A. Overview

B. Scientific Objectives

C. Cluster Instrumentation

D. Orbit and Separation Strategy

E. The Wide-Band Plasma Wave Investigation

     1. Scientific Objectives

     2. Description of the Wide-Band Receiver System

     3. WBD Operations

     4. WBD Measurements

The European Space Agency’s Cluster program is designed to study the small-scale spatial and temporal characteristics of the magnetospheric and near-Earth solar wind plasma. The program is composed of four identical spacecraft, which will be able to make physical measurements in three dimensions. The relative distance between the four spacecraft will be varied between 200 and 1800 km during the course of the mission.

The main goal of the Cluster mission is to study the small-scale plasma structures in space and time in the key plasma regions:

·        Solar Wind and Bow Shock: The Bow Shock is the region where the solar wind becomes decelerated from a supersonic to a subsonic speed before being deflected around the Earth. The length scales of various processes occurring at the bow shock, like the reflection and thermalization of the ions, are not yet known and a better knowledge requires multi-point measurements on various scales. Further upstream of the bow shock, the solar wind is the place where electromagnetic waves play a major role and a further understanding of these waves is needed, in particular their transmission through the bow shock and the magnetosheath

·        Magnetopause: The magnetopause is the thin plasma layer that separates the solar wind magnetic field from the Earth magnetic field. It is the locus where the plasma pressure of the solar wind is in equilibrium with the magnetic pressure inside the magnetosphere. Due to continuous variation of the solar wind pressure, this boundary moves continuously. Cluster with its four-point measurements, will help to characterize this motion

·        Polar Cusp: Almost anywhere near the surface of the magnetopause, the Earth’s magnetic field is tangential to the magnetopause, acting like a natural barrier to the solar wind particles. There are only two regions, one in each hemisphere, where the magnetic field is approximately perpendicular to the magnetopause, namely the polar cusps, allowing a more direct entry of solar wind particles into the magnetosphere. Cluster will make measurements with high resolution of the reconnection of the interplanetary magnetic field with the Earth’s magnetic field at these regions, allowing full three dimensional ion and electron distribution functions to be measured every spin.

·        Magnetotail: The magnetotail is characterized by magnetic field lines stretched by the solar wind flow in the anti-sunward direction. The outer region consists of two magnetotail lobes and the inner region of the plasma sheet boundary layer and the central plasma sheet. The lobes are large reservoir of magnetic energy, which contain plasma of densities much less than one particle per cubic meter. A spacecraft in this very tenuous will charge to high positive potentials, producing electron measurements to be saturated by photo-electrons coming back to the spacecraft, and ions being repelled. The Cluster potential will be controlled by the Active Spacecraft Potential Control instrument (ASPOC) and hence, will allow measurements of full distribution functions of the ions and electrons in the lobes.

·        Auroral Zone: The Auroral Zone is a ring of light emission, around the magnetic pole, created by precipitation of particles in the atmosphere. The cusp and boundary layers on the dayside and the plasma sheet and plasma sheet boundary layer on the nightside are the sources of these precipitations. The four Cluster spacecraft will cross the cusp and the auroral field lines between 4 and 6 RE, with a time delay between the spacecraft ranging from a few minutes to up to 40 minutes, thus allowing the time variations of the transient events to be studied

The four Cluster spacecraft are identical and each contain 11 instruments, giving a total of 44 instruments built by the Principal Investigators:

1.      The fluxgate magnetometer (FGM) is designed to provide inter-calibrated measurements of the magnetic-field vector at the location of the four Cluster spacecraft. The combined analysis of the data from the four spacecraft will yield parameters such as the current density vector, wave vectors and he geometry and structure of discontinuities.

2.      The electron drift instrument (EDI) is based on the emission and subsequent detection of tracer electrons to derive the ambient electric field. The instrument consists of two sets of electron guns/detectors at 180º to each other. EDI employs two different methods to measure the electric field. In the case of strong ambient magnetic fields, the displacement of the electron over one gyration can be measured by a triangulation method using the directions of emission of the two electron beams. When the ambient magnetic field is small, the instrument works in a mode whereby two beams are emitted in opposite directions and the time-of-flight is then measured. In addition, by varying the electron energy, the instrument can determine gradients in the local magnetic field.

