Javier Peralta

Solar migrating atmospheric tides in the winds of the polar region of Venus

We discuss methods currently in use for determining the significance of peaks in the periodograms of time series. We discuss some general methods for constructing significance tests, false alarm probability functions, and the role played in these by independent random variables and by empirical and theoretical cumulative distribution functions. We also discuss the concept of “independent frequencies” in periodogram analysis. We propose a practical method for estimating the significance of periodogram peaks, applicable to all time series irrespective of the spacing of the data. This method, based on Monte Carlo simulations, produces significance tests that are tailor-made for any given astronomical time series. Subject headings: Methods: data analysis — Methods: statistical — Stars: oscillations

Dayside temperatures in the Venus upper atmosphere from Venus Express/VIRTIS nadir measurements at 4.3 μm

In this work, we analysed nadir observations of atmospheric infrared emissions carried out by VIRTIS, a high-resolution spectrometer on board the European spacecraft Venus Express. We focused on the ro-vibrational band of CO 2 at 4.3 µm on the dayside, whose fluorescence originates in the Venus upper mesosphere and above. This is the first time that a systematic sounding of these non-local thermodynamic equilibrium (NLTE) emissions has been carried out in Venus using this geometry. As many as 143,218 spectra have been analysed on the dayside during the period 14/05/2006 to 14/09/2009. We designed an inversion method to obtain the atmospheric temperature from these non-thermal observations, including a NLTE line-by-line forward model and a pre-computed set of spectra for a set of thermal structures and illumination conditions. Our measurements sound a broad region of the upper mesosphere and lower thermosphere of Venus ranging from 10 −2 –10 −5 mb (which in the Venus International Reference Atmosphere, VIRA, is approximately 100–150 km during the daytime) and show a maximum around 195 ± 10 K in the subsolar region, decreasing with latitude and local time towards the terminator. This is in qualitative agreement with predictions by a Venus Thermospheric General Circulation Model (VTGCM) after a proper averaging of altitudes for meaningful comparisons, although our temperatures are colder than the model by about 25 K throughout. We estimate a thermal gradient of about 35 K between the subsolar and antisolar points when comparing our data with nightside temperatures measured at similar altitudes by SPICAV, another instrument on Venus Express (VEx). Our data show a stable temperature structure through five years of measurements, but we also found episodes of strong heating/cooling to occur in the subsolar region of less than two days.

Venus's major cloud feature as an equatorially trapped wave distorted by the wind

The superrotation of the atmospheres of slowly rotating bodies is a long-standing problem yet unsolved in atmospheric dynamics. On Venus, the most extreme case known of superrotation, this is accompanied and influenced by a recurrent planetary-scale cloud structure, known as the Y feature. So far, no model has simultaneously reproduced its shape, temporal evolution, related wind field, nor the relation between its dynamics and the unknown UV-absorbing aerosol that produces its dark morphology. In this paper we present an analytical model for a Kelvin-like wave that offers an explanation of these peculiarities. Under Venus cyclostrophic conditions, this wave is equatorially and vertically trapped where zonal winds peak and extends 7 km in altitude, and its vertical wind perturbations are shown to produce upwelling of the UV absorber. The Y-feature morphology and its 30 day evolution are reproduced as distortions of the wave structure by the Venus winds.

Stationary waves and slowly moving features in the night upper clouds of Venus

Venus Express wind measurements at Venus’s cloud top during the night show a different picture than dayside. Both fast and slow motions are detected (there are only fast ones during the day) as well as many stationary waves related to surface relief.

VENUS CLOUD MORPHOLOGY AND MOTIONS FROM GROUND-BASED IMAGES AT THE TIME OF THE AKATSUKI ORBIT INSERTION*

We report Venus image observations around the two maximum elongations of the planet at 2015 June and October. From these images we describe the global atmospheric dynamics and cloud morphology in the planet before the arrival of JAXAs Akatsuki mission on 2015 December 7. The majority of the images were acquired at ultraviolet wavelengths (380–410 nm) using small telescopes. The Venus dayside was also observed with narrowband filters at other wavelengths (890 nm, 725–950 nm, 1.435 μm CO2 band) using the instrument PlanetCam-UPV/EHU at the 2.2 m telescope in Calar Alto Observatory. In all cases, the lucky imaging methodology was used to improve the spatial resolution of the images over the atmospheric seeing. During the April–June period, the morphology of the upper cloud showed an irregular and chaotic texture with a well-developed equatorial dark belt (afternoon hemisphere), whereas during October–December the dynamical regime was dominated by planetary-scale waves (Y-horizontal, C-reversed, and ψ-horizontal features) formed by long streaks, and banding suggesting more stable conditions. Measurements of the zonal wind velocity with cloud tracking in the latitude range from 50°N to 50°S shows agreement with retrievals from previous works.

