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This is an open access article under the CC BY-NC-ND license., This work uses a subset of “quiet” MaRS ionospheric dayside observations (MaRSquiet, 2004–2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRSquiet. Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation, The Mars Express Radio Science Experiment (MaRS) is funded by the German Space Agency (DLR) under the Grant 50QM1802. Support for Mars Express Radio Science at Stanford University is provided by NASA through a JPL Contract. Support for the Multimission Radio Science Support Team is provided by NASA/JPL. Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. We thank everyone involved in the Mars Express project at ESTEC, ESOC, ESAC, JPL, and the ESTRACK and DSN ground stations for their continuous support. K. P. and M. P. acknowledge funding for this project by the Deutsche Forschungsgemeinschaft (DFG) under Grant PA 525/11-1, PA525/11-2 and PA 525/14-1. F.G.G. is funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades, the Agencia Estatal de Investigacion and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). Solar Irradiance Platform historical irradiances are provided courtesy of W. Kent Tobiska and Space Environment Technologies. These historical irradiances have been developed with partial funding from the NASA UARS, TIMED, and SOHO missions. We acknowledge support for work conducted on the development of the Mars Climate Database from the European Space Agency (under ESTEC Contract 11369/95/NL/JG(SC)) and from CNES. CNRS (LMD group), the IAA and the UK Particle Physics and Astronomy Research Council (AOPP, Oxford Group) also provided support during the development of the Martian General Circulation Models.

Solar X-ray flare responses in the E region of Mars' ionosphere were observed from Mars Global Surveyor (MGS) on 6 April 2001 and 17 March 2003. We have developed a coupled model for the calculation of Ionospheric Electron Content (IEC), flare and non-flare electron density profiles due to impact of X-rays (0.5–90 A) and Galactic Cosmic Rays (GCR) in the D and E regions of Mars' ionosphere simultaneously. In the non-flare profile the D layer is produced at 25 km due to impact of GCR and hard X-rays (0.5–3 A) with electron densities 1.0 × 102 cm−3 and 8 × 102 cm−3 respectively while E layer is produced at 100–110 km with electron density ~ 4–5 × 104 cm−3 due to impact of soft X-rays (3–90 A). The D flare peak is produced at 30 km with electron density ~ 2–4 × 104 cm−3. The E flare peak is produced at 100 km with electron density 1–2 × 105 cm−3. The predicted flare E-peak density is higher by a factor of ~2 than the measurements carried out by MGS. The D peak density of flare profile is larger by 1–2 orders of magnitude than that produced for non-flare profiles. Our model suggests that hard X-ray is a major source of the D layer in the Martian ionosphere.

The ionization of the lower Martian atmosphere and the presence of charged species are fundamental in the understanding of atmospheric electricity phenomena, such as electric discharges, large scale electric currents and Schumann resonances. The present photochemical model of the lower ionosphere of Mars (0–70 km) is developed to compute the concentration of the most abundant charged species (cluster-ions, electrons and charged aerosols) and electric conductivity, at the landing site and epoch of the ExoMars 2016 mission. The main sources of ionization are galactic cosmic rays (during daytime as well as nighttime) and photoionization of aerosols due to solar UV radiation during daytime. Ion and electron attachment to aerosols is another major source of aerosol charging. The steady state concentration of charged species is computed by solving their respective balance equations (also known as continuity equations), which include the source and sink terms of their photochemical reactions. Since the amount of suspended dust can vary considerably and it has an important effect on atmospheric properties, several dust scenarios, in addition to the day-night variations, are considered to characterize the variability of the concentration of charged species. It has been found that, during daytime, aerosols tend to become positively charged due to electron photoemission, and during nighttime, tend to charge negatively due to electron attachment. The most dominant day-night variability in ion and electron concentration occurs when the amount of suspended dust is the largest. The electric conductivity has been found to vary in the 10−13–10−7 S/m range, depending on the altitude, dust scenario and local time.

