Disk Brightness Research Articles

Properties of the Hermean regolith: IV. Photometric parameters of Mercury and the Moon contrasted with Hapke modelling

A comparison of the photometric properties of Mercury and the Moon is performed, based on their integral phase curves and disk-resolved image data of Mercury obtained with the Swedish Vacuum Solar Telescope. Proper absolute calibration of integral V-band magnitude observations reveals that the near-side of the Moon is 10–15% brighter than average Mercury, and 0–5% brighter for the “bolometric” wavelength range 400–1000 nm. As shown, this is supported by recent estimates of their geometric albedos. Hapke photometric parameters of their surfaces are derived from identical approaches, allowing a contrasting study between their surface properties to be performed. Compared to the average near-side Moon, Mercury has a slightly lower single-scattering albedo, an opposition surge with smaller width and of marginally smaller amplitude, and a somewhat smoother surface with similar porosity. The width of the lobes of the single-particle scattering function are smaller for Mercury, and the backward scattering anisotropy is stronger. In terms of the double Henyey–Greenstein b– c parameter plot, the scattering properties of an average particle on Mercury is closer to the properties of lunar maria than highlands, indicating a higher density of internal scatterers than that of lunar particles. The photometric roughness of Mercury is well constrained by the recent study of Mallama et al. (2002, Icarus 155, 253–264) to a value of about 8°, suggesting that the surfaces sampled by the highest phase angle observations (Borealis, Susei, and Sobkou Planitia) are lunar mare-like in their textural properties. However, Mariner 10 disk brightness profiles obtained at intermediate phase angles indicate a surface roughness of about twice this value. The photometric parameters of the Moon are more difficult to constrain due to limited phase angle coverage, but the best Hapke fits are provided by rather small surface roughnesses. Better-calibrated, multiple-wavelength observations of the integral and disk-resolved brightnesses of both bodies, and obtained at higher phase angle values in the case of the Moon, are urgently needed to arrive at a more consistent picture of the contrasting light scattering properties of their surfaces.

Resonant Structure in the Kuiper Disk: An Asymmetric Plutino Disk

In order to develop a dynamical model of the Kuiper disk, we run numerical integrations of particles originating from source bodies trapped in the 3 : 2 external mean motion resonance with Neptune to determine what percentage of particles remain in the resonance for a variety of particle and source body sizes. The dynamical evolution of the particles is followed from source to sink with Poynting-Robertson light drag, solar wind drag, radiation pressure, the Lorentz force, neutral interstellar gas drag, and the effects of planetary gravitational perturbations included. We find that the number of particles in the 3 : 2 resonance increases with decreasing β (i.e., increasing particle size) for the cases in which the initial source bodies are small (≤10 km in diameter) and that the percentage of particles in resonance is not significantly changed by either the addition of the Lorentz force, as long as the potential of the particles is small (≈5 V), or the effect of neutral interstellar gas drag. The brightness of the entire Kuiper disk is calculated using a model composed of 500 μm diameter particles and fits well with upper limits to the Kuiper disk brightness and previous estimates. A disk with a size-frequency distribution weighted toward large particles, which are more likely to remain in resonance, may have a stronger, more easily identifiable resonant signature than a disk composed of small particles.

Multiepoch Interferometric Study of Mira Variables. I. Narrowband Diameters of RZ Pegasi and S Lacertae

As part of the long-term monitoring of Mira variables at the Palomar Testbed Interferometer, we report high-resolution narrowband angular sizes of the oxygen-rich Mira S Lac and the carbon-rich Mira RZ Peg in the near-infrared. The data set spans three pulsation cycles for S Lac and two pulsation cycles for RZ Peg (a total of 1070 25 s observations) and represents the first study to correlate multiepoch narrowband interferometric data of Mira variables. When the calibrated visibility data are fitted using a uniform disk brightness model, differences are seen in their angular diameters as a function of wavelength within the K band (2.0-2.4 μm), the source of which is believed to be molecular absorptions in or above the photospheres of the two chemically different Miras. Using visible photometric data provided by the Association Francaise des Observateurs d'Etoiles Variables (AFOEV), the continuum minimum size of RZ Peg lags this by 0.28 ± 0.02 in pulsation, similar to the phase lag found in Correlation Radial Velocities (CORAVEL) data. However, for S Lac, the continuum minimum size tracks the visual maximum brightness. Based on the mean of the continuum angular diameter cycloids, basic stellar parameters are computed for both RZ Peg and S Lac, with both showing maximum atmospheric extension with respect to the 2.0 and 2.4 μm diameters near phase 0.9. Using the mean value of the fitted cycloids, RZ Peg has a radius Rmean = 377 ± 111 R☉ and a mean Teff = 2706 ± 36 K; S Lac has a radius Rmean = 292 ± 73 R☉ and a mean Teff = 2605 ± 47 K. The dominant source of error in the radii is the large uncertainty in the distances to these two stars.

