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The ROSETTA/ESA probe will probably rendezvous Comet 67P/Churyumov-Gerasimenko in 2014 and launch a Lander at the nucleus surface. The CONSERT instrument will perform the sounding of the comet nucleus by measuring a 90 MHz electromagnetic wave propagation from the Lander to the orbiter throughout the nucleus. The goal of this sounding is to determine the internal structures of the comet nucleus at different scales and to deduce information on its composition (density, type and abundance of the refractor). CONSERT is an original concept of spaceborne transmission radar based on the propagation throughout the nucleus while the classical radars are based on the reflection. This instrument fulfills three functions: a chronometer to measure the propagation delay, an imager based on the multipath propagation and a radiometer to estimate the wave attenuation The calibration of the instrument should characterize these three functions but due to the novelty of the instrument concept we have no classical set of parameters to quantify the instrument performances from a data-inversion point of view. The instrument design and the absence of calibration channel due to mass and power constraints require specific signal processing and calibration protocol. In this paper, we review the instrument, its objectives and its time transponder structure. Then we present the philosophy of the calibration. In particular we present the results of the instrument ground calibration and the planed in-fly calibration.

International audience; This paper presents a 3-D simulation of the COmet Nucleus Sounding Experiment by Radiowave Propagation (CONSERT) radiotomography experiment of Comet 67P/Churyumov-Gerasimenko. This experiment is part of the ROSETTA space mission. Our simulation is based on a ray tracing algorithm and takes into account multiple internal reflections, power losses, phase delays and polarization distortions. We emulate as closely as possible the true CONSERT data and discuss the results of the simulation.

[1] A major finding in exploration of Mars is the indication that a large quantity of water has been present on Mars. To discover the distribution of this quantity of water, the Mars Express spacecraft is carrying a spaceborne radar sounder, called Mars Advance Radar for Subsurface and Ionospheric Sounding (MARSIS), in order to map Mars subsurface dielectric characteristics. The returned radar echoes from Mars will contain both surface and subsurface reflection components, but the part of the signal we are interested in is the reflected signal from subsurface layers. To retrieve these weak deep echoes from the radar signal, a signal processing algorithm needs to be developed. In this paper, we present a computationally efficient radar signal simulation based on the use of the Facet Method as a surface modeling scheme. This simulator will be used to validate the MARSIS ground processing software and to support the interpretation of MARSIS data. The first step in the simulation algorithm definition is to model the Mars surface. Many surface modeling methods have been developed and can be found in the literature, but considering the fact that Mars surface is very smooth, the modeling algorithm we have developed uses the Facet Method. In this paper, we show that the Facet Method is an efficient scheme for modeling a relatively smooth surface, such as the Mars' surface. The instrument simulation we define makes strict reference to the MARSIS radar parameters; however, it may be used to model any radar sounder.

International audience; We summarize the scientific objectives of the CONSERT experiment. We point up the practical realisation of the instrument describing the main points necessary to understand how the instrument is working. The transponder structure and the principle of the temporal synchronization are explained. The calibration of the instrument is discussed and its performances as results of tests are shown. We also discuss future operations which have to be planned so as to prepare the in flight operations and to maintain the instrument knowledge for 11 years. Finally, the CONSERT ability to explore the 67P/Churyumov-Gerasimenko comet is discussed.

International audience; In view to prepare Mars human exploration, it is necessary to promote and lead, at the international level, a highly interdisciplinary program, involving specialists of geochemistry, geophysics, atmospheric science, space weather, and biology. The goal of this program will be to elaborate concepts of individual instruments, then of integrated instrumental packages, able to collect exhaustive data sets of environmental parameters from future landers and rovers of Mars, and to favour the conditions of their implementation. Such a program is one of the most urgent need for preparing human exploration, in order to develop mitigation strategies aimed at ensuring the safety of human explorers, and minimizing risk for surface operations. A few main areas of investigation may be listed: particle and radiation environment, chemical composition of atmosphere, meteorology, chemical composition of dust, surface and subsurface material, water in the subsurface, physical properties of the soil, search for an hypothesized microbial activity, characterization of radio-electric properties of the Martian ionosphere. Scientists at the origin of the present paper, already involved at a high degree of responsibility in several Mars missions, and actively preparing in situ instrumentation for future landed platforms (Netlander - now cancelled, MSL-09), express their readiness to participate in both ESA/AURORA and NASA programs of Mars human exploration. They think that the formation of a Mars Environment working group at ESA, in the course of the AURORA definition phase, could act positively in favour of the program, by increasing its scientific cross-section and making it still more focused on human exploration. © 2004 Published by Elsevier Ltd on behalf of COSPAR.

