The Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) Experiment

The Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment is one of five experiments which together comprise the Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, and a spectrum analyser to calculate in near-real time aboard the spacecraft, the complete auto- and cross-spectral matrices using the three magnetic and two electric components of the electromagnetic field. The magnetic waveform at frequencies below either 10 Hz or 180 Hz is also transmitted. The sensitivity of the search coil is adapted to the phenomena theo be studied: the values 3 × 10^-3 nT Hz^-1/2 and 3 × 10^-5 nT Hz^-1/2 are achieved respectively at 1 Hz and 100 Hz. The dynamic range of the STAFF instruments is about 96 dB in both waveform and spectral power, so as to allow the study of waves near plasma boundaries. Scientific objectives of the STAFF investigations, particularly those requiring four point measurements, are discussed. Methods by which the wave data will be characterised are described with emphasis on those specific to four-point measurements, including the use of the Field Energy Distribution function.     

THE DIGITAL WAVE-PROCESSING EXPERIMENT ON CLUSTER

The wide variety of geophysical plasmas that will be investigated by the Cluster mission contain waves with a frequency range from DC to over 100 kHz with both magnetic and electric components. The characteristic duration of these waves extends from a few milliseconds to minutes and a dynamic range of over 90 dB is desired. All of these factors make it essential that the on-board control system for the Wave-Experiment Consortium (WEC) instruments be flexible so as to make effective use of the limited spacecraft resources of power and telemetry-information bandwidth. The Digital Wave Processing Experiment, (DWP), will be flown on Cluster satellites as a component of the WEC. DWP will coordinate WEC measurements as well as perform particle correlations in order to permit the direct study of wave/particle interactions. The DWP instrument employs a novel architecture based on the use of transputers with parallel processing and re-allocatable tasks to provide a high-reliability system. Members of the DWP team are also providing sophisticated electrical ground support equipment, for use during development and testing by the WEC. This is described further in Pedersen et al. (this issue).     

THE WAVE EXPERIMENT CONSORTIUM (WEC)

In order to get the maximum scientific return from available resources, the wave experimenters on Cluster established the Wave Experiment Consortium (WEC). The WEC's scientific objectives are described, together with its capability to achieve them in the course of the mission. The five experiments and the interfaces between them are shown in a general block diagram (Figure 1). WEC has organised technical coordination for experiment pre-delivery tests and spacecraft integration, and has also established associated working groups for data analysis and operations in orbit. All science operations aspects of WEC have been worked out in meetings with wide participation of investigators from the five WEC teams.     

The FAST Spacecraft Instrument Data Processing Unit

The Fast Auroral Snapshot Explorer (FAST) is the second of the Small Explorer Missions which are designed to provide low cost space flight opportunities to the scientific community. FAST performs high time resolution measurements of the auroral zone in order to resolve the microphysics of the auroral acceleration region. Its primary science objectives necessitate high data volume, real-time command capability, and control of science data collection on suborbital time scales. The large number of instruments requires a sophisticated Instrument Data Processing Unit (IDPU) to organize the data into the 1 Gbit solid state memory. The large data volume produced by the instruments requires a flexible memory capable of both high data rate snapshots (12 Mbit s^–1) and coarser survey data collection (0.5 Mbit s^–1) to place the high rate data in context. In order to optimize the science, onboard triggering algorithms select the snapshots based upon data quality. This paper presents a detailed discussion of the hardware and software design of the FAST IDPU, describing the innovative design that has been essential to the FAST mission's success.     

THE ELECTRIC FIELD AND WAVE EXPERIMENT FOR THE CLUSTER MISSION

The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s^-1. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes up to 700 mV m^-1 with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.     

The Polar plasma wave instrument

The Plasma Wave Instrument on the Polar spacecraft is designed to provide measurements of plasma waves in the Earth's polar regions over the frequency range from 0.1 Hz to 800 kHz. Three orthogonal electric dipole antennas are used to detect electric fields, two in the spin plane and one aligned along the spacecraft spin axis. A magnetic loop antenna and a triaxial magnetic search coil antenna are used to detect magnetic fields. Signals from these antennas are processed by five receiver systems: a wideband receiver, a high-frequency waveform receiver, a low-frequency waveform receiver, two multichannel analyzers; and a pair of sweep frequency receivers. Compared to previous plasma wave instruments, the Polar plasma wave instrument has several new capabilities. These include (1) an expanded frequency range to improve coverage of both low- and high-frequency wave phenomena, (2) the ability to simultaneously capture signals from six orthogonal electric and magnetic field sensors, and (3) a digital wideband receiver with up to 8-bit resolution and sample rates as high as 249k samples s^–1.     

