1,143 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
Upflows in the central dark lane of sunspot light bridges
We use high spatial and spectral resolution observations obtained with the
CRisp Imaging SpectroPolarimeter at the Swedish 1-m Solar Telescope to analyze
the velocity profile of granular light bridges in a sunspot. We find upflows
associated with the central dark lanes of the light bridges. From bisectors in
the Fe I 630.15 nm line we find that the magnitude of the upflows varies with
height with the strongest upflows being deeper in the atmosphere. Typical
upflow velocities measured from the 70% bisector are around 500 m/s with peaks
above 1 km/s. The upflows in the central dark lane are surrounded by downflows
of weaker magnitude, sometimes concentrated in patches with enhanced velocities
reaching up to 1.1 km/s. A small spatial offset between the upflows and the
continuum dark lane is interpreted as a line-of-sight effect due to the
elevated nature of the dark lane and the light bridge above the umbral
surroundings. Our observations show that the central dark lane in granular
light bridges is not equivalent to the intergranular lanes of normal
photospheric granulation that host convective downflows. These results support
recent MHD simulations of magneto-convection in sunspot atmospheres.Comment: Accepted for publication in Astrophysical Journal 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
Flare induced penumbra formation in the sunspot of NOAA 10838
We have observed formation of penumbrae on a pore in the active region
NOAA10838 using Dunn Solar Telescope at NSO,Sunpot,USA. Simultaneous
observations using different instruments (DLSP,UBF,Gband and CaK) provide us
with vector magnetic field at photosphere, intensity images and Doppler
velocity at different heights from photosphere to chromosphere. Results from
our analysis of this particular data-set suggests that penumbrae are formed as
a result of relaxation of magnetic field due to a flare happening at the same
time. Images in \Halpha\ show the flare (C 2.9 as per GOES) and vector magnetic
fields show a re-orientation and reduction in the global value (a
measure of twist). We feel such relaxation of loop structures due to
reconnections or flare could be one of the way by which field lines fall back
to the photosphere to form penumbrae.Comment: 4 pages, Presented at IAU symposium 273- Physics of Sun and Starspot
A novel optical intracellular imaging approach for potassium dynamics in astrocytes.
Astrocytes fulfill a central role in regulating K+ and glutamate, both released by neurons into the extracellular space during activity. Glial glutamate uptake is a secondary active process that involves the influx of three Na+ ions and one proton and the efflux of one K+ ion. Thus, intracellular K+ concentration ([K+]i) is potentially influenced both by extracellular K+ concentration ([K+]o) fluctuations and glutamate transport in astrocytes. We evaluated the impact of these K+ ion movements on [K+]i in primary mouse astrocytes by microspectrofluorimetry. We established a new noninvasive and reliable approach to monitor and quantify [K+]i using the recently developed K+ sensitive fluorescent indicator Asante Potassium Green-1 (APG-1). An in situ calibration procedure enabled us to estimate the resting [K+]i at 133±1 mM. We first investigated the dependency of [K+]i levels on [K+]o. We found that [K+]i followed [K+]o changes nearly proportionally in the range 3-10 mM, which is consistent with previously reported microelectrode measurements of intracellular K+ concentration changes in astrocytes. We then found that glutamate superfusion caused a reversible drop of [K+]i that depended on the glutamate concentration with an apparent EC50 of 11.1±1.4 µM, corresponding to the affinity of astrocyte glutamate transporters. The amplitude of the [K+]i drop was found to be 2.3±0.1 mM for 200 µM glutamate applications. Overall, this study shows that the fluorescent K+ indicator APG-1 is a powerful new tool for addressing important questions regarding fine [K+]i regulation with excellent spatial resolution
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
Stabilization by rough noise for an epitaxial growth model
In this article we study a model from epitaxial thin-film growth. It was
originally introduced as a phenomenological model of growth in the presence of
a Schwoebbel barrier, where diffusing particles on a terrace are not allowed to
jump down at the boundary.
Nevertheless, we show that the presence of arbitrarily small space-time white
noise due to fluctuations in the incoming particles surprisingly eliminates all
nonlinear interactions in the model and thus has the potential to stabilize the
dynamics and suppress the growth of hills in these models
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
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