Expérience IAP – Instrument Analyseur Plasma

IAP Experiment – Plasma Analyser Instrument

1- Scientific Objectives.

The objective of the IAP experiment is to characterize the state of the ionospheric plasma, in order to detect perturbations that may be associated with seismic activity and to provide the plasma parameters that are needed to analyze the data from the plasma wave instruments, ICE and IMSC. As a secondary objective, near real time ionospheric data can be provided for space weather purposes.

Over the last two decades, a number of publications have dealt with the detection of ionospheric perturbations associated with the Earth's seismic activity, most of them related to effects following earthquakes, but a few of them describing also pre-seismic events. Initial studies used ground-based measurements such as ionospheric sounders or incoherent radars. More recently, satellite experiments have brought a rapidly growing number of interesting observations, giving in particular access to Total Electron Content (TEC) measurements deduced from the analysis of GPS data or from beacon satellite transmitters. In many cases, however, not only the analysis of the phenomena but even their characterization have proved to be difficult due to the weakness of the effects, the relative paucity of observations and hence their subsequent lack of statistics and the difficulty of removing natural signals that can mix with, and indeed sometimes mask, those induced by seismic activity.

Dedicated in-situ measurements of the ionospheric plasma, with nearly permanent observations along the orbit, and correlated TEC measurements on dense networks of stations will very significantly add to the earlier observations. They will improve the description of the phenomena and allow a better understanding of the underlying physical mechanisms through data assimilation in ionospheric models.

The aim of the IAP experiment is thus to measure the ion composition, density, temperature and bulk velocity with a time resolution up to ~ 360 ms and the ion density fluctuations up to ~ 160 Hz.

2- Description of the experiment.

Following the concept proposed by W.B. Hanson [Hanson, W.B. and R.A. Heelis, Techniques for measuring bulk gas motions from satellites, Space Sci. Instrum. 1, 493-524, 1975], the IAP experiment makes use of two analyzers: APR (for Analyseur à Potentiel Retardateur, Retarding Potential Analyzer) performs the energy analysis of the rammed ionospheric ions and ADV (for Analyseur pour Direction de Vitesse, Velocity Direction Analyzer) determines the average angles of arrival of the ions. APR measurements allow to determine the density, temperature and bulk energy of the ionospheric ions. Along the ~ 700 kilometer altitude circular orbit of DEMETER, the ion species of interest are H⁺, He⁺ and O⁺ and, in periods of magnetic storms and at high enough latitudes, the molecular ion group N₂⁺, NO⁺, O₂⁺. ADV provides the direction of the ram velocity vector. Schematic diagrams showing the principle of operation of the two analyzers are given in figures 1 and 2.

2.1- APR analyzer.

Ions travel through the APR analyzer through several high transparency grids. The first two grids, G1 and G2, at ground potential, shield the plasma from the potentials applied to the other grids inside the analyzer, thus preventing any disturbance of the ion trajectories before they enter the instrument. The double grid G3 + G4 is used to perform the energy analysis of the incoming ions through the applied retarding potential VGR. VGR is swept between a lower and an upper limit which lie in the range 0, +22V. The values of these lower and upper sweep voltages depend on the IAP specific mode of operation and are controlled by telecommand

Grids G5 and G6 are polarized at fixed potentials of 0V and –12V respectively. The negative voltage on G6 prevents photo-electrons or auroral electrons with energy lower than 12 eV to reach the collector. It also repels photo-electrons emitted by the collector when submitted to UV fluxes from the Sun, or fainter sources such as the geo-coronna or the auroral neutral atmosphere, which, otherwise, would represent a source of parasitic currents. In addition, G5 and G6, which are maintained at fixed voltages, act as an electrostatic shield to suppress the capacitive coupling between the double grid G3 and G4 and the collector, preventing strong disturbances on the current measured by the collector due to the sweep in potential of G3 + G4.

2.2- ADV analyzer.

Figure 2a displays a schematic view of the principle of operation of the ADV analyzer.

