Transit of Venus Observations Kevin Reardon (Queen’s University Belfast / Osservatorio di Arcetri)

Jay Pasachoff, Bryce Babcock (Williams College) Glenn Schneider (Steward Observatory, University of Arizona)

Pascal Hedelt (Observatoire de Bordeaux) Serge Koutchmy (Institut d'Astrophysique de Paris) Mihalis Mathioudakis (Queen’s University Belfast)

Paolo Tanga (Observatoire de la Cote d'Azur) Thomas Widemann (Observatoire de Paris, CNRS)

Introduction: This proposal describes an observing program designed to advance our understanding of planetary atmospheres, focusing on two key topics. These studies will take advantage of the transit of Venus visible from Sacramento Peak and Kitt Peak on 05 June 2012. This event will allow us to probe the dynamics and structure of the atmosphere of Venus through measurements of the refracted, scattered, and transmitted solar radiation. Observations of the transit of a planet with a known atmospheric composition will allow us to develop better methods to characterize, predict, and explain the details of exoplanet transits, providing direct support to a fundamental research topic in astrophysics in the coming decades. The transit of Venus, not to be visible from Earth for 105 years, is a unique opportunity to address these important questions. Venus, unlike Mercury, has a thick atmosphere and orbits within our solar system’s habitable zone, yet is life-hostile. This makes it an important case study to examine as we seek ways to probe our neighborhood for signs of life. The proposed observations make use of NSO’s comprehensive and unique set of resources to acquire high quality data of both immediate and historical value.

Atmospheric Studies of a Habitable Zone Planet: While very similar in their size, composition, and location within the habitable zone of our solar system, Venus and Earth are distinctly different in several crucial ways. The atmosphere of Venus is extremely dense (67 kg/m3 at the surface, compared to 1.2 kg/m3 on Earth) and is predominantly composed of CO2 (96.5%) rather than nitrogen (78%) as on Earth. Given these divergent conditions on the two planets, Venus shows significant differences in the physical processes at work in structuring its atmosphere. Significant progress has been made in identifying these mechanisms using terrestrial and spacecraft

localized ‘weather’ phenomena, the overall organization of the atmo-spheric circulation. Three broad regimes are clearly present in themiddle and lower atmosphere, with convective and wave-dominatedmeteorology in the lower latitudes and an abrupt transition tosmoother, banded flow at middle to high latitudes5. The latter ter-minates at about 30u from the pole, where the cold polar collar dis-covered by earlier missions lies. This encloses a vast vortex-typestructure several thousand kilometres across with a complex double‘eye’ that rotates every 2.5–2.8 Earth days. Simultaneous observationsin the ultraviolet and thermal infrared spectral ranges show corre-lated patterns, indicating that the contrasts at both wavelengths,although representing different atmospheric levels, are driven bythe same circumpolar dynamical regime5,6. Spectroscopic observa-tions indicate marked changes in the temperature and cloud struc-ture in the vortex, with the cloud top in the polar collar located at analtitude of 70–72 km, about 5 km or one scale height higher than inthe eye. Night-side observations in the transparent spectral windowsshowed that the vortex structure and circulation exist at as least asgreat a depth as the lower cloud deck at 50–55 km, although its‘dipole’ appearance seems to be confined to the cloud-top region6.The edge of the polar collar at 50–60u latitude apparently marks thepoleward limit of the Hadley circulation, the planet-wide overturn-ing of the atmosphere in response to the concentration of solar heat-ing in the equatorial zones (Fig. 2a). Indirect evidence of suchmeridional circulation is provided by monitoring of the latitudedistribution of minor constituents, especially carbon monoxide, asdynamical tracers in the lower atmosphere.

The mesopause on Venus at 100–120 km altitude marks anothertransition between different global circulation regimes, this time inthe vertical. The predominance of zonal super-rotation in the loweratmosphere below the mesopause is replaced by solar to antisolarflow in the thermosphere above, as revealed by non-LTE (non-localthermodynamic equilibrium) emission in the spectral band of O2 at1.27 mm that originates from the recombination of oxygen atoms indescending flow on the night side (Fig. 2b). The observed emissionpatterns are highly variable, with the maximum at about the anti-solar point and the peak altitude at about the mesopause7. A meso-spheric temperature maximum is observed on the night side8,produced by adiabatic heating in the subsiding branch of the thermo-spheric solar to anti-solar circulation.

