114 research outputs found
What determines the penumbral size and Evershed flow speed?
Using Hinode SP and G-band observations, we examined the relationship between
magnetic field structure and penumbral size as well as Evershed flow speed. The
latter two are positively correlated with magnetic inclination angle or
horizontal field strength within 1.5 kilogauss, which is in agreement with
recent magnetoconvective simulations of Evershed effect. This work thus
provides direct observational evidence supporting the magnetoconvection nature
of penumbral structure and Evershed flow in the presence of strong and inclined
magnetic field.Comment: 5 pages, 5 figures, IAU 273 proceedings, in pres
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
On Molecular Hydrogen Formation and the Magnetohydrostatic Equilibrium of Sunspots
We have investigated the problem of sunspot magnetohydrostatic equilibrium
with comprehensive IR sunspot magnetic field survey observations of the highly
sensitive Fe I lines at 15650 \AA\ and nearby OH lines. We have found that some
sunspots show isothermal increases in umbral magnetic field strength which
cannot be explained by the simplified sunspot model with a single-component
ideal gas atmosphere assumed in previous investigations. Large sunspots
universally display non-linear increases in magnetic pressure over temperature,
while small sunspots and pores display linear behavior. The formation of
molecules provides a mechanism for isothermal concentration of the umbral
magnetic field, and we propose that this may explain the observed rapid
increase in umbral magnetic field strength relative to temperature. Existing
multi-component sunspot atmospheric models predict that a significant amount of
molecular hydrogen (H2) exists in the sunspot umbra. The formation of H2 can
significantly alter the thermodynamic properties of the sunspot atmosphere and
may play a significant role in sunspot evolution. In addition to the survey
observations, we have performed detailed chemical equilibrium calculations with
full consideration of radiative transfer effects to establish OH as a proxy for
H2, and demonstrate that a significant population of H2 exists in the coolest
regions of large sunspots.Comment: 17 pages, 19 figures, accepted for publication in Ap
Solar Atmospheric Oscillations and the Chromospheric Magnetic Topology
We investigate the oscillatory properties of the quiet solar chromosphere in
relation to the underlying photosphere, with particular regard to the effects
of the magnetic topology. We perform a Fourier analysis on a sequence of
line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0
nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained
from full spectroscopic data acquired at high spatial resolution with the
Interferometric BIdimensional Spectrometer (IBIS). The field of view
encompasses a full supergranular cell, allowing us to discriminate between
areas with different magnetic characteristics. We show that waves with
frequencies above the acoustic cut-off propagate from the photosphere to upper
layers only in restricted areas of the quiet Sun. A large fraction of the quiet
chromosphere is in fact occupied by ``magnetic shadows'', surrounding network
regions, that we identify as originating from fibril-like structures observed
in the core intensity of the Ca II line. We show that a large fraction of the
chromospheric acoustic power at frequencies below the acoustic cut-off,
residing in the proximity of the magnetic network elements, directly propagates
from the underlying photosphere. This supports recent results arguing that
network magnetic elements can channel low-frequency photospheric oscillations
into the chromosphere, thus providing a way to input mechanical energy in the
upper layers.Comment: 4 pages, 3 figure, A&A Letters in pres
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