Experiments on the photophoretic motion of chondrules and dust aggregates—Indications for the transport of matter in protoplanetary disks

In a set of 16 drop tower experiments the motion of sub-millimeter to millimeter-sized particles under microgravity was observed. Illumination by a halogen lamp induced acceleration of the particles due to photophoresis. Photophoresis on dust-free chondrules, on chondrules, glass spheres and metal spheres covered with SiC dust and on pure SiC dust aggregates was studied. This is the first time that photophoretic motion of millimeter-sized particles has been studied experimentally. The absolute values for the photophoretic force are consistent with theoretical expectations for spherical particles. The strength of the photophoretic force varies for chondrules, dust covered particles and pure dust from low to strong, respectively. The measurements support the idea that photophoresis in the early Solar System can be efficient to transport solid particles outward.     

Migration of Dust Particles and Delivery of Volatiles to the Terrestrial Planets

Results of computer simulations of the migration of small bodies and dust in the Solar System showed that a relative fraction of cometary and trans-Neptunian dust particles in the total balance of the dust encountering the terrestrial planets can be significant. The contribution of dust particles to the delivery of volatiles to the terrestrial planets is estimated to be 3–4 orders of magnitude less than that of small bodies. However, the dust particles could be most efficient in the delivery of organic or even biogenic matter to the Earth, because they experience substantially weaker heating when passing through the atmosphere.     

A model of the origin of the Jovian ring

Starting with the assumption that the micron-sized particles which make up the bright Jovian ring are fragments of erosive collisions between micrometeoroid projectiles and large parent bodies, a physical model of the ring is calculated. The physics of high-velocity impacts leads to a well-defined size distribution for the ejecta, the optical properties of which can be compared with observation. This gives information on the ejecta material (very likely silicates) and on the maximum size of the projectiles, which turns out to be about 0.1 μm. The origin of these projectiles is discussed, and it is concluded that dust particles ejected in volcanic activity from Io are the most likely source. The impact model leads quite naturally to a distribution in ejecta sizes, which in turn determines the structure of the ring. The largest ejecta form the bright ring, medium-sized ejecta form a disk extending all the way to the Jovian atmosphere, and the small ejecta form a faint halo, the structure of which is dominated by electromagnetic forces. In addition to the Io particles, interaction with interplanetary micrometeoroids is also considered. It is concluded that μm-sized ejecta from this source have ejection velocities which are several orders of magnitude too large, and thus cannot contribute significantly to the observed bright ring. However, the total mass ejection rate is significant. Destruction of these ejecta by the Io particles may provide additional particles for the halo.     

The shape of the interstellar absorption band

Profiles of the 4430 band are calculated for resonant absorbers distributed within graphite particles, silicate particles and solid H₂ grains. The sizes of grains adopted are those which give agreement with the interstellar extinction. Only in the latter two cases can satisfactory agreement be obtained with recent observational data.     

Size of ambiplasma domains in the Universe

Assuming that the initial state of the Universe can be simulated by a thin large-scale homogeneous ambiplasma described by fluid type equations for charged particles (both matter and antimatter particles), stability calculations have been carried out of the various normal modes which might be supported by such a system. The present simplified analysis does not lead to the formation of matter-antimatter symmetric domains smaller than the size of the proto-metagalaxy (Alfvén, 1981) from the initial state of the Universe.     

The charged particle in the magnetosphere of a rotating neutron star

The motion of charged particles in a pulsar magnetosphere is examined in the present paper. Using the non-relativistic approximation, the trajectories of the charged particles are investigated qualitativley both in the case of axial and in the case of incline rotator. The obtained results can be used for the construction of the pulsar magnetosphere.     

Coulomb-trapped particles in the electromagnetic fields of an oblique magnetic rotator

An oblique, rotating magnetized sphere emits electromagnetic waves which, for large magnetization, can quickly accelerate charged particles to very high energies. A central, attractive Coulomb force can trap particles in the region beyond the light cylinder by balancing the accelerating influence of the radiation on the particles. We sample some of the particle orbits possible under these dynamical conditions. A general feature of these orbits is that non-interacting particles started with random initial conditions in the domain of attraction of these orbits will arrange themselves on a curve corotating with the axis of magnetization. Such particle configurations can be a source of pulsed radiation. In the idealized case of no interparticle interactions the spectral index for the radiation emitted by one frequently occurring configuration is found to be –2/3, for emission from radio to -ray frequencies. The dynamical conditions in this simple model closely match those prevalent in outer pulsar magnetospheres, making it possible that part of the radiation from pulsars is emitted by trapped plasma in the region beyond the light cylinder.     

