4,503 research outputs found
An investigation of Fe XVI emission lines in solar and stellar EUV and soft X-ray spectra
New fully relativistic calculations of radiative rates and electron impact
excitation cross sections for Fe XVI are used to determine theoretical
emission-line ratios applicable to the 251 - 361 A and 32 - 77 A portions of
the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A
comparison of the EUV results with observations from the Solar
Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals
excellent agreement between theory and experiment. However, for emission lines
in the 32 - 49 A portion of the soft X-ray spectral region, there are large
discrepancies between theory and measurement for both a solar flare spectrum
obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and
observations of Capella from the Low Energy Transmission Grating Spectrometer
(LETGS) on the Chandra X-ray Observatory. These are probably due to blending in
the solar flare and Capella data from both first order lines and from shorter
wavelength transitions detected in second and third order. By contrast, there
is very good agreement between our theoretical results and the XSST and LETGS
observations in the 50 - 77 A wavelength range, contrary to previous results.
In particular, there is no evidence that the Fe XVI emission from the XSST
flare arises from plasma at a much higher temperature than that expected for Fe
XVI in ionization equilibrium, as suggested by earlier work.Comment: 6 pages, 4 tables, 1 figure, MNRAS in pres
An assessment of Fe XX - Fe XXII emission lines in SDO/EVE data as diagnostics for high density solar flare plasmas using EUVE stellar observations
The Extreme Ultraviolet Variability Experiment (EVE) on the Solar Dynamics
Observatory obtains extreme-ultraviolet (EUV) spectra of the full-disk Sun at a
spectral resolution of ~1 A and cadence of 10 s. Such a spectral resolution
would normally be considered to be too low for the reliable determination of
electron density (N_e) sensitive emission line intensity ratios, due to
blending. However, previous work has shown that a limited number of Fe XXI
features in the 90-60 A wavelength region of EVE do provide useful
N_e-diagnostics at relatively low flare densities (N_e ~ 10^11-10^12 cm^-3).
Here we investigate if additional highly ionised Fe line ratios in the EVE
90-160 A range may be reliably employed as N_e-diagnostics. In particular, the
potential for such diagnostics to provide density estimates for high N_e
(~10^13 cm^-3) flare plasmas is assessed. Our study employs EVE spectra for
X-class flares, combined with observations of highly active late-type stars
from the Extreme Ultraviolet Explorer (EUVE) satellite plus experimental data
for well-diagnosed tokamak plasmas, both of which are similar in wavelength
coverage and spectral resolution to those from EVE. Several ratios are
identified in EVE data which yield consistent values of electron density,
including Fe XX 113.35/121.85 and Fe XXII 114.41/135.79, with confidence in
their reliability as N_e-diagnostics provided by the EUVE and tokamak results.
These ratios also allow the determination of density in solar flare plasmas up
to values of ~10^13 cm^-3.Comment: 7 pages, 3 figures, 2 tables, MNRAS in pres
The Area Distribution of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) are among the smallest observable objects on
the solar photosphere. A combination of G-band observations and numerical
simulations is used to determine their area distribution. An automatic
detection algorithm, employing 1-dimensional intensity profiling, is utilized
to identify these structures in the observed and simulated datasets. Both
distributions peak at an area of 45000 km, with a sharp decrease
towards smaller areas. The distributions conform with log-normal statistics,
which suggests that flux fragmentation dominates over flux convergence.
Radiative magneto-convection simulations indicate an independence in the MBP
area distribution for differing magnetic flux densities. The most commonly
occurring bright point size corresponds to the typical width of intergranular
lanes.Comment: Astrophysical Journal, accepte
Search for Rapid Changes in the Visible-Light Corona during the 21 June 2001 Total Solar Eclipse
Some 8000 images obtained with the SECIS fast-frame CCD camera instrument
located at Lusaka, Zambia, during the total eclipse of 21 June 2001 have been
analyzed to search for short-period oscillations in intensity that could be a
signature of solar coronal heating mechanisms by MHD wave dissipation. Images
were taken in white- light and Fe XIV green-line (5303 A) channels over 205
seconds (frame rate 39 s-1), approximately the length of eclipse totality at
this location, with a pixel size of four arcseconds square. The data are of
considerably better quality than were obtained during the 11 August 1999 total
eclipse, observed by us (Rudawy et al.: Astron. Astrophys. 416, 1179, 2004), in
that the images are much better exposed and enhancements in the drive system of
the heliostat used gave a much improved image stability. Classical Fourier and
wavelet techniques have been used to analyze the emission at 29518 locations,
of which 10714 had emission at reasonably high levels, searching for periodic
fluctuations with periods in the range 0.1-17 seconds (frequencies 0.06-10 Hz).
While a number of possible periodicities were apparent in the wavelet analysis,
none of the spatially and time-limited periodicities in the local brightness
curves was found to be physically important. This implies that the pervasive
Alfven wave-like phenomena (Tomczyk et al.: Science 317, 1192, 2007) using
polarimetric observations with the CoMP instrument do not give rise to
significant oscillatory intensity fluctuations.Comment: Accepted by Solar Physics; 16 figure
Building research capacity in a practitioner network: the National Action Research Network on researching and evaluating Personal Development Plannin
Propagating Wave Phenomena Detected in Observations and Simulations of the Lower Solar Atmosphere
We present high-cadence observations and simulations of the solar
photosphere, obtained using the Rapid Oscillations in the Solar Atmosphere
imaging system and the MuRAM magneto-hydrodynamic code, respectively. Each
dataset demonstrates a wealth of magneto-acoustic oscillatory behaviour,
visible as periodic intensity fluctuations with periods in the range 110-600 s.
