123 research outputs found
Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations
We study lithium depletion in low-mass and solar-like stars as a function of
time, using a new diffusion coefficient describing extra-mixing taking place at
the bottom of a convective envelope. This new form is motivated by
multi-dimensional fully compressible, time implicit hydrodynamic simulations
performed with the MUSIC code. Intermittent convective mixing at the convective
boundary in a star can be modeled using extreme value theory, a statistical
analysis frequently used for finance, meteorology, and environmental science.
In this letter, we implement this statistical diffusion coefficient in a
one-dimensional stellar evolution code, using parameters calibrated from
multi-dimensional hydrodynamic simulations of a young low-mass star. We propose
a new scenario that can explain observations of the surface abundance of
lithium in the Sun and in clusters covering a wide range of ages, from
50 Myr to 4 Gyr. Because it relies on our physical model of convective
penetration, this scenario has a limited number of assumptions. It can explain
the observed trend between rotation and depletion, based on a single additional
assumption, namely that rotation affects the mixing efficiency at the
convective boundary. We suggest the existence of a threshold in stellar
rotation rate above which rotation strongly prevents the vertical penetration
of plumes and below which rotation has small effects. In addition to providing
a possible explanation for the long standing problem of lithium depletion in
pre-main sequence and main sequence stars, the strength of our scenario is that
its basic assumptions can be tested by future hydrodynamic simulations.Comment: 7 pages, 3 figures, Accepted for publication in ApJ Letter
Assessment of parameters describing representativeness of air quality in-situ measurement sites
The atmospheric layer closest to the ground is strongly influenced by variable surface fluxes (emissions, surface deposition) and can therefore be very heterogeneous. In order to perform air quality measurements that are representative of a larger domain or a certain degree of pollution, observatories are placed away from population centres or within areas of specific population density. Sites are often categorised based on subjective criteria that are not uniformly applied by the atmospheric community within different administrative domains yielding an inconsistent global air quality picture. A novel approach for the assessment of parameters reflecting site representativeness is presented here, taking emissions, deposition and transport towards 34 sites covering Western and Central Europe into account. These parameters are directly inter-comparable among the sites and can be used to select sites that are, on average, more or less suitable for data assimilation and comparison with satellite and model data. Advection towards these sites was simulated by backward Lagrangian Particle Dispersion Modelling (LPDM) to determine the sites' average catchment areas for the year 2005 and advection times of 12, 24 and 48 h. Only variations caused by emissions and transport during these periods were considered assuming that these dominate the short-term variability of most but especially short lived trace gases. The derived parameters describing representativeness were compared between sites and a novel, uniform and observation-independent categorisation of the sites based on a clustering approach was established. Six groups of European background sites were identified ranging from <i>generally remote</i> to more polluted <i>agglomeration</i> sites. These six categories explained 50 to 80% of the inter-site variability of median mixing ratios and their standard deviation for NO<sub>2</sub> and O<sub>3</sub>, while differences between group means of the longer-lived trace gas CO were insignificant. The derived annual catchment areas strongly depended on the applied LPDM and input wind fields, the catchment settings and the year of analysis. Nevertheless, the parameters describing representativeness showed considerably less variability than the catchment geometry, supporting the applicability of the derived station categorisation
An ensemble-based approach to climate reconstructions
Data assimilation is a promising approach to obtain climate reconstructions that are both consistent with observations of the past and with our understanding of the physics of the climate system as represented in the climate model used. Here, we investigate the use of ensemble square root filtering (EnSRF) – a technique used in weather forecasting – for climate reconstructions. We constrain an ensemble of 29 simulations from an atmosphere-only general circulation model (GCM) with 37 pseudo-proxy temperature time series. Assimilating spatially sparse information with low temporal resolution (semi-annual) improves the representation of not only temperature, but also other surface properties, such as precipitation and even upper air features such as the intensity of the northern stratospheric polar vortex or the strength of the northern subtropical jet. Given the sparsity of the assimilated information and the limited size of the ensemble used, a localisation procedure is crucial to reduce "overcorrection" of climate variables far away from the assimilated information
One-dimensional thermal pressure-driven expansion of a pair cloud into an electron-proton plasma
Recently a filamentation instability was observed when a laser-generated pair
cloud interacted with an ambient plasma. The magnetic field it drove was strong
enough to magnetize and accelerate the ambient electrons. It is of interest to
determine if and how pair cloud-driven instabilities can accelerate ions in the
laboratory or in astrophysical plasma. For this purpose, the expansion of a
localized pair cloud with the temperature 400 keV into a cooler ambient
electron-proton plasma is studied by means of one-dimensional particle-in-cell
(PIC) simulations. The cloud's expansion triggers the formation of electron
phase space holes that accelerate some protons to MeV energies. Forthcoming
lasers might provide the energy needed to create a cloud that can accelerate
protons.Comment: 5 pages 4 figures, accepted for publication in Physics of Plasma
Three-dimensional structure and stability of discontinuities between unmagnetized pair plasma and magnetized electron-proton plasma
We study with a 3D PIC simulation discontinuities between an
electron-positron pair plasma and magnetized electrons and protons. A pair
plasma is injected at one simulation boundary with a speed 0.6 along its
normal. It expands into an electron-proton plasma and a magnetic field that
points orthogonally to the injection direction. Diamagnetic currents expel the
magnetic field from within the pair plasma and pile it up in front of it. It
pushes electrons, which induces an electric field pulse ahead of the magnetic
one. This initial electromagnetic pulse (EMP) confines the pair plasma
magnetically and accelerates protons electrically. The fast flow of the
injected pair plasma across the protons behind the initial EMP triggers the
filamentation instability. Some electrons and positrons cross the injection
boundary and build up a second EMP. Electron-cyclotron drift instabilities
perturb the plasma ahead of both EMPs seeding a Rayleigh-Taylor-type
instability. Despite equally strong perturbations ahead of both EMPs, the
second EMP is much more stable than the initial one. We attribute the rapid
collapse of the initial EMP to the filamentation instability, which perturbed
the plasma behind it. The Rayleigh-Taylor-type instability transforms the
planar EMPs into transition layers, in which magnetic flux ropes and
electrostatic forces due to uneven numbers of electrons and positrons slow down
and compress the pair plasma and accelerate protons. In our simulation, the
expansion speed of the pair cloud decreased by about an order of magnitude and
its density increased by the same factor. Its small thickness implies that it
is capable of separating a relativistic pair outflow from an electron-proton
plasma, which is essential for collimating relativistic jets of pair plasma in
collisionless astrophysical plasma.Comment: 25 pages, 12 figures, provisionally accepted for publication by the
New Journal of Physic
Comparison of two- and three-dimensional compressible convection in a pre-main sequence star
Extending our recent studies of two-dimensional stellar convection to 3D, we
compare three-dimensional hydrodynamic simulations to identically set-up
two-dimensional simulations, for a realistic pre-main sequence star. We compare
statistical quantities related to convective flows including: average velocity,
vorticity, local enstrophy, and penetration depth beneath a convection zone.
