphere, in the visiblerange, which corresponds to the altitude of 67 km. We present Doppler velocitymeasurements performed with the high resolution spectrometer MTR of the Solartelescope THEMIS (Teide Observatory, Canary Island) on the sodium D2 solar line(5890 A). Observations lasted only 49 min because of cloudy weather. However, wecould assess the instrumental velocity sensitivity, 31 m s−1 per pixel of 1 arcsec,and give a value of the amplitude of zonal wind at equator at 151 ±16 m s−1.
Table 2Standard deviation Σ of velocity in each column of the Venus map. The meanstandard deviation value is equal to 31 m s−1. The third line indicate the number ofspectra averaged in order to build each column. We have averaged column 11 and12 because column 11 alone have only 5 spectra, which affect the global noise level.The standard deviation increases in column 5 and 6 and 7 despite a major numberof averaged spectra, because of the wind velocity strong variation in these columns.
Fig. 1. Venus appearance during observations, on November 7th, 2007 at 13:06:54h (UTC). The planetary radius is about 10.88 arcsec and the phase angle about83.59◦. SEP and SSP stand for sub-Earth point and sub-solar point.
equator
terminator
Entrance slit
(0.5 x 100)''
Cut "C"
x
Night side
Maximum extension
of the scanned region
eq
Day side
θ
Fig. 2. Schematic review of notations used in this paper and illustration of thespatial coverage of Venus by our observations. The slanted lines indicate the regionwhich is not covered by observations. θ indicates the latitude and C the cut alongthe planet, through the entrance slit (in pixels). The maximum extension of thespatial coverage reaches θ = 45◦ on the planetary limb.
12
0 10 20 30 40 50593
594
595
596
597
598
599
Date (minutes)
Mea
nin
tensi
ty(A
DU
)
Fig. 3. Mean intensity measured on Earth’s skylight background on each spectrumof the temporal series. The mean value is equal to 597.6 ADU, whereas the standarddeviation of the points is equal to Σ = 58.16 ADU.
Fig. 4. Top: D2 sodium line on Sun spectrum as function of the wavelength (BASS2000 database, http://bass2000.obspm.fr). Intensity has been normalized to 1. TheDoppler sensitivity is related to the slope of the considered line. It reaches its max-imum at the transmission level of 30%. Bottom: the slope of the upper figure,converted in meter per second. In average, the Doppler velocity sensitivity is about< δI/I >= 0.5 10−4 per m s−1 for a bandwidth of 60 mA.
Fig. 5. Left: raw spectrum of Venus, centered on D2 sodium solar line. The y-axiscorresponds the spatial dimension, while the x-axis squares with the spectral dimen-sion. Venus corresponds to the dark region on the detector, with highest intensity,whereas the light background corresponds to the Earth’s sky spectrum. The D2 lineon Venus is clearly shifted with respect to the Earth’s. The slight curvature acrossthe whole image is estimated by fitting the D2 sodium line scattered by Earth’satmosphere with a second order polynomial (dot line). Right: clean spectrum. Afterstraightening out the distorsion, skylight sodium light has been averaged over thebackground, and subtracted to Venus. Thinner lines correspond to telluric absorp-tion lines.
wavelength (A)
y(a
rcse
c)
Spectrum 276
5888 5889 5890 5891 5892 5893
20
30
40
50
60
wavelength (A)
Spectrum 277
5888 5889 5890 5891 5892 5893
20
30
40
50
60
20 25 30 35 40 45 50 55 60
0
500
1000
1500
2000
2500
y (arcsec)
Mea
nin
tensi
ty
Spectrum 276Spectrum 277
Fig. 6. Top: two consecutive spectra (labelled 276 and 277 among the 318 spectratemporal series), which have been acquired with an interval of 10 s. Bottom: pro-jection of both spectra along the y-axis, in order to evaluate the spatial extensionof the planet selected by the entrance slit. Width determination is the only methodto locate the slit projection on Venus.
14
0 10 20 30 40 5060
65
70
75
80
85
90
95
Date (minutes)
Cut
Calo
ng
Ven
us
(pix
el)
Fig. 7. Spatial extension estimate “C” as function of time, along the observation run.The cut extension has been calculated on full resolution images, in order to keepthe original accuracy; 1 pixel corresponds to 0.2 arcsec. The extension is definedby the width at half maximum. The solid line represents the measurement of thespatial extension, while the dashed line represents the polynomial fitting of Venuscut. The standard deviation of points around the mean is equal to 2.46 pixels.