The MIMOSA radar is a spaceborne unfocused nadir looking synthetic aperture P-Band radar designed to penetrate the Earth superficial layers and proposed in the frame of the Earth Explorer Opportunity Missions (ESA). This paper reviews all the mission aspects. The scientific objectives are presented: 3D mapping of Antarctic ice sheet, biomass monitoring, and prospective objectives. The instrument concept is developed. The physics of the measurement is analyzed to show that the expected signal is a mixture of deep echoes-and surface clutter. A clutter rejection processing is proposed and some results of signal simulation are given for typical ice surfaces. A preliminary instrument design is described, based on the physics analysis and simulations results.

International audience; The simulation of CONSERT instrument which is on-board the International Rosetta mission is described here. The final technical solutions adopted for the instrument are studied for the first time with LabVIEW (© N. I. Corp.) which allows the global simulation of systems containing both analog and digital components. The signal is sampled at 1 GHz, which is sufficient to simulate the analog signal with the filters, amplifiers and modulators used. These modelings are essential in order to reach the complete description of the built system and they are very helpful to understand the test results during the instrument development. The study of non-linearity in the system which-can affect the instrument performances is discussed.

Recent studies of the ion energy balance in the mid-latitude ionosphere have led to the suggestion that hot neutral atomic oxygen may play a significant role; the presence of a population of hot O could explain some of the problems met in balancing the ion energy budget for Incoherent Scatter (IS) observations. The aim of the present study is to look at such effects by using numerical simulation. The TRANSCAR model is a time-dependent, 13-moment ionosphere model developed for high latitude studies. It was first adapted for mid-latitude conditions. In a first step the model was calibrated and cross-checked with St. Santin IS measurements for the winter case of 27 January 1972 around noon using, in particular, the MSIS neutral atmosphere model. This provides a reference diurnal variation of the ionosphere. The second step investigated the influence of a maxwellian population of hot neutral atomic oxygen introduced in addition to the standard neutral atmosphere. The paper describes the initial comparison between the model and St. Santin IS data, and then the effects induced by a hot atomic oxygen population.Key words. Ionosphere (ionosphere-atmosphere interactions; ion chemistry and composition; mid-latitude ionosphere)

With the regular access of landers to the surface at each launch window during the next decade, one of the major objectives of the exploration of Mars will be to unravel the many unknown in the geological and hydrological history of the planet. Among the presently planned missions, the NETLANDER project offers a unique opportunity to explore simultaneously the deep interior of the planet and the first layers of the subsurface in 4 landing sites displaying different geophysical morphologies. To take advantage of this opportunity we have proposed to fly on the 4 landers a ground-penetrating radar (GPR); this experiment aims at characterizing the geological structures in the vicinity of the landers and at detecting possible water reservoirs under the form of ground ice or even, at some depth, of liquid water. The GPR will operate at a frequency of ∼2 MHz and will include an original assembly of 3 electric and 3 magnetic antennas as receivers. This design will enable to determine not only the distance at which the signal has been reflected but also its direction and thus will provide a 3D imaging of the uppermost layers of the subsurface down to a depth of ∼2.5 km . A model of the electromagnetic properties of the anticipated layering of the Martian megaregolith including a ground ice layer was built and used to predict through a 3D electromagnetic numerical simulation the performances of the GPR. After a review of the scientific objectives of the experiment, this paper presents a technical description of the instrument and display some of the results of the numerical simulation. The GPR will provide as a by-product a sounding of the ionosphere at the fixed operating frequency and will also be used to measure the radioelectric background expected from possible electrical discharges in the atmosphere during dust storms.

