Interstellar dust in the solar system

Dust is an important component of galactic stucture and the cyclic processing of particulate matter leads to stellar and planetary formation. Though astronomical methods using analysis of dust-penetrating starlight can provide some limited information about the dust, the prospect of its in-situ sampling within the Solar System by spacecraft and its remote sensing by ground-based techniques open up a new field in galactic exploration.     

Magnetic polarimetric refraction in the solar corona

We present expressions which describe the angular displacement of radio sources due to refraction in a magnetized plasma. The main objective of the present paper is to take into account the combined effect of gradients of the electron density and the magnetic field. We use the geometrical optics approximation for the determination of the angular broadening of the radiation. The expressions obtained are applied to the case of the solar corona.     

The out of ecliptic distribution of interplanetary dust and its relation to other bodies in the solar system

We discuss the out-of-ecliptic component of the interplanetary dust cloud and its relation to the other small bodies in the solar system. The determination of the mass loss of comets, so far is quite uncertain and doesn't allow a finite study of the mass input to the dust cloud. However it is shown, that the dust particles in the inner solar system, i.e. within the earth orbit are most probable produced from a collisional evolution of larger, meteoroid, fragments of cometary origin. A further component of interstellar dust is especially important in the outer solar system and perhaps for the collisional evolution of the small bodies.     

Penetration of nearby supernova dust in the inner solar system

We investigate the method by which nearby supernovae – within a few tens of pc of the solar system – can penetrate the solar system and deposit live radioactivities on earth. The radioactive isotopic signatures that could potentially leave an observable geological imprint are in the form of refractory metals; consequently, it is likely they would arrive in the form of supernova-produced dust grains. Such grains can penetrate into the solar system more easily than the bulk supernova plasma, which gets stalled and deflected near the solar system due to the solar wind plasma pressure. We therefore examine the motion of charged grains as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation, and gravitational fields. We characterize the dust trajectories with analytical approximations which display the roles of grain size, initial velocity, and surface voltage. These results are verified with full numerical simulations for wide ranges of dust properties. We find that supernova dust grains traverse the inner solar system nearly undeflected, if the incoming grain velocity – which we take to be that of the incident supernova remnant – is comparable to the solar wind speeds and much larger than the escape velocity at 1 AU. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability and the dust capture onto the earth should have a geometric cross section. Our results cast in a new light the terrestrial deposition of radioisotopes from nearby supernovae in the geological past. For explosions beyond ～10 pc from earth, dust grains can still deliver supernova ejecta to earth, and thus the amount of supernova material deposited is set by the efficiency of dust condensation and survival in supernovae. Turning the problem around, we use observations of live ⁶⁰Fe in both deep-ocean and lunar samples to infer a conservative lower bound iron condensation efficiency of M_dust,Fe/M_tot,Fe ? 4  × 10^?4 for the supernova which apparently produced these species 2–3 Myr ago.     

The role of presolar dust in the formation of the solar system

Isotopic studies have revealed several types of presolar material in chondritic meteorites (e.g., Ne-E, various components of O, Ti, Ca, Mg). In fact, examples of presolar material are found in all meteorites whose components have not been completely altered by secondary processing. This paper suggests that presolar dust was the primary building material for the meteorites and terrestrial planets. To make this case, the characteristics of presolar dust are discussed and the material in the sun's parent molecular cloud is divided into eight reservoirs. Then the meteorites most likely to preserve their original constituents are identified, and it is shown that dust from several presolar material reservoirs is present in the primitive chondrites. Components that may have formed directly from presolar dust are also identified. Presolar dust and objects made from processed dust make up the vast majority of the material in primitive chondrites. Since there is no obvious reason to believe that other meteorites formed from fundamentally different material than did the primitive chondrites, it is reasonable to conclude that presolar dust, thermally processed but not evaporated and recondensed, was the parent material for the meteorites.In the second part of the paper, various processes that could have affected the presolar dust are identified. It is then shown that: (1) the chemical and oxygen isotopic variations between meteorite classes; (2) the formation of chondrules; and (3) accretion of chondrites and parent body metamorphism are consistent with relatively simple models that use presolar dust as the starting material. These models are presented, not as detailed solutions to the problems, but to exemplify a way of looking at the solar system that may lead to significant advances in our understanding.     

Polarimetric study of levitating dust aggregates with the PROGRA experiment

In order to interpret polarimetric remote observations of solar system dust clouds (e.g. cometary coma dust), laboratory measurements are needed. Three samples composed of aggregates are studied: crystallized enstatite, pyrogenic alumina and titanium oxide. The new version of the PROGRA² instrument allows to obtain polarimetric images of the samples under levitation. The dependence of polarization with phase angle and particle size is studied, as well as the effect of the porosity of the particles. Values of polarization at small phase angles are also discussed. The polarization near 90° decreases when the agglomerate size increases and when the porosity increases.     

