Plasma temperatures from alouette 1 electron density profiles

Values of plasma temperature and vertical temperature gradient were obtained by fitting theoretical models to 60,000 observed electron density profiles, at heights of 400–1000 km. Results show the diurnal and seasonal changes in temperature from 75°S to 85°N near solar minimum. At night the temperature and temperature gradient are both low inside the plasmapause and high outside. Day-time temperatures increase almost linearly with latitude, from 1500 K at the magnetic equator to a maximum of 3500 K at the plasmapause. There is also a sharp peak at 77° latitude, beneath the magnetospheric cleft. Mean vertical temperature gradients are $\text{ca. 0.5}\text{K}\text{km}$ at night, and 1–4 K/km during the day. The downwards flow of heat, during the day, increases from about zero at 10° latitude to a maximum of $\text{4 × 10}{}^9\text{eV}\text{cm}{}^2\text{sec}$ at the plasmapause. Night-time flows are 5–20 times less, inside the plasmasphere. Increases in magnetic activity cause a temperature increase at 400 km, of about 70 K per unit increase in K_p at all latitudes greater than 65°. The temperature peaks at the plasmapause and the magnetospheric cleft show little increase with magnetic activity, but move equatorwards by ca. 2° in latitude per unit K_p.     

The seasonal variation of the thermal structure of the atmosphere of Neptune

Previous work on the atmosphere of Uranus is extended to Neptune. The variation of effective temperature with latitude and season is evaluated within the approximations that the redistribution of internal heat in the interior results in the temperature at fixed pressure near the top of the convective region being independent of latitude and time, and that the transport of heat in the atmosphere is by means of radiation and convection. Meridional heat transport in the atmosphere is neglected. It is found that as the absorbed solar flux varies with season the flux of internal heat varies in the opposite sense such that the variation in the sum of the two is much smaller than the variation in either. The resulting variation in the flux radiated out the top of the atmosphere, which responds to the sum of the internal and absorbed solar fluxes, is substantially smaller than for Uranus because of the much larger flux of internal heat. For Neptune, the time-averaged effective temperature at the pole is ≈0.2°K greater than at the equator and the seasonal variation in the polar effective temperature is ≈0.8°K.     

Interpretation of the lunar microwave brightness temperature spectrum: Feasibility of orbital heat flow mapping

A detailed model of the lunar regolith is analyzed to examine the feasibility of an orbital mapping of heat flow using microwave radiometers. For regolith thermal and electrical properties which are representative of Apollo findings, brightness temperature observations in the bandλ = 5–30 cm would be required for heat flow analysis. Spectral variations shortward of 5 cm are controlled primarily by the temperature dependencies of the thermal conductivity and electrical absorption within the diurnal-varying layer. For wavelengths longer than 30 cm, unwanted emission from high impedance subregolith layers can be significant and size limitations on spacecraft radiometers is a factor. Over the 5- to 30-cm band, lunation-averaged brightness temperature increases of 2–10°K are predicted for heat flow values representative of the Apollo measurements. The magnitude of this increase depends directly on the value of regolith microwave absorption. For absorption values consistent with Apollo laboratory measurements, a spectral increase of 5°K is predicted. This value is considered marginally sufficient for an orbital heat flow measurement. However, important non-heat flow effects must be accounted for. Spectral variations can occur due to surface topography and subsurface scattering. For nadir viewing, surface roughness effects are not expected to be significant and topographic effects are nearly constant with wavelength for λ > 5cm. Volume scattering due to subsurface rock fragments can cause emission darkening of 1–6°K. However, spectral variations will not be large unless the distribution of scatterer sizes is sharply skewed. For the Moon, the most serious spurious effect appears to be emissivity variations due to the near-surface density gradient. A brightness temperature decrease of 10°K is predicted from centimeter to decameter wavelengths. If the transition from porous surface fines to compacted regolith soil occurs rapidly (within the upper 3–5 cm), most of the emissivity decrease will occur in the 5- to 30-cm wavelength band. It is recommended that complementary radar measurements be utilized to augment constraints on regolith emissivity and scattering properties.     

