ESA Science & Technology - Instruments

Introduction

Ulysses hosts a suite of science instruments dedicated to the study of a wide range of topics: the heliospheric magnetic field, heliospheric radio and plasma waves, the solar wind plasma including its minor heavy ion constituents, solar and interplanetary energetic particles, galactic cosmic rays and the anomalous cosmic ray component.

Other investigations are directed towards studies of cosmic dust and interstellar neutral gas, as well as solar x-rays and cosmic gamma-ray bursts.  Radio science experiments to probe the solar corona and to conduct a search for gravitational waves have also been carried out exploiting the spacecraft's radio communications links with the ground stations.

Summary

Overview of the nine instruments and additional experiments, the type of measurements and the Principal Investigators.

VHM/FGM

Vector Helium Magnetometer / Flux Gate Magnetometer

A fundamental feature of the heliosphere is the three dimensional structure of the interplanetary magnetic field. The simple, spherically symmetric and time-independent model of hydrodynamic expansion of the solar corona proposed originally by E.N. Parker in the early 1960's has provided a useful framework for describing interplanetary observations of the solar wind and of the interplanetary magnetic field. However, observations of the Sun and of the solar corona, as well as direct and indirect interplanetary measurements clearly indicate that there are very significant departures from both spherical symmetry and time independence.

The magnetic field investigation on Ulysses aims at determining the large scale features and gradients of the field, as well as the heliolatitude dependence of interplanetary phenomena so far only observed near the ecliptic plane.

The Ulysses magnetometer uses two sensors, one a Vector Helium Magnetometer (VHM), the other a Flux Gate Magnetometer (FGM) to measure the heliospheric magnetic field. The two magnetometers are located on the radial boom of the Ulysses spacecraft: the VHM sensor at the end of the 5m boom and the FGM sensor at 1.2m inboard from the VHM.

Both magnetometers are tri-axial sensors, but they use different physical principles to measure three orthogonal components of the magnetic field vector.

VHM - Vector Helium Magnetometer
The VHM was developed from instruments flown successfully on the Pioneers 10 and 11 missions and on the International Sun-Earth Explorer 3/International Cometary Explorer probe. The operating principle of the VHM is based on the effect an ambient magnetic field has on the efficiency with which a metastable population of He gas in the triplet ground state can be optically pumped.

Feedback currents provide highly linear measures of the three orthogonal components of the magnetic field vector in a reference frame defined with respect to the geometry of the sensor. The VHM has two operating ranges: +/-8.190 nT, equivalent to a sensitivity of 1.60 nT/V; and +/-65.52 nT, equivalent to 12.8 nT/V.

FGM - Flux Gate Magnetometer
The FGM sensor consists of three identical single axis ring-core fluxgate sensors, arranged in an orthogonal triad. The feedback signals to the three sensors represent in magnitude and sign the three components of the ambient magnetic field vector from the sensor.

Switchable feedback paths and output amplifier provide four ranges for the FGM measurements: +/-8 nT, +/-64 nT, +/-2.048 µT, and for pre-launch tests in the Earth's magnetic field, +/-44 µT. Range switching for the FGM is by ground command only.

Summary of Objectives

The prime scientific function of the instrument is to provide a time series of samples of the magnetic field vector along the trajectory of Ulysses. The types of questions which the Ulysses magnetic field observations will help answer are:

• questions concerning the large scale structure, in particular to what extent the Parker model is modified by the effects of the mid-latitude and polar field structures during the descending phase of the solar cycle; this heading also includes the latitude gradients in the field
• questions concerning dynamical features arising from different velocity solar wind streams interacting in interplanetary space
• questions concerning waves, discontinuities and transient phenomena, in particular the latitudinal extent and propagation of shocks in the heliosphere

Finally, the Jupiter flyby needed for achieving the out of ecliptic orbit provided an opportunity to explore magnetic field features in the high latitude dusk regions of the Jovian magnetosphere.

In cooperation with other investigations, the magnetic field observations will also provide the framework for studies of energetic particle propagation in the high latitude heliosphere, in particular to address questions concerning the access and propagation of galactic cosmic rays.

SWOOPS

Solar Wind Observations Over the Poles of the Sun

The solar wind plasma experiment on Ulysses accurately characterizes the bulk flow and internal state conditions of the interplanetary plasma in three dimensions as a function of solar latitude.

Solar wind electrons and ions are measured simultaneously with two completely separate instruments. Both instruments make use of a curved-plate electrostatic analyzer equipped with multiple Channel Electron Multipliers (CEMs). The CEMs are arranged to detect particles at chosen polar angles from the spacecraft spin axis; resolution in spacecraft azimuth is obtained by timing measurements with the spacecraft Sun clock as the spacecraft spins.

