452 research outputs found
Velocity fields in and around sunspots at the highest resolution
The flows in and around sunspots are rich in detail. Starting with the
Evershed flow along low-lying flow channels, which are cospatial with the
horizontal penumbral magnetic fields, Evershed clouds may continue this motion
at the periphery of the sunspot as moving magnetic features in the sunspot
moat. Besides these well-ordered flows, peculiar motions are found in complex
sunspots, where they contribute to the build-up or relaxation of magnetic
shear. In principle, the three-dimensional structure of these velocity fields
can be captured. The line-of-sight component of the velocity vector is
accessible with spectroscopic measurements, whereas local correlation or
feature tracking techniques provide the means to assess horizontal proper
motions. The next generation of ground-based solar telescopes will provide
spectropolarimetric data resolving solar fine structure with sizes below 50 km.
Thus, these new telescopes with advanced post-focus instruments act as a "zoom
lens" to study the intricate surface flows associated with sunspots.
Accompanied by "wide-angle" observations from space, we have now the
opportunity to describe sunspots as a system. This review reports recent
findings related to flows in and around sunspots and highlights the role of
advanced instrumentation in the discovery process.Comment: 6 pages, 1 figure, to be published in "Physics of Sun and star
spots", Proc. IAU Symp. 273, D.P. Choudhary and K.G. Strassmeier (eds.
Center-to-Limb Variation of Radio Emissions from Thermal-Rich and Thermal-Poor Solar Flares
A statistical analysis of radio flare events was performed by using the event
list of Nobeyama Radioheliograph in 1996-2009. We examined center-to-limb
variations of 17GHz and 34GHz flux by dividing the flare events into different
groups with respect to the 'thermal plasma richness' (ratio of the peak flux of
soft X-ray to non-thermal radio emissions) and the duration of radio bursts. It
is found that peak flux of 17 and 34GHz tend to be higher toward the limb for
thermal-rich flares with short durations. We propose that the thermal-rich
flares, which are supposed to be associated with an efficient precipitation of
high energy particles into the chromosphere, have a pitch angle distribution of
non-thermal electrons with a higher population along the flare loop.Comment: 12 pages, 5 figure
Small-scale chromospheric jets above a sunspot light bridge
High-resolution broadband filtergrams of active region NOAA 11271 in Ca ii H
and G band were obtained with the Solar Optical Telescope on board Hinode to
identify the physical driver responsible for the dynamic and small-scale
chromospheric jets above a sunspot light bridge. We identified the jets in the
Ca images using a semi-automatic routine. The chromospheric jets consist of a
bright, triangular-shaped blob that lies on the light bridge, while the apex of
this blob extends into a spike-like structure that is bright against the dark
umbral background. Most of the jets have apparent lengths of less than 1000 km
and about 30% of them have lengths between 1000-1600 km. They are oriented
within +/-35 deg. to the normal of the light bridge axis. Many of them are
clustered near the central part within a 2 arcsec area. The jets are seen to
move rapidly along the light bridge and most of them cannot be identified in
successive images taken with a 2 min cadence. The jets are primarily located on
one side of the light bridge and are directed into the umbral core. The Stokes
profiles at or close to the location of the blobs on the LB exhibit both a
significant net circular polarization and multiple components, including
opposite-polarity lobes. The magnetic field diverges from the light bridge
towards the umbral cores that it separates. In the photosphere there is a
predominantly uni-directional flow with speeds of 100-150 m/s along the light
bridge which is interrupted by a patch of weak motions that also moves along
the light bridge. The dynamic short-lived jets above the LB seem to be guided
by the magnetic field lines. Reconnection events are a likely trigger for such
phenomenon since they occur at locations where the magnetic field changes
orientation sharply. We find no clear relation between the jets and the
photospheric flow pattern.Comment: Accepted for publication in A&A, 9 pages, 7 figure
Magnetic Structure of Umbral Dots Observed with Hinode Solar Optical Telescope
High resolution and seeing-free spectroscopic observation of a decaying
sunspot was done with the Solar Optical Telescope aboard Hinode satellite. The
target was NOAA 10944 located in the west side of the solar surface from March
2 to March 4, 2007. The umbra included many umbral dots (UDs) with size of ~300
km in continuum light. We report the magnetic structures and Doppler velocity
fields around UDs, based on the Milne-Eddington inversion of the two iron
absorption lines at 6302 angstrom.
The histograms of magnetic field strength(B), inclination angle(i), and
Doppler velocity(v) of UDs showed a center-to-limb variation. Observed at disk
center, UDs had (1)slightly smaller field strength (Delta B=-17 Gauss) and
(2)relative blue shifts (Delta v=28 m s-1) compared to their surroundings. When
the sunspot got close to the limb, UDs and their surroundings showed almost no
difference in the magnetic and Doppler values. This center-to-limb variation
can be understood by the formation height difference in a cusp-shaped
magnetized atmosphere around UDs, due to the weakly magnetized hot gas
intrusion. In addition, some UDs showed oscillatory light curves with multiple
peaks around 10 min, which may indicate the presence of the oscillatory
convection. We discuss our results in the frameworks of two theoretical models,
the monolithic model (Schussler & Vogler 2006) and the field-free intrusion
model (Spruit & Scharmer 2006).Comment: 8 pages, 8 figures, accepted for publication in PAS
Hydrogen and oxygen isotope ratios of thermal waters of Okayama Prefecture, Japan
Hydrogen and oxygen isotope rations of thermal waters from 46 spas in Okayama Prefecture range from -62.6 to -29.2% in δD and from -10.0 to -4.4% in δ18O, respectively. The isotope rations indicate that all but one of the thermal water in Okayama prefecture are meteoric in origin. The Ofuku thermal water is the only exception, which is probably a mixture of seawater and meteoric water with the ratio of about 1.
Sulfur isotope rations of dissolvel sulfate in the thermal waters range from -6.2 ti 59.3% in δ34S. The high δ34S values observed in some thermal waters may be due to bacterial reduction of sulfate
Magnetic Flux Loss and Flux Transport in a Decaying Active Region
We estimate the temporal change of magnetic flux perpendicular to the solar
surface in a decaying active region by using a time series of the spatial
distribution of vector magnetic fields in the photosphere. The vector magnetic
fields are derived from full spectropolarimetric measurements with the Solar
Optical Telescope aboard Hinode. We compare a magnetic flux loss rate to a flux
transport rate in a decaying sunspot and its surrounding moat region. The
amount of magnetic flux that decreases in the sunspot and moat region is very
similar to magnetic flux transported to the outer boundary of the moat region.
The flux loss rates [] of magnetic elements with positive and
negative polarities are balanced each other around the outer boundary of the
moat region. These results suggest that most of the magnetic flux in the
sunspot is transported to the outer boundary of the moat region as moving
magnetic features, and then removed from the photosphere by flux cancellation
around the outer boundary of the moat region.Comment: 16 pages, 7 figures, Accepted for publication in Ap
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