Mutual Impedance Probe Research Articles

A new approach for estimating Titan's electron conductivity based on data from relaxation probe sensors on the Huygens experiment

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.

Structure of Titan's low altitude ionized layer from the Relaxation Probe onboard HUYGENS

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.

Electron conductivity and density profiles derived from the mutual impedance probe measurements performed during the descent of Huygens through the atmosphere of Titan

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.

RPC: The Rosetta Plasma Consortium

The Rosetta Plasma Consortium (RPC) will make in-situ measurements of the plasma environment of comet 67P/Churyumov-Gerasimenko. The consortium will provide the complementary data sets necessary for an understanding of the plasma processes in the inner coma, and the structure and evolution of the coma with the increasing cometary activity. Five sensors have been selected to achieve this: the Ion and Electron Sensor (IES), the Ion Composition Analyser (ICA), the Langmuir Probe (LAP), the Mutual Impedance Probe (MIP) and the Magnetometer (MAG). The sensors interface to the spacecraft through the Plasma Interface Unit (PIU). The consortium approach allows for scientific, technical and operational coordination, and makes optimum use of the available mass and power resources.

RPC-MIP: the Mutual Impedance Probe of the Rosetta Plasma Consortium

The main objective of the Mutual Impedance Probe (MIP), part of the Rosetta Plasma Consortium (RPC), is to measure the electron density and temperature of Comet 67P/Churyumov-Gerasimenko’s coma, in particular inside the contact surface. Furthermore, MIP will determine the bulk velocity of the ionised outflowing atmosphere, define the spectral distribution of natural plasma waves, and monitor dust and gas activities around the nucleus. The MIP instrumentation consists of an electronics board for signal processing in the 7 kHz to 3.5 MHz range and a sensor unit of two receiving and two transmitting electrodes mounted on a 1-m long bar. In addition, the Langmuir probe of the RPC/LAP instrument that is at about 4 m from the MIP sensor can be used as a transmitter (in place of the MIP ones) and MIP as a receiver in order to have access to the density and temperature of plasmas at higher Debye lengths than those for which the MIP is originally designed.

Evaluation of a mutual impedance probe to search for water ice in the Martian shallow subsoil

This work analyzes the performance of a mutual impedance probe used to measure the complex permittivity of a soil, and detect the presence of water ice buried in the subsurface. The sensor consists of four electrodes deployed above the ground, equally spaced along a straight line array. The quadrupole accuracy has been evaluated through a suitable error analysis which considered the errors associated with the electronics used to measure the mutual impedance of the system, and the position uncertainties of the electrode array. It has been found that the quadrupole accuracy is mainly limited by the electrode positioning. In particular, the electrode vertical misplacements perturb the measurements much more than the horizontal ones. As a case study, a two-layers soil with the deeper stratum containing 20% of water ice at 190 K, which could represent a realistic Martian soil scenario, has been analyzed. It was found that the ice detection requires an electrode vertical position accuracy better than 0.001 times the interelectrode distance. This requirement is far too difficult to be achieved in a fully automated rover mission. Such a conclusion poses significant doubts about the feasibility of projects which foresee the use of electric quadrupoles to detect water ice in the Martian subsoil.

Detection of subsurface ice and water deposits on Mars with a mutual impedance probe

A mutual impedance probe, also called quadrupolar probe or permittivitymeter, measures the complex permittivity of materials with a spatial resolution comparable to the average separation between its four sensors. This instrument is ideally suited for the detection of subsurface water deposits at shallow depths on Mars, since water mixtures are generally characterized by relatively large dielectric constant and conductivity. Permittivitymeters have been developed for commercial and space applications. An instrument identical to that which will land on Titan in 2004 has been tested with success in the field, and the results obtained on humid sand and in dry snow are presented. The possible applications of mutual impedance probes to the localization of water on Mars are discussed.

