provide a detailed description of the JANUS instrument. Here we only give a short summary of the instrument characteristics.

JANUS is a camera with an effective focal length of 467 mm and is equipped with a 1504 × 2000 pixel CMOS detector, resulting in a plate scale of 15 µrad per pixel and a FOV of 1.29° × 1.72°. The rows of pixels are read out in rolling shutter mode with a line read-out time of 0.2215 ms. A filter wheel allows to select among 13 individual filters whose characteristics were selected to study the Jupiter system. Table  summarizes the wavelengths and band-passes of these filters.

The JANUS panchromatic filter gives access to imaging under very low illumination conditions. Broad band filters are sensitive to color, and narrow band filters are sensitive to the vertical structure of Jovian clouds through the combination of filters inside methane absorption bands and at close spectral distances. Additional filters in JANUS were selected for investigations of the Jovian moons. A relevant filter for Earth observations is the JANUS F8 filter at 940 nm. This filter lies within a strong water-absorption band and provides sensitivity to vertical cloud heights on Earth. Observations of Earth clouds in this filter play a role comparable to that of the F7 filter on Jupiter, which probes Jupiter's atmosphere in the strong methane-absorption band at 889 nm. Additionally, filters F5, F6, F7, F11 and F12 are also weakly sensitive to water absorption bands.

A total of 140 images of the Earth were acquired on 20 August 2024 between 21:19:03 and 21:50:03 UTC. At the closest approach, approximately at a distance of 8400 km from Earth's surface, the images achieved a spatial resolution of 125 m per pixel. During the imaging sequence, the spacecraft moved relative to Earth's center with a horizontal velocity of 5.02–6.82 kms-1. Exposure times ranged from 0.222 to 162 ms, generally shorter than the JANUS dwell time for those velocities. The time interval between the start of consecutive images varied from 0.46 to 50 s, depending on exposure duration and filter-change requirements. A few night-side test images acquired with narrow-band filters were smeared by 3–5 pixels due to spacecraft motion.

The high velocity flyby at a very short distance to the Earth's surface, together with other operational constraints on the attitude of JUICE, resulted in the acquisition of a small number of multi-filter sequences. These were acquired in the South China Sea and the West Pacific. Also, only a few locations were observed twice with the same filter. Those observations were separated in time by only a few tens of seconds, making wind retrievals from cloud tracking extremely challenging. Pointing was inertial and the observation sequence was constrained to phase angles of 90° due to thermal constraints on the spacecraft. The images were lossless compressed. Tables A1 to A3 in the Appendix provide a list of the LEGA images from 20 August 2024 selected for this study. Images are identified by an image ID composed of an image session ID with 10 digits, an image number during the observing sequence made from 4 digits, and two digits that code the filter used from 01 to 13.

The data acquired on 9 September 2024 consisted of a sequence of 84 images acquired at a distance of 564 300 km under a phase angle of 70.0° and with a spatial resolution of 84.66 km per pixel at the sub-spacecraft point. Images were obtained in the 13 filters available to JANUS using six exposure times from 0.222 to 28.795 ms. All the images kept the same pointing and included both the Earth and the Moon. We focus our attention on the portion of the images covering the Earth. Table A4 lists the distant Earth observations analyzed in this paper.

The first image of the Earth was an observation of the limb of the planet during nighttime at a distance to the horizon of 18 010 km (Fig. a). The image was acquired with a exposure time of 36.1 ms with the F1 filter (panchromatic). The short-time exposure was selected due to the short dwell-time associated with the fast flyby and the unexpected nature of what the observation would show.

Given the low-exposure of the image, stray light from sunlight resulted in a circular artifact likely created from the illumination of the internal vanes of the long external baffle. This artifact appears in most of the nighttime images with different contrast. Stray light reduction strategies are under evaluation see for instance, and additional data is being acquired during periodic checkouts of the instrument in the cruise to Jupiter. In the airglow image, the ring patterns created by stray light have an influence on the determined radiance of about 0.17 mWm-2sr-1. In addition, the standard radiometric calibration process (see , this issue) results in the addition of a noisy regular pattern in the darkest regions of the images (see Fig. b). This is because of the behavior of the calibration algorithm that may introduce artifacts in case the signal is very low (less than 10 digital units in the original raw image). These effects are being studied for correction in future versions of the calibration pipeline, and efforts are being done to capture new data on stray light conditions to better characterize this effect.

