Polarimetry as a Tool for Observing Orographic Gravity Waves on Venus

Gourav Mahapatra,Maxence Lefèvre,Aymeric Spiga,Loïc Rossi,Daphne M Stam

doi:10.3847/psj/abf4cf

Abstract

Abstract Planet-wide stationary gravity waves have been observed with the thermal camera on the Akatsuki spacecraft. These waves have been attributed to the underlying surface topography and have successfully been reproduced using the Institut Pierre Simon Laplace (IPSL) Venus Mesoscale Model (VMM). Here, we use numerical radiative transfer computations of the total and polarized fluxes of the sunlight that is reflected by Venus under the conditions of these gravity waves to show that the waves could also be observed in polarimetric observations. To model the waves, we use the density perturbations computed by the IPSL VMM. We show the computed wave signatures in the polarization for nadir-viewing geometries observed by a spacecraft in orbit around Venus and as they could be observed using an Earth-based telescope. We find that the strength of the signatures of the atmospheric density waves in the degree of polarization of the reflected sunlight depends not only on the density variations themselves, but also on the wavelength and the cloud top altitude. Observations of such wave signatures on the dayside of the planet would give insight into the occurrence of the waves and possibly into the conditions that govern their onset and development. The computed change in degree of polarization due to these atmospheric density waves is about 1000 ppm at a wavelength of 300 nm. This signal is large enough for an accurate polarimeter to detect.

Highlights

Gravity waves are thought to play a key role in the transfer of energy and momentum from the lower to the upper parts of the Venus atmosphere, and through this would influence the super-rotation in the atmosphere (Hou & Farrell 1987; Peralta et al 2008)
Before discussing the signatures of gas density variations due to gravity waves in reflected sunlight, we discuss how the total and polarized fluxes F and Q and the degree of polarization PS depend on the gas optical thickness bm
Our numerical simulations indicate that the polarized flux and the degree of polarization P of sunlight that is reflected by Venus are sensitive to variations in the gas density, and through this, to variations of the gas optical thickness bm above the Venusian clouds, while the total flux F is more sensitive to the properties of the clouds themselves

Summary

Introduction

Gravity waves are thought to play a key role in the transfer of energy and momentum from the lower to the upper parts of the Venus atmosphere, and through this would influence the super-rotation in the atmosphere (Hou & Farrell 1987; Peralta et al 2008). Multiple and different types of observations have shown that the upper atmosphere of Venus carries various types of gravity waves (Kasprzak et al 1988; Piccialli et al 2014; Peralta et al 2017). The JAXA Akatsuki spacecraft’s Longwave Infrared Camera observed huge waves covering the planet almost from the southern to the northern poles (Fukuhara et al 2017) on 2015 December 7, just five hours after the spacecraft’s orbital insertion. These waves and the associated local density variations are what we focus on in this paper.

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Results

Conclusion