The WIND magnetic field investigation

The magnetic field experiment on WIND will provide data for studies of a broad range of scales of structures and fluctuation characteristics of the interplanetary magnetic field throughout the mission, and, where appropriate, relate them to the statics and dynamics of the magnetosphere. The basic instrument of the Magnetic Field Investigation (MFI) is a boom-mounted dual triaxial fluxgate magnetometer and associated electronics. The dual configuration provides redundancy and also permits accurate removal of the dipolar portion of the spacecraft magnetic field. The instrument provides (1) near real-time data at nominally one vector per 92 s as key parameter data for broad dissemination, (2) rapid data at 10.9 vectors s^–1 for standard analysis, and (3) occasionally, snapshot (SS) memory data and Fast Fourier Transform data (FFT), both based on 44 vectors s^–1. These measurements will be precise (0.025%), accurate, ultra-sensitive (0.008 nT/step quantization), and where the sensor noise level is <0.006 nT r.m.s. for 0–10 Hz. The digital processing unit utilizes a 12-bit microprocessor controlled analogue-to-digital converter. The instrument features a very wide dynamic range of measurement capability, from ±4 nT up to ±65 536 nT per axis in eight discrete ranges. (The upper range permits complete testing in the Earth's field.) In the FTT mode power spectral density elements are transmitted to the ground as fast as once every 23 s (high rate), and 2.7 min of SS memory time series data, triggered automatically by pre-set command, requires typically about 5.1 hours for transmission. Standard data products are expected to be the following vector field averages: 0.0227-s (detail data from SS), 0.092 s (detail in standard mode), 3 s, 1 min, and 1 hour, in both GSE and GSM coordinates, as well as the FFT spectral elements. As has been our team's tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the solar wind: (1) as a collisionless plasma laboratory, at all time scales, macro, meso and micro, but concentrating on the kinetic scale, the highest time resolution of the instrument (=0.022 s), (2) as a consequence of solar energy and mass output, (3) as an external source of plasma that can couple mass, momentum, and energy to the Earth's magnetosphere, and (4) as it is modified as a consequence of its imbedded field interacting with the moon. Since the GEOTAIL Inboard Magnetometer (GIM), which is similar to the MFI instrument, was developed by members of our team, we provide a brief discussion of GIM related science objectives, along with MFI related science goals.     

The Galileo Dust Detector

The Galileo Dust Detector is intended to provide direct observations of dust grains with masses between 10^-19 and 10^-9 kg in interplanetary space and in the Jovian system, to investigate their physical and dynamical properties as functions of the distances to the Sun, to Jupiter and to its satellites, to study its interaction with the Galilean satellites and the Jovian magnetosphere. Surface phenomena of the satellites (like albedo variations), which might be effects of meteoroid impacts will be compared with the dust environment. Electric charges of particulate matter in the magnetosphere and its consequences will be studied; e.g., the effects of the magnetic field on the trajectories of dust particles and fragmentation of particles due to electrostatic disruption. The investigation is performed with an instrument that measures the mass, speed, flight direction and electric charge of individual dust particles. It is a multicoincidence detector with a mass sensitivity 10⁶ times higher than that of previous in-situ experiments which measured dust in the outer solar system. The instrument weighs 4.2 kg, consumes 2.4 W, and has a normal data transmission rate of 24 bits s^-1 in nominal spacecraft tracking mode. On December 29, 1989 the instrument was switched-on. After the instrument had been configured to flight conditions cruise science data collection started immediately. In the period to May 18, 1990 at least 168 dust impacts have been recorded. For 81 of these dust grains masses and impact speeds have been determined. First flux values are given.     

