Dust dynamics during the plasma afterglow

The charge and dynamics of dust particles in an afterglow plasma are studied using a 1D model in the diffusion approximation, taking into account the transition from ambipolar to free diffusion. It is analyzed how external conditions (dust particle size, neutral gas pressure and initial electron density) affect the dust motion. The dust particle dynamics has been examined in microgravity conditions and in presence of gravity. Without gravity, the location of dust particles in plasma volume may change essentially during the afterglow if the dust size and pressure are small (≤ 10 nm and ≤ 30 mTorr, respectively). At small pressures, in the very beginning of afterglow, small nanoparticles move to the plasma boundary because the ion drag force dominates over the electric force. At afterglow times when the electron temperature becomes time-independent, the ion drag force decreases faster with time than the electric force due to the ion density decrease, and dust particles may move to the slab center. In presence of gravity, the effect of gravity force on dust particles is important only at large afterglow times (t ≥ 10 ms), when the electric and ion drag forces are small. The dust dynamics depends essentially on the initial plasma density. If the density is large (~ 1012 cm-3), small nanoparticles (≤ 10 nm) may deposit on plasma walls in the beginning of plasma afterglow because of an enhancement of the ion drag force.

Late Quaternary Dust, Loess and Desert Dynamics in Upwind Areas of the Chinese Loess Plateau

As a key global climate and dust archive, the nature of Chinese loess generation, transport and deposition remains debated. The lack of consensus on dust dynamics from sources to leeward regions fundamentally limits interpretation of the preserved past climate and dust record. Here, we investigate chronostratigraphic variability of aeolian deposits in upwind regions of the modern Chinese Loess Plateau (CLP) and attempt to understand dust dynamics that potentially affects loess deposition downwind. The strata consist of alternating layers of typical loess, well-sorted sand, and sandy loess, with obvious unconformities occurring at the transitions from loess to sand. We suggest that pre-existing typical loess in regions to the northwest of the modern CLP was eroded by wind, providing a significant source of homogeneous dust for the dust deposits downwind. The sand deposits interbedded with typical loess at the study sites suggests that proximal deserts have greatly expanded and contracted repeatedly prior to the Holocene. However, the spatial extents of the deserts, as inferred from the sections here, have not markedly diminished after the major expansion during the Last Glacial Maximum. Such a pattern of proximal desert dynamics plays an important role in regulating dust emission and transport, strongly affecting dust sequences on the CLP. Our results suggest a complex scenario of dust dynamics in upwind regions of the CLP at least over the Late Quaternary; the involved processes have to be considered when using conventional proxies from Chinese loess deposits to recover the history of climate and dust changes.

Abrupt last glacial loess-dust deposition over Southeast England coupled with dynamics of the British-Irish Ice Sheet

<p>Loess deposits are globally important dust archives but are often limited by imprecise chronological control. In particular, loess records adjacent to former ice sheets seldom have detailed, independent age models yet have the potential to elucidate the causes of past high latitude (>50° N in Northern Hemisphere) coarse dust emission close to former ice sheets, a relatively poorly known aspect of past dust dynamics. Loess deposits in southern Britain were formed in close proximity to western parts of the last glacial Eurasian ice sheets. However, currently their age and accumulation rate remain poorly known, limiting interpretation of the controls on last glacial coarse dust emission and deposition in the region.</p><p>Here we apply high sampling resolution quartz optically stimulated luminescence (OSL) to constrain the timing of dust accumulation and loess formation at the Pegwell Bay site in east Kent, SE England. The OSL ages and Bayesian (Bacon) age modelling results are the most detailed to date for western European loess, and show that loess began to accumulate around c. 25 ka, coinciding with Heinrich event 2 and the coupling of Fennoscandian and British-Irish ice sheets. There were two phases of greatly enhanced dust accumulation at the site, at 25-23.5 ka and 20-19 ka, separated by a lower accumulation rate period. Loess accumulation appears to have stopped or been dramatically reduced after 19-18 ka. We propose that the dynamics of the British-Irish and Fennoscandian Ice Sheets, associated glacial lake drainage, and linked reorganisations of atmospheric circulation, act to control loess accumulation at the site. In particular, we argue that both periods of enhanced dust accumulation were caused by advance-retreat phases of the North Sea ice lobe, and associated drainage of Dogger Lake. These events would have led to abrupt input of sediment-rich ice dammed lake and melt water from northern and eastern England and the North Sea into the exposed southern North Sea basin. This would have dramatically increased sediment availability for transport and deposition as loess in SE England. Easterly and north-easterly winds that could have transported this dust to SE England would have been enhanced by presence of an ice sheet anticyclone, enlarged during Fennoscandian and British-Irish ice sheet coalescence, as well as katabatic winds and easterly flow occurring on the northern side of Atlantic cyclones forced south of southern Britain by the extended western British-Irish ice sheet. As such, last glacial dust dynamics and loess accumulation in Britain is highly influenced by the interaction of the British-Irish and Fennoscandian ice sheets, Atlantic storm tracks, and the topography and drainage of the exposed North Sea basin.</p>