We present the different aspects of the development of an HF GPR which will be devoted to the characterization of geological structures and the detection of possible water reservoirs under the form of ground ice or even of liquid water on Mars.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The field-aligned neutral oscillations in the F-region (altitudes between 165 and 275 km) were compared using data obtained simultaneously with two independent instruments: the European Incoherent Scatter (EISCAT) UHF radar and a scanning Fabry-Perot interferometer (FPI). During the night of February 8, 1997, simultaneous observations with these instruments were conducted at Tromsø, Norway. Theoretically, the field-aligned neutral wind velocity can be obtained from the field-aligned ion velocity and by diffusion and ambipolar diffusion velocities. We thus derived field-aligned neutral wind velocities from the plasma velocities in EISCAT radar data. They were compared with those observed with the FPI (λ=630.0 nm), which are assumed to be weighted height averages of the actual neutral wind. The weighting function is the normalized height dependent emission rate. We used two model weighting functions to derive the neutral wind from EISCAT data. One was that the neutral wind velocity observed with the FPI is velocity integrated over the entire emission layer and multiplied by the theoretical normalized emission rate. The other was that the neutral wind velocity observed with the FPI corresponds to the velocity only around an altitude where the emission rate has a peak. Differences between the two methods were identified, but not completely clarified. However, the neutral wind velocities from both instruments had peak-to-peak correspondences at oscillation periods of about 10–40 min, shorter than that for the momentum transfer from ions to neutrals, but longer than from neutrals to ions. The synchronizing motions in the neutral wind velocities suggest that the momentum transfer from neutrals to ions was thought to be dominant for the observed field-aligned oscillations rather than the transfer from ions to neutrals. It is concluded that during the observation, the plasma oscillations observed with the EISCAT radar at different altitudes in the F-region are thought to be due to the motion of neutrals.Key words: Ionosphere (Ionosphere–atmosphere interactions) – Meteorology and atmospheric dynamics (thermospheric dynamics; waves and tides)

In this paper, we describe the CONSERT experiment for the international ROSETTA mission (Verdant and Schwehm, 1998). CONSERT aims at obtaining radio images of the nucleus of comet 46P/Wirtanen by transmitting radio waves through the comet nucleus, between the Lander and the Orbiter (Kofman et al., 1998). We describe the principle of the instrument, its main parameters, its modes of operation, as well as technical implications and trade-off. The compatibility of the science requirements and the technical solutions is shown.

The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downshifted plasma lines of the incoherent-scatter plasma lines at different wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for low-frequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat flow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed.Key words. Non-Maxwellian electron velocity distribution · Incoherent scatter plasma lines · EISCAT · Dielectric response function

International audience; Several authors have published models of auroral conductances in the past. Some of them are based on theoretical approaches; others are the result of observations. The data bases for the latter models range from 8 h to 3 years of experiments. In this paper, we show the results of our own modeling, based on a coupled ki- netic/fluid approach. We compare these results to the sta- tistical model based on 3 years of experiment. We then model the auroral ionospheric conductances above EISCAT and the EISCAT-Svalbard Radar for different solar-activity indices. These modeled conductances are fitted with a simple law depending on the f10.7 index and on the solar zenith angle.

[1] To prepare the Comet Nucleus Sounding Experiment using Radio wave Transmission during the Rosetta mission, we study the electromagnetic wave propagation through a comet nucleus model and tomographic inversion in a two-dimensional setting. For the propagation, the Ray Tracing Method (RTM) is validated with respect to the Pseudo-Spectral Time Domain (PSTD) method. For the inverse problem, a Tikhonov-like inverse RTM method based on weak permittivity assumptions is used, with synthetic data derived from the PSTD algorithm. Reconstruction results show that the Consert data will permit a reliable tomography of the comet nucleus. Surface data will enhance the quality of the imaging.

We present results of the first plasma-line measurement of the incoherent spectrum using the alternating-code technique with the EISCAT VHF radar. This technique, which has earlier mostly been used to measure high-resolution E-region ion-line spectra, turned out to be a very good alternative to other techniques for plasma-line measurements. The experiment provides simultaneous measurement of the ion line and downshifted and upshifted plasma-line spectra with an altitude resolution of 3 km and a temporal resolution of 10 s. The measurements are taken around the peak of the F region, but not necessarily at the peak itself, as is the case with the long-pulse technique. The condition for success is that the scale height should be large enough such that the backscattered signal from the range extent of one gate falls inside the receiver filter. The data are analysed and the results are combined with the results of the ion-line data analysis to estimate electron mean drift velocity and thereafter electric currents along the line of sight of the radar using both the standard dispersion relation assuming a Maxwellian electron velocity distribution and the more recent model including a heat-flow correction term.