Rosetta spacecraft influence on the mutual impedance probe frequency response in the long Debye length mode

During the Rosetta flyby of comet 46P/Wirtanen from 2011, plasma observations will be obtained from a number of instruments, the mutual impedance probe (MIP) is one of them. The mutual impedance technique is based on the measurements, as a function of the frequency, of the quasi-electrostatic coupling between an emitter and two receivers separated from the emitted source by at least twice the local plasma Debye length. The electron number density is deduced from the electron plasma frequency at which the transfer function reaches its maximum, and the Debye length, λ D, is deduced from the positions of the minima above the plasma frequency. The long Debye length (LDL) mode, which operates in the 7–168 kHz range, is a secondary mode of MIP that has been designed to probe cometary plasmas when λ D is longer than 70 cm. In that case, the Rosetta spacecraft presence cannot be neglected due to its conductive structures, the dimensions of which are of order of the emitter–receiver distance. A numerical simulation of the LDL mode is then necessary. The discrete surface charge distribution (DSCD) method, which is well adapted to the electric antenna problems in a kinetic plasma, would be suitable for the Rosetta flyby measurements. Here, all the conductive surfaces (spacecraft, solar panels and antennae) are compared with an alternating charge distribution that contributes to the LDL mode transfer function. The preliminary results show that in the early parts of the Rosetta mission, the cometary plasma can reasonably be considered to be Maxwellian, homogeneous, isotropic, collisionless and unmagnetized in the range of λ D from 0.7 to 2.5 m. The numerical results are compared with those obtained by ignoring the spacecraft influence. It appears that the resonance peak at the plasma frequency is sharpest and strongest when the spacecraft influence is considered. Moreover, the antiresonance frequencies which occur on both sides of the plasma frequency depend on the Debye length of the surrounding plasma. Hence, the LDL mode should be able to measure the electron temperature in the range from about 10 4– 2×10 5 K . Plasmas with electron number densities lying between 22 and 180 cm −3 should also be probed. This will allow the LDL mode to distinguish the boundary between the Wirtanen's cometary plasma and the Solar Wind in the prime investigations of the Rosetta mission, and this will complete the measurements made by the principal MIP mode in the non-magnetic cavity of the comet.

Rosetta Mission Mutual Impedance Probe Modelling: The Short and Long Debye Length Plasma Cases

The Rosetta spacecraft (S/C), which is planned to meet comet 46P/Wirtanen in 2011, will carry a set of five wave and plasma instruments (i.e. the Rosetta Plasma Consortium). This is to measure the cometary plasma properties from the minimum value of activity of the comet to its maximum value at perihelion. The mutual impedance probe, MIP, is one of those (Trotignon et al., 1999) five.

Surface and sub-surface electrical measurement of titan with the PWA-HASI experiment on HUYGENS

Abstract 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.

The Rosetta plasma consortium: Technical realization and scientific aims

The Rosetta spacecraft will rendez-vous with comet Wirtanen round 2011 and will study it for a period of nearly two years, thus providing a unique opportunity to monitor its behaviour over a wide range of distances from the Sun. A plasma and wave package, the Rosetta Plasma Consortium, RPC, will be part of the Rosetta orbiter payload. RPC is a highly integrated package that consists of five sensors: the Langmuir Probe, LAP, the Ion and Electron Sensor, IES, the Ion Composition Analyzer, ICA, the Fluxgate Magnetometer, MAG, and the Mutual Impedance Probe, MIP. RPC also includes the common instrument control, spacecraft interface, and power management unit, PIU, plus the Electrical Ground Support Equipment, EGSE. The prime objectives of RPC are to investigate: (1) the physical properties of the cometary nucleus and its surface, (2) the inner coma structure, dynamics, and aeronomy, (3) the development of cometary activity, and the microscopic and macroscopic structure of the solar-wind interaction region as well as the formation and development of the cometary tail. In addition, the planned asteroid, possibly Otawara and Siwa, flybys will provide an excellent opportunity to study in detail the physics of the solar wind-asteroid interaction. The magnetic and electric conductivity properties of the asteroid will also be studied.

