34 research outputs found
Production of thermal photons in viscous fluid dynamics with temperature-dependent shear viscosity
We compute the spectrum of thermal photons created in Au+Au collisions at
GeV, taking into account dissipative corrections in
production processes corresponding to the quark--gluon plasma and hadronic
phases. To describe the evolution of the fireball we use a viscous fluid
dynamic model with different parametrizations for the temperature--dependence
of . We find that the spectrum significantly depends on the values of
in the QGP phase, and is almost insensitive to the values in the
hadronic phase. We also compare the influence of the temperature--dependence of
on the spectrum of thermal photons to that of using different
equations of state in the fluid dynamic simulations, finding that both effects
are of the same order of magnitude.Comment: 16 pages, 4 figures. Accepted for publication in Mod. Phys. Lett.
Features of collisionless turbulence in the intracluster medium from simulated Faraday Rotation maps
Observations of the intracluster medium (ICM) in galaxy clusters suggest for
the presence of turbulence and the magnetic fields existence has been proved
through observations of Faraday Rotation and synchrotron emission. The ICM is
also known to be filled by a rarefied weakly collisional plasma. In this work
we study the possible signatures left on Faraday Rotation maps by collisionless
instabilities. For this purpose we use a numerical approach to investigate the
dynamics of the turbulence in collisionless plasmas based on an
magnetohydrodynamical (MHD) formalism taking into account different levels of
pressure anisotropy. We consider models covering the sub/super-Alfv\'enic and
trans/supersonic regimes, one of them representing the fiducial conditions
corresponding to the ICM. From the simulated models we compute Faraday Rotation
maps and analyze several statistical indicators in order to characterize the
magnetic field structure and compare the results obtained with the
collisionless model to those obtained using standard collisional MHD framework.
We find that important imprints of the pressure anisotropy prevails in the
magnetic field and also manifest in the associated Faraday Rotation maps which
evidence smaller correlation lengths in the collisionless MHD case. These
points are remarkably noticeable for the case mimicking the conditions
prevailing in ICM. Nevertheless, in this study we have neglected the decrease
of pressure anisotropy due to the feedback of the instabilities that naturally
arise in collisionless plasmas at small scales. This decrease may not affect
the statistical imprint differences described above, but should be examined
elsewhere.Comment: 24 pages, 15 figures, MNRAS accepte
Magnetothermal instabilities in magnetized anisotropic plasmas
Using the transport equations for an ideal anisotropic collisionless plasma
derived from the Vlasov equation by the 16-moment method, we analyse the
influence of pressure anisotropy exhibited by collisionless magnetized plasmas
on the magnetothermal (MTI) and heat-flux-driven buoyancy (HBI) instabilities.
We calculate the dispersion relation and the growth rates for these
instabilities in the presence of a background heat flux and for configurations
with static pressure anisotropy, finding that when the frequency at which heat
conduction acts is much larger than any other frequency in the system (i.e.
weak magnetic field) the pressure anisotropy has no effect on the MTI/HBI,
provided the degree of anisotropy is small. In contrast, when this ordering of
timescales does not apply the instability criteria depend on pressure
anisotropy. Specifically, the growth time of the instabilities in the
anisotropic case can be almost one order of magnitude smaller than its
isotropic counterpart. We conclude that in plasmas where pressure anisotropy is
present the MTI/HBI are modified. However, in environments with low magnetic
fields and small anisotropy such as the ICM the results obtained from the
16-moment equations under the approximations considered are similar to those
obtained from ideal MHD.Comment: v3: 16 pages, 2 figures, fixed typos, added references and a final
note on related wor
The role of pressure anisotropy in the turbulent intracluster medium
In low-density plasma environments, such as the intracluster medium (ICM),
the Larmour frequency is much larger than the ion-ion collision frequency. In
such a case, the thermal pressure becomes anisotropic with respect to the
magnetic field orientation and the evolution of the turbulent gas is more
correctly described by a kinetic approach. A possible description of these
collisionless scenarios is given by the so-called kinetic magnetohydrodynamic
(KMHD) formalism, in which particles freely stream along the field lines, while
moving with the field lines in the perpendicular direction. In this way a
fluid-like behavior in the perpendicular plane is restored. In this work, we
study fast growing magnetic fluctuations in the smallest scales which operate
in the collisionless plasma that fills the ICM. In particular, we focus on the
impact of a particular evolution of the pressure anisotropy and its
implications for the turbulent dynamics of observables under the conditions
prevailing in the ICM. We present results from numerical simulations and
compare the results which those obtained using an MHD formalism.Comment: 7 pages, 14 figures, Journal of Physics: Conference Serie
Progressive transformation of a flux rope to an ICME
The solar wind conditions at one astronomical unit (AU) can be strongly
disturbed by the interplanetary coronal mass ejections (ICMEs). A subset,
called magnetic clouds (MCs), is formed by twisted flux ropes that transport an
important amount of magnetic flux and helicity which is released in CMEs. At 1
AU from the Sun, the magnetic structure of MCs is generally modeled neglecting
their expansion during the spacecraft crossing. However, in some cases, MCs
present a significant expansion. We present here an analysis of the huge and
significantly expanding MC observed by the Wind spacecraft during 9 and 10
November, 2004. After determining an approximated orientation for the flux rope
using the minimum variance method, we precise the orientation of the cloud axis
relating its front and rear magnetic discontinuities using a direct method.