Measurements of the phase velocity of ionospheric plasma waves using the STARE radar system are compared with the electron drift velocity and the ion acoustic velocity calculated from data measured by EISCAT. This comparison shows that for those flow angles that are smaller than the largest flow angle (θs) for which the component of the electron drift velocity along the radar line of sight equals the ion acoustic velocity, the phase velocity observed by STARE tends to be limited by, or lower than, the ion acoustic velocity. Using observations with flow angles larger than θs, one can infer the electron drift from STARE measurements by the well-known cosine relation.

Sondrestrom observations show that a characteristic F‐region signature of the interaction between the magnetosphere and ionosphere is a narrow band of elevated electron temperatures. Its location is associated with the ion convection reversal in the morning and evening convection cells. Typically, near 500 km altitude, the temperature is 3500 to 4000 K. However, on 24 April 1983, a geomagnetically very active day with Kp values of 6‐ during the period of interest, the electron temperature reached 6000 K in the afternoon convection reversal. The ion velocities were between 1 and 2 km/s on both sides of the reversal. There was considerable soft particle precipitation and a large downward heat flux of 0.3 erg/cm²‐s at 450 km. These high temperature electrons then transferred 0.5 erg/cm²‐s to ions and neutrals between 175 and 550 km, which represents a very sizeable perturbation to the thermosphere.

The plasma oscillations that can be observed by the French incoherent scatter system have small phase velocities and are excited by low energy photoelectrons, typically 2–5 eV. Consequently, the method used to determine the energy photoelectron distribution from plasma line measurements made at other observatories (e.g. Cicerone, 1974) cannot be applied here: it is necessary to chose a model energy distribution with a small number of parameters. The energy shape of the flux is assumed Maxwellian and the angular shape is assumed linear with the cosine of the pitch angle. Total flux values and mean energies are obtained as a function of altitude, in agreement with other determinations, and the difference between upshifted and downshifted plasma line intensities lead to the determination of the anisotropy of the photoelectron flux.

A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project to record the first D -region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods of enhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 × 10 10 m −3 in the upper D -region. The measured meridional winds were lower than 10 m s −1 . A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined to be as low as 130 K. This detailed analysis of the spectral width profile indicates that below about 77–80 km the ratio of negative ions to electrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations.

In the F 1-region, temperature and ion composition measurements at EISCAT are usually obtained with a transmitted pulse of 360 μs, corresponding to a diffusive volume much larger than the ionospheric scale height. A special correlator program has been used in order to estimate the errors due to the smearing effect of the diffusive volume. We show that these errors can be as large as 50% in electron temperature and 20% in ion temperature and that they depend on electron density gradients in the diffusive volume. From ion composition results, we deduce that the transition altitude between molecular ions and oxygen ions is correctly estimated when using a 360 μs pulse, and that a diffusive volume of the order of ± 25 km would be good for obtaining correct composition values above 200 km altitude.

In this paper we describe measurements by the incoherent scatter technique in the low thermosphere and compare the results to models. We find that the observed neutral temperature in winter at 100 km is lower by about 15K than predicted by the MSIS model and closer to the temperature predicted by the Jacchia 71 model. We also show that the neutral mass, which we inferred from the temperature and scale height, is about 26–28 a.m.u. and that this value is also lower than that used in the MSIS model. Finally, we show data for a very disturbed day, with very strong anomalous heating, for which the assumption of thermal equilibrium between electrons and ions is not correct.

This work deals with experimental results showing practical methods of estimation of the slope and time of arrival of FM(Pc1)-type signals. The sensitivity of measurements to phase coherency is emphasized showing the usefulness of generalized coherency in those kinds of measurements.