Venus mesosphere and thermosphere temperature structure: II. Day-night variations

A two-dimensional nonlinear hydrodynamic model has been developed for studying the global scale winds, temperature, and compositional structure of the mesosphere and thermosphere of Venus. The model is driven by absorption of solar radiation. Ultraviolet radiation produces both heating and photodissociation. Infrared solar heating and thermal cooling are also included with an accurate NLTE treatment. The most crucial uncertainty in determining the solar drive is the efficiency by which $\text{λ < 1080}\text{A}\text{?}$ solar radiation is converted to heat. This question was analyzed in Part I, where it was concluded that essentially all hot atom and O(¹D) energy may be transferred to vibrational-rotational energy of CO₂ molecules. If this is so, the minimum possible euv heating occurs and is determined by the quenching of the resulting excess rotational energy. The hydrodynamic model is integrated with this minimum heating and neglecting any small-scale vertical eddy mixing. The results are compared with predictions of another model with the same physics except that it assumes that 30% of $\text{λ < 1080}\text{A}\text{?}$ radiation goes into heat and that the heating from longer-wavelength radiation includes the O(¹D) energy. For the low-efficiency model, exospheric temperatures are ?300°K on the dayside and drop to < 180°K at the antisolar point. For the higher-efficiency model, the day-to-night temperature variation is from ?600°K to ?250°K. Both versions of the model predict a wind of several hundred meters per second blowing across the terminator and abruptly weakening to small values on the nightside with the mass flow consequently going into a strong tongue of downward motion on the nightside of the terminator. The presence of this circulation could be tested observationally by seeing if its signature can be found in temperature measurements. Both versions of the model indicate that a self-consistent large-scale circulation would maintain oxygen concentrations with ?5% mixing ratios near the dayside F-1 ionospheric peak but ?40% at the antisolar point at the same pressure level.     

Absolute measurements and computed values for Martian irradiance between 10.5 and 12.5 μm

The combination of seasonal and orbital changes in Martian insolation result in complex latitude dependent surface temperature variations that effect the total radiance of the planet as seen from the earth. These surface temperature variations have been calculated, based upon a computer simulation of the thermal environment of the planet. The temperature variations are then integrated to yield the total radiance of the planet as seen from the earth as a function of time. The absolute radiance of Mars was measured on April 4, 1971, with a balloon-borne radiometer system operating in the wavelength range between 10.5 and 12.5 μm. The average brightness temperature of the Mars disk determined from these measurements was 254°K with a 1 σ error of 4°K.     

Eolian intracrater deposits on Mars: Physical properties and global distribution

An investigation of Martian intracrater materials has been made using their thermophysical properties as derived from Viking IRTM observations. Over one-fourth of all craters larger than 25 km in diameter between ?50°S and 50°N have localized deposits of coarse material on the floor which are associated with dark “splotches” seen visually. Assuming homogeneous, unconsolidated materials, the measured thermal inertias of these deposits ($\text{I = 0.003 × 10}{}^{\mathrm{?3}}\text{to}\text{0.026 × 10}{}^{\mathrm{?3}}\text{cal cm}{}^{\mathrm{?2}}\text{sec}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{°}\text{K}{}^{\mathrm{?1}}$) imply effective grain sizes ranging from 0.1 mm to 1 cm, with a modal value of 0.9 mm. These deposits are coarser and darker than the surrounding terrains and the majority of the Martian surface, but are not compositionally distinct from materials with similar albedos. They occur more frequently in the south, in regions of relatively coarse material (0.2 to 2 mm), and in relatively dark areas. These features most likely formed by entrapment of marginally mobile material which can be transported into, but not out of, crater depressions by the wind. Very few have recognizable dune forms: those that do have effective grain sizes less than 0.5 mm. The majority of the “splotch” deposits are coarser than the dune-forming materials found in the north polar region and inside extreme southern latitude craters and probably form low, broad zibar dunes or lag deposits. Intracrater deposits are noticeably lacking from the interior of the large, northern hemisphere low-inertia region of Arabia (?10°S to 30°N, 300° to 360°W), interpreted to be a sink for suspended dust, but do occur around the perimeter of this region. This distribution suggests that the intracrater features have been buried in the interior of Arabia and that the dust deposit is less extensive at the margins and may currently be expanding. The occurrence of regional dust deposits in the north may be related to the maximum wind activity currently occurring in the southern hemisphere and suggests that the location of regional sinks may migrate with time as the solar insolation maximum migrates.     