Relationship between the thickness of stellar disks and the relative mass of a dark galactic halo

We analyze the R-and K s-band photometric profiles for two independent samples of edge-on galaxies. The thickness of old stellar disks is shown to be related to the relative masses of the spherical and disk components of galaxies. The radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with increasing total mass-to-light ratio of the galaxies, which reflects the relative contribution of the dark halo to the total mass, and with decreasing central deprojected disk brightness (density). Our results are in good agreement with numerical models of collisionless disks that evolved to a marginally stable equilibrium state. This suggests that, in most galaxies, the vertical stellar-velocity dispersion, on which the equilibrium-disk thickness depends, is close to a minimum value that ensures disk stability. The thinnest edge-on disks appear to be low-brightness galaxies in which the dark-halo mass far exceeds the stellar-disk mass.

Metallicity and Kinematics of M31’s Outer Stellar Halo from a Keck Spectroscopic Survey

We present first results from a spectroscopic survey designed to examine the metallicity and kinematics of individual red giant branch stars in the outer halo of the Andromeda spiral galaxy (M31). This study is based on multislit spectroscopy with the Keck II 10 m telescope and Low Resolution Imaging Spectrograph of the Ca II near-infrared triplet in 99 M31 halo candidates in a field at R = 19 kpc on the southeast minor axis with brightnesses from 20 < I < 22. The spectra are used to isolate M31 halo red giants from foreground Milky Way dwarf stars, faint compact background galaxies, and M31 disk giants. The observed distribution of radial velocities is well fitted by an equal mix of foreground Milky Way dwarf stars, drawn from a standard Galactic model and with velocities v 0 km s-1, and M31 halo giants represented by a Gaussian of width σ ~ 150 km s-1 centered on its systemic velocity of v ≈ -300 km s-1. A secure sample of 29 M31 red giant stars is identified on the basis of radial velocity (v < -220 km s-1) and, in the case of four intermediate-velocity stars (-160 < v < -220 km s-1), broadband B-I color. For this sample of objects, there is rough agreement between the metallicities derived in independent ways: two different calibrations of the Ca II absorption-line strength and a photometric estimate based on fitting model stellar isochrones to an object's location in a (B-I, I) color-magnitude diagram. The [Fe/H] distribution of M31 halo giants has an rms spread of at least 0.6 dex and spans the 2 dex range over which the abundance measurement methods are calibrated. The mean/median metallicity of the M31 halo is about [Fe/H] = -1.9 to -1.1 dex (depending on the details of metallicity calibration and sample selection) and possibly higher: the high-metallicity end of the distribution is poorly constrained by our data since the selection function for the secure M31 sample excludes over 80% of the giants in solar/supersolar metallicity range. Possible reasons are explored for the apparent discrepancy between the mean [Fe/H] found in our spectroscopic survey (corrected for metallicity selection bias) and the slightly higher mean values found in earlier photometric studies. Field halo red giants in M31 appear to be somewhat more metal-rich on average than their Milky Way counterparts. The M31 halo [Fe/H] distribution is comparable to that of M31 globular clusters, Galactic globular clusters, and Local Group dwarf satellite galaxies. The data in this 19 kpc outer halo field are broadly consistent with a scenario in which the halo is built from the accretion of small stellar subsystems. There are four stars in the secure M31 sample that have particularly strong Ca II lines, indicating solar metallicity, at a common velocity of ≈-340 km s-1 close to the galaxy's systemic velocity, similar to what might be expected for M31 disk giants on the minor axis. An extrapolation of the inner disk brightness profile, however, falls far short of accounting for these four stars—the disk would instead have to be very large (Rdisk 80 kpc) and/or warped. More likely, these four stars represent a metal-rich debris trail from a past accretion event in the halo.