The most striking differences of the results obtained by initial observations of heater-induced plasma line enhancements with the European Incoherent Scatter Radar (EISCAT) facilities, by comparison with those from Arecibo, are in their far more pronounced overshoot at the time of the heater turn-on, together with the fashion in which the higher-order spectral features show a clear line structure. It is also noted that the EISCAT spectra show a distinct signature of the 'cascade' modes at odd numbers of ion acoustic frequency offsets from the HF offset, and on the center frequency side of this offset. 7 references.

The plasma lines observed by the French incoherent scatter radar during the period 1973–1974 are studied. Two methods are used to determine the steady-state photoelectron flux from plasma line measurements; one using a Maxwellian model for the photoelectron distribution and the other by solving (numerically) the differential equation that is satisfied by the distribution. The direct numerical calculation of the photoelectron flux is used to obtain theoretical kT p values which are compared with those from the plasma line observation. The comparison leads to the conclusion that there must be a sharp increase of the photoelectron flux when the energy decreases below 4 ∼ 5 eV. This result, in agreement with rocket and satellite measurements of the low energy photoelectron flux, is used to bear a new insight to the problem of the electron-gas heat balance: the problem is reduced to the need of an additional photoelectron flux production below 5 eV.

A method for obtaining O(+) relative abundance directly from the autocorrelation functions measured by EISCAT is presented no assumption is made regarding the temperatures. Results of O(+) relative abundance and temperatures obtained simultaneously are shown for a 20-hour period on May 9, 1982, and the time variations of O(+) relative abundance are compared to previous work. 16 references.

The electron density, ion temperature, and velocity measured on November 18, 1981, from 1100 to 1700 UT by EISCAT facilities exhibited clearly periodic behavior. An attempt to interpret this feature as a manifestation of gravity waves is presented. November 18 was a day of high magnetic activity, and it was difficult to separate completely the local effects from those induced by the gravity waves. The observed waves had a period of about 75-80 min and vertical wavelength of the order of 800 km. They were possibly generated in the auroral oval, about 1000 km from Tromso. 14 references.

On etudie un systeme adaptatif non boucle permettant d’estimer en temps reel un signal perturbe additivement par un bruit. Le bruit perturbateur du signal est une transformation lineaire du bruit sourcedont revolution temporelle est accessible. La transformation n’etant pas connue, il est necessaire de l’estimer pour pouvoir obtenir le bruit perturbateur a partir du bruit source,puis le signal. Les resultats de l’etude theorique des caracteristiques de ce systeme sont compares aux resultats d’experiences sur un correlofiltre realisees en laboratoire. On observe une bonne concordance entre la theorie et la pratique. Enfin, une discussion des limites theoriques du systeme adaptables a des processus qui peuvent etre consideres comme stationnaires sur un temps fini et de sa complexite en fonction des performances desirees est presentee.

Many theoretical studies using different collision models have been devoted to calculating ion velocity distributions in the high latitude ionosphere. In the presence of strong convection electric fields, the ions can be accelerated to drift speeds which exceed the thermal speed of the neutral gas leading to velocity distribution functions distorted from the Maxwellian shape. With EISCAT we are able to measure three independent components of the ion velocity vector and temperature in the same scattering volume. The ion temperature observed at about 300 km during strong enhancements of the ion velocity is characterized by different temperatures parallel and perpendicular to the magnetic field with T⊥ > T⫽. For an electric strength of 50 mV/m, an anisotropy factor of about 1.5 was measured.