This method takes into account the conservation of the azimuthal magnetic flux
between the in- and out-bound branches, and is valid for a finite impact
parameter (i.e., not necessarily a small distance between the spacecraft
trajectory and the cloud axis). Moreover, using the direct method, we find that
the ICME is formed by a flux rope (MC) followed by an extended coherent
magnetic region. These observations are interpreted considering the existence
of a previous larger flux rope, which partially reconnected with its
environment in the front. These findings imply that the ejected flux rope is
progressively peeled by reconnection and transformed to the observed ICME (with
a remnant flux rope in the front part).Comment: Solar Physics (in press
Expansion of magnetic clouds in the outer heliosphere
A large amount of magnetized plasma is frequently ejected from the Sun as
coronal mass ejections (CMEs). Some of these ejections are detected in the
solar wind as magnetic clouds (MCs) that have flux rope signatures. Magnetic
clouds are structures that typically expand in the inner heliosphere. We derive
the expansion properties of MCs in the outer heliosphere from one to five
astronomical units to compare them with those in the inner heliosphere. We
analyze MCs observed by the Ulysses spacecraft using insitu magnetic field and
plasma measurements. The MC boundaries are defined in the MC frame after
defining the MC axis with a minimum variance method applied only to the flux
rope structure. As in the inner heliosphere, a large fraction of the velocity
profile within MCs is close to a linear function of time. This is indicative
of} a self-similar expansion and a MC size that locally follows a power-law of
the solar distance with an exponent called zeta. We derive the value of zeta
from the insitu velocity data. We analyze separately the non-perturbed MCs
(cases showing a linear velocity profile almost for the full event), and
perturbed MCs (cases showing a strongly distorted velocity profile). We find
that non-perturbed MCs expand with a similar non-dimensional expansion rate
(zeta=1.05+-0.34), i.e. slightly faster than at the solar distance and in the
inner heliosphere (zeta=0.91+-0.23). The subset of perturbed MCs expands, as in
the inner heliosphere, at a significantly lower rate and with a larger
dispersion (zeta=0.28+-0.52) as expected from the temporal evolution found in
numerical simulations. This local measure of the expansion also agrees with the
distribution with distance of MC size,mean magnetic field, and plasma
parameters. The MCs interacting with a strong field region, e.g. another MC,
have the most variable expansion rate (ranging from compression to
over-expansion)
Dynamical evolution of a magnetic cloud from the Sun to 5.4 AU
Magnetic Clouds (MCs) are a particular subset of Interplanetary Coronal Mass
Ejections (ICMEs), forming large scale magnetic flux ropes. In this work we
analyze the evolution of a particular MC (observed on March 1998) using {\it in
situ} observations made by two spacecraft approximately aligned with the Sun,
the first one at 1 AU from the Sun and the second one at 5.4 AU. We study the
MC expansion, its consequent decrease of magnetic field intensity and mass
density, and the possible evolution of the so-called global ideal-MHD
nvariants. We describe the magnetic configuration of the MC at both spacecraft
using different models and compute relevant global quantities (magnetic fluxes,
helicity and energy) at both helio-distances. We also track back this structure
to the Sun, in order to find out its solar source. We find that the flux rope
is significantly distorted at 5.4 AU. However, we are able to analyze the data
before the flux rope center is over-passed and compare it with observations at
1 AU. From the observed decay of magnetic field and mass density, we quantify
how anisotropic is the expansion, and the consequent deformation of the flux
rope in favor of a cross section with an aspect ratio at 5.4 AU of (larger in the direction perpendicular to the radial direction from the
Sun). We quantify the ideal-MHD invariants and magnetic energy at both
locations, and find that invariants are almost conserved, while the magnetic
energy decays as expected with the expansion rate found. The use of MHD
invariants to link structures at the Sun and the interplanetary medium is
supported by the results of this multispacecraft study. We also conclude that
the local dimensionless expansion rate, that is computed from the velocity
profile observed by a single spacecraft, is very accurate for predicting the
evolution of flux ropes in the solar wind.Comment: 16 two-column pages, 8 figures. Accepted for publication in A&
A Helicity-Based Method to Infer the CME Magnetic Field Magnitude in Sun and Geospace: Generalization and Extension to Sun-Like and M-Dwarf Stars and Implications for Exoplanet Habitability
Patsourakos et al. (Astrophys. J. 817, 14, 2016) and Patsourakos and
Georgoulis (Astron. Astrophys. 595, A121, 2016) introduced a method to infer
the axial magnetic field in flux-rope coronal mass ejections (CMEs) in the
solar corona and farther away in the interplanetary medium. The method, based
on the conservation principle of magnetic helicity, uses the relative magnetic
helicity of the solar source region as input estimates, along with the radius
and length of the corresponding CME flux rope. The method was initially applied
to cylindrical force-free flux ropes, with encouraging results. We hereby
extend our framework along two distinct lines. First, we generalize our
formalism to several possible flux-rope configurations (linear and nonlinear
force-free, non-force-free, spheromak, and torus) to investigate the dependence
of the resulting CME axial magnetic field on input parameters and the employed
flux-rope configuration. Second, we generalize our framework to both Sun-like
and active M-dwarf stars hosting superflares. In a qualitative sense, we find
that Earth may not experience severe atmosphere-eroding magnetospheric
compression even for eruptive solar superflares with energies ~ 10^4 times
higher than those of the largest Geostationary Operational Environmental
Satellite (GOES) X-class flares currently observed. In addition, the two
recently discovered exoplanets with the highest Earth-similarity index, Kepler
438b and Proxima b, seem to lie in the prohibitive zone of atmospheric erosion
due to interplanetary CMEs (ICMEs), except when they possess planetary magnetic
fields that are much higher than that of Earth.Comment: http://adsabs.harvard.edu/abs/2017SoPh..292...89