HIGH-RESOLUTION SATELLITE IMAGING OF THE 2004 TRANSIT OF VENUS AND ASYMMETRIES IN THE CYTHEREAN ATMOSPHERE

Transits of Venus have been observed from Earth only in 1639, 1761, 1769, 1874, and 1882 prior to 21st century's 2004 and 2012 transits. Historically, transits of Venus were expected to provide measurement of the astronomical unit. The method was proposed after the Mars-parallax successful observation by Cassini, Richer, and Picard from Paris and Cayenne in 1672. Halley (1716) suggested that the distance to the Sun could be directly obtained by the observation of a transit of Venus from distant stations on Earth, making these international expeditions integral to arguably the most important problem in astronomy for over two centuries, since Kepler's law provided only proportional spacings of the planets (e.g., Woolf 1959; van Helden 1995; Chapman 1998; Ferguson 1999; Maunder & Moore 1999; Maor 2000; Hughes 2004; Sellers 2001; Pasachoff & Filippenko 2007).

We have provided a history of prior observations of transits of Venus in Schneider et al. (2001, 2004, 2005) and Pasachoff et al. (2003, 2005a), in which we used spacecraft observations of the 1999 transit of Mercury to solve the long-standing problem of the black-drop effect, showing its origin in both a point-spread function and the extreme solar limb darkening at the extreme limb. We also discussed the history of the black-drop effect during transits of Venus in Pasachoff et al. (2003, 2005a); though our ground-based observations were from Greece, to be able to observe both black drops with favorable weather predictions, our paper appeared in the proceedings of the IAU Symposium (Kurtz & Bromage 2005) that was held during the 2004 transit at the site in England at which Jeremiah Horrocks had, in 1639, first observed a transit of Venus (Horrocks 1639, as cited in Hevelius 1662).

Our series of observations of transits of Mercury in 1999 and 2003 and, later, of Venus in 2004 had been motivated by Schaefer (2000, 2001), who had pointed out that more than half the recent published mentions of the black-drop effect were incorrect in discussing its cause, mistakenly attributing the effect to Venus's atmosphere, which is too narrow to provide the observed effect. Motivated also by the historical accounts of previously observed transits of Venus and our previous work on the black-drop effect (Pasachoff et al. 2005a), we took advantage of the first transit of Venus to be seen from Earth since the ascending node pair of 1874 and 1882, which accounted for both black-drop and special atmospheric effects (Figure 1).

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Beside revisiting those long-standing issues resulting from the extreme scarcity of transit events (a couple of events, spaced by 8 yr, every 121.5 then 105.5 yr), we also desired to use the 2004 June 8 event as a surrogate to test observing methods, strategies, and techniques which have since been implemented by the CoRoT and Kepler space missions to detect and characterize extrasolar terrestrial planets as they transit their host stars.

We have previously reported (Schneider et al. 2005, 2006) on the detection of the 2004 transit of Venus in the Sun's total solar irradiance as an 0.1% dip lasting hours, with its ingress, egress, and mid-transit variations providing a direct analog to the exoplanet transit observations, though with much higher accuracy and detail (Schneider & Pasachoff 2006; Pasachoff et al. 2008).

It is usually stated that Venus's atmosphere was discovered by Lomonosov at Saint-Petersburg's Observatory on 1761 May 26 (O.S.) = June 6 during the descending node transit couple of 1761–1769 (Marov 2005). Lomonosov reported an aureole of light on the planet's limb, which he related to an atmosphere around the planet. Sheehan (2011, private communication) notes that Lomonosov's actual wording seems to correspond better to a black drop than to a Cytherean atmosphere and credits Johann Schröter in the 1790s for first firmly establishing the existence of Venus's atmosphere. Atmospheric effects have been subsequently detected at various, non-transit Venus inferior conjunctions in the crescent's cusps (H. N. Russell 1899). Historical reviews are provided, e.g., by Link (1969), Tanga (2004), and Sheehan & Westfall (2004).

