M. Baguhl

Solar wind magnetic field bending of Jovian dust trajectories.

From September 1991 to October 1992, the cosmic dust detector on the Ulysses spacecraft recorded 11 short bursts, or streams, of dust. These dust grains emanated from the jovian system, and their trajectories were strongly affected by solar wind magnetic field forces. Analyses of the on-board measurements of these fields, and of stream approach directions, show that stream-associated dust grain masses are of the order of 10−18 gram and dust grain velocities exceed 200 kilometers per second. These masses and velocities are, respectively, about 103 times less massive and 5 to 10 times faster than earlier reported.

Reduction of Galileo and Ulysses dust data

Abstract The reduction procedures which are applied to raw data from the Galileo and Ulysses Dust Detectors are described in order to obtain physical parameters (mass and velocity) for the recorded dust impacts. Both detectors are impact ionization detectors which measure the charge released from an impact onto a solid target. From the rise times of the signals, impact speeds from 2 to 70 km s−1 can be derived with an accuracy of about a factor 2. Electronic impact charges are measured from 10−14 to 10−8 C, which refer to a speed dependent mass range (e.g. 4 × 10−15−4 × 10−9 g at 20 km s−1 impact speed). Larger particles are recorded with saturated signals. Data processing performed both on board the spacecraft and on the ground is described. The processing allows dust impacts to be identified and separated from noise events. Supplementary information, such as impact time and sensor pointing at the time of impact, is also provided.

Three years of Galileo dust data

Abstract From its launch in October 1989 until the end of 1992, the Galileo spacecraft traversed interplanetary space from Venus to the asteroid belt and successfully executed close flybys of Venus, the Earth, and the asteroid Gaspra. The dust instrument has been operating most of the time since it was switched on in December 1989. Except for short time intervals near Earth, data from the instrument were received via occasional (once per week to once per month) memory read outs containing 282–818 bytes of data. All events (impacts or noise events) were classified by an onboard program into 24 categories. Over the three-year time span, the dust detector recorded 469 “big” dust impacts. These were counted in 21 of the 24 event categories. The three remaining categories of very low amplitude events contain mostly noise events. The impact rate varied from 0.2 to 2 impacts per day depending on heliocentric distance and direction of spacecraft motion with respect to the interplanetary dust cloud. Because the average data transmission rate was very low, some data were not received on the ground. Complete data sets for 358 “big” impacts were received, but the other 111 “big” impacts were only counted. The observed impact rates are compared with a model of the meteoroid complex.

Two years of Ulysses dust data

Abstract From October 18, 1990 to February 8, 1992 the Ulysses spacecraft traversed interplanetary space between the Earth and Jupiter; at Jupiter the spacecraft was deflected below the ecliptic onto a highly-inclined orbit ( i ∼80°). Here, we report on dust impact data obtained from launch until the end of 1992, nearly a year after the Jupiter flyby. During that time (792 days), the Ulysses dust detector recorded 968 impacts of dust particles with masses ranging from 10 −16 g to 10 −18 g. The impact rate varied from as low as one impact per week during quiet times to more than one per minute during the dust stream of March 10–11, 1992. In this paper, we present and describe the complete data set including both raw and reduced data. The performance of the sensor, which has been very satisfactory so far, is discussed in detail together with the noise discrimination scheme employed. The instruments detection threshold is given as a function of both the particles mass and its speed relative to Ulysses . The derived impact rates and the distribution of particle masses, speeds and impact directions are compared to a model of the meteoroid complex.

Identification of “small” dust impacts in the Ulysses dust detector data

Abstract Since October 1990, 3 weeks after the launch of the Ulysses spacecraft, the dust detector onboard recorded impacts of cosmic dust particles. Besides dust impacts, the detector recorded noise from a variety of sources. So far, a very rigid scheme had been applied to eliminate noise from impact data. The data labeled “big” dust impacts previously led to the identification of interstellar dust and of dust streams from Jupiter. The analysis presented here is concerned with data of signals of small amplitudes which are strongly contaminated by noise. Impacts identified in this data set are called “small” impacts. It is shown that dust impacts can be clearly distinguished from noise for most of the events due to the multi-coincidence characteristics of the instrument. 516 “small” impacts have been identified. For an additional 119 events, strong arguments can be given that they are probably small dust impacts. Thereby, the total number of dust impacts increases from 333 to 968 in the time period from 28 October 1990 to 31 December 1992. This increase permits a better statistical analysis, especially of the Jupiter dust streams which consist mostly of small and fast particles. Additional dust streams have been identified between the already known streams before and after Jupiter flyby. The dependence of the deflection from the Jupiter direction, the stream intensity and width on Jupiter distance support the assertion that they have been emitted from the Jovian system. The masses of the 635 “small” dust particles range from 6 × 10−17 to 3 × 10−10 g with a mean value of 1 × 10−12 g, which compares to a range from 1 × 10−16 to 4 × 10−9 g with a mean value of 2 × 10−11 g for the previously identified 333 “big” dust particles.

