Bo Ake Sture Gustafson

Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10−10 to 10−7 kilograms, and 48 grains of mass 10−5 to 10−2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

A generalized multiparticle Mie-solution: further experimental verification

Abstract We further test our electromagnetic multisphere-scattering solution developed earlier by comparing theoretical predictions from the theory with a set of laboratory measurements of microwave analog to light scattering by aggregated spheres. This solution is an extension of Mie theory to the multisphere case, generally applicable to an arbitrary aggregate of spherical and/or nonspherical particles. It is demonstrated once again that the theory is in a uniform agreement with experiment, convincingly confirming the veracity of the multiparticle-scattering formulation. The computer code for the calculation of the scattering by an aggregate of spheres in a fixed orientation and the experimental data havebeen made publically available.

Microwave analog to light scattering measurements: A modern implementation of a proven method to achieve precise control

Abstract The experimental determination of scattered electromagnetic radiation from a known target illuminated by a known source remains an essential tool to test new scattering theories and to investigate the scattering by particles for which a theory has not yet been devised. This article describes a modern broad-band microwave scattering facility capable of determining the elements of the scattering matrix from 0 to 168 ° scattering angle under automated control. The laboratory measurements can cover the size range from near the Rayleigh limit to geometric optics. Both phase and intensity are routinely measured at 85–501 discrete wavelengths from 2.7 to 4 mm, which allows the derivation of all elements in the scattering matrix. The unwanted background radiation can be removed through vector subtraction and the result verified using the technique of “time-gating” based on inverse Fourier transformation and time domain analysis. Measurements are shown to be repeatable and accurate. This is primarily due to the use of a “clean” mechanical and electronic design in combination with high mechanical and thermal stability.

Systematic studies of light scattering. 1: Particle shape

The light scattering properties of twenty-eight particles, spanning four sizes (near the resonance region) and seven related shapes (a 4:1 cylinder, 4:1 and 2:1 prolate spheroids, a sphere, 2:1 and 4:1 oblate spheroids, and a 4:1 disk), are presented for a common index of refraction, m = 1.61-i0.004, representing silicates. Microwave analog and theoretical methods were used to derive the scattered intensity and degree of polarization as a function of the scattering angle along with the extinction. All results refer to an ensemble or a cloud of identical particles because averages have been taken over random particle orientations. The degree of polarization, backscatter, and the radiation-pressure cross section are most sensitive to particle shape, implying that the use of Mie theory may be inappropriate for many applications.

Comet ejection and dynamics of nonspherical dust particles and meteoroids

This paper generalizes the formalism for calculating the ejection velocity of meteoroids and dust from comets and the forces to which such objects are subject in interplanetary space, including the dust tail of comets. It is found that spheres have the smallest cross section of any geometrical figures of the same valume averaged over random orientations, so for a fixed volume and mass, both the ejection velocity and beta reaches a minimum for bodies of spherical shapes. Flakes in random orientation are ejected near 70 percent of the highest ejection velocity for any orientation. Needles in random orientation escape a comet at nearly 90 percent of their maximum velocities. Randomly oriented cylinders of finite thickness escape at lower velocities that are slightly closer to their maximum velocities. The average beta acting on spin-aligned, perfectly absorbing needles is more than half that acting on a sphere of the same material and radius. 16 references.

Comparison between Multisphere Light-scattering Calculations: Rigorous Solution and Discrete-Dipole Approximation

We present the comparison of light-scattering calculations between a rigorous solution and the discrete-dipole approximation (DDA) for two-sphere aggregates. We also compare theoretical predictions with laboratory scattering measurements to examine the validity of the numerical solutions. It is found that there are cases in which the DDA solution, while satisfying the validity criterion for interdipole spacing to be small compared with the wavelength of incident radiation, deviates significantly from the rigorous solution and the experimental results. We show that the DDA works reasonably well for small-volume structures and that its validity is challenged, at least as it is currently implemented, when used on larger structures. We also show that, besides its advantages in reliability, the rigorous solution approach is far superior to the approximation method in computing efficiency as well.

Scattering by aggregates with and without an absorbing mantle: microwave analog experiments

We present angular scattering functions for loosely packed aggregates of 250 and 500 identical spheres near the Rayleigh size limit before and after the application of successive layers of an absorbing mantle. All measurements were obtained by using the microwave analog technique. Gross features of the scattering by aggregates without a mantle can be interpreted in terms of coherent scattering from the unit spheres acting independently of each other. The largest deviations from this approximation occur after the first minimum in forward scattering and extend to a scattering angle of 60° or 80° for our models. This intermediate range is also where the largest differences occur in the scattering from one aggregate to another. The angular extent of the range is largest for aggregates with the smallest dimensions. The scattering function is usually flat in the backscattering hemisphere and has little or no backscattering increase. The coherent scattering approximation breaks down when the aggregates are coated, and an equivalent spheres approximation becomes a better representation. The maximum degree of polarization near a scattering angle of 90° first decreases and then increases again as the mantle grows thicker.