Electron Analyzer Experiment
Electrons with central energies extending from 0.86 eV to 814 eV are detected at seven polar angles and various combinations of azimuth angle to cover the unit sphere comprehensively, so as to enable computation of the pertinent electron velocity distribution parameters. As the average electron flux level changes with heliocentric distance, command control of the CEM counting intervals is used to extend the dynamic range.

Ion Analyzer Experiment
Ions are detected between 255 eV/q and 34.4 keV/q using appropriate subsets of 16 CEMs at spin angles designed to provide matrices of counts as a function of energy per charge, azimuth angle, and polar angle centred on the average direction of solar-wind flow. Data matrices are obtained every 4 minutes when the spacecraft is actively transmitting and every 8 minutes during data store periods. These matrices contain sufficient energy and angle resolution to permit a detailed characterization of the ion velocity distributions, from which ion bulk parameters are derived.

As the average ion flux intensity changes with heliocentric distance, the entrance aperture size is periodically optimized by command selection from a set of seven apertures on a disk driven by a stepping motor. Changes in the average solar wind flow direction relative to the Earth-pointing spacecraft spin axis are accommodated by command selection of the proper measurement matrix from a set of 11 matrices. In a separate mode of operation and under favourable conditions, heavy ions of oxygen, silicon, and iron at various charge levels are resolved.

Summary of Objectives

The primary objective of the Ulysses solar wind plasma investigation is to investigate and establish bulk flow parameters and internal state conditions of the solar wind as a function of solar latitude. Further important goals include studies of radial gradients of solar wind properties between Earth and Jupiter and investigations of the solar wind interaction with the Jovian magnetosphere. The important goals of SWOOPS are:

• Determine systematic variations in the solar wind bulk flow with solar latitude
• Examine physical processes important for driving the coronal expansion
• Investigate variations in the evolution of high speed streams with latitude
• Investigate changes in the nature of transient disturbances with latitude
• Determine latitudinal variations in the relative abundance and charge state composition of solar wind minor ions
• Search for and identify plasma mechanisms regulating the electron heat flux ion beam relative flow velocities, and the He⁺⁺ to H⁺ ion temperature ratio
• Survey latitudinal variations in Alfvén wave amplitudes and the relative numbers of tangential and rotational discontinuities
• Determine coronal hole temperatures at which high speed stream solar wind Fe ionization states are established
• Estimate the electrostatic potential and electron collision length dependence on latitude
• Identify local heating mechanisms of solar wind ions
• Study the interaction of interstellar neutrals with the solar wind
• Study large amplitude hydromagnetic waves
• Evaluate radial and meridional gradients in solar wind electron temperature and anisotropy

SWICS

Solar Wind Ion Composition Spectrometer

SWICS is designed to determine uniquely the elemental and ionic-charge composition, and the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms^-1 (protons) to 1280 kms^-1 (Fe⁸⁺).

The SWICS sensor, which covers an energy per charge range from 0.16 to 59.6 keV/q in ~ 13 minutes, is based on the technique of particle identification using a combination of electrostatic deflection, post-acceleration, and a time-of-flight (TOF) and energy measurement. The operating principle of the sensor and the functions of the five basic sensor elements employed are:

• Ions of kinetic energy E, mass m and charge (ionization state) q enter the sensor through a large area, multi-slit collimator which selects proper entrance trajectories for the particles
• The electrostatic deflection analyzer serves as an energy- per-charge (E/q) filter, allowing only ions within a given energy-per-charge interval (determined by a stepped deflection voltage) to enter the TOF versus Energy system
• Ions are post-accelerated by a ~ 30 kV potential drop just before entering the TOF versus Energy system. The energy they gain is sufficient to be measured adequately by the solid-state detectors, which typically have a ~ 30 keV energy threshold. An energy measurement is essential for determining the elemental composition of an ion population and ions with energies below ~ 30 keV must be accelerated if their mass is to be identified
• In the time-of-flight system the velocity of each ion is determined by measuring the travel time τ of the particle between the start and stop detectors separated by a distance of 10 cm
• The particle identification is completed by measuring the residual energy of the ions in a conventional low-noise solid-state detector

Summary of Objectives

The measurements made by SWICS will have an impact on many areas of solar and heliospheric physics, in particular providing essential and unique information on:

