Theoretical and experimental studies of electromagnetic resonances in the ionospheric cavities of planets and satellites : instrument and mission perspectives

• Auteur / Autrice : Fernando Antonio Simoes
• Direction : Michel Hamelin
• Type : Thèse de doctorat
• Discipline(s) : Physique
• Date : Soutenance en 2007
• Etablissement(s) : Paris 6

Mots clés

Mots clés libres

Sciences planétaires

Mission Cassini-Huygens

Ondes électromagnétiques

Résonance de Schumann

Electricité athmosphérique

Instrumentation spatiale

Résumé

L’étude des ondes d’extrêmement basses fréquences dans les cavités ionosphériques des planètes et satellites dotés d’atmosphère suit une approche similaire à celle suivie pour la Terre. Elle contribue à la caractérisation du circuit électrique atmosphérique, des sources d’énergie associées et des limites des cavités. Un modèle numérique à éléments finis a été développé et appliqué à ces corps planétaires en vue en particulier d’étudier les résonances de Schumann. La pertinence d’un modèle de la cavité de Titan a été testée par rapport aux mesures de l’instrument PWA de la sonde Huygens. La découverte d’une couche ionisée à basse altitude et l’évaluation des propriétés diélectriques de la surface après l’atterrissage sont exposées. L’expérience acquise est appliquée à la conception de nouveaux instruments, ARES et SP2, pour étudier l’atmosphère et le sol de la planète Mars dans le cadre du projet ExoMars et pour d’autres corps lors de futurs projets spatiaux.

The study of extremely low frequency electromagnetic wave propagation in the ionospheric cavities of celestial bodies in the Solar System follows an approach similar to that developed for Earth. It contributes to the characterization of the atmospheric electric circuit and associated energy sources, and to the identification of the inner and outer cavity boundaries. A wave propagation finite element model is developed and applied to all planets and satellites surrounded by an atmosphere, with the aim of studying, in particular, the Schumann resonance phenomenon. The input parameters of the model are: (a) the geometry of the cavity, (b) the ionized atmosphere characteristics, (c) the neutral atmosphere refractivity and (d) the top subsurface complex permittivity. The simulation yields the eigenfrequency and Qfactors of the resonance and the distribution of the electric field in the cavity. The cavities of Venus and Titan are studied in more detail. The former is highly asymmetric and a significant splitting of the eigenfrequency is predicted. The latter has been explored by the Huygens Probe and, additionally, the low conductivity of Titan’s soil opens the door to subsurface investigations. The validity of a model of Titan’s cavity is scrutinized against the in situ measurements performed by the Permittivity, Waves and Altimetry (PWA) analyzer, onboard the Huygens Probe. The PWA instrument measured the ion and electron conductivity profiles using the Mutual Impedance (MI) and relaxation technique, and identified a conductive layer at an altitude of about 60 km; the relative permittivity and conductivity of the surface measured by the MI probe at the landing site are ~2 and ~10-10-10-9 Sm-1, respectively. No evidence of any lightning event or thunder clapping was found; but strong electric signal at around 36 Hz was observed throughout the descent. This narrow band emission is probably not an artefact. Modelling the cavity with an appropriate set of input parameters indicates that this signal is possibly a natural resonance of the cavity. The acquired experience is then applied to the design of novel electrical probes, ARES and SP2, to study the atmosphere and the ground of the planet Mars, in the forthcoming ExoMars mission, and of other celestial bodies in future space missions. It is proposed to take advantage of the polar characteristics of the water molecule and to apply the MI technique to the detection of subsurface ice in the Martian regolith.