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See detailThe compositional evolution of C/2012 S1 (ISON) from ground-based high-resolution infrared spectroscopy as part of a worldwide observing campaign
Dello Russo, N.; Vervack, R. J.; Kawakita, H. et al

in Icarus (2016), 266

Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/Δλ ∼ 2.5 × 10[SUP]4[/SUP ... [more ▼]

Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/Δλ ∼ 2.5 × 10[SUP]4[/SUP]) infrared spectroscopy as part of a worldwide observing campaign. Spectra were obtained on UT 2013 October 26 and 28 with NIRSPEC at the W.M. Keck Observatory, and UT 2013 November 19 and 20 with CSHELL at the NASA IRTF. H[SUB]2[/SUB]O was detected on all dates, with production rates increasing markedly from (8.7 ± 1.5) × 10[SUP]27[/SUP] molecules s[SUP]-1[/SUP] on October 26 (R[SUB]h[/SUB] = 1.12 AU) to (3.7 ± 0.4) × 10[SUP]29[/SUP] molecules s[SUP]-1[/SUP] on November 20 (R[SUB]h[/SUB] = 0.43 AU). Short-term variability of H[SUB]2[/SUB]O production is also seen as observations on November 19 show an increase in H[SUB]2[/SUB]O production rate of nearly a factor of two over a period of about 6 h. C[SUB]2[/SUB]H[SUB]6[/SUB], CH[SUB]3[/SUB]OH and CH[SUB]4[/SUB] abundances in ISON are slightly depleted relative to H[SUB]2[/SUB]O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C[SUB]2[/SUB]H[SUB]2[/SUB], HCN and OCS abundances relative to H[SUB]2[/SUB]O appear to be within the range of mean values, whereas H[SUB]2[/SUB]CO and NH[SUB]3[/SUB] were significantly enhanced. There is evidence that the abundances with respect to H[SUB]2[/SUB]O increased for some species but not others between October 28 (R[SUB]h[/SUB] = 1.07 AU) and November 19 (R[SUB]h[/SUB] = 0.46 AU). The high mixing ratios of H[SUB]2[/SUB]CO/CH[SUB]3[/SUB]OH and C[SUB]2[/SUB]H[SUB]2[/SUB]/C[SUB]2[/SUB]H[SUB]6[/SUB] on November 19, and changes in the mixing ratios of some species with respect to H[SUB]2[/SUB]O between October 28 to November 19, indicates compositional changes that may be the result of a transition from sampling radiation-processed outer layers in this dynamically new comet to sampling more pristine natal material as the outer processed layer was increasingly eroded and the thermal wave propagated into the nucleus as the comet approached perihelion for the first time. On November 19 and 20, the spatial distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H[SUB]2[/SUB]O, HCN, C[SUB]2[/SUB]H[SUB]6[/SUB], C[SUB]2[/SUB]H[SUB]2[/SUB], and H[SUB]2[/SUB]CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH[SUB]3[/SUB] and OCS may be consistent with extended sources for these species. Abundances of HCN and C[SUB]2[/SUB]H[SUB]2[/SUB] on November 19 and 20 are insufficient to account for reported abundances of CN and C[SUB]2[/SUB] in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributions for NH[SUB]3[/SUB] and NH[SUB]2[/SUB] are similar, suggesting that NH[SUB]3[/SUB] is the primary source of NH[SUB]2[/SUB] with no evidence of a significant dust source of NH[SUB]2[/SUB]; however, the higher production rates derived for NH[SUB]3[/SUB] compared to NH[SUB]2[/SUB] on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH[SUB]2[/SUB] as a distributed source and accounts for its emission mechanism is needed for future work. [less ▲]

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See detailSimultaneous multi-scale and multi-instrument observations of Saturn's aurorae during the 2013 observing campaign
Melin, H.; Badman, S.; Stallard, T. et al

in Icarus (2016)

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See detailSaturn’s auroral morphology and field-aligned currents during a solar wind compression
Badman, S.V.; Provan, G.; Bunce, E.J. et al

in Icarus (2016)

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See detailRecurrent pulsations in Saturn’s high latitude magnetosphere
Mitchell, D.G.; Carbary, J.F.; Bunce, E.J. et al

in Icarus (2016)

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See detailQuasi-periodic injections of relativistic electrons in Saturn's outer magnetosphere
Roussos, E.; Krupp, N.; Mitchell, D. G. et al

in Icarus (2016), 263

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See detailSPICAM observations and modeling of Mars aurorae
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Libert, Ludivine ULg et al

in Icarus (2016), 264

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the ... [more ▼]

