251 research outputs found
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
Bloom-Gilman duality of inelastic structure functions in nucleon and nuclei
The Bloom-Gilman local duality of the inelastic structure function of the
proton, the deuteron and light complex nuclei is investigated using available
experimental data in the squared four-momentum transfer range from 0.3 to 5
(GeV/c)**2. The results of our analysis suggest that the onset of the
Bloom-Gilman local duality is anticipated in complex nuclei with respect to the
case of the protonand the deuteron. A possible interpretation of this result in
terms of a rescaling effect is discussed with particular emphasis to the
possibility of reproducing the damping of the nucleon-resonance transitions
observed in recent electroproduction data off nuclei.Comment: revised version, to appear in Physical Review
A model for the dependence of polarized structure functions
We present an update of a phenomenological model for the spin dependent
structure functions of the proton and neutron. This model is based
on a broken SU(6) wavefunction parametrized by the unpolarized structure
functions. The two free parameters of the model are choosen to fulfill the
Bjorken and Ellis--Jaffe sum rules. The model respects isospin symmetry and has
zero strange sea polarization. Using new values for from hyperon beta
decay the resulting dependent asymmetries are in perfect agreement
with the existing data. Therefore we do not see any evidence for a ``spin
crisis''. With two choices for the dependence of and
is predicted and shown to be small for both cases.Comment: 18 pages and 11 figures as uudecoded ps file
Spontaneous formation of flux concentrations in a stratified layer
The negative effective magnetic pressure instability discovered recently in
direct numerical simulations (DNS) may play a crucial role in the formation of
sunspots and active regions in the Sun and stars. This instability is caused by
a negative contribution of turbulence to the effective mean Lorentz force (the
sum of turbulent and non-turbulent contributions) and results in formation of
large-scale inhomogeneous magnetic structures from initial uniform magnetic
field. Earlier investigations of this instability in DNS of stably stratified,
externally forced, isothermal hydromagnetic turbulence in the regime of large
plasma beta are now extended into the regime of larger scale separation ratios
where the number of turbulent eddies in the computational domain is about 30.
Strong spontaneous formation of large-scale magnetic structures is seen even
without performing any spatial averaging. These structures encompass many
turbulent eddies. The characteristic time of the instability is comparable to
the turbulent diffusion time, L^2/eta_t, where eta_t is the turbulent
diffusivity and L is the scale of the domain. DNS are used to confirm that the
effective magnetic pressure does indeed become negative for magnetic field
strengths below the equipartition field. The dependence of the effective
magnetic pressure on the field strength is characterized by fit parameters that
seem to show convergence for larger values of the magnetic Reynolds number.Comment: 14 pages, 8 figures, submitted to special issue "Advances of European
Solar Physics" in Solar Physic
Filtro anaeróbio associado à membrana de microfiltração tratando lixiviado de aterro sanitário
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Radiative Cooling in MHD Models of the Quiet Sun Convection Zone and Corona
We present a series of numerical simulations of the quiet Sun plasma threaded
by magnetic fields that extend from the upper convection zone into the low
corona. We discuss an efficient, simplified approximation to the physics of
optically thick radiative transport through the surface layers, and investigate
the effects of convective turbulence on the magnetic structure of the Sun's
atmosphere in an initially unipolar (open field) region. We find that the net
Poynting flux below the surface is on average directed toward the interior,
while in the photosphere and chromosphere the net flow of electromagnetic
energy is outward into the solar corona. Overturning convective motions between
these layers driven by rapid radiative cooling appears to be the source of
energy for the oppositely directed fluxes of electromagnetic energy.Comment: 20 pages, 5 figures, Solar Physics, in pres
SPIN-DEPENDENT NUCLEAR STRUCTURE FUNCTIONS: GENERAL APPROACH WITH APPLICATION TO THE DEUTERON
We study deep-inelastic scattering from polarized nuclei within a covariant
framework. A clear connection is established between relativistic and
non-relativistic limits, which enables a rigorous derivation of convolution
formulae for the spin-dependent nuclear structure functions g_1^A and g_2^A in
terms of off-mass-shell extrapolations of polarized nucleon structure
functions, g_1^N and g_2^N. Approximate expressions for g_{1,2}^A are obtained
by expanding the off-shell g_{1,2}^N about their on-shell limits. As an
application of the formalism we consider nuclear effects in the deuteron,
knowledge of which is necessary to obtain accurate information on the
spin-dependent structure functions of the neutron.Comment: 26 pages RevTeX, 9 figures available upon reques
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