2,975 research outputs found
Solar Atmospheric Oscillations and the Chromospheric Magnetic Topology
We investigate the oscillatory properties of the quiet solar chromosphere in
relation to the underlying photosphere, with particular regard to the effects
of the magnetic topology. We perform a Fourier analysis on a sequence of
line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0
nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained
from full spectroscopic data acquired at high spatial resolution with the
Interferometric BIdimensional Spectrometer (IBIS). The field of view
encompasses a full supergranular cell, allowing us to discriminate between
areas with different magnetic characteristics. We show that waves with
frequencies above the acoustic cut-off propagate from the photosphere to upper
layers only in restricted areas of the quiet Sun. A large fraction of the quiet
chromosphere is in fact occupied by ``magnetic shadows'', surrounding network
regions, that we identify as originating from fibril-like structures observed
in the core intensity of the Ca II line. We show that a large fraction of the
chromospheric acoustic power at frequencies below the acoustic cut-off,
residing in the proximity of the magnetic network elements, directly propagates
from the underlying photosphere. This supports recent results arguing that
network magnetic elements can channel low-frequency photospheric oscillations
into the chromosphere, thus providing a way to input mechanical energy in the
upper layers.Comment: 4 pages, 3 figure, A&A Letters in pres
Direct UV observations of the circumstellar envelope of alpha Orionis
Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
pres
SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Inter-network Chromosphere
On 12 March 1996 we obtained observations of the quiet Sun with the SUMER
instrument. The observa- tions were sequences of 15-20 second exposures of
ultraviolet emission line profiles and of the neighboring continua. These data
contain signatures of the dynamics of the solar chromosphere that are uniquely
useful because of wavelength coverage, moderate signal-to-noise ratios, and
image stability. The dominant observed phenomenon is an oscillatory behavior
that is analogous to the 3 minute oscillations seen in Ca II lines. The
oscillations appear to be coherent over 3-8". At any time they occur over
approx. 50 % of the area studied, and they appear as large perturbations in the
intensities of lines and continua. The oscillations are most clearly seen in
intensity variations in the UV (lambda > 912 A) continua, and they are also
seen in the intensities and velocities of chromospheric lines of C I, N I and O
I. Intensity brightenings are accompanied by blueshifts of typically 5 km
s. Phase differences between continuum and line intensities also
indicate the presence of upward propagating waves. Three minute intensity
oscillations are occasionally seen in second spectra (C II 1335), but never in
third spectra (C III and Si III). Third spectra and He I 584 show oscillations
in velocity that are not simply related to the 3 minute oscillations. The
continuum intensity variations are consistent with recent simulations of
chromospheric dynamics (Carlsson & Stein 1994) while the line observations
indicate that important ingredients are missing at higher layers in the
simulations. The data show that time variations are crucial for our
understanding of the chromosphere itself and for the spectral features formed
there.Comment: 8 pages, 3 figs, AASTeX, Accepted for publication in APJ letter
Canonically conjugate pairs and phase operators
For quantum mechanics on a lattice the position (``particle number'')
operator and the quasi-momentum (``phase'') operator obey canonical commutation
relations (CCR) only on a dense set of the Hilbert space. We compare exact
numerical results for a particle in simple potentials on the lattice with the
expectations, when the CCR are assumed to be strictly obeyed. Only for
sufficiently smooth eigenfunctions this leads to reasonable results. In the
long time limit the use of the CCR can lead to a qualitativel wrong dynamics
even if the initial state is in the dense set.Comment: 4 pages, 5 figures. Phys. Rev. A, in pres
Detection of supersonic downflows and associated heating events in the transition region above sunspots
IRIS data allow us to study the solar transition region (TR) with an
unprecedented spatial resolution of 0.33 arcsec. On 2013 August 30, we observed
bursts of high Doppler shifts suggesting strong supersonic downflows of up to
200 km/s and weaker, slightly slower upflows in the spectral lines Mg II h and
k, C II 1336 \AA, Si IV 1394 \AA, and 1403 \AA, that are correlated with
brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout
the 2 hr observation, with average burst durations of about 20 s. The locations
of these short-lived events appear to be the umbral and penumbral footpoints of
EUV loops. Fast apparent downflows are observed along these loops in the SJIs
and in AIA, suggesting that the loops are thermally unstable. We interpret the
