On the reality of the Venus winds

An examination of the effect of assumptions in the interpretation of the Venera wind data is made as a rebuttal to the suggestion by A.T. Young that the 140 m/sec Venera 8 horizontal wind at 45 km may be either spurious or anomalous. The Venera measurements of wind speed along with the Mariner measurements of a lower region of strong turbulence are evidence for a wide band of variable high-speed retrograde horizontal winds which girdle Venus at the equator. In the prevalent interpretation of the Mariner 10 uv photographs, the region of the top of the visible cloud is characterized by variable high-speed retrograde horizontal winds which orbit Venus with an average period of 4 Earth days, and by many features indicating vertical convection. This interpretation, together with the possibility of atmospheric corotation due to frictional coupling, suggests that the Venera-Mariner band of winds at 45 km extends well beyond the top of the visible cloud, and that the upper region of strong turbulence detected by the Mariners may result in part from vertical convection currents carried along by high-speed horizontal winds. In an alternate interpretation of the Mariner 10 uv photographs Young suggests that the predominant motions may be traveling wavelike disturbances with a 4-day period rather than bulk motion of the atmosphere. For this case the upper region of strong turbulence is interpreted as due mostly to vertical wind shear resulting from a rapid decrease in wind speed within a relatively short distance above the Venera-Mariner band of high-speed winds.     

Dynamical properties of the Venus mesosphere from the radio-occultation experiment VeRa onboard Venus Express

The dynamics of Venus’ mesosphere (60–100 km altitude) was investigated using data acquired by the radio-occultation experiment VeRa on board Venus Express. VeRa provides vertical profiles of density, temperature and pressure between 40 and 90 km of altitude with a vertical resolution of few hundred meters of both the Northern and Southern hemisphere. Pressure and temperature vertical profiles were used to derive zonal winds by applying an approximation of the Navier–Stokes equation, the cyclostrophic balance, which applies well on slowly rotating planets with fast zonal winds, like Venus and Titan. The main features of the retrieved winds are a midlatitude jet with a maximum speed up to 140 ± 15 m s^?1 which extends between 20°S and 50°S latitude at 70 km altitude and a decrease of wind speed with increasing height above the jet. Cyclostrophic winds show satisfactory agreement with the cloud-tracked winds derived from the Venus Monitoring Camera (VMC/VEx) UV images, although a disagreement is observed at the equator and near the pole due to the breakdown of the cyclostrophic approximation. Knowledge of both temperature and wind fields allowed us to study the stability of the atmosphere with respect to convection and turbulence. The Richardson number Ri was evaluated from zonal field of measured temperatures and thermal winds. The atmosphere is characterised by a low value of Richardson number from ～45 km up to ～60 km altitude at all latitudes that corresponds to the lower and middle cloud layer indicating an almost adiabatic atmosphere. A high value of Richardson number was found in the region of the midlatitude jet indicating a highly stable atmosphere. The necessary condition for barotropic instability was verified: it is satisfied on the poleward side of the midlatitude jet, indicating the possible presence of wave instability.     

Rocket studies of sporadic-E ionization and ionospheric winds

Simultaneous observations of positive ion density (by means of positive ion probes) and high-altitude winds (by scattering of sunlight in trails of alkali metal vapour) were made in three rocket firings at Hammaguir, Algeria in order to study the correlation of sporadic-E layers and high-altitude wind shears. On one occasion, a sporadic-E layer was found to occur at 95 km where the E-W component of the wind was zero and the wind shear was in the sense predicted by the theory of Whitehead. However, on two occasions, layers were also observed near 115 km where the E-W wind component was zero, but the wind shear was in the opposite sense. This appears to suggest that some modification of the theory is required.     

