A very simple and easily manufactured collimated anamorphotic system with two cylindrical mirrors in spectrograph AApS[formula omitted] (1992) 288–292

Abstract

Pluto’s tenuous atmosphere exhibits remarkable seasonal change as a result of the planet’s substantial obliquity and highly eccentric orbit. Over the past two decades, occultations have revealed that the atmospheric pressure on Pluto has increased substantially, perhaps by a factor as large as 2 to 4, as the planet has moved from equinox towards solstice conditions. These data have also shown variations in the strength of the dynamical activity in the atmosphere, as revealed by the varying abundance and amplitude of spikes in the occultation light curves resulting from refractive focussing by atmospheric waves. Toigo et al. (Toigo et al. [2010]. Icarus, 208, 402–411) explored the possibility that these waves are caused by solar-induced sublimation and diurnal deposition from N2 frost patches, driven by weak vertical winds resulting from the rising and sinking gas as it is released from or deposited onto the surface. Here, we extend this model to account explicitly for seasonal variations in average insolation and for the significant damping of vertical wave propagation by kinematic viscosity and thermal diffusivity (Hubbard et al. [2009]. Icarus, 204, 284–289). Damping is extremely effective in suppressing vertical propagation of waves with vertical wavelengths of a few kilometers or less, and the dominant surviving tidal modes have characteristic vertical wavelengths λ∼10–13km. We estimate the expected strength and regional characteristics of atmospheric tides over the course of Pluto’s orbit for a variety of assumed spatial distributions of surface frost and atmospheric surface pressure. We compute the predicted strength of tide-induced wave activity based on the actual frost distribution observed on Pluto from Hubble Space Telescope (HST) observations (Stern et al. [1997]. Astron. J., 113, 827; Buie et al. [2010]. Astron. J., 139, 1128–1143), and compare the results to calculations for volatile transport models of Young (Young [2013]. Astrophys. J., 766, L22) and Hansen et al. (Hansen et al. [2015]. Icarus, 246, 183–191). We develop simple scaling rules to estimate the variation of the strength of tidal activity with surface pressure PS and solar declination δ⊙, and show that the maximum expected temperature perturbation at an atmospheric pressure of P=0.1Pa scales as dTmax∝cosδ⊙/PS. Wave activity is strongest in the near-equatorial region (latitude|ϕ|≲30°), being only weakly dependent on the detailed frost distribution. Using a 3-D time-dependent geometric optics ray-tracing code, we compute model light curves for the geometric circumstances of three high-SNR occultations (2002 August 21, 2006 June 12, and 2012 July 18), taking into account the detailed three-dimensional characteristics of the tides as different regions of the atmosphere are probed over the course of each occultation chord. We compare the strength and abundance of the scintillations in the models with those seen in the data, using both the HST frost maps and the volatile transport model predictions. The striking asymmetries in the strengths of spikes between ingress and egress seen in some events are reproduced in the tidal model simulations, due primarily to the latitudes probed during the occultation: occultations at high northern or southern latitudes uniformly have much weaker wave activity than more equatorial events. A surface pressure range of PS=1–2Pa provides the best match between models and observations. With the impending arrival of the New Horizons spacecraft at Pluto in 2015, we predict that wave activity in the upper atmosphere will be strongest at equatorial regions, and controlled in amplitude primarily by the surface pressure and damping effects, rather than by the detailed frost distribution. If Pluto’s atmosphere begins to collapse in the coming decades, we expect that future stellar occultations will provide evidence for greatly enhanced atmospheric wave activity.