With the advantage of contemporary fast photometry instrumental techniques, our primary programmatic objectives for the 2004 transit were three fold: using spatially resolved Transition Region and Coronal Explorer (TRACE) images to revisit previously described black-drop and atmospheric aureole effects on one part, using the spatially unresolved Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on board ACRIMsat for the solar irradiance (Pasachoff et al. 2005b; Schneider et al. 2006).

• 1.  
  Black drop: we intended to use the space-based imagery to demonstrate that the—little-cited—conjecture that ground-based historical reports of "black drops" (e.g., Bergman 1761 and Figure 1; see Pasachoff et al. 2005a) at inner-planet transits are in fact a result of convolution of the instrumental (telescopic/optical) with atmospheric point-spread function in conjunction with the limb-darkened photospheric illumination function, as we showed was the case to explain the black-drop effect for the 1999 transit of Mercury (see Schneider et al. 2004);
• 2.  
  Refraction of solar light: our second purpose was to obtain the first, space-borne fast-cadence images of the circum-Cytherian "aureole," which results from refraction of sunlight in the observer's direction by layers of the upper atmosphere of Venus, with high spatial resolution and image stability, with the purpose of providing a throughout brightness latitudinal distribution as a function of time, prior to contact II and after contact III. This aureole results from refraction of sunlight in the observer's direction, by layers of the upper atmosphere of Venus. This objective has acquired a renewed interest in the context of the Venus Express orbiter investigation of the upper atmosphere structure of Venus in the polar region (García Muñoz et al. 2009; Ignatiev et al. 2009; Widemann et al. 2010a, 2010b).
• 3.  
  Testbed for photometry of an exoplanet transit: in a separate study using the un-spatially resolved ACRIM instrument, we followed the effect of the transit of Venus on the total solar irradiance and related it to current and future exoplanet observations, providing a sort of ground truth showing an analog in our solar system to effects observable only with light curves in other solar systems with the Kepler and CoRoT missions and ground-based exoplanet-transit observations (Schneider et al. 2005b).

In order to meet the science objectives (1) and (2) requiring imagery at high spatial resolution, we used NASA's TRACE spacecraft (Golub & Pasachoff 2009) to image the 2004 June 8 transit of Venus across the undiscriminated visible spectrum from UV to near-IR (see Section 2.1), at very high temporal cadence during (and flanking) the planet's crossing of the solar limb and while on the heavily limb-darkened portion of the solar disk (see Section 2.3). We simultaneously observed in Greece (Pasachoff et al. 2004b) and coordinated with observations with the Swedish 1-m Solar Telescope on La Palma in the Canary Islands, Spain (Pasachoff et al. 2006), in order to supplement the space-borne study with ground-based measurements (Pasachoff 2004).

2.1. TRACE Wavelengths

2.2. Spatial Scale and Resolution

2.3. Temporal Cadence

2.4. Images, "Movies," and Stereograms

We present here the first photometrically rigorous results from the ingress and egress imaging sequence taken in TRACE's WL passband. The TRACE WL optical channel, so we discovered, suffers from a ∼1% intensity optical "ghosting." Normally, this ghosting is not of concern for TRACE's solar observation programs, but as our interest rests primarily in the "bottom" 1% of the (12 bit) dynamic sampling range, this ghosting presented a significant photometric "challenge." With detailed characterization we found the "ghost" to be double, with different degrees of afocality and intensity, plus an additional diffuse (not afocally specular) component more difficult to analytically characterize. For each frame, we built and subtracted models of the instrumentally scattered light to remove these artifacts (examples of this will be added later). The images presented here have been post-processed in this manner, and while not "perfect," this processing very effectively mitigates the instrumentally ghosted/scattered light. This procedure, however, cannot recover intensity information with sufficient photometric precision in a ∼2 pixel (10–15) wide region exterior to the photosphere where the solar limb is bifurcated by the Cytherian disk. This is the reason for the "gap" you will note in the image of Venus presented here as Venus transits the solar limb.