ExoMars 2016 Schiaparelli Module Trajectory and Atmospheric Profiles Reconstruction

On 19^th October 2016 Schiaparelli module of the ExoMars 2016 mission flew through the Mars atmosphere. After successful entry and descent under parachute, the module failed the last part of the descent and crashed on the Mars surface. Nevertheless the data transmitted in real-time by Schiaparelli during the entry and descent, together with the entry state vector as initial condition, have been used to reconstruct both the trajectory and the profiles of atmospheric density, pressure and temperature along the traversed path.The available data-set is only a small sub-set of the whole data acquired by Schiaparelli, with a limited data rate (8 kbps) and a large gap during the entry because of the plasma blackout on the communications.This paper presents the work done by the AMELIA (Atmospheric Mars Entry and Landing Investigations and Analysis) team in the exploitation of the available inertial and radar data. First a reference trajectory is derived by direct integration of the inertial measurements and a strategy to overcome the entry data gap is proposed. First-order covariance analysis is used to estimate the uncertainties on all the derived parameters. Then a refined trajectory is computed incorporating the measurements provided by the on-board radar altimeter.The derived trajectory is consistent with the events reported in the telemetry and also with the impact point identified on the high-resolution images of the landing site.Finally, atmospheric profiles are computed tacking into account the aerodynamic properties of the module. Derived profiles result in good agreement with both atmospheric models and available remote sensing observations.     

Satellite measurements of high latitude convection electric fields

This paper reviews the first results of satellite experiments to measure magnetospheric convection electric fields using the double-probe technique.The earliest successful measurements were made with the low-altitude (680–2530 km) polar orbiting Injun-5 spacecraft (launched August, 1968). The Injun-5 data are discussed in detail. The Injun-5 results are compared with the initial findings of the electric field experiment on the polar orbiting OGO-6 satellite (400–1100 km, launched June, 1969).In addition to electric fields, the Injun-5 spacecraft also measures electric antenna impedance and thermal and energetic charged particle densities. Knowledge of these parameters makes possible a detailed investigation of the operation of the electric antenna system. We report on this investigation and discuss errors attributed to sunlight shadows on the probes, wake effects, and other factors. The Injun-5 experiment can generally determine electric fields to an accuracy of about ±30 mV m^-1, and under favorable conditions, accuracies of ±10 mV m^-1 can be obtained.Reversals in the electric field at auroral zone latitudes are the most significant convection electric field effect discovered in the Injun-5 data. Electric field magnitudes of typically 30 mV m^-1, and sometimes 100 mV m^-1, are associated with reversals. Electric field reversals occur on 36% of auroral zone traversals, at about 70° to 80° invariant latitude, at all local times, and in both hemispheres. The latitude of a reversal often changes markedly on time scales less than 2 h. Electric potentials of greater than 40 keV are associated with these high latitude electric fields. Reversals occur at the boundary of measurable intensities of >45 keV electrons and are coincident with inverted V type low energy electron precipitation events. In almost all cases the E×B/B ² plasma convection velocities associated with reversals are directed east or west, with anti-sunward components at higher latitudes and sunward components at lower latitudes. Maximum convection velocities are typically 1.5 km s^-1 and ordinarily occur at the auroral zone near the reversal.Two extreme (and many intermediate) configurations of anti-sunward plasma convection have been observed to occur on the high latitude side of electric field reversals: (1) Ordinarily, >0.75 kms^-1 convection is limited to narrow (5° INV wide) zones adjacent to the reversal. (2) For 14% of reversals >0.75 km s^-1 anti-sunward convection has been observed across the entire polar cap along the trajectory of the Injun-5 spacecraft. A summary pattern of >0.75 km s^-1 polar thermal plasma convection is presented.Electric field measurements from the OGO-6 satellite have substantiated many of the initial Injun-5 observations with improved accuracy and sensitivity. The OGO-6 detector revealed the persistent occurrence of anti-sunward convection across the polar cap region at velocities (<0.75 km s^-1) not generally detectable with the Injun-5 experiment. The OGO-6 observations also provided information indicating that the location of the electric field reversal shifts equatorward during periods of increased magnetic activity.The implications of the electric field measurements for magnetosphericand auroral structure are summarized, and a list of specific recommendations for improving future experiments is presented.     

