Michael E. Summers

First confirmation that water ice is the primary component of polar mesospheric clouds

Polar mesospheric clouds (PMCs) have been measured in the infrared for the first time by the Halogen Occultation Experiment (HALOE). PMC extinctions retrieved from measurements at eight wavelengths show remarkable agreement with model spectra based on ice particle extinction. The infrared spectrum of ice has a unique signature, and the HALOE-model agreement thus provides the first physical confirmation that water ice is the primary component of PMCs. PMC particle effective radii were estimated from the HALOE extinctions based on a first order fit of model extinctions.

Three‐dimensional plasma simulation of Io's interaction with the Io plasma torus: Asymmetric plasma flow

A three-dimensional, stationary, two-fluid plasma model for electrons and one ion species was developed to understand the local interaction of Ios atmosphere with the Io plasma torus and the formation of Ios ionosphere. Our model calculates, self-consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a given neutral atmosphere and imposed Io plasma torus conditions but assumes for the magnetic field the constant homogeneous Jovian field. With only photoionization in a pure SO2 atmosphere it is impossible to correctly model the plasma measurements by the Galileo spacecraft. With collisional ionization and photoionization the observations can be successfully modeled when the neutral atmospheric column density is Ncol = 6 × 1020 m−2 and the atmospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy for the maintenance of the collisional ionization process and thus the formation of Ios ionosphere. Anisotropic conductivity is shown numerically as well as analytically to be essential to understand the convection patterns and current systems across Io. The electric field is very greatly reduced, because the ionospheric conductances far exceed the Alfven conductance ΣA, and also strongly twisted owing to the Hall effect. We find that the electric field is twisted by an analytic angle tan Θtwist = Σ2/(Σ1 + 2ΣA) from the anti-Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Σ1 and Σ2 within a circle encompassing Ios ionosphere. Because the electron velocity is approximately equal to the E × B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, with E and B the electric and magnetic fields, respectively. Since Σ1 ∼ Σ2, the electron flow is strongly asymmetric during convection across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupiter when passing Io. Large electric currents flow in Ios ionosphere owing to these substantially different flow patterns for electrons and ions, and our calculations predict that a total electric current of 5 million A was carried in each Alfven wing during the Galileo flyby. We also find a total Joule heating rate dissipated in Ios ionosphere of P = 4.2 × 1011 W.

Titan's upper atmosphere - Structure and ultraviolet emissions

Abstract The Voyager 1 ultraviolet spectrometer (UVS) solar occultation and airglow data obtained during the Titan flyby are analyzed for composition and thermal structure of the upper atmosphere and relative importance of airglow excitation processes. From optical depth profiles inferred by Smith et al. (1982, J. Geophys. Res. 87, 1351–1359) for the evening terminator, the entrance occulatation, an asymptotic temperature of T∞ = 175 K, and a CH4 mixing ratio increasing from 0.06 ± 0.01 at 1000 km to 0.20 ± 0.02 at 1400 km are derived. The optical depth profiles are not of sufficient accuracy to extrapolate and accurately infer the tropopause CH4 mixing ratio, but our preference is for high values ∼0.026–0.034. The homopause occurs high in the atmosphere (∼1000 km) with an eddy diffusion coefficient K0 ∼ (4–8) × 108 cm2 sec−1 at this level and an approximate altitude dependence K = K0 exp (ζ/1.6), where ζ is the normalized geopotential height. From the shape of the observed bright limb profile for N2 emission (924–998 A) the magnetospheric power dissipation is, at most, 10% of the solar EUV power input to Titans extended atmosphere (= 2 × 10 9 W for λ A ) and consistent with our estimates of magnetospheric power delivered to the ionopause by curvature drift to thermal and suprathermal electrons. This value of magnetospheric power input is a factor of 25 lower than the original estimate of Strobel and Shemansky (1982, J. Geophys. Res. 87, 1361–1368). Our calculated absolute intensities for the N+ (1085 A) multiplet are consistent with the UVS observed intensities after downward revision by the Holberg et al. (1982, Astrophys. J., 257, 656–671) inflight calibration scheme, whereas the calculated N 2 c′ 4 (0-0, 958 A ) plus (0–1, 981 A) band intensities (the 970-A feature) are only ∼50% of the observed values. The nonthermal N atom escape rate is estimated to be ≤1025 sec−1. An upper limit on the tropopause Ar mixing ratio of 0.14 is derived from a comparison of the calculated relative intensities of the Ar resonance lines at 1048 and 1067 A with the N+ (1085-A) multiplet to the observationally inferred upper limit of 0.5.

