2,044,065 research outputs found
Mechanism of spontaneous formation of stable magnetic structures on the Sun
One of the puzzling features of solar magnetism is formation of long-living
compact magnetic structures; such as sunspots and pores, in the highly
turbulent upper layer of the solar convective zone. We use realistic radiative
3D MHD simulations to investigate the interaction between magnetic field and
turbulent convection. In the simulations, a weak vertical uniform magnetic
field is imposed in a region of fully developed granular convection; and the
total magnetic flux through the top and bottom boundaries is kept constant. The
simulation results reveal a process of spontaneous formation of stable magnetic
structures, which may be a key to understanding of the magnetic
self-organization on the Sun and formation of pores and sunspots. This process
consists of two basic steps: 1) formation of small-scale filamentary magnetic
structures associated with concentrations of vorticity and whirlpool-type
motions, and 2) merging of these structures due to the vortex attraction,
caused by converging downdrafts around magnetic concentration below the
surface. In the resulting large-scale structure maintained by the converging
plasma motions, the magnetic field strength reaches ~1.5 kG at the surface and
~6 kG in the interior; and the surface structure resembles solar pores. The
magnetic structure remains stable for the whole simulation run of several hours
with no sign of decay.Comment: 13 pages, 4 figures, submitted to the Astrophysical Journa
Structure formation in electromagnetically driven granular media
We report structure formation in submonolayers of magnetic microparticles
subjected to periodic electrostatic and magnetic excitations. Depending on the
excitation parameters, we observe the formation of a rich variety of
structures: clusters, rings, chains, and networks. The growth dynamics and
shapes of the structures are strongly dependent on the amplitude and frequency
of the external magnetic field. We find that for pure ac magnetic driving at
low densities of particles, the low-frequency magnetic excitation favors
clusters while high frequency excitation favors chains and net-like structures.
An abrupt phase transition from chains to a network phase was observed for a
high density of particles.Comment: 4 pages, 5 figure
Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystalline polymers
Structural characteristics in membranes formed by diffusion induced phase separation processes are discussed. Established theories on membrane formation from ternary systems can be extended to describe the effects of high or low molecular weight additives. A mechanism for the formation of nodular structures in the top layer of ultrafiltration membranes is presented. In the last part structures arising from polymer crystallization during immersion precipitation are discussed
The Effect of Cosmological Background Dynamics on the Spherical Collapse in MOND
The effect of background dynamics of the universe on formation of large scale
structures in the framework of Modified Newtonian Dynamics (MOND) is
investigated. A spherical collapse model is used for modeling the formation of
the structures. This study is done in two extreme cases: ({\it i}) assuming a
universe with a low-density baryonic matter without any cold dark matter and
dark energy; ({\it ii}) a dark energy dominated universe with baryonic matter,
without cold dark matter. We show that for the case ({\it ii}) the structures
virialize at lower redshifts with larger radii compared to the low-density
background universe. The dark energy slow downs the collapse of the structures.
We show that our results are compatible with recent simulations of the
structure formation in MOND.Comment: 16 pages, 4 Figures, accepted by New Astronom
Triggered Star Formation in the Environment of Young Massive Stars
Recent observations with the Spitzer Space Telescope show clear evidence that
star formation takes place in the surrounding of young massive O-type stars,
which are shaping their environment due to their powerful radiation and stellar
winds. In this work we investigate the effect of ionising radiation of massive
stars on the ambient interstellar medium (ISM): In particular we want to
examine whether the UV-radiation of O-type stars can lead to the observed
pillar-like structures and can trigger star formation. We developed a new
implementation, based on a parallel Smooth Particle Hydrodynamics code (called
IVINE), that allows an efficient treatment of the effect of ionising radiation
from massive stars on their turbulent gaseous environment. Here we present
first results at very high resolution. We show that ionising radiation can
trigger the collapse of an otherwise stable molecular cloud. The arising
structures resemble observed structures (e.g. the pillars of creation in the
Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of
gravitation we find small regions that can be identified as formation places of
individual stars. We conclude that ionising radiation from massive stars alone
can trigger substantial star formation in molecular clouds.Comment: 4 pages, 2 figures. To appear in: "Triggered Star Formation in a
Turbulent ISM", IAU Symposium 237, Prague, Czech Republic, August 2006; eds.
B.G.Elmegreen & J. Palou
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