An extensive series of spectral measurements has been made in the auroral E region with the Chatanika incoherent scatter radar. Becasue of the small scale length for variations of electron density, temperatures, and ion-neutral collisions we used the operating mode with the best possible range resolution—9 km. About 5% of the time the data exhibited an unusual spectral shape that was most pronounced at 105 and 110 km. Instead of being almost Gaussian with only a small hint of two peaks, the spectra are much wider, with two well-developed peaks. After carefully considering the validity of the measurements and their interpretation, we conclude that the unusual spectra are due to greatly enhanced electron temperatures. At 110 km, the electron temperature may increase from 250 K to 800 K, while the ion temperature remains near 250 K. This enhancement of the electron temperature extends from 99 km to at least 116 km. We show that the temperature increase is too large to be accounted for by auroral particle precipitation, though it coincides in time with ion temperature enhancements at altitudes above 125 km. Because these latter enhancements are believed to be due to joule heating, we deduce that electric fields of 24–40 mV/m are present and that the electrons are moving through the ions and neutrals at speeds of 500–800 m/s. Despite these velocities, we find that joule heating of the electrons also cannot account for the elevated electron temperatures. Several consequences of the elevated electron temperatures are discussed. One is that the rate constants for molecular recombination are reduced. Another is that during periods of significant joule heating, the deduced electron density profile, when fully corrected for temperatures, has a significantly lower peak altitude and greater density than that deduced under the usual assumption of equal electron and ion temperatures. Since conductivities, currents, ionization rates, and differential energy spectra are dependent upon the density profile, care must be taken to account properly for the temperature effects when deriving these quantities.

The incoherent scatter radar gives the possibility to study the ionospheric parameters of E and F-regions, at the same time, from the altitude of 90 km up to 1000 km with a good time and spatial resolution. In this paper, we describe the measurements technique and data analysis strategy. We continue by discussing the main energy sources and their influence on the auroral ionosphere. We show the average behavior of the low thermosphere studied with large amount of data. On some examples we describe the answer of the auroral ionosphere to the energy input, electric field and particle precipitations. The modification of the ion composition due to the energy deposition is illustrated and discussed. We study some unusual events due to very large electric fields. The observations of very large ion temperatures rising up to 10.000 K are explained. During the same period, we observed very large electron temperatures occurring between 100 and 120 km. These phenomena are due to a non-linear wave heating. The effect of non-Maxwellian ion velocity distribution, due to large electric field on measurements and on data interpretation, is discussed. Finally, we discuss the new trends in the incoherent scatter technique.

Incoherent scatter spectra for a plasma with a negative ion component are calculated successively by the kinetic and hydrodynamic approaches. It is shown that mesospheric negative ions enhance significantly the scattered power and the width of the ion component of the incoherent scatter spectrum. A simplified method is proposed for determining the mesospheric parameters from the measured incoherent scatter spectrum.

We investigate the influence of magnetic mirroring and elastic and inelastic scattering on the angular redistribution in a proton/hydrogen beam by using a transport code in comparison with observations. H-emission Doppler profiles viewed in the magnetic zenith exhibit a red-shifted component which is indicative of upward fluxes. In order to determine the origin of this red shift, we evaluate the influence of two angular redistribution sources which are included in our proton/hydrogen transport model. Even though it generates an upward flux, the redistribution due to magnetic mirroring effect is not sufficient to explain the red shift. On the other hand, the collisional angular scattering induces a much more significant red shift in the lower atmosphere. The red shift due to collisions is produced by Key words. Auroral ionosphere · Particle precipitation

This paper describes the LMS adaptive filter which compensates for the effect of a linear filter on a signal. The problem that we had to solve was to compensate a propagation channel with an adaptive filter, which has to be connected in series and in front of the channel. The adaptive filter estimates simultaneously the filter response and its inverse. We show the computer simulations of this system applied to the multipath transmission channel.

Les observations de diffusion Thomson UHF sur des ondes de Langmuir entrainant une modulation lineaire de frequence d'un emetteur UHF et une demodulation du signal recu montrent que les emissions HF puissantes utilisees dans la modification du plasma ionospherique cree instantanement des petites lacunes de plasma. Pour une energie de 8•10 -5 W/m 2 les lacunes sont de l'ordre de 3% a 55%

In this preliminary report on low-energy (0.3 to 3 eV) secondary electrons in the auroral E layer (90 to 150 km), we compare intensities of plasma lines observed with the Chatanika radar to theoretical predictions obtained from a detailed numerical model. The model calculations are initiated with a flux of energetic auroral primary electrons which enter the atmosphere and lose energy to electrons, ions, and neutrals through a combination of elastic and inelastic collisions. This flux is chosen in order that the total calculated ionization rate matches one that is deduced from the radar measurements. From these same calculations the steady state secondary electron flux is deduced as a function of altitude, energy, and pitch angle. This flux is used to calculate plasma line intensities which are then compared with observed intensities. Initial comparisons suggest that the plasma line theory, when applied to low altitudes, must include the effect of electron-neutral collisions. When this is done, the good agreement obtained between theory and experiment indicates the promise of this approach for the study of low-energy auroral electrons.

