The influence of the dust-free zone on F-corona brightness

<p>This presentation is related to model calculations of the circumsolar dust brightness that is seen in the F-corona and inner Zodiacal light. We calculate the brightness integral that includes the size distribution of the interplanetary dust, the spatial distribution, and the scattering properties. The scattering properties are estimated with Mie calculations of spherical particles consisting of astronomical silicate. We consider different size distributions of the dust particles with sizes between 1 nanometre - 100 micrometre. It was recently discussed that the extension of the dust-free zone can be inferred from the slope of the F-corona brightness seen in new observations received from the WISPR instrument on the NASA Parker Solar Probe (Stenborg et al., 2020). We, therefore, investigate the influence of the dust-free zone on the brightness and compare it to the influence that the dust size distribution has.</p><p>References</p><p>1. G. Stenborg, R. A. Howard, P. Hess, B. Gallagher, PSP/WISPR observations of dust density depletion near the Sun I. Remote observations to 8 Rsol from an observer between 0.13-0.35 AU, A&A, Forthcoming article, 2020. DOI: 10.1051/0004-6361/202039284</p>

Less atmospheric radiative heating due to aspherical dust with coarser size

Mineral dust aerosols cool and warm the atmosphere by scattering and absorbing both solar (short-wave: SW) and thermal (long-wave: LW) radiation. However, large uncertainties remain in dust radiative effects, largely due to differences in the dust size distribution and optical properties simulated in Earth system models. Here, we improve the simulated dust properties with datasets that leverage measurements of size-resolved dust concentration and asphericity factor (improved simulation) in a coupled global chemical transport model (IMPACT) with a radiative transfer module (RRTMG) (default simulation). The global and annual average of dust aerosol optical depth at 550 nm (DAOD550) from the improved simulation (0.029) falls within the range of a semi-observation-based estimate (0.030 ± 0.005), in contrast to that (0.023) of the default simulation. Improved agreement against semi-observation-based estimate of the radiative effect efficiency was obtained using less absorptive SW and more absorptive LW dust refractive indices. Our sensitivity simulations reveal that the improved simulation leads to a similar net global dust radiative effect at the Top Of Atmosphere (TOA) on a global scale to the default simulation (−0.08 vs. −0.09 W ·m−2) but results in less cooling at the surface (−0.23 vs. −0.88 W ·m−2), because of enhanced LW warming by coarser aspherical dust. Our results thus suggest less atmospheric radiative heating due to aspherical dust with coarser size over the major source regions (0.15 vs. 0.79 W ·m−2 on a global scale).

Effects of dust size distribution on nonlinear magnetosonic wave in a dusty plasma

Both the linear and the nonlinear magnetosonic wave in a multi-component dusty plasma are studied in the present paper. The dependence of the dispersion relation of the linear waves on the dust size distribution are given. It seems that the larger the difference between the maximum and the minimum radius of the dust grains, the lower the wave frequency for all cases of the dust size distribution. Furthermore, it is noted that the width, the amplitude and the propagation velocity of the KdV solitary wave depend on the dust size distribution, especially it depend on whether the number density of the larger sized dust grain is larger or smaller than that of the smaller sized dust grain. For the power law dust size distribution, the width and the propagation velocity of the KdV solitary wave between the maximum and the minimum radius of the dust grains is larger than that of mono-sized dusty plasma.

Dust coagulation feedback on magnetohydrodynamic resistivities in protostellar collapse

Context. The degree of coupling between the gas and the magnetic field during the collapse of a core and the subsequent formation of a disk depends on the assumed dust size distribution. Aims. We study the impact of grain–grain coagulation on the evolution of magnetohydrodynamic (MHD) resistivities during the collapse of a prestellar core. Methods. We use a 1D model to follow the evolution of the dust size distribution, out-of-equilibrium ionisation state, and gas chemistry during the collapse of a prestellar core. To compute the grain–grain collisional rate, we consider models for both random and systematic, size-dependent, velocities. We include grain growth through grain–grain coagulation and ice accretion, but ignore grain fragmentation. Results. Starting with a Mathis-Rumpl-Nordsieck (MRN) size distribution (Mathis et al. 1977, ApJ, 217, 425), we find that coagulation in grain–grain collisions generated by hydrodynamical turbulence is not efficient at removing the smallest grains and, as a consequence, does not have a large effect on the evolution of the Hall and ambipolar diffusion MHD resistivities, which still drop significantly during the collapse like in models without coagulation. The inclusion of systematic velocities, possibly induced by the presence of ambipolar diffusion, increases the coagulation rate between small and large grains, removing small grains earlier in the collapse and therefore limiting the drop in the Hall and ambipolar diffusion resistivities. At intermediate densities (nH ~ 108 cm−3), the Hall and ambipolar diffusion resistivities are found to be higher by 1 to 2 orders of magnitude in models with coagulation than in models where coagulation is ignored, and also higher than in a toy model without coagulation where all grains smaller than 0.1 μm would have been removed in the parent cloud before the collapse. Conclusions. When grain drift velocities induced by ambipolar diffusion are included, dust coagulation happening during the collapse of a prestellar core starting from an initial MRN dust size distribution appears to be efficient enough to increase the MHD resistivities to the values necessary to strongly modify the magnetically regulated formation of a planet-forming disk. A consistent treatment of the competition between fragmentation and coagulation is, however, necessary before reaching firm conclusions.

Polydisperse streaming instability – I. Tightly coupled particles and the terminal velocity approximation

ABSTRACT We introduce a polydisperse version of the streaming instability (SI), where the dust component is treated as a continuum of sizes. We show that its behaviour is remarkably different from the monodisperse SI. We focus on tightly coupled particles in the terminal velocity approximation and show that unstable modes that grow exponentially on a dynamical time-scale exist. However, for dust to gas ratios much smaller than unity, they are confined to radial wavenumbers that are a factor $\sim 1/{\overline{\rm St}}$ larger than where the monodisperse SI growth rates peak. Here ${\overline{\rm St}}\ll 1$ is a suitable average Stokes number for the dust size distribution. For dust to gas ratios larger than unity, polydisperse modes that grow on a dynamical time-scale are found as well, similar as for the monodisperse SI and at similarly large wavenumbers. At smaller wavenumbers, where the classical monodisperse SI shows secular growth, no growing polydisperse modes are found under the terminal velocity approximation. Outside the region of validity for the terminal velocity approximation, we have found unstable epicyclic modes that grow on ∼104 dynamical time-scales.