Dust dynamics in planet-driven spirals

Context. Protoplanetary disks are known to host spiral features that are observed in scattered light, the ALMA continuum, and more recently in CO gas emission and gas dynamics. However, it is unknown whether spirals in gas and dust trace the same morphology. Aims. We aim to study the morphology and amplitude of dusty spirals as function of the Stokes number and the underlying mechanisms that cause a difference between dusty spirals and gas spirals. We then construct a model to relate the deviation from Keplerian rotation in the gas to a perturbation in surface density of the gas and dust. Methods. We used FARGO-3D with dust implementation to numerically study the spirals, after which the results were interpreted using a semi-analytical model. This model was tested on observational data to predict the perturbation of the spiral in gas dynamics based on the continuum data. Results. We find that the pitch angle of a spiral does not differ significantly between gas and dust. The amplitude of the dust spiral decreases with the Stokes number (St) and starts to fade out at a typical St > 0.1 as the dust becomes decoupled from the gas. The semi-analytical model provides an accurate and fast representation of the difference in the surface density of the spiral in dust and gas. We find a spiral in the TW Hya velocity residual map, never seen before, which is a feature in the vertical velocity and has a kink at the continuum gap, yielding strong evidence for a planet at 99 au. Conclusions. We built a model that gives an estimate of the underlying dynamics of dust in a spiral, which can serve as evidence of the planetary origin of spirals and can be a probe for the Stokes number in the disk.

Inner dusty regions of protoplanetary discs – II. Dust dynamics driven by radiation pressure and disc winds

ABSTRACT We explore dust flow in the hottest parts of protoplanetary discs using the forces of gravity, gas drag, and radiation pressure. Our main focus is on the optically thin regions of dusty disc, where the dust is exposed to the most extreme heating conditions and dynamical perturbations: the surface of optically thick disc and the inner dust sublimation zone. We utilize results from two numerically strenuous fields of research. The first is the quasi-stationary solutions on gas velocity and density distributions from mangetohydrodynamical (MHD) simulations of accretion discs. This is critical for implementing a more realistic gas drag impact on dust movements. The second is the optical depth structure from a high-resolution dust radiation transfer. This step is critical for a better understanding of dust distribution within the disc. We describe a numerical method that incorporates these solutions into the dust dynamics equations. We use this to integrate dust trajectories under different disc wind models and show how grains end up trapped in flows that range from simple accretion on to the star to outflows into outer disc regions. We demonstrate how the radiation pressure force plays one of the key roles in this process and cannot be ignored. It erodes the dusty disc surface, reduces its height, resists dust accretion on to the star, and helps the disc wind in pushing grains outwards. The changes in grain size and porosity significantly affect the results, with smaller and porous grains being influenced more strongly by the disc wind and radiation pressure.

Dust trapping around Lagrangian points in protoplanetary disks

Aims. Trojans are defined as objects that share the orbit of a planet at the stable Lagrangian points L4 and L5. In the Solar System, these bodies show a broad size distribution ranging from micrometer (μm) to centimeter (cm) particles (Trojan dust) and up to kilometer (km) rocks (Trojan asteroids). It has also been theorized that earth-like Trojans may be formed in extra-solar systems. The Trojan formation mechanism is still under debate, especially theories involving the effects of dissipative forces from a viscous gaseous environment. Methods. We perform hydro-simulations to follow the evolution of a protoplanetary disk with an embedded 1–10 Jupiter-mass planet. On top of the gaseous disk, we set a distribution of μm–cm dust particles interacting with the gas. This allows us to follow dust dynamics as solids get trapped around the Lagrangian points of the planet. Results. We show that large vortices generated at the Lagrangian points are responsible for dust accumulation, where the leading Lagrangian point L4 traps a larger amount of submillimeter (submm) particles than the trailing L5, which traps mostly mm–cm particles. However, the total bulk mass, with typical values of ~Mmoon, is more significant in L5 than in L4, in contrast to what is observed in the current Solar System a few gigayears later. Furthermore, the migration of the planet does not seem to affect the reported asymmetry between L4 and L5. Conclusions. The main initial mass reservoir for Trojan dust lies in the same co-orbital path of the planet, while dust migrating from the outer region (due to drag) contributes very little to its final mass, imposing strong mass constraints for the in situ formation scenario of Trojan planets.