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Among the multiple questions that the CASSINI/HUYGENS mission tries to answer is the likelihood of electric discharges in Titan's atmosphere. The instruments “Huygens Atmospheric Structure Instrument” and “Radio and Plasma Wave Science” will probe the electromagnetic emissions during the Huygens descent and Cassini flybys, respectively. Although no lightning was observed during Voyager's encounters with Titan in 1980 and 1981, this does not exclude the existence of lightning phenomena. Recent investigations show that lightning discharges could occur in the lower atmosphere, such as the detection of methane condensation clouds in the troposphere and the theoretical prediction of an electric field that would be sufficient enough to cause lightning. We present a numerical model of Titan's atmosphere with the aim of calculating the resonance frequencies and the atmospheric transparency to electromagnetic waves. The detection and measurement of these resonances, Schumann frequencies, by the Huygens probe, would show the existence of electric activity connected with lightning discharges in the atmosphere. As it happens with the Schumann frequencies of Earth, losses associated with the electric conductivity will make these frequencies to be lower than the theoretically predicted, the fundamental one being located between 11 and 15 Hz. An analytical study shows that the strong losses associated with the high conductivity make it impossible that an electromagnetic wave generated near the surface with a frequency of 10 MHz or lower reaches the outer part of Titan's atmosphere. Therefore the detection of electromagnetic waves coming from Titan's lower atmosphere by the RPWS instrument is very unlikely.

The HUYGENS Atmospheric Structure Instrument (HASI) was designed to characterise the atmosphere of Titan during the descent of the HUYGENS probe in the framework of the NASA/ESA CASSINI/HUYGENS mission in November 2004. A balloon campaign was conducted in Leon, Spain, in December 1995, in order to test the HASI hardware and software in the terrestrial atmosphere and investigate the influence of the HUYGENS probe on the electrical measurements in a real environment. The subsystems from the HASI instrument: the Pressure Profile, the Accelerometer, the Temperature Profile and the Permittivity, Wave and Altimetry packages with their corresponding sensors were accommodated on a HUYGENS 1:1 mock-up and launched by a stratospheric balloon crossing a distance of 340 km and reaching a maximum altitude of around 30 km. The Huygens mission at Titan was simulated by a drop test; the probe was separated from the balloon in order to descent to ground dragged by a parachute. Measurements have been performed both in the ascending and descending phases.

The permittivity wave and altimetry (PWA) instrument is a part of the CASSINI/HUYGENS HASI experiment and was designed to determine the electrical parameters of the atmosphere of Titan in 2004. In December 1995, a balloon campaign was conducted in Leon, Spain, to test the HASI onboard hardware and software using a HUYGENS probe mock-up in an electromagnetic-disturbance-free environment (mainly from power emission lines at 50 Hz ). This work is concerned with the measurements of small ion polar conductivities and DC fields using the PWA relaxation probes (RP). The two RP electrodes were periodically set to ±5 V relative to the conductive surface of the mock-up and allowed to discharge in the surrounding atmosphere. The polar components of conductivity are calculated from the discharge time, and the DC field from the floating potential differences once the electrodes reach equilibrium. In spite of some observed effects, such as mock-up charging or oscillations in the measurement of potential, the conductivity measurements are coherent and in good agreement with the obtained results in other experiments. The conductivity data were collected in ‘fair-weather’ conditions, up to 30 km during a 4-h flight, every 72 s , giving an altitude resolution better than 400 m . We also discuss the DC field data that do not lead, in presence of charging effects, to a straightforward measurement of the natural DC field. The Comas Sola balloon flight, first real test of the PWA experiment in the terrestrial atmosphere, confirmed the validity of the ionic conductivity measurements but raised the problem of a reliable interpretation of the DC field.

The penetration of the Huygens probe through the atmosphere of Titan in 2004 will provide a unique opportunity to measure the electrical properties of its lower atmosphere. Many studies have been carried out to predict the density of electrons and the distribution of the major positive ions, but there is no recent analysis concerning the electrical conductivity that will be measured by Huygens. In this paper we present predictions of negative and positive conductivity by using the present models of the lower ionosphere. The predicted conductivity is delimited between a minimum and a maximum profile and compared with the capability of the PWA-HASI instrument on board Huygens to measure it. Special attention is paid to the influence of electrophilic species on the conductivity and on the decrease of the negative conductivity depending on the concentration of electrophilic species. We present an analytic expression to calculate the concentration of electrophilic species, if any, depending on the measurements of positive and negative conductivity that will measure PWA-HASI.