Almost no propagating waves with periods less than 140s and 110s are detected
in the observational and simulated datasets, respectively. High concentrations
of power are found in highly magnetised regions, such as magnetic bright points
and intergranular lanes. Radiative diagnostics of the photospheric simulations
replicate our observational results, confirming that the current breed of
magneto-hydrodynamic simulations are able to accurately represent the lower
solar atmosphere. All observed oscillations are generated as a result of
naturally occurring magnetoconvective processes, with no specific input driver
present. Using contribution functions extracted from our numerical simulations,
we estimate minimum G-band and 4170 Angstrom continuum formation heights of 100
km and 25 km, respectively. Detected magneto-acoustic oscillations exhibit a
dominant phase delay of -8 degrees between the G-band and 4170 Angstrom
continuum observations, suggesting the presence of upwardly propagating waves.
More than 73% of MBPs (73% from observations, 96% from simulations) display
upwardly propagating wave phenomena, suggesting the abundant nature of
oscillatory behaviour detected higher in the solar atmosphere may be traced
back to magnetoconvective processes occurring in the upper layers of the Sun's
convection zone.Comment: 13 pages, 9 figures, accepted into Ap
Stable Umbral Chromospheric Structures
Aims. To understand the morphology of the chromosphere in sunspot umbra. We
investigate if the horizontal structures observed in the spectral core of the
Ca II H line are ephemeral visuals caused by the shock dynamics of more stable
structures, and examine their relationship with observables in the H-alpha
line. Methods. Filtergrams in the core of the Ca II H and H-alpha lines as
observed with the Swedish 1-m Solar Telescope are employed. We utilise a
technique that creates composite images and tracks the flash propagation
horizontally. Results. We find 0"15 wide horizontal structures, in all of the
three target sunspots, for every flash where the seeing was moderate to good.
Discrete dark structures are identified that are stable for at least two umbral
flashes, as well as systems of structures that live for up to 24 minutes. We
find cases of extremely extended structures with similar stability, with one
such structure showing an extent of 5". Some of these structures have a
correspondence in H-alpha but we were unable to find a one to one
correspondence for every occurrence. If the dark streaks are formed at the same
heights as umbral flashes then there are systems of structures with strong
departures from the vertical for all three analysed sunspots. Conclusions.
Long-lived Ca II H filamentary horizontal structures are a common and likely
ever-present feature in the umbra of sunspots. If the magnetic field in the
chromosphere of the umbra is indeed aligned with the structures, then the
present theoretical understanding of the typical umbra needs to be revisited.Comment: Accepted to Astronomy and Astrophysics. Online material (Fig3.mov and
Fig4.mov) will be available at A&
Fe XIII emission lines in active region spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph
Recent fully relativistic calculations of radiative rates and electron impact
excitation cross sections for Fe {\sc xiii} are used to generate emission-line
ratios involving 3s3p--3s3p and
3s3p--3s3p3d transitions in the 170--225 \AA and 235--450 \AA
wavelength ranges covered by the Solar Extreme-Ultraviolet Research Telescope
and Spectrograph (SERTS). A comparison of these line ratios with SERTS active
region observations from rocket flights in 1989 and 1995 reveals generally very
good agreement between theory and experiment. Several new Fe {\sc xiii}
emission features are identified, at wavelengths of 203.79, 259.94, 288.56 and
290.81 \AA. However, major discrepancies between theory and observation remain
for several Fe {\sc xiii} transitions, as previously found by Landi (2002) and
others, which cannot be explained by blending. Errors in the adopted atomic
data appear to be the most likely explanation, in particular for transitions
which have 3s3p3d D as their upper level. The most useful Fe
{\sc xiii} electron density diagnostics in the SERTS spectral regions are
assessed, in terms of the line pairs involved being (i) apparently free of
atomic physics problems and blends, (ii) close in wavelength to reduce the
effects of possible errors in the instrumental intensity calibration, and (iii)
very sensitive to changes in N over the range 10--10
cm. It is concluded that the ratios which best satisfy these conditions
are 200.03/202.04 and 203.17/202.04 for the 170--225 \AA wavelength region, and
348.18/320.80, 348.18/368.16, 359.64/348.18 and 359.83/368.16 for 235--450 \AA.Comment: 11 pages, 8 figures, 10 tables, MNRAS, in pres
Association between body mass index and mental health among Scottish adult population: a cross-sectional study of 37,272 participants
<b>Background:</b> The evidence is conflicting as to whether body mass index (BMI) is associated with mental health and, if so, to what extent it varies by sex and age. We studied mental health across the full spectrum of BMI among the general population, and conducted subgroup analyses by sex and age.<p></p>
<b>Method:</b> We undertook a cross-sectional study of a representative sample of the Scottish adult population. The Scottish Health Survey provided data on mental health, measured by the General Health Questionnaire-12 (GHQ), BMI, demographic and life-style information. Good mental health was defined as a GHQ score <4, and poor mental health as a GHQ score ≥4. Logistic regression models were applied.
Results Of the 37 272 participants, 5739 (15.4%) had poor mental health. Overall, overweight participants had better mental health than the normal-weight group [adjusted odds ratio (OR) 0.93, 95% confidence interval (CI) 0.87–0.99, p = 0.049], and individuals who were underweight, class II or class III obese had poorer mental health (class III obese group: adjusted OR 1.26, 95% CI 1.05–1.51, p = 0.013). There were significant interactions of BMI with sex (p = 0.013) and with age (p < 0.001). Being overweight was associated with significantly better mental health in middle-aged men only. In contrast, being underweight at all ages or obese at a young age was associated with significantly poorer mental health in women only.<p></p>
<b>Conclusions:</b> The adverse associations between adiposity and mental health are specific to women. Underweight women and young women who are obese have poorer mental health. In contrast, middle-aged overweight men have better mental health.<p></p>
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
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