A plasma wave investigation for the Voyager Mission

The Voyager Plasma Wave System (PWS) will provide the first direct information on wave-particle interactions and their effects at the outer planets. The data will give answers to fundamental questions on the dynamics of the Jupiter and Saturn magnetospheres and the properties of the distant interplanetary medium. Basic planetary dynamical processes are known to be associated with wave-particle interactions (for instance, solar wind particle heating at the bow shock, diffusion effects that allow magnetosheath plasma to populate the magnetospheres, various energization phenomena that convert thermal plasma of solar wind origin into trapped radiation, and precipitation mechanisms that limit the trapped particle populations). At Jupiter, plasma wave measurements will also lead to understanding of the key processes known to be involved in the decameter bursts such as the cooperative mechanisms that yield the intense radiation, the observed millisecond fine-structure, and the Io modulation effect. Similar phenomena should be associated with other planetary satellites or with Saturn's rings. Local diagnostic information (such as plasma densities) will be obtained from wave observations, and the PWS may detect lightning whistler evidence of atmospheric electrical discharges. The Voyager Plasma Wave System shares the 10-meter PRA antenna elements, and the signals are processed with a 16-channel spectrum analyzer, covering the range 10 Hz to 56 kHz. At selected times during the planetary encounters, the PWS broadband channel will operate with the Voyager video telemetry link to give complete electric field waveforms over the frequency range 50 Hz to 10 kHz.     

The GGS/POLAR magnetic fields investigation

The magnetometer on the POLAR Spacecraft is a high precision instrument designed to measure the magnetic fields at both high and low altitudes in the polar magnetosphere in 3 ranges of 700, 5700, and 47000 nT. This instrument will be used to investigate the behavior of fieldaligned current systems and the role they play in the acceleration of particles, and it will be used to study the dynamic fields in the polar cusp, magnetosphere, and magnetosheath. It will measure the coupling between the shocked magnetosheath plasma and the near polar cusp magnetosphere where much of the solar wind magnetosphere coupling is thought to take place. Moreover, it will provide measurements critical to the interpretation of data from other instruments. The instrument design has been influenced by the needs of the other investigations for immediately useable magnetic field data and high rate (100+vectors s^–1) data distributed on the spacecraft. Data to the ground includes measurements at 10 vectors per second over the entire orbit plus snapshots of 100 vectors per second data. The design provides a fully redundant instrument with enhanced measurement capabilities that can be used when available spacecraft power permits.     

Dynamic tensile behavior of AZ31B magnesium alloy at ultra-high strain rates

The samples having {0001} parallel to extruding direction(ED) present a typical true stress–true strain curve with concave-down shape under tension at low strain rate. Ultra-rapid tensile tests were conducted at room temperature on a textured AZ31 B magnesium alloy. The dynamic tensile behavior was investigated. The results show that at ultra-high strain rates of 1.93 · 102 s 1and 1.70 · 103 s 1, the alloy behaves with a linear stress–strain response in most strain range and exhibits a brittle fracture. In this case, {10-12} 10-11 extension twinning is basic deformation mode. The brittleness is due to the macroscopic viscosity at ultra-high strain rate, for which the external critical shear stress rapidly gets high to result in a cleavage fracture before large amounts of dislocations are activated. Because {10-12} tension twinning, {10-11} compressive twinning,basal a slip, prismatic a slip and pyramidal c + a slip have different critical shear stresses(CRSS), their contributions to the degree of deformation are very differential. In addition,Schmid factor plays an important role in the activity of various deformation modes and it is the key factor for the samples with different strain rates exhibit various mechanical behavior under dynamic tensile loading.     

不同温度下IC10合金的本构关系

 利用材料试验机（MTS809）测得IC10合金在很宽的温度范围（25~800 ℃）和不同应变率（10^-5~10^-2s^-1）内的应力 应变曲线。试验结果表明：室温下IC10合金的流变行为对应变率不太敏感;相同应变率（10^-4s^-1）下,流变行为对温度较敏感;在不同应变率、25~800 ℃温度范围内,IC10合金的屈服应力变化很小。基于试验数据,修正并拟合了Johnson-Cook方程,并利用该方程对不同温度和不同应变率下IC10合金的流变应力进行预测。与试验数据对比表明,修正后的Johnson-Cook方程能较好地描述IC10合金在不同的温度和应变率条件下的流变行为。     

The Grain Impact Analyser and Dust Accumulator (GIADA) Experiment for the Rosetta Mission: Design, Performances and First Results

The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10⁻¹⁰ ÷ 4.0 × 10⁻⁴ kg m s⁻¹ for velocities up to about 300 m s⁻¹. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10⁻¹⁰ g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts demonstrate that GIADA is behaving as expected according to the design specifications. The International GIADA Consortium (I, E, UK, F, D, USA).     

