The effect of ammonia ice on the outgoing thermal radiance from the atmosphere of Jupiter

We examine the effects of NH₃ ice particle clouds in the atmosphere of Jupiter on outgoing thermal radiances. The cloud models are characterized by a number density at the cloud base, by the ratio of the scale height of the vertical distribution of particles (H_p) to the gas scale height (H_g), and by an effective particle radius. NH₃ ice particle-scattering properties are scaled from laboratory measurements. The number density for the various particle radius and scale height models is inferred from the observed disk average radiance at 246 cm^?1, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH₃ absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm^?1. We find lower limits on the particle size of 3 μm if H_p/H_g = 0.15, or 10μmif H_p/H_g = 0.50 or 0.05. NH₃ ice particles are relatively dark near the far-infrared and 8.5-μm atmospheric windows, and the outgoing thermal radiances are not very sensitive to various assumptions about the particle-scattering function as opposed to radiances at 5 μm, where particles are relatively brighter. We examined observations in these three different spectral window regions which provide, in principle, complementary constraints on cloud parameters. Characterization of the cloud scale height is difficult, but a promising approach is the examination of radiances and their center-to-limb variation in spectral regions where there is significant opacity provided by gases of known vertical distribution. A blackbody cloud top model can reduce systematic errors due to clouds in temperature sounding to the level of 1K or less. The NH₃ clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level.     

Scattering in the atmosphere of Venus: III. Line profiles and phase curves for Rayleigh scattering

Spectral line profiles, curves of growth, and curves for the equivalent width of a line as a function of Venus phase angle have been computed for a Rayleigh scattering cloud and compared with those for a cloud of isotropic scatterers. The results are very similar for the two kinds of scattering, with the exception of the curves of equivalent width as a function of Venus phase angle. These latter curves exhibit the “inverse phase effect” and rule out the possibility that the scale height of the clouds can be much less than half the scale height of the gas. The optical depth of the clouds, τ_c, is approximately 100.     

A bulk cloud parameterization in a Venus General Circulation Model

A condensing cloud parameterization is included in a super-rotating Venus General Circulation Model. A parameterization including condensation, evaporation and sedimentation of mono-modal sulfuric acid cloud particles is described. Saturation vapor pressure of sulfuric acid vapor is used to determine cloud formation through instantaneous condensation and destruction through evaporation, while pressure dependent viscosity of a carbon dioxide atmosphere is used to determine sedimentation rates assuming particles fall at their terminal Stokes velocity. Modifications are described to account for the large range of the Reynolds number seen in the Venus atmosphere.Two GCM experiments initialized with 10 ppm-equivalent of sulfuric acid are integrated for 30 Earth years and the results are discussed with reference to “Y” shaped cloud structures observed on Venus. The GCM is able to produce an analog of the “Y” shaped cloud structure through dynamical processes alone, with contributions from the mean westward wind, the equatorial Kelvin wave, and the mid-latitude/polar Mixed Rossby/Gravity waves. The cloud top height in the GCM decreases from equator to pole and latitudinal gradients of cloud top height are comparable to those observed by Pioneer Venus and Venus Express, and those produced in more complex microphysical models of the sulfur cycle on Venus. Differences between the modeled cloud structures and observations are described and dynamical explanations are suggested for the most prominent differences.     

Retrieval of ammonia abundances and cloud opacities on Jupiter from voyager IRIS spectra

A method is formulated to retrieve gaseous ammonia abundance and cloud opacities at 45 and 5 μm from Voyager IRIS data using a simplified atmospheric model and a two-stream radiative transfer approximation. Our goal is to obtain sufficient computational efficiency to permit global mapping of the relative horizontal variations of these parameters. A single cloud layer is invoked with a base pressure of 680 mbar and a scale height equal to 0.14 times the gas scale height. The NH₃ vertical distribution is modeled with a scale height equal to that of the cloud above 680 mbar and with a mole fraction independent of height at deeper levels. Measurements of brightness temperature as a function of emission angle from selected locations on the planet are used to verify the validity of the model and to constrain certain model parameters. It is found that the cloud particles can be treated as pure absorbers at 45 μm, but scattering must be included at 5 μm where a single scattering albedo of ∼0.75 is inferred. These results are used to develop a simple algorithm for the retrieval of ammonia abundance and cloud optical depths at 45 and 5 μm from measurements at 216, 225, and 2050 cm⁻¹.     

