The atypical generation mechanism of Titan's Schumann resonance

Abstract

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. (2007Béghin et al. ( , 2009Béghin et al. ( , 2010Béghin et al. ( , 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.

Self-consistent modeling of induced magnetic field in Titan’s atmosphere accounting for the generation of Schumann resonance

“…The most relevant constrained conditions involve the day-side hemisphere exposed to the up-stream magnetized plasma flow of the co-rotating Saturn's magnetosphere. The latter is assumed to display a nominal bipolar configuration (Simon et al, 2013) applicable to the Huygens descent after seasonal and solar activity considerations (Béghin, 2014). In these conditions, the frozen field lines of the incoming plasma are piled-up and draped around two symmetrical lobes transported downstream in the wake-side (Neubauer et al, 1984).…”

“…The input parameters of the upper atmosphere model specifically relevant to this work, taken from Béghin (2014), are determined as follows. The initial conditions involve the average altitude profile of the incoming magnetospheric field (B o ) which is assumed obeying the MHD balance conditions between the magnetic and thermal pressures (e.g., Keller et al, 1994).…”

“…The current sheets are subsequently modulated by the ELF ion-acoustic instability occurring as soon as the ion-electron drift velocity exceeds the adiabatic ion-acoustic speed (Béghin, 2014). In our model, each of the two conjugated current sheets is defined by its longitudinal length (L) and latitudinal width (Y), as sketched in Fig.…”

“…The relevant observations were subsequently reported and interpreted by Béghin et al (2009Béghin et al ( , 2010Béghin et al ( , 2012Béghin et al ( , 2014 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 the latter peculiarity, the major differences between Earth and Titan SR are: (i) the type and the location of the current sources of the emission; (ii) the electromagnetic generation mechanism; (iii) the modal equations of frequency distribution of the eigenmodes; and (iv), the absence of perfectly conductive surface on Titan, contrary to Earth.…”

“…1) accounting for the feeding source of Titan's SR (Béghin, 2014), a quantitative assessment of the physical structures and electric properties of the ionopause and deep atmosphere is proposed in Section 2. The altitude profile of the current-induced magnetic field is derived in Section 3 from the usual Biot-Savart law applied to a model of current-sheet.…”