3.      The accurate measurement of the cold plasma population demands that the electrostatic potential of the spacecraft with respect to the ambient plasma be maintained at a very low level. Cluster will be equipped with an ion emitter, the Active Spacecraft Potential Control (ASPOC) experiment, to routinely control this potential. ASPOC is specifically designed to stabilize the fluctuating potential by emission of indium ions with an energy between 5 and 8 keV and a total current of 50 mA.

4.      The cluster ion spectrometry (CIS) experiment employs two sensors to obtain the full three-dimensional ion distribution of the major species with high time resolution and mass per charge plasma composition. One sensor, the time-of-flight ion composition and distribution function analyzer (CODIF) will measure the distribution of the major ion species from 0 to 40 keV/q with an angular resolution of 22.5º x 10.25º and two different sensitivities. CODIF also uses a retarding potential analyzer to make more accurate measurements below 15 eV/q. The other sensor, the hot ion analyzer (HIA), will measure the distribution of the ions without distinction mass from 5 eV/q to 32 eV/q with an angular resolution 5.6º x 5.6º and two different sensitivities. HIA is specifically designed for the highly directional ion beams observed in the solar wind. A specific feature of the CIS is the double sensitivity of the sensors, which will allow the precise measurement of the large flux of ion beams in the solar wind as well as the low flux of ions in the lobes of the magnetosphere.

5.      The plasma electron measurements will be performed by the plasma electron and current experiment (PEACE). The instrument consists of two sensors: The low energy electron analyzer (LEEA) and the high-energy electron analyzer (HEEA). The detection of cold electrons requires a very careful design to eliminate spurious effects introduced by the photo electrons which are known to be abundant near the spacecraft skin. LEEA is designed to measure the low-energy electrons from 0.7 to 10 eV but is also capable of covering the full energy range up to 30 keV. HEEA, with a geometric factor five times higher than LEEA, covers the full energy range from 0.7 eV to 30 keV. The two sensors are mounted opposite to each other on the spacecraft which allows the three dimensional distribution function to be measured every half of spacecraft spin in the energy part common to both sensors.

6.      Energetic particles are sensitive probes for remote-sensing techniques in nearly all plasma regions of geospace. These particles help to identify distant acceleration regions and they can be used to trace plasma flows and magnetic field line topologies. The research with adaptive particle imaging detectors (RAPID), consists of two spectrometers, each containing position-sensitive solid-state detectors: the imaging ion mass spectrometer (IIMS) which measures the ion distribution function from 30 to 1500 keV/q with distinction of mass and the imaging electron spectrometer (IES) which measures the electron from 20 to 450 keV.

A suite of five instruments, EFW, STAFF, WHISPER, WBD and DWP forms the Wave Experiment Consortium WEC:

7.      The electric field and wave (EFW) experiment has been specifically designed to study the fast time and space varying vectorial electric fields, including the DC to low-frequency range. In addition the instrument will measure the spacecraft potential and the electron density and temperature.

8.      The spatio-temporal analysis of field fluctuation (STAFF) experiment has two main parts: The search coil which measures the magnetic component of the electromagnetic fluctuations and the spectrum analyzer which performs auto and cross correlation between electric and magnetic components. Such measurements will determine the shape, current density and motion of small-scale current structures and identify the source of plasma waves and turbulence.

9.      The waves of high frequency and sounder probing of electron density by relaxation (WHISPER) is an intermittent transmitter/receiver instrument that can also be operated in a passive (receive-only) mode. The transmitter emits a short pulse to stimulate plasma resonances. After each emission, the receiver part is activated to detect plasma density in the range of 0.2-80 cm^-3.

10.  The objective of the Wide Band Data (WBD) receiver system is to provide high-resolution electric field waveforms and frequency-time spectrograms of terrestrial plasma waves and radio emissions. WBD will also perform unique measurements, which are the Very Long Baseline Interferometer (VLBI) measurements with up to four spacecraft. VLBI will give new information on the angular size and motion of the sources of terrestrial radio emissions such as auroral kilometric radiation.

11.  The digital wave processing (DWP) instrument coordinates WEC measurements and performs particle correlations. These correlations consist of auto-correlation functions of the time series of particle detector counts, as a function of energy and pinch angle. A hardwired link with PEACE provides the electron count measurements. These correlation functions will allow the investigation of nonlinear wave-particle interactions, which are believed to be the source process of many plasma transport mechanisms.