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the altitudes and the intensities of the auroral emissions. The CO (a3P–X1R) Cameron bands between 190 and 270 nm, the CO Fourth Positive system (CO 4P) between 135 and 170 nm, the CO2+ doublet at 289 nm, the OI at 297.2 nm and the 130.4 nm OI triplet emissions have been identified in the spectra and in the time variations of the signals. The intensities of these auroral emissions have been quantified and the altitude of the strongest emission of the CO Cameron bands has been estimated to be 137 ± 27 km. The locations of these auroral events have also been determined and correspond to the statistical boundary of open-closed magnetic field lines, in cusp-like structures. The observed altitudes of the auroral emissions are reproduced by a Monte-Carlo model of electron transport in the Martian thermosphere for mono-energetic electrons between 40 and 200 eV. No correlation between electron fluxes measured in the upper thermosphere and nadir auroral intensity has been found. Here, we simulate auroral emissions observed both at the limb and at the nadir using electron energy spectra simultaneously measured with the ASPERA-3/ELS instrument. The simulated altitudes are in very good agreement with the observations. We find that predicted vertically integrated intensities for the various auroral emissions are overestimated, probably as a consequence of the inclination and curvature of the magnetic field line threading the aurora. However, the relative brightness of the CO and CO2+ emissions is in good agreement with the observations. [less ▲]

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See detailIs the O2(a1Δg) Venus nightglow emission controlled by solar activity ?
Soret, Lauriane ULg; Gérard, Jean-Claude ULg

in Icarus (2015), 262

Several past studies showed that the O2(a1Δg) Venus nightglow emission at 1.27 μm is highly variable on a timescale of hours. We examine whether the intensity of this emission shows a more global trend ... [more ▼]

Several past studies showed that the O2(a1Δg) Venus nightglow emission at 1.27 μm is highly variable on a timescale of hours. We examine whether the intensity of this emission shows a more global trend linked to solar activity. [less ▲]

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See detailThe small binary asteroid (939) Isberga
Carry, B.; Matter, A.; Scheirich, P. et al

in Icarus (2015), 248

In understanding the composition and internal structure of asteroids, their density is perhaps the most diagnostic quantity. We aim here at characterizing the surface composition, mutual orbit, size, mass ... [more ▼]

In understanding the composition and internal structure of asteroids, their density is perhaps the most diagnostic quantity. We aim here at characterizing the surface composition, mutual orbit, size, mass, and density of the small main-belt binary asteroid (939) Isberga. For that, we conduct a suite of multi-technique observations, including optical lightcurves over many epochs, near-infrared spectroscopy, and interferometry in the thermal infrared. We develop a simple geometric model of binary systems to analyze the interferometric data in combination with the results of the lightcurve modeling. From spectroscopy, we classify Ibserga as a Sq-type asteroid, consistent with the albedo of 0.14<SUB>-0.06</SUB><SUP>+0.09</SUP> (all uncertainties are reported as 3-σ range) we determine (average albedo of S-types is 0.197 ± 0.153, see Pravec et al. (Pravec et al. [2012]. Icarus 221, 365-387). Lightcurve analysis reveals that the mutual orbit has a period of 26.6304 ± 0.0001 h, is close to circular (eccentricity lower than 0.1), and has pole coordinates within 7° of (225°, +86°) in Ecliptic J2000, implying a low obliquity of 1.5<SUB>-1.5</SUB><SUP>+6.0</SUP> deg . The combined analysis of lightcurves and interferometric data allows us to determine the dimension of the system and we find volume-equivalent diameters of 12.4<SUB>-1.2</SUB><SUP>+2.5</SUP> km and 3.6<SUB>-0.3</SUB><SUP>+0.7</SUP> km for Isberga and its satellite, circling each other on a 33 km wide orbit. Their density is assumed equal and found to be 2.91<SUB>-2.01</SUB><SUP>+1.72</SUP> gcm<SUP>-3</SUP> , lower than that of the associated ordinary chondrite meteorites, suggesting the presence of some macroporosity, but typical of S-types of the same size range (Carry [2012]. Planet. Space Sci. 73, 98-118). The present study is the first direct measurement of the size of a small main-belt binary. Although the interferometric observations of Isberga are at the edge of MIDI capabilities, the method described here is applicable to others suites of instruments (e.g., LBT, ALMA). [less ▲]