Sequences of ultraviolet and infrared images have been used tomeasure the wind speeds at different altitudes by tracking themotions of contrast features in the clouds. Zonal winds at the cloudtops (,70 km) derived from the ultraviolet imaging are in the range1006 10m s21 at latitudes below 50u (ref. 5), in good agreement withthe earlier observations9,10. The new data, which penetrate the brightupper haze obscuring the main cloud at middle latitudes, find thatthe cloud-top winds quickly decline poleward of 50u. The infraredobservations6 sound the dynamics in the main cloud deck at,50 kmaltitude on the night side, finding strong vertical wind shear of about3m s21 km21 below 50u, and no shear poleward of this latitude, whencompared with the higher-altitude ultraviolet-derived winds. Thewind velocity profiles on Venus are found to be roughly, althoughnot exactly, in agreement with those predicted by the cyclostrophic

Table 1 | The scientific payload of Venus Express

Name (acronym) Description Measured parameters

ASPERA-4 Detection and characterization of neutral and charged particles Electrons1 eV–20keV; ions0.01–36 keV/q; neutral particles0.1–60keVMAG Dual sensor fluxgate magnetometer, one sensor on a 1-m-long boom B field 8 pT–262 nT at 128HzPFS Planetary Fourier Spectrometer (currently not operating) Wavelength 0.9–45 mm; spectral resolving power about 1,200SPICAV/SOIR Ultraviolet and infrared spectrometer for stellar and solar occultation

measurements and nadir observationsWavelengths 110–320 nm, 0.7–1.65mm and 2.2–4.4 mm; spectralresolving power up to 20,000

VeRa Radio Science investigation for radio-occultation and bi-static radar measurements X- and S-band Doppler shift, polarization and amplitude variationsVIRTIS Ultraviolet–visible–infrared imaging spectrometer and high-resolution infrared

spectrometerWavelength 0.25–5mm for the imaging spectrometer and 2–5 mm forthe high-resolution channel; resolving power about 2,000

VMC Venus Monitoring Camera for wide-field imaging Four parallel channels at 365, 513, 965 and 1010 nm

These instruments are expected to produce more than 2 terabits of data during the design lifetime of four Venus sidereal days (about 1,000 Earth days). Venus Express is operating in an ellipticalpolar orbit with a period of 24 h and an apocentre altitude of 66,000 km. The pericentre altitude is maintained between 250 and 400 km approximately over the north pole. q is elementary charge.

Polar vortexSub-solar to anti-solar cell

Hadley cell

Cold

Cold

Warm Warm

Polar collar

Solar heating

EUV flux

Recombinationof O atomsinto O2 (!)

Night-sideairglow

CO2photodissociation

a

b

Figure 2 | Schematic view of the general circulation of Venus’s atmosphere.a, Themain feature is a convectively drivenHadley cell, which extends fromtheequatorial region up to about 60u of latitude in each hemisphere. The trend ispolewards at all levels that can be observed by tracking the winds (at about50–65 kmaltitude above the surface), so the returnbranchof the cellmust be inthe atmospherebelowthe clouds.Acold ‘polar collar’ is foundaroundeachpoleat about 70u latitude; theHadley circulation evidently feeds amid-latitude jet atits poleward extreme, inside which there is a circumpolar belt characterized byremarkably low temperatures and dense, high clouds. Inside the collar athinning of the upper cloud layer forms a complex and highly variable feature,called the ‘polar dipole’ in earlier literature describing poorly resolvedobservations, which appears bright in the thermal infrared6. Because in generalterms thinner-than-average or lower-than-average cloud is often associatedwith a descending air mass, and vice versa, the vortex may represent a second,high-latitude circulation cell, resembling winter hemisphere behaviour onEarth.b, Above about 100 kmaltitude the circulation regime onVenus changescompletely to a sub-solar to anti-solar pattern. Oxygen airglow emission at1.27mm reveals the recombination of oxygen atoms into molecular oxygenwhile descending to lower altitudes in the anti-solar region.Additional evidenceof this circulation is givenbytheupper-atmospheretemperatureprofiles,whichshow a pronounced temperature maximum on the night side that is due tocompressional heating in the downward branch of the circulation cell8.