Earth's airglow is the most notable feature in Fig. a. The airglow appears as a bright halo detached over the surface of the planet with a radiance level that peaks at 90 km above the surface with values of 0.15 mWm-2sr-1. Additional airglow extends over altitudes around 250 km (Fig. d–f). Multiple stars of magnitudes 8.1–10.8 are visible inside and outside the mean airglow emissions. Photometric measurements of these stars, corrected for background contributions from airglow and stray light using aperture mask photometry results in an estimated surface brightness of 18 magarcsec-2 for the main airglow layer at 90 km height. This value compares well with typical estimates of night-sky brightness observed from the ground, with brightness values of about 21.6 magarcsec-2 for observations of the zenith . The extended emission at 250 km competes in brightness with stray light, and its spatial structure and overall brightness are more complex to characterize. In the wavelength range of the panchromatic filter, the most intense emissions of Earth's airglow come from atomic oxygen (OI) at 630 nm, sodium (NaD) at 588.9–589.6 nm, and hydroxyl (OH) (all peaking near 90 km). The extended emission around 250 km is most likely contributed by OI emissions at 630.0 and 636.4 nm .

Besides airglow, the JANUS Earth observations in Fig.  show scattered light over the horizon from the surface to an altitude of about 25 km. However, the low radiance level and low signal to noise ratio of the image do not allow for further exploration of light scattering from the atmosphere on the nightside limb.

Finally, close to Earth's horizon, the image also shows clouds illuminated by the Moon and bright lights in the west coast of Madagascar. These bright lights are observed again at the same geographical positions in a JANUS image acquired 50.2 s later with more coverage of the ground and less coverage of the sky (Fig. c). In this second image, large cloud systems illuminated by the full Moon are visible over the Indian Ocean. From the calibrated JANUS panchromatic image, the radiance levels of these clouds and the overall Moonshine in this location is ∼0.3 mWm-2sr-1, which is in overall agreements with expected values from typical ground measurements of lunar illumination .

JANUS will image Jupiter's nightside limb several times on its orbital tour . The recent discovery of a widespread Jovian airglow in the near-IR by the James Webb Space Telescope opens the possibility to search for visible counterparts to those emissions. However, the key observational target on JANUS observations of nightside limb are auroras. Both, the Galileo orbiter and the Juno mission have been able to observe auroras on Jupiter in the visible range . Galileo observations showed complex structures over extended arcs 250 km above the 1 bar level. The auroral emissions extended over vertical distances of 120 km, and were observed at a spatial resolution of 26 km per pixel. Juno observations of auroras and Jupiter's limb trace strong and weak emissions at multiple latitudes . JANUS will image Jupiter's limb at a pixel scale of 10–20 km in several perijoves over the JUICE mission. Thus, we expect to map not only the spatial extent of auroras, but also to characterize their vertical structure and obtain information about its variability at different time-scales. Jupiter auroral emissions in the visible observed by Galileo with near nadir viewing had intensities of ∼ 80 kR ∼ 300 kR . These are about 10–30 brighter than the airglow emissions observed in JANUS images of the Earth. In addition, Jupiter Observations at perijove will have much larger dwell times than in the fast Earth flyby allowing for the acquisition of well exposed detailed views of the Jovian auroras.

Replicating nighttime observations of the scattering of moon light above the horizon observed on these images of Earth's limb might be difficult on Jupiter, where the Galilean moons receive ∼25 times less light. However, Jupiter's extended hazes in the equatorial and polar latitudes will increase light scattering effects, and equivalent observations of Jupiter by JANUS will be run under much longer integration times. Thus, detached haze layers as those observed by Galileo at high phase angles might be observed on JANUS images of the nightside when JANUS frames include the limb of the planet.

Further, JANUS may be able to observe visible counterparts to the UV auroras of Ganymede detected by the Hubble Space Telescope and Juno , with auroras in the visible observable on Ganymede's nightside . These auroral observations might provide key information about Ganymede's internal ocean.

Nighttime images of the Earth were acquired over tropical latitudes from Madagascar to Thailand. Many of these night-time images contain bright sources from city lights, fires and ships, most of them much weaker than typical lightning events. Detailed analysis shows that only one of the images contains a potential candidate for lightning. However, the detection of these nighttime compact light sources illustrate the potential of the instrument to detect lightning on Jupiter, where images will cover much larger fields of view and lightning events encompass a wide range of optical powers e.g.. Thus, these observations, and calibration of the light sources detected, help us to guide the design of surveys of lightning on Jupiter.