The cloud structure of the jovian atmosphere as seen by the Cassini/CIRS experiment

We analyze the thermal infrared spectra of Jupiter obtained by the Cassini-CIRS instrument during the 2000 flyby to infer temperature and cloud density in the jovian stratosphere and upper troposphere. We use an inversion technique to derive zonal mean vertical profiles of cloud absorption coefficient and optical thickness from a narrow spectral window centered at 1392 cm⁻¹ (7.18 μm). At this wavenumber atmospheric absorption due to ammonia gas is very weak and uncertainties in the ammonia abundance do not impact the cloud retrieval results. For cloud-free conditions the atmospheric transmission is limited by the absorption of molecular hydrogen and methane. The gaseous optical depth of the atmosphere is of order unity at about 1200 mbar. This allows us to probe the structure of the atmosphere through a layer where ammonia cloud formation is expected. The results are presented as height vs latitude cross-sections of the zonal mean cloud optical depth and cloud absorption coefficient. The cloud optical depth and the cloud base pressure exhibit a significant variability with latitude. In regions with thin cloud cover (cloud optical depth less than 2), the cloud absorption coefficient peaks at 1.1±0.05 bar, whereas in regions with thick clouds the peak cloud absorption coefficient occurs in the vicinity of 900±50 mbar. If the cloud optical depth is too large the location of the cloud peak cannot be identified. Based on theoretical expectations for the ammonia condensation pressure we conclude that the detected clouds are probably a system of two different cloud layers: a top ammonia ice layer at about 900 mbar covering only limited latitudes and a second, deeper layer at 1100 mbar, possibly made of ammonium hydrosulfide.     

Stellar Occultations by turbulent planetary atmospheres: A wave-optical theory including a finite scale height

A generalized wave-optical theory of stellar occultations by a turbulent planetary atmosphere is developed. The finite scale height of the atmosphere is retained for the first time. It is found that the finite scale height of the atmosphere affects the scintillations observed during the occultation in a number of ways which are most easily understood in terms of an effective Fresnel scale. We demonstrate the validity of a phase-changing screen approximation for occultation by a turbulent atmosphere in parameter ranges of general interest. Using this approximation various statistical properties of the fluctuating intensity are calculated explicitly. We present expressions for the total scintillation power, correlation function of the intensity, the cross-correlation at two frequencies, and its application to refractivity determinations. All of these expressions are given as a function of occultation depth and of parameters of the mean atmosphere and turbulence.     

A 1D microphysical cloud model for Earth,and Earth-like exoplanets: Liquid water and water ice clouds in the convective troposphere

One significant difference between the atmospheres of stars and exoplanets is the presence of condensed particles (clouds or hazes) in the atmosphere of the latter. In current 1D models clouds and hazes are treated in an approximate way by raising the surface albedo, or adopting measured Earth cloud properties. The former method introduces errors to the modeled spectra of the exoplanet, as clouds shield the lower atmosphere and thus modify the spectral features. The latter method works only for an exact Earth-analog, but it is challenging to extend to other planets.The main goal of this paper is to develop a self-consistent microphysical cloud model for 1D atmospheric codes, which can reproduce some observed properties of Earth, such as the average albedo, surface temperature, and global energy budget. The cloud model is designed to be computationally efficient, simple to implement, and applicable for a wide range of atmospheric parameters for planets in the habitable zone.We use a 1D, cloud-free, radiative–convective, and photochemical equilibrium code originally developed by Kasting, Pavlov, Segura, and collaborators as basis for our cloudy atmosphere model. The cloud model is based on models used by the meteorology community for Earth’s clouds. The free parameters of the model are the relative humidity and number density of condensation nuclei, and the precipitation efficiency. In a 1D model, the cloud coverage cannot be self-consistently determined, thus we treat it as a free parameter.We apply this model to Earth (aerosol number density 100 cm^?3, relative humidity 77%, liquid cloud fraction 40%, and ice cloud fraction 25%) and find that a precipitation efficiency of 0.8 is needed to reproduce the albedo, average surface temperature and global energy budget of Earth. We perform simulations to determine how the albedo and the climate of a planet is influenced by the free parameters of the cloud model. We find that the planetary climate is most sensitive to changes in the liquid water cloud fraction and precipitation efficiency.The advantage of our cloud model is that the cloud height and the droplet sizes are self-consistently calculated, both of which influence the climate and albedo of exoplanets.     