Altair’s Oblateness and Rotation Velocity from Long‐Baseline Interferometry

We present infrared interferometric angular size measurements for the A7IV-V star Altair that indicate a noncircular projected disk brightness distribution. Given the known rapid rotation of this star, we model the data as arising from an elongated rigid atmosphere. To first order, an ellipse may be fitted to our interferometric diameter measurements, with major and minor axes of 2a = 3.461 ± 0.038 mas and 2b = 3.037 ± 0.069 mas, respectively, for a difference of 424 ± 79 μas between 2a and 2b, and with an axial ratio of a/b = 1.140 ± 0.029. Assuming that the apparent oblateness of the photosphere is due to the star's rapid rotation, a more rigorous evaluation of the observation data in the context of a rigidly rotating Roche model shows that an estimate of v sin i = 210 ± 13 km s-1 can be derived that is independent of spectroscopic techniques. Also derived are values for the mean effective temperature and the mean linear radius, and an observational constraint upon the relationship between rotation velocity and stellar inclination is established. Altair is the first main-sequence star for which direct observations of an oblate photosphere have been reported and the first star for which v sin i has been established from observations of the star's photospheric geometry.

4.16. A model of the Galactic disk by a central hole

We present a model aimed to reproduce the infrared brigtness distribution of the Galaxy, as observed by the COBE-DIRBE experiment, and by the Japanese balloon flights (eg. Hayakawa et al., 1981). Based on the disk brightness profiles of a sample of spiral galaxies, and on the observed brigthness distribution of bars, we adopted new mathematical expressions to represent each component (bulge, bar, disk, spiral arms).

Whole-Disk Observations of Jupiter, Saturn, and Mars in Millimeter/Submillimeter Bands

Whole disk brightness ratios for Jupiter, Saturn, and Mars are reported at 5.7, 9.5, 16.4, and 22.5 cm$^{-1}$. Using models for the brightness temperature of Mars, the whole disk brightness temperatures for Jupiter and Saturn are also given for the four frequencies.

Inner-Truncated Disks – Are They Tied to Bars?

An inner-truncated disk (ITD) brightness profile is one in which the inward extrapolation of the outer disk surface brightness significantly exceeds the observed surface brightness in a region near the galaxy center. Freeman (1970) identified these profiles as Type II profiles. Fourteen of Freeman’s (1970) sample of 36 external galaxies are designated as Type II (ITD); seven of the ITD’s are classified as barred galaxies and ITD’s are found at all morphological types.Ohta, Hamabe, &amp; Wakamatsu (1990) report that luminosity profiles perpendicular to the bars of six early-type galaxies all exhibit the Freeman Type II shape, while the feature is lost when azimuthally averaged profiles of the same galaxies are studied. They postulate that the bar formation resulted in an azimuthal redistribution of the stars, but no net radial redistribution.

Hubble Space Telescope and R-Band Eclipse Maps of the UX Ursae Majoris Accretion Disk