A reanalysis of Venus winds at two cloud levels from Galileo SSI images

The global circulation of the Venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts: orbiters, balloons and probes. Among them, the Galileo spacecraft monitored the Venus atmosphere in a flyby in February 1990 in its route toward Jupiter. Since the flyby was almost equatorial, published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the SSI camera [Belton, M.J.S., and 20 colleagues, 1991. Science 253, 1531-1536] stop at latitudes 50° north and south. In this paper we present new results on Venus winds based on a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud. Our main result is that we have been able to extend the zonal wind profile up to the polar latitudes: 70° N and 70° S at 418 nm and 70° N at 986 nm. Binned and smoothed profiles are given in tabular form. We show that the zonal winds drop in their velocity poleward of latitudes 45° N and 50° S where an intense meridional wind shear develops at the two cloud levels. Our data confirm the magnitude of this shear, retrieved previously from radio occultation data, but disagrees with it in the latitudinal location of the sheared region. The new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption. The meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region.     

Observation of vertical E-region neutral winds in two intense auroral arcs

Vertical winds have been observed by optical Doppler measurements of the 557.7 nm emission in the aurora, using a Fabry-Perot spectrometer. Both upward and downward winds were observed, of 15 m s^?1 magnitude. The upward winds were associated with westward overhead currents, and with low altitude aurora (～ 110 km) as determined by the auroral temperature, while a high altitude aurora (～ 135 km) and eastward currents were associated with the downward wind. The Lorentz force of these currents has the wrong direction to act as a direct forcing mechanism. It is concluded that Joule heating is directly responsible for the upward winds, while the divergence of horizontal winds is responsible for the downward winds.     

Mesospheric O3, H and H2O at high latitudes: A theoretical model

The high latitude thermosphere is characterized by a large heat input which produces a strong wind field. In a previous work, the modification of the vertical transport mechanisms produced by high latitude horizontal winds was studied and the resulting concentration profiles are used here to study the influence at mesospheric levels. We obtain an improved agreement with experimental measurements. High solar zenithal angles could explain other differences between the high and middle latitude's mesosphere, especially below 80 km, approximately.     

110 km neutral zonal wind patterns

In the height range between 105 and 115 km sporadic E formation is due exclusively to the zonal (E-W) neutral winds and both theory and experiment indicate sporadic E will occur very close to a reversal point of this zonal wind. By studying the observed heights of sporadic E-layers from a global distribution of stations we can deduce some of the regular properties of the zonal winds at 110 km. The semidiurnal zonal wind pattern is shown to be well defined, is principally the 2,2 mode, and agrees well with theoretical predictions. The diurnal zonal wind pattern is less clearly defined and does not closely resemble any theoretical mode. Steady components agree with those found by other methods.     

Martian winds and dust clouds

Previous calculations of the surface wind stress required to raise dust on Mars are reconsidered and the threshold friction velocity is found to be about 2.0 m sec^?1 with particles of 200–300 μm being the most easily lifted. With this friction velocity, the planetary resistance law yields a corresponding wind at the top of the Ekman layer of 60 m sec^?1, and the logarithmic wind law yields a corresponding wind at the top of the Prandtl layer of 38 m sec^?1. These speeds are somewhat lower than those used by previous investigators.Various mechanisms for producing such strong winds are examined and it is concluded that the general circulation, thermal effects of topography, mechanical effects of topography and dust devils are all capable of doing so.Dust storms associated with small-scale disturbances are found to be incapable of growth. A scaling analysis of the equations of horizontal motion and of hydrostatic balance shows that a dust cloud at least 10 km thick and several tens of km in radius can, by absorption of sunlight, generate temperature gradients that, in turn, produce winds capable of raising more dust. Thus, a feedback mechanism is suggested in which an initial dust cloud exceeding certain critical dimensions can grown to planetary size. The preference of large dust storms to occur at southern hemisphere summer solstice is attributed to the maximum of insolation at that time. It is suggested that the frequent origin in the Noachis-Hellas region may be due to orographic features of the right scale and to low height in that area.     

Wind estimates near 150 km from the variation in inclination of low-perigee satellite orbits

Precise orbit determinations of five Air Force low-altitude satellites are used to estimate winds near 150 km from variations in the satellite orbital inclinations. Zonal winds determined by this method range from 25 to 200 m/sec during quiet to moderately disturbed geomagnetic conditions, to winds on the order of 300–600 m/sec during active geomagnetic conditions. Comparisons are made with other wind data and appropriate theories.     

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.     

