Astronomy and Astrophysics Supplement Series, Ulysses Instruments Special Issue, Vol. 92, No. 2, pp. 411-423, Jan. 1992. Copyright © 1992 European Southern Observatory. Reprinted by permission.

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Astron. Astrophys. Suppl. Ser: 92, 411-423 (1992)

The Ulysses dust experiment

E. Grün1, H. Fechtig1, R.H. Giese2*, J. Kissel1, D. Linkert1, D. Maas1,3, J.A.M. McDonnell4, G.E. Morfill5, G. Schwehm6 and H.A. Zook7

1 Max-Planck- Institut für Kernphysik, Heidelberg, Germany
2 Ruhr Universität, Bochum, Germany
3 Kernforschungszentrum Karlsruhe, Karlsruhe, Germany
4 University of Kent, Canterbury, U.K.
5 Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany
6 ESA ESTEC, Noordwijk, The Netherlands
7 NASA Johnson Space Center, Houston, Texas, U.S.A.

 * in memoriam.

 Received April 24; accepted June 27, 1991

Abstract. -- The Ulysses dust experiment is intended to provide direct observations of dust grains with masses between 10-16 g and 10-6 g in interplanetary space, to investigate 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 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 3.8 kg, consumes 2.2 W, and has a normal data transmission rate of 8 bits/s in nominal spacecraft tracking mode. On 27th October 1990 the instrument was switched-on. The instrument was configured to flight conditions and science data collection started immediately. In the period to 13th January 1991 at least 44 dust impacts have been recorded. Flux values are given covering the heliocentric distance range from 1.04 to 1.7 AU.

Key words: artificial satellites, space probes--instrumentation interplanetary medium.

1. Introduction and scientific objectives.

There are several methods to study various aspects of interplanetary dust (for a review see Leinert & Grün 1990).Observations of the scattered sunlight from interplanetary dust and its thermal emission (both components are called zodiacal light) reveal the large scale spatial distribution of particles in the 10 µm to 1 millimeter size range. Meteor observations refer to millimeter and larger objects, the orbits of which intersect the Earth. Interplanetary dust particles collected in the stratosphere allow us to obtain compositional and morphological information. From lunar microcrater studies, the size distribution of sub micron to mm sized particles was determined. Only very limited information is available from these Earth-based methods on dust in the outer solar system and on dust outside the ecliptic plane. Complementary to the above mentioned methods are remote sensing and in-situ studies by instruments on board interplanetary spacecraft (McDonnell 1978). For the original two-spacecraft International Solar Polar Mission (ISPM) two complementary dust instruments were selected. A zodiacal light photometer on the NASA spacecraft and an in-situ dust detector on the ESA spacecraft.

Unfortunately, after the reduction of ISPM to the pre-sent single spacecraft Ulysses mission this complementary approach could no longer be continued and only the in-situ dust detector remained on board the spacecraft.

In-situ dust detectors give information on physical and orbital properties of individual particles as well as size and flux distributions of small particles. The Ulysses mission provides an unique opportunity for the study of various aspects of the interplanetary dust complex. During its six-year long interplanetary cruise it will cover a heliocentric distance range from 1 to 5.4 AU and an ecliptic latitude range from -80 to +80 degrees. Measurements by a twin dust detector on the Galileo spacecraft (Grün et al. 1992)which was launched in October 1989 will serve as an in-ecliptic base line for the measurements by Ulysses.

The Ulysses and Galileo dust detectors are descendants of the dust detector flown on the HEOS-2 satellite(Dietzel et al. 1973). 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 (Hoffmann et al. 1975; Fechtig et al. 1979). These dust detectors are based on the impact ionization phenomenon (Friichtenicht & Slattery, 1963; Fechtig et al. 1978), which provides extremely high sensitivity for recording small dust particles. Table 1 compares characteristics of different interplanetary dust detectors.

