Instrument Overview
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The instrument consists of a 0.1 mm thick gold foil of hemispherical shape with three grids at the entrance (entrance grid, charge grid, and shield), as well as an ion collector and channeltron detector. The maximum sensitive area (for particles moving parallel to the sensor axis) is 0.1 m**2. Upon impact the particle produces a plasma, whose charge carriers are separated by an electric field between the target and the ion collector. Negative charges (mainly electrons) are collected at the target. The positive charges are collected partly by the ion collector and partly by a channeltron. The channeltron is used as it is insensitive to electric and vibrational noise. See Gruen et al. (1992b) for more information concerning the instrument.
Science Objectives Summary
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The objective of the Ulysses dust experiment is to investigate the physical and dynamical properties of small dust particles (10**-16 to 10**-6g) as a function of ecliptic latitude and heliocentric distance, and the study of their interrelation with interplanetary/interstellar phenomena. The parameters to be determined include the mass, speed, flight direction and electric charge of individual particles. Specific objectives are:
- To determine the impact rate, size frequency, and the distribution of flight directions and electric charges of interplanetary dust particles
- To classify particle orbits into bound orbits around the Sun or hyperbolic orbits leaving or entering the solar system
- To study the distributions of orbital elements (semi-major axis, eccentricity, inclination ) of particles in bound orbits
- To determine as functions of heliocentric distance and ecliptic latitude the spatial density of the interplanetary large particle population which generally moves in 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 in 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 traversing 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 amount of dust produced 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 to the properties of the ambient plasma (plasma density, energy spectrum), the solar radiation spectrum and magnetic fields
Instrument Measurements
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Positively or negatively charged particles entering the sensor are first detected via the charge which they induce in the charge grid while flying between the entrance and shield grids. The grids adjacent to the charge pick-up grid are kept at the same potential in order to minimize the susceptibility of the charge measurement to mechanical noise. 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 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 particles are derived by using empirical correlations between these four quantities.
Detector Description
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The sensor consists of a grid system for the measurement of the particle charge, an electrically grounded target (hemisphere) and a negatively biased ion collector. A charged dust particle entering the sensor will induce a charge in the charge grid, which is connected to a charge sensitive amplifier. The output voltage of this amplifier rises until the particle passes this grid, and falls off to zero when it reaches the shield grid. The peak value (Q_p) is stored for a maximum of 600 microseconds and is only processed if an impact is detected by the impact ionization detector within this time. A dust particle hitting the hemispherical target produces electrons and ions, which are separated by the electric field between hemisphere and ion collector into negative charges (electrons and negative ions) and positive ions. The negative charges are collected at the hemisphere and measured by a charge sensitive amplifier (Q_e). Positive ions are collected and measured at the negatively biased ion collector with a charge sensitive amplifier (Q_i). Some of the ions penetrate the ion collector (which is partly transparent - total transmission approximately 40 percent), are further accelerated, and hit the entrance cone of an electron multiplier (channeltron). Secondary electrons are produced, amplified, and measured by a charge sensitive amplifier (Q_c). Other quantities measured are the rise times of both the positive and negative charge pulses. The measurement of the time delay between electron pulse and ion pulse serves as a means for distinguishing impact events from noise. Impact events have time delays of 2-50 microseconds, while mechanical noise has a time delay of milliseconds. These signal amplitudes and times of a single recorded event are digitized and stored in an Experiment Data Frame (EDF). A measurement cycle is initiated if either the negative charge Q_e on the hemispherical target, or the positive charge Q_i on the ion-collector, or the positive charge Q-c on the channeltron exceeds a threshold. Since the hemisphere has a large area which is directly exposed to interplanetary plasma and high-energy radiation, this may cause some interference for the Q_e measurement. To avoid this interference during high activity times, it is possible to switch by command to a mode in which a measurement cycle is initiated if only the charge on the ion collector Q_i (small area and not directly exposed) or channeltron signal Q_c exceeds the threshold. If more than one event occurs within the transmission time of one EDF, then these events are counted by several amplitude-dependent counters. The dead-time caused by the measurement cycles is 5 milliseconds.
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.
