Eclipse Research Articles

The seven-lobed shape of the outer edge of Saturn’s A ring

Using the complete set of stellar and radio occultation data from the Cassini mission, we fit a multimode model to the outer edge of Saturn’s A ring, similar to that previously applied to the B ring edge by Spitale and Porco (2010) and Nicholson et al. (2014a). Our model takes into account the coorbital libration of the satellite Janus, whose 7:6 Lindblad resonance is believed to be responsible for maintaining the edge at its observed location. Consistent with previous analyses (Spitale and Porco, 2009; El Moutamid et al., 2016), we find that the shape of the ring’s edge is dominated by a 7-lobed radial distortion that rotates with the same angular velocity as Janus during the periods when the satellite is on the inner leg of its 8-yr libration. The amplitude of this distortion is ∼12 km, and one of the seven minima is aligned within a few degrees of the satellite’s mean longitude. At times when Janus is on the outer leg of its libration, however, the 7-lobed pattern disappears completely. In addition to this resonantly-forced distortion, our data reveal the presence of a rich spectrum of normal modes sculpting the ring edge. When the 7-lobed pattern is present, the principal secondary mode has m=5, while when the 7-lobed pattern is absent, the shape of the edge is dominated by modes with m=9 and m=12, all with radial amplitudes of 4–6 km. The data strongly suggest that the m=5 mode actually persists, but at an undetectable level, throughout the latter period. Lower-amplitude modes are also seen with m=3, 4, 6, 8, 10 and 18, though not at all times. The normal mode frequencies are consistent with a simple analytic model whereby each mode exists within a resonant cavity near the edge of the ring (Borderies et al., 1985; Longaretti, 2018), from which we estimate an average surface mass density in this region of ∼20g cm−2, consistent with that derived from weak density waves (Tiscareno and Harris, 2018). Despite the relative complexity of the best-fitting model, the RMS deviations between it and the observed edge radii range from 1.7 to 5.5 km, substantially exceeding the measurement errors of ∼0.3 km and strongly suggesting the existence of additional, as-yet-uncharacterized perturbations. Contrary to previous analyses, we find no evidence for beating between the strong m=7 signature due to Janus and a weaker signature due to its coorbital companion Epimetheus (Spitale and Porco, 2009) and only weak evidence for an m=−3 mode driven by a gravity anomaly within Saturn (El Moutamid et al., 2016).

A comparative DFT study on the π-π stacking interaction of some N-based organic fused heterocycles with eclipsed and staggered conformations

As most of the antimicrobial, anticancer, antibacterial or antiviral agents are made of nitrogen(N)-based organic fused heterocyclic system; therefore, the study of such system has a great impact on pharmacology and medicinal chemistry research. Quantum mechanical study on the non-covalent or π-π stacking interaction of any organic system always plays a significant role to analyze the proper conformation of such stacked models. It has been observed that some bioactive N-based fused heterocycles; viz. quinoline, isoquinoline, cinnoline, quinazoline, phthalazine, quinoxaline and petridine; shows favored stacked homo dimer and results in effective π-π stacking interaction that occurs within themselves. Usually, non-covalent interaction follows intercalation mechanism via π-stacking which results in very strong interaction within heterocyclic systems. Herein, some of the stacked dimers of N-based fused heterocyclic systems have been investigated using DFT method, as it is the most accurate and appropriate tool for the prediction of molecular interaction. We also carried out the stacking interaction of such system with different molecular conformations i.e., eclipsed and staggered models. In our investigation, it has revealed that for most of the N-based stacked heterocyles, staggered conformation (with intermolecular rotation 120° and 180°) shows favored stacked conformation than that of eclipsed conformation (with intermolecular rotation 0°).

Solar occultation observations of Saturn's rings with Cassini UVIS

The Ultraviolet Imaging Spectrograph (UVIS) instrument onboard the Cassini spacecraft observed 41 solar occultations by Saturn's rings through its extreme ultraviolet (56–118 nm) spectrographic channel. These solar occultations complement the set of stellar occultations measured by the broadband UVIS High Speed Photometer in the far ultraviolet (110–190 nm). We provide a uniform calibration of all of the UVIS solar occultation data across the full ring system. We have compiled complete catalogs of these occultations as high-level data products delivered to the Planetary Data System (PDS) Ring Moon Systems Node. This manuscript serves as a guide to this dataset and the corresponding high-level data products outlining their value for analysis of Saturn's rings. These products include light curves and optical depth profiles as functions of ring radius. In addition to providing key parameters of interest obtained through each of these occultations, we show that the optical depths of various ring regions derived from solar occultations are consistent with those from stellar occultations. We also demonstrate that fits to self-gravity wake parameters are similar for solar and stellar occultations. Lastly, we identify a variety of potential spectral trends at various ring locations that future investigations may further explore to address key questions pertaining to the properties of Saturn's rings.