Disintegration of dust aggregates in interstellar shocks and the lifetime of dust grains in the ISM

We have studied to influence of grain porosity on the dust destruction mechanism in interstellar shocks. Our results show, that fluffy aggregates of dust grains can be broken up easily in shocks.     

Comparative studies of meteoroid-planet interaction in the inner solar system

We review the current state of studies in planet–meteoroid interactions, a relatively new discipline in planetary science. Recent observations of phenomena such as meteor trails in the atmosphere of Mars and impact flashes on the Moon have prompted new theoretical work in the field. However, our ability to test these new models and advance our understanding of the processes involved is being inhibited by the lack of systematic long-term observations with instruments dedicated to the task. Here we consider the different types of meteoroid effects on a planetary environment. The current state of knowledge leads us to expect signatures detectable by existing instrumentation, either serendipitously or, in a more targeted fashion, by employing such apparatus in innovative ways and making use of already available model predictions. These will result in near-term advances in the field, to be used towards incorporating meteoroid-effect-detecting capabilities explicitly into future planetary instrumentation or building dedicated instruments.     

Interaction of relativistic dust grains with the solar radiation field

The effects of the solar radiation field on the propagation of relativistic dust grains are evaluated. It is concluded that relativistic iron grains with energies 10¹⁹ eV will melt in the solar radiation field before they reach the Earth's orbit around the Sun. However iron grains with lower energies will reach the Earth's orbit but grains travelling from the direction of the Sun will melt. This directional anisotropy or fingerprint may be used to search for relativistic dust grains in the primary cosmic rays. The fact that no significant solar system anisotropy has been detected places constraints on the hypothesis that the initiating particles of the extensive air showers are relativistic iron grains.     

Some studies on the solar microwave bursts in relation to the slowly varying component

The occurrences of 5772 microwave bursts recorded by the Sagamore Hill and Manilla Solar Radio Observatories over the period January 1968 to July 1970, covering the maximum phase of the current solar cycle at frequencies 2695, 4995 and 8800 MHz and their energy excesses have been examined in relation to the S-component of solar radio emission. The average slowly varying component has been determined by the superposed epoch method commonly known as the Chree analysis. Similar treatment of the bursts, data, mentioned above has been made to examine any probable 27-day variation and the results obtained have been compared with that of the S-component. Further, spectra of the microwave bursts under the so-called spectral type - inverted U, particularly those having a peak at 4995 MHz, have also been examined and compared with the average spectrum of the S-component. Some of the important results obtained from the present analysis are: (1) the nature of variation of both the average number of occurrences and energy excesses of the microwave bursts follow in general the average 27-day variation of the S-component, (2) the number of occurrences and energy excesses of the microwave bursts are comparatively greater in the ascending phase of the 27-day cycle than those in the descending phase, (3) bursts at progressively higher frequencies originate at lower levels in the solar atmosphere than those of the associated S-component, and (4) the average spectrum of the microwave bursts of inverted U spectral type having a peak at 4995 MHz is quite identical in nature to that of the S-component.     

A calculation of the Rosseland mean opacity of dust grains in primordial solar system nebulae

The Rosseland mean opacity was determined for an ensemble of dust grain species though to have been present in the early solar nebula, as well as in the primordial nebulae that helped to form the outer planets. In performing these calculations, we have derived and used more general equation for the Rosseland opacity that allows for anisotropic scattering. The identity of the major particle species and their relative abundances were found from thermodynamic equilibrium and solar elemental abundances. The optical constants of these materials were defined at wavelengths ranging from near-UV to the radio domain. Calculations were performed for a very wide range of particle size distributions, including a nominal one based on that of interstellar dust grains. In addition, asymptotic expressions for the Rosseland opacity are derived in the limits of very small and very large sized particles. Results are presented for nebular temperatures varying from 10 to 2500°K and for nebular gas densities varying from 10^?14 to 1 g/cm³. The values of the Rosseland mean opacity do not depend sensitively on the choice of the particle size distribution function, provided that there are few particles having sizes in excess of several tens of microns. At low temperatures that lie within the stability field of condensed water (?200°K), this opacity varies approximately as the square of the temperature for the nominal size distribution and for all distributions having few particles larger than several tens of microns. However, the Rosseland opacity has a much weaker temperature dependence at higher temperatures for this class of size distributions and at all temperatures for size distributions containing numerous particles larger than several tens of microns. As a result, thermal convection in primordial nebulae occurs over broader ranges of altitudes at low temperatures and for size distributions for which extensive aggregation has not yet occurred.     