PROGRESS NATUREjVol 450j29 November 2007

630Nature ©2007 Publishing Group

Figure 1: The general circulation pattern of the atmosphere of Venus, showing the meridional flow driven by solar heating on the sunward side. From Svedhem et al, 2007

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observations. Most recently, ESA’s Venus Express mission has provided new details on the atmospheric dynamics. In an interesting parallel to the global flows seen on the Sun, the atmosphere of Venus shows a significant meridional flow, driven in this case by the solar heating from above in the equatorial zone (see overview in Svedhem et al., 2007). This drives an observed polarward flow in the troposphere (h< 60 km), with a return flow inferred to occur in the denser regions of the lower atmosphere. This poleward flow terminates at a latitude of 60-70° where a cold, jetlike circulation, called the “polar collar,” is found, encompassing polar vortices. Venus' middle atmosphere (60-120 km, also known as the mesosphere) is a transition region between this lower atmosphere circulation and the thermosphere (h > 120km), where diurnal heat and pressure gradients drive dynamic flows between the sunward and night–side of the planet. These cross-terminator1 flows occur both at the equator and over the poles (Drossart et al., 2007). Monitoring of thermal profiles and winds in the mesosphere has revealed significant time variability in the transition region where different types of circulation coexist. Questions remain, however, about the composition, vertical structure, and variability of the atmosphere. Aspects of these questions are powerfully addressed by transit observations. The transit geometry is unique because it provides for the simultaneous observation of the entire terminator of Venus with a constant solar zenith angle, compared with single-direction observations made from the Venus Express spacecraft. This simultaneous observation around the entire limb of the planet will allow determination of the latitudinal variation of the temperature or density structure of the mesosphere. This will help calibrate the Venus Express observations, in particular the lower mesosphere temperature structure using limb solar occultations or radio occultations by the SOIR and VeRa experiments (Vandaele et al., 2008, Piccialli et al. 2011). The line-of-sight velocities around the full limb, caused by nightward flows or polar vortices, still have not been fully characterized. Because of the large background flux from the Sun, molecular absorption lines or refraction from the atmosphere of Venus can be measured with high spatial resolution and signal-to-noise in just a few seconds. The June transit will therefore provide a once-in-a-lifetime opportunity to apply the powerful tool of imaging spectroscopy to probe new dynamical timescales in the Cytherean atmosphere. Exoplanet Analogues: Planets transiting across their host stars are now routinely observed throughout our galactic neighborhood. The signature of the geometric transit in the light curve has been used to identify more than a thousand extra–solar planet candidates with observations from the Kepler satellite (with the number still growing). In order to provide a “truth test” for these detections, the comparable measurement was performed during the 2004 1 The terminator is the boundary between the day and night side of the planet

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transit of Venus using the sun-as-a-star irradiance measurement of the ACRIMsat satellite (Schneider et al. 2006; Pasachoff et al. 2011) to recover the “unknown” transit geometry. In the eight years since that first transit, however, the goal of detecting the exoplanet atmospheric signature through transmission spectroscopy has now become practical. The strong molecular bands in planetary atmospheres produce broad absorption features that can be detected, with sufficient integration, during the planetary transits. These atmospheric signatures have been detected for hot Jupiters (e.g., Charbonneau et al. 2002; Vidal-Madjar et al. 2003, 2011; Snellen et al. 2010). Similar signatures are expected for Earth-sized planets, though with a reduced magnitude because of the smaller size of the planet and atmosphere with respect to the host star. This field will see increasing attention and resources in coming years. Indeed it is listed as a key priority in the latest Astronomy and Astrophysics decadal survey. Several new missions under study –NASA’s FINESSE (Fast INfrared Exoplanet Survey Explorer) and ESA’s EChO (Exoplanet Characterisation Observatory) – have complementary science goals to detect atmospheres around rocky planets, the characterization of their chemical composition, and the identification of biomarkers in the atmosphere. In this context, Venus is our closest model for telluric exoplanet targets. Obtaining its transmission spectrum during its transit across the Sun will serve both as a comparison basis for transiting Earth-mass exoplanets to be observed in the future, and a proof of feasibility that such observations can effectively probe the atmospheres of exoplanets in this mass range. A related experiment will be carried out during the ToV by a French-led group with which we are collaborating. They will obtain observations of the moon by the HST to detect these spectral signatures in the reflected solar light. Such observations are crucial to help test and validate the sensitive detection techniques. A successful series of test observations of the lunar regions was made by HST in January 2012.