Two types of impact detectors were used for interplanetary dust measurements: impact ionization detectors with detection thresholds of 10-16 to 10-13 g and penetration detectors with detection thresholds of 10-9 and 10-8 g. The dust detector on board the VEGA mission has a detection threshold of 10- 12 g and combines aspects of both detectors (Simpson et al. 1989). The given detection thresholds refer to a typical impact speed of 20 km/s. The penetration detectors on board Pioneers 10 and 11 (Humes 1980) have large geometric factors, i.e. sensitive areas and effective solid angles (for reference, a flat plate has [pi] sr effective solid angle). Most impact ionization detectors have sensitive areas of only 0.01 m2, except the Galileo and Ulysses instruments which have a ten times larger sensitive area. Because of their limited effective solid angles, HEOS 2, Helios 1/2 (Dietzel et al. 1973), Ulysses and Galileo detectors provide some directional information. For example, in an isotropic flux half of the particles are recorded from within a cone of 32 degrees half angle around the axis of the Ulysses sensor. The Pioneer 8 and 9 detectors (Berg & Richardson 1969) could record approximate directions for a few time-of-flight events; most of the impacts, however, were recorded by the wide angle front film sensor (Berg & Grün 1973). The dynamic range of the instrument describes the range over which particle masses can be determined at a given impact speed. For larger particles the instruments reach saturation and only lower mass limits can be stated. Dynamic range of 1 for the Pioneer 10 and 11 instruments implies that only lower mass limits for all impacts can be determined. Since the interplanetary dust flux is not isotropic the actual viewing geometries of the detectors are of importance.

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:

2. Scientific background.

2.1. THREE-DIMENSIONAL DISTRIBUTION OF THE ZODIACAL DUST CLOUD.

Zodiacal light observations have so far provided the primary means of studying the large scale out-of-ecliptic distribution of the interplanetary dust cloud. By inversion of the observations it has been derived that the grain properties depend on their elevation above the ecliptic plane, i.e. upon the inclination of their orbits (Levasseur-Regourd 1991). There are at present various three - dimensional models of the Zodiacal dust cloud predicting equidensity surfaces of fan-like or ellipsodial shape, as well as more complicated (two-lobe) models (Giese et al. 1985 and 1986). The Ulysses dust experiments will set stringent limits on the model parameters by requiring compatibility between the fluxes measured and the observed intensity, polarisation and color or zodiacal light. It has been shown (Whipple 1967; Grün et al. 1985) that the meteoritic dust cloud at 1 AU is self-destructive on a time scale of the order of 105 years. Therefore, to maintain the meteoritic dust complex efficient sources are required to replenish it. The in- situ measurement of the three-dimensional spatial and orbital distribution of dust particles will yield information on the relative significance of the three possible sources, i.e. asteroids, comets and interstellar dust.

The two dust experiments onboard the Pioneer 10 and 11 spacecraft provided information on the radial dependence of the spatial density of large dust grains (~>= 20 µm diameter outside the Earth's orbit. Between 1 and 3.3 AU these experiments detected a decrease of the dust abundance proportional to r-1.5 (Humes et al., 1974; Hanner et al., 1976). The photometer did not record any scattered sunlight above background beyond the asteroidal belt while the penetration experiment recorded dust impacts out to about 18 AU distance from the Sun at an almost constant rate (Humes 1980). The seemingly contradictory measurements outside the asteroid belt may be explained in such a way that the registered dust grains which are on high inclination orbits are most likely of cometary origin. From the Halley investigations it is known that cometary grains have low albedos (Keller et al. 1986). Therefore, a possible explanation for the dust population outside the asteroid belt is that these dust grains are comparatively young cometary dust grains of extremely low albedos on "cometary" orbits (Fechtig 1989).

2.2. DYNAMICS OF INTERPLANETARY DUST.

Recent measurements of dust in the inner solar system with the Pioneer 8/9 and Helios space probes and the HEOS-2 satellite have shown that there are several populations of dust particles possessing different dynamical properties. The orbital distribution of larger meteoroids is best known from meteor observations.

According to Sekanina & Southworth (1975), sporadic meteoroids move on orbits with an average eccentricity of 0.4 and an average semi-major axis of 1.25 AU at the Earth's orbit. Helios measurements allowed us to identify an interplanetary dust population (Grün et al. 1980) which consists of particles on highly eccentric orbits (e > 0.4) and with semi-major axes > 0.5 AU. Pioneer 11 in-situ data obtained between 4 and 5 AU are best explained by meteoroids moving on highly eccentric orbits (Humes 1980). These particle populations resemble most closely the sporadic meteor population.

A second population of slow-moving, small (10- 13 g ~<= m ~<= 10-11 g) particles has been observed by the Pioneer 8/9 (Berg & Grün 1973) and HEOS 2 (Hoffmann et al. 1975) dust experiments. The particles were found to arrive from the apparent apex direction at a relative speed of approx. 10km/s (at 1 AU). The Helios dust experiment (Grün et al. 1980) has seen these particles which orbit the Sun on low eccentric orbits (e < 0.4). These low angular momentum "apex" particles are thought (Grün & Zook 1980) to originate from collisional break-up of larger meteoroids in the inner solar system.