Stabilization of Zn group dimers: A theoretical study

AbstractQuantum chemical calculations have been carried out to investigate the possibility of Group 12 (Zn, Cd, Hg) dimer formation supported by the B3ring. All these complexes have significantly shorter M‐M (M = Zn, Cd, Hg) distances compared to their bare dimers, M2. The staggered and eclipsed forms of these complexes were found to be nearly degenerate. They have significant bond strengths as revealed from the significantly higher values of Wiberg bond indices as well as bond dissociation energies. The topological feature of electron density reveals significant covalency in the M‐M interaction which has also been confirmed from natural orbital for chemical valence analysis coupled with extended transition state (ETS‐NOCV) calculations.

A stellar occultation by the transneptunian object (50000) Quaoar observed by CHEOPS

Context. Stellar occultation is a powerful technique that allows the determination of some physical parameters of the occulting object. The result depends on the photometric accuracy, the temporal resolution, and the number of chords obtained. Space telescopes can achieve high photometric accuracy as they are not affected by atmospheric scintillation. Aims. Using ESA’s CHEOPS space telescope, we observed a stellar occultation by the transneptunian object (50000) Quaoar. We compare the obtained chord with previous occultations by this object and determine its astrometry with sub-milliarcsecond precision. Also, we determine upper limits to the presence of a global methane atmosphere on the occulting body. Methods. We predicted and observed a stellar occultation by Quaoar using the CHEOPS space telescope. We measured the occultation light curve from this dataset and determined the dis- and reappearance of the star behind the occulting body. Furthermore, a ground-based telescope in Australia was used to constrain Quaoar’s limb. Combined with results from previous works, these measurements allowed us to obtain a precise position of Quaoar at the occultation time. Results. We present the results obtained from the first stellar occultation by a transneptunian object using a space telescope orbiting Earth; it was the occultation by Quaoar observed on 2020 June 11. We used the CHEOPS light curve to obtain a surface pressure upper limit of 85 nbar for the detection of a global methane atmosphere. Also, combining this observation with a ground-based observation, we fitted Quaoar’s limb to determine its astrometric position with an uncertainty below 1.0 mas. Conclusions. This observation is the first of its kind, and it shall be considered as a proof of concept of stellar occultation observations of transneptunian objects with space telescopes orbiting Earth. Moreover, it shows significant prospects for the James Webb Space Telescope.

Physical properties of the trans-Neptunian object (38628) Huya from a multi-chord stellar occultation

Context. As part of our international program aimed at obtaining accurate physical properties of trans-Neptunian objects (TNOs), we predicted a stellar occultation by the TNO (38628) Huya of the star Gaia DR2 4352760586390566400 (mG = 11.5 mag) on March 18, 2019. After an extensive observational campaign geared at obtaining the astrometric data, we updated the prediction and found it favorable to central Europe. Therefore, we mobilized half a hundred of professional and amateur astronomers in this region and the occultation was finally detected by 21 telescopes located at 18 sites in Europe and Asia. This places the Huya event among the best ever observed stellar occultation by a TNO in terms of the number of chords. Aims. The aim of our work is to determine an accurate size, shape, and geometric albedo for the TNO (38628) Huya by using the observations obtained from a multi-chord stellar occultation. We also aim to provide constraints on the density and other internal properties of this TNO. Methods. The 21 positive detections of the occultation by Huya allowed us to obtain well-separated chords which permitted us to fit an ellipse for the limb of the body at the moment of the occultation (i.e., the instantaneous limb) with kilometric accuracy. Results. The projected semi-major and minor axes of the best ellipse fit obtained using the occultation data are (a′, b′) = (217.6 ± 3.5 km, 194.1 ± 6.1 km) with a position angle for the minor axis of P′ = 55.2° ± 9.1. From this fit, the projected area-equivalent diameter is 411.0 ± 7.3 km. This diameter is compatible with the equivalent diameter for Huya obtained from radiometric techniques (D = 406 ± 16 km). From this instantaneous limb, we obtained the geometric albedo for Huya (pV = 0.079 ± 0.004) and we explored possible three-dimensional shapes and constraints to the mass density for this TNO. We did not detect the satellite of Huya through this occultation, but the presence of rings or debris around Huya was constrained using the occultation data. We also derived an upper limit for a putative Pluto-like global atmosphere of about psurf = 10 nbar.