D. Ehrenreich et al.: Venus as a transiting exoplanet

cross-sections are calculated from the HITRAN 2004 linelist (Rothman et al. 2005) following Rothman et al. (1998,Sect. A.2.4) but using Voigt functions instead of Lorentzian lineprofiles. The UV cross-sections of CO2, O2, SO2, and O3 aretaken from the AMOP2 and AMP3 data repositories. The pho-toabsorption cross sections are plotted in Fig. 2a.

Scattering – Important additional absorption is caused bydi!usion processes: Rayleigh scattering from atmospheric CO2and, to a lesser extent, N2, CO, and H2O, emerging abovethe main cloud deck at 70 km, and Mie scattering caused byH2SO4 droplets in the upper haze (70–90 km). Rayleigh dif-fusion, which e"ciency increases toward the blue following!!4, results from the scattering of light by particles with sizesr much smaller than the wavelength (x = 2"r/! " 1), typi-cally, molecules composing the atmospheric gases. In the caseof Venus, the di!usion is mainly caused by CO2: the scatteringcross section is 8/3x4(n2!1)2/(n2+1)2 (see, e.g., Lecavelier desEtangs et al. 2008a), where n is the refractive index of the gas.For CO2, we use the formula of Sneep & Ubachs (2005) to calcu-late n as a function of wavelength. The upper haze is composedby particles with sizes larger than the wavelength (4 <# x <# 300).Consequently, the calculation of the extinction (scattering + ab-sorption) cross section by the bimodal distribution of upper hazeparticles is based on Mie theory. The complex refractive indexof H2SO4 droplets is taken from Hummel et al. (1988). Below3 µm, the imaginary part of the refractive index is negligibleand the scattering dominates the Mie extinction. The absorptioncross section, linked to the imaginary part of the refractive in-dex, becomes non-negligible above #3 µm, whereas the scatter-ing cross section drops. The extinction cross section is the sumof the scattering and absorption. It is calculated for log-normalparticle size distributions with the set of light scattering routinesavailable at the University of Oxford Physics Department4 andplotted as a function of wavelength for hazes with di!erent par-ticle size distributions in Fig. 2a.

3. Results and discussion

The transmission spectrum of Venus is shown with a reso-lution of 1 nm in Fig. 2b, as relative absorption ##!(!) =#!(!) ! #!(!min), where #!(!) = {[R! + h(!)]/R$}2 and !minis defined as the wavelength where the transit depth is minimal;it is 1.67, 2.50, and 2.65 µm for the haze-free, mode-1 haze,and modes-1+2 haze models, respectively. The e!ective heightof absorption h(!) has the same meaning as in Kaltenegger &Traub (2009) and is plotted in Fig. 2c. It can be retrieved on-line(Table A.1). The amplitude of the spectrum reaches #25 ppm forthe prominent CO2 UV bands below 0.2 µm. From 0.2 µm andabove, the spectrum is dominated by Mie scattering (<2.7 µm)and absorption (>2.7 µm) by upper haze particles, which cansignificantly impact on the amplitude of the absorption from theother spectral features, such as the CO2 transitions around 2 µmor the Hartley UV band of O3, depending on the assumed par-ticle size distribution. The maximum amplitude of this e!ect(between UV and IR) is #6 to 8 ppm for mode-1 and modes-1+2 haze models, respectively. In the infrared, the most no-ticeable feature is the $3 vibrational band of CO2 at #4.3 µm(#15 ppm). For a real exoplanet transit, these absorption would

2 http://amop.space.swri.edu/3 http://www.cfa.harvard.edu/amp/ampdata/4 http://www-atm.physics.ox.ac.uk/code/mie/index.html

Fig. 2. a) Absorption cross sections of the considered atmosphericgases, Rayleigh scattering cross section of CO2 (dashed red line), andMie extinction cross sections for mode-1 (thick blue line) and mode-2(thick violet line) haze particles. b) Transit spectrum of Venus transit-ing in front of the Sun, as seen from Earth, in relative absorption andc) e!ective height of absorption. The di!erent transit spectra shownare models with no upper haze (black), mode-1 upper haze (red), andmodes-1+2 upper haze (green). The top of the main cloud deck is set at70 km. The transit spectrum of the Earth calculated by Kaltenegger &Traub (2009) is overplotted (blue) and shifted by +100 km for clarity.

be a factor 12.3 smaller because there is no parallax e!ect – thedistance to the exoplanet equals the distance to the transited star.