• conditions and processes in the region of the corona where the solar wind is accelerated
• the location of the source regions of the solar wind in the corona
• coronal heating processes
• the extent and causes of variations in the composition of the solar atmosphere
• plasma processes in the solar wind
• the acceleration of energetic particles in the solar wind
• the thermalization and acceleration of interstellar ions in the solar wind, and their composition
• the composition, charge states and behaviour of the plasma in various regions of the Jovian magnetosphere

URAP

Unified Radio & Plasma Wave Investigation

URAP is designed to detect both distant radio emissions, as well as locally generated plasma waves. The sensors consist of a 72.5 m electric field dipole antenna in the spin plane, a 7.5-m electric field monopole along the spin axis and a pair of orthogonal search coil magnetic antennas. The various receivers, designed to encompass specific needs of the investigation, cover the frequency range from DC to 1 MHz and are summarised below together with the relaxation sounder and DC measurements:

Radio Astronomy Receivers (RAR)
The radio receivers consist of four superheterodyne receivers whose frequency stepping is programmed in ROM memory and controlled by telecommand. Two receivers, Z_L and Z_H, are connected to the spin-axis (Z) preamplifier; the two others, SL and SH, are connected to the combination of the signals from the spin-plane (±X) preamplifiers and the Z preamplifier to form the electronically synthesized tilted dipole. In each case, one receiver is tuned to low frequencies (1.25-48.5 kHz), and the other to high frequencies (52-940 kHz).

Plasma Frequency Receiver (PFR)
The PFR is intended to monitor the wide spectrum of plasma phenomena with constant frequency coverage, large dynamic range, and good frequency resolution. Two such receivers are supplied, one for E_x and one for E_z. The frequency range, 0.57 to 35 kHz, is covered in 32 logarithmic frequency steps, with a corresponding separation of 14%.

Wave Form Analyser (WFA)
The WFA or FFT-DPU (Fast Fourier Transform Data Processing Unit) provides spectral analysis in the frequency range from 0.08 to 448 Hz of signals received from the plasma wave and magnetic preamplifiers. The spectral analysis is performed separately for frequencies below 10 Hz and between 10 Hz and 448 Hz. The > 10 Hz processing is done by three microprocessors, each dedicated to one of the E_x, B_y, and the selected B_z or E_z sources. The signals are analysed in 12 logarithmically-spaced bands.  Signals for the < 10 Hz processing are obtained from the E_x and the selected B_y or B_z sources which have been low-pass filtered and converted with a 10-bit analog-to-digital converter. The signals are also analysed in 12 logarithmically-spaced bands.

Fast Envelope Sampler (FES)
The purpose of the FES is to capture transient, rapidly varying phenomena, at sample rates up to the order of one sample per millisecond, and store them in a memory for later telemetry. Two filter channels (Hi: 600 Hz – 60 kHz; and Lo: 10 Hz – 20 kHz) are used concurrently, with respectively 3 and 4 commandable filters for the isolation of one phenomenon in the presence of others, but otherwise FES has no frequency resolution, and only samples the envelope of the detected signal.

Sounder
The major purpose of a relaxation sounder is to provide a reliable measure of the local electron plasma density, through the detection of the resonance excited close to the electron plasma frequency.  The sounder was designed around use of the low frequency radio receiver, with the only specific hardware items being two simple transmitters, small additions to the radio receivers, and extra ROM for the microprocessors. The signal is analyzed on board by the URAP Data Processing Unit, using a modified Walsh transform.

DC voltage measurements
DC signals refer to measurements of the instantaneous potential or potential difference of the antennas. Three potentials are measured: the potential on the Z antenna with respect to the spacecraft E_ZDC, the potential of the +X antenna with respect to the spacecraft E_XAN, and the potential difference between the two X antennas E_XDC. DC voltage measurements are made using the plasma wave preamplifiers and some signal conditioning within the same box and are digitized by the Spacecraft Data Handling System. There are two sets of measurements made for X_DC and Z_DC each, fixed and scan. Fixed means that the samples are made at fixed angles with respect to the spacecraft rotation and scan means that the sample angle is changed each time by an amount such that after 512 samples each angle (512th of a spin) has been seen. This permits a reconstitution of the average field around the spacecraft.

Summary of Objectives

The scientific objectives of URAP experiment are twofold:

1. the determination of the direction, angular size, and polarization of radio sources for remote sensing of the heliosphere and the Jovian magnetosphere
2. the detailed study of local wave phenomena, which determine the transport coefficients of the ambient plasma

The tracking of solar radio bursts, for example, can provide three dimensional snap-shots of the large scale magnetic field configuration along which the solar exciter particles propagate.

URAP observations of Jovian radio emissions greatly improve the determination of source locations and consequently our understanding of the generation mechanism(s) of planetary radio emissions.