Pattern Recognition Using SVM for the Classification of the Size and Distance of Trans-Neptunian Objects Detected by Serendipitous Stellar Occultations

We present a new pipeline based on the Support Vector Machine algorithm to confirm the detection and perform classification of small solar system objects by serendipitous stellar occultations. This pipeline is designed to analyze light curves and to identify the occultation events and the classification of the occulting bodies according to their size, typically from a fraction to a few kilometers, and their distance from the Sun, typically a few tens of astronomical units. The input light curves for this pipeline were obtained from the event simulator for the Trans-Neptunian Automated Occultation Survey (TAOS II). We explore parameters affecting occultation light curves such as spectral type, apparent magnitude and finite angular size of the occulted star, angle from opposition, and readout cadence for the observations; also we assumed a Poisson noise distribution as expected from the TAOS II project. We find that occultation events, especially by trans-Neptunian objects with diameters ≥2 km are detected with 99.99%, 99.53%, and 86% efficiency for stars with a visual apparent magnitude of 12, 14, and 16, respectively at 0.05 s of exposure time. In terms of size and distance classification the overall accuracy is 94%. However, for smaller objects the confirmation and classification depends mostly upon the signal-to-noise ratio.

A spectroscopic approach to identifying the out-of-plane conformations of Ni(II) meso-tetraphenylporphyrin in solution

Ni(II) meso-tetraphenylporphyrin (NiTPP) is a model molecule whose substituent causes out-of-plane (oop) deformations of the porphyrin macrocycle. Although the conformers of NiTPP in the solution had been tentatively explored, the determination of its specific conformers was controversial. In this work, based on the conformational search through molecular dynamics (MD), three energy lowest-lying conformers of NiTPP were sorted out, namely, the S4 (staggered) conformer, the C2 (eclipsed) conformer, and the C1 (eclipsed-staggered) conformer. According to the high-precision free energy calculation based on density functional theory (DFT), the Raman and 1H-NMR spectra of the three conformers were calculated. The actual spectra of NiTPP in solution resulted from the sum spectra of the mixture conformers, so with the consideration of the corresponding conformation weights, the experimental spectra could be stimulated. Our results showed that only at the low-frequency ∼215 cm−1, did the Raman spectrum in solution differ significantly from that in solid powder. On the other hand, the 1H-NMR spectra of the NiTPP conformers were remarkably different, and none of the single conformer's 1H-NMR range could match the experimental 1H-NMR spectrum. However, with the consideration of the existence of three dominant conformations, the observed 1H-NMR spectrum could be well simulated, confirming the rational mixture of the three dominant conformers existing in the aqueous solution. Therefore, this work has not only resolved the different conformers of NiTPP in solution, but also provided an effective approach to combining DFT, Raman, and 1H-NMR analytical tools for identifying different oop conformations of metalloporphyrins in solution.

Design and Simulation of Stellar Occultation Infrared Band Constellation

This study provides an in-depth analysis of the characteristics of stellar occultation events. Using 10 target star sources, the influence of orbital elements on the number, duration, and distribution of stellar occultation events was simulated and analyzed, and the constellation configuration was designed. The results showed the following points: (1) the orbital inclination had the greatest influence on the number of occultation events, with obvious upward and downward trends in the range of 10–40° and 150–180°, and the amount of occultation data remained at about 303 times under the other angle conditions. The orbital height had an effect on the number of occultations, but the amplitude was small. (2) The use of four orbits had an impact on the occultation duration. The duration decreased with an increase in the orbit height and inclination, the distribution was symmetrical with the perigee angular distance, and it increased with an increase in the ascending intersection right ascension. (3) The higher the orbital height, the less comprehensive the longitudinal and latitudinal distribution of occultation events. With an orbital inclination of less than 150°, the greatest occultation event was covered to encompass the entire world. The other two orbital elements had negligible effects on the longitudinal and latitudinal distribution of occultation events. (4) The elevation of the occultation event increased with an increase in the orbital altitude, but the azimuth showed no obvious change trends. A considerable number of normal occultations can be obtained with an orbital inclination of less than 120°. The other two orbital elements had a negligible effect on the distribution of altitude and azimuth of occultation events. A stellar occultation constellation configuration was designed based on the simulation results, and the results showed that the following parameters of satellites can be used to realize the global distribution of occultation events: orbital height of 500 km, orbital inclination of 97.3771°, perigee angular distance of 40°, and ascending node right ascension steps of 40°. This configuration will ensure that an adequate number of normal occultations are obtained, which will ensure the quality of data inversion under the condition of 152 infrared target star sources.

