346 research outputs found
How to optimize nonlinear force-free coronal magnetic field extrapolations from SDO/HMI vector magnetograms?
The SDO/HMI instruments provide photospheric vector magnetograms with a high
spatial and temporal resolution. Our intention is to model the coronal magnetic
field above active regions with the help of a nonlinear force-free
extrapolation code. Our code is based on an optimization principle and has been
tested extensively with semi-analytic and numeric equilibria and been applied
before to vector magnetograms from Hinode and ground based observations.
Recently we implemented a new version which takes measurement errors in
photospheric vector magnetograms into account. Photospheric field measurements
are often due to measurement errors and finite nonmagnetic forces inconsistent
as a boundary for a force-free field in the corona. In order to deal with these
uncertainties, we developed two improvements: 1.) Preprocessing of the surface
measurements in order to make them compatible with a force-free field 2.) The
new code keeps a balance between the force-free constraint and deviation from
the photospheric field measurements. Both methods contain free parameters,
which have to be optimized for use with data from SDO/HMI. Within this work we
describe the corresponding analysis method and evaluate the force-free
equilibria by means of how well force-freeness and solenoidal conditions are
fulfilled, the angle between magnetic field and electric current and by
comparing projections of magnetic field lines with coronal images from SDO/AIA.
We also compute the available free magnetic energy and discuss the potential
influence of control parameters.Comment: 17 Pages, 6 Figures, Sol. Phys., accepte
Thin accretion disc with a corona in a central magnetic field
We study the steady-state structure of an accretion disc with a corona
surrounding a central, rotating, magnetized star. We assume that the
magneto-rotational instability is the dominant mechanism of angular momentum
transport inside the disc and is responsible for producing magnetic tubes above
the disc. In our model, a fraction of the dissipated energy inside the disc is
transported to the corona via these magnetic tubes. This energy exchange from
the disc to the corona which depends on the disc physical properties is
modified because of the magnetic interaction between the stellar magnetic field
and the accretion disc. According to our fully analytical solutions for such a
system, the existence of a corona not only increases the surface density but
reduces the temperature of the accretion disc. Also, the presence of a corona
enhances the ratio of gas pressure to the total pressure. Our solutions show
that when the strength of the magnetic field of the central neutron star is
large or the star is rotating fast enough, profiles of the physical variables
of the disc significantly modify due to the existence of a corona.Comment: Accepted for publication in Astrophysics & Space Scienc
Comment on "Resolving the 180-deg Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions"
In a recent paper, Leka at al. (Solar Phys. 260, 83, 2009)constructed a
synthetic vector magnetogram representing a three-dimensional magnetic
structure defined only within a fraction of an arcsec in height. They rebinned
the magnetogram to simulate conditions of limited spatial resolution and then
compared the results of various azimuth disambiguation methods on the resampled
data. Methods relying on the physical calculation of potential and/or
non-potential magnetic fields failed in nearly the same, extended parts of the
field of view and Leka et al. (2009) attributed these failures to the limited
spatial resolution. This study shows that the failure of these methods is not
due to the limited spatial resolution but due to the narrowly defined test
data. Such narrow magnetic structures are not realistic in the real Sun.
Physics-based disambiguation methods, adapted for solar magnetic fields
extending to infinity, are not designed to handle such data; hence, they could
only fail this test. I demonstrate how an appropriate limited-resolution
disambiguation test can be performed by constructing a synthetic vector
magnetogram very similar to that of Leka et al. (2009) but representing a
structure defined in the semi-infinite space above the solar photosphere. For
this magnetogram I find that even a simple potential-field disambiguation
method manages to resolve the ambiguity very successfully, regardless of
limited spatial resolution. Therefore, despite the conclusions of Leka et al.
(2009), a proper limited-spatial-resolution test of azimuth disambiguation
methods is yet to be performed in order to identify the best ideas and
algorithms.Comment: Solar Physics, in press (19 pp., 5 figures, 2 tables
Nonlinear force-free reconstruction of the global solar magnetic field: methodology
We present a novel numerical method that allows the calculation of nonlinear
force-free magnetostatic solutions above a boundary surface on which only the
distribution of the normal magnetic field component is given. The method relies
on the theory of force-free electrodynamics and applies directly to the
reconstruction of the solar coronal magnetic field for a given distribution of
the photospheric radial field component. The method works as follows: we start
with any initial magnetostatic global field configuration (e.g. zero, dipole),
and along the boundary surface we create an evolving distribution of tangential
(horizontal) electric fields that, via Faraday's equation, give rise to a
respective normal field distribution approaching asymptotically the target
distribution. At the same time, these electric fields are used as boundary
condition to numerically evolve the resulting electromagnetic field above the
boundary surface, modelled as a thin ideal plasma with non-reflecting,
perfectly absorbing outer boundaries. The simulation relaxes to a nonlinear
force-free configuration that satisfies the given normal field distribution on
the boundary. This is different from existing methods relying on a fixed
boundary condition - the boundary evolves toward the a priori given one, at the
same time evolving the three-dimensional field solution above it. Moreover,
this is the first time a nonlinear force-free solution is reached by using only
the normal field component on the boundary. This solution is not unique, but
depends on the initial magnetic field configuration and on the evolutionary
course along the boundary surface. To our knowledge, this is the first time
that the formalism of force-free electrodynamics, used very successfully in
other astrophysical contexts, is applied to the global solar magnetic field.Comment: 18 pages, 5 figures, Solar Physic
Flat galaxies with dark matter halos - existence and stability
We consider a model for a flat, disk-like galaxy surrounded by a halo of dark
matter, namely a Vlasov-Poisson type system with two particle species, the
stars which are restricted to the galactic plane and the dark matter particles.