The electron density distributions of the lower ionospheres of Mars and Venus are mainly dependent on the solar X-ray and EUV flux and the solar zenith angle. The influence of an increasing solar flux is clearly seen in the increase of the observed peak electron density and total electron content (TEC) of the main ionospheric layers. The model “Ionization in Atmospheres” (IonA) was developed to compare ionospheric radio sounding observations, which were performed with the radio science experiments MaRS on Mars Express and VeRa on Venus Express, with simulated electron density profiles of the Mars and Venus ionospheres. This was done for actual observation conditions (solar flux, solar zenith angle, planetary coordinates) from the bases of the ionospheres to ∼160 km altitude. IonA uses models of the neutral atmospheres at ionospheric altitudes (Mars Climate Database (MCD) v4.3 for Mars; VenusGRAM/VIRA for Venus) and solar flux information in the 0.5–95 nm wavelength range (X-ray to EUV) from the SOLAR2000 data base. The comparison between the observed electron density profiles and the IonA profiles for Mars, simulated for a selected MCD scenario (background atmosphere), shows that the general behavior of the Mars ionosphere is reproduced by all scenarios. The MCD “low solar flux/clear atmosphere” and “low solar flux/MY24” scenarios agree best (on average) with the MaRS set of observations, although the actual Mars atmosphere seemed to be still slightly colder at ionospheric altitudes. For Venus, the VenusGRAM model, based on VIRA, is too limited to be used for the IonA simulation of electron density profiles. The behavior of the V2 peak electron density and TEC as a function of solar zenith angle are in general reproduced, but the peak densities and the TEC are either over- or underestimated for low or high solar EUV fluxes, respectively. The simulated V2 peak altitudes are systematically underestimated by 5 km on average for solar zenith angles less than 45° and the peak altitudes rise for zenith angles larger than 60°. The latter is the opposite of the observed behavior. The explanation is that VIRA and VenusGRAM are valid only for high solar activity, although there is also very poor agreement with VeRa observations from the recent solar cycle, in which the solar activity increases to high values. The disagreement between the observation and simulation of the Venus electron density profiles proves, that the true encountered Venus atmosphere at ionospheric altitudes was denser but locally cooler than predicted by VIRA.

On January 14, 2005, the Relaxation Probe (RP) on the Huygens experiment measured the electrical conductivity in the lower atmosphere of Titan, the largest Saturnian satellite, from 140 km to 40 km of altitude. The Huygens experiment provided a unique opportunity to compare results from the Mutual Impedance Probe (MIP) with the Relaxation Probe (RP) sensor. A geometrical effect related to the size of the electrode and the Debye length of charged species was previously included in the data processing of the signals of the RP. Now, better theoretical estimations of the resistances, enable us to present critical remarks for the data processing of the RP and the different causes which may contaminate the data: a) the model of the RP, b) the fluid structure interaction, c) the stability of the reference potential of the gondola, d) the floating potential of the parachute or a balloon. It is hoped that our remarks will help future planetary or terrestrial missions.

[1] We report that radio science (RS) experiment onboard Mars Express (MEX) has observed three plasma layers in the nighttime ionosphere of Mars at altitudes ~80–100 km, ~120 km, and ~160 km, which are reproduced by model calculation due to impact of meteoroid, solar wind proton, and electron, respectively. The densities of 21 ions (Mg+, Fe+, Si+, MgO+, MgCO2+, MgO2+, MgN2+, FeO+, FeO2+, FeN2+, FeCO2+, SiO+, SiCO2+, SiN2+, SiO2+, CO2+, N2+, O+, O2+, CO+, and NO+) have been computed between altitude 50 km and 200 km. The model shows that all atmospheric ions (CO2+, N2+, O+, CO+, O2+, and NO+) are produced above 100 km due to solar wind electron and proton impact ionizations. The metallic ions are formed between 50 km and 100 km due to ablation of micrometeoroids. It is found that mass ~3.0 × 10−4 g of incoming meteoroid is sufficient for meteor ablation and its characteristic flux ~4.0 × 10−15 cm−2s−1 could produce the nighttime metallic layer observed by MEX. The calculated electron densities are also compared with the occultation measurements made by Mars 4/5 in the nighttime ionosphere of Mars.

