An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe

Abstract

A numerical simulation of Titan's electromagnetic cavity in the VLF band is carried out using the Transmission Line Matrix (TLM) method, with the aim of assessing the VLF electric field spectra sent by the Huygens probe. In an Earth‐like model, successive peaks would be expected in the spectra, associated with multiple reflections of the electromagnetic wave on the external surfaces of Titan's electromagnetic cavity, formed by the ionosphere and a conductive ground or underground surface. However, owing to high losses conferred by the electrical conductivity to Titan's atmosphere, the direct numerical and experimental spectra are decreasing functions of the frequency without resonances or other special forms. The weak atmospheric resonances are extracted from the data measured by the Huygens's sensors after a procedure based on analysis of the late time response is applied to the experimental spectra. The new late time spectra present the appearance expected, with successive peaks and a mean between adjacent resonances of 0.67 kHz, in agreement with the late time TLM results when the Borucki et al. (2006) conductivity profile with aerosols is introduced in the numerical model. This gap in frequency between successive peaks gives an effective optical distance between the ground and the conductive ionosphere of approximately 220 km, considerably higher than the terrestrial one where this distance is around 75 km.

Comment on “An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe” by J. A. Morente et al.

“…The real part of the energy distribution has no physical significance on its own and its only merit is to provide a fake spectrum offering a series of apparent resonances separated by intervals that are, this time, controlled by the phase shift induced by the early time blanking out [ Berthelin et al , 2009]. Note the perfect similarity between Figure 1 (right) and the first spectrum in Figure 10d of Morente et al [2009b].…”

“…As discussed in section 3, as presented in Morente et al's Figure 10d the absolute value of the real part of the DFT as a VLF power spectrum [ Morente et al , 2009b] is totally unjustified from a physical point of view. Taking the study cases of Morente et al [2008, 2009b] as examples, we describe in sections 3 and 4 the mechanism that can generate artificial resonances and harmonics out of noisy signals and featureless spectra. We have developed numerical codes similar to those of Morente and coworkers, and we are able to duplicate the same spectra, supposedly observed during the descent.…”

“…This phase shift does not affect the DFT modulus but clearly influences the frequency distributions of its real and imaginary parts. This point is not relevant for the ELF signal analysis [ Morente et al , 2008], but it is of paramount importance for understanding the treatment of the VLF signal [ Morente et al , 2009b]. Since the VLF energy modulus distribution does not display sufficiently visible resonances, it seems that they show instead, without any explanation, the absolute value of the real part of the DFT.…”

“…In a first paper, they claim that they have detected in the ELF range (0–100 Hz) several harmonics of a global resonance allegedly generated by lightning activity in the spherical cavity guide formed by the surface of Titan and the inner boundary of the ionosphere, a phenomenon similar to the Schumann resonance observed at Earth [ Morente et al , 2008]. In the second paper dedicated to the VLF electric signal recorded by PWA, in the range 0–10 kHz, they argue that they can also bring out the transverse resonance and its harmonics, a more local phenomenon that develops around the excitation source and whose frequency is controlled by the separation between Titan's surface and the inner ionospheric boundary [ Morente et al , 2009b].…”

“…The analyses of the ELF and VLF signals by Morente and his coworkers are flawed, and their conclusions are shown here to be invalid. A more comprehensive and thorough assessment of the numerical approaches proposed by Morente et al [2008, 2009b] is given by Berthelin et al [2009].…”