I. Mann

The Electric Charging of Interstellar Dust in the Solar System and Consequences for Its Dynamics

The motion of the solar system relative to the local interstellar medium causes a flux of neutral gas and dust into the solar system. When interstellar dust particles penetrate into the solar system, the mass distribution of the grains is modified because smaller dust grains, which have higher charge-to-mass ratios, are deflected by the Lorentz force in the solar magnetic field. In order to investigate the conditions of interstellar dust particles streaming into the heliosphere, the equilibrium surface potentials of interstellar dust grains are calculated assuming that the grains are homogeneous spheres of different materials. The following charging processes are considered: photoelectron emission, sticking and penetration of plasma particles, and secondary electron emission due to bombardment of energetic plasma particles. We find that the dependences of the surface potentials on the heliocentric distance and the grain mass are weak except for the heliocentric distance range between the heliopause and the termination shock. We show that the influence of Lorentz forces on small dust grains with masses less than 10-19 kg is strong enough to cause a significant deflection of the grains from their original direction of motion. Hence, we can expect that a selection of particles in a certain size interval can take place already near the heliopause. Although the exact value of the charge varies with the material composition of dust, a selection effect in the material composition of dust grains entering the solar system cannot be expected. Even if interstellar dust grains that can enter the solar system are nonspherical, the surface potentials of the grains can be represented within the framework of the spherical dust model that is applied in this study.

Formation and Acceleration of Nano Dust in the Inner Heliosphere

We study the dynamics of nano dust grains in the region inward from 1?AU. Assuming that the grains are created with the velocities close to Keplerian, we find that, despite the strong coupling to magnetic field, there is a population of trapped nano grains within about 0.2?AU from the Sun. The nano dust grains produced outside of the trapped region are accelerated to high velocities, of the order of 300?km?s?1, provided that the charge to mass ratio is not much less than 10?5 e/m p . These values correspond to dust sizes equal or smaller than approximately 10?nm.

Galileo observes electromagnetically coupled dust in the Jovian magnetosphere

Measurements of dust coupled to the Jovian magnetosphere have been obtained with the dust detector on board the Galileo spacecraft. We report on data obtained during the first four orbits about Jupiter that had flybys of the Galilean satellites: Ganymede (orbits 1 and 2), Callisto (orbit 3), and Europa (orbit 4). The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by up to 2 orders of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileos position relative to the Jovian magnetic field. Around 20 RJ (Jupiter radius, RJ=71, 492 km) in bound a dip in the impact rate has been found consistently. At the same times, reversals by 180° in impact direction occurred. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. At times of satellite flybys, enhanced rates of dust impacts have been observed, which suggests that all Galilean satellites are sources of ejecta particles. Inside about 20 RJ impacts of micrometer-sized particles have been recorded that could be particles on bound orbits about Jupiter. (Less)

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.

Interstellar dust properties derived from mass density, mass distribution, and flux rates in the heliosphere

We apply recent in situ measurements of the mass density, mass distribution, and flux rate of dust particles that enter the solar system, i.e., the heliosphere, to study the properties and dynamics of the interstellar dust flux into the solar system. We show that depletion at the heliopause alters the flux for particles with mass, m 10−15 kg). We show that early measurements that provided evidence for the focusing are in agreement with present estimates of the interstellar dust flux. There is clear observational evidence for all listed effects and detailed studies would be possible on the basis of more extensive measurements. We compare the in situ measurements of absolute density and mass distribution to the parameters determined for the interstellar medium (ISM). While the lack of small particles in the measured mass distribution may be a result of their depletion, the detection of larger grains may support models of collisional particle growth in the ISM. As well, interstellar particles that were found in meteorites yield evidence for the existence of large grains in the ISM, although they may be of completely different origin. We infer from the absolute densities that the dust reflects the conditions of the ISM on large scales rather than those of the local interstellar medium (LISM) in which the Sun is embedded. This supports theoretical results that the gas-dust friction scales in interstellar space exceed the extension of the LISM. From calculations of radiation pressure forces and comparison to the detected mass distribution we conclude that the detected interstellar dust particles are better described as composite grains rather than as particles that consist of pure materials, which further supports the model of collisional particle growth.

RADIATION PRESSURE FORCES ON TYPICAL INTERPLANETARY DUST GRAINS

Abstract The radiation pressure force, that acts on dust in interplanetary space depends on size and composition of the particles. The knowledge of this effect is essential for the study of dust dynamics. Four models of “typical” interplanetary dust grains are constructed from common notions of their physical properties, and their possible parent bodies. The influence of radiation pressure forces on the particles is estimated by means of so-called β-values, that give the ratio of radiation pressure force to gravitation force in interplanetary space. The β-values and albedos are calculated using Mie theory for homogeneous and core-mantle spheres. The Maxwell-Garnett mixing rule is used to describe either the porosity of particle or the inclusion of another material. Derived albedos of mixed-material particles appear to be generally lower than those of grains consisting of pure, strongly absorbing substances, which has also influence on the radiation pressure forces. The calculations show that the assumption of extremely porous particles, often discussed as a description of cometary dust, leads to very high radiation pressure forces. Models applied for more compact particles, either of interstellar and asteroidal origin or produced by alteration of “fresh” cometary material show similar slopes of their beta values, which are lower than for the “young” cometary material. The study shows that only particles with masses m >10 −10 g can be assumed to behave dynamically (i.e. under influence of radiation pressure forces) like the “big” zodiacal particles ( m >10 −8 g).

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.

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.

Brightness of the solar F-corona

We discuss our present knowledge about the brightness of the solar F-corona in the wavelength range from the visible to the middle infrared. From the general trend of the observational data, the F-corona is regarded as the continuous extension of the zodiacal light at smaller elongation of the line of sight. A contribution of thermal emission from dust is indicated by the increasing F-coronal brightness in comparison to the solar spectrum towards longer wavelength. As compared with the F-coronal brightness, the polarization and color in the visible regime are not well determined due to the high sensitivity of these quantities to the observational accuracy. Aside from observational problems, our present interpretation of the F-coronal brightness is also limited due to ambiguities in the inversion of the line of sight integral. Nevertheless, the measurements and model calculations of the brightness can be used to deduce some physical properties of dust grains. We show that the hump of the near-infrared brightness at 4 solar radii, which was sometimes observed in the corona, is related rather to the physical properties of dust grains along the line of sight than to the existence of a dust ring as previously discussed. We also show that the appearance or disappearance of the near-infrared peak in the coronal brightness cannot be described in any periodic cycle for each wavelength range.

Dust measurements in the Jovian magnetosphere

Dust measurements have been obtained with the dust detector onboard the Galileo spacecraft inside a distance of about 60RJ from Jupiter (Jupiter radius, RJ = 71,492 km) during two periods of about 8 days around Galileos closest approaches to Ganymede on 27 June and on 6 Sept 1996. The impact rate of submicrometer-sized particles fluctuated by a factor of several hundred with a period of about 10 hours, implying that their trajectories are strongly affected by the interaction with the Jovian magnetic field. Concentrations of small dust impacts were detected at the times of Ganymede closest approaches that could be secondary ejecta particles generated upon impact of other particles onto Ganymedes surface. Micrometer-sized dust particles, which could be on bound orbits about Jupiter, are concentrated in the inner Jovian system inside about 20RJ from Jupiter.