Energetic Particles Investigation (EPI)

The Energetic Particles Investigation (EPI) instrument operates during the pre-entry phase of the Galileo Probe. The major science objective is to study the energetic particle population in the innermost regions of the Jovian magnetosphere — within 4 radii of the cloud tops — and into the upper atmosphere. To achieve these objectives the EPI instrument will make omnidirectional measurements of four different particle species — electrons, protons, alpha-particles, and heavy ions (Z > 2). Intensity profiles with a spatial resolution of about 0.02 Jupiter radii will be recorded. Three different energy range channels are allocated to both electrons and protons to provide a rough estimate of the spectral index of the energy spectra. In addition to the omnidirectional measurements, sectored data will be obtained for certain energy range electrons, protons, and alpha-particles to determine directional anisotropies and particle pitch angle distributions. The detector assembly is a two-element telescope using totally depleted, circular silicon surfacebarrier detectors surrounded by a cylindrical tungsten shielding with a wall thickness of 4.86 g cm^-2. The telescope axis is oriented normal to the spherical surface of the Probe's rear heat shield which is needed for heat protection of the scientific payload during the Probe's entry into the Jovian atmosphere. The material thickness of the heat shield determines the lower energy threshold of the particle species investigated during the Probe's pre-entry phase. The EPI instrument is combined with the Lightning and Radio Emission Detector (LRD) such that the EPI sensor is connected to the LRD/EPI electronic box. In this way, both instruments together only have one interface of the Probe's power, command, and data unit.     

THE CLUSTER MAGNETIC FIELD INVESTIGATION

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s^-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.     

A Lamb wave signal reconstruction method for high-resolution damage imaging

In Lamb wave-based Structural Health Monitoring(SHM), a high-enough spatial resolution is highly required for Lamb wave signals to ensure the resolution and accuracy of damage detection. However, besides the dispersion characteristic, the signal spatial resolution is also largely restricted by the space duration of excitation waveforms, i.e., the Initial Spatial Resolution(ISR)for the signals before travelling. To resolve the problem of inferior signal spatial resolution of Lamb waves, a Lamb Wave Signal Reconstruction(LWSR) method is presented and applied for highresolution damage imaging in this paper. In LWSR, not only a new linearly-dispersive signal is reconstructed from an original Lamb wave signal, but also the group velocity at the central frequency is sufficiently decreased. Then, both dispersion compensation and ISR improvement can be realized to achieve a satisfying signal spatial resolution. After the frequency domain sensing model and spatial resolution of Lamb wave signals are firstly analyzed, the basic idea and numerical realization of LWSR are discussed. Numerical simulations are also implemented to preliminarily validate LWSR. Subsequently, LWSR-based high-resolution damage imaging is developed. An experiment of adjacent multiple damage identification is finally conducted to demonstrate the efficiency of LWSR and LWSR-based imaging methods.     

The FAST Satellite Fields Instrument

We describe the electric field sensors and electric and magnetic field signal processing on the FAST (Fast Auroral SnapshoT) satellite. The FAST satellite was designed to make high time resolution observations of particles and electromagnetic fields in the auroral zone to study small-scale plasma interactions in the auroral acceleration region. The DC and AC electric fields are measured with three-axis dipole antennas with 56 m, 8 m, and 5 m baselines. A three-axis flux-gate magnetometer measures the DC magnetic field and a three-axis search coil measures the AC magnetic field. A central signal processing system receives all signals from the electric and magnetic field sensors. Spectral coverage is from DC to 4 MHz. There are several types of processed data. Survey data are continuous over the auroral zone and have full-orbit coverage for fluxgate magnetometer data. Burst data include a few minutes of a selected region of the auroral zone at the highest time resolution. A subset of the burst data, high speed burst memory data, are waveform data at 2×10⁶ sample s^–1. Electric field and magnetic field data are primarily waveforms and power spectral density as a function of frequency and time. There are also various types of focused data processing, including cross-spectral analysis, fine-frequency plasma wave tracking, high-frequency polarity measurement, and wave-particle correlations.