2,861 research outputs found
Quasi-Biennial variations in helioseismic frequencies: Can the source of the variation be localized?
We investigate the spherical harmonic degree (l) dependence of the "seismic"
quasi-biennial oscillation (QBO) observed in low-degree solar p-mode
frequencies, using Sun-as-a-star Birmingham Solar Oscillations Network (BiSON)
data. The amplitude of the seismic QBO is modulated by the 11-yr solar cycle,
with the amplitude of the signal being largest at solar maximum. The amplitude
of the signal is noticeably larger for the l=2 and 3 modes than for the l=0 and
1 modes. The seismic QBO shows some frequency dependence but this dependence is
not as strong as observed in the 11-yr solar cycle. These results are
consistent with the seismic QBO having its origins in shallow layers of the
interior (one possibility being the bottom of the shear layer extending 5per
cent below the solar surface). Under this scenario the magnetic flux
responsible for the seismic QBO is brought to the surface (where its influence
on the p modes is stronger) by buoyant flux from the 11-yr cycle, the strong
component of which is observed at predominantly low-latitudes. As the l=2 and 3
modes are much more sensitive to equatorial latitudes than the l=0 and 1 modes
the influence of the 11-yr cycle on the seismic QBO is more visible in l=2 and
3 mode frequencies. Our results imply that close to solar maximum the main
influence of the seismic QBO occurs at low latitudes (<45 degrees), which is
where the strong component of the 11-yr solar cycle resides. To isolate the
latitudinal dependence of the seismic QBO from the 11-yr solar cycle we must
consider epochs when the 11-yr solar cycle is weak. However, away from solar
maximum, the amplitude of the seismic QBO is weak making the latitudinal
dependence hard to constrain.Comment: 10 pages, 6 figures, accepted for publication in MNRA
Solar cycle variations of large frequency separations of acoustic modes: Implications for asteroseismology
We have studied solar cycle changes in the large frequency separations that
can be observed in Birmingham Solar Oscillations Network (BiSON) data. The
large frequency separation is often one of the first outputs from asteroseismic
studies because it can help constrain stellar properties like mass and radius.
We have used three methods for estimating the large separations: use of
individual p-mode frequencies, computation of the autocorrelation of
frequency-power spectra, and computation of the power spectrum of the power
spectrum. The values of the large separations obtained by the different methods
are offset from each other and have differing sensitivities to the realization
noise. A simple model was used to predict solar cycle variations in the large
separations, indicating that the variations are due to the well-known solar
cycle changes to mode frequency. However, this model is only valid over a
restricted frequency range. We discuss the implications of these results for
asteroseismology.Comment: 9 pages, 11 figures, accepted for publication in MNRAS, references
updated, corrections following proof
A new efficient method for determining weighted power spectra: detection of low-frequency solar p-modes by analysis of BiSON data
We present a new and highly efficient algorithm for computing a power
spectrum made from evenly spaced data which combines the noise-reducing
advantages of the weighted fit with the computational advantages of the Fast
Fourier Transform (FFT). We apply this method to a 10-year data set of the
solar p-mode oscillations obtained by the Birmingham Solar Oscillations Network
(BiSON) and thereby uncover three new low-frequency modes. These are the l=2,
n=5 and n=7 modes and the l=3, n=7 mode. In the case of the l=2, n=5 modes,
this is believed to be the first such identification of this mode in the
literature. The statistical weights needed for the method are derived from a
combination of the real data and a sophisticated simulation of the instrument
performance. Variations in the weights are due mainly to the differences in the
noise characteristics of the various BiSON instruments, the change in those
characteristics over time and the changing line-of-sight velocity between the
stations and the Sun. It should be noted that a weighted data set will have a
more time-dependent signal than an unweighted set and that, consequently, its
frequency spectrum will be more susceptible to aliasing.Comment: 11 pages, 7 Figures, accepted for publication in MNRAS, Figure 6 had
to be reduced in size to upload and so may be difficult to view on screen in
.ps versio
The underlying physical meaning of the relation
Asteroseismology of stars that exhibit solar-like oscillations are enjoying a
growing interest with the wealth of observational results obtained with the
CoRoT and Kepler missions. In this framework, scaling laws between
asteroseismic quantities and stellar parameters are becoming essential tools to
study a rich variety of stars. However, the physical underlying mechanisms of
those scaling laws are still poorly known. Our objective is to provide a
theoretical basis for the scaling between the frequency of the maximum in the
power spectrum () of solar-like oscillations and the cut-off
frequency (). Using the SoHO GOLF observations together with
theoretical considerations, we first confirm that the maximum of the height in
oscillation power spectrum is determined by the so-called \emph{plateau} of the
damping rates. The physical origin of the plateau can be traced to the
destabilizing effect of the Lagrangian perturbation of entropy in the
upper-most layers which becomes important when the modal period and the local
thermal relaxation time-scale are comparable. Based on this analysis, we then
find a linear relation between and , with a
coefficient that depends on the ratio of the Mach number of the exciting
turbulence to the third power to the mixing-length parameter.Comment: 8 pages, 11 figures. Accepted in A&
