6,368 research outputs found
Standing sausage modes in coronal loops with plasma flow
Magnetohydrodynamic waves are important for diagnosing the physical
parameters of coronal plasmas. Field-aligned flows appear frequently in coronal
loops.We examine the effects of transverse density and plasma flow structuring
on standing sausage modes trapped in coronal loops, and examine their
observational implications. We model coronal loops as straight cold cylinders
with plasma flow embedded in a static corona. An eigen-value problem governing
propagating sausage waves is formulated, its solutions used to construct
standing modes. Two transverse profiles are distinguished, one being the
generalized Epstein distribution (profile E) and the other (N) proposed
recently in Nakariakov et al.(2012). A parameter study is performed on the
dependence of the maximum period and cutoff length-to-radius
ratio in the trapped regime on the density parameters
( and profile steepness ) and flow parameters (magnitude
and profile steepness ). For either profile, introducing a flow
reduces relative to the static case. depends
sensitively on for profile N but is insensitive to for profile E. By
far the most important effect a flow introduces is to reduce the capability for
loops to trap standing sausage modes: may be
substantially reduced in the case with flow relative to the static one. If the
density distribution can be described by profile N, then measuring the sausage
mode period can help deduce the density profile steepness. However, this
practice is not feasible if profile E better describes the density
distribution. Furthermore, even field-aligned flows with magnitudes
substantially smaller than the ambient Alfv\'en speed can make coronal loops
considerably less likely to support trapped standing sausage modes.Comment: 11 pages, 9 figures, to appear in Astronomy & Astrophysic
Spatial damping of propagating sausage waves in coronal cylinders
Sausage modes are important in coronal seismology. Spatially damped
propagating sausage waves were recently observed in the solar atmosphere. We
examine how wave leakage influences the spatial damping of sausage waves
propagating along coronal structures modeled by a cylindrical density
enhancement embedded in a uniform magnetic field. Working in the framework of
cold magnetohydrodynamics, we solve the dispersion relation (DR) governing
sausage waves for complex-valued longitudinal wavenumber at given real
angular frequencies . For validation purposes, we also provide
analytical approximations to the DR in the low-frequency limit and in the
vicinity of , the critical angular frequency separating trapped
from leaky waves. In contrast to the standing case, propagating sausage waves
are allowed for much lower than . However, while able
to direct their energy upwards, these low-frequency waves are subject to
substantial spatial attenuation. The spatial damping length shows little
dependence on the density contrast between the cylinder and its surroundings,
and depends only weakly on frequency. This spatial damping length is of the
order of the cylinder radius for , where
and are the cylinder radius and the Alfv\'en speed in the
cylinder, respectively. We conclude that if a coronal cylinder is perturbed by
symmetric boundary drivers (e.g., granular motions) with a broadband spectrum,
wave leakage efficiently filters out the low-frequency components.Comment: 6 pages, 2 figures, to appear in Astronomy & Astrophysic
Some aspects of global Lambda polarization in heavy-ion collisions
Large orbital angular momentum can be generated in non-central heavy-ion
collisions, and part of it is expected to be converted into final particle's
polarization due to the spin-orbit coupling. Within the framework of A
Multi-Phase Transport (AMPT) model, we studied the vorticity-induced
polarization of hyperons at the midrapidity region in
Au-Au collisions at energies GeV. Our results show
that the global polarization decreases with the collisional energies and is
consistent with the recent STAR measurements. This behavior can be understood
by less asymmetry of participant matter in the midrapidity region due to faster
expansion of fireball at higher energies. As another evidence, we discuss how
much the angular momentum is deposited in different rapidity region. The result
supports our asymmetry argument.Comment: 6 pages, 4 figures, CPOD 2017 proceedin
Transition Form Factors in the PQCD approach
Under two different scenarios for the light scalar mesons, we investigate the
transition form factors of mesons decay into a scalar meson in the
perturbative QCD approach. In the large recoiling region, the form factors are
dominated by the short-distance dynamics and can be calculated using
perturbation theory. We adopt the dipole parametrization to recast the
dependence of the form factors. Since the decay constants defined by the scalar
current are large, our predictions on the form factors are much larger
than the transitions, especially in the second scenario. Contributions
from various light-cone distribution amplitudes (LCDAs) are elaborated and we
find that the twist-3 LCDAs provide more than a half contributions to the form
factors. The two terms of the twist-2 LCDAs give destructive contributions in
the first scenario while they give constructive contributions in the second
scenario. With the form factors, we also predict the decay width and branching
ratios of the semileptonic and decays. The
branching ratios of channels are found to have the order of
while those of have the order of . These
predictions can be tested by the future experiments.Comment: 20 pages, 31 figure
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