In terms of e!ective height of absorption h(!) (Fig. 2c), thecytherean limb could be probed from 70 to 150 km, i.e., fromthe top of the cloud deck up to above the mesopause, in casethe upper haze is only composed by mode-1 particles. If largermode-2 particles are involved, it won’t be possible to probe thelimb below #80 km, reached at 2.65 µm.

In fact, the lowest altitude that it is possible to reach withtransmission spectroscopy is set by the dominant di!usionregime, Rayleigh or Mie. In the case of Venus, the most re-markable and extended spectral signature is that of Mie scat-tering by the upper haze. This situation contrasts with the Earth,whose transmission spectrum is dominated at short wavelengthsby Rayleigh scattering from N2 and the broad Chappuis band ofO3, as modelled by Ehrenreich et al. (2006). This first basic mod-elling has been refined in the case of the Earth by Kaltenegger &Traub (2009), whose spectrum is reproduced in Fig. 2c.

In an atmosphere where Rayleigh scattering is the dom-inant di!usion process, it is fairly straightforward to assumethat, eventhough there is no spectral identification from spec-tral lines, the main carrier of the Rayleigh scattering is the mostabundant atmospheric gas. That would be H2 for a giant ex-oplanet (Lecavelier des Etangs et al. 2008b) or N2 and CO2,

L2, page 3 of 6

Figure 2: (top) the calculated transit spectrum for Venus in transit as seen from Earth; (bottom) the effective height of absorption of different atmospheric components. From Ehrenreich et al., 2012.

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Transit Circumstances: The transit of Venus as seen from Sacramento Peak will begin shortly after 16:00 local time (22:05 UT), with the Sun at an elevation of 47° (at an airmass of 1.35). Venus’s angular diameter will be 57.8” and will be moving relative to the Sun at a rate of 0.0667 arcseconds/s (geocentric). First contact will occur on the northeast limb of the Sun. The entire ingress, the time between first and second contact, will last 18 minutes. Approximately one hour after second contact, the Sun’s elevation will have dropped to 30º. Greatest transit will occur at 01:27 UT, approximately three hours after 2nd contact, with the Sun at an elevation of seven degrees. Finally, the Sun will set at 02:08 UT (8 PM local time), four hours after the first contact.

Local Circumstances

Sac Peak I = 22:05:44 II = 22:23:13 G = 01:25:35 Sunset = 02:08

Figure 3: The geocentric transit ephemeris prepared by Fred Espenak. Venus's elevation at different times during transit are marked in blue.

Kitt Peak I = 22:06:09 II = 22:23:38 G = 01:25:31 Sunset = 02:29

Figure 4: Map of worldwide visibility of 2012 transit

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Transit Observations As the Venus moves onto the solar disk at the start of a transit, the portion of the planet protruding above the limb is encircled by a bright ring, an effect often reported as far back as the 1761 transit by Lomonsov2. After the first digital recordings of the aureole at the 2004 transit (Pasachoff, Schneider, and Widemann, 2011), Tanga et al. (2012) performed a quantitative examination of the spatial structuring of the aureole. They found that refraction is the predominant cause of aureole, with the background solar illumination bent through the mesosphere of Venus. The scale height and altitude are parameters that can be adjusted to reproduce the observed intensity profile of the arc. The transit observations thus serve as a useful probe of the upper atmosphere of Venus, allowing determination of the radial temperature profile as well as latitudinal and temporal variations of the atmosphere. However, the data obtained in 2004 are not sufficient to fully explore the presence and latitudinal distribution of aerosols in the upper atmosphere.

At second contact, the disk of Venus is internally tangential to the solar limb and the “black-drop” effect is seen (Schneider et al. 2004, Pasachoff et al., 2005.). When the planet is on the disk it is fully illuminated from behind by the Sun. Below heights of approximately 60 km, the atmosphere of Venus is fully opaque at almost all wavelengths. However, as the solar radiation passes through the optically thin upper atmosphere of Venus it is selectively absorbed by the molecular species transitions. These absorption profiles encode information on the physical conditions in the atmosphere, including density, bulk flows, and unresolved “turbulent” broadening.

The transmission spectra of Venus was observed in 2004 at the VTT on Tenerife using the Tenerife Infrared Polarimeter (TIP) in the infrared near 1600 nm, as discussed in Hedelt et al. (2011). They were able to isolate several CO2 lines above the disk of Venus. They used LTE radiative transfer calculations to generate model spectra, which were then compared to the observed spectra to find the best–fit parameters for the temperature and radial distribution of CO2 in the Cytherean atmosphere. While observations were obtained with the slit in a variety of positions with respect to the planet, all these data were summed in order to get sufficient signal to noise. This however results in the loss of all information about latitudinal and temporal variations in the atmosphere.

