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1.
The meteoric influx explanation of superrotation (Mitra, 1974) is re-examined. It is shown that the excess orbital angular momentum of the meteoroids is transferred to the region below about 110 km, and thus can probably not account for the superrotation of the 150–400 km atmospheric layer.  相似文献   

2.
Mitra has suggested that the Superrotation of the upper atmosphere is caused by a deposition of meteoroids. The meteoroids are assumed to impart to the atmosphere the excess of their orbital angular momentum per unit mass over the Earth's angular momentum per unit mass. The process is to take place in the height region above 150 km. Only above this height is a Superrotation of the atmosphere observed. In this report the forces that tend to make the atmosphere corotate with the Earth are analysed. It is shown that the most important of these forces is ion drag, and not viscous drag as postulated by Mitra. As the net angular spin momentum imparted by the meteoroids seems to be less than Mitra's estimate and its main part is applied to the atmosphere at altitudes much lower than 150 km, the hypothesis that meteoroids provide a significant contribution to the Superrotation is rejected.  相似文献   

3.
A theory for the production of eddy diffusion in the upper atmosphere by the global deposition of meteoroids is presented. It is based on the assumption that meteoroids falling on the Earth carry, on the average, a greater amount of orbital angular momentum per unit mass than that corresponding to the Earth's orbit. This excess of orbital angular momentum of the meteoroids is deposited in some or the other form during their interaction with the Earth's atmosphere. The softer material deposits the excess of its orbital angular momentum in a region slightly higher than the harder material and is held responsible for the superrotation observed in the atmosphere. It is shown that the other population of meteoroids which is metallic in nature deposits the excess orbital angular momentum below 100 km altitude and produces eddies. The size and velocity of the eddies so formed give the value of the vertical eddy diffusion coefficient in agreement with the upper limit set by Johnson and Wilkins (1965) from the study of downward heat transport in the atmosphere.  相似文献   

4.
The theory of superrotation of the Earth's atmosphere by global deposition of meteoroids recently developed by the author (Mitra, 1974) is extended after a slight refinement to explain the rotation period of Venusian clouds. A satisfactory agreement with observations is obtained.  相似文献   

5.
The hypothesis that superrotation of the Earth's atmosphere results from meteoroid influx is untenable. Meteor observations are not of sufficient precision to substantiate direct rotation when the meteoroid influx is treated macroscopically. Detailed dynamic studies argue against any possible direct rotation.  相似文献   

6.
Jack D. Drummond 《Icarus》1982,49(1):143-153
A compilation of theoretical meteor radiants is presented for all numbered (through 2525) asteroids which approach the Earth's orbit to within 0.20 AU. On the basis of orbital similarity, asteroids associated with current meteor streams and Prairie Network fireballs are listed; plausible associations with medieval fireball radiants are also given. The best defunct comet candidates in terms of meteoric evidence appear to be 2101 Adonis and 2201 1947XC. Asteroids which may be either extinct comets or perturbed main belt asteroids accompanied by collisional debris (represented by fireballs) are 1917 Cuyo, 2202 Pele, 2061 Anza, and 2340 Hathor. 1566 Icarus and 1981 Midas are the only asteroids whose orbits approach to less than 0.07 AU of the Earth's orbit, have a northern radiant, and still show no certain meteoric activity. The majority of Atens, Apollos, and Amors do not pass sufficiently close (<0.07 AU) to the Earth's orbit for a reasonable expectation of meteoric activity, or have radiants south of ?20° declination, requiring southern hemisphere observations.  相似文献   

7.
It is shown in the paper that the mechanism of superrotation of the atmosphere of Venus consists of several interrelated processes, including the transport of angular momentum from the solid planet to the atmosphere at a wind flow over the planet’s surface relief, the upward transport of momentum by threedimensional turbulent vortices, the inverse energy cascade in a large-scale quasi-two-dimensional flow, and the horizontal transfer of angular momentum in the Hadley cell. These processes make different contributions to the superrotation at different altitudes.  相似文献   