On the thickness and evolution of the dust layer during the formation of the solar system

We discuss certain dynamical processes during the final stage of the sinking of the dust layer. We supposed that turbulance gave rise to a state of slow sinking (quasi-equilibrium) and evaluated the critical thickness at the onset of gravitational instability in the radial direction. We gave a precise numerical relation between 3 length-scales: $\text{〈|Z|〉c : h}{}_1\text{: λ}{}_{\mathrm{T}}\text{= 0.02107 : 0.1592 : 1}$, the first being the mean height of the dust particles at the onset of radial instability, the second being that value of the half-thickness and of the height at which the self-gravity of the dust layer is equal to the solar z-component, and the last being the longest wavelength at the onset of ring instability. We also calculated the time required for the formation of rings and found it to be far shorter than the sinking time.     

The influence of grain rotation on the structure of dust aggregates

We study the effect of rotation during the collision between dust aggregates, in order to address a mismatch between previous model calculations of Brownian motion driven aggregation and experiments. We show that rotation during the collision does influence the shape and internal structure of the aggregates formed. The effect is limited in the ballistic regime when aggregates can be considered to move on straight lines during a collision. However, if the stopping length of an aggregate becomes smaller than its physical size, extremely elongated aggregates can be produced. We show that this effect may have played a role in the inner regions of the solar nebula where densities were high.     

Sunspot populations and their relation with the solar cycle

The joint consideration of theoretical and observational arguments is used to conclude that two different spot populations co-exist in the Sun.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Theoretical,observational, and isotopic estimates of the lifetime of the solar nebula

Abstract— There are a variety of isotopic data for meteorites which suggest that the protostellar nebula existed and was involved in making planetary materials for some 10⁷ yr or more. Many cosmochemists, however, advocate alternative interpretations of such data in order to comply with a perceived constraint, from theoretical considerations, that the nebula existed only for a much shorter time, usually stated as ≤ 10⁶ yr. In this paper, we review evidence relevant to solar nebula duration which is available through three different disciplines: theoretical modelling of star formation, isotopic data from meteorites, and astronomical observations of T Tauri stars. Theoretical models based on observations of present star-forming regions indicate that stars like the Sun form by dynamical gravitational collapse of dense cores of cold molecular clouds in the interstellar medium. The collapse to a star and disk occurs rapidly, on a time scale of the order 10⁵ yr. Disks evolve by dissipating energy while redistributing angular momentum, but it is difficult to predict the rate of evolution, particularly for low mass (compared to the star) disks which nonetheless still contain enough material to account for the observed planetary system. There is no compelling evidence, from available theories of disk structure and evolution, that the solar nebula must have evolved rapidly and could not have persisted for more than 1 Ma. In considering chronologically relevant isotopic data for meteorites, we focus on three methodologies: absolute ages by U-Pb/Pb-Pb, and relative ages by short-lived radionuclides (especially ²⁶Al) and by evolution of ⁸⁷Sr/⁸⁶Sr. Two kinds of meteoritic materials-refractory inclusions such as CAIs and differentiated meteorites (eucrites and angrites)—appear to have experienced potentially dateable nebular events. In both cases, the most straightforward interpretations of the available data indicate nebular events spanning several Ma. We also consider alternative interpretations, particularly the hypothesis of radically heterogeneous distribution of ²⁶Al, which would avoid these chronological interpretations. The principal impetus for such alternative interpretations seems to be precisely the obviation of the chronological interpretation (i.e., the presumption rather than the inference of a short (≤1 Ma) lifetime of the nebula). Astronomical observations of T Tauri stars indicate that the presence of dusty disks is a common if not universal feature, that the disks are massive enough to accomodate a planetary system such as ours, and that at least some persist for 10⁷ yr or more. The results are consistent with the time scales inferred from the meteorite isotopic data. They cannot be considered conclusive with regard to solar nebula time scales, however, in part because it is difficult to relate disk observations to processes that affect meteorites, and in part because the ages assigned for these stars could be wrong by a factor of several in either direction. We conclude that the balance of available evidence favors the view that the nebula existed and was active for at least several Ma. However, because the evidence is not definitive, it is important that the issue be perceived to be an open question, whose answer should be sought rather than presumed.     