Calibration Description
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Impact tests with iron, carbon, and silicate particles were performed at the Heidelberg dust accelerator facility. The particles were in the speed range from 1 to 70 km/s and in the mass range from 1.0E-15 to 1.0E-10 grams. In addition to the projectile material variation, calibrations for iron particles with varying impact angles were done. See Goller and Gruen (1989) for more information.
To obtain calibrations without information about the impact angle and the composition of an impacting micrometeoroid, a set of curves (one for each measurement channel) was calculated, which were averaged over three different materials (iron, carbon, and silicate) and over the range of relevant impact angles (20 to 53 degrees). The measurements were done at different angles with iron particles and at one fixed angle (20 degrees) with carbon and silicate projectiles. Difficulties in accelerating glass and carbon projectiles and the low acceleration rate made it impossible to do tests at more than one angle.
A computer simulation of the detector exposed to an isotropic particle flux leads to the result that 50 percent of the particles hit the detector under an angle of 32 degrees or lower, relative to the sensor axis. Its effective viewing cone covers a solid angle of 1.4 sr. As the target is curved (hemispherical) the impact angle, measured relative to the target normal at the point of impact, is generally different from the angle of incidence (relative to the sensor axis). The direction of travel of the impacting particle can not be determined. From the computer simulation the most probable impact angle is 28 degrees, the average angle is 36 degrees. This information, used with the pointing of the instrument, can be used to obtain a rough estimate of the particle trajectory. The particle's flight path inside the detector was determined to be 20 +/- 5 cm.
There are three possibilities for the determination of a particle's speed (the risetimes and the ratio Q_c/Q_i). Using all three measurements and comparing them with the calibration curves, the speed can be determined with an accuracy of a factor of 1.6. Using only one the accuracy is given by a factor of 2.
With a known particle speed the mass can be determined from the charge yields Q_i/m and Q_e/m. If the speed is known within a factor of 1.6 and both yields are used for mass measurements the value can be measured with an uncertainty of a factor of 6. The main part of this error is caused by the limited accuracy of the speed measurement. The smallest impact charge Q_i detectable is about 10**14 coulomb which corresponds to a mass and speed dependent threshold that can be approximated by a power law (see Gruen et al., 1995a).
Instrument Modes
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Different instrument modes exist to alter the instrument's susceptibility to noise. These modes are changed by adjusting the thresholds of the detectors aboard the instrument. The thresholds are altered by telecommand from Earth. The threshold levels of the detectors are included within the dataset.
Onboard Processing
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See Gruen et al, 1992b and 1995c.
First, the instrument microprocessor, which controls the experiment measurement cycle, collects the buffered data and processes the data according to its onboard program. This takes about 5 ms. The signal amplitudes and times of a single recorded event (dust impact or noise) are digitized and stored in an Experiment Data Frame (EDF) of 16 bytes (i.e. 128 bits). Supplementary information like event time and instantaneous spin position are collected from the spacecraft and added in each EDF. Dead-time caused by the measurement cycle is 5 ms.
The instruments are designed to reliably operate under noisy conditions thereby allowing the reliable extraction of true dust impacts from noise events. True impacts can be detected at rates of as low as one per month. This is achieved by raising the threshold levels of all impact signals individually by telecommand which allows instrument sensitivity to be adapted to the actual noise environment on board the spacecraft. Coincidences between the signals are established which, along with the signal amplitudes, are used to classify each event.