Mesoscale linear streaks on Mars: environments of dust entrainment

Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes.     

Upper-atmosphere zonal winds: Variation with height and local time

The average rotation rate of the upper atmosphere can be found by analysis of the changes in the orbital inclinations of satellites, and results previously obtained have indicated that the atmospheric rotation rate appreciably exceeds the Earth's rotation rate at heights between 200 and 400 km.We have examined all such results previously published in the light of current standards of accuracy: some are accepted, some revised, and some rejected as inadequate in accuracy. We also analyse a number of fresh orbits and, adding these to the accepted and revised previous results, we derive the variation of zonal wind speed with height and local time. The rotation rate (rev/day) averaged over all local times increases from near 1.0 at 150 km height to 1.3 near 350 km (corresponding to an average west-to-east wind of 120 m/s), and then decreases to 1.0 at 400 km and probably to about 0.8 at greater heights. The maximum west-to-east winds occur in the evening hours, 18–24 h local time: these evening winds increase to a maximum of about 150 m/s at heights near 350 km and decline to near zero around 600 km. In the morning, 4–12 h local time, the winds are east to west, with speeds of 50–100 m/s above 200 km. We also tentatively conclude that, at heights above 350 km, the average rotation rate is greater in equatorial latitudes (0–25°) than at higher latitudes.     

The generation of vertical thermospheric winds and gravity waves at auroral latitudes—II. Theory and numerical modelling of vertical winds

Recent observations of strong vertical thermospheric winds and the associated horizontal wind structures, using the 01(3P-1D)nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia have been described in an accompanying paper (Paper I). The high latitude thermosphere at a height of 200–300 km displays strong vertical winds (30–50m ms^?1)of a persistent nature in the vicinity of the auroral oval even during relatively quiet geomagnetic conditions. During an auroral substorm, the vertical (upward) wind in the active region, including that invaded by a Westward Travelling Surge, may briefly(10–30 min)exceed 150 m s^?1. Very large and rapid changes of horizontal wind structure (up to 500 m^?1 in 30 min) usually accompany such large impulsive vertical winds. Magnetospheric energy and momentum sources generate large vertical winds of both a quasi-steady nature and of a strongly time-dependent nature. The thermospheric effects of these sources can be evaluated using the UCL three-dimensional, time-dependent thermospheric model. The auroral oval is, under average geomagnetic conditions, a stationary source of significant vertical winds (10–40 m s^?1). In large convective events (directly driven by a strong momentum coupling from the solar wind) the magnitude may increase considerably. Auroral substorms and Westward Travelling Surges appear to be associated with total energy disposition rates of several tens to more than 100 erg cm^?2s^?1, over regions of a few hours local time, and typically 2–5° of geomagnetic latitude (approximately centred on magnetic midnight). Such deposition rates are needed to drive observed time-dependent vertical (upward) winds of the order of 100–200m s^?1.The response of the vertical winds to significant energy inputs is very rapid, and initially the vertical lifting of the atmosphere absorbs a large fraction (30% or more) of the total substorm input. Regions of strong upward winds tend to be accompanied in space (and time) by regions of rather lower downward winds, and the equatorward propagation of thermospheric waves launched by auroral substorms is extremely complex.     

Circulation of the Venus upper mesosphere/lower thermosphere: Doppler wind measurements from 2001–2009 inferior conjunction,sub-millimeter CO absorption line observations

Sub-millimeter ¹²CO (346 GHz) and ¹³CO (330 GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120 km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115 km altitudes versus local time (～8 pm–4 am) and latitude (～60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of ¹²CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85 m/s retrograde) versus SSAS (65 m/s) circulation at the 1 μbar pressure (108–110 km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50 m/s, averaged over 5000 km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper, Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined ¹²CO and ¹³CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115 km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere.     

