647 research outputs found
First detection of linear polarization in the line profiles of active cool stars
The application of high-resolution spectropolarimetry has led to major
progress in understanding the magnetism and activity of late-type stars. During
the last decade, magnetic fields have been discovered and mapped for many types
of active cool stars using spectropolarimetric data. However, these
observations and modeling attempts are fundamentally incomplete since they are
based on the interpretation of the circular polarization alone. Taking
advantage of the newly built HARPS polarimeter, we have obtained the first
systematic observations of several cool active stars in all four Stokes
parameters. Here we report the detection of magnetically-induced linear
polarization for the primary component of the very active RS CVn binary HR 1099
and for the moderately active K dwarf epsilon Eri. For both stars the amplitude
of linear polarization signatures is measured to be ~10^{-4} of the unpolarized
continuum, which is approximately a factor of ten lower than for circular
polarization. This is the first detection of the linear polarization in line
profiles of cool active stars. Our observations of the inactive solar-like star
alpha Cen A show neither circular nor linear polarization above the level of
10^{-5}, indicating the absence of a net longitudinal magnetic field stronger
than 0.2 G.Comment: accepted for publication in Astrophysical Journal Letter
Properties of Magneto-Hydrodynamic Waves in the Solar Photosphere with Hinode
We report the observations of the magneto-hydrodynamic waves propagating
along magnetic flux tubes in the solar photosphere. We identified 20 isolated
strong peaks (8 peaks for pores and 12 peaks for inter-granular magnetic
structure) in the power spectra of the l.o.s. (line-of-sight) magnetic flux,
the l.o.s. velocity, and the intensity for 14 different magnetic
concentrations. The observation is performed with the spectro-polarimeter of
the Solar Optical Telescope aboard the \emph{Hinode} satellite. The observed
phase relation between the magnetic and the photometric intensity fluctuation
would not be consistent with that caused by the opacity effect, if the magnetic
field strength decreases with height along the oblique line of sight. We
suggest that the observed fluctuations are due to longitudinal (sausage mode)
and/or transverse (kink mode) MHD waves. The observed phase relation between
the fluctuations in the magnetic flux and the velocity is consistent with the
superposition of the ascending wave and the descending wave reflected at
chromosphere/corona boundary (standing wave). Even with such reflected waves,
the residual upward Poynting flux is estimated to be erg
cm s for a case of the kink wave. Seismology of the magnetic flux
tubes is possible to obtain various physical parameters from the observed
period and amplitude of the oscillations.Comment: 50 pages, 10 Postscript images, accepted for publication in
ApJ.(There are some changes in version 2 of our paper. Equations (65), (66),
(68) in page 40 are corrected. Figure 9 is replaced with new one following
the corrected equations. The calculated parameters in pages 32, 33, 42 are
corrected.
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
Effect of four premolar extractions on the vertical dimension of the face : A retrospective cephalometric study
PURPOSE
Adequate control of the vertical dimension is of great importance in orthodontic treatment. Although existing evidence is very limited, extraction of four premolars is thought to contribute towards improved control of anterior facial height compared with non-extraction treatment protocols. Thus, the aim of this retrospective cohort study was to compare the effect of fixed-appliance treatment with extraction of four premolars to non-extraction treatment on the skeletal vertical dimension.
METHODS
A consecutive sample of 76 children with skeletal hyperdivergence (49% male; mean age 11.9 years) was divided into two groups for treatment with either non-extraction (n = 31) or extraction of four premolars (n = 45). Baseline characteristics were comparable: overjet 5.1 ± 2.5 mm, overbite 2.4 ± 1.9 mm, ANB angle 4.6 ± 2.3°, and SN-ML angle 40.2 ± 3.5°. Patients were treated with standard edgewise fixed appliances with closing loops/sliding mechanics. Vertical skeletal and dental outcomes were measured on lateral cephalograms before and after treatment. Data were analyzed with linear regression at 5%.
RESULTS
Compared to non-extraction treatment, treatment with premolar extractions had no significant effect on the SN-ML angle (difference (Δ) = 0.07°; 95% confidence interval -0.90 to 1.01°; P = 0.88). Statistically significant changes between the extraction and non-extraction groups were only found for the parameters SNA (Δ -1.47°; P = 0.003), ANB (Δ -1.17°; P = 0.004), SN-OP (Δ -1.48°; P = 0.04), and L1-ML (Δ -6.39°; P < 0.001).
CONCLUSION
Orthodontic treatment of children with skeletal hyperdivergence using systematic extraction of four premolars had minimal effects on the vertical facial dimension compared to non-extraction treatment
On Shear-Driven Ventilation of Snow
A series of experiments have been made in a wind tunnel to investigate the ventilation of snow by shear. We argue that the zero-plane displacement can be used as a convenient indicator of ventilation, and that this can be obtained from measurements of mean velocity profiles in conditions of zero pressure gradient. Measurements made over a natural snow surface show a zero-plane displacement depth of less than 5mm, but practical considerations preclude extensive use of snow for these measurements. Instead, the influence of permeability is investigated using reticulated foams in place of snow. We demonstrate that the foam and snow have similar structure and flow-relevant properties. Although the surface of the foam is flat, the roughness lengths increase by two orders of magnitude as the permeability increases from 6 × 10−9 to 160 × 10−9 m2. The zero-plane displacement for the least permeable foams is effectively zero, but more than 15mm for the most permeable foams. Our data compare well to the few studies available in the literature. By analogy to conditions over snow surfaces, we suggest that shear-driven ventilation of snow is therefore limited to the upper few millimetres of snow surface
- …