Determination of zonal wind speeds at heights near 530 km from the orbit of Explorer 24 (1964-76A)

The orbit of Explorer 24 (1964–1976A) has been determined at 18 epochs during the five month period prior to its decay in October 1968, using the RAE orbit refinement computer program PROP6. As a balloon, the satellite was strongly influenced by atmospheric perturbations, despite its high perigee altitude near 490 km. It therefore provided an opportunity of determining atmospheric rotation rates at high altitude. The rotation rate, Λ rev day^?1, was estimated from the observed variation in orbital inclination, after the removal of perturbations including those due to solar radiation pressure.The mean rotation rates, averaged over local time, are Λ = 0.98 for 18 May to 18 August 1968 at 542 km; Λ = 1.06 for 18 May to 13 October 1968 at 533 km.For morning conditions, Λ = 0.9 for 22 June to 20 July 1968 at 540 km; Λ = 0.8 during September 1968 at 513 km.For evening conditions, Λ = 1.1 for 18 May to 15 June 1968, and for 26 July to 7 September 1968, at 540 km and 536 km respectively; Λ = 1.3 for 28 September to 13 October 1968 at 484 km.Further, the maximum W to E zonal wind has been estimated to occur at 20.5 h local time, during the period of the analysis.     

Upper-atmosphere zonal winds from satellite orbit analysis

In this paper we review and interpret the values of upper-atmosphere rotation rate (zonal winds) obtained by analysing satellite orbits determined from observations. The history of the method is briefly reviewed, the basic principles are explained, objections to the method are answered, and three examples are given. Existing analyses of the atmospheric rotation rate A are critically reviewed, and, after rejecting some and revising others, we are left with 85 values. These are divided according to local time and season, to give the variation of A with height in nine situations—namely morning, evening and average local time, for summer, winter and average season. These observational results indicate that the value of Λ (in rev/day), averaged over both local time and season, increases from 1.0 at 125 km to 1.22 at 325 km and then decreases to 1.0 at 430 km and 0.82 at 600 km. The value of Λ is higher in the evening (18–24 h), with a maximum value (near 1.4) corresponding to a West-to-East wind of 150 m s^?1 at heights near 300 km. The value of Λ is lower in the morning (06–12 h), with East-to-West winds of order 50 m s^?1 at heights of 200–400 km. There is also a consistent seasonal variation, the values of Λ being on average 0.15 higher in winter and 0.1 lower in summer than the average seasonal value. No significant variation with solar activity is found, but there is a slight tendency for a greater rotation rate at lower latitudes for heights above 300 km. Unexpectedly, the values for the 1960s are found to be significantly higher than those for the 1970s. Finally, these observational values are compared with the theoretical global model of Fuller-Rowell and Rees: there is complete agreement on the trends, though there are some differences in the mean values.     

Systematic winds at heights between 350 and 675 km from analysis of the orbits of four balloon satellites

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.     

Saturn's cloud morphology and zonal winds before the Cassini encounter

Analyses of Hubble Space Telescope (HST) images of Saturn obtained from August 2003 to March 2004, with extensive support from ground-based telescopes, have been used to characterize the cloud morphology and motions in its atmosphere few months before the Cassini encounter. We present data on the major meteorological features as potential targets for Cassini observations. We extend our previous measurements of the zonal winds during the 1996-2002 period (A. Sánchez-Lavega et al., 2003, Nature, 423, 623-625), confirming the strong change in the equatorial jet, and the high hemispheric symmetry of the zonal wind pattern.     

Characterization of zonal winds in the stratosphere of Titan with UVES

Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (4200-5200 Å) probes one scale height higher than the long-wavelength one (5200-6200 Å), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 m s⁻¹ at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds.     

Perturbations of the orbit of Explorer 19 due to solar radiation

The orbit of the balloon satellite, Explorer 19, is analysed to determine the effects of direct solar radiation pressure over one revolution of the satellite (111 min) for MJD 42822 and MJD 42966. At the earlier date, the satellite entered the Earth's shadow, presenting an opportunity to examine the effectiveness of two different shadow models. The reflectivity of the surface of the satellite was estimated from analysis of the variations in orbital eccentricity over a period of 236 days.Although many of the parameters associated with the shape and orientation of the satellite are unknown, the theory for a non-spherical satellite is applied using trial and error methods to determine the parameters of best fit. The paper concludes with an examination of the perturbations in orbital eccentricity and inclination due to incident, specularly reflected, and diffusely reflected radiation.     