The ion-neutral chemistry of the atmosphere of Mars has been studied with a one-dimensional model. Here we present the main reactions that lead to the formation of the most abundant positive and negative ions. Galactic cosmic rays ionize the atmospheric constituents, mainly CO 2 , and reactions of charge transfer of CO + 2 with oxygen produces O + 2 . The further ion-neutral chemistry is quite similar to that of D -region on Earth. The relative high abundance of water and its higher proton affinity compared with other molecules, lead to the expectation that the most abundant positive ions are hydronium hydrates, H + 3 O(H 2 O) n . The chemistry includes the production of negative ions, O − and O − 2 , by electron capture of oxygen-bearing molecules and sinks of negative ions by electron photodetachment and ion-ion recombination. Oxygen negative ions are transformed through a chain of reactions into the carbon and nitrogen-bearing ions, CO − n and NO − n . The formation of hydrated negative ions is also considered.

Towards the end of 2004 during the descent of the HUYGENS probe the Permittivity, Wave and Altimetry instrument (PWA) as part of the HASI experiment will investigate electrical properties and electric field fluctuations in the atmosphere of Titan below 170 km. We have modelled the propagation of low frequency electromagnetic waves in the lower ionosphere of Titan. The ionisation in this altitude range (below 400 km) is mainly produced by galactic cosmic rays. Electromagnetic waves could be generated by lightning phenomena at very low altitudes. A major emphasis of our work is on the calculation of the wave attenuation using the electron densities and collision frequencies from the model by Molina-Cuberos et al. (1999, J. Geophys. Res., 104, 21,997–22,024).

A coupled model of the upper atmosphere and ionosphere of Titan, which includes all relevant physical processes for neutral and ionized components, is presented for a solar zenith angle of 30°. The main results concerning the ionosphere are: (i) ionospheric peak of about 6 000 electrons per cc appears at 980 km, (ii) the most abundant component near the peak is HCNH + , but at lower altitudes heavy nitrilic and hydrocarbon ions dominate, (iii) termolecular reactions can play an important role in ionospheric chemistry at 700 km and below, only if reaction rates are as large as 5 × 10 −23 cm 6 s −1 .

International audience; The Permittivity Waves and Altimetry (PWA) experiment is part of the Huygens Atmospheric Structure Instrument (HASI) presently ‘en route’ toTitan as a part of the HUYGENS Probe carried by the Saturn orbiter CASSINI. When HUYGENS lands upon Titan, the Mutual Impedance (MI) probe, part of the PWA experiment will be set to a special mode to measure the complex permittivity of the surface. We discuss here the capabilities of the instrument relative to several possible surface models currently under discussion. For a liquid surface, PWA will provide additional permittivity measurements to the Surface Science Package (SSP) measurements. In the case of a solid surface, with an estimation of the probe penetration from SSP acceleration data and under the assumption of homogeneity, the complex permittivity can be derived. Moreover, if the surface layer is well known up to a few tens of centimeters, we would also have knowledge of the permittivity of the underlying material. Although PWA-MI provides only permittivity measurements it should contribute greatly to identifying Titan's surface structure and composition in synergy with the other HUYGENS instruments.

[1] We have used dust opacity values observed by the Thermal Emission Imaging System onboard Mars Odyssey to estimate the effect of dust aerosols in the D region of the Martian ionosphere. An ion-dust aerosol model has been developed to calculate ion concentrations and conductivity at midlatitudes during a dust storm in the Southern Hemisphere. We report that the concentration of the water cluster ions H+(H2O)n, NO2−(H2O)n, and CO3−(H2O)n are reduced by 2 orders of magnitude in the presence of dust aerosols. This indicates that during a dust storm, when the optical depth changes considerably, a large hole in the ion concentrations may appear until this anomalous condition returns to the normal condition after a period of about a few days. During such dust storms, the total ion conductivity is reduced by an order of magnitude.

[1] We report the first model results of zonal wind perturbations in the nighttime D region ionosphere of Mars during winter and summer seasons at latitude ranges 60°S–64°S and 64.7°N–67.3°N, respectively. The production rate, ion density, and electron density are calculated between longitudes 0° and 360°E due to impact of galactic cosmic rays on the nighttime troposphere of Mars at these latitudes. In this calculation, temperature and air density are used from the measurements made by the radio occultation experiment on board Mars Global Surveyor at low solar activity period. In these seasons the zonal distribution of temperature, air density, and electron density are fitted by a least squares method at altitudes near the surface, 15 km and 30 km. The modes of seasonal winds are also calculated from Fourier analysis at these heights. These measurements suggest mode of wave number 2 in northern high latitudes and wave numbers 3 and 8 in southern high latitudes, respectively. It is found that the nighttime ionosphere of Mars consists of a permanent D peak, which varies with east longitudes in both hemispheres. The modes 2 and 3 of semidiurnal tides dominate the D peak oscillation during the summer and winter seasons, respectively. The water cluster ions H3O+(H2O)n, NO2−(H2O)n and CO3−(H2O)n are the major species in the D region ionosphere of Mars.

