The atypical generation mechanism of Titan's Schumann resonance

摘要

The observation of a presumed Schumann resonance (SR) during the landing of the Huygens Probe in Titan's atmosphere on January 2005 was subsequently reported and interpreted by Béghin et al. (2007, 2009, 2010, 2012) as being generated through the interaction of Saturn's magnetosphere with Titan's ionosphere rather than from the lightning activity that prevails on Earth. Beyond this atypical behavior, the existence of a single mode instead of the usual multimodal structure of terrestrial SRs and a comprehensive analysis of the physical generation mechanism remained to be investigated. The purpose of the present work is to draw up the baselines of a global model reconciling the Huygens data and the proposed generation mechanism. Based upon relevant observations obtained after several tenths of Titan's flybys by the Cassini orbiter, the modeling involves macro plasma physics processes as well as a global analysis of the mechanisms at the moon's scale. The clue to the SR's generation mechanism is shown to be the low-frequency modulation of the Pedersen current sheets that are induced in the ionopause region by the corotating Saturn's magnetosphere. The modulation principle involves a wave coupling between the ion-acoustic instabilities driven by the longitudinal current sheets and the electromagnetic quasi-transverse whistler mode. The spectral distribution of the sole second eigenmode seen by the Huygens Probe is found to comply with the ionopause plasma parameters measured by Cassini during the bipolar configuration of Titan-Saturn interaction that is thought to have occurred during the probe descent in the atmosphere.