Upper-atmosphere zonal winds determined from orbital analysis of Cosmos 482, 1972-23A

The orbit of Cosmos 482 has been determined at 55 epochs during the period August 1975–October 1977. Inclined at 52° to the Equator, of eccentricity exceeding 0.3 and perigee height near 210 km, this high drag and high eccentricity orbit is the most eccentric as yet analysed by PROP. The combination of the orbital characteristics, restricted global coverage of observational data, and the many observations of angular measurement at extreme range proved to be troublesome. Cleared of other perturbations, the inclination is analysed to determine the atmospheric rotation rate, λ rev day^?1, of the zonal winds at a height near 235 km. Results reveal a diurnal and seasonal dependence, including a summer value, averaged over local time, of λ = 0.9 and an evening value of λ = 1.4 in the winter months. The resultant wind velocities vary between 48 m s^?1 East to West and 193 m s^?1 West to East, with an average of 48 m s^?1 West to East.     

Air density at heights near 400km from the orbit of 1963-27A

The Agena B upper-stage rocket 1963-27A was launched into a near-circular orbit, inclined at 82.3° to the Equator, on 29 June 1963. Its orbit is determined at 52 epochs over the 16 month interval prior to its decay on 26 October 1969. The resulting orbital elements are used to obtain 95 atmospheric density values, at heights near 400km. Corrected to fixed heights, and normalised to a common exospheric temperature, these values reveal the semi-annual variation in density. A comparison between the observed variation and that of a recent model atmosphere is made. Although agreement between the two is generally good, their principal differences are discussed.     

Air density at heights near 200 km from the orbit of 1969-20B

The rocket of Cosmos 268, 1969-20B, entered orbit on 5 March 1969, with an initial perigee height of 230 km and inclination of 48.40°. Accurate orbits were computed at RAE from all available observations. Using the values of perigee height from the RAE orbit and decay rates from Spacetrack bulletins, 103 values of density have been calculated between July 1969 and February 1970. On three occasions when geomagnetic activity was strong there were sudden increases in density. When the density was corrected to a fixed height, the semi-annual variation was apparent. There was a strong minimum in July 1969, a maximum in October–November 1969 and a weak minimum in January 1970.     

Air density at heights near 200km from the orbit of 1970-65D

Cosmos 359 rocket, 1970-65D, entered orbit on 22 August 1970, with an initial perigee height of 209 km and inclination 51·2°, and decayed on 6 October 1971. Using the values of perigee height from RAE orbits and decay rates from USAF Spacetrack bulletins, 146 values of air density have been calculated between August 1970 and September 1971, mainly at heights between 180 and 230 km.On ten occasions in 1971 when there were substantial geomagnetic disturbances there were sudden increases in density, the largest being about 32 per cent.When the density was corrected to a fixed height and allowance was made for the day-tonight variation and the effects of solar activity, the dominant feature was a semi-annual variation, with maxima in density centred at 6 November 1970 and 7 April 1971, and minima centred at 5 January and 28 July 1971. The maxima in density are nearly equal and exceed the minima by about 50 per cent.     

The effect of the equatorially symmetric zonal winds of Saturn on its gravitational field

The penetration depth of Saturn’s cloud-level winds into its interior is unknown.A possible way of estimating the depth is through measurement of the effect of the winds on the planet’s gravitational field.We use a self-consistent perturbation approach to study how the equatorially symmetric zonal winds of Saturn contribute to its gravitational field.An important advantage of this approach is that the variation of its gravitational field solely caused by the winds can be isolated and identified because the leading-order problem accounts exactly for rotational distortion,thereby determining the irregular shape and internal structure of the hydrostatic Saturn.We assume that(i)the zonal winds are maintained by thermal convection in the form of non-axisymmetric columnar rolls and(ii)the internal structure of the winds,because of the Taylor-Proundman theorem,can be uniquely determined by the observed cloud-level winds.We calculate both the variation △J_n,n=2,4,6...of the axisymmetric gravitational coefficients J_n caused by the zonal winds and the non-axisymmetric gravitational coefficients △J_(nm) produced by the columnar rolls,where m is the azimuthal wavenumber of the rolls.We consider three different cases characterized by the penetration depth 0.36 R_S,0.2 R_S and 0.1 R_S,where R_S is the equatorial radius of Saturn at the 1-bar pressure level.We find that the high-degree gravitational coefficient ( J_(12)+△J_(12)) is dominated,in all the three cases,by the effect of the zonal flow with |△J_(12)/J_(12)|100%and that the size of the non-axisymmetric coefficientsdirectly reflects the depth and scale of the flow taking place in the Saturnian interior.     