In order to meet the scientific objectives of the mission, the orbit was chosen with a perigee at 4 RE, an apogee at 19.6 RE, an inclination of 90º and a line of apsides around the ecliptic plane. The orbital period is 57 hours.

The Cluster Wide Band (WBD) Plasma Wave Investigation is designed to provide very high-resolution frequency-time measurements of plasma waves in the Earth’s magnetosphere. The WBD instrument is part of the Wave Experiment Consortium (WEC) and consists of a digital wide-band receiver that can provide electric or magnetic field waveforms over a wide range of frequencies.

The wide band technique involves transmitting band-limited waveforms directly to the ground using a high-rate data-link. The preliminary advantage of this approach is that continuous waveforms are available for detailed high-resolution frequency-time analysis. The frequency-time resolution is limited only by the uncertainty principle, (Dw Dt ~ 1) Since the frequency resolution (Dw) and time resolution (Dt) can be selected on the ground, the wide band technique has the advantage that the resolution can be adjusted to provide optimum analysis of the phenomena of interest.

The primary purpose of the WBD investigation is to support WEC science objectives by providing high-resolution spectral analysis. At boundaries and other regions with steep spatial gradients, WBD provides with high-time resolution single-spacecraft measurements for comparison with data from other instruments, such as the magnetometer and plasma instruments. From these data, waves produced from current-driven instabilities and other mechanisms involving spatial inhomogeneities can be clearly identified. In addition to single-point measurements, wide-band data from two or more spacecraft can also be used to resolve space-time ambiguities as the spacecraft pass through complex spatial structures.

 The WBD instrument

1.      Sensors and Sensor Interfaces: The Cluster plasma-wave sensors consist of two orthogonal spherical electric antennas located in the spin plane of the spacecraft, and a triaxial search coil magnetometer oriented with two axes in the spin plane and a third axis parallel to the spacecraft spin axis. The electric antennas, designated Ey and Ez, are provided by the EFW investigation and have sphere-to-sphere separations of 100 m when fully deployed. The spheres each contain a high-impedance preamplifier that provides signals to the EFW main electronics, to the WBD, and to the other wave instruments via buffer amplifiers. The EFW/WBD buffer amplifier is a low-noise, low-power design that maintains a flat response up to about 600 kHz. The boom-mounted three-axis search coil magnetometer (Bx, By and Bz, but only Bx and By are measured by WBD) is part of the STAFF instrumentation, and provides magnetic-field measurements up to 4 kHz. The WBD instrument has the capability of processing signals from only one of the four sensors. The sensor selection is controlled by spacecraft command via the DWP.

2.      Frequency Bands: The input frequency range of the wide-band receiver can be shifted by a frequency converter to any one of four frequency ranges, where the conversion frequency, f, determines the lower edge of the frequency range selected. The conversion frequency is obtained by dividing a 14 MHz reference oscillator that is located within the WBD instrument. To maintain phase stability in the entire system, this oscillator is synchronized to the spacecraft 220.t52 kHz high-frequency-clock. The high frequency clock is obtained by dividing the spacecraft Ultra-Stable Oscillator (USO), which operates at a frequency of 2²³ Hz, with a stability of 6.5 10^-7 % over a 12 hour period. A spacecraft command to select a particular frequency band causes DWP to switch the wide-band receiver to an appropriate input band pass filter and to a conversion frequency of 0, 125, 250 or 500 kHz. The bandwidth of the WBD output waveform is determined by one of three band pass filters selected in combination with a given output mode. The conversion frequencies and band pass filter ranges are summarized in the following table:

                                      WBD Instrument Parameters

3.      Gain Control: The gain of the wide-band receiver is determined by a set of four dual-gain amplifiers that may be selected to provide various gains in increments of 5 dB. The dual-gain amplifiers have gains of 0 or 5 dB, 0 or 10 dB, 0 or 20 dB, and 0 or 40 dB. The gain control has two modes of operation: fixed and auto-raging. The gain control modes are controlled by spacecraft command. In the fixed gain mode, the receiver gain can be se to any one of the sixteen levels (from 0dB to 75 dB). In the auto-raging mode, the output from the programmable amplifiers compared to a pair of reference amplitudes. If criteria for changing the gain are met, the gain is either increased or decreased in one step (5 dB). The gain is updated at a rate determined by the gain update clock, which is a DWP function selected by spacecraft command.