Figure  contains a detailed view of a small section of the full JANUS image shown in Fig. . Several compact lights can be seen near the horizon. Their geographical position and brightness are repeated in the next JANUS image obtained 50 s later. Panel (b) shows a map of the original observation in terms of longitude and geodetic latitude. Some of these compact lights correspond to the city of Mamoudzou in the island of Mayotte, East of Madagascar, and potentially, also to the city of Andoany and villages around Cap D'Ambre in the north coast of Madagascar. Most other lights, including the most intense sources, correspond to fires in Madagascar (fires are common in the area at that time of the year due to farming activities). The nature of these lights as fires in rural and bush areas was verified using data from NASA's Fire Information for Resource Management System (FIRMS; see https://firms.modaps.eosdis.nasa.gov/, last access: 9 July 2026), which provides daily maps of hot spots on the surface of the Earth from IR observations by Earth monitoring satellites. Similar light sources were observed in JANUS images over Cambodia and Vietnam, and again were verified to be caused by surface fires from comparisons with FIRMS data.

Nighttime observations of the Indian ocean showed an interesting case of a dim light spot over a convective cloud. Figure  shows a group of moderately bright pixels appearing on image 0777503922_0005_01 acquired at 21:23:13 UTC. This small light spot was located inside a convective system at 80.73° E, 2.32° N, south of Sri Lanka, and has a distribution of light close to the Point Spread Function (PSF) of the instrument for a punctual source. An image of the same area obtained 50 s later (image 0777503922_0006_01) does not show the same group of mildly bright pixels. Both images had a spatial resolution of 196 m and an exposure time of 36.1 ms. Meteorological satellites show the presence of a ∼ 4000-km long convective line at that region. In order to investigate the nature of this event, we examined lightning detections of the area captured by the Earth Networks Total Lightning Network (ENTLN). This network locates lightning activity from a global array of radio sensors capturing broadband radio-frequency emissions from lightning discharges . We also examined data from the popular Blitzortung network operated by volunteers . However, none of these networks show any signature of a lightning event in the 4 s around the time of the JANUS image acquisition. Also, the World-Wide Lightning Location Network (WWLLN) detecting very-low-frequency radio waves (“sferics”) that can travel thousands of kilometers from their lightning origin did not observe lightning on that specific location over the 21:00–22:00 UTC time range. These negative detections imply that if there was a lightning event it should have been a very weak one.

Since all JANUS images are calibrated in radiance, the optical energy of the group of bright pixels can be estimated. To do that we integrate the total flux of the group of pixels in the image using aperture photometry to remove the background illumination (12 mWm-2sr-1), we multiply by the pixel area (1962 m2) and by the exposure time of the image (36 ms). A multiplication by a geometric correction factor 4π is also included assuming isotropic emission. This results in an estimated detection of optical energy for this event of 210 J. This evaluation of the optical energy does not take into account light extinction within the cloud. Since exposure time and lightning development time-scales are comparable, the energy associated with these bright pixels represent a lower energy limit to the energy of the lightning candidate.

The optical energies of lightning flashes in the visible wavelength range is typically on the order of 1.2–3.6×106 J depending on lightning type , although substantially weaker flashes have also been reported. Thus, the energy detected in this event is about 4 orders of magnitude below typical values of optical energy of lightning in the visible and near-infrared. This order of magnitude estimate of the optical energy is also 2–4 orders of magnitude smaller than reported optical energies of sprites and is also much smaller than other Transient Luminous Events (TLEs) like ELVES, Halos or Blue Jets . The estimated read-out noise of this image is about 0.05 mWm-2sr-1, or about two orders of magnitude smaller than the estimated radiance of this event.

Since noise from a cosmic ray or particle hit at the detector cannot be disregarded, this event highlights the difficulties to identify the nature of weak light sources on nightside images. For the future JANUS investigation of Jupiter lightning, defining a detection threshold for lightning can be problematic when analyzing observations acquired from large distances in the more energetic environment of the Jovian magnetosphere. Systematic observations will acquire enough detections to provide an identification threshold based on statistics on the events observed.

Since noise from cosmic rays or particle hits at the detector cannot be fully disregarded, this event highlights the difficulty of identifying weak light sources in nightside images. In principle, cosmic rays may be distinguished from lightning flashes by their sharper spatial profiles compared to the instrument small point spread function (PSF) (currently being analyzed from stellar observations during check-out operations). For future JANUS observations of Jupiter's nightside, defining a robust detection threshold for lightning remains challenging in the energetic Jovian magnetospheric environment and systematic observations will be required to build a robust statistical identification criteria based on detected events.

Another image with interesting compact light sources is an image of the Gulf of Thailand (Fig. ). In the image showed in this figure, the scene is illuminated by the Moon, which is at low elevation on the WSW direction. Stray light effects create the large-scale circular pattern visible on the image. Several small and bright patches of light are observed over the sea and in coastal populations in two islands on the lower-left side of the image. The lights observed over the sea are located in densely traveled maritime routes, and the largest and brightest clusters of light are concentrated near the offshore Benchamas and Tantawan oil and gas fields, which suggest that the compact light sources are the industrial facilities of the offshore gas fields and large ships or groups of ships. The radiance of the spot identified at the geographical location of Benchamas is 27 mWm-2sr-1, and individual spots likely associated with individual ships or cluster of ships have radiances of 3–10 mWm-2sr-1 (e.g. each of them are similar individually to the event shown on Fig. , but their alignment over a heavy traveled maritime route reveals their nature).

