Christian Béghin

USING SCHUMANN RESONANCE MEASUREMENTS FOR CONSTRAINING THE WATER ABUNDANCE ON THE GIANT PLANETS—IMPLICATIONS FOR THE SOLAR SYSTEM'S FORMATION

The formation and evolution of the solar system is closely related to the abundance of volatiles, namely water, ammonia, and methane in the protoplanetary disk. Accurate measurement of volatiles in the solar system is therefore important for understanding not only the nebular hypothesis and origin of life but also planetary cosmogony as a whole. In this work, we propose a new remote sensing technique to infer the outer planets water content by measuring Tremendously and Extremely Low Frequency (TLF–ELF) electromagnetic wave characteristics (Schumann resonances) excited by lightning in their gaseous envelopes. Schumann resonance detection can be potentially used for constraining the uncertainty of volatiles of the giant planets, mainly Uranus and Neptune, because such TLF–ELF wave signatures are closely related to the electric conductivity profile and water content. Key words: planets and satellites: composition – planets and satellites: formation – planets and satellites: physical evolution – protoplanetary disks – space vehicles: instruments – waves Online-only material: color figures

The atypical generation mechanism of Titan's Schumann resonance

The observation of a presumed Schumann resonance (SR) during the landing of the Huygens Probe in Titans atmosphere on January 2005 was subsequently reported and interpreted by Beghin et al. (2007, 2009, 2010, 2012) as being generated through the interaction of Saturns magnetosphere with Titans 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 Titans flybys by the Cassini orbiter, the modeling involves macro plasma physics processes as well as a global analysis of the mechanisms at the moons scale. The clue to the SRs 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 Saturns 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.

Electrostatic Potential Radiated by a Pulsating Charge in a Two-Electron Temperature Plasma

Mutual impedance experiments have been developed to constrain the plasma bulk properties, such as density and temperature, of ionospheric and later space plasmas, through the electric coupling between an emitter and a receiver electric antennas. So far, the analytical modeling of such instruments has enabled to treat ionospheric plasmas, where charged particles are usually well characterized by Maxwellian electron distributions. With the growth of planetary exploration, mutual impedance experiments are or will be used to constrain space plasma bulk properties. Space plasmas are usually out of local thermodynamic equilibrium; therefore, new methods to calibrate and analyze mutual impedance experiments are now required in such non-Maxwellian plasmas. To this purpose, this work aims at modeling the electric potential generated in a two-electron temperature plasma by a pulsating point charge. A numerical method is developed for the computation of the electrostatic potential in a sum of Maxwellian plasmas. After validating the method, the results are used to build synthetic mutual impedance spectra and quantify the effect of a warm electron population on mutual impedance experiments, in order to illustrate how the method could be applied for recent and future planetary space missions, such as Rosetta, BepiColombo, and JUICE. In particular, we show how it enables to separate the densities and temperatures of two different electron populations using in situ measurements from the RPC-MIP mutual impedance experiment on board Rosetta.

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

Citation: Grard, R., S. Berthelin, C. Beghin, M. Hamelin, J.‐J. Berthelier, J. J. Lopez‐Moreno, and F. Simoes (2011), Commenton “An analysis of VLF electric field spectra measured in Titan’s atmosphere by the Huygens probe” by J. A. Morente et al.,J. Geophys. Res., 116, E05005, doi:10.1029/2009JE003555.

Electron temperature anisotropy associated to field-aligned currents in the Earth's magnetosphere inferred from Rosetta MIP-RPC observations during 2009 flyby

A new approach is proposed for data interpretation of the Mutual Impedance Probe (MIP) instrument from the Rosetta Plasma Consortium (RPC) during the 2009 Earths fly-by gravity assist through the magnetosphere, from dusk to dawn regions. The spacecraft trajectory of +/-8 Re (Earths radius) was crossing several structures of field aligned currents (FACs) and radiations belts on both legs of the closest approach (CA, 2.450 km altitude). As routinely revealed by several pioneering space missions, natural and forced electrostatic wave emissions called Fqs were observed over +/- 3 Re at around CA using a dedicated mode of the MIP instrument. These emissions are lying between consecutive harmonics of the electron-cyclotron frequency, and their wavelength is perpendicular to the magnetic field-lines. Provided that the Fqs wavelengths projected along the MIP antenna might be estimated, it is shown that the local value of the Larmor radius can be deduced, hence the electron temperature component perpendicular to the magnetic field is subsequently derived. On the other hand, during the time of Fqs observations, the presence of VLF hiss emissions usually observed in these regions, gives us the possibility to determine the electron temperature anisotropy associated to the electrostatic electron anisotropy instability according to the theoretical model proposed by Gary and Cairns (JGR, vol.104,1999). Significant dynamic constraints revealed by crossing successive series of FACs tubes are shown being controlled by this anisotropy, and the fact that the magnetic pressure is significantly larger than the thermal pressure suggests that the FACs lobes are non-force free.