view Abstract Citations (66) References (56) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope and R-Band Eclipse Maps of the UX Ursae Majoris Accretion Disk Baptista, R. ; Horne, Keith ; Hilditch, R. W. ; Mason, K. O. ; Drew, J. E. Abstract Hubble Space Telescope time-resolved spectroscopic and ground-based R-band photometric data of UX UMa during eclipse are presented and discussed. The spectroscopy covers the C IV λ1550 and He II λ1640 spectral regions. The eclipses in the UV continuum are deep, nearly reaching the zero level at mid-eclipse phases. Sharp ingress and egress features reveal the occultation of the white dwarf at disk center but there is no evidence of the orbital hump or of the bright spot. The steep ultraviolet eclipse light curves are used to estimate the radius of the white dwarf and to constrain the binary parameters. We find M1 = M2 = 0.47 Msun and i = 71°.0±0°.6. The white dwarf contributes with ≃25% of the light at λ1600 Å. If the inner disk is optically thick it has a temperature of Twd ≳ 65,000 K. Velocity-resolved light curves in the C IV λ1550 line show evidence of rotational disturbance and a strong orbital modulation is seen in the line center light curve. Maximum-entropy eclipse mapping techniques are used to solve for a map of the disk brightness distribution and for fluxes of an additional uneclipsed component in 18 narrow UV bands and in the R band. The angular scale of the disk and the distance to UX UMa are estimated from a color-magnitude diagram by a method similar to cluster main-sequence fitting. We find D = 345±34 pc. The radial temperature profiles derived from the UV continuum maps range from 8000 K in the outer disk regions to 30,000 K in the inner disk and reasonably follow the T ∝ R-¾ law for steady mass accretion at a rate of M = 10-8.0±0.2 Msun yr-1. The comparison of R-band eclipse maps from observations 2 months apart indicates changes in the structure of the outer disk which are probably due to small changes (≲20%) in the mass transfer rate from the secondary star. The maps in the C IV and He II line profiles are affected by emission from the disk wind. The radial temperature profiles of both line center maps show a flat slope. It is suggested that at low velocities the eclipsed part of the line emission arises from an opaque and extended region with a rather uniform spatial intensity distribution as projected in the plane of the sky. When plotted as a sequence of spatially resolved spectra these lines appear in absorption at disk center and change to emission in the outer disk regions besides showing large uneclipsed components. This behavior is similar to that found by Rutten et al. for the Balmer lines and suggests that these optical lines may also have a wind component. We obtain a revised ephemeris for the times of minimum and an upper limit for secular period changes of |Pdot| < 1.3 × 10-12 s s-1. The mass transfer rate inferred from this limit is an order of magnitude smaller than the value derived from the eclipse mapping, suggesting that the high mass transfer rate of UX UMa is not the consequence of its secular period decrease. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175970 Bibcode: 1995ApJ...448..395B Keywords: STARS: BINARIES: ECLIPSING; STARS: INDIVIDUAL CONSTELLATION NAME: UX URSAE MAJORIS; STARS: NOVAE; CATACLYSMIC VARIABLES; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (3) MAST (1) ESA (1)

Synthetic images of proto-planetary disks around young stars

We present synthetic images of accretion disks around young stars computed from a model where the disk's vertical structure is solved assuming hydrostatic equilibrium. The disk's brightness results from three emission processes: (1) the reprocessing of stellar photons in the optically thick disk's regions; (2) the scattering of stellar photons in the optically thin parts of the disk; and (3) the thermal emission of the disk due to viscous energy dissipation during the accretion process.

H I Lyman alpha emission from Saturn (1980‐1990)

Observations of Saturn made with the International Ultraviolet Explorer (IUE) satellite over the past decade have been analyzed to determine the characteristics of the H I Ly α dayglow and auroral emissions. The dayglow emission shows significant long‐ (years) and short‐term (days‐months) variability, with peak emissions in excess of 2 kR observed in 1980 and again in 1989; the disk brightness decreased after 1980 to its lowest value of ∼1 kR in January 1984; it then rose steadily and also exhibited increased short‐term variability between 1987 and 1989. The dayglow emission is strongly correlated (r&gt;0.7) with the solar Ly α flux, the solar He 10,830 Å equivalent width (EQW), and the solar 10.7‐cm radio flux. The amplitude of the long‐term variation best matches that of the solar 10.7‐cm radio flux, while the short‐term variability is more closely proportional to the solar Ly α flux and the He 10,830 Å EQW. Unless the interplanetary medium hydrogen density is larger than the majority of published values, a large discrepancy of more than a factor of 2 exists between the Ly α brightnesses measured by IUE and Voyagers 1 and 2 on nearly the same dates that cannot be explained by solar variability. Current models suggest that resonant scattering can account for less than one‐half of the observed dayglow emission. In addition, the ratio of solar maximum to solar minimum brightnesses is larger than can be explained by resonant scattering from hydrogen produced by solar extreme ultraviolet (EUV) processes. We conclude, based on current models, that the Saturn Ly α dayglow is not dominated by resonant scattering but nonetheless varies in direct proportion to solar activity. Postulations of a significant collisional source for the emission therefore remain plausible. Intermittent outbursts of up to 1 kR in auroral emission above disk brightness have also been observed over the past decade. Neither the dayglow nor the auroral emission exhibits any dependence on magnetic longitude. Several attempts to observe Saturn aurora associated with a major solar flare that produced unprecedented geomagnetic activity in March 1989 were unsuccessful.