Response of the intermediate complexity Mars Climate Simulator to different obliquity angles

A climate model of intermediate complexity, named the Mars Climate Simulator, has been developed based on the Portable University Model of the Atmosphere (PUMA). The main goal of this new development is to simulate the climate variations on Mars resulting from the changes in orbital parameters and their impact on the layered polar terrains (also known as permanent polar ice caps). As a first step towards transient simulations over several obliquity cycles, the model is applied to simulate the dynamical and thermodynamical response of the Martian climate system to different but fixed obliquity angles. The model is forced by the annual and daily cycle of solar insolation. Experiments have been performed for obliquities of φ=15^° (minimum), φ=25.2^° (present), and φ=35^° (maximum). The resulting changes in solar insolation mainly in the polar regions impact strongly on the cross-equatorial circulation which is driven by the meridional temperature gradient and steered by the Martian topography. At high obliquity, the cross-equatorial near surface flow from the winter to the summer hemisphere is strongly enhanced compared to low obliquity periods. The summer ground temperature ranges from 200 K (φ=15^°) to 250 K (φ=35^°) at 80^°N in northern summer, and from 220 K (φ=15^°) to 270 K (φ=35^°) at 80^°S in southern summer. In the atmosphere at 1 km above ground, the respective range is 195-225 K in northern summer, and 210-250 K in southern summer.     

The effect of orbital element variations on the mean seasonal daily insolation on Mars

In this paper we briefly study changes in the mean seasonal insolations on the planet Mars caused by significant large-scale variations in the following orbital elements: the eccentricity (e), the obliquity (ε) and the longitude of perihelion (λ _p ). Three orbital configurations have been investigated. In the first, the eccentricity equals successively 0, 0.075, and 0.15, whereas for the obliquity and the longitude of perihelion we took the present values which amount, respectively to 25° and 250°. In the second situation, ε=15, 25, and 35° for a circular orbit (e=0) and with λ _p =250°. In the last model we have sete=0.075 and ε=25° for λ _p =?90,0, and 90°. Although long-term periodic oscillations ofe (first case) and λ _p (third case) produce, respectively, very small or no variations in the average yearly insolation, fluctuations of the above mentioned planetary data strongly effect the mean summer and winter daily insolations. Indeed, the calculations reveal that between the two extreme values of the orbital elements used, the seasonal insolations exhibit a change in amplitude of about 15 to 20% difference over the entire latitude interval. Considering more particularly the second case it is found that the summertime insolation experiences a nearly similar variation as the mean annual daily insolation — i.e., a decrease of about 7% at the equator and a more than twofold increase at the poles. The corresponding mean winter daily insolation varies maximally by approximately 60% in the 60–80° latitude range.     

The upper atmosphere of Neptune: An analysis of occultation observations

An analysis of available observations of the April 7, 1968 occultation of BD ?17° 4388 by Neptune yields upper atmosphere temperatures of ～140°K near the 5 × 10¹⁴cm^?3 level. The temperature structure of the atmosphere at these levels is complicated and nonisothermal. Diurnal temperature variations are certainly less than 10°K and may be absent. The average temperature decreases by less than 15°K between 0° and 55° latitude.     

A seasonal model of the Saturnian upper troposphere: Comparison with Voyager infrared measurements

An extension of the seasonal climate model of R. D. Cess and J. Caldwell (1979, Icarus, 38, 349–357) to Saturn's upper troposphere is presented. The ring-modulated latitudinal dependence of the insolation, the ring thermal emission, the oblateness of the planet, the orbit eccentricity, and the latitudinal variation of the internal heat flux are taken into account. Calculations agree closely with the temperature—latitude profiles retrieved from Voyager IRIS measurements at atmospheric levels located above the 0.2-bar pressure level; they reproduce the observed large-scale hemispheric asymmetry which is then shown to result from the seasonally variable insolation. Aerosol absorption is found to be the dominant source of atmospheric solar heating in the troposphere and the model suggests an aerosol mean unit optical depth around the 0.25-bar level in the equatorial region and around the 0.35-bar level at other latitudes. The model fails to predict the retrieved temperature—latitude profiles below the 0.3-bar level. This discrepancy is attributed to the existence of clouds at these levels which are responsible for an additional far-infrared opacity not taken into account in the temperature retrieval. The cloud-top altitude would be about 0.3 bar except in the 20 to 40°N region where these clouds would be confined below the 0.6-bar level. The poor correlation between infrared measurements and visible images is discussed and a possible model of Saturn's cloud structure is proposed.     