The study of observed wave-particle interactions will improve our understanding of the processes that occur in the solar wind and at Jupiter and of radio wave generation.

EPAC/GAS

Energetic PArticles Composition / Interstellar Neutral Gas

The two sensor systems EPAC and GAS are dedicated to the interplanetary ions and neutral gas respectively. The GAS detector was originally to fly on the NASA satellite of the dual-spacecraft ISPM (International Solar-Polar Mission), but after cancellation of the US satellite it was incorporated in the Ulysses payload, closely interfaced with the EPAC instrument.

EPAC - Energetic PArticles Composition

The EPAC sensor is designed to measure the fluxes, angular distributions, energy spectra, and composition of ions in the energy range from 300 keV/nucleon to 25 MeV/nucleon. It comprises four telescopes that each use the dE/dx - E technique, where particles traverse a thin detector and then stop it in a second, much thicker detector. Particles that traverse the two detector stack, are eliminated by a third veto-detector.

The four telescopes (T1-T4) are mounted such that their central axes include angles of 22.5°, 67.5°, 112.5°, and 157.5°, with respect to the spacecraft spin axis. Each telescope has a field-of-view with a full angle of 35°. Thus the instrument covers 80% of the full sphere during one spacecraft rotation. Each telescope has a geometric factor of about 0.08 cm² sr.

Each telescope consists of three Si-surface barrier detectors, A, B, and C surrounded by a massive platinum shield. Background rejection is realised by using multiparameter analysis. All four telescopes operate in a self-calibrating dE/dx (detector A) versus E (detector B) mode, wherein particle tracks can be used to obtain conclusive absolute calibrations. Each telescope and associated electronics is able to measure the elemental composition of low-energy nuclei from hydrogen to iron.

The front detectors (A) are protected against sunlight by Al-layers, 80 µg cm^-2 thick, and are used with the Al-side facing the incoming particles. The B-detectors are installed with their Au-side facing the front detector to minimize radiation damage.

GAS - Interstellar Neutral Gas

The Solar System moves through the local interstellar medium (IM) with a relative velocity of 20 kms^-1. The Sun's magnetic field prevents the charged component of the IM from entering the Solar System, but not the neutral component. The properties of the local interstellar gas, represented by neutral helium penetrating the heliosphere, can therefore be measured in-situ by the Ulysses GAS instrument.

The neutral helium particles are detected via the secondary electrons or ions which are emitted upon particle impact from a freshly deposited lithium-fluoride (LiF) layer in the GAS detector.

Two nearly identical detector channels are housed in a vacuum-tight box of 9.1 × 5.6 × 3.4 cm. The fields of view are limited by two circular apertures each to full opening angles of 4.9° in channel (I) and 7.4° for channel (II). The outer apertures are protected by a simple, asymmetric baffle against direct sunlight.

Incoming particles first pass electrostatic deflection systems, which serve as filters against charged particles up to energies per charge of ~80 kV in channel (I) and ~50 kV in channel (II) due to a DC-voltage between the plates. The conversion plates consisting of reduced (black and conductive) lead glass are mounted on ceramic thick- film resistors, used as heaters to bake out the conversion plates at temperatures up to 200 °C. The plates, with an effective area of about 8 × 10 mm, are inclined by 45° to the optical axis towards the channeltrons and by 28° towards the furnace, which is mounted in the middle between the two channels.

The tiny furnace is filled with about 2 mm³ of LiF. On telecommand a helix (1 mm diameter, 3 mm length) of platinum wire heats the LiF efficiently up to about 600 °C, where a mild evaporation of the LiF starts. The evaporated LiF is deposited simultaneously on both the conversion plates and a small quartz crystal used to monitor the thickness of the deposited layers. The supply is sufficient for a total thickness of 150 nm of the deposited layers, or about 20 evaporation processes.

Secondary particles released from the conversion plates after particle impact are accelerated towards the funnel of the two (one for each channel) Channel Electron Multipliers (CEMs) by an electric field between the conversion plate and the plane grid in front of the CEM. Here the number of secondary particles is measured. The acceleration potential can be selected by telecommands to select the detection mode. It is about -2700 V for detection of the positive secondary ions or about +420 V for detection of secondary electrons. The two channels (I) and (II) are always operated simultaneously in the same mode (electron or ion detection).

The sensor head is mounted on a turntable, with the rotation axis oriented perpendicular to the spin axis of the spacecraft. This way the elevation angle between the spin axis and the optical axis of the sensor can be varied between 0° and 180° with a minimum step width of 1°. Then, together with the rotation of the spacecraft, the whole celestial sphere can be scanned.

