220,381 research outputs found
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
Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope
Aims: We aim to achieve high spatial resolution as well as high polarimetric
sensitivity, using an earth-based 1m-class solar telescope, for the study of
magnetic fine structure on the Sun. Methods: We use a setup with 3 high-speed,
low-noise cameras to construct datasets with interleaved polarimetric states,
particularly suitable for Multi-Object Multi-Frame Blind Deconvolution image
restorations. We discuss the polarimetric calibration routine as well as
various potential sources of error in the results. Results: We obtained near
diffraction limited images, with a noise level of approximately 10^(-3)
I(cont). We confirm that dark-cores have a weaker magnetic field and at a lower
inclination angle with respect to the solar surface than the edges of the
penumbral filament. We show that the magnetic field strength in
faculae-striations is significantly lower than in other nearby parts of the
faculae.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 11
figure
Mozambique Channel Eddies in GCMs: A question of resolution and slippage
Hydrographic observations in the 21st century have shown that the flow within the MozambiqueChannel is best described by a series of large poleward-propagating anticyclonic eddies, rather than, aspreviously thought, a continuous intense western boundary current. The portrayal of this region in various runs of the NEMO 75-level model is found to vary between those two descriptions depending upon the resolution used and the implementation of the model's lateral boundary conditions. In a comparison of 1/4 ? resolution runs, the change of these conditions from free-slip to no-slip leads to the mean southward flow moving further offshore, with greater variability in the zonal and meridional velocities as the flow organises itself into eddies, and a reduction in total transport. If a realization of a model is unable to get these aspects of the physical flow correct, then this will significantly reduce its ability to show a realistic biological signal or long-term response to climate change. Further south, beyond Durban, the application of no-slip conditions similarly causes the mean Agulhas Current to lie further offshore, making it much more able to simulate Natal Pulses.<br/
CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
We discuss penumbral fine structure in a small part of a pore, observed with
the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST),
close to its diffraction limit of 0.16 arcsec. Milne-Eddington inversions
applied to these Stokes data reveal large variations of field strength and
inclination angle over dark-cored penumbral intrusions and a dark-cored light
bridge. The mid-outer part of this penumbra structure shows 0.3 arcsec wide
spines, separated by 1.6 arcsec (1200 km) and associated with 30 deg
inclination variations. Between these spines, there are no small-scale magnetic
structures that easily can be be identified with individual flux tubes. A
structure with nearly 10 deg more vertical and weaker magnetic field is seen
midways between two spines. This structure is co-spatial with the brightest
penumbral filament, possibly indicating the location of a convective upflow
from below.Comment: Accepted for publication in ApJL 17 Oct 2008. One Figure adde
Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
Recent studies of methanol and ground-state OH masers at very high spectral
resolution have shed new light on small-scale maser processes. The nearby
source W3(OH), which contains numerous bright masers in several different
transitions, provides an excellent laboratory for high spectral resolution
techniques. We present a model of W3(OH) based on EVN observations of the
rotationally-excited 6030 and 6035 MHz OH masers taken at 0.024 km/s spectral
resolution. The 6.0 GHz masers are becoming brighter with time and show
evidence for tangential proper motions. We confirm the existence of a region of
magnetic field oriented toward the observer to the southeast and find another
such region to the northeast in W3(OH), near the champagne flow. The 6.0 GHz
masers trace the inner edge of a counterclockwise rotating torus feature.
Masers at 6030 MHz are usually a factor of a few weaker than at 6035 MHz but
trace the same material. Velocity gradients of nearby Zeeman components are
much more closely correlated than in the ground state, likely due to the
smaller spatial separation between Zeeman components. Hydroxyl maser peaks at
very long baseline interferometric resolution appear to have structure on
scales both smaller than that resolvable as well as on larger scales.Comment: 21 pages using emulateapj.cls including 16 figures and 2 tables,
accepted to Ap
Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers
Context. Global MHD simulations show Kelvin-Helmholtz (KH) instabilities at
the contact surface of two merging neutron stars. That region has been
identified as the site of efficient amplification of magnetic fields. However,
these global simulations, due to numerical limitations, were unable to
determine the saturation level of the field strength, and thus the possible
back-reaction of the magnetic field onto the flow. Aims. We investigate the
amplification of initially weak fields in KH unstable shear flows, and the
back-reaction of the field onto the flow. Methods. We use a high-resolution
ideal MHD code to perform 2D and 3D local simulations of shear flows. Results.