2Pasachoff and Sheehan (2012, in press) conclude that the effect was not, in fact, observed by M. Lomonosov at the St. Petersberg Observatory because his reports do not appear to match the timing of the 2004 observations.

Figure 5: The aureole of Venus observed with the Swedish 1-m Solar Telescope (SST) in 2004. The contrast of the region of the aureole has been enhanced by a factor of nine.

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Instead at this transit we propose acquiring imaging spectroscopy data with IBIS that will allow us to (a) study atmospheric dynamics at small spatial and temporal scales (the 0.1” pixel scale of IBIS corresponds to 21 km on Venus); and (b) compare the characteristics of the equatorial and polar regions, valuable in interpreting the measurements made in situ by Venus Express. These data will be analyzed in the same manner as Heldelt et al. (2011) to infer information on the temperature, density, and bulk velocities as a function of latitude. The observation of the atmospheric profiles around the full circumference of the planet will also provide valuable in validating models of transiting planets, as discussed in Ehrenreich et al. (2012). Observational Program – Sacramento Peak

Our primary goal is to obtain observations of Venus’s narrow atmospheric layer surrounding full planetary disk. We will isolate this region both spatially and spectrally. We intend to employ three different instruments multiplexed over separate spectral regimes in order to obtain as complete a dataset as possible. The three instruments are:

Instrument Spectral Range Image Scale Notes ROSA 3800, 3966, 4300, 4860 Å 0.08 arcsec/pixel FWHM 10–100 Å IBIS narrowband 5896, 6563, 7820, 8542 Å 0.1 arcsec/pixel R ~2–300,000 IBIS whitelight 6200 Å 0.1 arcsec/pixel FWHM 100 Å

FIRS 15000 – 16000 Å !" ~ 60 Å 0.3 arcsec/pixel

f/36 mode, 75” slit R ~200,000

The observations will be divided into two phases. The initial period will be that between first and second contact when the aureole of Venus is visible, going through to the brief period following second contact when the “black-drop” effect is visible. Once Venus is well inside the solar limb, observations dedicated to transmission spectroscopy can be obtained. Aureole

In order to address the question of the presence of aerosols in the upper atmosphere of Venus, we propose to perform multi–wavelength observations with ROSA through four different filters ranging from the near-UV to green. These will be used to look for the wavelength dependence of the scattering (as compared to the solar spectrum) that would be the signature of Cytherian atmospheric aerosol.

The surface brightness of the aureole increases as a greater fraction of the disk of Venus moves inside the solar limb (Tanga et al., 2012). Around first contact, this surface brightness can be anywhere from 10 to 100 times fainter than the intensity of the solar disk. Since the thickness of the refracting layer is close to our spatial resolution, the most important way to increase the detectability of the aureole arc is to maintain a high

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spatial resolution, in particular through the use of short exposures and adaptive optics/tip-tilt correction3.

ROSA will be run at a high frame rate in order to collect the multiple images needed for the application of post-facto image reconstruction. These techniques, in particular speckle reconstruction, have been shown to be photometrically correct and to work even on faint structures above the solar limb; they will be applied to these data in order to produce images with a maximum spatial resolution of 0.15 arcseconds, corresponding to 32 km at Venus.

The pixel scale of the images (~0.075 arcsec/pixel) is constrained by the need to fit the entire 60 arcsecond diameter disk of Venus, framed by a sufficient portion of the solar surface, on the 1000 x 1000 pixel ROSA detectors. For this reason we are also looking into bringing an additional large-format (2k x 2k), rapid-readout (50 fps) camera to complement the ROSA system. The ROSA observations will continue through second contact to get a continuous series of observations for further studies of the black-drop effect (indicatively about 8 minutes past first contact, when Venus will be one planetary radius away from the limb). We will also obtain several sets of observations on the disk as well in order to look for any signature of the aureole enhancement around the disk during these later phases.

Figure 5: Subfield of an image obtained at the Dutch Open Telescope during the 2004 transit (see Tanga et al. 2011). The width and altitude of the aureole (or Lomonosov arc) can be measured directly with an accuracy of 30 km or better.