Zonal winds near Venus' cloud top level: A model study of the interaction between the zonal mean circulation and the semidiurnal tide

A primitive equation wave-mean flow interaction model, designed by J. R. Holton and used originally to study Earth's middle atmosphere, has been adapted to Venus in order to clarify our understanding of the interaction between the semidiurnal tide and the thermally driven mean meridional circulation near the cloud top level. With or without the tide the model produces midlatitude jets whose structure is insensitive to vertical shear of the background angular velocity above and below the cloud top level, but it is sensitive to background angular velocity at the cloud top level. When this background angular velocity is close to that of Venus, the latitudes and speeds of these jets are similar to the latitudes and speeds of jets at the Venus cloud top level as inferred from observed temperatures and the cyclostrophic balance condition. In agreement with the hypothesis of Fels and Lindzen, the model tide accelerates the equatorial zonal wind near the cloud top level and decelerates it at higher levels. The tidal vertical wavelength, maximum amplitude, dissipative decay length, and zonal flow accelerations are sensitive functions of background angular velocity, in agreement with elementary gravity wave theory. In the equatorial cloud top region, tidal acceleration is comparable in magnitude to the decelerative effects of vertical advection and the model's Rayleigh friction damping. For sufficiently rapid initial zonal flow near the cloud top level, the area-weighted global mean cloud top level zonal wind increases with time over a 50-day model run as a result of tidal acceleration. Agreement between the model tide and the observed tide, or the tide determined in the more detailed calculations of Pechmann and Ingersoll, is best when the background angular velocity at the jet level is about 30% larger than that observed.     

Air density at heights near 435 km from the orbit of skylab 1 (1973-27A)

On February 8, 1974, Skylab 1 was manoeuvred into a near circular orbit of inclination 50.04° and perigee near 420km. Orbital parameters have been computed at forty-six epochs thereafter using all available observations. Using these orbital elements, supplemented by orbital decay rates derived from NORAD bulletins, 193 values of air density were determined between 23 February, 1974 and 11, August, 1976. Corrected to a fixed height and normalised with respect to exospheric temperature these values reveal the semi-annual variation, exhibiting maxima in March–April and October–November, and minima in January–February and July–August. For 1974–1976 the July minima are more pronounced than the January minima whilst the April and October maxima appear equal. Overall the variation is greater than that indicated by CIRA 1972.     

Variations in air density at heights near 150 km, from the orbit of the satellite 1966-101G

The satellite 1966-101G was launched on 2 November 1966 into an orbit with an initial perigee height of 140 km. A satellite with such a low perigee usually decays within a few days, but 1966-101G was exceptionally dense and remained in orbit until 6 May 1967. Analysis of the changes in its orbital period provides an unique opportunity for studying continuously for six months the variations in air density at a height near 150 km.

This paper records the results of such an analysis, applicable for the (medium) level of solar activity prevailing early in 1967. It is shown that at a height of 155 km the air density is greater by day than by night, with the maximum daytime density exceeding the minimum night-time density by a factor of 1.7: in contrast the COSPAR International Reference Atmosphere 1965 predicts that the density should be slightly greater by night than by day. It is also found that the night-time density increases as solar activity increases, and that the density scale height given by CIRA 1965 at heights near 150 km is too low, perhaps by about 20%.     

On the perturbation of the anomalistic period of a highly eccentric orbit satellite due to the zonal harmonics

In this note a simple formula is given for the perturbation of the anomalistic period of a highly eccentric orbit due to the zonal harmonics. This perturbation depends essentially only on the semi-major axisa, the eccentricitye (or pericentre radius r =a(1-e)) and the latitude of the pericentre.     