In addition to the images presented here, movies and three-dimensional (3D) stereograms of the transit made from selected TRACE imaging frames are available through the link http://www.transitofvenus.info.

The ingress sequence (Figures 2 and 3) is comprised of 98 frames from 050553 UT to 054751 UT, optimized for the atmosphere and for any black drop, respectively. The egress sequence (Figures 4 and 5) is comprised of 84 frames from 105231 UT to 112315 UT, optimized for the atmosphere and for any black drop, respectively. Both sequences have been assembled into movies with a playback rate of 10 frames per second. The movies (ingress atmosphere and black drop optimized and egress atmosphere and black drop optimized) are rendered as 16 bit (32 kilobyte-level gray scale). These animations track on the Sun; a third version for each ingress and egress has been computed to track on Venus's disk. Especially for astronomers at the times of the 2117 and 2124 transits, we are also providing the ingress and egress images in FITS formats as supplemental material.

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In the first six frames of the ingress sequence and in the "Venus Atmospheric Refraction" movies, a linear dynamic display range is used, from –15 (hard black) to +55 (hard white) instrumental counts (DN: data numbers) per pixel. (TRACE has a dynamic data range of 4096 DN.) The background sky has been median normalized to zero DN, and the 1σ noise in the background is ∼2 DN. In the bottom two frames of the ingress sequence, and in the "No Intrinsic Black Drop" movies, a display stretch over a larger dynamic range was made, to better define the limb of Venus against the "noise" of the solar granulation. The same two display stretches are used for the egress sequence images.

The solar photospheric radial limb-darkening profile has been removed from each of these images. This "flattens" the intensity of the photosphere and collapses the dynamic display range, thus very well shows the photospheric granulation. After "flattening," the intensity variation in the photospheric features is 20 times greater than the light seen, at its brightest, in the Cytherean atmospheric aureole.

3.1. Optimizing the Image Dynamic Display Range for the Black-drop Effect

3.2. Optimizing the Imaging Dynamic Display Range for Limb Atmospheric Refraction

To simultaneously visualize Venus's atmosphere against the sky, and Venus's limb against the photosphere, the solar disk (with Venus in front of it) has been reduced/scaled in intensity by a factor of 10. In Figures 2–5, the median level of the photosphere—after flattening by removing the radial limb-darkening profile (see Schneider et al. 2004; cf. Section 2.2 and Figures 7–9) and subsequent reduction in intensity by a factor of 10—has been set to +25 DN (rather than zero, i.e., offset from the sky) to enable, very easily, seeing the solar limb against the sky background. With this display stretch, the interior silhouetted disk of Venus appears hard black against the granular photosphere (i.e., similar to the contrast as it appears to the eye with the Sun appropriately filtered). Simultaneously, the disk of Venus against the sky appears as the same intensity as the sky, which as Venus is backlit, is appropriate.

The TRACE images very clearly show the emergence of sunlight (refracted) by the Cytherean atmosphere as Venus approaches the line of sight to the solar disk. The phenomenon is clearly asymmetric, as a small arc of atmosphere first becomes visible on the southern disk of Venus, near the Venus south pole, not on a line perpendicular to the solar limb. By "geometrical mid-ingress," when the solar limb is equally bifurcated by Venus, this arc of scattered light is readily apparent.

Stereograms showing the phenomena in 3D appear in Figure 6; higher-resolution versions are available in the online edition.

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About 12 minutes before second contact the atmospheric arc becomes "full" and contiguous, and grows brighter with decreasing angular distance to the solar limb. Upon egress the asymmetry in the brightness along the atmospheric arc is striking. Second (053850 UT image) and third (115849 UT image) contacts are geometrically "sharp" except for "blurring" by the 1" full-width at-half-maximum TRACE WL point-spread function and residual instrumentally scattered light in the TRACE satellite optical system.