A third population consisting of very small particles (m ~<= 10-13 g) has been detected by the Pioneer 8/9 and Helios space probes arriving at the sensors from approximately the solar direction. Existence of these particles was recently confirmed by Hiten (Igenbergs et al. 1991) measurements. These particles have been identified by Zook & Berg (1975) as small grains generated in the inner solar system and leaving the solar system on hyperbolic orbits due to the dominating effect of the radiation pressure force. The ratio of radiation pressure force (Frad) over solar gravity (Fgrav) is termed, ([beta] = Frad/Fgrav and depends only on particle properties (Burns et al. 1979). Particles with, [beta] ~ 1 are called [beta]-meteoroids.

Dust particles in space are electrically charged to an equilibrium potential given by the opposing effects of photoelectric emission caused by solar UV radiation and electron recombination from the ambiant plasma (solar wind or magnetospheric plasma). Theoretical studies of the interaction of charged dust particles with the interplanetary magnetic field (Grün et al. 1984; Morfill et al. 1986) have shown that considerable effects on the distribution and dynamics of dust in the solar system can be expected. Direct measurements of the charges of some large interplanetary dust particles have been performed with the Helios 1/2 micrometeoroid experiments (Leinert & Grün 1990).

2.3. INTERSTELLAR GRAINS IN THE VICINITY OF THE SUN.

The spatial density of interstellar dust grains in the vicinity of the solar system is estimated to be within the range 10-27 g cm-3 to 10-26 g cm- 3. The higher value is derived from the average interstellar extinction of 1 mag per kpc (Greenberg, 1973) while the lower value assumes that the solar system is currently surrounded by a low density warm interstellar medium of 2 10- 25 g cm-3, one percent of which is in dust (Wood et al. 1985). We will use this lower value in the following discussion. The Sun moves with respect to this material at a speed of 20 km/s which amounts to a mass flux of 2 x 10-17 g m-2 s-1. If we assume that most mass is in particles of radius s = 10-7 m (m ~ 10-14 g), then a flux of 2 x 10-3 m-2 s-1 is expected to arrive from the solar apex direction. The observed interplanetary dust flux of this sized particles at 1 AU distance form the Sun is 2.5 x 10-4 m-2 s-1 (Grün et al. 1985). McDonnell & Berg (1975) have shown that the interstellar component of cosmic dust is less than 0.03 of the interplanetary flux. These numbers result in an upper limit for the interstellar particle flux at the Earth's orbit of only 7.5 x 10- 6 m-2 s-1, which is a factor 300 below our estimate. What is wrong?

Interstellar grains entering the solar system are affected by solar gravity, radiation pressure and electro-magnetic effects. Therefore, small interstellar dust particles may be prevented from entering the inner solar system and are not accessible to in-situ observations at the Earth's orbit. At 2 AU, for example, particles whose flow speed is v = 20 km/s can only be observed if their [beta] <= 1.5. The interaction of charged interstellar grains with the interplanetary magnetic field may also prevent interstellar particles (a <= 10-7 m) from entering the inner solar system. The deflection of particles depends also on the ecliptic latitude b at which they enter the solar system. At |b| <= 15 deg., particles encounter the alternating sector structure of the interplanetary magnetic field and are thus less deflected on average than those particles entering at higher ecliptic latitudes, which encounter a magnetic field of constant polarity (Morfill & Grün 1979). In any case, the most favourable mission for the in-situ detection of interstellar dust is one which goes as far as possible away from the Sun and covers a wide range of ecliptic latitudes. The Ulysses mission is excellently suited for this purpose, since the spacecraft will reach large heliocentric distances in the ecliptic and covers a latitude range of 0 deg. to -70 deg. outside r = 2.5 AU.

2.4. COMETARY DUST.

As far as it is presently known, comets provide a large source of small interplanetary particles. The injection of dust into the interplanetary medium has been directly observed by the missions to comets Halley and Giacobini-Zinner. Dust which was just released from a comet can only be observed by an in-situ experiment if the comet passes between the spacecraft and the Sun close enough in space and time. A detection of dust from comet Kohoutek by the HEOS satellite has been reported by Hoffmann et al. (1976).