Air density at heights near 150 km in 1970, from the orbit of Cosmos 316 (1969-108A)

Cosmos 316 (1969-108A) was launched on 23 December 1969 into an orbit with an initial perigee height of 154 km at an inclination of 49.5° to the equator. Being very massive, Cosmos 316 had a longer lifetime than any previous satellite with such a low initial perigee: it remained in orbit until 28 August 1970. Because of its interest for upper-atmosphere research, the satellite was intensively observed, and accurate orbits are being determined at RAE from all available observations. Using perigee heights from the RAE orbits so far computed, and decay rates from Spacetrack bulletins, 102 values of air density have been obtained, giving a detailed picture of the variations in density at heights near 150 km between 24 December 1969 and 28 August 1970. The three strongest geomagnetic storms, on 8 March, 21 April and 17 August 1970, are marked by sudden increases in density of at least 23, 15 and 24 per cent respectively. With values of density extending over eight months, it is possible for the first time to examine a complete cycle of the semi-annual variation at a height near 150 km: the values of density, when corrected to a fixed height, exhibit minima in mid January and early August; at the intervening maximum, in April, the density is 30 per cent higher than at the minima.     

Air density at heights near 300 km,from analysis of the orbit of China 2 rocket (1971-18b)

China 2 rocket, 1971-18B, was launched on 3rd March 1971 into an orbit inclined at 69.9° to the Equator, with an initial perigee height of 265 km. Analysis of its orbit has yielded values of air density at average intervals of 6 days between July 1971 and January 1972. When corrected to a fixed height, the density exhibits a correlation with the geomagnetic index A_p and the solar 10.7-cm radiation. With values of density extending over seven months it is possible to examine a complete cycle of the semi-annual variation at a height near 300 km. The values of density, corrected for the day-to-night variation and for solar and geomagnetic activity, reveal minima in mid-August and late January; at the intervening maximum, in early November, the density is almost 40% higher than at the minima.     

Resonant asteroids between the main belt and Jupiter's orbit

Selected orbits of asteroids close to the 3/2 and 4/3 resonances in the outer belt are studied by extended numerical integrations of the four-body problem Sun-Jupiter-Saturn-asteroid. Jupiter's variable eccentricity causes strong or dominant effects in the asteroidal eccentricity, rate of perihelion, and critical argument. However, a suitable transformation removes most of these effects by the transition to respective new quantities. Cases of circulation can change to permanent libration in the new critical argument. This happens in the case of (1256) Normannia, one of the two objects found to be circulating near the 3/2 resonance in former work. The other object, (334) Chicago, remains a case of circulation and shows significant deviations from quasi-periodic motion, contrary to all the other studied Hildas (3/2 cases) and to (279) Thule (4/3). Temporary libration with respect to a resonance of high order is visible in the long-period evolution of Chicago's orbit. There are cases of some analogy further inward in the belt: (903) Nealley changes to permanent libration at the 2/1 resonance by the use of the new critical argument, and (522) Helga's orbit shows a non-quasi-periodic behaviour.     

Variations in air density at heights near 200 km during 1967–1969, from the orbit of 1967-31A

Air density at a height of 180–200 km from July 1967 to September 1969 has been determined from analysis of the high eccentricity orbit of satellite 1967-31A. The data show good correlation between sudden density increase and geomagnetic disturbance. The increases for disturbances of equal strength are approximately 40% greater during night-time than daytime hours. The day-night influence is also observed in the changes in density with changes in the solar flux index, F₁₀. The 27-day density variation is predominant mainly during night-time, although the atmospheric response to F₁₀ variations is quite variable regardless of local time. A semi-annual variation of approx. 40% is observed. Also found is a 25% diurnal variation for heights near 170–180 km, which is in good agreement with the CIRA 1972 atmosphere.     

Predicted electrical conductivity between 0 and 80 km in the Venusian atmosphere

Calculations of the space charge, ion density, and conductivity in the Venus atmosphere were made. The presence of the cloud particles on Venus causes a profound reduction in the calculated values of the ion density and conductivity compared to the values that are obtained without consideration of the cloud particles. When the cloud particles are included in the calculations, the results for the ion density and conductivity are approximately the same as those of the terrestrial atmosphere at the same pressure-altitude. Because the particles span such a large range of sizes and are abundant over a substantial range of pressure, the space charge varies strongly with altitude and particle size. Differential settling of the particles is expected to produce weak electric fields in the clouds.