observations as cool material falling from coronal heights, and especially
coronal rain produced along the thermally unstable loops, which leads to an
increase of intensity at the loop footpoints, probably indicating an increase
of density and temperature in the TR. The rain speeds are on the higher end of
previously reported speeds for this phenomenon, and possibly higher than the
free-fall velocity along the loops. On other observing days, similar bright
dots are sometimes aligned into ribbons, resembling small flare ribbons. These
observations provide a first insight into small-scale heating events in
sunspots in the TR.Comment: accepted by ApJ
Spectral simplicity and asymptotic separation of variables
We describe a method for comparing the real analytic eigenbranches of two
families of quadratic forms that degenerate as t tends to zero. One of the
families is assumed to be amenable to `separation of variables' and the other
one not. With certain additional assumptions, we show that if the families are
asymptotic at first order as t tends to 0, then the generic spectral simplicity
of the separable family implies that the eigenbranches of the second family are
also generically one-dimensional. As an application, we prove that for the
generic triangle (simplex) in Euclidean space (constant curvature space form)
each eigenspace of the Laplacian is one-dimensional. We also show that for all
but countably many t, the geodesic triangle in the hyperbolic plane with
interior angles 0, t, and t, has simple spectrum.Comment: 53 pages, 2 figure
The Influence of Arthritis in Other Major Joints and the Spine on the One-Year Outcome of Total Hip Replacement:A Prospective, Multicenter Cohort Study (EUROHIP) Measuring the Influence of Musculoskeletal Morbidity
Background
Whilst arthritis in other affected joints and back pain is known to lead to worse outcomes following total hip replacement surgery, these risk factors have not previously been operationalized as a musculoskeletal morbidity profile. The aim of this study was to measure the influence of other joints and spine (as grades of musculoskeletal morbidity) on the 1-year outcome of primary total hip replacement.
Methods
The European Collaborative Database of Cost and Practice Patterns of Total Hip Replacement study consists of 1,327 patients receiving primary THR for osteoarthritis across 20 European orthopedic centers. The primary outcome was the responder rate for THR at 12-months as measured by the relative effect per patient (REPP score), calculated for each patient using the total WOMAC score. The primary predictor of interest was combinations of arthritis of large joints and spine grouped into four musculoskeletal morbidity (MSM) grades: 1 (single-joint), 2 (multi-joints), 3 (single-joint and spine), 4 (multi-joints and spine). Confounders adjusted for were: age, sex, body mass index, living alone, years of hip pain, ASA grade, anxiety/depression, pre-operative WOMAC subscales.
Results
845 patients were included for this analysis with complete 12-month follow-up WOMAC scores. The mean age was 65.7 years and 55.2% were female. Increasing MSM grade was associated with worse outcomes of surgery, where the proportion of patients responding to THR were: 254 (92.4%) MSM grade 1, 272 (87.2%) MSM grade 2, 46 (80.7%) MSM grade 3, 142 (74.4%) MSM grade 4. This was confirmed in adjusted logistic regression models: MSM grade 4 vs. 1 relative risk ratio (RRR) 0.82 95% confidence interval (CI) (0.75, 0.90); MSM grade 3 vs. 1 OR 0.87 95%CI (0.77, 0.99); MSM grade 2 vs. 1 OR 0.95 95%CI (0.89, 1.00).
Conclusions
Other joints and spine measured as musculoskeletal morbidity have a strong influence on the 1-year outcome after THR. The effect size was large in comparison to other risk factors. Even so, the majority of patients in MSM grade 4 can still profit from surgery (>75% response to surgery)
Using Social Media to Promote STEM Education: Matching College Students with Role Models
STEM (Science, Technology, Engineering, and Mathematics) fields have become
increasingly central to U.S. economic competitiveness and growth. The shortage
in the STEM workforce has brought promoting STEM education upfront. The rapid
growth of social media usage provides a unique opportunity to predict users'
real-life identities and interests from online texts and photos. In this paper,
we propose an innovative approach by leveraging social media to promote STEM
education: matching Twitter college student users with diverse LinkedIn STEM
professionals using a ranking algorithm based on the similarities of their
demographics and interests. We share the belief that increasing STEM presence
in the form of introducing career role models who share similar interests and
demographics will inspire students to develop interests in STEM related fields
and emulate their models. Our evaluation on 2,000 real college students
demonstrated the accuracy of our ranking algorithm. We also design a novel
implementation that recommends matched role models to the students.Comment: 16 pages, 8 figures, accepted by ECML/PKDD 2016, Industrial Trac
- …