Summary of Objectives

EPAC
Among the range of investigations the EPAC measurements contribute to, are the studies of:

• Variations in the elemental abundances of low-energy charged particle populations close to the Sun:
  

  	-	The influence of various coronal structures upon coronal transport
  	-	The energy dependence of coronal storage
  	-	The overall characteristics of interplanetary propagation of these particles at different heliographic latitudes
  	-	The influence of the interplanetary magnetic field and its fluctuations on particle propagation (diffusion)
  	-	The effects of solar-wind streams (convection and adiabatic deceleration)
  	-	The importance of gradient and curvature drifts
  	-	The influence of the induced electric-field drift on low- energy particles
  	-	The dependence on heliographic latitude of all of the above components

• The acceleration mechanism of interplanetary energetic particles that are accelerated in interplanetary space out of the thermal and suprathermal tails of the ion energy distributions by for example interplanetary-propagating shock waves or corotating (with the Sun) interaction regions
• The occurence and latitudinal dependence of the anomalous cosmic ray (ACR) component (ACR particles originate at the edge of the heliopause, as opposed to from within our galaxy like most of the galactic cosmic rays). ACR are predominantly singly charged, moderate-velocity, high-rigidity ions that have their origin as interstellar neutrals which have been ionized in interplanetary space, convected outward by the solar wind and accelerated by the termination shock
• The penetration and streaming of galactic cosmic rays into the heliosphere

GAS
The main goal of the GAS instrument is to determine the density, bulk velocity relative to the Solar System, and temperature of the interstellar particles penetrating the Solar System.

HI-SCALE

Heliosphere Instrument for Spectra, Composition & Anisotropy at Low Energies

HI-SCALE is designed to make measurements of interplanetary ions and electrons throughout the entire Ulysses mission. The ions (E_i ≥ 50 keV) and electrons (E_e ≥ 30 keV) are identified uniquely and detected by five separate solid-state detector telescopes that are oriented to give nearly complete pitch-angle coverage from the spinning spacecraft.

Ion elemental abundances are determined by a ΔE vs E telescope using a thin (5 µm) front solid state detector element in a three-element telescope.

Inflight calibration is provided by radioactive sources mounted on telescope covers which can be closed for calibration purposes and for radiation protection during the course of the mission.

Ion and electron spectral information is determined using both broad-energy-range rate channels and a 32 channel pulse-height analyser (channels spaced logarithmically) for more detailed spectra.

Summary of Objectives

HI-SCALE is designed to make significant advances toward understanding physical processes involved in the solar control of low-energy ions and electrons in the heliosphere. The key scientific objectives include:

• Low energy solar particle fluxes will be used as probes of the morphological changes in coronal and interplanetary magnetic field structures as a function of heliolatitude
• Measurements of both relativistic and nonrelativistic energy electrons and nonrelativistic energy ions are used to model the physical conditions in flares and study solar-flare process
• Measurements of the chemical (atomic) composition of low-energy nuclei emitted from the Sun in the active region band and at high heliolatitudes will provide insight into the solar elemental abundances
• Determining the parameters of low-energy solar particle propagation in interplanetary space using measurements of the particle anisotropy and composition as functions of heliographic latitude. Of particular interest is the study of particle propagation in the vicinity of the neutral sheet in the interplanetary magnetic field
• Correlations of non-relativistic electron events with on-board Ulysses radio measurements provide quantitative physical parameters of outward- propagating wave-particle interactions at different latitudes in the heliolatitude-dependent interplanetary plasma
• Changes in particle energy distributions will be measured and the physical processes in the interplanetary medium that cause them will be identified. Particular emphasis will be placed on modelling the changes produced by shock, stochastic, and other possible mechanisms of particle acceleration as the Ulysses spacecraft moves away from the ecliptic plane
• The 'quiet-time' low-energy particle populations in the interplanetary medium will be measured, and the possible separation of the solar, galactic, and planetary magnetosphere components will be made by their different heliolatitude variations
• Measurements of the temporal and spatial variations in low-energy particle intensities and compositions in the vicinity of the Jovian magnetosphere will be made
• The new knowledge gained from HI-SCALE investigations of the global dynamics and structure of the heliosphere will be used to define in a more quantitative way the influences of solar activity on the terrestrial environment and its technological systems
  

COSPIN

COsmic Ray and Solar Particle INvestigation

COSPIN comprises a set of five telescope subsystems with a total of six charged particle telescopes to address a wide range of scientific objectives made possible by a mission to investigate the Sun and the heliosphere in three dimensions.

