SH51D-0292 0800h
Comet C/2002 V1 (NEAT): Behavior of a Cometary Dust Tail at 0.1 AU From the Sun
Comet C/2002 V1 (NEAT) was observed by the LASCO coronagraph aboard the ESA/NASA Solar and Heliospheric Observatory spacecraft during February 16-20, 2003. This period spanned the comet's perihelion passage at 0.1 AU from the Sun. The observing geometry was excellent, with the comet's orbital plane almost perpendicular to the line of sight. Images obtained using the instrument's five optical-near IR broadband filters reveal an ion tail and a highly structured dust tail. The comet's extremely high gas and dust production rates were similar to those of C/1995 O1 (Hale-Bopp), but peaked in a much more extreme solar wind environment. Two large coronal mass ejections erupted in the comet's general direction during the observations, the largest of which affected not only the ion tail, but also the striated dust tail. Significant changes were observed in the dust tail's structure that we interpret as evidence of an interaction between the CME and electrically-charged cometary dust. We shall present our observations of the dust component of the comet, and our interpretations of them, supported by the results of simulations.
SH51D-0293 0800h
A Dynamic Fountain Model for Lunar Dust
There is much evidence to show that lunar horizon glow observed at the terminator is caused by sunlight scattered by dust grains originating from the surface. The dust grains and lunar surface are electrically charged by the Moon's interaction with the local plasma environment and the photoemission of electrons due to solar UV and X-rays. This causes the like-charged surface and dust particles to repel each other, and creates a near-surface electric field. Previous models have explained dust observations at $\sim$10 cm above the surface, by suggesting that charged grains "levitate" in the local electric field; however this does not explain observations at $\sim$1 km altitude. In order to explain the high-altitude dust observations, we propose a dynamic "fountain" model in which charged dust grains follow ballistic trajectories, subsequent to being accelerated upwards through a narrow sheath region by the surface electric field. These dust grains may affect the optical quality of the lunar environment for astronomical observations and interfere with exploration activities.
SH51D-0294 0800h
Collective behaviour of a System of Emitting Dust Particles
In the present work we focus on the role of electron emission (either thermionic, secondary or photoelectric) in charging an object immersed in a plasma. Recent work [1] has shown how electron emission can fundamentally affect the shielding potential around the dust. In particular, depending on the physical parameters of the system, the shielding potential can develop an attractive potential well. The aim of the present work is two-fold. First, we will present a theory of the existence and properties of the attractive potential well to explain the conditions for the formation, as well as providing a description of the well in terms of Lennard-Jones (LJ) potentials. Second, we focus on the consequences of attraction among eqully charged dust particle and on the peculiar collective behaviour under the circumstances. Here, we present a series of simulations conducted with a new code designed to study a large system of weakly coupled dust particles, interacting with a LJ like potential. [1] G.L. Delzanno, G. Lapenta, M. Rosenberg, Phys. Rev. Lett., 92, 035002 (2004).
SH51D-0295 0800h
Power Fluctuations in Meteor Head Echoes Observed with the EISCAT VHF Radar
Meteor head echoes are radio wave reflections from the plasma generated by the interaction of meteoroids with the atmosphere at about 80-120 km altitude. The echoes are characterized by being transient and Doppler shifted. The received power is confined in range, as from a point source, and it moves with the line-of-sight velocity of the meteoroid. Head echoes arise and disappear as meteoroids pass through the radar beam, and usually last from one tenth to a few tenths of a second. Some meteor echoes in the data from a meteor experiment carried out with the EISCAT VHF radar at 224 MHz contain peculiar pulsations in received power. These pulsations are in the frequency range 50-100 Hz. The interpulse period of the experiment was 2.167 ms, i.e., meteoroid line-of-sight velocity and echo power was monitored at 461 Hz. This limits the observable power fluctuation frequency range to about 10-230 Hz. The process causing the echo power pulsations is at present not identified. Plasma effects are the most likely cause, a presumable mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate.