8.
B.A. Conway 《Icarus》1982,51(3):610-622
A frequency-dependent model of tidal friction is used in the determination of the time rate of change of the lunar orbital elements and the angular velocity of the Earth. The variational equations consider eccentricity, the solar tide on the Earth, Earth oblateness, and higher-order terms in the Earth's tidal potential. A linearized solution of the equations governing the precission of the Earth's rotational angular momentum and the lunar ascending node is found. This allows the analytical averaging of the variational equations over the period of relative precession which, though large, is necessarily small in comparison to the time step of the numerical integrator that yields the system history over geological time. Results for this history are presented and are identified as consistent with origin of the Moon by capture. This model may be applied to any planet-satellite system where evolution under tidal friction is of interest.  相似文献   

9.
Recent studies have attributed certain properties of the Earth's atmosphere to excess orbita angular momentum of impinging meteoroids. A realistic analysis of meteor observations does not support the existence of this excess.  相似文献   

10.
The Community Atmosphere Model (CAM), a 3-dimensional Earth-based climate model, has been modified to simulate the dynamics of the Venus atmosphere. The most current finite volume version of CAM is used with Earth-related processes removed, parameters appropriate for Venus introduced, and some basic physics approximations adopted. A simplified Newtonian cooling approximation has been used for the radiation scheme. We use a high resolution (1° by 1° in latitude and longitude) to take account of small-scale dynamical processes that might be important on Venus. A Rayleigh friction approach is used at the lower boundary to represent surface drag, and a similar approach is implemented in the uppermost few model levels providing a ‘sponge layer’ to prevent wave reflection from the upper boundary. The simulations generate superrotation with wind velocities comparable to those measured in the Venus atmosphere by probes and around 50-60% of those measured by cloud tracking. At cloud heights and above the atmosphere is always superrotating with mid-latitude zonal jets that wax and wane on an approximate 10 year cycle. However, below the clouds, the zonal winds vary periodically on a decadal timescale between superrotation and subrotation. Both subrotating and superrotating mid-latitude jets are found in the approximate 40-60 km altitude range. The growth and decay of the sub-cloud level jets also occur on the decadal timescale. Though subrotating zonal winds are found below the clouds, the total angular momentum of the atmosphere is always in the sense of superrotation. The global relative angular momentum of the atmosphere oscillates with an amplitude of about 5% on the approximate 10 year timescale. Symmetric instability in the near surface equatorial atmosphere might be the source of the decadal oscillation in the atmospheric state. Analyses of angular momentum transport show that all the jets are built up by poleward transport by a meridional circulation while angular momentum is redistributed to lower latitudes primarily by transient eddies. Possible changes in the structure of Venus’ cloud level mid-latitude jets measured by Mariner 10, Pioneer Venus, and Venus Express suggest that a cyclic variation similar to that found in the model might occur in the real Venus atmosphere, although no subrotating winds below the cloud level have been observed to date. Venus’ atmosphere must be observed over multi-year timescales and below the clouds if we are to understand its dynamics.  相似文献   

11.
The experiment measures the mass, speed and approximate influx direction of the micrometeoroids. They are detected by the plasma produced during their impact on the sensor. In the first year after launch 173 particles were registrated.The temporal distribution of the particles indicates three different categories. Category 1 consists of 3 large groups of particles (“swarms”) each of which encountered the detector within some tens of minutes. Category 2 includes 8 small groups of particles each of which was detected within hours. Category 3 is made up by the remaining particles which are randomly distributed. The swarms are of very recent origin, possibly supplied by (1) meteoroids grazing the Earth's atmosphere, (2) ejecta from the Moon, and (3) collisions of dust particles. The groups may be dissipated swarms or particles from the outer zone of swarms. The randomly distributed particles consist of sporadic interplanetary particles and completely dissipated groups.The comparison of both the “particle rates” (number of detected particles per day) and the cumulative particle flux curves for the Earth's apex, antiapex, ecliptic north and south directions shows that the rates vary only within a factor of 2, whereas the particle flux is extremely anisotropic. The flux for the apex direction at 10?12 g is 7 × 10?5 m?2 s?1 which is 1 to 2 orders of magnitude higher than the flux values for the other directions.  相似文献   