Our spatially resolved TRACE observations obtained during the 2004 transit of Venus, as illustrated by Figures 3(c) and (d) and 4(a), show no evidence of the classical/historical "black-drop" effect. This finding illustrates the success for the predicted negative result based on TRACE WL observations of the 1999 November 15 transit of Mercury (Schneider et al. 2004; Pasachoff et al. 2005a), where it has been demonstrated that the effect for both Mercury and Venus inner-planet transits was a result of the convolution of the instrumental (telescopic/optical) with the atmospheric point-spread function in conjunction with the limb-darkened photospheric illumination function. The centuries-old effect is therefore a non-intrinsic phenomenon (it is gone, now, the way of the Martian canals). Since for a transit of Mercury, the telescope's point-spread function alone was insufficient to explain the black-drop effect, and since we could explain the entire effect only by convolving the point-spread function with the sharply varying limb-darkening profile at the extreme solar limb (Schneider et al. 2004), there is every reason to extrapolate that result to transits of Venus.

The dry, 96.5% carbon dioxide (CO₂) and 3.5% nitrogen (N₂) atmosphere of Venus has been extensively surveyed by both orbiter and in situ measurements in the last three decades. Its study has been the subject of pioneering solar system exploration—Venus was the destination of the first interplanetary spacecraft, Venera-1, launched by the Soviets on 1961 February 12. Although the atmosphere was first claimed by Lomonosov in the 1700s, and CO₂ discovered in it by Adams & Dunham (1932), its 4–5 day superrotation was still debated in the 1970s and only fully recognized after the highly successful Pioneer Venus orbiter and multi-probe mission (1979–1992).

In situ measurements have been performed essentially in the mesosphere below 100 km and below the clouds. Information about minor atmospheric constituents, their concentration, reactions, sources, and sinks remains essentially incomplete, as only scarce measurements of localized vertical profiles have been performed above 100 km altitude, where temperature fields, especially in the polar regions, as well as their time variability, are still debated (see, e.g., Vandaele et al. 2008; Clancy et al. 2008). The upper cloud structure itself reveals distinct dynamical structure in the latitudinal direction, with convective, wave-dominated zonal flow in the lower latitude range near Venus's equator, and a significant transition to less convective banded flow between 45° up to about ±70°–80°. Ground-based Doppler wind velocities confirm the mean zonal flow deduced from tracking of cloud features (Widemann et al. 2007, 2008; Widemann & Liao 2010). The temperature field forms a bulge of cold air at 60°–80° latitude called the "cold collar region." This torus-like structure encloses a vast vortex-type structure several thousand kilometers across, with a slower rotation period (from –2.5 to –2.8 Earth days) and significantly depressed cloud top altitude, at about 62–65 km instead of 70–74 km closer to the equator (Svedhem et al. 2007; Ignatiev et al. 2009).

Simulation of the transit circumstances reveals that the tangent point at second contact was about 25° from the Cytherean equator and the tangent point at third contact was at latitude approximately 45°. Initial modeling of the effect of differential refraction on the deviation of solar light by an isothermal atmosphere, of scale height about 3.5–4 km (e.g., García Muñoz et al. 2009), in the direction of the observer indicates the rays originating from the solar photosphere cross a layer about 35–40 km above cloud tops (P. Tanga 2011, private communication). This geometrical property of the upper atmosphere is in agreement with the latitudinal increase of refracted sunlight in the transit aureole we imaged using the TRACE WL data and especially visible in Figures 2(b), 4(d), and 5(a) of this paper. The latitudinal extent on Venus south polar limb matches very well the polar vortex region southward of 70°–80°, where cloud tops are lower by about 8–10 km. Similar agreement can be traced back to Russell's drawings of 1874 (Figure 1) for the northern hemisphere of Venus.