Experimental and theoretical results of light scattering by aggregates of spheres

We present laboratory microwave scattering measurements for complex amplitude scattering matrices of three aggregates of 2, 8, and 27 identical spheres and compare them with theoretical predictions. Electromagnetic multiparticle-scattering calculations involve the determination of a large number of vector translation coefficients introduced by the addition theorems for vector spherical harmonics. For one of the two classes of vector translation coefficients there is an overall-sign discrepancy between two groups of formulations that exist in the literature. We compare our experimental data with the theoretical results from scattering calculations using the two different sets of formulas for computation of the translation coefficients. This comparison of experiment with theory reveals that Cruzans original research on the vector addition theorems [Q. Appl. Math. 20, 33-40 (1962)] is correct, although many authors believe that Cruzans formulation contains an overall-sign error.

Zodiacal Dust Bands

One of the outstanding problems in solar system science is the source of the particles that constitute the zodiacal cloud. The zodiacal dust bands discovered by IRAS have a pivotal role in this debate because, without doubt, they are the small, tail end products of asteroidal collisions. Geometrical arguments are probably the strongest and the plane of symmetry of the dust bands places their source firmly in the asteroid belt. A cometary source, Comet Encke for example, could exist at the distance of the mainbelt, but the dynamics of cometary orbits makes the formation of cometary dust bands impossible, unless, of course, there is a significant (comparable in volume to the asteroidal families) source of comets interior to the orbit of Jupiter with low (asteroidal) orbital eccentricities. We have suggested that the dust bands are associated with the prominent asteroidal families. The link with the Themis and Koronis families is good but the link with Eos remains to be proved. We show here by detailed modeling that even though the filtered infrared flux in the 25µm waveband associated with the dust bands is only ~1% of the total signal, this is only the “tip of the iceberg” and that asteroidal dust associated with the bands constitutes ~10% of the zodiacal cloud. This result, plus the observed size-frequency distribution of mainbelt asteroids and the observed ratio of the number of family to non-family asteroids allows us to estimate that asteroidal dust accounts for about one third of the zodiacal cloud. The discovery of the “leading-trailing” asymmetry of the zodiacal cloud in the IRAS data and our interpretation of this asymmetry in terms of a ring of asteroidal particles in resonant lock with the Earth is important for two reasons. (1) The existence of the ring strongly suggests that large (diameter ≥ 12µm) asteroidal particles (or particles with low orbital eccentricities) are transported to the inner solar system by drag forces. (2) The observed ratio of the trailing-leading asymmetry allows an independent estimate of the contribution of asteroidal particles to the zodiacal cloud. These new results have important implications for the source of the interplanetary dust particles (IDPs) collected at the Earth. Because asteroidal particles constitute about one third of the zodiacal cloud and are transported to the inner solar system by drag forces, gravitational focussing by the Earth that results in the preferential capture of particles from orbits with low inclinations and low eccentricities and the possible “funneling” effect of the ring itself, imply that nearly all of the unmelted IDPs collected at the Earth are asteroidal.

RESONANT STRUCTURE IN THE KUIPER DISK: AN ASYMMETRIC PLUTINO DISK

In order to develop a dynamical model of the Kuiper disk, we run numerical integrations of particles originating from source bodies trapped in the 3 : 2 external mean motion resonance with Neptune to determine what percentage of particles remain in the resonance for a variety of particle and source body sizes. The dynamical evolution of the particles is followed from source to sink with Poynting-Robertson light drag, solar wind drag, radiation pressure, the Lorentz force, neutral interstellar gas drag, and the effects of planetary gravitational perturbations included. We find that the number of particles in the 3 : 2 resonance increases with decreasing � (i.e., increasing particle size) for the cases in which the initial source bodies are small (� 10 km in diameter) and that the percentage of particles in resonance is not significantly changed by either the addition of the Lorentz force, as long as the potential of the particles is small (� 5 V), or the effect of neutral interstellar gas drag. The brightness of the entire Kuiper disk is calculated using a model composed of 500 lm diameter particles and fits well with upper limits to the Kuiper disk brightness and previous estimates. A disk with a size-frequency distribution weighted toward large particles, which are more likely to remain in resonance, may have a stronger, more easily identifiable resonant signature than a disk composed of small particles.