Chirality influence on the cytotoxic properties of anionic chiral bis(N-heterocyclic carbene)silver complexes

Complexes Na3[Ag(NHCR)2], 2a-e and 2b’-c’, where NHCR is a N-heterocyclic carbene of the 2,2′-(1H-2λ3,3λ4-imidazole-1,3-diyl)dicarboxylate type, were prepared by treatment of compounds HLR, 1a-e and 1b’-c’ (2-(1-(carboxyalkyl)-1H-imidazol-3-ium-3-yl)carboxylate), with silver oxide in the presence of aqueous sodium hydroxide. They were characterized by analytical, spectroscopic (infrared, IR, 1H and 13C nuclear magnetic resonance, NMR, and circular dichroism) and X-ray methods (2a). In the solid state, the anionic part of complex 2a, [Ag(NHCH)2]3−, shows a linear disposition of Ccarbene-Ag-Ccarbene atoms and an eclipsed conformation of the two NHC ligands. The proposed bis(NHC) nature of the silver complexes was maintained in solution according to NMR and density functional theory (DFT) calculations. The cytotoxic activity of compounds 2 was evaluated against four cancer cell lines and one non-cancerous cell line and several structure-activity correlations were found for these complexes. For instance, the activity decreased when the bulkiness of the R alkyl group in Na3[Ag(NHCR)2] increased. More interesting is the detected chirality-anticancer relationship, where complexes Na3[Ag{(S,S)-NHCR}2] (R = Me, 2b; iPr, 2c) showed better anticancer activity than those of their enantiomeric derivatives Na3[Ag{(R,R)-NHCR}2] (R = Me, 2b’; iPr, 2c’).

The multichord stellar occultation on 2019 October 22 by the trans-Neptunian object (84922) 2003 VS2

Context. Stellar occultations have become one of the best techniques to gather information about the physical properties of trans-Neptunian objects (TNOs), which are critical objects for understanding the origin and evolution of our Solar System. Aims. The purpose of this work is to determine, with better accuracy, the physical characteristics of the TNO (84922) 2003 VS2 through the analysis of the multichord stellar occultation on 2019 October 22 and photometric data collected afterward. Methods. We predicted, observed, and analyzed the multichord stellar occultation of the Second Gaia Data Release (Gaia DR2) source 3449076721168026624 (mυ = 14.1 mag) by the plutino object 2003 VS2 on 2019 October 22. We performed aperture photometry on the images collected and derived the times when the star disappeared and reappeared from the observing sites that reported a positive detection. We fit the extremities of such positive chords to an ellipse using a Monte Carlo method. We also carried out photometric observations to derive the rotational light curve amplitude and rotational phase of 2003 VS2 during the stellar occultation. Combining the results and assuming a triaxial shape, we derived the 3D shape of 2003 VS2. Results. Out of the 39 observatories involved in the observational campaign, 12 sites, located in Bulgaria (one), Romania (ten), and Serbia (one), reported a positive detection; this makes it one of the best observed stellar occultations by a TNO so far. Considering the rotational phase of 2003 VS2 during the stellar occultation and the rotational light curve amplitude derived (Am = 0.264 ± 0.017 mag), we obtained a mean area-equivalent diameter of DAeq = 545 ± 13 km and a geometric albedo of 0.134 ± 0.010. By combining the rotational light curve information with the stellar occultation results, we derived the best triaxial shape for 2003 VS2, which has semiaxes a = 339 ± 5 km, b = 235 ± 6 km, and c = 226 ± 8 km. The derived aspect angle of 2003 VS2 is θ = 59° ± 2° or its supplementary θ = 121° ± 2°, depending on the north-pole position of the TNO. The spherical-volume equivalent diameter is DVeq = 524 ± 7 km. If we consider large albedo patches on its surface, the semi-major axis of the ellipsoid could be ~ 10 km smaller. These results are compatible with the previous ones determined from the single-chord 2013 and four-chord 2014 stellar occultations and with the effective diameter and albedo derived from Herschel and Spitzer data. They provide evidence that 2003 VS2’s 3D shape is not compatible with a homogeneous triaxial body in hydrostatic equilibrium, but it might be a differentiated body and/or might be sustaining some stress. No secondary features related to rings or material orbiting around 2003 VS2 were detected.