Investigation of the polarization observed in infrared absorption bands in the spectra of protostars

We investigate the linear polarization in the two deepest infrared absorption bands observed in the spectra of protostars, the water-ice band with the center near 3.1 μm and the silicate band with the center near 9.7 μm, using a core-mantle confocal spheroid model with various axial ratios a/b and relative volumes of the core material. We consider the effect of the grain shape, structure, and type (oblate, prolate) as well as the type of grain orientation and its location relative to the incident ray of light and the magnetic field direction on the central wavelengths of the two bands and the polarizability in the bands. We have found that the observed relationships between the polarizability in the bands and the ratio of their optical depths at the band centers can be explained if we choose slightly oblate or prolate particles (a/b ≲2 for the silicate band and 1.3 ≲ a/b ≲ 2 for the ice band). For any type of orientation, the core-mantle confocal spheroid model requires different axial ratios for the ice and silicate bands to account for the observed polarization. We show that picket-fence-oriented particles can explain the observed polarization in the ice band at angles α between the particle rotation axis and the incident ray ≳30° and in the silicate band at any α. Perfectly Davis-Greenstein-oriented particles can explain the observed polarization in the ice band at angles Ω between the line of sight and the magnetic field direction ≳60° and in the silicate band at any Ω. The orientation parameter ζ (imperfect Davis-Greenstein orientation) must be no more than 0.5 (oblate particles) and 0.1 (prolate particles) for the ice band and can be arbitrary for the silicate band. Original Russian Text ? T.V. Zinov’eva, 2006, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2006, Vol. 32, No. 10, pp. 748–766.     

Stability of co-orbital ring material with applications to the Janus-Epimetheus system

An analytical model that describes the evolution of ring particles that are co-orbital with two larger bodies on near-circular and near-planar orbits has been formulated. This can be used to estimate the lifetime of the particles within the ring. All the examples investigated, such as the Janus-Epimetheus (JE) system, indicate that the particles should be removed from the co-orbital region within half a synodic period (∼4 years for JE). Numerical modelling confirms the predictions of the model. When the masses are on eccentric orbits the particles remain within the co-orbital system for longer. Our results suggest that the ring associated with Janus and Epimetheus must be continually fed with material, probably by meteoroid impacts on the two satellites.     

Acceleration of Pick-up Ions at the Solar Wind Termination Shock

It is generally accepted that pick-up ions act as a seed population for anomalous cosmic rays originating at the solar wind termination shock. We believe that the ion pre-acceleration process operating in the heliosphere up to the termination shock can be very important to inject the ions into the shock acceleration process. The pick-up ions pre-accelerated by solar wind turbulences have already a pronounced high energy tail when they reach the shock. Some fraction of these ions can experience further acceleration up to energies of anomalous cosmic rays by means of shock drift and diffusive acceleration. In the present paper the shock drift acceleration of pick-up ions suffering multiple reflection due to abrupt changes in both the strength and direction of the magnetic field through the shock is considered. The reflection process operates for high velocity particles different from the reflection by the electric cross-shock potential. During the first reflection the mean kinetic energy of pick-up ions increases by approximately a factor of 10. Reflected particles have highly anisotropic velocity distribution. Subsequent excursion of the particles in the turbulent upstream flow leads to diffusion in pitch-angle space and, as a result, the particles can return to the shock again suffering, thus, multiple encounters. In order to describe the motion of particles in the upstream and down streamparts of the flow we solve the Fokker-Plank transport equation for anisotropic velocity distribution function. This revised version was published online in July 2006 with corrections to the Cover Date.     

Atmospheric entry heating of micrometeorites at Earth and Mars: Implications for the survival of organics

The atmospheric entry heating of micrometeorites (MMs) can significantly alter their pre‐existing mineralogy, texture, and organic material. The degree of heating depends predominantly on the gravity and atmospheric density of the planet on which they fall. For particles falling on Earth, the alteration can be significant, leading to the destruction of much of the pre‐entry organics; however, the weaker gravity and thinner atmosphere of Mars enhance the survival of MMs and increase the fraction of particles that preserve organic material. This paper investigates the entry heating of MMs on the Earth and Mars in order to examine the MM population on each planet and give insights into the survival of extraterrestrial organic material. The results show that particles reaching the surface of Mars experience a lower peak temperature compared to Earth and, therefore, experience less evaporative mass loss. Of the particles which reach the surface, 68.2% remain unmelted on Mars compared to only 22.8% on Earth. Due to evaporative mass loss, unmelted particles that reach the surface of Earth are restricted to sizes <70 μm whereas particles >475 μm survive unmelted on Mars. Approximately 10% of particles experience temperatures below ~800 K, that is, the sublimation temperature of refractory organics found in MMs. On Earth, this fraction is significantly lower with less than 1% expected to remain below this temperature. Lower peak temperatures coupled with the larger sizes of particles surviving without significant heating on Mars suggest a much higher fraction of organic material surviving to the Martian surface.     