2.5. ASTEROIDAL DUST.

Within the asteroid belt, the Pioneer 10/11 dust experiments did not find any increase of the dust abundances as had previously been expected by some investigators. The conclusion of the experimentalists (Humes et al. 1974) was that not much dust is produced in the asteroid belt. Observation of asteroidal dust bands by the infrared satellite IRAS (Hauser et al. 1984) conversely found that the asteoroid belt is a significant source of dust in the solar system. The study of lunar microcraters indicates that the majority of the projectiles which produced lunar microcraters have densities between 3 and 8 g cm-3. These high density particles could well be predominately of asteroidal origin (Fechtig 1989). Inside 1 AU, however, measurements of the dust population indicate that most of the mass of the dust grains there is probably of cometary origin (Grün et al. 1985).

2.6. DUST IN JUPITER'S MAGNETOSPHERE.

Dust has been observed in the Jovian magnetosphere, both by in-situ experiments onboard Pioneer-10 and 11, and by remote sensing instrumentation onboard Voyager-1 and 2. Pioneer-10 and 11 measured a 1000-times increased flux of micrometeoroids in the vicinity of Jupiter compared with the interplanetary flux. The voyager imaging experiment detected a ring of particulates at distances out to 2.5 Jovian radii, as well as volcanic activity on Io, which may be able to eject small dust particles into the Jovian magnetoshpere. All of these phenomena may be related, possibly via dynamic effects of the dust particles (cf. Burns et al. 1984).

3. Experimental approach

3.1. INSTRUMENT DESCRIPTION.

The Ulysses dust detector is very similar to the Galileo dust detector which has been described in detail by Grün et al. (1992). Therefore, only the basic features of the instruments are repeated and some additional information is given in this paper. Both instruments detect individual particles impacting on the sensor and measure their mass, impact speed, electric charge, and determine the impact direction. Both instruments consist of an impact ionization sensor and the appropriate electronics. The Ulysses instrument weighs 3.8 kg and consumes 2.2 W.

The measuring principle of the Ulysses sensor is illustrated in Figure 1. Positively or negatively charged particles entering the sensor are first detected via the charge QP 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 QE and QI, 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 (approx. x 100) by an electron multiplier (channeltron). This signal QC serves as a control for the identification of dust impacts. The thresholds and the dynamic ranges of the different measurements are given in Table 2.

A measurement cycle is initiated if either QE, QI or QC exceed a threshold. In order to avoid interferences by noise on one or two of these measurement channels, it is possible to switch by command to a mode in which a measurement cycle is initiated if two or even only one of them exceed the respective threshold. Another measure to lower the susceptibility for noise is to raise the thresholds (up to a factor 10) of the different measurement channels by telecommand according to the actual noise environment on board the spacecraft. Coincidences between the different signals are derived and evaluated in an event classification scheme. The instrument and its operation are designed to reliably suppress noise and allow detection of as few as one impact event per month, the impact charge of which is close to the detection threshold.

The signal amplitudes and times of a single recorded event are digitized and stored in an Experiment Data Frame (EDF). Supplementary information like time of impact and instantaneous spin position are collected from the spacecraft and added in each EDF. If more than one event occurs before this EDF has been transmitted to ground or read to tape, then each event is counted by one of 24 amplitude dependent counters. The dead-time caused by the measurement cycle is 5 ms.

Figure 2 shows a block diagram of the Ulysses instrument. The signals from the sensor are conditioned and analysed. The microprocessor coordinates the experiment measurement cycle, collects the buffered measurement data and processes the data according to a program stored in the memory.

Each event recorded by the instrument is classified according to its ion signal amplitude into 6 amplitude intervals (A1 to A6) of about one decade width. In addition each event is categorized by one out of four event classes. Class C0 are all events (noise events and some unusual impact events - e.g. hits of the sensor's internal structure) which are not categorized in a higher class. For classes C1 through C3 events the measured parameters and their relation to each other are increasingly restricted, so that C3 represents generally only dust impact events. In the A-range of the instrument memory six EDFs containing information on discrete events of any class but of six different amplitude intervals are buffered. In addition eight EDFs containing information on the most recent class C3 events are stored in the instrument buffer. This classification scheme can be adapted to the in-flight noise situation by changing the classification parameters (set points) or by adjusting the onboard classification program by commands. Therefore, detailed information on noise is mandatory in order to optimize the memory utilization strategy, to evaluate the reliability of impact detection for the various event classes as well as to minimize the effect on dead-time. Beside storing complete information in the buffer each event is counted in one out of 24 counters (each counting from 0 to 255) according to its amplitude range and event class. The matrix of the 24 accumulators (ACs) is shown in Table 3. This way each event is counted even if the complete information is not received on ground.