[1] We have developed a model to calculate the production rates and densities of positive and negative ions in the troposphere of Mars at evening. Using this model, the densities of 35 ions (Ar+, O2+, CO2+, O2+CO2, NO+, H3O+, H3O+H2O, H3O+(H2O)2, H3O+(H2O)3, H3O+(H2O)4, H3O+HO, CO2+CO2, CO+, C+, N2+, NO+CO2, N+, O2+(CO2)2, O2+H2O, O2+(H2O)2, O4+, O+, CO3−H2O, CO3−(H2O)2, CO3−, CO4−, NO2−, NO2−H2O, NO2−(H2O)2, NO3−, NO3−H2O, NO3−(H2O)2, O2−, O3−, O−) are estimated in summer and winter at latitudes 67°N and 62°S for solar zenith angles 80° and 85°, respectively. The impact ionization source is taken as galactic cosmic rays. The model atmospheres at these locations are constructed from air density measured by radio occultation experiment aboard Mars Global Surveyor. The maximum electron densities in southern winter and northern summer are obtained at altitudes ∼25 km and ∼30 km, respectively, due to high efficiency of electron attachment to Ox molecules. Of the 35 ions considered in the model, the chemistry of 9 major ions (H3O+(H2O)n for n = 1, 2, 3, 4 and CO4−, CO3−, NO2−H2O and CO3−(H2O)n for n = 1, 2) are discussed in this paper. The ion densities are changed by factors of 5 to 10 between these two seasons.

The propagation of extremely low frequency (ELF) waves in the Earth surface-ionosphere cavity and the properties of the related Schumann resonances have been extensively studied in order to explain their relation with atmospheric electric phenomena. A similar approach can be used to understand the electric environment of Venus and, more importantly, search for the evidence of possible atmospheric lightning activity, which remains a controversial issue. We revisit the available models for ELF propagation in the cavity of Venus, recapitulate the similarities and differences with other planets, and present a full wave propagation finite element model with improved parameterization. The new model introduces corrections for refraction phenomena in the atmosphere; it takes into account the day-night asymmetry of the cavity and calculates the resulting eigenfrequency line splitting. The analytical and numerical approaches are validated against the very low frequency electric field data collected by Venera 11 and 12 during their descents through the atmosphere of Venus. Instrumentation suitable for the measurement of ELF waves in planetary atmospheres is briefly addressed. Copyright 2008 by the American Geophysical Union.

[1] The effect of the Earth-ionosphere electromagnetic cavity on the spectrum of an atmospheric signal generated by a broadband electrical current source is analyzed numerically by means of the Transmission Line Matrix (TLM) method. Two new TLM meshes are developed, one with transmission lines connected in parallel and the other with connections in series. The equations describing propagation through these parallel or series meshes are equivalent to the Maxwell equations for TEr or TMr modes in the spherical Earth-ionosphere cavity, respectively. The numerical algorithm obtains Schumann resonance frequencies very close to the experimental ones, confirming that this methodology is a valid numerical tool for predicting these resonances on other planets and moons. Finally, the TEr and TMr modes with a higher order than the Schumann resonances are also analyzed, finding that the effect of atmospheric conductivity is to shift the peak frequencies toward higher values than the eigenfrequencies corresponding to the lossless system. For daytime conditions, these peak frequencies appear around 2, 4, 6, 8… kHz, connected to an effective aboveground ionosphere height of approximately 75 km. In the night region, the shift is slightly smaller and the effective ionosphere height is around 85 km in agreement with smaller values in the conductivity profile.

[1] Schumann resonance frequencies are the resonance frequencies for an electromagnetic field inside the natural conducting cavity formed by the Earth's surface and its ionosphere. The spectrum peaks for the total electromagnetic energy stored inside the cavity define a unique value for each Schumann resonance. Because of the impossibility of measuring the total energy stored in the atmosphere, the experimental determination of Schumann resonances is carried out by a local measurement of the magnetic or electric field spectrum. However, peak frequencies in the amplitude of the electromagnetic field Fourier spectrum of a lightning discharge excited atmosphere may depend on height because this field depends on the atmosphere conductivity and this physical magnitude varies with height. Therefore it is reasonable to expect a slight shift between the peaks corresponding to the electric or magnetic field spectrum at a given height and those obtained by considering the total energy stored. In this paper the Earth's electromagnetic cavity is numerically modeled by using the transmission line matrix (TLM) numerical method. The altitude profile of the magnetic field spectrum remains constant from ground to 70-km height; however, a decay between 1.5 and 4 Hz is observed for heights between 70 and 100 km, which represents a medium reduction of 10%.