Amalthea: Implications of the temperature observed by Voyager

Voyager 1 IRIS observations of Amalthea, although initially indicating an unusually high temperature, now give a temperature of only 164 ± 5°K, a value consistent with the Earth-based measurement by G. H. Rieke [Icarus25, 333–334 (1975)] of 155 ± 15°K. We numerically modeled the temperature profile in the satellite's surface layer as a function of location and time of day, assuming a triaxial ellipsoid shape and thermal properties similar to those of the lunar soil. The major heat source is direct insolation, but temperatures are increased slightly by thermal radiation from Jupiter (?9°K), by sunlight reflected from the planet (?5°K), and by charged particle bombardment (?2°K). Maximum calculated temperatures reach 166°K, and we estimate that the temperature that Voyager would have measured under these circumstances is ≈160°K, in agreement with the observed temperature. Possible sources of error in the model are discussed in detail, including satellite shape effects, unusually low emissivity, uncommonly rough surface, abnormal thermal intertia, variability of the charged particle flux, and Joule heating. The IRIS observation strongly suggests that (i) the Amalthean surface has an emissivity near unity; (ii) the charged particle flux on the satellite at the time of observation was no more than 20 times larger than the flux indicated by Pioneer observations; and (iii) Joule heating of the satellite is insignificant (a conclusion also supported by rough calculations). The IRIS observation cannot, however, put any useful limits on the thermal inertia of the Amalthean surface layer.     

Tracking the edge of the south seasonal polar cap of Mars

The advance and retreat of the polar caps were one of the first observations that indicated Mars had seasons. Because a large portion of the atmosphere is cycled in and out of the seasonal caps during the year, the frost deposits play a significant role in regional and global atmospheric circulation. Understanding the nature of the seasonal polar caps is imperative if we are to understand the current Martian climate. In this study, we track the seasonal cap edges as a function of season and longitude for the fall and winter seasons (MY27), using data from the Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEX) ESA mission. Making use of the rapid rise (decrease) in surface temperature that occurs when CO₂ ice is removed (deposited), in a first approach, we defined the advancing cap edge to be where the surface temperature drops below 150 K, and the retreating cap edge where the surface temperature rises above 160 K. In this case, starting from Ls∼50°, the edge progression speed start to be longitude dependent. In the hemisphere that extends form the eastern limit of the Hellas basin to the western limit of the Argyrae basin (and containing the two) the edges progression speed is about a half than that of the other hemisphere; the cap is thus asymmetric and, unexpectedly, no CO₂ ice seems to be present inside the basins. This is because the above mentioned surface temperatures used in this approach to detect the cap edges are not adequate (too low) for the high-pressure regions inside the basins where, following the Clausius–Clapeyron's law, the CO₂ condensation temperature can be several degrees higher than that of the adjacent lower-pressure regions. In the second, final approach, special attention has been given to this aspect and the advancing and retreating cap edges are defined where, respectively, the surface temperatures drop below and rise above the CO₂ condensation temperature for the actual surface pressure values. Now, the results show an opposite situation than the previous one, with the progression speed being higher and the cap more extended (up to −30° latitude) in the hemisphere containing the two major Martian basins. During the fall season, up to Ls∼50° the South Martian polar cap consists of CO₂ frost deposits that advance towards lower latitudes at a constant speed of 10° of latitude per 15 degrees of Ls. The maximum extension (−40° latitude) of the South polar cap occurs somewhere in the 80°–90° Ls range. At the winter solstice, when the edges of the polar night start moving poleward, the cap recession has already started, in response to seasonal changes in insolation. The CO₂ ice South polar cap will recede with a constant speed of ∼5° of latitude every 25° degrees of Ls during the whole winter. The longitudinal asymmetries reduce during the cap retreat and completely disappear around Ls=145°.     

Slope angle and frost formation on Mars

The low partial pressure of water vapor in the Martian atmosphere implies a low (～200K) frost point for water on the surface. This, in turn, has made it difficult to accept the traditional water frost explanation of bright patches that sometimes occur near the equator. We show that, because insolation and surface albedo are so important in determining surface temperature, nighttime frost deposits on steep slopes facing away from the Sun during local winter may persist throughout the day, even at the equator. Above about 25° north latitude, permanent frost deposits may form on steep north-facing slopes. Frost deposits on slopes could explain some bright features, as well as act as a reservoir for atmospheric H₂O.     