JANUS also captured images of bright lights in different cities. Figure  shows Cambodia's capital Phnom Penh and the small city of Tuy Hòa in the coast of Vietnam. Phnom Penh and Tuy Hòa have populations of 2.0 million people and 160 000 people respectively. The city center near the riverside area in Phnom Penh and several large infrastructures can be identified in the JANUS image when comparing with maps of Phnom Penh. The Tuy Hòa image also shows a bright spot that corresponds to the most intense fire observed by JANUS, with its nature confirmed from a comparison with FIRMS data.

The image with Phnom Penh also shows two sets of bright pixels (one is highlighted with a red circle in Fig.  and the other is outside the FOV shown in this figure). The nature of these sources is discussed in the next subsection.

Values of the observed optical energy associated with the light sources observed in these images can be calculated following the same technique used for the lightning candidate previously discussed. Integrated energy from Phnom Penh and its city center are 8080 and 5570 J respectively (although these numbers are more uncertain than for compact light sources due to the extended source and background illumination from the Moon). The optical energy of the intense fire near Tuy Hóa is estimated as 1500 J, which is three orders of magnitude smaller than a typical lightning flash .

Most compact light sources observed on JANUS images of the Earth are created by fires or artificial sources, and the single lightning candidate observed over a convective cloud system had an estimated detected optical energy that even under conservative assumptions is several orders of magnitude lower than typical space-based detections of lightning flashes. However, we note that the intrinsic luminosity of the flash could be much larger if this were a low altitude lightning obscured by high opacity clouds.

When evaluating the lack of firm detections of lighning in these JANUS images, it is worthy to note that the efficiency of a camera-based lightning search depends on the total spatial and temporal coverage of the ensemble of observations (how much terrain has been observed and over how much time). The 20 JANUS nighttime images of the Earth covered a total area per time of ∼ 89 000 km² s (assuming for simplicity nadir pointing for all images and multiplying the area covered by each image by their exposure time and adding their totals), or ∼2.8×10-3 km2yr. Extensive observations of lightning from space show a lightning flash frequency of 20–30 flashes km⁻² yr⁻¹ in strongly lightning active regions, and values of about 5 flashes km⁻² yr⁻¹ for tropical latitudes . Thus, the observational efficiency of the JANUS nighttime images is very small compared with lightning rates. For instance, image 0777503922_0014_01 (Fig. ) of the Gulf of Thailand, which is known to be a very active site of lightning, covered 79 300 km² and was acquired with a exposure time of 25 ms, meaning that it had an efficiency of 6.3×10-5 km² yr. This translates in a probability for this image of containing a lightning flash of only 0.2 %. For the whole set of nightside observations, and considering a representative value of 5 flashes km⁻² yr⁻¹, we get a probability for the JANUS nightside dataset of ∼ 1.5 % of containing a single lightning event. In addition, diurnal flash rates are found to be near their minimum values in the 03:00–06:00 local time , further reducing the probability of observing lightning in the JANUS nocturnal data set, which was concentrated in that time range.

These aspects can be further examined observing the lightning activity over the area traversed by JANUS from networks of lightning detection based on radio receivers. Figure  shows the ensemble of lightning detections from spherics detected by the WWLLN and Blitzortung networks on the area of the Indian Ocean over the 21:00–22:00 UTC time range. For information on the coverage and detection efficiency of the WWLLN detections the reader can refer to . Only a few lightning strokes occurred over a full hour on some of these JANUS short-exposure frames, confirming the low probability to observe lightning events. However, lightning activity was occurring very close to the JANUS images shown in Fig. , and the nature of the local meteorological activity indicates that electric activity was indeed possible on that location. The JANUS image 777503922_0011_01 covered also a region of strong convection with lightning activity over the 1 h time interval shown in Fig. . A close inspection of this image did not show any lightning candidate to brightness levels at least four times smaller than the weak candidate shown in Fig. .