In 2D, the magnetic field is amplified in less than 0.01ms until it reaches
locally equipartition with the kinetic energy. Subsequently, it saturates due
to resistive instabilities that disrupt the KH vortex and decelerate the shear
flow on a secular time scale. We determine scaling laws of the field
amplification with the initial field strength and the grid resolution. In 3D,
this hydromagnetic mechanism may be dominated by purely hydrodynamic
instabilities limiting the amplification. We find maximum magnetic fields of
10^16 G locally, and r.m.s. maxima within the box of 10^15 G. However, such
strong fields exist only for a short period. In the saturated state, the
magnetic field is mainly oriented parallel to the shear flow for strong initial
fields, while weaker initial fields tend to lead to a more balanced
distribution of the field energy. In all models the flow shows small-scale
features. The magnetic field is at most in equipartition with the decaying
shear flow. (abridged)Comment: 26 pages, 22 figures (figure quality reduced); accepted for
publication in Astronomy & Astrophysic
The Photospheric Poynting Flux and Coronal Heating
Some models of coronal heating suppose that convective motions at the
photosphere shuffle the footpoints of coronal magnetic fields and thereby
inject sufficient magnetic energy upward to account for observed coronal and
chromospheric energy losses in active regions. Using high-resolution
observations of plage magnetic fields made with the Solar Optical Telescope
aboard the Hinode satellite, we investigate this idea by estimating the upward
transport of magnetic energy --- the vertical Poynting flux, S_z --- across the
photosphere in a plage region. To do so, we combine: (i) estimates of
photospheric horizontal velocities, v_h, determined by local correlation
tracking applied to a sequence of line-of-sight magnetic field maps from the
Narrowband Filter Imager, with (ii) a vector magnetic field measurement from
the SpectroPolarimeter. Plage fields are ideal observational targets for
estimating energy injection by convection, because they are: (i) strong enough
to be measured with relatively small uncertainties; (ii) not so strong that
convection is heavily suppressed (as within umbrae); and (iii) unipolar, so S_z
in plage is not influenced by mixed-polarity processes (e.g., flux emergence)
unrelated to heating in stable, active-region fields. In this plage region, we
found that the average S_z varied in space, but was positive (upward) and
sufficient to explain coronal heating, with values near (5 +/- 1) x 10^7
erg/cm^2/s. We find the energy input per unit magnetic flux to be on the order
of 10^5 erg/s/Mx. A comparison of intensity in a Ca II image co-registered with
the this plage shows stronger spatial correlations with both total field, B,
and unsigned vertical field, |B_z|, than either S_z or horizontal field, B_h.
The observed Ca II brightness enhancement, however, probably contains a strong
contribution from a near-photosphere hot-wall effect unrelated to atmospheric
heating.Comment: 30 pages, 11 figures, accepted by Pub. Astron. Soc. Japa
Positivity of Chern Classes for Reflexive Sheaves on P^N
It is well known that the Chern classes of a rank vector bundle on
\PP^N, generated by global sections, are non-negative if and vanish
otherwise. This paper deals with the following question: does the above result
hold for the wider class of reflexive sheaves? We show that the Chern numbers
with can be arbitrarily negative for reflexive sheaves of any
rank; on the contrary for we show positivity of the with weaker
hypothesis. We obtain lower bounds for , and for every
reflexive sheaf \FF which is generated by H^0\FF on some non-empty open
subset and completely classify sheaves for which either of them reach the
minimum allowed, or some value close to it.Comment: 16 pages, no figure
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