The arc of atmosphere seen includes the polar region of Venus and indicates that the asymmetry is due to the latitudinal dependence of upper cloud altitude northward of the polar collar in the polar vortex region of Venus's atmosphere. Additional constraints on Venus's latitudinal distribution of upper cloud altitude from the ESA Venus Express mission are being compared with ground-based observations and with our TRACE space-based observations by Tanga et al. (2011). We have set up a collaboration with Tanga to measure brightness of the atmospheric arc as functions of position along the arc and of time for both the 2004 observations and, one hopes, eventually the 2012 observations.

Regarding the photometry of the event compared to the total solar irradiance, using the spatially unresolved Venus transit light curve obtained by ACRIMsat (and a similar one obtained by SORCE/TIM) by Schneider et al. (2006), variations were found to be 0.0018% at the 1σ level (compared to a 0.0019% expected change in signal) with inter-region variations in internal dispersions of 0.00022%. Hence, a sensitivity to the presence versus the absence of a Venus-like opaque planetary atmosphere was tested at only a 1.05σ level of confidence, allowing the authors to discuss the ability to detect exoplanet stellar transits photometrically with scalably comparable instrumental sensitivities. Because newer generation programs to detect super-Earths and their atmospheric signature by exoplanet primary transits are currently submitted as future programs, our team has started an investigation of intermediate-spectral-resolution techniques to probe the molecular species content of the refractive layers near 110 km (80 km) in the polar vortex region by positioning a spectro-imaging device in the ingress and egress region from a solar observatory, without a coronographic system. This procedure would eventually lead us to more quantitative assessments on CO₂ broadband detection below 190 nm in the aureole spectrum, as well as upper limits of detectable species in the context, including the CO rovibration 2–0 band at 2.3 μm, and the 1–0 band at 4.7 μm.

Now that the Atmospheric Imaging Assembly on NASA's Solar Dynamics Observer (Golub & Pasachoff 2009) has replaced TRACE, its 12 s cadence for eight of its 10 filters and full-disk field of view with similar 0.5 arcsec pixels meaning that the transit of Venus observations on 2012 June 6 will be automatically made, without the need for the special arrangements that we made for TRACE cadence and pointing in 2004. Venus Express's and other observations of Cytherean clouds will be used for explaining any observed transit asymmetries for the 2012 transit, our last opportunity to do so from Earth until the next pair of transits of Venus, which will be in the ascending node, on 2117 December 11 and 2125 December 8.

We thank Karel Schrijver, Ted Tarbell, and other members of the TRACE science and operations teams for their invaluable assistance and efforts in planning and implementing these observations; and Leon Golub at the Harvard-Smithsonian Center for Astrophysics for advice on TRACE observations and data. Kayla Gaydosh (then at Bryn Mawr College) assisted with the image calibration and post-processing under a summer research program supported by the Keck Northeast Astronomy Consortium. David Butts, Joseph Gangestad, and Owen Westbrook also assisted on site in Thessaloniki, Greece, with our coordinated ground-based observations. We thank John Seiradakis of the Aristotle University of Thessaloniki for his collaboration. We also thank the Committee for Research and Exploration of the National Geographic Society for their research-grant support for the transit of Venus expedition. Pasachoff's solar observations were supported in part by Guest Investigator NASA grant NNG04GK44G for work with the TRACE spacecraft and are now supported in part by NASA Marshall grant NNX10AK47A. Pasachoff's planetary work is supported in part by grant NNX08AO50G from NASA Planetary Astronomy to Williams College. He thanks Michael Brown and the Department of Planetary Science at Caltech for hospitality during the completion of this paper.

The Transition Region and Coronal Explorer, TRACE, is a mission of the Stanford-Lockheed Institute for Space Research (a joint program of the Lockheed-Martin Advanced Technology Center's Solar and Astrophysics Laboratory and Stanford's Solar Observatories Group), and part of NASA's Small Explorer program.