Density measurements in key regions of the Earth's magnetosphere: Cusp and auroral region

Two active experiments, a relaxation sounder and a mutual impedance probe, have been implemented on board Viking. These active measurements, together with measurements from the electron spectrometers, are used to determine the plasma density in the cusp region, up to 13,500 km, the apogee of Viking, as well as in the sources of the auroral kilometric radiation (AKR). When active experiments are switched on in the cusp region, several plasma resonances are detected; they correspond to zero group velocity waves, indicative of a characteristic frequency of the plasma, namely the plasma frequency, and the fqn (n = 1, 2, 3,…n) series, above fuh, the upper hybrid frequency. The fqn series allow an independent estimate of the plasma density. Plasma density in the cusp proper is found to be much larger (> 100 cm−3 in some cases) than in the adjacent regions. An attempt is also made to estimate the respective densities and temperatures of the various components of the plasma. In contrast, auroral regions are low‐density regions (fpe/fce ≪ 1). The active sounding of the plasma by a relaxation sounder gives a resonance at fuh, which allows an estimate of the plasma density. Low‐frequency whistler mode emissions are commonly observed in the night sector. Their upper cutoff frequency has often been used for estimating the plasma frequency. Active experiments are used to test this method, which is shown to be valid most of the time in the high‐altitude auroral region, yet also sometimes misleading in other regions. Once the estimate of the density through the upper cutoff frequency of the hiss is validated, it can be used to follow with a good time resolution the sharp density variations experienced as Viking crossed AKR sources. It is found that the density within AKR sources is of the order of even less than 1 cm−3.

Electron plasma density deduced from lower oblique resonance measurements

In a magnetoplasma, the amplitude of the lower oblique resonance in the mutual impedance of two electric antennas is very sensitive to the plasma frequency. This property can be used to measure the electron density in a simple way when the electron temperature and the magnetic field are known, provided also that the probe has been properly calibrated. As an application, densities are deduced from measurements made by a mutual-impedance probe on a rocket in the auroral ionosphere. In the future, better practical design and theoretical modeling of the probe should make calibration unnecessary.

Global characteristics of the cold plasma in the equatorial plasmapause region as deduced from the Geos 1 Mutual Impedance Probe

Thermal plasma parameters derived by the mutual impedance experiment on GEOS are described. The experiment is well suited to the measurement of the electron density and temperature of the outer plasmasphere (when kTe/Ne < 1.6 eV/cm³). This investigation of the whole set of data supplied by GEOS 1 (4 < L < 8, 0000–2400 MLT) covers three main regions : the plasmasphere, an intermediate region of ionospheric refilling, and the plasma trough. In the plasmasphere, we observe profiles with Ne ∝ L−4, while Te stands around 10,000° K or less. The intermediate region, situated next to the plasmasphere and above it, is always present in the day sector, where the ionospheric source plays a leadingpart. In that zone, the plasma parameters, poorly known up to now, exhibit Ne values ∼ 2 to 20 cm−3, together with Te values of 20,000° K on the average, dispersed over a 5,000 to 100,000° K range during disturbances. In the night sector, the intermediate region is seen only during the recovery phase. The region of depleted density is observed at the higher L values in the night and morning MLT sectors. There, plasmas out of Maxwellian equilibrium are seen under disturbed conditions. The dynamic response of the thermal plasma parameters to temporal variations of the am index of magnetic activity follows a known scenario as concerns Ne, making apparent a night‐to‐day, MLT dependent time delay. As concerns Te, the dynamical study reveals striking features, such as the persistance of the Te modifications into the dusk sector, the interpretation of which remains to be clarified.

Continuous measurement of electron density in unmagnetised plasmas by a self-oscillating probe

We show experimentally that it if possible to measure continuously the electron plasma density using a mutual impedance probe working in a self-oscillating mode at the plasma frequency. The comparison of the experimental curves obtained in unmagnetised plasmas either in a classic sweep-frequency mode or in a self-oscillating mode emphasises the excellent accuracy of this new diagnostic method.