In the spring and fall of 1978 we made an extensive series of plasma line and correlative observations with the Chatanika incoherent scatter radar. To make these measurements, we greatly modified the radar receiving system. In addition to enlarging the plasma line filter bank the most significant change was the incorporation of a high-speed correlator provided by the French. This was the first use of a correlator in a monostatic radar to obtain the intensity spectra of naturally occurring plasma lines. In this paper we develop the signal-processing theory that we use to obtain the plasma line intensities from these measurements; we also show that these intensities compare well with those obtained from the filter bank. To show the richness of the phenomena and to explore the capabilities of the correlator, we examine a wide variety of spectra that have been enhanced by secondary electrons in the auroral E layer. From the other simultaneous measurements we are able to relate these spectra and their variations to the auroral situation. We also obtained the first measurements in the auroral region of photoelectron-excited plasma lines in the E and F layers. Perhaps most significant, in the plasma line spectra we detected a Doppler shift that we then used to determine the Birkeland current carried by ambient electrons. Although there is a large estimated uncertainty for this first determination, we obtained a downward Birkeland current of 10 μA/m² in the diffuse aurora in what is, most likely, the equatorward portion of the evening sector auroral oval.

International audience; The Consert instrument (Bistatic radio Sounder) is flying onboard the ESA-Rosetta mission to wards Comet 67P/Churyumov-Gerasimenko. This paper described the generation of simulated signals as resulting from the interaction of the electromagnetic waves transmetted from Consert, travelling through a modeled comet nucleus, and reaching the Consert Receiver.

This work aims at processing the data of CP1 and CP2 programs of EISCAT ionospheric radar from 1987 to 1994 using the "full profile" method which allows to solve the "temperature-composition" ambiguity problem in the lower F region. The program of data analysis was developed in the CEPHAG in 1995–1996. To improve this program, we implemented another analytical function to model the ion composition profile. This new function better reflects the real profile of the composition. Secondly, we chose the best method to select the initial conditions for the "full profile" procedure. A statistical analysis of the results was made to obtain the averages of various parameters: electron concentration and temperature, ion temperature, composition and bulk velocity. The aim is to obtain models of the parameter behaviour defining the ion composition profiles : z50 (transition altitude between atomic and molecular ions) and dz (width of the profile), for various seasons and for high and low solar activities. These models are then compared to other models. To explain the principal features of parameters z50 and dz, we made an analysis of the processes leading to composition changes and related them to production and electron density profile. A new experimental model of ion composition is now available.Key words. Auroral ionosphere · Ion chemistry and composition · Instruments and techniques · EISCAT

EGU2007-A-09791

We investigate the inverse problem of imaging the internal structure of comet 67P/Churyumov- Gerasimenko from radiotomography CONSERT data (radiowave transmitted back and forth from the Rosetta orbiter to the Philae lander through the comet) by using an approach based on the discretization of the interior dielectric permittivity and of the shape of the comet, as well as the interior / exterior electric field, along suitable basis of spectral functions (Zernike and Hankel functions), in the frame of the Helmholtz equation.

International audience; The MARSIS radar sounder on Mars Express completed a comprehensive mapping campaign of the south polar region. The radar signal typically penetrates to the base of the layered deposits. Terrain outside of the classic layered deposits also exhibits layering in the radar data.

International audience; The aim of the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) is the characterization of the inner structure and electrical properties of the Chuyurmov-Gerasimenko's nucleus. The instrument will sound the comet's nucleus between the lander Philae at the comet's surface and the Rosetta main spacecraft. A coarse three-dimensional model of the complex dielectric permittivity inside the nucleus will be reconstructed from the whole set of data obtained during the first science phase [3]. The work presented here show how a limited set of data acquired at grazing angles during a single low altitude fly-by can be used to characterize the shallow sub-surface of the nucleus. The study is based on simulated data obtained by two different electromagnetic models: the accurate pseudo spectral time-domain method and a much faster ray-based approximation taking into account material and path-loss.

International audience; Not Available