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A new chiral material composed of a two-dimensional lattice of five-segment cranks, with the same handedness is experimentally and numerically studied. The sample, formed by a single layer of such structures, exhibits huge electromagnetic activity and a negative refraction index for linearly polarised waves. Both the chirality parameter and the bandwidth with the negative refraction index are larger than those observed in chiral media composed of three-segment cranks. The material also shows pronounced electromagnetic activity (larger than 250° per wavelength). This observed behaviour is attributed to the fact that the five-segment configuration enhances the electromagnetic coupling compared with the three-segment configuration.

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.

A new EBG (Electromagnetic Band Gap) structure with chiral crank elements is presented and its effects on the electric field propagation along a microstrip line are analysed and compared with the effects produced by mushroom-type inclusions. Chiral crank inclusions are distributed along both sides of the strip and the corresponding results are compared with those obtained by other elements commonly used to create EBG media, mushroom-type structures. A single row of high chirality structures on both sides of a transmission line produces a band gap with a wider bandwidth than that obtained by the inclusion of other elements commonly used to create EBG media, such as mushroom-type structures. Unlike what happen with mushroomtype inclusions, more rows of chiral elements do not change significantly the central frequency and bandwidth of the band gap. The proposed structure works in X-Band (8.2-12.4 GHz) and it is easily adaptable to other frequencies by changing the size of the crank inclusions. Several samples have been made in FR-4 printed circuit board and a good agreement has been found between numerical analysis and experimental measurements. This work has been supported by the Ministerio de Economía y Competitividad of Spain (ERDF co-funded Research Projects PGC2018-098350-B-C21 and PGC2018-098350-B-C22.

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.

Negative refraction has many potential applications in the information and communications technology field. However, the high losses of most negative refraction materials prevent the use of this novel property in the implementation of practical devices in the HF range. In an attempt to overcome this inconvenience, we present two gammadion-fishnet chiral metamaterial structures, constituted by the combination of a lossy chiral metamaterial and different fishnet structures. The electromagnetic characterization of the proposed metamaterials provides a larger bandwidth with negative refraction as well as lower losses when compared with the initial structures. The results were obtained by using both numerical and experimental data, which support our designs. This work has been partially supported by the Spanish Government MINECO through the ERDF co-funded Research Projects TEC2014-55463-C3-1-P and TEC2014-55463-C3-3-P, PGC2018-098350-B-C21 and PGC2018-098350-B-C22.

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.

Producción Científica, This communication presents a novel diode switchable chiral metamaterial structure that manipulates the polarization in different ways depending on the active bias lines. Three different bias states that provide three totally different behaviors have been considered: polarization rotator, circular polarization converter and linear to circular converter., Ministerio de Economía, Industria y Competitividad (Project TEC2014-55463- C3-1-P, TEC2014-55463-C3-2-P and TEC2014-55463-C -3 - P)

A new free-wave experimental method for chiral media characterization is presented. The setup is based on a pair of dual horn antennas and a diplexer 90◦ phase shifter, which allows to produce both circularly and linearly polarized waves and to measure the co- and cross-polar components of the transmission and reflection coefficients of a chiral slab under normal illumination. Then, six experimental measurements are available to retrieve three constitutive parameters. Furthermore, by including the reflection coefficient of the backed metal sample an over determined system of equations is obtained, which leads to increase the accuracy of the material characterization. This work was supported by the Spanish Government (Projects TEC2014-55463-C3-1-P and TEC2014-55463-C3-2-P) and by the European Commission (ERDF). The authors thank C. Arévalo for helping with the paper edition.

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.

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.

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.

Materials with electromagnetic activity at microwave frequencies are usually made by spreading helical inclusions into a host medium. A new manufacturing technique of chiral media based on the inclusion of three segment wire hooks (cranks) is presented in this letter. We show that the random inclusion of many cranks into a host medium produces electromagnetically active materials. Based on a free-wave experimental system, we have found that the transmitted wave through the material is rotated with respect to the incident one. First results seem to be in agreement with the typical behavior of chiral media at the resonant frequency.

During recent decades, a great variety of novel and complex materials have been designed with promising practical applications. It has been found, however, that in many cases the relations between electric and magnetic fields cannot be described by standard constitutive equations. Among these new materials we may mention, for example, chiral, nonreciprocal, gyrotropic and negative refractive index media. Bi-isotropic media are the most general line‐ ar, homogeneous and isotropic materials, and they respond to electromagnetic excitation ac‐ cording to the following relations

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.

International audience; Some of the secrets of the atmosphere of Titan have been unveiled by the Huygens Probe. The Permitivity Wave and Altimetry system detected a hidden ionosphere much below the main ionosphere, that lies between 600 and 2000 km. Theoretical models predicted a low altitude ionosphere produced by cosmic rays that, contrary to magnetospheric particles and UV photons, are able to penetrate down in the atmosphere. Two sensors: Mutual Impedance (MI) and Relaxation Probe (RP) measured the conductivity of the ionosphere by two different methods and were able to discriminate the two branches of electrical conductivity due to the positive and negative charges. The measurements were made from 140 to 40 km and show a maximum of charge densities ≈2 × 109 m−3 positive ions and ≈450 × 106 m−3 electrons at around 65 km. Here we present the altitude distribution of the concentration of positive ions and electrons obtained from the RP and MI sensors.

[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.

[1] The study of the propagation of extremely low frequency (ELF) waves is essential for electromagnetic sounding investigations planned for some of the future Martian missions. Future surface stations will have the possibility of continuously recording low-frequency electromagnetic field fluctuations. Natural electromagnetic waves produced near the surface by electrostatic discharges in dust storms (dust devils) or by geological activity can be trapped in the resonant cavity formed by the surface and lower ionosphere as it occurs on the Earth. Low-frequency electromagnetic waves can also travel along the magnetic field lines of the recently discovered magnetic anomalies from the magnetosphere to the surface and may produce resonant structures in the cavity. The structure of the resonant frequencies, also called Schumann frequencies, is mainly determined by the geometry of the cavity and by the global electrical conductivity of the ionosphere/atmosphere. Measurements of Schumann frequencies by surface stations can be used for remote sensing of the electrical conductivity of the lower ionosphere/atmosphere. We present a numerical model of electromagnetic wave propagation based on the transmission line modeling (TLM) method with the aim of calculating the resonance frequencies on Mars and their dependence on solar activity and various possible ionization sources like meteoroids. The model has been previously validated by application to the terrestrial case. The numerical results obtained for the Earth are very close to the experimental ones, which supports our predictions on Mars. Our model can be used to study the global atmospheric conductivity using future real ELF measurements by surface stations or even balloons on Mars.

[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.

The free-wave characterization of metamaterials is usually carried out by illuminating a sample with a linearly polarized plane electromagnetic wave. At points before and after the sample, sensors are introduced to measure the transverse components of the field, in order to compute the reflection and transmission coefficients related with the co- and cross-polar field components. Based on this information, retrieval algorithms allow parameters like rotation angle, effective chirality and refraction index to be calculated. Here we propose to use the transmission signals under illumination with plane circularly polarized waves, without sensing the reflection signal, to calculate the chirality parameter and the rotation angle due to the electromagnetic activity of the material. This new method, which allows a simpler characterization of a chiral slab, is applied to the study of metamaterials composed of both periodic and random distributions of metallic structures with chiral symmetry. The experimental results are contrasted with simulations and alternative measurements obtained using linearly polarized waves.

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.