JANUS will extensively characterize lightning events in Jupiter's atmosphere. Lightning observations will have a combination of long exposure times, accumulating several flashes over the same area, and short exposures to observe individual flashes. Jovian lightning is far more intense than lightning on Earth, but it will be observed from much larger distances allowing statistical mapping and determination of the size of lightning spot sizes which constrains lightning depth . So far, the largest optical survey of lightning activity on Jupiter comes from the Galileo orbiter, which determined a flash rate above a given energy threshold of 4×10-3 flashes km⁻² yr⁻¹, with lightning sources concentrating in cyclonic regions . A lightning survey from the Stellar Reference Unit on Juno, which is a far more sensitive instrument than the ISS camera on Galileo, gave 6.1×10-2 flashes km⁻² yr⁻¹ for lightning observations over high latitudes . In addition, the lightning activity detected by radio instruments can be significantly higher , indicating the difficulties to observe in the visible weak or deep atmosphere lightning events. Thus, for its survey of Jovian lightning, JANUS will obtain repeated observations of large and small areas of Jupiter using a combination of short and long time exposures to maximize the efficiency of those surveys and understand the weak light spots from statistics of the results. Observations of hot pixels, cosmic rays and energetic particles in the high-energy Jovian environment will characterize those effects on the detector, helping to understand the capability of the instrument to unambiguously characterize weak lightning sources. Observations conducted at a variety of spatial resolutions will give information about the spatial distribution of lightning events, their depth and energy release. Dayside and nightside observations of the same areas will be planned similarly to storms observed by Galileo and will be a key part of the JANUS investigation of convective storms in Jupiter. In addition, synergetic observations with other JUICE instruments sounding the lower layers of the atmosphere with MAJIS and upper layers with the Ultraviolet Spectrometer instrument on JUICE , together with radio emissions with the RPWI (e.g. similarly to recent multi-instrument analyses with Juno, see or previous works in Saturn with Cassini, see ) will be particularly relevant to investigate the spatial distribution, energy output and impact of storms in the Jovian atmosphere.

JANUS images of the Earth show two moderately bright events that we could not associate to surface features, were too weak to imply lightning, and occurred in regions not covered by clouds. These two events appear on the JANUS image 0777503922_0015_01 over Cambodia. The brightest of these features is highlighted with a red circle in Fig. . A detailed image of the structure of both features at pixel scale is shown in Fig. . The total flux of the brightest of these two events is a third of the total flux of the large fire visible in Fig. b.

If these events were true detections of transient light sources, the detected energy required to create these streaks would be compatible with meteor streaks from small objects entering Earth's atmosphere. From the radiance values, pixel scale of 158 m per pixel, and 25 ms exposure time, the integrated optical energy of the two events in Fig.  would be 549 and 266 J respectively. For luminous efficiencies of 1 %–10 % , these values of optical energy can be released from meteors impacting Earth's atmosphere at speeds from 11–70 kms-1 with masses of 1–30 g.

However, a comparison of the sharpness of these bright features with the instrument PSF, which can be roughly approximated by a Gaussian with a FWHM of 1.6±0.4 pixels in the panchromatic filter from fits to stellar-sources imaged during check-out operations, suggests that these features might be created by cosmic rays or energetic particle hits on the detector. Thus, cosmic rays identification and rejection methods (e.g. , ) may need to be tested on JANUS images under a variety of observing conditions.

Even though these two events are most likely created by energetic particles or cosmic rays, there is a considerable interest to detect and characterize impacts in Jupiter from small-size objects leaving visible traces in the atmosphere that could be similar to these two groups of pixels. Jupiter receives a flux of impacts that is much greater than that of Earth. Voyager 1 observed one small impact in Jupiter from a distance of 555 000 km and at a pixel scale over Jupiter of 20 km from an object with an estimated size of 11 kg . The UVS instrument on the Juno mission detected a single bolide with an estimated mass of 250–5000 kg. The impact rate for objects of similar mass and size is considered to be around 24 000 yr⁻¹ . Juno also observed several candidates to TLEs in a proportion of 10 to 1 with respect to impacts . Super bolides observed in Jupiter from Earth with sizes of 5–25 m in diameter are expected to hit Jupiter 10–65 times per year lighting the atmosphere at the impact location for only a few seconds .

Planed JANUS observation of Jupiter's nightside will most likely observe dozens of impacts in the size category of the one observed by Voyager 1, and many more smaller given the more sensitive camera than the one in Voyager 1 and the larger coverage over the nightside of the planet over the 3.5 years orbital tour. It should also be able to observe several energetic impacts similar in size and energy to the one detected by Juno. Building a sufficiently large set of observations of impact events in Jupiter would help immensely in the determination of the current impact rate in the planet, the size distribution of meteoroids, and the role of these impacts in polluting the upper stratosphere. In order to efficiently identify small impacts from cosmic rays, JANUS will have to test or develop a cosmic-ray identification and rejection technique, appropriate to the characteristics of the JANUS detector and the energetic environment of the Jupiter observations.