The first two grids G1, at ground potential, and G2, at a fixed potential V2, shield the external plasma from the potentials applied on the various grids internal to the analyzer. The potential V2 is controlled through telecommand and can be given 2 values, 0V or +2V. In the first case all ions entering the instrument can get access to the collector. In the second case H⁺ ions are repelled due to their low ram energy and ADV essentially measures the direction of the bulk velocity of the. major heavy ions O⁺. As explained below, this allows to measure the ion bulk velocity direction with a better accuracy. Grids G3 and G4 act as a diaphragm, the dimension of which controls the transfer function of the analyzer. Grids G5 and G6, at ground potential, shield ions admitted through the diaphragm from the influence of the –12V voltage applied to G7. This last grid, which plays the same role as grid G6 in APR, prevent external electrons with energy less than 12 eV from reaching the collector and also repel the photo-electrons created on the collector by UV illumination.

The ADV collector is divided in 4 insulated quadrants, as shown in figure 2a. These 4 quadrants can be associated in pairs by means of high insulation switches in order to define two identical half collectors symmetrical either with respect to the X axis, when the associated quadrants are C1=A+B and C2=C+D, or with respect to the Y axis, when the associated quadrants are C1=A+C and C2=B+D. Simple analytical calculations show that the ratio I₁/I₂ of the ion currents falling on C1 and C2 in the first (respectively second) configuration is a function of the angle of the ion ram velocity with respect to the XZ (respectively YZ) plane. To reach the desired accuracy for the APR transfer function relating current ratios to velocity angles, a numerical computation of the ion collection by C1 and C2, has been performed using a detailed geometrical model of the instrument. Using this calculated transfer function and the measured ratio I₁/I₂, it is therefore possible to determine the two angles that define the direction of the ram velocity. In addition to measuring the I₁/I₂ current ratio, the instrument electronics also allows to measure directly the current I₁ collected by C1. Combining this latter measurement and the measured current ratio allows to determine the total ion current falling on the ADV collector and its temporal variations from which one can get access to the density variations along the orbit with an excellent temporal/spatial resolution of 6.4 ms in burst modes. Due to their higher Mach number, heavy ions such as O⁺ are more focused at the entrance of the instrument than H⁺ ions and the corresponding ratio I₁/I₂ displays a sharper variation as a function of the angle of the ion bulk velocity with respect to the normal to the instrument. ADV measurements are thus more accurate for heavy ions only than when H⁺ is present. This is the reason to prevent H⁺ ions to reach the collector when one has to measure very the small plasma velocities that are expected at mid latitudes.

2.1- Electronics.

Both analyzers include their respective front ends electronics, consisting of a single 4 decade logarithmic amplifier (10^-11 A to 10^-7 A) for APR and two 2 decade logarithmic amplifiers (10^-10 A to 10^-8 A) followed by a difference amplifier for ADV with also the necessary set of high insulation switches. In order to correct for possible variable offsets of the ADV logarithmic amplifiers, the I1 and I2 inputs are periodically interchanged. The noise level of the APR current measurements is about 10-¹³ A and the relative error on the I1/I2 ration of about 0.1%.

The rest of the electronics, which includes the swept voltage VGR generator, the ADC's and the FPGA used to command and control the operation of the instrument and interface the instrument with the scientific payload DPU, is integrated on a circuit board housed in the BANT module.

APR data are digitalized on 16 bits, ADV current ratio data on 12 bits and the C1 current from ADV on 16 bits.

2.4- Modes of operation.

The various modes of operation of IAP differ by the numbers of steps to sweep the VGR potential and the duration of each step. Sub-modes of APR differ by the minimum and maximum values of the VGR sweep. The single APR output and the two ADV outputs are sampled at each step of the VGR voltage.

IAP has 2 modes of operation in the DEMETER Survey mode, respectively labeled Survey 1 and Survey 2. Survey 1 corresponds to a medium energy resolution mode for APR with 28 VGR steps 12.8 ms long providing a complete set of plasma parameters every ~ 360 ms. Survey 2, with 56 VGR steps 12.8 ms long, corresponds to a high energy resolution mode with a lower temporal resolution providing a complete set of measurements every 720 ms. Density fluctuations from ADV measurements are available with a time resolution of 12.8 ms.

There is only one mode of operation in the DEMETER Burst mode, featuring high energy and high temporal resolution with 56, 6.4 ms long, VGR steps providing a complete set of plasma parameters every ~ 360 ms. Density fluctuations are available with a time resolution of 6.4 ms.