4.      A/D Converter and Format Generator: The output analogue waveform is sampled by an 8-bit analogue-to-digital converter that provides the sampling resolution and the data output rates.

WBD output modes

For sample rates where the bit rates exceeds the spacecraft telemetry rate (220 Kbps) the digitized wide-band data is duty-cycled by a format generator that reduces the average bit rate to 220 Kbps. The format generator organizes the digitized waveform data into a 1096-byte output frame, which includes appropriate timing and status information.

5.      WDB Data Interface: The WBD instrument utilizes two separate paths for transferring frames of digitized waveform data to the spacecraft data-handling system. The primary path supports real-time acquisition of WBD data by the NASA DSN. During real-time acquisition data (TDA Mode 8), the WBD data appears on a dedicated virtual telemetry channel embedded in the 1096-byte data field of the standard 1279-byte transfer frame. The WBD transfer frames are acquired by a DSN receiving station. The secondary path supports non real-time data acquisition via the spacecraft solid-state recorder. This interface supports a non-real-time mode of data collection (TDA Mode 5.2 or BM2) dedicated mainly to WBD. When the BM2 mode is enabled, WBD data are transferred to DWP at 220 Kbps, and the DWP, in turn, reduces the wide band data by a factor of three either by digital filtering or duty cycling. At the new average bit rate of 73 Kbps, the WBD data are transferred to the On-Board-Data-Handling (OBDH) system for recording and subsequent playback using the Solid State Recorder (SSR).

Although WBD can be commanded to a large number of internal configurations (antenna, bandwidth, gain select, etc.) the most significant WBD operational aspect relates to the manner of telemetry acquisitions. Because the wide-band approach carries a basic requirement for high data rates, WBD data do not appear in the low rate telemetry. Rather, two special high-data rate telemetry acquisition modes were implemented to support WBD operations. These are acquisition mode TDA 8, which involves real-time data reception by a DSN ground station, and acquisition mode TDA 5.2 which is a special record mode (BM2) dedicated to WBD. The use of these modes is constrained by various operational considerations, including the availability of appropriate ground-based receiving antennas.

During the Cluster mission, the primary mode for acquiring WBD data will be the single-spacecraft TDA 8 mode of operation using the DSN. By agreement DSN will provide a minimum of 2 hours of coverage per spacecraft per 57-hour orbit. Also, the bulk of the WBD coverage will be provided by the Canberra and Goldstone DSN stations. Since the TDA 8 utilizes the entire bandwidth of the downlink, data from the other Cluster experiments must be recorded on the solid-state recorder during the DSN acquisition periods.

The following are some of the measurements done by the WBD instrument at different positions in the orbit of Cluster around the Earth:

The overall spectrogram measured during this time slot is shown in the following picture. These measurements were taken on February 4 2001, between 13:30 and 14:30. We can see in this case the data for the four Cluster spacecraft:

(Overview1)

The following are some detailed parts from the previous measurements, taken in 30 seconds slots. It is very noticeable the presence of Whistlers emissions and some reflections corresponding to the originating whistler:

For Cluster C1 Antenna Ey

(2001035135428-S1)

(2001035135458-S1)

(2001035135528-S1)

For Cluster C2 Antenna Ey

(2001035135426-S2)

(2001035135455-S2)

(2001035135525-S2)

For Cluster C3 Antenna Ey

(2001035135428-S3)

(2001035135458-S3)

(2001035135528-S3)

For Cluster C4 Antenna Ey

(2001035135428-S4)

(2001035135458-S4)

(2001035135528-S4)

The following are some of the measurements done by the WBD instrument at different positions in the orbit of Cluster around the Earth:

The overall spectrogram measured during this time slot is shown in the following picture. These measurements were taken on February 4 2001, between 18:00 and 19:30. We can see in this case the data for Cluster spacecraft C3:

(Overview2)

The following are some detailed parts from the previous measurements, taken in 30 seconds slots. Here we can see the transition from the magnetosphere into the Auroral Zone, where we find the presence of the auroral hiss, which has a sharp frequency cutoff at the plasma frequency, starting at 5 kHz and moving towards 7.5 kHz as the spacecraft move in time:

All the data correspond to Cluster C3 Antenna Ez

(2001049183627)

(2001049183657)

(2001049183727)