Low Energy Telescope (LET)
LET measures the flux, energy spectra and elemental composition of solar energetic particles and low energy cosmic ray nuclei from hydrogen up to iron. The instrument covers an energy range from ~ 1 to ~ 75 MeV/n, using a double dE/dx versus E telescope. Comprehensive on-board particle identifier electronics and an event priority system enable rare nuclei to be analyzed in preference to the more common species. Isotope separation for light nuclei such as helium is also achieved.

Anisotropy Telescopes (AT)
The ATs sensor unit consists of two, identical charged-particle telescopes, each with a geometrical factor of 0.75 cm²sr, whose role is to measure the three-dimensional charged-particle distribution in the energy ranges 0.7 to 2.2 MeV for Z ≥ 1, 2.2 to 6.5 MeV for protons, and 3.1 to 23.0 MeV for Z ≥ 2. The three dimensional distribution measurements are achieved by inclining the two telescopes at independent angles (AT1 at 145° and AT2 at 60°) to the spin axis of the spacecraft and sectoring (8 sectors) the data outputs of the telescopes during each spin. Both telescopes have a 70° full-opening angle.

High Energy Telescope (HET)
HET is a large geometric factor cosmic ray telescope that uses particle trajectory determination together with the dE/dx versus residual E technique to measure the energy and identify the mass and charge of cosmic rays. For particles which stop in the detector stack, the telescope provides charge and mass resolution sufficient for studies of the chemical and isotopic composition of cosmic rays from hydrogen through nickel (1 ≤ Z ≤ 28). The trajectory of incident cosmic rays can be determined to an accuracy of better than 1°.

High Flux Telescope (HFT)
The HFT, which is mounted on top of the HET, consists of a single 25 mm² × 18 µm silicon surface-barrier detector, passively collimated by a fan-shaped aluminium collimator to give a viewing aperture of 17° × 60°, with a geometrical factor of 0.033 cm²sr. The collimator imposes a low-energy cut-off of 50 MeV for protons and 5 MeV for electrons incident from outside the viewing aperture.

Kiel Electron Telescope (KET)
The KET telescope is mounted to view perpendicular to the spacecraft spin axis and has an acceptance angle of 44.6° full cone with an auxiliary field of view of 106°. KET is designed to measure electron fluxes between 2.5 and 6000 MeV, and to determine energy spectra in the range 7 - 170 MeV. The telescope also provides measurements of the proton and alpha-particle fluxes in several energy windows between 3 and > 2100 MeV/nucleon. In addition, two low-energy electron and proton channels provide anisotropy information in 8 sectors.

Summary of Objectives

Examples of the COSPIN scientific goals include:

• For energetic charged particles of solar origin, to determine the role of coronal magnetic fields in their acceleration and propagation and to search for the origin of the enrichment of ³He and Fe nuclei observed in some solar particle events
• Using galactic cosmic radiation measurements, to explore the likely reduction or elimination of solar modulation in polar regions relative to the equator, to search for the origin of the anomalous nuclear component, and to determine the nucleosynthetic origins of nuclei at lowest measurable energies
• For energetic nuclei and electrons of interplanetary origin, to study the three-dimensional character of travelling shocks, corotating interaction regions and their associated charged particle acceleration
• As a secondary scientific objective at Jupiter encounter (closest approach 8 Feb., 1992), to characterize the energetic charged particle populations during the first traversal of the dusk side of the Jovian magnetosphere and to search for the mechanism producing the ~10 hour clock variation of Jovian electrons in the interplanetary medium
  

GRB

Gamma Ray Burst

The Ulysses solar X-ray/cosmic gamma-ray burst instrument comprises both soft and hard X-ray detectors. The design of the GRB experiment had to took into account several constraints. A radiation-hardened microprocessor was required to survive the passage through the Jovian radiation belts and the ones available during the design phase of the GRB experiment dictated simplified operating modes for the experiment. Another important constraint was the environment conditions imposed by the spacecraft's powering system: the Radioisotope Thermoelectric Generator (RTG). To minimize the interference from the RTG, the sensors had to be mounted on the magnetometer boom, and were required to be essentially amagnetic (the remnant field could not exceed 2 × 10^-5 G at 25 cm).

Hard X-ray detectors
The hard X-ray detector was designed to operate in the nominal energy range 15-150 keV. The sensor shape was chosen to have a nearly isotropic response, because being located on the magnetometer boom allows for a nearly all-sky visibility. The system consists of two 3-mm thick by 51-mm diameter CsI(Tl) crystals mounted via a plastic light guide to two photomultiplier tubes.