These statistics are produced during stationary, steady-state compressible
convection in the star's convection zone. Our simulations with the MUSIC code
confirm the common result that two-dimensional simulations of stellar
convection have a higher magnitude of velocity on average than
three-dimensional simulations. Boundary conditions and the extent of the
spherical shell can affect the magnitude and variability of convective
velocities. The difference between 2D and 3D velocities is dependent on these
background points; in our simulations this can have an effect as large as the
difference resulting from the dimensionality of the simulation. Nevertheless,
radial velocities near the convective boundary are comparable in our 2D and 3D
simulations. The average local enstrophy of the flow is lower for
two-dimensional simulations than for three-dimensional simulations, indicating
a different shape and structuring of 3D stellar convection. We perform a
statistical analysis of the depth of convective penetration below the
convection zone, using the model proposed in our recent study (Pratt et al.
2017). Here we analyze the convective penetration in three dimensional
simulations, and compare the results to identically set-up 2D simulations. In
3D the penetration depth is as large as the penetration depth calculated from
2D simulations.Comment: 13 pages, 8 figure
PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front
We study with particle-in-cell (PIC) simulations the stability of fast
magnetosonic shocks. They expand across a collisionless plasma and an
orthogonal magnetic field that is aligned with one of the directions resolved
by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed
when it enters a layer with a reduced density of mobile ions, which decreases
the shock speed by up to 15\% in 1D simulations. In the 2D simulations, the
density of mobile ions in the layer varies sinusoidally perpendicularly to the
shock normal. We resolve one sine period. This variation only leads to small
changes in the shock speed evidencing a restoring force that opposes a shock
deformation. As the shock propagates through the layer, the ion density becomes
increasingly spatially modulated along the shock front and the magnetic field
bulges out where the mobile ion density is lowest. The perturbed shock
eventually reaches a steady state. Once it leaves the layer, the perturbations
of the ion density and magnetic field oscillate along its front at a frequency
close to the lower-hybrid frequency; the shock is mediated by a standing wave
composed of obliquely propagating lower-hybrid waves. We perform three 2D
simulations with different box lengths along the shock front. The shock front
oscillations are aperiodically damped in the smallest box with the fastest
variation of the ion density, strongly damped in the intermediate one, and
weakly damped in the largest box. The shock front oscillations perturb the
magnetic field in a spatial interval that extends by several electron skin
depths upstream and downstream of the shock front and could give rise to
Whistler waves that propagate along the shock's magnetic field overshoot.
Similar waves were observed in hybrid and PIC simulations and by the MMS
satellite mission.Comment: 25 pages, 12 figures, accepted for publication in Physica Script
Extreme value statistics for two-dimensional convective penetration in a pre-Main Sequence star
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.We examine a penetration layer formed between a central radiative zone and a large convection zone in the deep interior of a young low-mass star. Using the Multidimensional Stellar Implicit Code (MUSIC) to simulate two-dimensional compressible stellar convection in a spherical geometry over long times, we produce statistics that characterize the extent and impact of convective penetration in this layer. We apply extreme value theory to the maximal extent of convective penetration at any time. We compare statistical results from simulations which treat non-local convection, throughout a large portion of the stellar radius, with simulations designed to treat local convection in a small region surrounding the penetration layer. For each of these situations, we compare simulations of different resolution, which have different velocity magnitudes. We also compare statistical results between simulations that radiate energy at a constant rate to those that allow energy to radiate from the stellar surface according to the local surface temperature. Based on the frequency and depth of penetrating convective structures, we observe two distinct layers that form between the convection zone and the stable radiative zone. We show that the probability density function of the maximal depth of convective penetration at any time corresponds closely in space with the radial position where internal waves are excited. We find that the maximal penetration depth can be modeled by a Weibull distribution with a small shape parameter. Using these results, and building on established scalings for diffusion enhanced by large-scale convective motions, we propose a new form for the diffusion coefficient that may be used for one-dimensional stellar evolution calculations in the large P\'eclet number regime. These results should contribute to the 321D link.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework (FP7/2007-2013)/ERC grant agreement no. 32047
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