Solar wind access to the plasma sheet along the flanks of the magnetotail

Low-energy particle trajectories in an idealized magnetotail magnetic field are investigated to determine the accessibility of magnetosheath protons and electrons to the plasma sheet along the flanks of the tail magnetopause. The drift motion of the positively (negatively) charged particles incident on the dawn (dusk) magnetotail flank causes such particles to penetrate deeper into the magnetotail. For certain combinations of particle energy, incident velocity vector and initial penetration point on the tail magnetopause, the incident particles can become trapped in the plasma sheet, after which their net drift motion then provides a current capable of supporting the entire observed magnetotail field. The results further indicate that the bulk of the solar wind plasma just outside the distant tail boundary, which streams preferentially in a direction along the magnetopause away from the Earth at velocities around 400 km s^?1, can be caught up in the tail if the initial penetration point is within about 2R_E, of the quasi-neutral sheet. It is suggested that a large fraction of the magnetotail plasma is composed of former solar wind particles which have penetrated the magnetospheric boundary at the tail flanks.     

Generalized adiabatic theory applied to the magnetotail current sheet

We use the generalized first adiabatic invariant, an extension of the magnetic moment for regions of large field gradients, to treat particles in the magnetotail current sheet. The equations of motion can be expressed in terms of drift parameters which vary slowly and smoothly at the drift rate, not at the gyration rate. The analysis leads to boundaries in phase space which form a generalized loss cone and separate particles drifting into and out of the layer from particles trapped within the layer. These boundaries can be used in the moment integrals for densities and currents when the drifting particles differ in temperature, or in other properties, from the trapped population, as has been suggested by observations. We give examples of how different kinds of particle orbits contribute to the spatial profiles of density and current and thus to the field structure of the current sheet. We find that the parallel pressure of the drifting particles must exceed the transverse pressure for self-consistent solutions to exist, and based on this result, we give examples of fully self-consistent solutions using bi-Maxwellian ion and Maxwellian electron distributions. We give a proof, using generalized adiabatic theory, of Cowley's (1978a) theorem that particles trapped in the current layer experience zero net drift.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Dynamical zodiacal cloud models constrained by high resolution spectroscopy of the zodiacal light

The simulated Doppler shifts of the solar Mg I Fraunhofer line produced by scattering on the solar light by asteroidal, cometary, and trans-neptunian dust particles are compared with the shifts obtained by Wisconsin H-Alpha Mapper (WHAM) spectrometer. The simulated spectra are based on the results of integrations of the orbital evolution of particles under the gravitational influence of planets, the Poynting-Robertson drag, radiation pressure, and solar wind drag. Our results demonstrate that the differences in the line centroid position in the solar elongation and in the line width averaged over the elongations for different sizes of particles are usually less than those for different sources of dust. The deviation of the derived spectral parameters for various sources of dust used in the model reached maximum at the elongation (measured eastward from the Sun) between 90° and 120°. For the future zodiacal light Doppler shifts measurements, it is important to pay a particular attention to observing at this elongation range. At the elongations of the fields observed by WHAM, the model-predicted Doppler shifts were close to each other for several scattering functions considered. Therefore the main conclusions of our paper do not depend on a scattering function and mass distribution of particles if they are reasonable. A comparison of the dependencies of the Doppler shifts on solar elongation and the mean width of the Mg I line modeled for different sources of dust with those obtained from the WHAM observations shows that the fraction of cometary particles in zodiacal dust is significant and can be dominant. Cometary particles originating inside Jupiter's orbit and particles originating beyond Jupiter's orbit (including trans-neptunian dust particles) can contribute to zodiacal dust about 1/3 each, with a possible deviation from 1/3 up to 0.1-0.2. The fraction of asteroidal dust is estimated to be ∼0.3-0.5. The mean eccentricities of zodiacal particles located at 1-2 AU from the Sun that better fit the WHAM observations are between 0.2 and 0.5, with a more probable value of about 0.3.     

Influence of a sudden compression of the magnetosphere on outer zone electron fluxes measured at arbitrary pitch-angle

Analytical laws of motion of individual particles are used to evaluate the effects of a sudden compression on the outer radiation zone. A modulation of fluxes can be detected by synchronous satellites. Its intensity increases with the pitch-angle of the particles and the inward convection of field lines.     

