343 research outputs found
Supersonic Downflows in a Sunspot Light Bridge
We report the discovery of supersonic downflows in a sunspot light bridge
using measurements taken with the spectropolarimeter on board the Hinode
satellite. The downflows occur in small patches close to regions where the
vector magnetic field changes orientation rapidly, and are associated with
anomalous circular polarization profiles. An inversion of the observed Stokes
spectra reveals velocities of up to 10 km/s, making them the strongest
photospheric flows ever measured in light bridges. Some (but not all) of the
downflowing patches are cospatial and cotemporal with brightness enhancements
in chromospheric Ca II H filtergrams. We suggest that these flows are due to
magnetic reconnection in the upper photosphere/lower chromosphere, although
other mechanisms cannot be ruled out.Comment: 4 pages, 5 figures, Published in ApJ Letter
Seeing-Induced Errors in Solar Doppler Velocity Measurements
Imaging systems based on a narrow-band tunable filter are used to obtain
Doppler velocity maps of solar features. These velocity maps are created by
taking the difference between the blue- and red-wing intensity images of a
chosen spectral line. This method has the inherent assumption that these two
images are obtained under identical conditions. With the dynamical nature of
the solar features as well as the Earth's atmosphere, systematic errors can be
introduced in such measurements. In this paper, a quantitative estimate of the
errors introduced due to variable seeing conditions for ground-based
observations is simulated and compared with real observational data for
identifying their reliability. It is shown, under such conditions, that there
is a strong cross-talk from the total intensity to the velocity estimates.
These spurious velocities are larger in magnitude for the umbral regions
compared to the penumbra or quiet-sun regions surrounding the sunspots. The
variable seeing can induce spurious velocities up to about 1 km/s It is also
shown that adaptive optics, in general, helps in minimising this effect.Comment: 14 page
High Resolution Observations using Adaptive Optics: Achievements and Future Needs
Over the last few years, several interesting observations were obtained with
the help of solar Adaptive Optics (AO). In this paper, few observations made
using the solar AO are enlightened and briefly discussed. A list of
disadvantages with the current AO system are presented. With telescopes larger
than 1.5m are expected during the next decade, there is a need to develop the
existing AO technologies for large aperture telescopes. Some aspects of this
development are highlighted. Finally, the recent AO developments in India are
also presented
Evershed clouds as precursors of moving magnetic features around sunspots
The relation between the Evershed flow and moving magnetic features (MMFs) is
studied using high-cadence, simultaneous spectropolarimetric measurements of a
sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler
velocities, magnetograms, and total linear polarization maps are calculated
from the observed Stokes profiles. We follow the temporal evolution of two
Evershed clouds that move radially outward along the same penumbral filament.
Eventually, the clouds cross the visible border of the spot and enter the moat
region, where they become MMFs. The flux patch farther from the sunspot has the
same polarity of the spot, while the MMF closer to it has opposite polarity and
exhibits abnormal circular polarization profiles. Our results provide strong
evidence that at least some MMFs are the continuation of the penumbral Evershed
flow into the moat. This, in turn, suggests that MMFs are magnetically
connected to sunspots.Comment: To appear in ApJ Letters, Vol 649, 2006 September 20 issu
Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots
High resolution spectropolarimetric observations of 3 sunspots taken with
Hinode demonstrate the existence of supersonic downflows at or close to the
umbra-penumbra boundary which have not been reported before. These downflows
are confined to large patches, usually encompassing bright penumbral filaments,
and have lifetimes of more than 14 hr. The presence of strong downflows in the
center-side penumbra near the umbra rules out an association with the Evershed
flow. Chromospheric filtergrams acquired close to the time of the
spectropolarimetric measurements show large, strong, and long-lived
brightenings in the neighborhood of the downflows. The photospheric intensity
also exhibit persistent brightenings comparable to the quiet Sun.