Substrate concentration dependence of the diffusion-controlled steady-state rate constant
The Smoluchowski approach to diffusion-controlled reactions is generalized to
interacting substrate particles by including the osmotic pressure and
hydrodynamic interactions of the nonideal particles in the Smoluchoswki
equation within a local-density approximation. By solving the strictly
linearized equation for the time-independent case with absorbing boundary
conditions, we present an analytic expression for the diffusion-limited
steady-state rate constant for small substrate concentrations in terms of an
effective second virial coefficient B_2*. Comparisons to Brownian dynamics
simulations excluding HI show excellent agreement up to bulk number densities
of B_2* rho_0 < 0.4 for hard sphere and repulsive Yukawa-like interactions
between the substrates. Our study provides an alternative way to determine the
second virial coefficient of interacting macromolecules experimentally by
measuring their steady-state rate constant in diffusion-controlled reactions at
low densities.Comment: 7 pages, 3 figure
An audit of the quality of inpatient care for adults with learning disability in the UK
OBJECTIVES: To audit patient hospital records to evaluate the performance of acute general and mental health services in delivering inpatient care to people with learning disability and explore the influence of organisational factors on the quality of care they deliver. SETTING: Nine acute general hospital Trusts and six mental health services. PARTICIPANTS: Adults with learning disability who received inpatient hospital care between May 2013 and April 2014. PRIMARY AND SECONDARY OUTCOME MEASURES: Data on seven key indicators of high-quality care were collected from 176 patients. These covered physical health/monitoring, communication and meeting needs, capacity and decision-making, discharge planning and carer involvement. The impact of services having an electronic system for flagging patients with learning disability and employing a learning disability liaison nurse was assessed. RESULTS: Indicators of physical healthcare (body mass index, swallowing assessment, epilepsy risk assessment) were poorly recorded in acute general and mental health inpatient settings. Overall, only 34 (19.3%) patients received any assessment of swallowing and 12 of the 57 with epilepsy (21.1%) had an epilepsy risk assessment. For most quality indicators, there was a non-statistically significant trend for improved performance in services with a learning disability liaison nurse. The presence of an electronic flagging system showed less evidence of benefit. CONCLUSIONS: Inpatient care for people with learning disability needs to be improved. The work gives tentative support to the role of a learning disability liaison nurse in acute general and mental health services, but further work is needed to confirm these benefits and to trial other interventions that might improve the quality and safety of care for this high-need group
Flicker as a tool for characterizing planets through Asterodensity Profiling
Variability in the time series brightness of a star on a timescale of 8
hours, known as 'flicker', has been previously demonstrated to serve as a proxy
for the surface gravity of a star by Bastien et al. (2013). Although surface
gravity is crucial for stellar classification, it is the mean stellar density
which is most useful when studying transiting exoplanets, due to its direct
impact on the transit light curve shape. Indeed, an accurate and independent
measure of the stellar density can be leveraged to infer subtle properties of a
transiting system, such as the companion's orbital eccentricity via
asterodensity profiling. We here calibrate flicker to the mean stellar density
of 439 Kepler targets with asteroseismology, allowing us to derive a new
empirical relation given by
. The calibration is valid for stars with
KK, and flicker estimates corresponding
to stars with . Our relation has a model error in the
stellar density of 31.7% and so has times lower precision than that
from asteroseismology but is applicable to a sample times greater.
Flicker therefore provides an empirical method to enable asterodensity
profiling on hundreds of planetary candidates from present and future missions.Comment: 6 pages, 3 figures, 1 table. Accepted to ApJ Letters. Code available
at https://www.cfa.harvard.edu/~dkipping/flicker.htm
Changes in the sensitivity of solar p-mode frequency shifts to activity over three solar cycles
Low-degree solar p-mode observations from the long-lived Birmingham Solar
Oscillations Network (BiSON) stretch back further than any other single
helioseismic data set. Results from BiSON have suggested that the response of
the mode frequency to solar activity levels may be different in different
cycles. In order to check whether such changes can also be seen at higher
degrees, we compare the response of medium-degree solar p-modes to activity
levels across three solar cycles using data from Big Bear Solar Observatory
(BBSO), Global Oscillation Network Group (GONG), Michelson Doppler Imager (MDI)
and Helioseismic and Magnetic Imager (HMI), by examining the shifts in the mode
frequencies and their sensitivity to solar activity levels. We compare these
shifts and sensitivities with those from radial modes from BiSON. We find that
the medium-degree data show small but significant systematic differences
between the cycles, with solar cycle 24 showing a frequency shift about 10 per
cent larger than cycle 23 for the same change in activity as determined by the
10.7 cm radio flux. This may support the idea that there have been changes in
the magnetic properties of the shallow subsurface layers of the Sun that have
the strongest influence on the frequency shifts.Comment: 6 pages, 3 figures, accepted by MNRAS 3rd July 201
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