Spectroscopic investigation of the chromosphere

The cloud model employed in the analysis of chromospheric contrast profiles is subject to two criticisms. The source function in the cloud may not be varied independently of the Doppler width in the case of Hα and the radiative coupling between the cloud and the underlying atmosphere cannot be ignored. These criticisms are investigated quantitatively with two simple extreme models. It is found that by taking account of both effects the cloud model may be reinstated. Observed chromospheric features may be understood in terms of clouds of varying parameters embedded in the uppermost regions of a generally undisturbed homogeneous atmosphere. The variable cloud parameters are the optical thickness, the Doppler width, the bulk velocity and the angular size viewed from the line forming regions of the underlying atmosphere. Without multidimensional models the distribution of these parameters in chromospheric features observed at supergranulation boundaries for instance cannot be determined. General considerations however allow the interpretation of plagettes as simply low-lying mottles and allow the chromospheric velocity distribution derived by the original cloud model analysis to be upheld.     

Cosmic ray ionization of lower Venus atmosphere

Cosmic ray particles passing through dense lower atmosphere of Venus decay giving rise to various charged and neutral particles. The flux and degradation of dominant cascade particles namely neutrinos and pions are computed and ionization contributions at lower altitudes are estimated. Using the height profile of pion flux, the muon flux is computed and used to estimate ionization at lower altitudes. It is shown that cosmic ray produced ionization descends to much lower altitudes intercepting the thickness of Venus cloud deck. The dynamical features of Venus cloud deck are used to allow the likely charging and charge separation processes resulting into cloud-to-cloud lightning discharges.     

Magnetic deformation of the white dwarf surface structure

The influence of strong, large‐scale magnetic fields on the structure and temperature distribution in white dwarf atmospheres is investigated. Magnetic fields may provide an additional component of pressure support, thus possibly inflating the atmosphere compared to the non‐magnetic case. Since the magnetic forces are not isotropic, atmospheric properties may significantly deviate from spherical symmetry. In this paper the magnetohydrostatic equilibrium is calculated numerically in the radial direction for either for small deviations from different assumptions for the poloidal current distribution. We generally find indication that the scale height of the magnetic white dwarf atmosphere enlarges with magnetic field strength and/or poloidal current strength. This is in qualitative agreement with recent spectropolarimetric observations of Grw+10°8247. Quantitatively, we .nd for e.g. a mean surface poloidal field strength of 100 MG and a toroidal field strength of 2‐10 MG an increase of scale height by a factor of 10. This is indicating that already a small deviation from the initial force‐free dipolar magnetic field may lead to observable effects. We further propose the method of finite elements for the solution of the two‐dimensional magnetohydrostatic equilibrium including radiation transport in the diffusive approximation. We present and discuss preliminary solutions, again indicating on an expansion of the magnetized atmosphere.     

Processing of refractory meteorite inclusions (CAIs) in parent-body atmospheres

The refractory meteorite inclusions known as CAIs (calcium-aluminum rich inclusions) display melted rims that were produced by thermal events of only a few seconds duration. We show that gas dynamic deceleration in a temporary atmosphere around an accreting parent body, produced by gas release during accretion, could provide a regime of sufficiently high gas density and small scale height to achieve partial melting of the CAIs. In addition, the presence of dust in the atmosphere would increase the gradient of pressure with height (i.e., effectively reduce the scale height), lower the rate of blowoff (thus keeping more gas around the body), as well as allow dust particles to become trapped in the partially melted material as is observed in some cases. Thus, CAIs may be regarded as probes of a primitive atmosphere by virtue of the thermal and mineralogical alteration that occurred upon their passage through the atmosphere.     