Soft X-ray detectors
The soft X-ray sensors were designed as solar X-ray monitors for the energy range ≈ 5-20 keV. They consist of two 500-µm thick, 0.5-cm²-area Si surface barrier detectors. A 100 mg cm^-2 beryllium foil front window rejects low energy X-rays and defines a conical field of view of 75° half-angle. The amplified pulses of the two Si detectors are analyzed by a hybrid stack of six-level discriminators which define four differential energy channels and two integral channels.

Summary of Objectives

The three main scientific objectives of the GRB experiment are:

• The study and monitoring of solar flare X-ray emission
• The detection and localization of cosmic gamma-ray bursts
• The in-situ detection of Jovian auroral X-ray radiation

DUST

Cosmic Dust

The Ulysses dust experiment provides direct observations of dust grains in interplanetary space and allows for investigation of their physical and dynamical properties as functions of heliocentric distance and ecliptic latitude. Of special interest is the question of what portion is provided by comets, asteroids and interstellar particles.

The Ulysses dust detector is a descendant of the dust detector flown on the HEOS-2 satellite. This instrument carried out measurements in the near-Earth space and observed various effects of the Earth's magnetosphere and the Moon on the interplanetary dust population.

DUST 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. It consists of an impact ionization sensor and the appropriate electronics and allows for measuring the mass, speed, flight direction and electric charge of individual dust particles.

Positively or negatively charged particles entering the sensor are first detected via the charge Q_P which they induce to the charge grid while flying between the entrance and shield grids. All dust particles - charged or uncharged - are detected by the ionization they produce during the impact on the hemispherical impact sensor. After separation by an electric field, the ions and electrons of the plasma are accumulated by charge sensitive amplifiers (CSA), thus delivering two coincident pulses Q_E and Q_I, of opposite polarity. The rise times of the pulses, which are independent of the particle mass, decrease with increasing particle speed. From both the pulse heights and rise times, the mass and impact speed of the dust particle are derived by using empirical correlations between these four quantities. A third independent signal originates from part of the positive impact charge which is detected and amplified (~ 100×) by an electron multiplier (channeltron). This signal Q_C serves as a control for the identification of dust impacts.

Summary of Objectives

The overall objective of the Ulysses dust experiment is the investigation of the physical and dynamical properties of small dust particles (10^-16 - 10^-6 g) as a function of ecliptic latitude and heliocentric distance, and the study of their interrelation with interplanetary/interstellar phenomena. The parameters to be measured include the mass, speed, flight direction and electric charge of individual particles. The impact rate, size frequency, and the distribution of flight directions and electric charges will be determined. Specific objectives are:

• To allow us to classify particle orbits into bound orbits around the Sun or hyperbolic orbits leaving or entering the Solar System. The distributions of orbital elements (semi-major axis, eccentricity, inclination) of particles in bound orbits will be studied
• To determine as functions of heliocentric distance and ecliptic latitude the spatial density of the interplanetary large particle population which generally moves on bound orbits around the Sun, and to determine the relative significance of comets and asteroids as sources for these zodiacal dust particles
• To measure the flux and velocity of particles coming on hyperbolic orbits from the general direction of the Sun
• To identify interstellar dust particles and perform direct measurements of the spatial density, heliocentric distribution, velocity and mass of interstellar grains transiting the Solar System
• To observe enhancements of cometary dust particles during the transit of the spacecraft through the plane of a comet's orbit
• To investigate the spatial density of dust particles within the asteroid belt and determine the dust production by collisions in the asteroid belt
• To investigate the influence of the Jovian gravitational field on the interplanetary dust population
• To measure electric charges of dust particles and establish the relationship of these charges with properties of the ambient plasma (plasma density, energy spectrum), the solar radiation spectrum and magnetic fields

SCE

Solar Corona Experiment

The SCE performs coronal-sounding measurements of the Sun's corona during the spacecraft's solar conjunctions. These investigations exploit Ulysses' radio communications links with Earth to extract information on the structure of otherwise virtually inaccessible media throughout the heliosphere.

Spacecraft Radio Telecommunications Subsystem
The SCE instrumentation consists of the same equipment on the spacecraft and in the ground stations that is used for radio communications (command, telemetry, and navigation). During SCE data recording intervals the telecommunications subsystem is configured to receive an S-band signal from the ground station and transmit S- and X-band signals coherent with the received signal. The coherence ratio (transponder multiplier from S-band up to S-band down) is 240/221. The ratio of the two downlink frequencies, X-band to S-band, is 11/3. The signals are transmitted to the ground station via the dual-feed high gain antenna.