3.2. INSTRUMENT DATA.

In this paragraph the instrument data, its data transmission mode and its initial on-ground processing steps are described. All information on events recorded by the instrument (dust impacts or noise) is contained in packets (EDFs) of 16 bytes (1 byte=8 bits) of science data. The transmission of seven EDFs constitute an instrument read-out cycle (six A-range events and one of the subcommutated class C3 events as well as all 24 accumulators) which is continuously repeated

The Ulysses mission is designed to provide continuous data coverage even when data transmission to Earth is only possible during one pass of approx. 8 hours per day. Continuous coverage is achieved by storing data from the instruments at a low rate into an on-board memory which is read-out at high rate together with real-time 2 data transmission during a pass. At a spacecraft data transmission rate of 1024 bps one EDF is sent every 16 s. Lower bit rates down to 128 bps during storage or real time transmission periods are possible. Apart from science data there are also other types of data which are transmitted to ground: (1) housekeeping information (temperatures, currents and voltages) is regularly read out, (2) instrument memory content and (3) set point data are read-out only on request by telecommand.

Although continuous science data coverage is anticipated, there are several reasons why it is not achieved. There are gaps in the data transmission or storage due to anomalies on-board or on the ground. Even scheduled occurrences (like engineering data transmission for trouble shooting) may not be covered by on board science data storage due to operational constraints. In addition, bad data is received because of imperfections of the transmission link from the spacecraft to the computer on the ground.

The goal of the instrument data system is to transmit to ground full information on any dust impact or noise event which is recorded by the instrument. This goal is achieved when the event rates in each of the six amplitude ranges are smaller than one per instrument read-out cycle (112 s at 1024 bps) and when all data are received on ground. If the event rate is higher than that, then each event is only counted in one of the 24 accumulators. Because of the limited number of events which can be counted in each accumulator the maximum rate at which the full information can be transmitted to ground is 2.3 s- 1 at 1024 bps. If the event rate is larger than ~ 200 s- 1 dead time is generated. Each measurement (the generation of an EDF) causes about 5 ms instrumental dead time before the next event can be completely processed. In order to determine this type of dead time each threshold exceeding at any measurement channel is counted and intermittently transmitted to ground within an EDF. The mean resolution of these counters is 0.6 ms, 2.6 ms, and 5.0 ms for the P-, E-, I- and C-channels (named after their signal designations), respectively. Of course, for the dead time determination only those channels count which are allowed to initiate a measurement cycle. The aim of instrument operations is the supression of noise to the extent that data from most events is completely received on ground.

Data received from the dust experiment on ground will be initially evaluated according to the following scheme:

  1. Impact events, potential impact and noise events are extracted from the raw data and put into an Event File. Count rates of the different events are accumulated over fixed time intervals and put into an Accumulator File.

  2. Accounting for missing data and statistical analysis of the noise data will result in a Dead-time History File, with the help of which impact rates can be accurately determined.

  3. Identification of all dust impact data and calibration of this data will result in a Physical Parameter File, which then can be used for further analysis by the investigator team.

3.3. GEOMETRIC AND KINEMATIC SELECTION EFFECTS.

The geometric detection probability is defined by the sensitivity of the detector for particles impinging from different directions in an isotropic flux of particles. Directions are determined by an angle [theta] to the sensor axis. The maximum angle at which particles can impact onto the target is 70 degrees. The maximum area at [theta] = 0 deg. is 0.10 m2. The effective solid angle interval covered by the detector is 1.45 sr (cf. Göller & Grün 1989). The average impact angle is 36 degrees. The average flight path of a particle (corresponding to flight time measurement tPE) from the charge sensing grid to the hemispherical target is 20 cm ± 5 cm.