0 5 10 15 20 25 30 35 40 45 50−0.4
−0.35
−0.3
Sun
vel
oci
ty(k
ms−
1)
0 5 10 15 20 25 30 35 40 45 5013.25
13.3
13.35
Ven
us
vel
oci
ty(k
ms−
1)
time (minutes)
Sun Venus
Fig. 8. Relative velocities with respect to Teide Observatory between 13:06:52 hand 13:55:34 h (UT) on November, 7th 2007. Initial date squares with 13:06:00h, velocities are expresses in km s−1. Left y-axis indicates Sun relative velocity,which mean amplitude is about −0.35 km s−1. Right y-axis indicates Venus relativevelocity, which mean is about +13.3 km s−1. Consequently, the mean Doppler shiftbetween D2 sodium line scattered by Venus’ and Earth’ atmospheres is equal to+13.65 km s−1 (www.imcce.fr).
15
5887 5888 5889 5890 5891 58920
0.2
0.4
0.6
0.8
1
1.2
wavelength (A)
Inte
nsi
ty(a
rbit
rary
unit
)
Earth atmosphereVenus atmosphere
10 15 20 25 303.1
3.15
3.2
3.25
3.3
3.35x 10
7
x-axis (pixels)
Cro
ssco
rrel
ati
on
(arb
itra
ryunit
)
Data 4th degree
Fig. 9. Left: reference mean spectrum (dashed line) and Venus spectrum as a func-tion of the wavelength (full line). Reference spectrum is calculated for each spec-trum by averaging all the skylight spectrum. Venus spectrum is obtained on a 1arcsec spatial resolution spectrum, and correspond to the planetary equator. Bothspectrum have been normalized with respect to their maximum value and Venusspectrum has been offset, only for graphical reasons. Right: 4th order polynomialfit of the maximum of the cross correlation between Venus and reference spectra.The Maximum position is equal to 20.51 pixels.
Time (minutes)
y(a
rcse
c)
Doppler shift (104 m s−1)
0 5 10 15 20 25 30 35 40 45
20
30
40
50
60 1.15
1.2
1.25
0 5 10 15 20 25 30 35 40 45 501.18
1.19
1.2
1.21
1.22
1.23
1.24
1.25
Time (minutes)
Dopple
rsh
ift
(10
4m
s−1)
−400 −200 0 200
20
30
40
50
60
y(a
rcse
c)
Doppler shift (m s−1)
Fig. 10. Top left: Doppler shift diagram as a function of time (x-axis) and space(y-axis). Doppler shift is expressed in 104 m s−1. Bottom left: mean Doppler shiftwith time. Top right: mean Doppler shift with spatial dimension. The dashed lineon the bottom left plot indicates the fitted estimate obtained by a weighted movingaverage, which has been used to characterize the vertical distorsion of the Doppler“surface” plotted in bottom right figure.
16
Time (minutes)
y(a
rcse
c)
Doppler shift (104 m s−1)
0 5 10 15 20 25 30 35 40 45
20
30
40
50
60 1.15
1.2
1.25
Fig. 11. Clean Doppler diagram, obtained after subtraction of the spurious distorted“surface” enlightened in the raw diagram (Fig 10).
x (arcsec)
y(a
rcse
c)
Doppler shift (m s−1)
5 10 15 20
5
10
15
20
−100
−50
0
50
100
150
200
x (arcsec)
y(a
rcse
c)
Doppler shift (m s−1)
5 10 15 20
5
10
15
20
−100
−50
0
50
100
150
200
Fig. 12. Left: Relative velocity map obtained after summation of Doppler shiftwithin 1-arcsec intervals. The dot line circle indicates the planetary diameter. Themaximum latitude extension reaches ±45◦, while the maximum longitude reaches55 at pixels (5, 7) and (5, 17). Right: the same Doppler map where pixels have beenaveraged within a regular latitude-longitude grid, spaced by 10◦. Latitude range is[−45◦, 45◦] while longitude range is [0◦, 55◦].