The atmosphere of Pluto as observed by New Horizons

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto’s atmosphere is cold, rarefied, and made mostly of nitrogen and methane, with layers of haze. INTRODUCTION For several decades, telescopic observations have shown that Pluto has a complex and intriguing atmosphere. But too little has been known to allow a complete understanding of its global structure and evolution. Major goals of the New Horizons mission included the characterization of the structure and composition of Pluto’s atmosphere, as well as its escape rate, and to determine whether Charon has a measurable atmosphere. RATIONALE The New Horizons spacecraft included several instruments that observed Pluto’s atmosphere, primarily (i) the Radio Experiment (REX) instrument, which produced near-surface pressure and temperature profiles; (ii) the Alice ultraviolet spectrograph, which gave information on atmospheric composition; and (iii) the Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC), which provided images of Pluto’s hazes. Together, these instruments have provided data that allow an understanding of the current state of Pluto’s atmosphere and its evolution. RESULTS The REX radio occultation determined Pluto’s surface pressure and found a strong temperature inversion, both of which are generally consistent with atmospheric profiles retrieved from Earth-based stellar occultation measurements. The REX data showed near-symmetry between the structure at ingress and egress, as expected from sublimation driven dynamics, so horizontal winds are expected to be weak. The shallow near-surface boundary layer observed at ingress may arise directly from sublimation. The Alice solar occultation showed absorption by methane and nitrogen and revealed the presence of the photochemical products acetylene and ethylene. The observed nitrogen opacity at high altitudes was lower than expected, which is consistent with a cold upper atmosphere. Such low temperatures imply an additional, but as yet unidentified, cooling agent. A globally extensive haze extending to high altitudes, and with numerous embedded thin layers, is seen in the New Horizons images. The haze has a bluish color, suggesting a composition of very small particles. The observed scattering properties of the haze are consistent with a tholin-like composition. Buoyancy waves generated by winds flowing over orography can produce vertically propagating compression and rarefaction waves that may be related to the narrow haze layers. Pluto’s cold upper atmosphere means atmospheric escape must occur via slow thermal Jeans’ escape. The inferred escape rate of nitrogen is ~10,000 times slower than predicted, whereas that of methane is about the same as predicted. The low nitrogen loss rate is consistent with an undetected Charon atmosphere but possibly inconsistent with sublimation/erosional features seen on Pluto’s surface, so that past escape rates may have been much larger at times. Capture of escaping methane and photochemical products by Charon, and subsequent surface chemical reactions, may contribute to the reddish color of its north pole. CONCLUSION New Horizons observations have revolutionized our understanding of Pluto’s atmosphere. The observations revealed major surprises, such as the unexpectedly cold upper atmosphere and the globally extensive haze layers. The cold upper atmosphere implies much lower escape rates of volatiles from Pluto than predicted and so has important implications for the volatile recycling and the long-term evolution of Pluto’s atmosphere. MVIC image of haze layers above Pluto’s limb. About 20 haze layers are seen from a phase angle of 147°. The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. For example, white arrows on the left indicate a layer ~5 km above the surface, which has descended to the surface at the right. Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto’s atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto’s atmosphere to space. It is unclear whether the current state of Pluto’s atmosphere is representative of its average state—over seasonal or geologic time scales.

Two‐dimensional model calculations of nitric oxide transport in the middle atmosphere and comparison with Halogen Occultation Experiment data

A two-dimensional chemical transport model has been used to examine the physical processes governing the transport of high levels of thermospheric nitric oxide (NO) downward into the middle atmosphere. Three different facets of this transport are studied. The first facet involves diffusion from the thermosphere to the summertime mesopause region. The second facet involves downward advection by the mean meridional circulation in the wintertime mesosphere and the effects of planetary wave mixing on the latitudinal gradient of NO. The third facet is the residual amount of NO deposited in the springtime upper stratosphere and its senstivity to the magnitude and duration of the unmixed descent which occurred the previous winter. Comparison of the model with observations by the Halogen Occultation Experiment (HALOE) suggest the following: (1) A clear auroral enhancement in summertime NO exists at 89 km. Model calculations suggest this results from both in situ ionization and dissociation of N2 as well as downward diffusion from the thermosphere above 100 km. (2) Using HALOE CH4 observations as a tracer, enhanced NO in the wintertime mesosphere is seen to be transported to latitudes as far equatorward as 30°–40°. The model is in good agreement with these observations when planetary wave mixing is included. Without this mixing, the enhanced NO remains confined to high latitudes that are not observed by HALOE in winter. (3) The model overestimates the net NO deposited into the upper stratosphere. This appears to be related to the model springtime warming being delayed relative to the real atmosphere. Inclusion of an additional source of drag in the polar stratosphere in late winter yields better agreement with observations.