12.
This paper investigates the exchange of global mean angular momentum between an atmosphere and its underlying planet by a simple model. The model parameterizes four processes that are responsible for zonal mean momentum budget in the atmospheric boundary layer for a rotating planet: (i) meridional circulation that redistributes the relative angular momentum, (ii) horizontal diffusion that smoothes the prograde and retrograde winds, (iii) frictional drag that exchanges atmospheric angular momentum with the underlying planet, and (iv) internal redistribution of the zonal mean momentum by wave drag. It is shown that under a steady-state or a long-term average condition, the global relative angular momentum in the boundary layer vanishes unless there exists a preferred frictional drag for either the prograde or the retrograde zonal wind. We further show quantitatively that one cannot have either a predominant steady prograde or retrograde wind in the boundary layer of a planetary atmosphere. The parameter dependencies of the global relative angular momentum and the strength of the atmospheric circulation in the boundary layer are derived explicitly and used to explain the observational differences between the atmospheres of Earth and Venus.  相似文献   

13.
G.S. Golitsyn 《Icarus》1984,60(2):289-306
A brief survey is given of the observational data on wind speeds in the atmosphere of Venus, as well as results of the theory of similitude and of a scale analysis for estimation of the wind speeds and temperature contrasts. It is shown that, in the lower portion of the atmosphere, containing roughly half of the mass, the first method produces results which are in somewhat better agreement with the measurements. Measurements of the wind distribution in the atmosphere are discussed. It is shown that, in the slowly rotating atmosphere of Venus, we should expect the Solberg mechanism of inertial instability of the circulation to be effective with respect to axisymmetrical perturbations. The numerical experiments of G.P. Williams (1968, J. Atmos. Sci., 25, 34–1045; 1970, Geophys. Fluid Dyn., 1, 357–369) indicate that in this case the circulation in the meridional plane can be broken down into a series of forward and reverse cells. The existence of such cells can serve to preserve the angular momentum of the planet with its atmosphere—the total momentum of the atmospheric frictional forces against the surface should on the average equal zero. This supports the hypothesis of G. Schubert et al. (1980, J. Geophys. Res., 85, 8007–8025) concerning the multicellular structure of the meridional circulation. Data are analyzed with regard to the time variability of the circulation. If the angular momentum of Venus′ atmosphere can change by 30% (which is not excluded by the presently available data; in Earth's atmosphere seasonal variations of the momentum reach 50%), then the relative variations in the length of a Venusian day will attain 10?3, i.e., several hours. The surface boundary layer is considered. It is shown that, due to the small transparency of the atmosphere to thermal radiation, heat transfer between the surface and the atmosphere should basically take place by turbulent heat exchange. The basic parameters of the dynamic and thermal regimes of this layer are estimated. Questions of light refraction in the boundary layer are discussed. A strict theory of refraction, developed for these conditions, confirms the preliminary rough estimates of V.I. Moroz (1976, Cosmic Res., 14, No. 5, 691–692; Space Sci. Rev., 25, 3–127), viz, that the horizon is visible on the panorama at a distance of order 100m, due to a relatively sharp negative gradient near the surface.  相似文献   

14.
Xun Zhu  Darrell F. Strobel 《Icarus》2005,176(2):331-350
Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 m s−1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 m s−1 Titan-day−1 to maintain peak prograde winds in excess of 100 m s−1 at the 1 mbar level.  相似文献   

15.
We consider the kinetics of a rarefied rotating planetary atmosphere. The spatial distributions of the atmospheric-gas density and mean angular velocity were determined by analyzing the exact solution of the two-dimensional kinetic equation. We show that the angular velocity of the gas at some distance from the planet could be higher than that in the initial layer starting from which the atmosphere is rarefied. Our model calculations elucidate the superrotation mechanism under consideration.  相似文献   

16.
We use a simplified terrestrial general circulation model as a nonlinear process model to investigate factors that influence the extent of equatorial superrotation in statically stable atmospheres on slowly rotating planets such as Titan and Venus. The possibility of multiple equilibria is tested by running the same model to equilibrium from vastly different initial conditions. The final state is effectively independent of initial state, reinforcing the impression that equatorial superrotation is inevitable on slowly rotating planets with stable radiative equilibrium structures. Of particular interest is the fact that at Titan rotation, the model equilibrates with strong prograde winds even when initialized with strong retrograde winds. This suggests that reliable remote sensing inferences of latitudinal temperature gradients on Titan can unambiguously be interpreted as evidence for superrotation. We also demonstrate for the first time that significant equatorial superrotation can be produced at Venus' rotation rate in such models, given sufficient numerical precision. The strength of superrotating zonal winds increases with rotation rate in the slowly rotating regime when other parameters are held fixed. However, the efficiency of superrotation relative to the angular momentum of an atmosphere corotating with the solid planet increases with decreasing rotation rate instead, because the Hadley cell strengthens and expands poleward. This allows for the formation of stronger high latitude jets, which ultimately serve as the source for equatorial superrotation via barotropic instability. Estimates of relevant parameter settings for Triton and Pluto tentatively imply that their atmospheres may marginally be in the superrotating regime, but only if temperature decreases with height near the surface.  相似文献   

