LORRI observations of waves in Pluto's atmosphere

Abstract

Observations during the New Horizons (NH) spacecraft flyby of Pluto in July 2015 revealed that Pluto's atmosphere supports an extensive circumplanetary haze with embedded layers, suggesting several possible microphysical and/or dynamical excitation processes. The purpose of this paper is to build upon existing observations and analyses of Pluto's atmosphere—specifically of the complex haze layer structures—to identify wave structure in Pluto's atmosphere. Here three NH/Long Range Reconnaissance Imager (LORRI) image sequences from the flyby at high phase angles (148°–169°) and three different resolutions (0.093 km/pix, 0.96 km/pix, and 3.86 km/pix) are analyzed. Several haze layer characteristics were extracted, namely—slope, amplitude, waveform, and the associated power spectral densities (PSDs); and their variations with local geography. These are then explored in the context of possible wave types in Pluto's atmosphere, such as tidal and orographically driven inertia-gravity (buoyancy) waves. PSD peaks at 8–10 km and 18–22 km vertical wavelength are found in NH images, which is consistent with the perturbations seen in Earth-based stellar occultations of Pluto's atmosphere. The 8–10 km signals are localized to low-latitudes and equatorial regions and the 18–22 km signals are more globally distributed. Haze layer background relative amplitudes were found to be around 0.01–0.04. Slopes of layers were found to be correlated with the emergence and disappearance of a 25 km layer around 30°N. An amplitude increase of oscillations below 30 km altitude exists in the high-resolution image sequence. These findings indicate the possibility of waves in Pluto's atmosphere and motivate further studies of wave dynamics combining NH data with state-of-the-art models of Pluto's atmosphere. These results are important because they can provide strong constraints to models and to the type of waves that can be present in Pluto's atmosphere.