The colour of the solar corona and dust grains in it

The photometry of coronal colour negatives is carried out. The films were obtained at the March 7, 1970 and July 10, 1972 eclipses. A distribution of the coronal brightness in the red (635 nm), green (545 nm), and blue (455 nm) wavelength intervals up to distances of (6–7)R is deduced (Figure 1). Colour indexes of the corona (the emission ratio red/blue - C _rb and green/blue - C _gb) have been obtained. We assume C _rb = C _gb = 1 in the inner corona (2R ). The maximum of colour indexes of the 1970 corona are at the distances of about 4R (C _rb 1.9 and C _gb 1.7). A slight reddening within the limits of the errors was found in the 1972 corona.There is a correlation between colour indexes and diffuse external reinforcements (RED) brightness. The analysis of the results leads to the conclusion that RED consists of dust grains with radii 1 m. RED brightness is evaluated to be 4 × 10^-10 B . There is 1 grain of dust in the elementary volume with cross section of 1 cm² along the line of sight. The intensity of dust emission in wavelength interval 10 m deduced by the authors is approximately 1 W cm^-2 s m^–1. That is in agreement with Mankin et al. (1974) and Léna et al. (1974) observations. The whole dust mass of RED is 1% of the coronal gas mass contained within RED region. The dust grain number density is about 10^-11 cm^-3.Determinations of the colour of the solar corona have been made by a number of scientists (Tikhov, 1940, 1957; Allen, 1946; Blackwell, 1952; Michard, 1956; Sharonov, 1958; Nay et al., 1961). The corona colour was found to be somewhat redder than the Sun's. However this question is not finally settled to date.     

Modeling of aggregation of fractal dust clusters in a laminar protoplanetary disk

The evolutionary hydrodynamic model for the formation and growth of loose dust aggregates in the aerodisperse medium of a laminar disk, which was originally comprised of the gas and solid (sub)micrometer particles, is considered as applied to the problem of the formation of planetesimals in the Solar protoplanetary cloud. The model takes into account the fractal properties of dust clusters. It is shown that the clusters partly merge in the process of cluster-cluster coagulation, giving rise to the formation of large fractal aggregates that are the basic structure-forming elements of loose protoplanetesimals arising as a result of physicochemical and hydrodynamic processes similar to the processes of growth of the fractal clusters. Earlier, the modeling was conventionally performed in an “ordinary” continuous medium without considering the multifractional structure of the dust component of the protoplanetary cloud and the fractal nature of the dust clusters being formed during its evolution. Instead, we propose to consider a complex of loose dust aggregates as a special type of continuous medium, namely, the fractal medium for which there exist points and regions that are not filled with its particles. We suggest performing the hydrodynamic modeling of this medium, which has a noninteger mass dimensionality, in a fractional integral model (its differential form) that takes the fractality into account using fractional integrals whose order is determined by a fractal dimensionality of the disk medium.     

Growth and sedimentation of dust grains in the primordial solar nebula

Of the formation processes in the solar system, the process of growth and sedimentation of dust grains in the primordial solar nebula is investigated for a region near the Earth's orbit. The growth equation for dust grains, which are sinking as well as being in thermal motion, is solved numerically in the wide mass range between 10^?12 and 10⁶ g. Any turbulent motions in the nebula are assumed to have already decayed when the sedimentation begins. The numerical simulation shows that the growth and sedimentation proceed faster than was found by Kusaka et al. (1970) but in accordance with the estimate of Safronov (1969) owing to a cooperative interaction of the growth and the sedimentation; that is, at about 3 × 10³ years after the beginning of the growth and sedimentation a dust layer, composed of centimeter-sized grains, is formed at the equator of the solar nebula. Furthermore, the mass density of dust grains floating in the outer layers of the nebula is found to be of the order of 10^?5 after 10⁵ years compared with that before the sedimentation. From these results, it can be estimated that at about 5 × 10³ years after the beginning of sedimentation the dust layer breaks up owing to the onset of gravitational instability.     

Theoretical intensities of Fexiv in the solar EUV spectrum

Equilibrium population of Fexiv levels in coronal conditions was calculated including configurations 3s ²3p, 3s3p ², 3s ²3d, 3p ³, 3s3p3d, 3s ²4s, 3s ²4p, 3s ²4d, 3s ²4f. Relative populations of selected levels are given in Table VII. Figure 1 shows the dependence of relative intensities of the strongest lines on electron density. Certain line ratios can be used for the determination of N _e .E.g., at T=2 × 10⁶ K and with a dilution factor 0.4, the intensity ratio of λ211.3 and λ219.0 changes by a factor of 65 if N _e increases from 10⁷ to 10¹¹ (Table VIII). Cascades from the 3s3p3d and 3p ³ configurations are important in the population of some levels of 3s3p ² (Table VI). A possibility of identification of additional lines in the solar spectrum is indicated. NAS-NRC Resident Research Associate.     

Properties of flux tubes and the relation with solar irradiance variability

At the solar surface the magnetic field is bundled into discrete elements of concentrated flux, often referred to as magnetic flux tubes, which cover only a small fraction of the solar surface. Flux tubes span a whole spectrum of sizes, ranging from sunspots to features well below the best currently obtainable spatial resolution. Whereas sunspots have been well studied, our knowledge of the true brightness of small-scale magnetic features is hampered by the insufficient spatial resolution of the observations. A better understanding of the thermal and magnetic properties of these small-scale features, however, is crucial for an understanding of (climate-relevant) long-term solar irradiance variations.