Recovery of the mean Jovian temperature structure from inversion of spectrally resolved thermal radiance data

The mean thermal structure of the Jovian atmosphere near the temperature minimum (0.1 bar) is recovered by inversion of thermal radiance data. Improvements over previous studies of this type are made in two respects. (1) Care is taken to select data sources which are on a consistent calibration scale and from a frequency region which minimizes the variance in the recovered temperature. (2) The accuracy in the temperature recovery vs the vertical resolution capability is studied quantitatively. The contributions of various sources of systematic error, which generate significant uncertainty in the recovered thermal structure, are assessed. Sources of systematic error include assumptions about the stratospheric NLTE source function, the extent of cloud cover, the methane mixing ratio, and the calibration scale. Future investigations are outlined which would reduce such uncertainties and provide consistency with a wider range of data on the Jovian upper atmosphere.     

Infrared limb darkening of the Venus atmosphere

An analysis of the limb darkening component obtained by Ingersoll and Orton [Icarus21 (1974), 121–128] from the thermal infrared maps of Venus published by Murray, Wildey, and Westphal [J. Geophys. Res.68 (1963), 4813–4818] and Westphal, Wildey, and Murray [Astrophys. J.142 (1965), 799–802] shows that the Cytherean cloud tops were close to radiative equilibrium in 1962. A method for obtaining the optical depth, the extinction coefficient, and the extinction scale height from such data is derived and values are extracted from Marov's [Icarus16 (1972), 415–461] standard model of the Venus atmosphere.     

The low-latitude circulation of Mars

A simple linear, baroclinic and primitive equation model which includes both parameterized dissipation and a fairly realistic basic state is used to study the seasonal response of the Martian atmosphere to the steady-state influence of the orography of Mars. It is argued that the orography possesses a thermal and mechanical influence upon the state of the atmosphere. The thermal influence, which has a maximum at low latitudes, is a result of the temperature anomaly introduced into the atmosphere throughout the troposphere by the orographic feature. The resultant heat sources are shown to possess time scales which are much longer than diurnal, thus allowing a steady-state background circulation to develop. Using thermal and mechanical forcing derived from simple laws, the model is solved numerically to provide seasonal distributions of the steady-state circulation.The steady-state solutions are dominated by the thermal forcing of the Tharsis Ridge region and to a lesser degree, by that of the Olympus Mons region. Mechanical orographic forcing appears to possess an insignificant role in determining the low-latitude circulation. The states of the winter midlatitudes and tropics and the summer midlatitudes are very different, with the former region the most energetic. In the winter midlatitudes the kinetic energy is seen to increase with height with the excitation of large-scale and geostrophic near-barotropic eddies. In the tropics, the kinetic energy decreases with height and the response is nearly completely confined to the longitude-height plane. The transition between these two states occurs abruptly in the subtropics.Some of these features are similar to the planetary scale and long-period circulation of the low latitudes of the terrestrial atmosphere. Other features require consideration of properties inherent in the Martian atmosphere. To study these a simple, continuous analytic model is introduced which contains strong dissipation of time scales characteristic of Mars. It is shown that one solution, the equatorial Kelvin wave, is modified considerable by the strong damping and that it dominates the low-latitude circulation. Besides decaying rapidly with height, the vertical wave scale is stretched considerably with height by the dissipative processes. Such a stretching is shown to be scale selective and the longest horizontal modes are stretched the most in the vertical. Besides allowing an explanation of some features of the Martian atmosphere, the predominant vertical scale of the equatorial Kelvin wave allows some confidence in the choice of a two-layer model for the numerical study.     

Long nonlinear waves in a compressible magnetically structured atmosphere

The Benjamin-Ono equation is derived for long slow sausage waves propagating in a vertical magnetic slab embedded into a stratified atmosphere, provided that the slab thickness is much smaller than the scale height of the atmosphere. The soliton propagation in a nonstratified atmosphere is discussed. The approximate formulas describing the slow evolution of the amplitude and the length of a soliton propagating in a very weakly stratified atmosphere are obtained. The exact soliton-like solution for an atmosphere with a linearly growing temperature is found.     