Ozone vertical distribution in Mars Years 27–30 from SPICAM/MEX UV occultations

The SPICAM/MEX ultraviolet spectrometer probed the Martian atmosphere with the occultation method from 2004 until 2014. SPICAM/MEX performed both stellar and solar occultations during in total four Martian Years with good spatial and seasonal coverages. We have analyzed these occultations and performed a rigorous quality check of the retrievals to eliminate false detections. We present the observed features of the vertical distribution of Martian ozone, a key chemical species. Stellar occultations probe the nightside atmosphere, whereas solar occultations are acquired at the terminator (sunrise or sunset), enabling the study of the day–night transition of this photochemically active species. Comparison of the observations with a global climate model show a good overall agreement. However, quantitative differences are found in certain regions, possibly related to difficulties in correct modeling of the water cycle. Our dataset allows us to study certain particular features of Martian ozone. The low- and midlatitude ozone layer forming during northern spring is mapped in both hemispheres and its night–terminator variations are probed with the combination of stellar and solar occultations. The southern polar winter vortex shows hints of the well-known mid-altitude ozone layer already detected previously. During the northern polar spring, SPICAM observes the top of the lower atmosphere ozone layer above 10 km, showing O3 concentrations that the model reproduces quite well. SPICAM observations are in good agreement with previously published observations from other instruments.

Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn’s Ionosphere

For orbits 288 and 292 of Cassini’s Grand Finale, clear dips (sharp and narrow decreases) are visible in the densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini’s position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn’s C ring. The calculated shadowed altitudes along Cassini’s trajectory line up well with the dips in the data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini’s trajectory. The resulting estimate of ∼0.3 km s−1 at an altitude of ∼3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

A theoretical density functional theory calculation-based analysis of conformers of p-xylene

Different conformers of many aliphatic compounds such as ethane, butane, cyclohexane and their derivatives have been studied to find the most reactive as well as the most stable conformer. For the first time, two conformers of p-xylene were found using theoretical DFT calculation and the vibrational modes, Raman activity, and other spectra of each conformer were also studied. The most significant data that clearly distinguished both conformers was depolarization spectra. Besides, many other parameters were found different in both conformers of p-xylene such as Mulliken charge’s, optimization energy, HOMO’s of both conformers. Also, the presented study predicts, why eclipsed conformer of p-xylene is more reactive than staggered conformer. The reactivity of the eclipsed form is explained on the basis of HOMO-LUMO energy gap. Also, the presented study opens the door for future work to be done because each conformer can produce a specific product. Moreover, the rates of reaction are also dependent on the conformers and their relative stability.

Occultation portal: A web-based platform for data collection and analysis of stellar occultations

ABSTRACT Recording a stellar occultation is one powerful method that gives direct information about the physical properties of the occulting Solar system object. In order to obtain reliable and accurate results, simultaneous observations from different locations across-track of the projected path are of great importance. However, organizing all the observing stations, aggregating, and analysing the data is time-consuming and not that easy. We have developed a web portal named Occultation Portal (OP) to manage all those occultation observation campaigns from a central server. With this portal, the instrumental and observational information of all observers participating in a stellar occultation campaign and the concerned data are archived systematically in a standard format. The researchers can then visualize the archived data on an event basis. The investigators can also extract the light curve for each data set with the added reduction pipeline to the portal base. This paper describes in detail the portal structure and the developed features.

Sounding Rocket Observation of Nitric Oxide in the Polar Night

AbstractAn altitude profile of Nitric Oxide (NO) in the 80–110 km altitude range was measured in the polar night from a sounding rocket on 27 January 2020. The observations were made using the technique of stellar occultation with a UV spectrograph observing the γ (1,0) band of NO near 215 nm. The tangent point for the altitude profile was at 74° latitude, a location that had been in darkness for 80 days. The retrieved slant column density profile is interpreted using an assumed four‐parameter analytic profile shape. Retrievals of the fitting parameters yield a profile with a peak NO concentration of 2.2 ± 0.7 × 108 cm−3 at 93.5 ± 4.1 km. The observations were made during a time of minimum solar and geomagnetic activity. The NO maximum retrieved from the rocket profile is significantly larger in abundance and lower in altitude than other observations on the same day at nearby latitudes just outside the polar night. These rocket‐borne results are consistent with NO that is created over the course over the polar winter and is confined to high latitudes in the polar night by the mesospheric polar vortex. During the course of that confinement the abundance increases due to the lack of photodissociation, allowing the NO to descend. We show that the observed descent can be explained by eddy diffusion‐driven transport, though vertical advection cannot be ruled out.

