454 research outputs found
Resolving the internal magnetic structure of the solar network
We analyze the spectral asymmetry of Stokes V (circularly polarized) profiles
of an individual network patch in the quiet Sun observed by Sunrise/IMaX. At a
spatial resolution of 0.15"-0.18", the network elements contain substructure
which is revealed by the spatial distribution of Stokes V asymmetries. The area
asymmetry between the red and blue lobes of Stokes V increases from nearly zero
at the core of the structure to values close to unity at its edges (one-lobed
profiles). Such a distribution of the area asymmetry is consistent with
magnetic fields expanding with height, i.e., an expanding magnetic canopy
(which is required to fulfill pressure balance and flux conservation in the
solar atmosphere). Inversion of the Stokes I and V profiles of the patch
confirms this picture, revealing a decreasing field strength and increasing
height of the canopy base from the core to the periphery of the network patch.
However, the non-roundish shape of the structure and the presence of negative
area and amplitude asymmetries reveal that the scenario is more complex than a
canonical flux tube expanding with height surrounded by downflows.Comment: accepted for publication in ApJ Letter
Orphan penumbrae: Submerging horizontal fields
We investigate the properties of orphan penumbrae, which are photospheric
filamentary structures observed in active regions near polarity inversion lines
that resemble the penumbra of regular sunspots but are not connected to any
umbra. We use Hinode data from the Solar Optical Telescope to determine the
properties of orphan penumbrae. Spectropolarimetric data are employed to obtain
the vector magnetic field and line-of-sight velocities in the photosphere.
Magnetograms are used to study the overall evolution of these structures, and
G-band and Ca II H filtergrams are to investigate their brightness and apparent
horizontal motions. Orphan penumbrae form between regions of opposite polarity
in places with horizontal magnetic fields. Their magnetic configuration is that
of -shaped flux ropes. In the two cases studied here, the
opposite-polarity regions approach each other with time and the whole structure
submerges as the penumbral filaments disappear. Orphan penumbrae are very
similar to regular penumbrae, including the existence of strong gas flows.
Therefore, they could have a similar origin. The main difference between them
is the absence of a "background" magnetic field in orphan penumbrae. This could
explain most of the observed differences. The fast flows we detect in orphan
penumbrae may be caused by the siphon flow mechanism. Based on the similarities
between orphan and regular penumbrae, we propose that the Evershed flow is also
a manifestation of siphon flows.Comment: 15 pages, 15 figure
Excitation of Slow-Modes in Network Magnetic Elements Through Magnetic Pumping
From radiation magnetohydrodynamic simulations of the solar atmosphere we
find a new mechanism for the excitation of longitudinal slow modes within
magnetic flux concentrations. We find that the convective downdrafts in the
immediate surroundings of magnetic elements are responsible for the excitation
of slow modes. The coupling between the external downdraft and the plasma
motion internal to the flux concentration is mediated by the inertial forces of
the downdraft that act on the magnetic flux concentration. These forces, in
conjunction with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric and
chromospheric layers of the magnetic element. Subsequent to the transient
pumping phase, the atmosphere rebounds, causing a slow mode traveling along the
magnetic flux concentration in the upward direction. It develops into a shock
wave in chromospheric heights, possibly capable of producing some kind of
dynamic fibril. We propose an observational detection of this process.Comment: 5 pages, 4 figures, accepted for publication in ApJ Lette
The properties of penumbral microjets inclination
We investigate the dependence of penumbral microjets inclination on the
position within penumbra. The high cadence observations taken on 10 November
2006 with the Hinode satellite through the \ion{Ca}{ii} H and G--band filters
were analysed to determine the inclination of penumbral microjets. The results
were then compared with the inclination of the magnetic field determined
through the inversion of the spectropolarimetric observations of the same
region. The penumbral microjet inclination is increasing towards the outer edge
of the penumbra. The results suggest that the penumbral microjet follows the
opening magnetic field lines of a vertical flux tube that creates the sunspot.Comment: 4 pages, 4 figures, A&A Letter in pres
HINODE Observations of Chromospheric Brightenings in the Ca II H Line during small-scale Flux Emergence Events
\ion{Ca}{2} H emission is a well-known indicator of magnetic activity in the
Sun and other stars. It is also viewed as an important signature of
chromospheric heating. However, the \ion{Ca}{2} H line has not been used as a
diagnostic of magnetic flux emergence from the solar interior. Here we report
on Hinode observations of chromospheric \ion{Ca}{2} H brightenings associated
with a repeated, small-scale flux emergence event. We describe this process and
investigate the evolution of the magnetic flux, G-band brightness, and
\ion{Ca}{2} H intensity in the emerging region. Our results suggest that energy
is released in the chromosphere as a consequence of interactions between the
emerging flux and the pre-existing magnetic field, in agreement with recent 3D
numerical simulations.Comment: 12 Pages, 6 Figures, Accepted for publication in ApJ Letter
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