78 research outputs found
Explanation of the sea-serpent magnetic structure of sunspot penumbrae
Recent spectro-polarimetric observations of a sunspot showed the formation of
bipolar magnetic patches in the mid penumbra and their propagation toward the
outer penumbral boundary. The observations were interpreted as being caused by
sea-serpent magnetic fields near the solar surface (Sainz Dalda & Bellot Rubio
2008). In this Letter, we develop a 3D radiative MHD numerical model to explain
the sea-serpent structure and the wave-like behavior of the penumbral magnetic
field lines. The simulations reproduce the observed behavior, suggesting that
the sea-serpent phenomenon is a consequence of magnetoconvection in a strongly
inclined magnetic field. It involves several physical processes: filamentary
structurization, high-speed overturning convective motions in strong, almost
horizontal magnetic fields with partially frozen field lines, and traveling
convective waves. The results demonstrate a correlation of the bipolar magnetic
patches with high-speed Evershed downflows in the penumbra. This is the first
time that a 3D numerical model of the penumbra results in downward directed
magnetic fields, an essential ingredient of sunspot penumbrae that has eluded
explanation until now.Comment: 9 pages, 3 figures, submitted to ApJ Letter
Global Twist of Sunspot Magnetic Fields Obtained from High Resolution Vector Magnetograms
The presence of fine structures in the sunspot vector magnetic fields has
been confirmed from Hinode as well as other earlier observations. We studied 43
sunspots based on the data sets taken from ASP/DLSP, Hinode (SOT/SP) and SVM
(USO). In this \emph{Letter}, (i) We introduce the concept of signed shear
angle (SSA) for sunspots and establish its importance for non force-free
fields. (ii) We find that the sign of global (force-free parameter) is
well correlated with the global SSA and the photospheric chirality of sunspots.
(iii) Local patches of opposite signs are present in the umbra of each
sunspot. The amplitude of the spatial variation of local in the umbra
is typically of the order of the global of the sunspot. (iv) We find
that the local is distributed as alternately positive and negative
filaments in the penumbra. The amplitude of azimuthal variation of the local
in the penumbra is approximately an order of magnitude larger than
that in the umbra. The contributions of the local positive and negative
currents and in the penumbra cancel each other giving almost no
contribution for their global values for whole sunspot. (v) Arc-like structures
(partial rings) with a sign opposite to that of the dominant sign of
of the umbral region are seen at the umbral-penumbral boundaries of some
sunspots. (vi) Most of the sunspots studied, belong to the minimum epoch of the
23 solar cycle and do not follow the so-called hemispheric helicity
rule.Comment: 15 pages, 3 figures, 1 table; Accepted for publication in the ApJ
Letter
Seismology of the Sun : Inference of Thermal, Dynamic and Magnetic Field Structures of the Interior
Recent overwhelming evidences show that the sun strongly influences the
Earth's climate and environment. Moreover existence of life on this Earth
mainly depends upon the sun's energy. Hence, understanding of physics of the
sun, especially the thermal, dynamic and magnetic field structures of its
interior, is very important. Recently, from the ground and space based
observations, it is discovered that sun oscillates near 5 min periodicity in
millions of modes. This discovery heralded a new era in solar physics and a
separate branch called helioseismology or seismology of the sun has started.
Before the advent of helioseismology, sun's thermal structure of the interior
was understood from the evolutionary solution of stellar structure equations
that mimicked the present age, mass and radius of the sun. Whereas solution of
MHD equations yielded internal dynamics and magnetic field structure of the
sun's interior. In this presentation, I review the thermal, dynamic and
magnetic field structures of the sun's interior as inferred by the
helioseismology.Comment: To be published in the proceedings of the meeting "3rd International
Conference on Current Developments in Atomic, Molecular, Optical and Nano
Physics with Applications", December 14-16, 2011, New Delhi, Indi
Stochastic excitation of nonradial modes II. Are solar asymptotic gravity modes detectable?
Detection of solar gravity modes remains a major challenge to our
understanding of the innerparts of the Sun. Their frequencies would enable the
derivation of constraints on the core physical properties while their
amplitudes can put severe constraints on the properties of the inner convective
region. Our purpose is to determine accurate theoretical amplitudes of solar g
modes and estimate the SOHO observation duration for an unambiguous detection.
We investigate the stochastic excitation of modes by turbulent convection as
well as their damping. Input from a 3D global simulation of the solar
convective zone is used for the kinetic turbulent energy spectrum. Damping is
computed using a parametric description of the nonlocal time-dependent
convection-pulsation interaction. We then provide a theoretical estimation of
the intrinsic, as well as apparent, surface velocity. Asymptotic g-mode
velocity amplitudes are found to be orders of magnitude higher than previous
works. Using a 3D numerical simulation, from the ASH code, we attribute this to
the temporal-correlation between the modes and the turbulent eddies which is
found to follow a Lorentzian law rather than a Gaussian one as previously used.
We also find that damping rates of asymptotic gravity modes are dominated by
radiative losses, with a typical life-time of years for the
mode at Hz. The maximum velocity in the considered
frequency range (10-100 Hz) is obtained for the mode at Hz and for the at Hz. Due to uncertainties in the
modeling, amplitudes at maximum i.e. for at 60 Hz can range from
3 to 6 mm s.Comment: 18 pages, 19 figures, accepted for publication in Astronomy &
Astrophysic
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
Influence of organofunctional organosilicon crosslinking agents on the rate of cure of the polydimethylsiloxane-polyphenylsilsesquioxane block copolymers
ChemInform Abstract: POLYMERISATION UND WAERMEBESTAENDIGKEIT VON POLYORGANOSILOXANEN MIT SILOXAN- UND SILICIUM-KOHLENSTOFF-GLIEDERN
Thermal and thermal-oxidative stability of polyorganocarbosiloxanes with a net-like structure
Mass spectroscopic investigation of radical polymerization initiated by dicumyl peroxide
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