The Ulysses spacecraft spins at a rate of approx. 5 revolutions per minute. The spin axis (angular momentum vector) is anti- parallel to the centerline of the antenna dish which points towards the Earth. The sensor is mounted to the spacecraft body with its axis at an angle [gamma]U = 95 degrees with respect to the positive spin axis. Figure 3 shows the average exposed sensor areas as a function of this angle [gamma] for both the Ulysses and Galileo dust detectors. The sensor areas are averaged over a spin period. For a directed dust flux the maximum areas are 0.020 m2 at [gamma] = 95 deg. and 0.025 m2 at [gamma] = 50 deg. with respect to the spin axes of the Ulysses and Galileo sensors, respectively

The detection probability of the Ulysses dust detector for dust particles on a given orbit varies significantly between the in- ecliptic leg from Earth to Jupiter and the post-Jupiter out-of-ecliptic leg of the Ulysses trajectory. The necessary condition for detecting particles by the dust detector is that their orbits cross the orbit of Ulysses. In addition the angle between their relative velocity vectors and the Ulysses spin axis has to be within the sensitivity range shown in Figure 3. The detection probability for interplanetary particles on low inclination, low eccentricity orbits is low on the in-ecliptic leg but very high on the out-of-ecliptic leg, provided that their inclination is large enough to reach the latitude of Ulysses. The detection probability for interplanetary dust on highly eccentric orbits is medium to high on both legs. There are three periods during which [beta]-meteoroids are best observed: right after launch, during the latter portion of the in-ecliptic leg and during the ecliptic crossing between both solar polar passes. High kinematic probabilities for detecting interstellar particles entering the solar system exist close to their perihelion distances during the in-ecliptic leg and during all of the out-of-ecliptic leg of Ulysses orbit.

3.4. CALIBRATION AND PERFORMANCES PARAMETERS.

Extensive calibration tests have been performed with the Ulysses and Galileo dust detectors (Göller & Grün 1985, Göller & Grün 1989) at the Heidelberg dust accelerator. Calibration tests were performed with iron, carbon, and silicate particles. The particles were in the speed range from 1 km/s to 70 km/s and in the mass range from 10-15 g to 10-10 g. Figure 4 shows the positive charge (ion) yield for glass projectiles as a function of the impact speed.

The impact speed is determined by the rise times tI, tE and the charge ratio QC/QI at known channeltron amplification. When the particle speed is known, the mass can be determined from the charge yields QI/m and QE/m. Using only a single measured parameter (one rise time or the charge ratio) for the speed determination then the accuracy is about a factor 2, if all three methods are used the accuracy improves to a factor 1.6. If the speed is known within a factor of 2 and both charge yields are used for mass determinations, the mass value can be measured with an uncertainty of a factor of 10.

The measurable particle mass ranges from 10-16 g < m < 10-10 g at 40 km/s to 10-12 g < m < 10-6 kg at 3 km/s impact speed. The detectable impact speed is v ~>= 1 km/s. For larger particles the detector operates as a threshold detector. The particles electric charges are measured from 10-14 C to 10- 10 C for negative charges and 10-14 C to 10- 12 C for positive charges. The impact rate will be measured from 3 x 10-7 s-1 to a few per second in order to cover both the impact rates expected in interplanetary space and during times of high activity.

4. Initial performance.

On 19th October 1990 the cover of the dust instrument was successfully deployed as indicated by the change of the sensor temperature. After deployment the sensor temperature increased because the Sun was shining into the sensor during part of a spin revolution. The sensor was allowed to outgas for over one week. On 27th October the instrument was switched-on for the first time. All functions were successfully checked-out within the first two weeks and the instrument was set to the measuring mode. Low amplitude noise on three (EA, CA and PA) out of four charge measuring channels was detected. This noise predominantly occurred at the beginning and end of the period when the Sun illuminated the interior of the sensor during a spin revolution. By help of real-time commanding the instrument was brought into a state where the types of noise encountered initially were reduced and could easily be recognized in the data. This was done, (1) by allowing only the ion and channeltron signals to trigger a measurement cycle, (2) by reducing the channeltron voltage from the nominal value to a lower value at which only low noise rates were observed, and (3) by adjusting the set-point values used in the onboard program to define the event classes. During the later course of the mission this solar noise diminished and was found to follow the amount of sensor illumination.