Ground stations: the DSN radio science system
With the exception of the 34-m station (DSS 12) in Goldstone, California, all DSN antennas are equipped with both closed-loop and open-loop receivers. These receivers are supplied with frequency and timing reference signals from a highly stable hydrogen maser frequency standard.

• Closed loop
  As part of each station's receiver-exciter subsystem, the closed-loop receiver acquires and tracks the spacecraft carrier signal using a phase-locked loop feedback scheme. The tracking subsystem estimates and reports the Doppler shift after comparing the phase-locked loop frequency output with a reference from the station's frequency standard, which is also used to generate the uplink signal. In addition to the Doppler measurements, the spacecraft's range can be determined using the sequential ranging assembly. Modulated with range code, the S-band uplink signal is transmitted to the spacecraft where it is detected and transponded back to the station. The round-trip light time, directly related to the spacecraft's distance, is computed in the ground station by comparing the received code with the transmitted code
• Open loop
  Used primarily for Radio Science experiments and VLBI, the open-loop receiver downconverts the filtered carrier signal to baseband where it is digitized and recorded. The receivers at the 70-m stations have four channels, thereby enabling simultaneous recording of both S- and X-band downlinks in both polarizations (left- and right-hand circularly polarized)
  

Summary of Objectives

The main science objective of SCE is to derive the plasma parameters of the solar atmosphere:

• To derive the 3-D distribution of the coronal electron density from the SCE ranging and Doppler data products. The large-scale structure can be inferred from the total electron content I obtained from dual-frequency ranging
• In addition to the quasi-static structure of the outer corona, the dual-frequency Doppler data will be used to characterize the level and spectral index of coronal turbulence
• Use Multi-station observations to derive the plasma bulk velocity at solar distances where the solar wind is expected to undergo its greatest acceleration

In addition, it is anticipated that the interplanetary vestiges of a Coronal Mass Ejection will be occasionally detected as a significant perturbation in the ranging and Doppler data.

Profiting from the favourable geometry during the Jupiter encounter on 8 February 1992, the SCE was also used to measure the electron density of the Io Plasma Torus.

GWE

Gravitational Wave Experiment

Since the optimum size of a gravitational wave detector is the wave length, interplanetary dimensions are needed for detecting gravitational waves in the mHz range. Doppler tracking of Ulysses provides sensitive detections of gravitational waves in this low frequency band. The driving noise source is the fluctuations in the refractive index of interplanetary plasma. This dictates the timing of the experiment to be near solar opposition and sets the target accuracy for the fractional frequency change at 3.0 × 10^-14 for integration times of the order of 1000 seconds.

In the spacecraft Doppler tracking method, the Earth and spacecraft constitute the two objects whose time-varying separation is monitored to detect a passing gravitational wave. The monitoring is accomplished with high-precision Doppler tracking in which a constant frequency microwave radio signal (S-band) is transmitted from the Earth to the spacecraft (uplink); the signal is transponded (received and coherently amplified) at the spacecraft; and then transmitted back to Earth (downlink) in both S- and X-band signals. This Dual frequency downlink is required in order to calibrate the interplanetary media which affects the two frequency bands differently. The downlink signal is recorded at Earth and its frequency is compared to the constant uplink frequency f₀ to extract the Doppler signal, δf / f₀.

Summary of Objectives

The objective of the gravitational wave investigation on Ulysses is to search for low frequency gravitational waves crossing the Solar System. Because of the great distance to the spacecraft, this method is most sensitive to wave periods between about 100 - 8000 seconds, a band which is not accessible to ground-based experiments which are superior for periods below 1 second.

Interdisciplinary Investigations

These investigations, although providing no hardware of their own, combine data from several Ulysses experiments to study specific scientific questions of relevance to the mission.

Directional Discontinuities
This investigation looks at large- and small-scale plasma irregularities or inhomogeneities in the solar wind, which is not the well-behaved, uniform medium normally considered by theoreticians. Depending on the type of variation shown by the magnetic-field and plasma parameters, a directional discontinuity can be either a tangential discontinuity, a shock, or a true rotational discontinuity. New theoretical models of these phenomena are developed as part of the Ulysses investigation, and the magnetic-field and plasma observations obtained by Ulysses are used to check these models and identify fundamental plasma-physical processes.

Mass Loss and Ion Composition
This investigation studies the dependence of coronal mass loss on heliographic latitude, with a view to improving the understanding of the energy and momentum balance of the outer layers of the Sun. Secondly, a search is made for a latitudinal dependence of the solar wind ion composition.