JANUS images obtained near dawn show illumination effects created by tangent illumination close to the terminator. Figure  shows high cumulus clouds observed before sunrise, during the morning astronomical twilight. Here, scattered light from the distant dawn terminator travels over the upper atmosphere and illuminates the uppermost clouds in the scene half an hour before local sunrise. A comparison with later observations during morning hours over the tropical latitudes covered by JUICE suggests these cumulus clouds may have high cloud tops at heights of 10–14 km. Several cloud systems are observed in the lower troposphere with very low illumination and without projecting observable shadows.

As the spacecraft moved towards dawn, the illumination increased rapidly and elevated clouds started to receive direct illumination from the Sun refracted on the atmosphere. Figure  contains two consecutive images close to the dawn terminator showing a strong gradient in the illumination of the scene. The left panel shows that the light increases by a factor of 3.5 over a distance of 280 km approaching dawn. The next JANUS image acquired in the sequence and displayed in the center panel, shows an additional enhancement of brightness by a factor of 4.0 over the next 280 km. This image saturates near dawn (all elevated clouds and some low terrains appear saturated in the upper part of the image, closest to dawn). Both images show crepuscular rays, (CRs), which are alternating dark and light bands in the sky that take the form of light streaks as sunlight filters through cloud gaps . The crepuscular rays in these two images are a combination of shadows and forward scattering on the elevated aerosols present in the images . The long shadow highlighted in this figure reaches a length of 510 km.

Figure  shows details over the north coast of Luzon island. The lateral illumination by a rising sun shows strong contrasts associated with the mountainous terrain, visible rivers and the coast. Dense clouds appear saturated and thin cirrus with gravity waves are also visible in the image. A long-dark feature in the sea is probably an internal ocean wave, common in the Luzon strait . This small portion of the original image demonstrates the high-resolution of the JANUS images, and the capacity to observe very faint contrasts as those coming from the cirrus clouds and from the linear features in the coast produced by the parallel alignment to the coast of local populations. It is remarkable that the high clouds in Fig.  do not project shadows over the terrain because of the almost total horizontal illumination, which contrasts with the CRs on Fig.  in which CRs propagate at altitudes near the elevation of the original clouds casting shadows and leaving gaps over thin layers of elevated aerosols. Because the terrain is mostly illuminated by diffuse dawn light, mountain shadows are not as strong as they would be if the Sun had fully risen.

Observations of the Earth terminator were acquired under a 90° phase angle. A similar illumination will be common in many JUICE perijoves in which images of the planet at spatial resolution of 15 km per pixel will be common. With Earth's terminator showing complex illumination effects over 500 km, and Jupiter's larger radius by a factor of 11, JANUS images of the terminator may observe similar extreme illumination conditions in Jupiter helping to identify and separate upper and lower clouds like those observed in Figs.  and . Such illumination effects at the terminator will cover a significant portion of the JANUS detector allowing to explore these effects if appropriate exposure times to the challenging illumination can be determined.

Earth observations obtained soon after local dawn show a variety of cirrus and cumulus clouds that project long crepuscular rays and shadows.

Two sequences of multi-filter images were obtained minutes after local sunrise with nearly tangent illumination and nadir viewing. Figure  shows part of one of these sequences obtained about 12 min after local sunrise. The images show cumulus clouds and elevated cirrus in color and in the JANUS filter F8 centered at 940 nm. Observations in this filter are strongly affected by the wide H₂O absorption band at 820–940 nm. The color image shows a bright convective cloud, most likely a cumulonimbus, elevated dense cirrus, and multiple bright clouds reaching lower heights. Long crepuscular rays dominate the scene in short wavelengths, and shadows projected by the central cumulus cloud are clearly observable in most filters. The F8 image shows much higher contrast associated with the high dense cirrus and the bright convective system, but most other bright clouds in the color image are almost invisible in the F8 image implying shallow cumulus over most of the image.

No shadows are observed in the image within the F8 filter. This implies that the massive large cloud system observed in the F8 image is at altitudes comparable to or above the convective bright cumulus. The low elevation of the Sun, combined with the homogeneous distribution of brightness in the F8 image, imply that water is concentrated in the lower part of the troposphere (an equivalent image of Jupiter's high clouds with tangential illumination using the JANUS F7 filter in a methane band would result in an image with a gradient in illumination from the vertically well-mixed atmospheric methane). When examining the full multi-filter sequence, the contrast of the shadow and crepuscular rays is maximum in blue and violet wavelengths (most likely because of the forward scattering effects of the crepuscular rays), and is much lower in the F7, F11 and F12 filters, which are centered at wavelengths of 889, 910 and 1015 nm respectively, which are wavelengths also sensitive to weak water absorption bands.