17.
Six values of the rate of rotation of the Earth's upper atmosphere have been obtained by analysis of the orbital inclinations of four balloon satellites in the intervals just before the final decay of their orbits. The effective heights of these results range from about 350 to about 675 km. The values themselves range from 0·8 to 1·4 times the Earth's rotation and correspond to zonal wind speeds between 100 m/sec westward and 200 m/sec eastward. All the results correspond to fairly specific local times and are consistent with a diurnal wind pattern in low latitudes having a strong eastward maximum near local midnight and a lesser westward maximum near 10:00 LT. They argue against the contention of a sharp decrease in the rate with respect to that of the Earth, which is supposed to begin at about 360 km. The factors involved in the determination of these values and the method used are discussed in considerable detail.  相似文献   

18.
The chemistry and evolution of Titan's atmosphere is reviewed in the light of the scientific findings from the Voyager mission. It is argued that the present N2 atmosphere may be Titan's initial atmosphere rather than photochemically derived from an original NH3 atmosphere. The escape rate of hydrogen from Titan is controlled by photochemical production from hydrocarbons. CH4 is irreversibly converted to less hydrogen rich hydrocarbons, which over geologic time accumulate on the surface to a layer thickness of ～0.5 km. Magnetospheric electrons interacting with Titan's exosphere may dissociate enough N2 into hot, escaping N atoms to remove ～0.2 of Titan's present atmosphere over geologic time. The energy dissipation of magnetospheric electrons exceeds solar e.u.v. energy deposition in Titan's atmosphere by an order of magnitude and is the principal driver of nitrogen photochemistry. The environmental conditions in Titan's upper atmosphere are favorable to building up complex molecules, particularly in the north polar cap region.  相似文献   

19.
Jack J. Lissauer 《Icarus》1984,57(1):63-71
Ejecta from impacts of micrometeoroids on Saturn's ring particles will, in most cases, remain in orbit about Saturn and eventually be reaccreted by the rings, possibly at a different radial location. The resulting mass transport has been suggested as the cause of some of the features observed in Saturn's rings. Previous attempts to model this transport have used numerical simulations which have not included the effects of the angular momentum transport coincident with mass transport. An analytical model for ballistic mass transport in Saturn's rings is developed. The model includes the effects of angular momentum advection and shows that the net material movement due to angular momentum advection is comparable to that caused by direct ballistic mass transport.  相似文献   

20.
TitanWRF general circulation model simulations performed without sub-grid-scale horizontal diffusion of momentum produce roughly the observed amount of superrotation in Titan’s stratosphere. We compare these results to Cassini-Huygens measurements of Titan’s winds and temperatures, and predict temperature and winds at future seasons. We use angular momentum and transformed Eulerian mean diagnostics to show that equatorial superrotation is generated during episodic angular momentum ‘transfer events’ during model spin-up, and maintained by similar (yet shorter) events once the model has reached steady state. We then use wave and barotropic instability analysis to suggest that these transfer events are produced by barotropic waves, generated at low latitudes then propagating poleward through a critical layer, thus accelerating low latitudes while decelerating the mid-to-high latitude jet in the late fall through early spring hemisphere. Finally, we identify the dominant waves responsible for the transfers of angular momentum close to northern winter solstice during spin-up and at steady state. Problems with our simulations include peak latitudinal temperature gradients and zonal winds occurring ∼60 km lower than observed by Cassini CIRS, and no reduction in zonal wind speed around 80 km, as was observed by Huygens. While the latter may have been due to transient effects (e.g. gravity waves), the former suggests that our low (∼420 km) model top is adversely affecting the circulation near the jet peak, and/or that we require active haze transport in order to correctly model heating rates and thus the circulation. Future work will include running the model with a higher top, and including advection of a haze particle size distribution.  相似文献   

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