Interestingly, the orientation of the penumbral filaments at the site of the
downflows is similar to that resulting from the reconnection process described
by Ryutova et al. The existence of such downflows in the inner penumbra
represents a challenge for numerical models of sunspots because they have to
explain them in terms of physical processes likely affecting the chromosphere.Comment: Accepted for publication in Ap
Properties of Umbral Dots as Measured from the New Solar Telescope Data and MHD Simulations
We studied bright umbral dots (UDs) detected in a moderate size sunspot and
compared their statistical properties to recent MHD models. The study is based
on high resolution data recorded by the New Solar Telescope at the Big Bear
Solar Observatory and 3D MHD simulations of sunspots. Observed UDs, living
longer than 150 s, were detected and tracked in a 46 min long data set, using
an automatic detection code. Total 1553 (620) UDs were detected in the
photospheric (low chromospheric) data. Our main findings are: i) none of the
analyzed UDs is precisely circular, ii) the diameter-intensity relationship
only holds in bright umbral areas, and iii) UD velocities are inversely related
to their lifetime. While nearly all photospheric UDs can be identified in the
low chromospheric images, some small closely spaced UDs appear in the low
chromosphere as a single cluster. Slow moving and long living UDs seem to exist
in both the low chromosphere and photosphere, while fast moving and short
living UDs are mainly detected in the photospheric images. Comparison to the 3D
MHD simulations showed that both types of UDs display, on average, very similar
statistical characteristics. However, i) the average number of observed UDs per
unit area is smaller than that of the model UDs, and ii) on average, the
diameter of model UDs is slightly larger than that of observed ones.Comment: Accepted by the AP
Downflows in sunspot umbral dots
We study the velocity field of umbral dots at a resolution of 0.14". Our
analysis is based on full Stokes spectropolarimetric measurements of a pore
taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We
determine the flow velocity at different heights in the photosphere from a
bisector analysis of the Fe I 630 nm lines. In addtion, we use the observed
Stokes Q, U, and V profiles to characterize the magnetic properties of these
structures. We find that most umbral dots are associated with strong upflows in
deep photospheric layers. Some of them also show concentrated patches of
downflows at their edges, with sizes of about 0.25", velocities of up to 1000
m/s, and enhanced net circular polarization signals. The downflows evolve
rapidly and have lifetimes of only a few minutes. These results appear to
validate numerical models of magnetoconvection in the presence of strong
magnetic fields.Comment: Final published version. For best quality figures, please download
the PS versio
Searching for overturning convection in penumbral filaments: slit spectroscopy at 0.2 arcsec resolution
Recent numerical simulations of sunspots suggest that overturning convection
is responsible for the existence of penumbral filaments and the Evershed flow,
but there is little observational evidence of this process. Here we carry out a
spectroscopic search for small-scale convective motions in the penumbra of a
sunspot located 5 deg away from the disk center. The position of the spot is
very favorable for the detection of overturning downflows at the edges of
penumbral filaments. Our analysis is based on measurements of the Fe I 709.0 nm
line taken with the Littrow spectrograph of the Swedish 1 m Solar Telescope
under excellent seeing conditions. We compute line bisectors at different
intensity levels and derive Doppler velocities from them. The velocities are
calibrated using a nearby telluric line, with systematic errors smaller than
150 m/s. Deep in the photosphere, as sampled by the bisectors at the 80%-88%
intensity levels, we always observe blueshifts or zero velocities. The maximum
blueshifts reach 1.2 km/s and tend to be cospatial with bright penumbral
filaments. In the line core we detect blueshifts for the most part, with small
velocities not exceeding 300 m/s. Redshifts also occur, but at the level of
100-150 m/s, and only occasionally. The fact that they are visible in high
layers casts doubts on their convective origin. Overall, we do not find
indications of downflows that could be associated with overturning convection
at our detection limit of 150 m/s. Either no downflows exist, or we have been
unable to observe them because they occur beneath tau=1 or the spatial
resolution/height resolution of the measurements is still insufficient.Comment: Accepted for publication in Ap
Radiative transfer effects on Doppler measurements as sources of surface effects in sunspot seismology
We show that the use of Doppler shifts of Zeeman sensitive spectral lines to
observe wavesn in sunspots is subject to measurement specific phase shifts
arising from, (i) altered height range of spectral line formation and the
propagating character of p mode waves in penumbrae, and (ii) Zeeman broadening
and splitting. We also show that these phase shifts depend on wave frequencies,
strengths and line of sight inclination of magnetic field, and the polarization
state used for Doppler measurements. We discuss how these phase shifts could
contribute to local helioseismic measurements of 'surface effects' in sunspot
seismology.Comment: 12 pages, 4 figures, Accepted for publication in the Astrophysical
Journal Letter
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