Dispersion in Neptune’s zonal wind velocities from NIR Keck AO observations in July 2009

We report observations of Neptune made in H-(1.4–1.8 μm) and K’-(2.0–2.4 μm) bands on 14 and 16 July 2009 from the 10-m W.M. Keck II Telescope using the near-infrared camera NIRC2 coupled to the Adaptive Optics (AO) system. We track the positions of 54 bright atmospheric features over a few hours to derive their zonal and latitudinal velocities, and perform radiative transfer modeling to measure the cloud-top pressures of 50 features seen simultaneously in both bands. We observe one South Polar Feature (SPF) on 14 July and three SPFs on 16 July at ～65?°S. The SPFs observed on both nights are different features, consistent with the high variability of Neptune’s storms. There is significant dispersion in Neptune’s zonal wind velocities about the smooth Voyager wind profile fit of Sromovsky et al. (Icarus, 105:140, 1993), much greater than the upper limit we expect from vertical wind shear, with the largest dispersion seen at equatorial and southern mid-latitudes. Comparison of feature pressures vs. residuals in zonal velocity from the smooth Voyager wind profile also directly reveals the dominance of mechanisms over vertical wind shear in causing dispersion in the zonal winds. Vertical wind shear is not the primary cause of the difference in dispersion and deviation in zonal velocities between features tracked in H-band on 14 July and those tracked in K’-band on 16 July. Dispersion in the zonal velocities of features tracked over these short time periods is dominated by one or more mechanisms, other than vertical wind shear, that can cause changes in the dispersion and deviation in the zonal velocities on timescales of hours to days.     

Exospheric zonal winds between 540 and 620 km from the orbit of Explorer 24

Data on the variation of the orbital inclination of the balloon satellite Explorer 24 (1964-76A) from 1964 to 1968 have been used to determine zonal winds between 540 and 620 km. In this height region the effect of zonal winds on the orbital inclination may become very small compared to other perturbations like accelerations due to the geopotential, lunisolar gravitation and the solar radiation pressure. It is demonstrated especially that the solar radiation pressure may become the most significant force changing the orbital inclination. The diurnal mean zonal winds derived from Explorer 24 point to an exospheric rotation rate which is about 6–10% less than the rotation rate of the Earth in the analyzed height region. Since the possible errors of the data analysis are of a similar order of magnitude, it can not be excluded that the exosphere corotates with the Earth. Furthermore, a local time dependence of the zonal winds could be detected. The diurnal varitation of the zonal wind is shown to be in good agreement with the theoretical model of Blum and Harris. Our results are discussed and compared with all previous investigations of orbital inclination changes of satellites above 350 km.     

Simulations of high-latitude spots on Jupiter: Constraints on vortex strength and the deep wind

We combine high-resolution observations of the dynamical behavior of small vortices (diameters ?5000 km) located at latitude 60°N on Jupiter with forward modeling, using the EPIC atmospheric model, to address two open questions: the dependence of the zonal winds with depth, and the strength of vortices that are too small to apply cloud tracking to their internal structure. The observed drift rates of the vortices can only be reproduced in the model when the zonal winds increase slightly with depth below the cloud tops, with a vertical shear that is less than was measured at 7°N at the southern rim of a 5-μm hotspot by the Galileo Probe Doppler Wind Experiment (DWE). This supports the idea that Jupiter's vertical shear may vary significantly with latitude. Our simulations suggest that the morphology of the mergers between vortices mainly depends on their maximum tangential velocities, the best results occurring when the tangential velocity is close to the velocity difference of the alternating jets constraining the zone in which the vortices are embedded. We use this correlation, together with the high-resolution data available for the White Ovals, to derive an empirical relationship between the maximum tangential velocity of a jovian vortex and its size, normalized by the strength and size of the encompassing shear zone. The Great Red Spot stands out as a significant anomaly to this relationship, but interestingly it is becoming less so with time.     

Thermal structure of Saturn from Voyager infrared measurements: Implications for atmospheric dynamics

Saturn atmospheric temperatures at the 150-mbar level retrieved from Voyager IRIS measurements indicate the presence of small-scale meridional gradients which are approximately symmetric with respect to the equator, but are superposed on a large-scale hemispheric thermal asymmetry. Under the assumption that the retrieved values at this atmospheric level represent kinetic temperatures on a constant pressure surface, it is suggested that the small-scale structure is produced by a meriodional circulation associated with the dissipative decay of the zonal winds with height, while the hemispheric asymmetry represents a thermal response to the seasonally varying insolation. The small-scale gradients are correlated with zonal winds derived from Voyager images at mid and high latitudes through the thermal wind relation; the calculated thermal wind shears suggest a decay with height of the jet system toward a state of uniform eatward flow. The existence of the approximately symmetric zonal winds and associated temperature gradients in the presence of a large-scale seasonal thermal response suggests that the jet system is driven at depths substantially below the levels where seasonally modulated insolation is important ($\text{p?0.5}\text{bar}$).     