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See detailEvidence that Pluto's atmosphere does not collapse from occultations including the 2013 May 04 event
Olkin, C. B.; Young, L. A.; Borncamp, D. et al

in Icarus (2015), 246

Combining stellar occultation observations probing Pluto's atmosphere from 1988 to 2013, and models of energy balance between Pluto's surface and atmosphere, we find the preferred models are consistent ... [more ▼]

Combining stellar occultation observations probing Pluto's atmosphere from 1988 to 2013, and models of energy balance between Pluto's surface and atmosphere, we find the preferred models are consistent with Pluto retaining a collisional atmosphere throughout its 248-year orbit. The occultation results show an increasing atmospheric pressure with time in the current epoch, a trend present only in models with a high thermal inertia and a permanent N<SUB>2</SUB> ice cap at Pluto's north rotational pole. [less ▲]

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See detailThe binary near-Earth Asteroid (175706) 1996 FG3 - An observational constraint on its orbital evolution
Scheirich, P.; Pravec, P.; Jacobson, S. A. et al

in Icarus (2015), 245

Using our photometric observations taken between April 1996 and January 2013 and other published data, we derived properties of the binary near-Earth Asteroid (175706) 1996 FG<SUB>3</SUB> including new ... [more ▼]

Using our photometric observations taken between April 1996 and January 2013 and other published data, we derived properties of the binary near-Earth Asteroid (175706) 1996 FG<SUB>3</SUB> including new measurements constraining evolution of the mutual orbit with potential consequences for the entire binary asteroid population. We also refined previously determined values of parameters of both components, making 1996 FG<SUB>3</SUB> one of the most well understood binary asteroid systems. With our 17-year long dataset, we determined the orbital vector with a substantially greater accuracy than before and we also placed constraints on a stability of the orbit. Specifically, the ecliptic longitude and latitude of the orbital pole are 266 ° and - 83 ° , respectively, with the mean radius of the uncertainty area of 4 ° , and the orbital period is 16.1508 ± 0.0002 h (all quoted uncertainties correspond to 3σ). We looked for a quadratic drift of the mean anomaly of the satellite and obtained a value of 0.04 ± 0.20 deg /yr<SUP>2</SUP> , i.e., consistent with zero. The drift is substantially lower than predicted by the pure binary YORP (BYORP) theory of McMahon and Scheeres (McMahon, J., Scheeres, D. [2010]. Icarus 209, 494-509) and it is consistent with the tigidity and quality factor of μQ = 1.3 ×10<SUP>7</SUP> Pa using the theory that assumes an elastic response of the asteroid material to the tidal forces. This very low value indicates that the primary of 1996 FG<SUB>3</SUB> is a 'rubble pile', and it also calls for a re-thinking of the tidal energy dissipation in close asteroid binary systems. [less ▲]

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See detailLatitudinal structure of the Venus O2 infrared airglow: A signature of small-scale dynamical processes in the upper atmosphere
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Piccioni, Giuseppe et al

in Icarus (2014), 236

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the ... [more ▼]

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the latitudinal distribution of the O2(a1D) airglow limb profiles at 1.27 lm to characterize its distribution and variability. We show that the instantaneous structure of the emission is very different from the statistical global view of an enhanced emission near the equator, decreasing in brightness and slightly increasing in altitude toward the poles. The peak intensity of the limb profiles varies by a factor up to 50 between the brightest spots and the darkest regions. The bright airglow spots correspond to regions of enhanced downward flow of oxygen atoms originating from the dayside. Considerable variations in brightness and morphology are observed in the altitude–latitudinal distribution over a 24-h period. Analysis of the limb profiles indicates that secondary airglow peaks located at altitudes higher than the mean value of 96 km are observed on about 30% of the latitudinal cuts, but they are concentrated in narrow latitude areas extending over a few hundred kilometers. Most of them occur in transition regions between two altitude regimes in the 50 to 60 N region, possibly associated with the drop of the cloud top altitude observed equatorward of the ‘‘cold collar’’. We interpret these results as an indication that the strength of vertical transport in this mesosphere–thermosphere transition region is very variable both in location and time. This variability, also observed in nadir airglow images and wind measurements, is a key characteristic of the mesosphere–thermosphere transition region. It may be caused by fluctuations of the global day-to-night circulation generated by gravity waves. We show with a one dimensional model that local enhancements of eddy transport is a possibility. This variability is currently not accounted for by global circulation models that predict a single stable region of enhanced airglow in the vicinity of the antisolar point. [less ▲]