Aperture synthesis observations of Saturn and its rings at 2.7-mm wavelength

We present 2.7-mm interferometric observations of Saturn made near opposition in June 1984 and June 1985, when the ring opening angle was 19° and 23°, respectively. By combining the data sets we produce brightness maps of Saturn and its rings with a resolution of 6″. The maps show flux from the ring ansae, and are the first direct evidence of ring flux in the 3-mm wavelength region. Modelfits to the visibility data yield a disk brightness temperature of 156 ± 5° K, a combined A, B, and C ring brightness temperature of 19 ± 3° K, and a combined a ring cusp (region of the rings which block the planet's disk) brightness temperature of 85 ± 5° K. These results imply a normal-to-the-ring optical depth for the combined ABC ringof 0.31 ± 0.04, which is nearly the same value found for wavelenghts from the UV to 6 cm. About 6°K of the ring flux is attributed to scattered planetary emission, leaving an intrinsic thermal component of ∼13°K. These results, together with the ring particle size distributions found by the Voyager radio occultation experiments, are consistent with the idea that the ring particles are composed chiefly of water ice.

High-resolution maps of the 1.5 GHz emission from Jupiter's disk and radiation belt

VLA maps of four different faces of Jupiter made with a resolution of about 0.3 Jovian radius show new features of the radiation belt emission. A synchrotron model which reproduces these features serves to define the major characteristics of the relativistic electrons in the radiation belt. The observations provide the best determination to date of the atmospheric emission at 1.5 GHz and yield a disk brightness temperature of 425 + or - 100 K.

On the origin of the discrete character of the solar disk brightness in the 160 nanometer continuum

view Abstract Citations (39) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the origin of the discrete character of the solar disk brightness in the 160 nanometer continuum Foing, B. ; Bonnet, R. M. Abstract The performance of the Transition Region Camera (TRC) is evaluated with respect to spatial resolution in order to arrive at a more refined description of the 160 nm features. The origins of the ultraviolet-bright grains observed in the interior of the supergranulation cells and of the bright elements that delineate the network are investigated. It is believed that the network elements and the cell grains have different origins. The network elements may appear bright as a consequence of the Wilson effect at the top of concentrated magnetic flux tubes. The magnetic field intensity deduced on the assumption that the tubes are thermalized with their surroundings is 120 G, which corresponds to an intensity of 1.5 kG at the photospheric level. The cell grains cannot be of magnetic origin because the magnetic field intensity they would require would be too high. It is suggested that they are the trace of a local dissipation of energy contained in periodic motions. Publication: The Astrophysical Journal Pub Date: April 1984 DOI: 10.1086/161955 Bibcode: 1984ApJ...279..848F Keywords: Brightness Distribution; Chromosphere; Modulation Transfer Function; Solar Magnetic Field; Ultraviolet Photography; Solar Temperature; Statistical Analysis; Sun; Solar Physics full text sources ADS |

VLA observations of the Galilean satellites

Jupiter's Galilean satellites I–IV, Io, Europa, Ganymede, and Callisto have been observed with the VLA at 2 and 6 cm. The Jovian system was about 4.46 AU from the Earth at the time the observations were taken. The flux densities for satellites I–IV at 2 cm are 15 ± 2, 5.6 ± 1.2, 22.3 ± 2.0, and 26.0 ± 2.5 mJy, respectively, which corresponds to disk brightness temperatures of 92 ± 13, 47 ± 10, 67 ± 6, and 92 ± 9°K, respectively. At 6 cm flux densities of 1.10 ± 0.2, 0.55 ± 0.12, 2.0 ± 0.2, and 3.15 ± 0.2 mJy were found, corresponding to temperatures of 65 ± 11, 44 ± 10, 55 ± 6, and 105 ± 7°K, respectively. The radio brightness temperatures are lower than the infrared, the latter generally being consistent with the temperature derived from equilibrium with absorbed insolation. The radio temperature are qualitatively consistent with the equilibrium temperature for fast rotating bodies considering the high radio reflectivity (low emissivity) as determined from radar measurements by S. J. Ostro (1982). In Satellites of Jupiter (D. Morrison, Ed.). Univ. of Arizona Press, Tucson).