The upper atmosphere of Uranus: A critical test of isotropic turbulence models

Observations of the 15 August 1980 Uranus occultation of KM 12, obtained from Cerro Tololo InterAmerican Observatory, European Southern Observatory, and Cerro Las Campanas Observatory, are used to compare the atmospheric structure at points separated by ～140 km along the planetary limb. The results reveal striking, but by no means perfect, correlation of the light curves, ruling out isotropic turbulence as the cause of the light curve spikes. The atmosphere is strongly layered, and any acceptable turbulence model must accommodate the axial ratios of $\text{?60}$ which are observed. The mean temperature of the atmosphere is 150 ± 15°K for the region near number density 10¹⁴ cm^?3. Derived temperature variations of vertical scale ～ 130km and amplitude ±5°K are in agreement for all stations, and correlated spikes correspond to low-amplitude temperature variations with a vertical scale of several kilometers.     

Thermal effects of insolation propagation into the regoliths of airless bodies

We have investigated thermal models for planetary surfaces composed of particles that are bright and optically thin in the visual, and dark and opaque in the thermal infrared. The models incorporate the assumption that insolation is absorbed over a finite distance in the regolith, predicting lower daytime and higher nighttime temperatures than those predicted if the insolation were a absorbed only at the surface. The magnitude of the effect depends on the scale length for absorption of insolation relative to the diurnal skin depth for thermal diffusion, and can be significant when insolation penetrates to a depth comparable to the diurnal skin depth. In particular, for bodies like Enceladus and Europa, the maximum daytime temperature depression and nighttime temperature elevation can be 10°K or more for penetration-depth scales ∼ 1.5 cm. If insolation penetrates deeply enough into a surface, and the thermal-infrared opacity of its constituent particles is very high (e.g., in a regolith composed of particles of water ice), a solid-state greenhouse can result! This has important implications for geophysical models of high-albedo, icy bodies because actual boundary-layer temperatures may in fact be significantly higher than those assumed in previous studies, making it easier to melt the interiors of such bodies. Another important implication of the models is that where insolation- penetration is significant, thermal inertias inferred from models that do not allow for this effect will be upper limits to the real thermal inertia.     

Variations of the stratospheric temperature along the limb of Uranus: Results of the 22 April 1982 stellar occultation

Stratospheric temperature profiles of Uranus were derived from the stellar occultation of 22 April 1982 in the pressure range 5–30 μbar. The observations were made at the European Southern Observatory, Chile, and at the Observatoire du Pic du Midi et de Toulouse, France with two telescopes in both sites. The study of these profiles confirms that Uranus' stratosphere is warmer than had been expected from radiative models (J. F. Appleby, 1980, Atmospheric Structures of the Giant Planets from Radiative-Convective Equilibrium Models. PhD. Thesis, State University of New York at Stony Brook) and that there has been a general increase of temperature since 1977 (R. G. French, J. L. Elliot, E. W. Dunham, D. A. Allen, J. H. Elias, J. A. Frogel, and W. Liller, 1983, Icarus53, 399–414). Furthermore, the profiles exhibit a nonisothermal feature with a maximum temperature around the 8-μbar pressure level. The amplitude of this feature increases linearly with the diurnally averaged insolation 〈D〉 up to the observed value 〈D〉 ～ 0.15. Moreover, the temperature at 8 μbar, as well as the mean stratospheric temperature, reaches a plateau around the equator of the planet which is far from maximum insolation. For a nominal abundance of methane η_CH4 ～ 3 × 10^?5 and normal incidence, the UV absorption could compete with the IR methane absorption bands at the pressure level 8 μbar. However, the high temperatures observed even at grazing incidence imply important circulation phenomena to isothermalize distant regions of the planet. Alternatively, the observed profiles may suggest that an optically thin aerosol layer distributed over one scale height is responsible for the temperature maximum at 8 μbar. The total mass of dust necessary to heat this region up significantly would be a small fraction (6 × 10¹⁰ g vs 5 × 10¹⁸ g) of the Uranian ring system, which appears then as a possible reservoir of dust. However, a falling rate of ～1 msec^?1 would deplete the rings in a short time (≈2 × 10⁵ years) so that a dynamical process is needed to sustain the aerosol layer.     