Each measured event (noise or impact) is classified according to the strength of its ion signal (IA) into one of six amplitude ranges (AR=1 to 6). Each amplitude range correspond roughly to one decade in electronic charge, Q_I. In addition, each event is categorized into one of four event classes (described by the class number CLN). The event classification scheme, which defines criteria that must be satisfied for each class, is shown:
--------------------------------------------------------------------------
Parameters: | CLN=0 | CLN=1 | CLN=2 | CLN=3
--------------------------------------------------------------------------
IA | IA > 0 | IA > 0 | IA > 0 | IA > SP16
-------------| or | or |----------------------------------------
EA | EA > 0 | EA > 0 | EA > 0 | EA > SP14
-------------| or |--------------------------------------------------
CA | CA > 0 | CA > 0 | CA > 0 | CA > SP15
--------------------------------------------------------------------------
ET | | | SP03 <= ET <= SP04 | SP03 <= ET <= SP04
--------------------------------------------------------------------------
IT | | | SP01 <= IT <= SP02 | SP01 <= IT <= SP02
--------------------------------------------------------------------------
EIC | | | EIC = 0 | EIC = 0
--------------------------------------------------------------------------
ICC | | | ICC = 1 | ICC = 1
--------------------------------------------------------------------------
Noise counter| | | |
of: | | | |
EN | | | | EN <= SP11
IN | | | | IN <= SP09
CN | | | | CN <= SP10
--------------------------------------------------------------------------
Within each class these conditions are connected by logical 'and'
except where noted. Class 0 (CLN = 0) includes all events that are not
categorized in a higher class (typically noise and unusual impact events
- e.g. impacts onto the sensor's internal structure other than the impact
target). In classes 1 through 3, the criteria become increasingly
restricted so that CLN = 3 generally represents true dust impact events
only. Some of the set point values (SP01 to SP15), which can be set by
ground command, are used in the classification scheme. The set points are
as follows:
SP01 = 1
SP02 = 15
SP03 = 1
SP04 = 15
SP09 = 2
SP10 = 8
SP11 = 8
SP14 = 0
SP15 = 0
SP16 = 0
The on board classification can be adapted to the in-flight noise
environment by changing the thresholds and classification parameters
(set points) or by adjusting the onboard classification program through
telecommands. Detailed information on noise is mandatory in order to
evaluate the reliability of impact detection for the various event
categories, to minimize the effect on dead-time and to optimize memory
utilization.
The memory is divided into separate ranges in which various data is given priority. The A-range of instrument memory stores the six most recent EDFs - one for each amplitude range regardless of class. The E range, graphically depicted below, stores the last 8 events occurring within class 3. These events satisfy the most stringent constraints and are almost certainly true impacts.
The above four classes, together with six amplitude ranges, constitute twenty-four separate categories. Each of these categories has its own 8-bit accumulator:
| | Class number (CLN)
|Amplitude|
IA | Range | 0 1 2 3
-------------------------------------------
0- 7 | AR = 1 | AC01 | AC11 | AC21 | AC31
8-15 | AR = 2 | AC02 | AC12 | AC22 | AC32
16-23 | AR = 3 | AC03 | AC13 | AC23 | AC33
24-32 | AR = 4 | AC04 | AC14 | AC24 | AC34
48-55 | AR = 5 | AC05 | AC15 | AC25 | AC35
56-63 | AR = 6 | AC06 | AC16 | AC26 | AC36
As long as the respective accumulator does not overflow, each event is
counted even if the complete information is not received on ground.
Generally, the event rate is so low (even in the low amplitude and low
class ranges) that the true increment can be reliably determined. All
categories and corresponding accumulators - excluding AC01, AC11 and AC02
- contain primarily impact events. Even in these latter categories, true
impacts can be identified and separated from noise events if the complete
data set for an event is available (Baguhl et al., 1993).
The transmission of seven EDFs constitute an instrument read-out cycle (six A-range events and one of the subcommutated class 3 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 approximately 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 a high rate together with real-time data transmission during a pass. At a spacecraft data transmission rate of 1024 bps, one EDF is sent every 16 seconds. Lower bit rates down to 128 bps during storage or real time transmission periods are possible.
Data processing on the ground
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After receiving the partially processed data from the spacecraft, the following data processing steps are performed on the ground:
(1) instrument health check
(2) generation of accumulator histories
(3) extraction of discrete events
(4) reduction of impact data
(5) generation of data products
The instrument health check involves inspection of instrument house keeping data such as temperatures, voltages, currents and a check of the test pulse data. If, for example, the temperature readings are too high, the heater power level can be set accordingly.
If excessive noise is detected then appropriate measures, such as changing the thresholds or channeltron high voltage by telecommand, can be taken. Occasionally, tests of different instrument modes are performed in order to probe the actual noise environment; the instrument parameters can then be adjusted accordingly.
The preparation of data products is the final routine step of dust data processing. A number of separate files are produced which reflect various stages of data processing.
Instrument Mounting
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The instrument is located on the equipment platform of the spacecraft body and its axis is at an angle of 85 degrees with respect to the positive z axis, where the z axis is the rotation axis of the spacecraft and the positive direction is where the axis points roughly towards Earth. The Ulysses dust detector weighs 3.8 kg and consumes 2.2 W.