Seasonal variation of middle atmospheric CH4 and H2O with a new chemical‐dynamical model

A new zonally averaged, chemical-dynamical model of the middle atmosphere is used to study the processes which control the distributions and seasonal variability of CH4 and H2O. This model incorporates a nondiffusive, nondispersive advection scheme, a time-dependent linear model of planetary wave drag and horizontal mixing (Kyy), a new parameterization of gravity wave drag and vertical mixing (Kzz), and an explicit treatment of LTE (local thermodynamic equilibrium) and non-LTE IR cooling. Model chemistry is calculated using a Newton-Raphson iterative scheme, which allows consistent simulations of species with highly nonuniform chemical lifetimes. In this study we focus on the sensitivity of model CH4 and H2O to the magnitude of tropospheric latent heat release, planetary wave and gravity wave activity, and the methane oxidation rate. Model results show that in the tropical stratosphere their vertical distributions are strong functions of both the methane oxidation rate and the ascent rate, the latter driven by a combination of tropospheric latent heat release and atmospheric drag. At low latitudes HALOE observations and model results both show conservation of “potential H2” (2×CH4+H2O) below ∼50 km. However, the conservation of potential H2 from HALOE observations breaks down above ∼55 km, while the model shows conservation well into the middle mesosphere (∼70 km). This may suggest serious inadequacies in our understanding of the photochemistry of water vapor and mesospheric HOx, in particular those processes which control the partitioning of H2 and H2O. At high latitudes, H2O model/data comparisons suggest that horizontal mixing is important in determining the observed latitudinal gradient in mesospheric water vapor. We also find that inside the polar winter vortex, while the strength of tropical latent heat forcing and planetary wave drag influence the descent rate, both horizontal mixing and the methane photochemistry play important roles in determining the CH4 mixing ratio. Finally, we suggest that the observed interhemispheric asymmetry in the seasonal cycle of mesospheric H2O may be linked to larger values of Kzz in the southern winter mesosphere. This represents a key difference between mesospheric and stratospheric tracer transport. In the stratosphere, greater net unmixed descent in the southern hemisphere directly translates into lower tracer values relative to the northern hemisphere, while mesospheric tracer transport shows the opposite behavior.

Descent of long‐lived trace gases in the winter polar vortex

Recent observations of CH4 and HF from the UARS Halogen Limb Occultation Experiment (HALOE) suggest that vigorous descent occurs within the polar winter vortex with “mesospheric” values of CH4 evident down to 30 mbar. This study shows that a highly accurate two-dimensional model advection scheme coupled with a modern radiation scheme, parameterized planetary and gravity wave drag algorithms can produce tracer distributions consistent with HALOE observations. The modeled tracer distribution within the polar vortex is found to be principally dependent on the strength of dynamical drag in the middle atmosphere and the strength of the planetary wave forcing. However, the strong downward transport of tracers at the poles during winter can be disrupted in midwinter by planetary wave mixing. Thus the weaker planetary wave forcing in the southern hemisphere winter allows for a more coherent descent of long-lived tracers from the mesosphere than during the northern hemisphere winter. Multiple-year integrations of the model reveal a general circulation of the stratosphere which lofts tracers to mesospheric altitudes. Material removed from the mesosphere returns to the stratosphere principally within the polar regions. Upward vertical transport of material is found to be enhanced by horizontal planetary wave mixing.

New Horizons: Anticipated Scientific Investigations at the Pluto System

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N2:CH4 atmospheres (such as Titan, Triton, and the early Earth).

Declining trend of total ozone column over the northern parts of India

Ozone is one of the important atmospheric trace gases that absorbs both incoming solar near‐ultraviolet and outgoing infrared radiation from the Earths surface. After the discovery of the ‘ozone hole’, assessment of the long‐term trend of ozone in different regions of the globe has become a frontline topic of research. The present study deals with the variability in the total ozone column over the Indian subcontinent using satellite and limited ground observations. The linear regression technique was applied to the Nimbus and Earth Probe Total Ozone Mapping Spectrometer (EP‐TOMS) data to study the trends during 1997–2003. The rate of decline of ozone is found to be higher in recent years over the northern parts of India, covering the Indo‐Gangetic basin, compared with other parts of India.

Satellite observations of upper stratospheric and mesospheric OH: The HOxdilemma

We report the first observations of the vertical distribution of hydroxyl (OH) from the upper stratosphere to the mesopause. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) made these measurements in August 1997. The data confirm the results from the earlier November 1994 MAHRSI mission that were confined to altitudes above 50 km, namely that mesospheric OH densities are 25 to 35% lower than predicted by standard photochemical theory. However, the new observations show that below 50 km the OH density increases rapidly and at 43 km altitude it is larger than that expected from standard theory. This represents a serious dilemma for our understanding of odd-hydrogen chemistry because the same key reactions are thought to dominate OH/HO2 partitioning in both regions. We show that neither standard photochemical theory nor any previously proposed changes are adequate to explain the OH observations in both the upper stratosphere and mesosphere.