Numerical experiments on the stability of preplanetary disks

Gravitational stability of gaseous protostellar disks is relevant to theories of planetary formation. Stable gas disks favor formation of planetesimals by the accumulation of solid material; unstable disks allow the possibility of direct condensation of gaseous protoplanets. We present the results of numerical experiments designed to test the stability of thin disks against large-scale, self-gravitational disruption. The disks are represented by a distribution of about 6 × 10⁴ point masses on a two-dimensional (r, φ) grid. The motions of the particles in the self-consistent gravity field are calculated, and the evolving density distributions are examined for instabilities. Two parameters that have major influences on stability are varied: the initial temperature of the disk (represented by an imposed velocity dispersion), and the mass of the protostar relative to that of the disk. It is found that a disk as massive as 1M_⊙, surrounding a 1M_⊙ protostar, can be stable against long-wavelength gravitational disruption if its temperature is about 300°K or greater. Stability of a cooler disk requires that it be less massive, but even at 100°K a stable disk can have an appreciable fraction ($\text{～}\text{1}\text{3}$) of a solar mass.     

Sodic plagioclase thermometry of type 6 ordinary chondrites: Implications for the thermal histories of parent bodies

Abstract— The structural states of sodic plagioclase crystals of ～50 μm in size from three H6, two L6, and one LL6 chondritic meteorites have been determined by measuring the Δ131 parameter with a Gandolfi camera after analyzing chemical compositions. The temperature for each sodic plagioclase crystal has been determined by plotting the Δ131 parameter, corrected for the influence of K, on the relation diagram between the Δ131 parameter and the temperature of synthesis of sodic plagioclase by Smith (1972). The temperature obtained is assigned to the crystallization temperature of sodic plagioclase, and the maximum plagioclase temperature for each meteorite can be assumed to correspond to the maximum temperature attained by each meteorite during metamorphism. The maximum metamorphic temperatures estimated are 725–742 °C for the H6 chondrites, 808–820 °C for the L6 chondrites, and 800 °C for the LL6 chondrite. These temperatures are lower than those based on Ca contents of clinopyroxenes (Dodd, 1981; McSween et al., 1988) but are consistent with those based on Ca contents of orthopyroxenes (McSween and Patchen, 1989; Langenhorst et al., 1995; Jones, 1997). The K content of sodic plagioclase correlates with the temperature obtained from the structural state. This positive correlation suggests that sodic plagioclase has formed in the course of equilibration processes of alkali elements in prograde metamorphism (i.e., during heating processes). The results of this study (i.e., the maximum metamorphic temperature of the H6 chondrites is lower than that of the L6 chondrites by ～80 °C, and meteorites of the same chemical group show very similar maximum metamorphic temperatures) are in accordance with the predictions of calculations based on the ²⁶Al heat source and the onion-shell structure model of the parent bodies.     

General circulation and transport in Saturn’s upper troposphere and stratosphere

We present a model for the general circulation and dynamical transport in Saturn’s upper troposphere and stratosphere and derive the effective advective circulation and eddy transport coefficients required for use in two-dimensional (latitude–altitude) photochemistry–transport models. A three-dimensional Outer-Planet General Circulation Model (OPGCM) is used to generate the transport data. We find that the OPGCM adequately captures the global-scale, pole-to pole temperature contrast, but overestimates mid- and high-latitude temperatures in the summer hemisphere by ～5 K. In addition, the model reproduces the local temperature minimum seen at the equator in Cassini Composite Infrared Spectrometer (CIRS) 0.1-mbar data but not the local maximum in 1-mbar temperatures, suggesting that it is capturing the phase of Saturn’s Semiannual Oscillation associated with a temperature minimum at the equator but not the opposite phase. The meridional circulation at low latitudes is found to be dominated by a seasonally reversing Hadley circulation, characterized by upwelling near the equator, cross-equatorial flow from summer to winter hemisphere, and strong subsidence centered near 25° latitude in the winter hemisphere. The cross-equatorial flow induces an asymmetry in which the equatorial jet is found to be stronger in the winter than in the summer stratosphere. The location of the subsidence near 25°N for L_s ～ 310° coincides with local maxima in acetylene, diacetylene, and methylacetylene mixing ratios measured by Cassini/CIRS (Guerlet, S., Fouchet, T., Bézard, B., Moses, J.I., Fletcher, L.N., Simon-Miller, A.A., Flasar, F.M. [2010]. Icarus 209, 682–695). This result supports the suggestion by Guerlet et al. (2010) that the hydrocarbon abundances are enhanced at this latitude by pronounced downward transport of hydrocarbon-rich air from above. The lateral eddy diffusion coefficient is found to typically be ～10⁵–10⁶ m² s^?1 at mid-latitudes, implying meridional eddy transport time scales of order 100–1000 years.