A quantitative interpretation of the altitudes associated with these cloud systems cannot be directly obtained from the multi-filter observations because water abundance probably varies over the scene and the observation does not provide access to different illumination conditions. However, the altitudes of the clouds can be investigated from the shadows casted by the elevated clouds. Figure  shows two geometrical analysis of shadows projections. The first one is a simplified version of the geometry assuming no parallax effects. This is correct for an observer at an infinite distance observing a small FOV over the planet under a plane-parallel approximation. Under these conditions, the height h of a cloud can be retrieved from the total extension of its shadow d as: 1h=dtan⁡i+tan⁡e, where i and e are the incidence and emission angles respectively.

For a close observer (in the images shown in Fig.  the sub-spacecraft altitude was 9060 km, the main incidence angle was 86.85° and the main emission angle was 3.56°), an evaluation of heights from shadows requires taking into account parallax and the ellipsoidal geometry of the surface. Figure b shows a sketch of the relevant geometry. The cloud altitude evaluation is solved by finding the intersection point between vectors P1O and P2S. Here, points O and S are the positions of the observer and the Sun respectively, and P1 and P2 are the position of the bright cloud and end of its tail respectively. Expressing vectors also with respect to the center of the planet, (i.e. transforming longitudes and latitudes of a point like P1 into the vector CP1) then: 2CO=CP3+P3OP1O=CO-CP1P2S≈CS

For an ellipsoidal planet like Jupiter the method works in the same way by taking the coordinates of points P1, P2 and P3 referred to the center of the planet and considering the local radius.

For the geometry of the observations shown in Fig. , calculating cloud altitudes from shadows in this way results in an increase of altitudes of about 20 % with respect to the use of Eq. (). It should also be noted that altitudes estimates from shadows in this image are minimum values, since the end of the shadows over the ocean are difficult to determine with the tangential illumination. Several additional calculations of cloud altitudes from shadows are showed in the next section.

Figure  shows a high resolution image of active convection on cumulus clouds imaged with a spatial resolution of 130 m per pixel and observed 1 h after the local sunrise. The cumulus are projecting clouds to distances of up to 65 km, and the cloud tops reach heights above the ocean of up to 14 km. Only observations in a green filter covered this region. No anvil clouds were observed, which suggest, when combined with the early local hour, that convection had initiated near sunrise and had not reached a fully developed mature stage. Because of the illumination and viewing geometry, the cumulus appears tilted towards the bottom part of the image, and the cloud tops and east side of the clouds can be observed simultaneously nearly down to the base of the convective towers. Patchy cloud structures at 1–5 km scales indicate stratocumulus and active boundary layer convection after sunrise.

Figure  shows examples of mature cumulus developing active convection. The spatial resolution of these images is 126 m per pixel, and the field of view covers a location 3 h after the local sunrise. These storms are under a mature stage and show fully developed anvil clouds with longitudinal extensions of 20 to 85 km and cloud tops peaking at altitudes of 12–14 km. A multi-filter sequence of images of this area was obtained allowing to build color images. The apparent brown-red color of the ocean is due to Sun-glint conditions affecting these observations (i.e., near-specular reflection of sunlight; in these observations, the incidence angle of the central pixel was 51° and the emission angle was 40°, values close enough to induce glint effects).

In the image sequence that covered the area shown in Fig. , two images separated by 34 s overlap over the three large convective cumulus cloud at the bottom of the image. Map projections of these images show strong apparent motions in the highest clouds, while features at the base near the ocean remain stationary. Those motions could be indicative of divergence and expanding clouds. With the spatial resolution of these images and the time difference between both image acquisitions, a 1-pixel motion implies velocities of 3.7 ms-1. However, the strong parallax effects associated with the changing view point of the observations, and the combination of horizontal and vertical motion, do not allow to measure winds from cloud tracking, either manually or with image correlation software. The expansion of the anvil clouds appears very limited, and expansion rates larger than 30 ms-1 can be ruled out.

The comparison of these color observations with images in the F8 filter in the water absorption band at 940 nm shows bright clouds at the convective cores and large anvil clouds, dark clouds for moderately elevated cumulus and a dark ocean.

Figure  shows a more complex scene contiguous with the one in the previous figure. Here elevated cirrus extending over a horizontal distance of 190 km appear as wispy cloud systems close to active cumulus towers. The bottom panels in this figure show a magnified view of an isolated active cumulus observed in a nearby location in the image sequence. This cloud system corresponds to the brightest region in the F8 image, and the cloud morphology shows overshooting convection, with the F8 brightest part of the cloud ascending over the darker anvil cloud by about 2.0–2.5 km.

We were not able to identify changes in the brightness of the ocean in the F8 filter, which is strongly sensitive to water absorption. This suggests a very homogeneous distribution of water vapor over the tropics in the regions sampled by these images.