The low-frequency data collected with the antenna of the Permittivity, Wave and Altimetry experiment on board the Huygens Probe that landed on Titan on 14 January 2005 have been thoroughly analyzed considering different possible natural and artificial effects. Although a definite conclusion is still subject to the outcome of complementary inquiries, it results from our analysis that the observations can be explained, for the most part, in term of natural phenomena rather than being artifacts. Extremely-low frequency waves generated in the ionosphere of Titan, driven by the corotating Saturn's frozen plasma flow, are assumed to be the most likely source for the observation of the second eigenmode of a Schumann-like resonance at around 36 Hz in the moon-ionosphere cavity. This particular mode is thought to be enhanced with respect to other harmonics because of the particular location of the landing site with respect to that of the supposed sources. The power budget of the observed wave amplitude seems to be consistent with a rough model of the global current of the wake-ionosphere circuit. Broadband low-frequency noise events which are observed sporadically during the descent are probably due to shot noise on the antenna when the Probe is crossing aerosol clouds, an interpretation supported by post-flight ground tests. Contrary to the situation encountered on Earth, atmospheric lightning does not appear to be the source of a conventional Schumann resonance on Titan.

The propagation of extremely low frequency (ELF) electromagnetic waves in the Earth's ionospheric cavity and the associated resonance phenomena have been extensively studied, in relation with lightning activity. We perform a similar investigation for Titan, the largest moon of Saturn. There are important differences between Earth and Titan, as far as the cavity geometry, the atmospheric electron density profile, and the surface conductivity are concerned. We present an improved 3D finite element model that provides an estimate of the lowest eigenfrequencies, associated quality factors (Q-factors), and ELF electric field spectra. The data collected by the electric antenna of the Permittivity, Waves, and Altimetry (PWA) instrument reveals the existence of a narrow-band signal at about 36 Hz during the entire descent of Huygens upon Titan. We assess the significance of these measurements against the model predictions, with due consideration to the experimental uncertainties.

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.

A numerical modeling of the electromagnetic characteristics of Titan’s atmosphere is carried out by means of the TLM numerical method, with the aim of calculating the Schumann resonant frequencies of Saturn’s satellite. The detection and measurement of these resonances by the Huygens probe, which will enter Titan’s atmosphere at the beginning of 2005, is expected to show the existence of electric activity with lightning discharges in the atmosphere of this satellite. As happens with the Schumann frequencies on Earth, losses associated with electric conductivity will make these frequencies lower than theoretically expected, the fundamental frequency being located between 11 and 15 Hz. This numerical study also shows that the strong losses associated to the high conductivity make it impossible for an electromagnetic wave with a frequency of 10 MHz or lower, generated near the surface, to reach the outer part of Titan’s atmosphere.

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).

The paper deals with a coupled model of the upper atmosphere and ionosphere of Titan. The neutral atmosphere is described by a comprehensive photochemical model for 30° solar zenith angle with molecular and turbulent diffusive transport taken into account. At the upper boundary (1430 km) escaping flux of light molecules is included in boundary conditions, while near the lower boundary (32 km) condensation of less volatile species is introduced. The atmosphere is coupled with the ionosphere via a large set of bi- and termolecular ion-neutral reactions. The ionosphere is considered to be in chemical equilibrium; in addition to ion-neutral reactions, recombination loss and ionization by solar photons as well as by saturnian magnetospheric electrons and photoelectrons appear in the mass balance equations. In the most advanced models, from several considered, heavy hydrocarbon ions and heavy nitriles are grouped and represented as two pseudoions. The peak ion density of 5200–6000 cm −3 , depending on the employed model, is obtained at a height of 970–1000 km, i.e., slightly lower than in previous ionospheric studies, which, however, were carried out for 60° solar zenith angle. The main ionospheric component near the peak is HCNH + , but at lower altitudes heavy nitriles followed by heavy hydrocarbons are more abundant, while near the upper boundary (>1400 km) CH + 5 and C 2 H 5 + compete with HCNH + . When termolecular reaction rates are increased to about 5×10 −23 cm 6 s −1 , as suggested by V. G. Anicich and M. J. McEwan (1997, Planet. Space Sci. 45 , 897–923) for a nitrogen-dominated atmosphere, ion density profiles change significantly (50%) in the lower part of the ionosphere, at 700–800 km. The astonishing result by J. L. Fox and R. V. Yelle (1997, Geophys. Res. Lett. 24 , 2179–2192), whose ionospheric model prefers heavy pseudoions as main constituents near the peak, can, in part, be reproduced if the density of C 4 H 2 , a neutral molecule that controls heavy ion production rate, is increased by a factor of 100. The ionosphere is quite sensitive (up to 30% change in density) to variation in recombination rates and/or electron temperature profile. As it can be expected, densities of certain minor neutral components are severely influenced by coupling with ions, e.g., by an order of magnitude for CH 3 C 2 H. Surprisingly enough, even some major neutral species, like CH 4 , show a 50% change in the thermospheric density profile when ion–neutral reactions are introduced. This proves that for molecules in a shallow chemical equilibrium with other neutral species, even a small perturbation caused by reactions with ions can result in a significant change of the density. An interesting and reassuring conclusion has been obtained for HCN: to reach the agreement with the observed mixing ratio profile in the thermosphere, the loss due to polymerization should be considered together with a strongly productive interaction with ions. In view of the results obtained for the neutral atmosphere it seems that coupling with ions can be as important as other factors that shape the neutral density profiles and should be taken into account in future models that will aim at data interpretation from the Cassini/Huygens mission.