These constituents interact only through the gravitational potential which
stars and dark matter create collectively. Using a variational approach we
prove the existence of steady state solutions and their nonlinear stability
under suitably restricted perturbations.Comment: 39 page
Can We Improve the Preprocessing of Photospheric Vector Magnetograms by the Inclusion of Chromospheric Observations?
The solar magnetic field is key to understanding the physical processes in
the solar atmosphere. Nonlinear force-free codes have been shown to be useful
in extrapolating the coronal field upward from underlying vector boundary data.
However, we can only measure the magnetic field vector routinely with high
accuracy in the photosphere, and unfortunately these data do not fulfill the
force-free condition. We must therefore apply some transformations to these
data before nonlinear force-free extrapolation codes can be self-consistently
applied. To this end, we have developed a minimization procedure that yields a
more chromosphere-like field, using the measured photospheric field vectors as
input. The procedure includes force-free consistency integrals, spatial
smoothing, and -- newly included in the version presented here -- an improved
match to the field direction as inferred from fibrils as can be observed in,
e.g., chromospheric H images. We test the procedure using a model
active-region field that included buoyancy forces at the photospheric level.
The proposed preprocessing method allows us to approximate the chromospheric
vector field to within a few degrees and the free energy in the coronal field
to within one percent.Comment: 22 pages, 6 Figur
Thermodynamics and collapse of self-gravitating Brownian particles in D dimensions
We address the thermodynamics (equilibrium density profiles, phase diagram,
instability analysis...) and the collapse of a self-gravitating gas of Brownian
particles in D dimensions, in both canonical and microcanonical ensembles. In
the canonical ensemble, we derive the analytic form of the density scaling
profile which decays as f(x)=x^{-\alpha}, with alpha=2. In the microcanonical
ensemble, we show that f decays as f(x)=x^{-\alpha_{max}}, where \alpha_{max}
is a non-trivial exponent. We derive exact expansions for alpha_{max} and f in
the limit of large D. Finally, we solve the problem in D=2, which displays
rather rich and peculiar features
Locating current sheets in the solar corona
Current sheets are essential for energy dissipation in the solar corona, in
particular by enabling magnetic reconnection. Unfortunately, sufficiently thin
current sheets cannot be resolved observationally and the theory of their
formation is an unresolved issue as well. We consider two predictors of coronal
current concentrations, both based on geometrical or even topological
properties of a force free coronal magnetic field. First, there are
separatrices related to magnetic nulls. Through separatrices the magnetic
connectivity changes discontinuously. Coronal magnetic nulls are, however, very
rare. At second, inspired by the concept of generalized magnetic reconnection
without nulls, quasi-separatrix layers (QSL) were suggested. Through QSL the
magnetic connectivity changes continuously, though strongly. The strength of
the connectivity change can be quantified by measuring the squashing of the
flux tubes which connect the magnetically conjugated photospheres.
We verify the QSL and separatrix concepts by comparing the sites of magnetic
nulls and enhanced squashing with the location of current concentrations in the
corona. Due to the known difficulties of their direct observation we simulated
the coronal current sheets by numerically calculating the response of the
corona to energy input from the photosphere heating a simultaneously observed
EUV Bright Point. We did not find coronal current sheets not at the
separatrices but at several QSL locations. The reason is that although the
geometrical properties of force free extrapolated magnetic fields can indeed,
hint at possible current concentrations, a necessary condition for current
sheet formation is the local energy input into the corona
Self-gravitating Brownian particles in two dimensions: the case of N=2 particles
We study the motion of N=2 overdamped Brownian particles in gravitational
interaction in a space of dimension d=2. This is equivalent to the simplified
motion of two biological entities interacting via chemotaxis when time delay
and degradation of the chemical are ignored. This problem also bears some
similarities with the stochastic motion of two point vortices in viscous
hydrodynamics [Agullo & Verga, Phys. Rev. E, 63, 056304 (2001)]. We
analytically obtain the density probability of finding the particles at a
distance r from each other at time t. We also determine the probability that
the particles have coalesced and formed a Dirac peak at time t (i.e. the
probability that the reduced particle has reached r=0 at time t). Finally, we
investigate the variance of the distribution and discuss the proper form
of the virial theorem for this system. The reduced particle has a normal
diffusion behaviour for small times with a gravity-modified diffusion
coefficient =r_0^2+(4k_B/\xi\mu)(T-T_*)t, where k_BT_{*}=Gm_1m_2/2 is a
critical temperature, and an anomalous diffusion for large times
~t^(1-T_*/T). As a by-product, our solution also describes the growth of
the Dirac peak (condensate) that forms in the post-collapse regime of the
Smoluchowski-Poisson system (or Keller-Segel model) for T<T_c=GMm/(4k_B). We
find that the saturation of the mass of the condensate to the total mass is
algebraic in an infinite domain and exponential in a bounded domain.Comment: Revised version (20/5/2010) accepted for publication in EPJ
Thermodynamic Description of the Relaxation of Two-Dimensional Euler Turbulence Using Tsallis Statistics
Euler turbulence has been experimentally observed to relax to a
metaequilibrium state that does not maximize the Boltzmann entropy, but rather
seems to minimize enstrophy. We show that a recent generalization of
thermodynamics and statistics due to Tsallis is capable of explaining this
phenomenon in a natural way. The maximization of the generalized entropy
for this system leads to precisely the same profiles predicted by the
Restricted Minimum Enstrophy theory of Huang and Driscoll. This makes possible
the construction of a comprehensive thermodynamic description of Euler
turbulence.Comment: 15 pages, RevTe
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