1,993 research outputs found
On deriving p-mode parameters for inclined solar-like stars
Thanks to their high quality, new and upcoming asteroseismic observations -
with CoRoT, Kepler, and from the ground... - can benefit from the experience
gained with helioseismology. We focus in this paper on solar-like oscillations,
for which the inclination of the rotation axis is unknown. We present a
theoretical study of the errors of p-mode parameters determined by means of a
maximum-likelihood estimator, and we also analyze correlations and biases. We
have used different, complementary approaches: we have performed either
semi-analytical computation of the Hessian matrix, fitting of single mean
profiles, or Monte Carlo simulations. We give first analytical approximations
for the errors of frequency, inclination and rotational splitting. The
determination of the inclination is very challenging for the common case of
slow rotators (like the Sun), making difficult the determination of a reliable
rotational splitting. Moreover, due to the numerous correlations, biases - more
or less significant - can appear in the determination of various parameters in
the case of bad inclination fittings, especially when a locking at 90 degrees
occurs. This issue concerning inclination locking is also discussed.
Nevertheless, the central frequency and some derived parameters such as the
total power of the mode are free of such biases.Comment: 9 pages, 6 figures, to appear in A&
Spectroscopic measurements of temperature and plasma impurity concentration during magnetic reconnection at the Swarthmore Spheromak Experiment
Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT [J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications (2004)] to yield the electron temperature in the plasma with 1 µs time resolution. Average T_e is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion Doppler spectrometer (IDS) [C. D. Cothran et al., Rev. Sci. Instrum. 77, 063504 (2006)], likewise rises during spheromak merging, peaking at ~22 eV, but a similar increase in T_i is seen during single spheromak discharges with no merging. The VUV emission line measurements are also used to constrain the concentrations of various impurities in the SSX plasma, which are dominated by carbon, but include some oxygen and nitrogen. A burst of soft x-ray emission is seen during reconnection with a new four-channel detector (SXR). There is evidence for spectral changes in the soft x-ray emission as reconnection progresses, although our single-temperature equilibrium spectral models are not able to provide adequate fits to all the SXR data
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 Octave (Birmingham - Sheffield Hallam) automated pipeline for extracting oscillation parameters of solar-like main-sequence stars
The number of main-sequence stars for which we can observe solar-like
oscillations is expected to increase considerably with the short-cadence
high-precision photometric observations from the NASA Kepler satellite. Because
of this increase in number of stars, automated tools are needed to analyse
these data in a reasonable amount of time. In the framework of the asteroFLAG
consortium, we present an automated pipeline which extracts frequencies and
other parameters of solar-like oscillations in main-sequence and subgiant
stars. The pipeline uses only the timeseries data as input and does not require
any other input information. Tests on 353 artificial stars reveal that we can
obtain accurate frequencies and oscillation parameters for about three quarters
of the stars. We conclude that our methods are well suited for the analysis of
main-sequence stars, which show mainly p-mode oscillations.Comment: accepted by MNRA
Gender, Anxiety, And Depressive Symptoms: A Longitudinal Study Of Early Adolescents
Does anxiety lead to depression more for girls than for boys? This study prospectively examines gender differences in the relationship between anxiety and depressive symptoms in early adolescence. One hundred thirteen 11-to 14-year-old middle school students complete questionnaires assessing depressive symptoms and three dimensions of anxiety (worry and oversensitivity, social concerns and concentration, and physiological anxiety) as well as total anxiety symptoms at an initial assessment and 1 year later. Total anxiety and worry and oversensitivity symptoms are found to predict later depressive symptoms more strongly for girls than for boys. There is a similar pattern of results for social concerns and concentration symptoms, although this does not reach statistical significance. Physiological anxiety predicts later depressive symptoms for both boys and girls. These findings highlight the importance of anxiety for the development of depression in adolescence, particularly worry and oversensitivity among girls
A thorough analysis of the short- and mid-term activity-related variations in the solar acoustic frequencies
The frequencies of the solar acoustic oscillations vary over the activity
cycle. The variations in other activity proxies are found to be well correlated
with the variations in the acoustic frequencies. However, each proxy has a
slightly different time behaviour. Our goal is to characterize the differences
between the time behaviour of the frequency shifts and of two other activity
proxies, namely, the area covered by sunspots and the 10.7cm flux. We define a
new observable that is particularly sensitive to the short-term frequency
variations. We then compare the observable when computed from model frequency
shifts and from observed frequency shifts obtained with the Global Oscillation
Network Group (GONG) for cycle 23. Our analysis shows that on the shortest
time-scales the variations in the frequency shifts seen in the GONG
observations are strongly correlated with the variations in the area covered by
sunspots. However, a significant loss of correlation is still found. We verify
that the times when the frequency shifts and the sunspot area do not vary in a
similar way tend to coincide with the times of the maxima of the quasi-biennial
variations seen in the solar seismic data. A similar analysis of the relation
between the 10.7cm flux and the frequency shifts reveals that the short-time
variations in the frequency shifts follow even more closely those of the 10.7cm
flux than those of the sunspot area. However, a loss of correlation between
frequency shifts and 10.7cm flux variations is still found around the same
times.Comment: 7 pages, 6 figures, accepted for publication in MNRA
On the relation between activity-related frequency shifts and the sunspot distribution over the solar cycle 23
The activity-related variations in the solar acoustic frequencies have been
known for 30 years. However, the importance of the different contributions is
still not well established. With this in mind, we developed an empirical model
to estimate the spot-induced frequency shifts, which takes into account the
sunspot properties, such as area and latitude. The comparison between the model
frequency shifts obtained from the daily sunspot records and those observed
suggests that the contribution from a stochastic component to the total
frequency shifts is about 30%. The remaining 70% is related to a global,
long-term variation. We also propose a new observable to investigate the short-
and mid-term variations of the frequency shifts, which is insensitive to the
long-term variations contained in the data. On the shortest time scales the
variations in the frequency shifts are strongly correlated with the variations
in the total area covered by sunspots. However, a significant loss of
correlation is still found, which cannot be fully explained by ignoring the
invisible side of the Sun when accounting for the total sunspot area. We also
verify that the times when the frequency shifts and the sunspot areas do not
vary in a similar way tend to coincide with the times of the maximum amplitude
of the quasi-biennial variations found in the seismic data.Comment: 4 pages, 2 figures, proceedings of the Joint TASC2 - KASC9 Workshop -
SPACEINN - HELAS8 Conference "Seismology of the Sun and the Distant Stars
2016: Using Today's Successes to Prepare the Future". To be published by the
EPJ Web of Conference
Total cross sections for positrons scattered elastically from helium based on new measurements of total ionization cross sections
An improved technique is presented for employing the 2.3m spectrometer to measure total ionization cross sections, Q sub ion, for positrons incident on He. The new ionization cross section agree with the values reported earlier. Estimates are also presented of total elastic scattering cross section, Q sub el, obtained by subtracting from total scattering cross sections, Q sub tot, reported in the literature, the Q sub ion and Q sub Ps (total positronium formation cross sections) and total excitation cross sections, Q sub ex, published by another researcher. The Q sub ion and Q sub el measured with the 3m high resolution time-of-flight spectrometer for 54.9eV positrons are in accord with the results from the 2.3m spectrometer. The ionization cross sections are in fair agreement with theory tending for the most part to be higher, especially at 76.3 and 88.5eV. The elastic cross section agree quite well with theory to the vicinity of 50eV, but at 60eV and above the experimental elastic cross sections climb to and remain at about 0.30 pi a sub o sq while the theoretical values steadily decrease
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