Water and the Martian W cloud

The diurnal brightening of the W cloud region of Mars during the flyby of Mariners 6 and 7 is likely due to the formation of water ice clouds. The water required in the cloud and as vapor in the atmosphere to produce the observed brightening is estimated to be roughly between 0.1 and 10μm. Either a qualified local source with this daily production over the W cloud area or a more probable daily uplift and saturation of existing atmosphere are compatible with observations.     

Observation of Thunderstorm Ground Enhancements with intense fluxes of high-energy electrons

The high altitude (∼3200 m above sea level) of Aragats Space Environmental Center (ASEC) and low elevation of the thunderclouds provides a good opportunity to detect Thunderstorm Ground Enhancements (TGEs), particles of which rapidly attenuate in the atmosphere. In 2012, we have estimated the energy spectra of several TGEs and revealed significant electron fluxes extended till 30–40 MeV. Measured in the one and the same event gamma ray and electron fluxes allow to estimate the height of the thundercloud above the detector. Proceeding from the energy spectra and the height of the cloud we estimate the electron spectra on the exit from the electric field of the thundercloud, the number of excess electrons in the cloud and avalanche multiplication rate.     

Solar flux in Saturn's atmosphere: Penetration and heating rates in the aerosol and cloud layers

In this work, we describe an analysis of the internal solar radiation fields in Saturn's atmosphere. The aim of this paper is to study how the solar radiation flux in optical wavelengths (0.25-1.0 μm) is attenuated, primarily by the effect of the aerosols located close to the tropopause level, retrieving also the corresponding solar heating rates. We use a doubling-adding method and previous results on the vertical cloud and haze structure of Saturn's atmosphere. Our study shows that the maximum penetration level (∼250 mbar) for these wavelengths is substantially higher than previously expected because of the huge optical thickness of the tropospheric haze described in all vertical cloud structure models. We compare our results with previous estimates and parameterizations for seasonal climate models and propose a new approach for future models, with an intense and concentrated heating rate close to the top level of the tropospheric haze. Given that our spectral range accounts for about the 70% of the total solar flux, and using previous estimates for the penetration levels of infrared radiation in Saturn's atmosphere, we conclude solar radiation effect is negligible at levels below 600 mbar. This result is fundamental for understanding the role of solar radiation in the general atmospheric circulation of Saturn.     

Radiative instability of a cloudy planetary atmosphere

A cloudy planetary atmosphere at rest is shown to be unstable to disturbances of large horizontal scale. The energy source for the instability is the change in radiative heat flux associated with vertical displacement near the emitting level. A simple model is described in which Q∞ δz, where Q is the net heating rate in the cloud and δz is vertical displacement. The constant of proportionality may be either positive or negative. Disturbances may take the form of either quasi-steady geostrophic motions or amplified inertia-gravity waves. The model is applied to Jupiter's zonal winds and to motions near the Venus cloud tops, and provides a possible explanation for many important features of these two flows.     

Formation of spectral lines in planetary atmosphere IV. Theoretical evidence for structure of the jovian clouds from spectroscopic observations of methane and hydrogen quadrupole lines

The theory of formation of pressure-broadened methane lines and collision-narrowed hydrogen quadrupole lines in a Jovian atmosphere is studied in detail for a physically realistic model of the planet's lower atmosphere. Only observations of the center-to-limb (CTL) variations of the equivalent width of absorption lines for both of these molecules can identify the structure of the visible cloud layers. Observations of the CTL variation of methane and hydrogen quadrupole lines are the most suitable for studying the Jovian atmosphere. The CTL variations for hydrogen are much greater and more sensitive to variations of the properties of the thin upper tropospheric cloud layer than the corresponding observations of methane lines. A detailed comparison of hydrogen quadrupole with methane lines is made for the same continuum conditions, enabling us to develop a detailed understanding of the formation of the collision-narrowed hydrogen quadrupole lines in a Jovian atmosphere.