Evidence of electrical activity on Titan drawn from the Schumann resonances sent by Huygens probe

A procedure is shown for extracting weak resonances from the responses of electromagnetic systems excited by electric discharges. The procedure, based on analysis of the late-time system response, is first checked using an analytical function and later with the data for the electric field generated by the computational simulation of Titan's atmosphere using the Transmission Line Matrix (TLM) method. Finally, the low frequency spectrum of the natural electric field in Titan's atmosphere sent by the mutual impedance sensor (MIP) included in the Huygens probe is analyzed employing this technique. The MIP sensor was initially designed to measure the horizontal component of the electric field during a quiet descent. Fortunately, the swinging that occurred during the descent allowed the MIP to measure the radial component of the electric field mixed with the horizontal one. Application of the late-time analysis technique shown in this paper confirms the signature of lightning reported by preliminary data observations, bringing out the expected eigenfrequencies of the Titan-ionosphere electromagnetic cavity, known as Schumann resonances. These resonances are the resonant frequencies of the lower TMr (transverse magnetic to r) modes, which are quasi-transverse electromagnetic modes because they present vertical or radial components of the electric field two orders of magnitude higher than the associated horizontal, azimuthal and zenithal, components. The sequence of Schumann resonances is unique for each celestial body with an ionosphere, since these resonances are fully determined by the dimensions of the planet or satellite and the corresponding atmospheric conductivity profile. Detecting these frequencies in an atmosphere is clear proof of electrical activity, since it implies the existence of an electromagnetic-energy source, which is essential to create and maintain them.