Seasonal Variations of Chemical Species and Haze in Titan's Upper Atmosphere

Seasonal variation is significant in Titan's atmosphere owing to the large change of solar insolation resulting from Titan's 26.7° axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400–1200 km, which includes the mesosphere and the lower thermosphere, over more than one-fourth of Titan's year (2006–2014, L S = 318°–60°), using 18 stellar occultation observations obtained by Cassini/Ultraviolet Imaging Spectrograph. Vertical profiles of eight chemical species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, HC3N) and haze particles are retrieved from these observations using an instrument forward model, which considers the technical issue of pointing motion. The Markov Chain Monte Carlo algorithm is used to obtain the posterior probability distributions of parameters in the retrieval, which inherently tests the extent to which species profiles can be constrained. The results show that no change of the species profiles is noticeable before the equinox, while the decrease of atmospheric temperature and significant upwelling in the summer hemisphere are found five terrestrial years afterward. Altitude of the detached haze layer decreases toward the vernal equinox and then disappears, and no reappearance is identified within the time range of our data, which is consistent with observations from Cassini/Imaging Science Subsystem. This study provides observational constraints on the seasonal change of Titan's upper atmosphere and suggests further investigations of the atmospheric chemistry and dynamics therein.

Milliarcsecond Astrometry for the Galilean Moons Using Stellar Occultations

A stellar occultation occurs when a Solar System object passes in front of a star for an observer. This technique allows the sizes and shapes of the occulting body to be determined with kilometer precision. In addition, this technique constrains the occulting body’s positions, albedos, densities, and so on. In the context of the Galilean moons, these events can provide their best ground-based astrometry, with uncertainties in the order of 1 mas (∼3 km at Jupiter’s distance during opposition). We organized campaigns and successfully observed a stellar occultation by Io (JI) in 2021, one by Ganymede (JIII) in 2020, and one by Europa (JII) in 2019, with stations in North and South America. We also re-analyzed two previously published events: one by Europa in 2016 and another by Ganymede in 2017. We then fit the known 3D shape of the occulting satellite and determine its center of figure. This resulted in astrometric positions with uncertainties in the milliarcsecond level. The positions obtained from these stellar occultations can be used together with dynamical models to ensure highly accurate orbits of the Galilean moons. These orbits can help when planning future space probes aiming at the Jovian system, such as JUICE by ESA and Europa Clipper by NASA. They also allow more efficient planning of flyby maneuvers.

The Molecular Oxygen Density Structure of the Lower Thermosphere as Seen by GOLD and Models

AbstractOxygen chemistry plays a central role in the production of plasma in the ionosphere. While molecular oxygen (O2) is the third most abundant constituent in the thermosphere, molecular and atomic oxygen densities are difficult to quantify accurately. Consequently, modeled densities have significant uncertainties. New stellar occultation measurements from the Global‐scale Observations of Limb and Disk (GOLD) mission provide data that can quantify, with high precision, the O2 density and its daily and local time variations. These observations are compared to the Mass Spectrometer Incoherent Scatter radar family of empirical models and the physics based Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension. While GOLD observations show a strong diurnal structure with local time, the models display a semidiurnal structure in O2. The local time structure of O2 is critical for accurately modeling plasma densities in the ionosphere.

Report on Three Stellar Occultations by the Excited Kuiper Belt Object 2002 MS4.

Abstract We report on three stellar occultation events by the Kuiper Belt object 2002 MS4, observed using a 300 mm telescope on Anarchist Mountain, BC, Canada. Using the three events we have determined chord diameters of (810 ± 70) km, (630 ± 140) km, and (780 ± 70) km. All of these diameters are consistent with those reported from previous occultation events. Based on these data we estimate that 2002 MS4 is at least 810 ± 70 km in diameter These events also provide high-precision sky positions for 2002 MS4 at the exact time of the occultation, which can be used to refine the orbital ephemeris of this target, improving predictions of future events. All data and processing outputs described in this Research Note can be retrieved from: 10.11570/22.0001