The first determination of the 15N/14N ratio in Titan’s atmosphere comes from Earth-based millimetric wavelength spectroscopic observations of HC15N/HC14N. These measurements indicate that the bulk nitrogen is enriched in the heavy isotope 15N by about 4.5 times relative to the Earth value. A 15N/14N anomaly on Mars of about 1.6 times the terrestrial value has been established previously resulting from nonthermal atmospheric escape processes (e.g. dissociative recombination of N+2 ions). We investigated electron dissociative recombination of N+2 ions, electron impact dissociation of N2 molecules, nonthermal exothermic ionosphere-related photochemical reactions, atmospheric sputtering via solar wind and magnetospheric particles, solar wind pick-up and the loss of 14N to 14C via cosmic rays as possible sources of nitrogen isotope fractionation in Titan’s atmosphere where this molecule is the principal constituent. Using a Monte Carlo method we have shown that electron impact dissociation and dissociative recombination in the low energy range of molecular nitrogen and N+2 ions could lead to an isotope fractionation since the energy of the newly released 15N isotope is slightly smaller than the necessary escape energy, or in the case of dissociative recombination, close to Titan’s escape energy. We show that the isotope fractionation for the other more efficient escape processes like atmospheric sputtering is negligibly small, since the energy of both newly released nitrogen isotopes is much greater than Titan’s escape energy. We found that diffusive separation of 15N/14N according to their atomic mass is very important in the solution of this isotope anomaly. Further indications of a much greater particle output or much higher solar wind mass flux of the early Sun during the first half billion years are presented which could explain the observed enrichment of 15N. We found that atmospheric sputtering and pick-up caused by a high solar wind particle outflow during a Post T-Tauri phase could be responsible for the observed nitrogen anomaly. Our study indicates that the mass of Titan’s early atmosphere was at least 30 times greater than the present value. An explanation of this anomaly is important for enabling us to estimate the total nitrogen reservoir required to produce the present Titan atmosphere. In situ measurements and confirmation of the Earth-based 15N/14N isotope anomaly observations will be possible with the Gas Chromatograph and Mass Spectrometer (GCMS) instrument on board of the Huygens probe. They will be of great importance for understanding the formation and evolution of atmospheres around bodies in the solar system.

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.

International audience; The Huygens Probe measured the electrical conductivity of Titan atmosphere from about 140 km down to the surface, employing relaxation and mutual impedance techniques. Previous analyses have shown some differences on the conductivity measurements obtained with two independent sensors - Relaxation Probe (RP) and Mutual Impedance Probe (MIP). A 20-fold maximum discrepancy occurred around the conductivity peak at 60-70 km. To understand the nature of such discrepancy, we reassess the RP data by taking into account a geometrical factor related to the electrode finite size and the Debye length of the ionized medium. The present analysis implies replacing the standard Laplace field distribution by a more elaborated model considering the Poisson equation and the resistance between the RP electrodes and the medium. Although a complete understanding of the conductivity profile is still missing, this work brings RP and MIP data to a much better agreement. The conductivity maximum difference derived from the two sensors is now lower a factor of 2. This reassessment is also useful for future instruments and missions.

[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.