Long-term evolution of the aerosol debris cloud produced by the 2009 impact on Jupiter

We present a study of the long-term evolution of the cloud of aerosols produced in the atmosphere of Jupiter by the impact of an object on 19 July 2009 (Sánchez-Lavega, A. et al. [2010]. Astrophys. J. 715, L155-L159). The work is based on images obtained during 5 months from the impact to 31 December 2009 taken in visible continuum wavelengths and from 20 July 2009 to 28 May 2010 taken in near-infrared deep hydrogen-methane absorption bands at 2.1-2.3 μm. The impact cloud expanded zonally from ∼5000 km (July 19) to 225,000 km (29 October, about 180° in longitude), remaining meridionally localized within a latitude band from 53.5°S to 61.5°S planetographic latitude. During the first two months after its formation the site showed heterogeneous structure with 500-1000 km sized embedded spots. Later the reflectivity of the debris field became more homogeneous due to clump mergers. The cloud was mainly dispersed in longitude by the dominant zonal winds and their meridional shear, during the initial stages, localized motions may have been induced by thermal perturbation caused by the impact’s energy deposition. The tracking of individual spots within the impact cloud shows that the westward jet at 56.5°S latitude increases its eastward velocity with altitude above the tropopause by 5-10 m s⁻¹. The corresponding vertical wind shear is low, about 1 m s⁻¹ per scale height in agreement with previous thermal wind estimations. We found evidence for discrete localized meridional motions with speeds of 1-2 m s⁻¹. Two numerical models are used to simulate the observed cloud dispersion. One is a pure advection of the aerosols by the winds and their shears. The other uses the EPIC code, a nonlinear calculation of the evolution of the potential vorticity field generated by a heat pulse that simulates the impact. Both models reproduce the observed global structure of the cloud and the dominant zonal dispersion of the aerosols, but not the details of the cloud morphology. The reflectivity of the impact cloud decreased exponentially with a characteristic timescale of 15 days; we can explain this behavior with a radiative transfer model of the cloud optical depth coupled to an advection model of the cloud dispersion by the wind shears. The expected sedimentation time in the stratosphere (altitude levels 5-100 mbar) for the small aerosol particles forming the cloud is 45-200 days, thus aerosols were removed vertically over the long term following their zonal dispersion. No evidence of the cloud was detected 10 months after the impact.     

Plasma dynamics in the tail of Comet Kohoutek 1973 XII

Photographs of Comet Kohoutek 1973 XII from the period 1974, Jan. 19, 0 UT to Jan. 21, 3 UT, collected from many different observatories and assembled in a unified format, are studied. During this time a large-scale tail disturbance was observed which coincides with the passage of a high-speed solar wind stream and an interplanetary sector boundary. Superimposed on a regular outward motion of tail condensations of a speed less than or about 100 km/sec, a kink moves down the tail with almost solar wind velocity. From the shape of the kink the direction of the solar wind adjacent to the tail is deduced. Of particular interest are tail segments where the solar wind flows across the tail. A waviness on the windward side of the tail is explained by differential acceleration, i.e., dense tail clouds are more massive and therefore less accelerated by the solar wind. On the leeward side tail rays point into the down-wind direction. During the large-scale disturbance the overall plasma density seems to be enhanced. While a tail disconnection does not occur in the event studied it is proposed that the tail disconnection observed in other, more dramatic events is caused by the differential acceleration mechanism combined with changes in the ion source. These are possibly due to enhanced charge exchange of cometary neutrals in the compression region in front of the high speed stream. The problem of tail ray formation near tail condensations is discussed but no solution is offered.