[1] A numerical modeling of Earth's atmosphere is carried out by means of the Transmission Line Matrix (TLM) numerical method with the aim of calculating the Schumann resonance frequencies. The numerical results obtained are very close to the experimental ones and those obtained with the widely accepted two-scale-height ionospheric model, which allows us to affirm that this is a valid numerical tool for predicting the Schumann frequencies in the atmospheres of other planets and moons. The great flexibility of the TLM numerical method also allows the study of slight shifts in the Schumann resonance frequencies due to an increase in electrical conductivity at the Earth's poles, originating from high-energy particle precipitation emitted from the Sun in conjunction with solar flares. A slight increase in the first Schumann resonance frequency is observed during these events, which is associated with a reduction in the dimensions of the electromagnetic cavity.

[1] The ion neutral chemistry of the lower Martian atmosphere is more complex than any other ionospheric region and similar, in some aspects, to the terrestrial D region. Two ionization sources operate at these altitude levels: solar radiation, which is the most important one below around 70 km, and galactic cosmic radiation. The interaction of positive ions and electrons produced by these ionization sources with the Martian atmosphere, which is abundant in species able to be charged negatively, produces a high variety of positive and negative ions. We present here a new model of the ion chemistry developed to calculate the concentration of electrons and most abundant ions in the atmosphere of Mars below 100 km. The present knowledge of Mars' atmosphere allows us to include new neutral and ionic species, such as nitrogen derivated, in a more detailed ion-chemistry calculation than previous lower ionopheric models of Mars. We obtain that the electron density profile has a low-altitude secondary peak below the main photon-induced maximum. This peak is placed at around 35 km and is due to the high efficiency of the electron attachment to Ox molecules, which entails that the concentration of negative ions is higher than that of electrons below around 30 km. The most abundant ionic species predicted are positive ions like hydronimum hydrates H3O+(H2O)n and O2+ and negative ions like CO3−(H2O)n and NO3−(H2O)n.

[1] Titan, the largest satellite of Saturn, possesses a dense N2-CH4 atmosphere rich in organics with a complex chemistry that could be similar to Earth's primordial atmosphere. Due to its massive atmosphere and the long distance to the Sun, cosmic rays are the main energetic source of the lower part of the atmosphere. Cosmic radiation can reach Titan's troposphere (z < 40 km) and activate the chemistry at depths where UV light and high-energy electrons cannot penetrate. The interaction of cosmic rays with the neutral constituents of Titan produces a complex ion chemistry that leads to the formation of a high variety of massive ions and organic molecules. The low temperatures and high pressures prevailing in Titan's atmosphere result in a high efficiency of three-body reactions and on the formation of ion clusters. Association of neutral molecules with HCNH+, CH3CNH+, and HC3NH+ produces ions containing the CN group, and the electron recombination of these clusters can produce a high variety of N-containing compounds. Here we propose schemes to synthesize nitrile-type molecules from the dissociative recombination ion clusters produced by cosmic rays. Our ion chemistry model can explain the existence of some of the nitriles found in laboratory studies, such as propenenitrile, propanenitrile, butanenitrile, pentanenitrile, and 10 more as well as predict the existence of new species. Experimental simulations of Titan's atmosphere yield the formation of a high number of species, which are not considered in the chemical schemes of current photochemical models.

Titan's lower ionosphere (from 1 to 400 km) has been studied with a one-dimensional ion-neutral model. In this region of the atmosphere, galactic cosmic rays (GCRs) are the main ionization source. They penetrate to the deeper atmosphere and ionize the neutral constituents of Titan's atmosphere (mainly N2, CH4, Ar, H2, and CO) to produce N2+, N+, Ar+, CH4+, CH3+, CH2+, H2+, H+, and CO+. Fast reactions with the neutrals convert these ions into ions such as CH5+, C2H5+, and N2H+. Different pathways are proposed to obtain the ion and electron densities. The most abundant ions are cluster ions, like CH5+·CH4, HCO+·H2, and HCNH+·C2H4, and long chain hydrocarbon ions. In atmospheres very rich in N2, such as Titan's, ions like H4C7N+ and CH3CNH+ also represent an important contribution to the total positive ion density. Three-body reactions may play an important role in the dense atmosphere of Titan, and special attention is devoted to them. The calculated electron density in the lower atmosphere reaches a peak of ≈ 2150 cm−3 at an altitude of 90 km.