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See detailTime variations of O2(a1Delta) nightglow spots on the Venus nightside and dynamics of the upper mesosphere
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

in Icarus (2014), 237

The dynamical regime of the Venus upper atmosphere is mainly decomposed into three regions. The first one, located below 65 km of altitude is governed by the retrograde superrotational zonal (RSZ ... [more ▼]

The dynamical regime of the Venus upper atmosphere is mainly decomposed into three regions. The first one, located below 65 km of altitude is governed by the retrograde superrotational zonal (RSZ) circulation. The second region above 130 km is dominated by the subsolar to antisolar (SS–AS) circulation. The dynamics of the transition region in between are still not fully understood. However, the O2(a1D) nightglow emission at 1.27 lm, whose emitting layer is located at 96 km, can be used as a tracer of the dynamics in this transition region and the imaging spectrometer VIRTIS-M on board Venus Express, orbiting Venus since April 2006, acquired a large amount of nadir observations at this wavelength. Several previous studies showed that the O2(a1D) nightglow emission is statistically located near the antisolar point. In this study, individual VIRTIS-M nadir observations have been analyzed to investigate the variability of the phenomenon. Bright patches of 1.27 lm airglow have been extracted from every observation. It appears that the location of the bright patch is highly variable, even though the brightest patches occur near the antisolar point. Nadir observations have also been divided into time series, allowing generating animations to follow the intensity and the displacement of bright patches over time. Apparent wind velocities and characteristic decay/rise times and have been deduced from these time series. The speed of the displacements varies from 0 up to 213 m s 1, with a mean value of 54 m s 1. Owing to the high variability of the direction of the displacements both in the short and the long terms, no clear trend of a global motion at 96 km can be deduced from these observations. The mean decay time is 750 min while the mean rise time is 1550 min. The decay time can be explained as a combination of radiative decay and atomic oxygen transport. [less ▲]

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See detailOpen flux in Saturn's magnetosphere
Badman, Sarah; Jackman, Caitriuna; Nichols, Jonathan et al

in Icarus (2014), 231

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See detailThe tumbling spin state of (99942) Apophis
Pravec, P; Scheirich, P; Ďurech, J et al

in Icarus (2014), 233

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See detailHubble observations of Jupiter’s north–south conjugate ultraviolet aurora
Gérard, Jean-Claude ULg; Grodent, Denis ULg; Radioti, Aikaterini ULg et al

in Icarus (2013), 226

Comparisons of the northern and southern far ultraviolet (UV) auroral emissions of Jupiter from the Hubble Space Telescope (HST) or any other ultraviolet imager have mostly been made so far on a ... [more ▼]

Comparisons of the northern and southern far ultraviolet (UV) auroral emissions of Jupiter from the Hubble Space Telescope (HST) or any other ultraviolet imager have mostly been made so far on a statistical basis or were not obtained with high sensitivity and resolution. Such observations are important to discriminate between different mechanisms responsible for the electron acceleration of the different components of the aurora such as the satellite footprints, the «main oval» or the polar emissions. The field of view of the ACS and STIS cameras on board HST is not wide enough to provide images of the full jovian disk. We thus compare the morphology of the north and south aurora observed 55 min apart and we point out similarities and differences. On one occasion HST pointed successively the two polar regions and auroral images were seen separated by only 3 min. This makes it possible to compare the emission structure and the emitted FUV power of corresponding regions. We find that most morphological features identified in one hemisphere have a conjugate counterpart in the other hemisphere. However, the power associated with conjugate regions of the main oval, diffuse or discrete equatoward emission observed quasi-simultaneously may be different in the two hemispheres. It is not directly nor inversely proportional to the strength of the B-field as one might expect for diffuse precipitation or field-aligned acceleration with equal ionospheric electron density in both hemispheres. Finally, the lack of symmetry of some polar emissions suggests that some of them could be located on open magnetic field lines. [less ▲]

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See detailComparative analysis of airglow emissions in terrestrial planets, observed with VIRTIS-M instruments on board Rosetta and Venus Express
Migliorini, A.; Piccioni, G.; Capaccioni, F. et al

in Icarus (2013), 226

Airglow emissions are optimal processes to investigate the chemistry and dynamics in planetary atmospheres. In this study, we focus on the O2 and OH airglow emissions in Venus, Earth, and Mars atmospheres ... [more ▼]

