Modeling thunder propagation and detectability on Titan.

Abstract

This research is part of a study investigating the characteristics of thunder on Saturn’s largest moon, Titan. In tandem with electromagnetic signatures, thunder can corroborate and quantify lightning discharges. A physical model for the propagation of thunder on Titan, based on the most recent data collected by the Cassini–Huygens mission, is being developed. The model approximates a tortuous 20 km cloud-to-ground lightning channel by an angle-wise random walk of small discharge segments, each generating a strong cylindrical shock wave, which acquires an N-wave shape after it travels through the relaxation radius, into the acoustic regime. These acoustic waves are then propagated to the far-field detector where they are added linearly to form long-range thunder. The detectability of thunder signatures by a sensor in Titan’s lower atmosphere depends on the moon’s atmospheric structure. In order to constrain the fraction of acoustic energy reaching a detector in Titan’s troposphere, the model accounts for the upward-refracting sound speed profile up to the inversion point at ∼45 km and also for ground effects. The sound speed and attenuation are computed along the length of the lightning channel (20 km) using altitude-dependent pressure, density, and temperature measurements by Cassini–Huygens, as well as thermophysical parameters (specific heats, viscosity, thermal conductivity, and diffusivity) extracted from NIST’s Chemistry WebBook.