On 4th November a new type of high rate, high amplitude, intermittent noise was observed. This noise on three channels (IA, EA and PA) was found to coincide with the operation of the sounder which is part of the Unified Radio and Plasma Wave (URAP) experiment. Figure 5 shows the pulse height distributions of 241 discrete impact and normal noise events (5a) which were recorded during the first 78 days of dust instrument operation in comparison to 893 discrete sounder noise events (5b) which were recorded in just 7 days. Sounder noise signals reach amplitude values of 15 which correspond to charge levels a factor 100 above the sensitivity threshold. About 60 coincident (IA and EA) noise events are recorded during the 128 s of each sounder cycle which affect two subsequent data read-out cycles of the dust instrument of 112 sec duration each. I.e. two discrete sounder noise events are transmitted to ground the rest is counted in the accumulators. Initially, the sounder was operated at least once every 17 minutes which produced more than 20% dead-time in the dust instrument. Starting from November 15th the sounder is operated only once every 3 hours which produces only about 2% dead-time. However, additional dead-time is caused when the sounder is operated during gaps in the data transmission since noisy data swamp all internal data storage capacity of the dust instrument. Similar noise has never been observed during ground testing since the sounder did not participate in integrated system tests of the spacecraft and experiments.

Figure 5a shows the ion amplitude (IA) distribution of all 241 discrete events (IA > 0 and not sounder noise) found in the experiment data records of the dust experiment until 13th January 1991. 167 of these events (hatched area) had non-zero electron amplitudes (EA) and for 42 of these (cross-hatched area) both signals occured within 16 µs of each other. The latter ones are considered dust impact events since also other parameters (e.g. IT and ET) are also compatible with values obtained in laboratory tests of dust impacts. For bigger impacts (IA >= 8) the coincidence time can be relaxed so that all 19 events with both electron and ion signals are considered dust impacts. These defitions of dust impact events are also applicable to the Galileo dust experiment (Grün et al. 1992). Additionally, some of the other events may have been caused by dust impacts. However, only careful analysis of all information available and comparison with laboratory dust impacts will allow us to finally identify a few additional probable dust impacts in this data set. No electric charge carried by dust particles has been identified so far above the noise background.

Channeltron (CA)-information has not yet been used because we have not yet accurately determined the channeltron amplification yet. This is only possible with enough statistics in the impact data. An effect of the low channeltron voltage is the increased amplitude range in which class-number-0-events occur. Therefore, discrete data sets of small impact events (IA <= 15) are not complete since they may have been overwritten by (sounder) noise events especially during gaps in the data transmission.

Complete information on the event rate recorded by the dust experiment can be obtained from the accumulated data. Table 3 gives the actual counter values as of 13th January 1991. Three of the accumulators (AC01, AC11 and AC02) have been overflown (at count 255) because of high (sounder) noise counts. During the initial configuration of the instrument (on 6th and 7th Nov. 1990) high noise rates at the channeltron output in addition to the sounder noise produced noise counts also in accumulators AC21 all 7 counts, AC31 2 counts and AC12 8 counts. All other counts in these and the remaining accumulators are caused by dust impacts. Therefore, all accumulators except AC01, AC11 and AC02 are used to determine the impact rate.

A summary of the recorded impact rate as a function of time is displayed in Figure 6. The heavy lines show a rate which is deduced from the accumulated data and refers to big particles impacts (corresponding to a mass threshold of approx. 10- 14 g at 20 km/s impact speed) since the two lowest amplitude ranges are under represented. The rate is very low, namely only 0.13 impacts per day. This rate corresponds to a dust flux onto the Ulysses dust sensor of 1.5 x 10-5 m-2 s-1. On 13th Jan. 1991 Ulysses had reached a heliocentric distance of 1.7 AU. The Galileo dust instrument observed in about the same region of space a factor ten higher fluxes. This difference can be explained by the different viewing geometries of both instruments and the assumption that the particles recorded move on low eccentricity, low inclination orbits (McDonnell 1978 and Grün & Zook 1980) and hence the effective detection area of Ulysses is much reduced.

The Galileo and Ulysses dust fluxes should be compared with the dust fluxes measured by the Pioneer 10 and 11 penetration sensors inside 1.7 AU (Humes et al. 1974 and Humes 1980). Fluxes of 1.6 x 10-15 and 8 x 10-6 m-2 s-1 with gaps of no impacts recorded between 1.2 and 1.4 AU and between 1.2 and 2.3 AU have been observed by the Pioneer 10 and 11 sensors, respectively. These fluxes correspond to threshold masses of 2 x 10-9 and 10-8 g at impact speeds of 20 km/s for both sensors, respectively. The viewing geometries of both Pioneer instruments correspond more to the Galileo case than to the Ulysses case. It is concluded that the mass spectrum has to be very flat in the corresponding mass range. This has been observed also in the near 1 AU dust environment (Grün et al. 1985).