Airglow emissions are optimal processes to investigate the chemistry and dynamics in planetary atmospheres. In this study, we focus on the O2 and OH airglow emissions in Venus, Earth, and Mars atmospheres, which are controlled by chemical reactions common to the three planets. By studying these phenomena on Venus, Earth, and Mars using similar instruments, we are able to derive information about their photochemistry and the physical conditions of the atmospheres, but also to constrain the dynamics responsible for transport of atomic oxygen, ozone and other minor species. After a review of the nightglow emissions observed in the Venus atmosphere, we analyze the O2 and OH airglow emissions in the Earth's atmosphere observed during the 3 swing-bys of our planet by the Rosetta spacecraft. We also report the detection of the O2 nightglow emission in the Mars atmosphere, observed in February 2007 during the Rosetta fly-by of the planet. The airglow characteristics are in agreement with the measurements obtained from sensors on board Mars Express. [less ▲]

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See detailThe characteristics of the O2 Herzberg II and Chamberlain bands observed with VIRTIS/Venus Express
Migliorini, A.; Piccioni, G.; Gérard, Jean-Claude ULg et al

in Icarus (2013)

The oxygen Venus nightglow emissions in the visible spectral range have been known since the early observations from the Venera spacecraft. Recent observations with the VIRTIS instrument on board Venus ... [more ▼]

The oxygen Venus nightglow emissions in the visible spectral range have been known since the early observations from the Venera spacecraft. Recent observations with the VIRTIS instrument on board Venus Express allowed us to re-examine the Herzberg II system of O2 and to further study its vertical distribution, in particular the (0–m00 with m00 = 7–13) bands. The present work describes the vertical profile of the observed bands and relative intensities from limb observation data. The wavelength-integrated intensities of the Herzberg II bands, with m00 = 7–11, are inferred from the recorded spectra. The resulting values lie in the range of 84–116 kR at the altitudes of maximum intensity, which are found to lie in the range of 93–98 km. Three bands of the Chamberlain system, centered at 560 nm, 605 nm, and 657 nm have been identified as well. Their emission peak is located at about 100 km, 4 km higher than the Herzberg II bands. For the first time, the O2 nightglow emissions were investigated simultaneously in the visible and in the IR spectral range, showing a good agreement between the peak position for the Herzberg II and the O2ða1Dg—X3R g Þ bands. An airglow model, proposed by Gérard et al. (Gérard, J.C., Soret, L., Migliorini, A., Piccioni, G. [2012]. Icarus.) starting from realistic O and CO2 vertical distributions derived from Venus-Express observations, allows reproduction of the observed profiles for the three O2 systems. [less ▲]

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See detailOxygen nightglow emissions of Venus: Vertical distribution and collisional quenching
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Migliorini, Alessandra et al

in Icarus (2013)

We compare the altitude of three O2 night airglow emissions observed at the limb of Venus by the VIRTIS spectral imager with the values predicted by a model accounting for the different radiative ... [more ▼]

We compare the altitude of three O2 night airglow emissions observed at the limb of Venus by the VIRTIS spectral imager with the values predicted by a model accounting for the different radiative lifetimes and collisional deactivation of the upper O2 states. The O and CO2 density profiles are based on remote sensing observations from the Venus Express spacecraft. Effective production efficiencies of the involved O2 metastable states and quenching coefficients by oxygen and carbon dioxide are adjusted to provide the best match with the measured emission limb profiles. We find values in general good agreement with earlier studies for the c1Σ-u state which gives rise to the Herzberg II bands. In particular, we confirm the low net yield of the c state production and the importance of its deactivation by CO2, for which we derive a quenching coefficient of 3x10-16 cm-3 s-1. The ∼4.5 km higher altitude of the Chamberlain band emission also recently detected by VIRTIS and the ratio of the Herzberg II/Chamberlain bands observed with Venera are well reproduced. To reach agreement, we use a 12% yield for the A’3Δu production following O atom association and quenching coefficients by O and CO2 of 1.3x10-11 cm-3 s-1 and 4.5x10-13 cm-3 s-1 respectively. We conclude that the different peak altitudes of the IR Atmospheric, Herzberg II and the Chamberlain bands reflect the relative importance of radiative relaxation and collisional quenching by O and CO2. [less ▲]

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