Three years of Galileo dust data. II. 1993-1995

Abstract Between January 1993–December 1995, the Galileo spacecraft traversed interplanetaryspace between Earth and Jupiter and arrived at Jupiter on 7 December 1995. The dust instrumentonboard the spacecraft was operating during most of the time and data from the instrument wereobtained via memory readouts which occurred at rates between twice per day and once per week.All events were classified by an onboard program into 24 categories. Noise events were usuallyrestricted to the lowest categories (class 0). During Galileos passage through Jupiters radiationbelts on 7 December 1995, several of the higher categories (classes 1 and 2) also show evidencefor contamination by noise. The highest categories (class 3) were noise-free all the time. Arelatively constant impact rate of interplanetary and interstellar (big) particles of 0.4 impacts perday was detected over the whole three-year time span. In the outer solar system (outside about2.6 AU) they are mostly of interstellar origin, whereas in the inner solar system they are mostlyinterplanetary particles. Within about 1.7 AU from Jupiter intense streams of small dust particleswere detected with impact rates of up to 20,000 per day whose impact directions are compatiblewith a Jovian origin. Two different populations of dust particles were detected in Jovianmagnetosphere: small stream particles during Galileos approach to the planet and big particlesconcentrated closer to Jupiter between the Galilean satellites. There is strong evidence that thedust stream particles are orders of magnitude smaller in mass and faster than the instrumentscalibration, whereas the calibration is valid for the big particles. Because the data transmissionrate was very low, the complete data set for only a small fraction (2525) of all detected particlescould be transmitted to Earth; the other particles were only counted. Together with the 358particles published earlier, information about 2883 particles detected by the dust instrumentduring Galileos six years journey to Jupiter is now available.

In situ measurements of cosmic dust

In-situ measurements of cosmic dust provide information on the spatial and orbital distributions, and on the physical and chemical properties of dust in interplanetary space. Pioneers 8 through 11, Helios, Galileo and Ulysses spaceprobes measured interplanetary dust from 0.3 to 18 AU distance from the Sun. The Earth satellites HEOS, Hiten, as well as other spacecraft, determined the flux of micrometeoroids at 1 AU. The size distribution from a few micrometer to millimeter range was also characterized by analysis of lunar micro craters and later verified by near-Earth satellites like LDEF. Distinctly different populations of dust particles exist throughout the solar system. In the inner solar system, out to about 3 AU, zodiacal dust particles are observed by in-situ detection from spaceprobes. These particles orbit the Sun on low inclination (i 10 μm) outside about 3 AU. The dust detectors onboard the Ulysses and Galileo spaceprobes identified micrometer sized interstellar dust sweeping through the solar system. Within a distance from Jupiter of about 2 AU Ulysses and Galileo observed streams of tiny grains originating from within the jovian system.

Galileo and Ulysses dust measurements: Fz Venus to Jupiter

The Galileo and Ulysses spaceprobes carry two similar dust detectors through interplanetary space from Venus to Jupiter. We report here on impacts which correspond to dust particles above a mass threshold of about 10−13g for which we have complete records. Between December 1989 and January 1992 Galileo repeatedly traversed interplanetary space between 0.7 and 2.26 AU and recorded 374 impacts. The observed impact rates ranged from 0.1 to about 3 impacts per day strongly dependent on whether the spacecraft moved towards or away from the Sun. From October 1990 to January 1992 the Ulysses spacecraft had reached a distance of 5.17 AU from the Sun and had recorded 72 impacts at rates between 0.1 and 0.5 per day. Inside about 2 AU the observed fluxes are compatible with a population of interplanetary dust particles moving on low to moderately eccentric (e = 0.1 to 0.5) and low inclination (i = 0 deg. to 30 deg.) orbits. Outside this distance a dust particle population on different orbits is required in order to explain the Ulysses data.

Three years of Ulysses dust data: 1993-1995

The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (i = 79°). After its Jupiter flyby in 1992 at a heliocentric distance of 5.4 AU, the spacecraftreapproached the inner solar system, flew over the Suns south polar region in September 1994,crossed the ecliptic plane at a distance of 1.3 AU in March 1995, and flew over the Suns northpolar region in July 1995. We report on dust impact data obtained with the dust detector onboardUlysses between January 1993 and December 1995. We publish and analyse the complete dataset of 509 recorded impacts of dust particles with masses between 10-16 g-10-7 g. Together with 968 dust impacts from launch until the end of 1992 published earlier ([Gruuml;n et al., 1995c]), information about 1477 particles detected with theUlysses sensor between October 1990 and December 1995 is now available. The impact ratemeasured between 1993 and 1995 stayed relatively constant at about 0.4 impacts per day andvaried by less than a factor of ten. Most of the impacts recorded outside about 3.5 AU arecompatible with particles of interstellar origin. Two populations of interplanetary particles havebeen recognized: big micrometer-sized particles close to the ecliptic plane and smallsub-micrometer-sized particles at high ecliptic latitudes. The observed impact rate is comparedwith a model for the flux of interstellar dust particles which gives relatively good agreement withthe observed impact rate. No change in the instruments noise characteristics or degradation of thechanneltron could be revealed during the three-year period.

Ice sublimation below artificial crusts: results from comet simulation experiments

Abstract Dust mantles or non-volatile mineral crusts most probably exist on large parts of the surface of many cometary nuclei. Even when such a layer is only a few millimetres thick and porous, its existence reduces substantially the gas emission rate of the underlying ice. In the present paper we report the results of systematic laboratory experiments, where a sample of porous, granular ice covered by a dark steel plate with holes was irradiated under vacuum conditions by an artificial light source simulating the Sun. The dark steel plate containing holes of defined size was intended to simulate the effect of a porous non-volatile cometary crust or dust mantle. We observed the build-up of vapour pressure below the artificial crust and measured the temperature profile developing in the ice sample for different hole sizes. In all experiments a drastic reduction of the gas emission rate (compared with the case of free sublimation from a dark icy surface of the same albedo and emissivity) was observed. The main effect of the porous crust is a much faster heating of the underlying ice due to suppression of gas outflow. The experimental results are interpreted in terms of a new heat conduction model that allows for all conduction modes that may act in such a structured ice, in particular Knudsen gas diffusion, infrared radiation, and solid-state heat conduction via intergranular connection points.