330 research outputs found
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
Post-training load-related changes of auditory working memory: An EEG study
Working memory (WM) refers to the temporary retention and manipulation of information, and its capacity is highly susceptible to training. Yet, the neural mechanisms that allow for increased performance under demanding conditions are not fully understood. We expected that post-training efficiency in WM performance modulates neural processing during high load tasks. We tested this hypothesis, using electroencephalography (EEG) (N = 39), by comparing source space spectral power of healthy adults performing low and high load auditory WM tasks. Prior to the assessment, participants either underwent a modality-specific auditory WM training, or a modality-irrelevant tactile WM training, or were not trained (active control). After a modality-specific training participants showed higher behavioral performance, compared to the control. EEG data analysis revealed general effects of WM load, across all training groups, in the theta-, alpha-, and beta-frequency bands. With increased load theta-band power increased over frontal, and decreased over parietal areas. Centro-parietal alpha-band power and central beta-band power decreased with load. Interestingly, in the high load condition a tendency toward reduced beta-band power in the right medial temporal lobe was observed in the modality-specific WM training group compared to the modality-irrelevant and active control groups. Our finding that WM processing during the high load condition changed after modality-specific WM training, showing reduced beta-band activity in voice-selective regions, possibly indicates a more efficient maintenance of task-relevant stimuli. The general load effects suggest that WM performance at high load demands involves complementary mechanisms, combining a strengthening of task-relevant and a suppression of task-irrelevant processing
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
EUV Sunspot Plumes Observed with SOHO
Bright EUV sunspot plumes have been observed in five out of nine sunspot
regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. In the other
four regions the brightest line emissions may appear inside the sunspot but are
mainly concentrated in small regions outside the sunspot areas. These results
are in contrast to those obtained during the Solar Maximum Mission, but are
compatible with the Skylab mission results. The present observations show that
sunspot plumes are formed in the upper part of the transition region, occur
both in magnetic unipolar-- and bipolar regions, and may extend from the umbra
into the penumbra.Comment: 8 pages, 3 figures, to be published in ApJ Letter
The discrepancy in G-band contrast: Where is the quiet Sun?
We compare the rms contrast in observed speckle reconstructed G-band images
with synthetic filtergrams computed from two magneto-hydrodynamic simulation
snapshots. The observations consist of 103 bursts of 80 frames each taken at
the Dunn Solar Telescope (DST), sampled at twice the diffraction limit of the
telescope. The speckle reconstructions account for the performance of the
Adaptive Optics (AO) system at the DST to supply reliable photometry. We find a
considerable discrepancy in the observed rms contrast of 14.1% for the best
reconstructed images, and the synthetic rms contrast of 21.5% in a simulation
snapshot thought to be representative of the quiet Sun. The areas of features
in the synthetic filtergrams that have positive or negative contrast beyond the
minimum and maximum values in the reconstructed images have spatial scales that
should be resolved. This leads us to conclude that there are fundamental
differences in the rms G-band contrast between observed and computed
filtergrams. On the basis of the substantially reduced granular contrast of
16.3% in the synthetic plage filtergram we speculate that the quiet-Sun may
contain more weak magnetic field than previously thought.Comment: 16 pages, 8 figure
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
Convection and the origin of Evershed flows in sunspot penumbrae
We discuss a numerical 3D radiation-MHD simulation of penumbral fine
structure in a small sunspot. This simulation shows the development of short
filamentary structures with horizontal flows, similar to observed Evershed
flows, and an inward propagation of these structures at a speed compatible with
observations. We conclude that the Evershed flow represents the horizontal flow
component of overturning convection in gaps with strongly reduced field
strength. The top of the flow is always directed outward--away from the umbra--
because of the broken symmetry due to the inclined magnetic field. Upflows
occur in the inner parts of the gaps and most of the gas turns over radially
(outwards and sideways), and descends back down again. The ascending, cooling
and overturning flow tends to bend magnetic field lines down, forcing a
weakening of the field that makes it easier for gas located in an adjacent
layer--further in--to initiate a similar sequence of motion, aided by lateral
heating, thus causing the inward propagation of the filament.Comment: 4 pages, 1 figure. Submitted to ApJ
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