International audience; The propagation of extremely low frequency (ELF) electromagnetic waves and resonance phenomena in the Earth atmosphere has been extensively studied, in relation with ionospheric dynamics, and thunderstorm and lightning activities. A similar investigation can be performed for any other planet and satellite environment, provided this body is wrapped into an ionosphere. There are, however, important differences between Earth and other bodies, regarding the surface conductivity, the atmospheric electron density, the ionospheric cavity geometry, and the sources of electromagnetic energy. In a first approximation, the size of the cavity defines the range of the resonance frequency; the electron density profile, up to the upper atmospheric boundary, controls the wave attenuation; the nature of the electromagnetic sources influences the field distribution in the cavity; and the body surface conductivity, which gives the reflection and transmission coefficients, indicates to what extent the subsurface can be explored. The knowledge of the frequencies and attenuation rates of the principal eigenmodes provides unique information about the electric properties of the cavity. Instruments capable of monitoring the electromagnetic environment in the ELF range are, therefore, valuable payload elements on balloons, descent probes and landers. We develop models for selected inner planets, gaseous giants and their satellites, and review the propagation process of ELF electromagnetic waves in their atmospheric cavities, with a particular emphasis on the application of the Schumann resonance observation to subsurface studies. The instrumentation suitable for monitoring the electromagnetic environment in geophysical cavities is briefly addressed.

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.

During the descent of the Huygens probe through the atmosphere of Titan, on January 14th, 2005, the permittivity, waves and altimetry (PWA) subsystem, a component of the Huygens atmospheric structure instrument (HASI), detected an ionized layer at altitudes around 63 km with two different instruments, the relaxation probe (RP) and the mutual impedance probe (MIP). A very detailed analysis of both data sets is required, in order to correct for environmental effects and compare the two independent estimates of the electrical conductivity. The present work is dedicated to the MIP data analysis. New laboratory tests have been performed to validate or improve the available calibration results. Temperature effects have been included and numerical models of the MIP sensors and electric circuitry have been developed to take into account the proximity of the Huygens probe body. The effect of the vertical motion of the vessel in the ionized atmosphere is estimated in both analytical and numerical ways. The peculiar performance of the instrument in the altitude range 100–140 km is scrutinized. The existence of a prominent ionized layer, and of enhancements in the conductivity and electron density profiles at 63 km, are discussed in the light of previous theoretical predictions.

[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.

The permittivity, waves and altimetry (PWA) instrument was designed for the investigation of the electric properties and other related physical characteristics of the atmosphere of Titan, from an altitude around 140 km down to the surface. PWA carried sensors to measure the atmospheric conductivity, and record electromagnetic and acoustic waves up to frequencies of 11.5 and 6.7 kHz, respectively. PWA also measured the relief roughness during the descent and the permittivity of the surface after touchdown. The measurements and the results of the preliminary analysis are presented. An ionized layer is detected at altitudes above 50 km, using two independent techniques, and the presence of free electrons in the upper atmosphere is confirmed. An electric signal at around 36 Hz is observed throughout the descent, but it is not yet confirmed that this emission is unambiguously related to a resonance of the ionospheric cavity. The relative dielectric constant of Titan's surface material is nearly 2 and the electric conductivity 4×10−10 S m−1. The electric properties of the surface seem to evolve after touch-down, possibly due to a local warming of the landing site by the Huygens Probe body.

[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.

Cosmic ray radiation is the main mechanism for ionizing the lower atmosphere of Titan. Their higher penetration power, in comparison with solar photons, allows cosmic rays to penetrate deep into the atmosphere of Titan, ionizing the neutral molecules and generating an ionosphere with an electron density peak, placed at around 90 km, similar in magnitude to the ionospheric peak produced by solar radiation in the upper atmosphere. In the lower atmosphere, the electron density profile, in the absence of a magnetic field, depends mainly on the modulation of cosmic rays by the solar wind and on the nature of the ionizable particles. We present here the first results of a new numerical model developed to calculate the concentration of electrons and most abundant ions in the Titan lower atmosphere. The present knowledge of Titan’s atmosphere permits us to include new neutral and ionic species, such as oxygen derivates, in a more detailed ion-chemistry calculation than previous lower ionospheric models of Titan. The electron density peaks at 90 km with a magnitude of 2150 cm −3 . The ion distribution obtained predicts that cluster cations and hydrocarbon cations are the most abundant ions below and above the electron density peak, respectively. We also discuss the effect of solar activity at the distance of the Saturn orbit on the spectrum of the cosmic particles. We obtain that from solar minimum to solar maximum the ionization rate at the energy deposition peak changes by a factor of 1.2 at 70 km, and by a factor of 2.6 at altitudes as high as 400 km. The electron density at the concentration peak changes by a factor of 1.1 at 90 km, and by a factor of 1.6 at 400 km.