Large-scale storms powered by moist convection are a common feature in Jupiter's atmosphere with a major role in the atmospheric dynamics . JANUS will study the occurrence rate and dynamics of convective storms of multiple scales in Jupiter allowing studies of their onset, development and cycle of activity . The combination of dayside and nightside observations will allow to determine their electric activity . Observations with filters sensitive to different altitudes, combined with radiative transfer models (e.g. ) will allow us to determine their vertical structure. JANUS observations of Jupiter acquired at sufficient spatial resolution will enable to measure divergence rates and turbulent fluxes through the atmosphere , including convective storms and eddies. Thus, JANUS will perform a full investigation of the role of convective storms in shaping the atmospheric dynamics of the Jovian atmosphere.

Some of the JANUS images show gravity waves over elevated cirrus. Figure  shows a long cirrus cloud system (>160 km in length), where several gravity wave packets with wavelengths of 1.6 to 2.2 km are observable. The waves are aligned perpendicularly to the elongated cirrus suggesting a main direction for the winds flow and strong stratification. The cirrus is not dense enough to project shadows that could be used to determine its height, and this area was only observed on images using the F3 (green) filter. The low-level clouds present in the image are indicative of stratocumulus in the early morning, and the absence of larger vertically extended cumulus over the area also indicates local stability.

Figure  shows several dark features over the ocean with arc-shaped fronts, narrow elongated arcs and wavy dark and bright patterns. These type of features appear on multiple JANUS images of the ocean acquired with the F3 filter (none of these areas was also covered by additional filters to characterize their color). The elongated arcs with scales of 30-80 km, and the regularly spaced bands of features, are consistent with internal waves in the ocean . Internal waves can form from interactions of sea currents along the thermocline, and by tidal interactions with the bathymetry. They can also be formed from variations in sea surface roughness created by wave currents moving with different phase speeds . The Izu-Ogasawara Trench in the western North Pacific, which is considered to be one location driving intense internal waves generation, is located at around 1000 km west of the location of the internal waves imaged by JANUS. In addition, the visibility of internal waves is also affected by sunglint effects and the location of these images also contains partial sunglint from differences between the incidence and emission angle of 20°.

We expect that JANUS will image several wave systems at Jupiter, characterizing their properties, physical origin, and influence on the atmosphere. Past observations show that gravity waves in the cloud fields of Jupiter can appear in the upper hazes of the planet in methane-band images and in the lower clouds in the ammonia cloud layer . JANUS will be able to characterize their frequency, wavelength, and phase speeds allowing for several studies of atmospheric waves in Jupiter's atmosphere.

At the end of the observation sequence, JANUS imaged the limb of the Earth illuminated by a nearly zenithal sun. Figure  shows a JANUS image of the dayside horizon with incidence angle varying from 12 to 2° over the full image. This geometry shows extensive cloud systems over the North-Central Pacific, and scattered light over the horizon, where the vertical spatial resolution is 0.177 km. The image also shows the south coast of the Big Island of Hawaii covered by dense clouds and with a dark coast.

The structure of the atmospheric limb in this image is particularly interesting. At the spatial resolution of this image, the limb reveals the vertical structure of Earth's atmosphere. Discrete low clouds above Earth's spheroid are visible at altitudes of 1–5 km. Above these lower clouds, scattered light shows the presence of a detached double layer of aerosols at altitudes of 12–15 km near the tropopause. This overall structure is observed all over the limb. Scattered light at altitudes above the tropopause rapidly diminishes with increasing altitude. Diffuse light is observed to vary suddenly at altitudes of about 20 km, showing evidences of atmospheric structure even at that height. These results are similar to those obtained by instruments specifically designed to profile the limb and investigate the vertical structure of aerosols .

Dayside limb observations of the Earth are similar to Galileo observations of Jupiter's hazes under limb viewing geometries . Galileo showed that at high phase angles near 140–150°, the equatorial and polar hazes can display their vertical structure when observed at sufficiently high spatial resolution (Galileo observed these effects with a spatial resolution of 15 km per pixel). JANUS will acquire a large number of similar observations of Jupiter's limbs in multiple filters at different latitudes and with different illumination conditions and with a sufficient spatial resolution to study the vertical distribution of aerosols. By combining these observations with radiative transfer models, we should obtain a very valuable dataset able to inform about the vertical distribution of clouds and aerosols and particle size at different heights through the planet. With a maximum spatial resolution of about 10 km in the closest perijoves, structures equivalent to one third of the vertical scale height H will be resolved. This will provide direct information on the extent and properties of the equatorial and polar hazes. If science operations planning allows it, similar JANUS observations could also determine the vertical structure above specific atmospheric systems covered by dense upper hazes like the Great Red Spot and other major ovals.