SH51D-0296 0800h
Small Interstellar Dust Grains Near the Heliosphere and Their Transport Into the Solar System
In distinction to larger grains that penetrate easily into the inner solar system, small interstellar dust grains, with much larger values of charge-to-mass ratio, are coupled efficiently to the interstellar magnetic field and so mostly prevented from entering the heliosphere. We present the results of simulations of small charged dust particle motion in the transition region between the undisturbed interstellar medium and the heliosphere. The velocity distributions of small dust grains near the heliosphere are found to be highly sensitive to the presence of sharp shock waves in the transition region, like the heliospheric bow shock. We also simulate the motion of those small charged grains that were able to cross the heliopause and find that, in the region close to the ecliptic plane, their further penetration into the inner solar system is facilitated by the presence of wavy heliospheric current sheet.
SH51D-0297 0800h
Can cosmic rays amorphize silicate dust particles?
Interstellar and interplanetary dust plays a key role in the evolution of the Interstellar Medium (ISM) in galaxies and the formation of stars, planets and possibly life. The dust composition is determined by physical processes, including ion bombardment. We have irradiated forsterite single crystals dust analogs with swift heavy ions with a large electronic energy deposition. Amorphization of the crystals was monitored with Rutherford Back-Scattering (RBS) and verified by transmission electron microscopy (TEM) observations. Modeling of ion tracks also shows a similar behavior. IR spectra of the irradiated samples agree well with observed astronomical spectra. Based on this evidence, and taking into account accepted ion fluxes we can predict that silicate grains can be amorphized by heavy cosmic rays.
SH51D-0298 0800h
Self-Magnetization of the Early Solar System Matter
The remnant magnetization of chondrite type meteorite matter indicates the existence of $10^{-5}$ - $10^{-3}$ Tesla magnetic fields in the early solar system accretion disk. Taking into account parameter regimes being typical for this early evolutionary stage of the partially ionized dusty proto-solar accretion disk we performed self-consistent three-dimensional multi fluid plasma - neutral gas - dust simulations. The results of our systematic simulations show that shear flow driven collisional interactions are able to yield self-induced magnetic fields of the order of $10^{-5}$ - $10^{-3}$ Tesla in a dusty proto-planetary accretion disk on very short time scales of the order of a few years. Thus, shear flow induced self-magnetization can yield a significant contribution to the magnetization of the early solar system and is able to explain the measured remnant magnetization of meteorite material.
SH51D-0299 0800h
Secondary Electron Emission from Aggregate Dust Particles
Dust particles immersed in plasma inevitably carry electric charges on their surfaces as a consequence of their interactions with plasma particles. Secondary electron emission is one of the most important charging mechanisms in the region of high-temperature plasma such as the vicinity of the sun and the heliospheric boundary. Furthermore, its importance is elevated for small grains whose radii are comparable to the escape length of electrons. Available information indicates that dust particles in the solar system are fluffy aggregates consisting of submicron grains rather than compact spheres. The number of secondary electrons emitted from fluffy aggregate particles is expected to be smaller than that from compact spherical particles, because electrons emitted toward a void in a fluffy particle can be absorbed within the particle again. Therefore, one needs to take into account the morphology of dust grains for evaluating the secondary electron yield. We will present a simple method to numerically estimate the secondary electron yield for fluffy dust aggregates and discuss the importance of particle morphology for the determination of the secondary electron current.
SH51D-0300 0800h
New constraints from the inner source pickup ions on the dust population near the sun
There are two main sources of pickup ions (PUIs) in the heliosphere: the interstellar source and the inner source, which is produced due to solar wind interaction with heliospheric grains. The inner-source PUIs are found to dominate inside 1 AU, and have a peak source rate between 10 and 30 $R_s$. The inner-source PUIs are currently thought to be generated when solar wind passes through extremely small grains. The exiting solar wind particles, mainly neutrals at solar wind energies, are then ionized and picked up by the solar wind. The IDPs in this case must be small enough, a few tens of a nm at most, to act as thin foils by allowing the solar wind ions to pass through the grains without sputtering significant mass. The inner-source PUIs have been studied thus far mainly at solar minimum. We present a comparison between three selected periods at solar minimum and solar maximum. During these periods the inner source was remarkably stable. Moreover, we show that the inner source is consistent with randomly distributed sources. These new results add two constraints to the existing constraints from the inner source on the dust population near the sun. We drew a matrix considering these constraints and evaluating the mechanisms that have been proposed.