Analytical Solution for Waves in Planets with Atmospheric Superrotation. I. Acoustic and Inertia-Gravity Waves

This paper is the first of a two-part study devoted to developing tools for a systematic classification of the wide variety of atmospheric waves expected on slowly rotating planets with atmospheric superrotation. Starting with the primitive equations for a cyclostrophic regime, we have deduced the analytical solution for the possible waves, simultaneously including the effect of the metric terms for the centrifugal force and the meridional shear of the background wind. In those cases when the conditions for the method of the multiple scales in height are met, these wave solutions are also valid when vertical shear of the background wind is present. A total of six types of waves have been found and their properties were characterized in terms of the corresponding dispersion relations and wave structures. In this first part, only waves that are direct solutions of the generic dispersion relation are studied—acoustic and inertia-gravity waves. Concerning inertia-gravity waves, we found that in the cases of short horizontal wavelengths, null background wind, or propagation in the equatorial region, only pure gravity waves are possible, while for the limit of large horizontal wavelengths and/or null static stability, the waves are inertial. The correspondence between classical atmospheric approximations and wave filtering has been examined too, and we carried out a classification of the mesoscale waves found in the clouds of Venus at different vertical levels of its atmosphere. Finally, the classification of waves in exoplanets is discussed and we provide a list of possible candidates with cyclostrophic regimes.

Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki

The Venusian atmosphere is in a state of superrotation where prevailing westward winds move much faster than the planet’s rotation. Venus is covered with thick clouds that extend from about 45 to 70 km altitude, but thermal radiation emitted from the lower atmosphere and the surface on the planet’s night-side escapes to space at narrow spectral windows of near-infrared. The radiation can be used to estimate winds by tracking the silhouettes of clouds in the lower and middle cloud regions below about 57 km in altitude. Estimates of wind speeds have ranged from 50 to 70 m/s at low- to mid-latitudes, either nearly constant across latitudes or with winds peaking at mid-latitudes. Here we report the detection of winds at low latitude exceeding 80 m/s using IR2 camera images from the Akatsuki orbiter taken during July and August 2016. The angular speed around the planetary rotation axis peaks near the equator, which we suggest is consistent with an equatorial jet, a feature that has not been observed previously in the Venusian atmosphere. The mechanism producing the jet remains unclear. Our observations reveal variability in the zonal flow in the lower and middle cloud region that may provide new challenges and clues to the dynamics of Venus’s atmospheric superrotation.

Venus's winds and temperatures during the MESSENGER's flyby: An approximation to a three‐dimensional instantaneous state of the atmosphere

Even though many missions have explored the Venus atmospheric circulation, its instantaneous state is poorly characterized. In situ measurements vertically sampling the atmosphere exist for limited locations and dates, while remote sensing observations provide only global averages of winds at altitudes of the clouds: 47, 60, and 70 km. We present a three-dimensional global view of Venuss atmospheric circulation from data obtained in June 2007 by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and Venus Express spacecrafts, together with ground-based observations. Winds and temperatures were measured for heights 47–110 km from multiwavelength images and spectra covering 40°N–80°S and local times 12 h–21 h. Dayside westward winds exhibit day-to-day changes, with maximum speeds ranging 97–143 m/s and peaking at variable altitudes within 75–90 km, while on the nightside these peak below cloud tops at ∼60 km. Our results support past reports of strong variability of the westward zonal superrotation in the transition region, and good agreement is found above the clouds with results from the Laboratoire de Meteorologie Dynamique (LMD) Venus general circulation model.

Venus Spectrophotometry During the MESSENGER Mission Fly-By

The NASA mission MESSENGER fly-byed planet Venus on June 2007 on its route to Mercury. This chance was took to produce coordinated observations between Messenger and ESA Venus Express spacecrafts. This work shows spectra in the wavelength range between 320 and 1450nm retrieved with the instrument MASCS (Mercury Atmospheric and Surface Composition Spectrometer). Spectra are calibrated in absolute reflectivity (diffuse reflection by Venus clouds) and wavelength, and they are navigated in order to retrieve their position in the planet’s disk. Comparing synthetic spectra with these ones for each viewing geometry we will obtain information on the vertical distribution of cloud particulates between 60 and 75km height, approximately, as well as the SO2 abundance, among others. This will be combined with almost simultaneous data gathered by the visible and infrared spectrograph VIRTIS onboard Venus Express spacecraft. The results of the atmospheric modeling will be presented elsewhere.