Thermal effects of insolation propagation into the regoliths of airless bodies

We have investigated thermal models for planetary surfaces composed of particles that are bright and optically thin in the visual, and dark and opaque in the thermal infrared. The models incorporate the assumption that insolation is absorbed over a finite distance in the regolith, predicting lower daytime and higher nighttime temperatures than those predicted if the insolation were a absorbed only at the surface. The magnitude of the effect depends on the scale length for absorption of insolation relative to the diurnal skin depth for thermal diffusion, and can be significant when insolation penetrates to a depth comparable to the diurnal skin depth. In particular, for bodies like Enceladus and Europa, the maximum daytime temperature depression and nighttime temperature elevation can be 10°K or more for penetration-depth scales ∼ 1.5 cm. If insolation penetrates deeply enough into a surface, and the thermal-infrared opacity of its constituent particles is very high (e.g., in a regolith composed of particles of water ice), a solid-state greenhouse can result! This has important implications for geophysical models of high-albedo, icy bodies because actual boundary-layer temperatures may in fact be significantly higher than those assumed in previous studies, making it easier to melt the interiors of such bodies. Another important implication of the models is that where insolation- penetration is significant, thermal inertias inferred from models that do not allow for this effect will be upper limits to the real thermal inertia.     

Automated classification of interplanetary dust particles: Johnson Space Center Cosmic Dust Catalog Volume 15

Abstract– The “Cosmic Dust Catalog,” published by the NASA Johnson Space Center (JSC), describes thousands of interplanetary dust particles subjected to preliminary analysis and with labels indicating their origin. However, only about 80% of the particles are assigned unambiguous labels, the labels of the remaining 20% being uncertain. In addition, the Stardust mission results opened up the possibility that some particles classified as terrestrial contaminants are instead of cosmic (cometary) origin. In this article, we present a methodology for automatic classification of particles on the basis of similarity of their X‐ray energy dispersive spectrometry spectra. The method is applied to the 467 particles constituting Volume 15 of the catalog. A first part of the analysis is to digitize the spectra from their scanned images. The digitized spectra are subjected to agglomerative clustering, which reveals 16 distinct clusters or compositional types of particles. The Sammon’s map is used to visualize the relationship between different clusters; 6 clusters corresponding to cosmic particles and 10 clusters corresponding to terrestrial contaminants are clearly separated on the map indicating overall differences between diverse spectra of cosmic and terrestrial particles. By reconciling labels with the clustering structures, we propose the relabeling of 155 particles including the relabeling of 31 terrestrial contaminants into cosmic particles. The proposed relabeling needs to be confirmed by in‐depth study of these particles. The paucity of particles with firmly determined cometary or asteroidal origin makes it difficult to establish whether the spectra based autoclassification can be utilized to discriminate between cometary and asteroidal particles. The methodology presented here can be used to classify all particles published in the catalog, as well as different samples for which comparable spectra are available.     

Nonresonant generation of MHD disturbances during supernova explosions in an inhomogeneous interstellar medium

We analyze the nonresonant generation of large-scale magnetic inhomogeneities near a shock front by accelerated particles. The MHD disturbances are generated by the electric current excited by relativistic particles in the preshock medium in the presence of weak large-scale density inhomogeneities. The MHD modes considered can be amplified by other resonant and nonresonant mechanisms related to the presence of relativistic particles. We estimate the magnetic fields and the energies to which charged particles can be accelerated in different phases of the interstellar medium by taking into account the random magnetic fields generated by the mechanism considered.     

Modeling of Proton Acceleration in a Magnetic Island Inside the Ripple of the Heliospheric Current Sheet

Crossings of the heliospheric current sheet (HCS) at the Earth’s orbit are often associated with observations of anisotropic beams of energetic protons accelerated to energies from hundreds of keV to several MeV and above. A connection between this phenomenon and the occurrence of small-scale magnetic islands (SMIs) near reconnecting current sheets has recently been found. This study shows how pre-accelerated protons can be energized additionally due to oscillations of multiple SMIs inside the ripple of the reconnecting HCS. A model of the electromagnetic field of an oscillating 3D SMI with a characteristic size of ~0.001 AU is developed. A SMI is supposed to be bombarded by protons accelerated by magnetic reconnection at the HCS to energies from ~1keV to tens of keV. Numerical simulations have demonstrated that the resulting longitudinal inductive electric fields can additionally reaccelerate protons injected into a SMI. It is shown that there is a local “acceleration” region within the island in which particles gain energy most effectively. As a result, their average escape energies range from hundreds of keV to 2 MeV and above. There is almost no particle acceleration outside the region. It is shown that energies gained by protons significantly depend on the initial phase and the place of their entry into a SMI but weakly depend on the initial energy. Therefore, low-energy particles can be accelerated more efficiently than high-energy particles, and all particles can reach the total energy limit upon their escape from a SMI. It is also found that the escape velocity possesses a strong directional anisotropy. The results are consistent with observations in the solar wind plasma.