The rate of small particles ( 10-15 g, thin lines) dropped rapidly with time corresponding to the change in spacecraft attitude. At the beginning of the period shown the spin axis was at approx. right angle with respect to the solar direction while on DOY (Day-Of-Year) 360-1990 the spin axis was pointing away from the Sun. The sensor pointings at time of impacts showed a strong concentration towards the solar direction. This is a property expected for [beta]-meteoroids (Berg & Grün 1973) which are believed to be created by collisions inside the spacecraft's orbit and leaving the solar system on hyperbolic orbits because of radiation pressure (Zook & Berg 1975). Assuming an effective sensor area of 0.020 m2 the maximum flux of, [beta]-meteoroids corresponds to 9 x 10-4 m-2 s-1 if we take into account that the data covered only 80% of the available time.

Acknowledgements.

The development, implementation and testing of this instrument and its data processing system was only made possible by the efforts of a great number of individuals from various departments of the Max-Planck-Institut für Kernphysik and from outside institutions and companies. The authors are especially indebted to G. Baust, H. Geldner, J.R. Göller, R. Hofacker, O. Kress, G. Linkert, G. Matt, C. Nehls, D. Roemer, H. Sallmutter, G. Schäfer, W. Schneider, N. Siddique, O. Titze, F. Veloso and A. Zahlten for their personal involvement during that campaign. Important contributions came from P. Gammelin, F. Eckl and D. Semder of ARGE PEES, W. Fischer and G. Pahl and A. Kaiser. Careful attention and effective coordination and support has been provided by H. Schneppe of DLR and by P. Casseley, W. Frank, R. Grün, H. Schaap and G. Tomaschek of ESTEC and the Ulysses project teams. We are grateful for the support received from the project scientists K.P. Wenzel and E.J. Smith. This work has been supported by the Bundesminister für Forschung and Technologie, under the grants 01 ON 048-WRK 275/4-7.15 and 01 ON 87029.

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TABLE 1. In situ dust detectors in interplanetary space,(1) heliocentric distance (AU), (2) detector type#, (3)mass threshold (g) at 20 km/s, (4) sensitive area (m2),(5) effective solid angle (sr), (6) dynamic mass range and(7) viewing geometry##.

 

TABLE 2. Signals measured by the Ulysses dust detector upon impact of a dust particle onto the sensor and related particle parameters.

 

TABLE 3. Event classification scheme and accumulator (AC) matrix. The numbers shown refer to the actual values as of 13 Jan 1991. Additional counts occured in AC01, AC02 and AC03 (one count) which were eliminated because of a counter reset at two days after switch-on.

 

FIGURE 1. Sensor configuration (schematic) and measured signals upon impact of a positively charged dust particle.

 

FIGURE 2. Functional block diagram of the Ulysses dust detector.

 

FIGURE 3. Average sensor areas as a function of the angle [gamma] between the impact direction and the positive spin axis for the Ulysses and Galileo dust detectors. The sensor areas are averaged over a spin period. The sensor axis has an angle of [gamma]U = 95 and [gamma]G = 55 degrees for the Ulysses and Galileo dust detectors, respectively.

 

FIGURE 4. Ion yield for glass projectiles as a function of the impact speed (according to Göller & Grün 1989). The rhombs give mean values of the ion yield within small speed intervals which were measured during the calibration runs.

 

FIGURE 5. Normalized pulse height (ion amplitude IA) distributions of discrete events recorded by the dust experiment with non-zero ion amplitudes. An increment of 8 in IA-value corresponds to a factor of ten in the corresponding charge signal. Hatched areas indicate non-zero electron amplitudes: cross-hatched areas indicate events for which the ion signal followed the electron signal within 16 µs. 5a) All events - except sounder noise events - which were recorded from 28th Oct. 1990 until 13th Jan. 1991. 5b) sounder noise events recorded during 5th through 11th Nov. 1190.

 

FIGURE 6. Impact rate recorded by the Ulysses dust experiment during the first 78 days of operation until 13th Jan. 1991 as a function of time (DOY = Day-Of-Year). The heavy lines show the rate which is deduced from the accumulated data (except AC01, AC11 and AC02). This rate corresponds to big particles impacts (>= 10-14 g) except for time interval DOY 310 through 313 during which all 17 recorded counts were noise events. The thin lines show the rate of small impact events ( >= 10-15 g) which is obtained from discrete data. The rate of events with non-zero ion and electron amplitudes which occured within 16 µs of each other is given. A dead-time correction has not been applied.