Radio emission from Io

view Abstract Citations (4) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio emission from Io de Pater, I. ; Jaffe, W. J. ; Brown, R. A. ; Berge, G. L. Abstract Radio observations of Io taken with the VLA at 2, 6, and 21 cm show flux densities of 11.6 + or - 0.2 mJy, and less than 0.5 mJy, respectively, where the latter is a 2 sigma upper limit. These flux densities correspond to Io disk brightness temperatures of T(b) (2 cm) = 98 + or - 17 K, T(b) (6 cm) = 85 + or - 16 K, and T(b) (21 cm) less than 400 K, respectively. These radio brightness temperatures are consistent with thermal emission expected from Io's surface on the basis of its infrared brightness temperature and its radio emissivity derived from radar studies. No evidence for nonthermal radio emission from Io is found, such as has been reported by Mingaliev et al (1979). By using a model for the generation of synchrotron emission at 21 cm by energetic electrons in a hypothetical Io magnetosphere, an upper limit of Io's dipole magnetic moment less than 2 x 10 to the 25th gauss/cu cm is found. Publication: The Astrophysical Journal Pub Date: October 1982 DOI: 10.1086/160350 Bibcode: 1982ApJ...261..396D Keywords: Io; Radio Astronomy; Brightness Temperature; Earth Magnetosphere; Nonthermal Radiation; Planetary Radiation; Radiant Flux Density; Synchrotron Radiation; Thermal Emission; Lunar and Planetary Exploration full text sources ADS |

On the demise of disk galaxies in rich clusters

view Abstract Citations (35) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the demise of disk galaxies in rich clusters Kent, S. M. Abstract Attention is given to the observational results of Dressler (1980) concerning the properties and distributions of spiral, S0 and elliptical galaxies in rich clusters as a function of the local density of galaxies. A hypothesis is proposed in which it is assumed that the only fundamental property of galaxies affected by the cluster environment is the brightness of the galactic disks, which is reduced in regions of high galactic density. It is shown that this phenomenon, together with a few well defined selection effects, is capable of explaining quantitatively the observed changes in population distribution, absolute galactic bulge magnitudes and bulge/disk ratios with local density. Possible causes for a correlation between disk brightness and density are discussed in the context of current theories of galaxy formation, and it is argued that the formation of disks is most likely disrupted by the collapse of a cluster. Publication: The Astrophysical Journal Pub Date: May 1981 DOI: 10.1086/158855 Bibcode: 1981ApJ...245..805K Keywords: Disk Galaxies; Elliptical Galaxies; Galactic Clusters; Galactic Evolution; Spiral Galaxies; Galactic Bulge; Galactic Structure; Gravitational Collapse; Luminosity; Tides; Astrophysics full text sources ADS |

Spatial imaging of hydrogen Lyman-alpha emission from Jupiter

A sounding rocket measurement of the H I L-alpha emission from Jupiter made on Dec. 1, 1978 shows limb darkening and an average disk brightness of 13 kR. This brightness is significantly higher than in previous measurements, and was confirmed by an IUE observation on Dec. 10, 1978. Comparison with a plane-parallel hydrogen layer model indicates that there is enhanced emission from the equatorial regions, reaching a peak near 80 deg longitude.

Sulfur dioxide in the Venus atmosphere: Distribution and implications

The Pioneer Venus Orbiter Ultraviolet Spectrometer sees variable disk brightness features similar to the well‐known "UV markings" seen at longer wavelengths. The bright features are consistent with a homogeneous cloud of H2SO4 aerosols. The darker features show the presence of a broad‐band absorber, which is at some depth in the cloud layer. Additional contrast arises from SO2 absorption. The observed strength of the SO2 absorption as a function of wavelength rules out a uniform mixing ratio for the SO2. The data are well fitted by an inhomogeneous light scattering model in which the SO2 scale height is one‐fifth of the CO2 scale height and the mixing ratio of SO2 at 40 mb is 1 × 10−7. A model of the oxidation of sulfur dioxide in the upper cloud reproduces the observed vertical distribution of SO2 and indicates that SO2 alone is sufficient to produce the observed amount of H2SO4 in this region.