While high-resolution images were obtained from the solar telescopes in the Canaries and from TRACE in 2004, no spectroscopy of the aureole has yet been performed. The imaging spectroscopy capabilities of IBIS are well suited to rapidly recording the scattered spectral profiles in the arc. The modification of the profiles after passing through atmosphere of Venus could provide additional information higher levels of the atmosphere than accessible with transmission spectroscopy later in the transit.

We propose to use IBIS to perform high-spatial-resolution imaging spectroscopy of the full aureole arc in several different spectral lines. These will include the solar Na D1 5896, Hα 6563, and Fe I 7090 Å lines, as well as the CO2 7822 Å lines formed directly in the Cytherean atmosphere (see below). This latter may reveal interesting features due the integration over a long refracted path length through the atmosphere. We will

3 The ability to use the adaptive optics system during the ingress phase needs to be further evaluated. More likely, only a tip tilt correction, using the on-disk edge of Venus or the limb of the Sun, will be possible.

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evaluate whether the IBIS observations of the off–limb arc should be acquired in the single Fabry-Perot (FPI) mode to increase throughput and allow short (5-10 msec) exposures. Once Venus is mostly or fully on the disk, the narrow FPI could be quickly reinserted into the beam and the observations continue. Transmission Spectroscopy:

Once Venus is well inside the limb (approximately 15 minutes after 2nd contact), we will perform spectroscopy of the atmosphere seen in absorption. Here we will use several CO2 lines inside one of the molecular bands present in the transmission spectrum. The use of the molecular lines has the significant advantage that all the CO2 absorption can be ascribed to the atmosphere of Venus, with no confusion from corresponding solar lines (as in the transit of Mercury, when the sodium absorption from the exosphere lies in the wing of the much stronger solar sodium absorption). We have identified a sequence of CO2 lines at 7822 Å in a clean region of the solar spectrum with few solar or telluric lines to conflate the transit signal. A dedicated filter is being ordered from Andover Corporation for this spectral region. We will perform repeated spectral scans of several of the CO2 lines in this range. We expect to be able to achieve a velocity resolution on the order 0.1 km/sec at all azimuthal angles around the planet’s back-lit limb. We will perform two different sets of observations, one optimized to high temporal resolution, with repeated scans through a single CO2 line with only a single image at each wavelength postion. Another set will be acquired with approximately 10 images per wavelength position. With this latter set, image

Figure 6: The calculated transmission spectrum of the atmosphere Venus in the 782.2 nm CO2 band during transit (purple). The spectrum is the combination of the solar, Cytherean, and terrestrial spectrum convolved with the 35 mÅ FWHM bandpass of IBIS. The solar atlas spectrum (red) and terrestrial absorption spectrum (green) show almost no features in this spectral range. The yellow shows the theoretical passband of the 782.3 nm prefilter.

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reconstruction techniques can be applied to the narrowband data to assure the highest spatial resolution (at the expense of the temporal cadence). We will also use FIRS to observe the CO2 lines in the infrared region. This observation is comparable to the observations performed at the VTT during the 2004 transit and analyzed by Hedelt et al. (2011). Since the multiple slits of FIRS (with a separation of ~43” separation) will not efficiently sample the disk of Venus, we will instead remove the DWDM4 filter and operate in the single slit mode. This will allow us to achieve a larger spectral coverage, approximately 50 Å with a 0.05 Å sampling, which is important in detecting as many CO2 lines to model as possible. In the spatial direction the slit will cover 75 arcseconds with a spatial sampling of 0.08 arcsec/pixel. Coordinated Observations: The proposed observations are already being directly coordinated with the observations from several other solar telescopes, including: McMath–Pierce / NAC SOLIS / ISS SOLIS / VSM SOLIS / FDP Mees Solar Observatory Hinode / SOT Hinode / XRT SDO / AIA SDO / HMI Yunnan Observatory, China This broad set of complementary observations will be assembled into a cohesive dataset for analysis many different groups. We envision making the DST data freely available for other researchers. This work is funded by a National Geographic Society, Committee for Research and Exploration Grant, a NASA/AAS Small Research Grant (for purchase of the CO2 filter, and a NSF grant for the analysis of the acquired data at Williams College.

4 Dense wavelength division multiplexing

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atmosphere of Venus as revealed by VIRTIS on Venus Express,” Nature, 450, 7170, 641-645.

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Lilensten, J.; Lecavelier Des Etangs, A.; Arnold, L., 2012, “Transmission spectrum of Venus as a transiting exoplanet”, Astron. & Astrophys., 537, id.L2

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