Searching for g modes Appourchaux, T.; Boumier, P.; Leibacher, J. W. ...
Astronomy and astrophysics (Berlin),
09/2018, Volume:
617
Journal Article
Peer reviewed
Open access
Context
. The recent claims of
g
-mode detection have restarted the search for these potentially extremely important modes. These claims can be reassessed in view of the different data sets available ...from the SoHO instruments and ground-based instruments.
Aims
. We produce a new calibration of the GOLF data with a more consistent
p
-mode amplitude and a more consistent time shift correction compared to the time series used in the past.
Methods
. The calibration of 22 yr of GOLF data is done with a simpler approach that uses only the predictive radial velocity of the SoHO spacecraft as a reference. Using
p
modes, we measure and correct the time shift between ground- and space-based instruments and the GOLF instrument.
Results
. The
p
-mode velocity calibration is now consistent to within a few percent with other instruments. The remaining time shifts are within ±5 s for 99.8% of the time series.
Issue Title: The physics of solar and solar-like oscillations; guest editors T. Bedding and J. Leibacher In the modern era of helioseismology we have a wealth of high-quality data available, e.g., ...more than 6 years of data collected by the various instruments on board the SOHO mission, and an even more extensive ground-based set of observations covering a full solar cycle. Thanks to this effort a detailed picture of the internal rotation of the Sun has been constructed. In this paper we present some of the actions that should be done to improve our knowledge of the inner rotation profile discussed during the workshop organized at Saclay on June 2003 on this topic. In particular we will concentrate on the extraction of the rotational frequency splittings of low- and medium-degree modes and their influence on the rotation of deeper layers. Furthermore, for the first time a full set of individual |m|-component rotational splittings is computed for modes ≤4 and 1<ν<2 mHz, opening new studies on the latitudinal dependence of the rotation rate in the radiative interior. It will also be shown that these splittings have the footprints of the differential rotation of the convective zone which can be extremely useful to study the differential rotation of other stars where only these low-degree modes will be available.PUBLICATION ABSTRACT
Acoustic rates of excitation of solar p modes can be estimated from observations in order to place constraints on the modelling of the excitation process and the layers where it occurs in the star. ...For several reasons (including a poor signal to noise ratio and mode overlap), this estimation is difficult. In this work, we use three completely independent datasets to obtain robust estimates in the solar case for $\ell=1$ modes. We also show that the height in the solar atmosphere where the modes are observed must be taken into account. Our three sets of results are shown to be consistent, particularly in the lower part of the p-mode spectrum (from 1.8 mHz to 2.8 mHz). At higher frequencies, the agreement is not as good, because of a larger dispersion of the measurements and also because of some systematic differences which might be due to observation height estimation or to a systematic influence of the noise.
Aims.Long-lived stellar oscillation modes are usually detected using Fourier transforms of time series of stellar radial velocities or brightness. It is commonly thought that one could use the cross ...spectrum of the two signals, or alternatively use the interleaved series of a single signal, to considerably improve the detection level by reducing the noise level. Methods.We use a statistical analysis of the cross spectrum to compute its mean value and rms value, and use the associated signal-to-noise ratio for stochastically excited modes. Results.Here, we demonstrate that the gain in the signal-to-noise ratio can indeed be improved for modes with a shorter lifetime than the observation time, but not those with a longer lifetime than the observation time.
Solar flares presumably have an impact on the deepest layers of the solar atmosphere and yet the observational evidence for such an impact is scarce. Using ten years of measurements of the Na D
1
and ...Na D
2
Fraunhofer lines, measured by GOLF onboard SOHO, we show that this photospheric line is indeed affected by flares. The effect of individual flares is hidden by solar oscillations, but a statistical analysis based on conditional averaging reveals a clear signature. Although GOLF can only probe one single wavelength at a time, we show that both wings of the Na line can nevertheless be compared. The varying line asymmetry can be interpreted as an upward plasma motion from the lower solar atmosphere during the peak of the flare, followed by a downward motion.
Context. CoRoT is a space telescope dedicated to stellar seismology and the search for extrasolar planets. The mission is led by the CNES in association with French laboratories and has a large ...international participation. The European Space Agency (ESA), Austria, Belgium, and Germany contribute to the payload, and Spain and Brazil contribute to the ground segment. Development of the spacecraft, which is based on a PROTEUS low earth orbit (LEO) recurrent platform, commenced in October 2000, and the satellite was launched on December 27, 2006. Aims. The instrument and platform characteristics prior to launch have been described in ESA publication (SP-1306). In the present paper we explain the behaviour in flight, based on raw and corrected data. Methods. Five runs have been completed since January 2007. The data used here are essentially those acquired during the commissioning phase and from a long run that lasted 146 days. These enable us to give a complete overview of the instrument and platform behaviour for all environmental conditions. The ground based data processing is not described in detail because the most important method has been published elsewhere. Results. We show that the performance specifications are easily satisfied when the environmental conditions are favourable. Most of the perturbations, hence data corrections, are related to LEO perturbations: high energy particles inside the South Atlantic Anomaly (SAA), eclipses and temperature variations, and line of sight fluctuations due to the attitude control system. Straylight due to the reflected light from the earth, which is controlled by the telescope and baffle design, appears to be negligible.
This study was conducted to identify early predictors of the total cost of inflammatory arthritis (IA). One hundred and eighty patients affected by undifferentiated arthritis (UA) or rheumatoid ...arthritis (RA) were included in the French Very Early rheumatoid Arthritis (VErA) cohort between 1998 and 2001. Health economic data for 2003 were collected using a patient self‐questionnaire. Results were analysed in terms of direct, indirect and total costs in 2003 euros (2003€) for the population as a whole and in diagnostic subgroups. A payor perspective (the French National Health Insurance, in this case) was adopted. Multiple linear regression models were used to identify predictors of total cost from among the criteria assessed on recruitment. Results of the study showed that for the study population as a whole, the mean total cost was €4700 per patient. The costs attributable to the RA and UA sub‐groups were €5928 and €2424 per patient, respectively. In a univariate analysis, certain parameters were significantly correlated with a higher cost of illness. In the multivariate analysis, some of these parameters were further identified as being predictive of higher cost. Two strong significant, early predictors of total cost were identified: higher pain (P = 0.002) and the presence of rheumatoid factor (P = 0.004). In the RA sub‐group, lower grip strength of the dominant hand (P = 0.039) was another predictor of the illness’s subsequent economic impact. In conclusion, our data show that simple clinical and laboratory parameters can be used early in the course of IA to predict the condition’s impact on healthcare budgets.
We report the observations of a main-sequence star, HD 42618 (Teff 5765 K, G3V) by the space telescope CoRoT. This is the closest star to the Sun ever observed by CoRoT in term of its fundamental ...parameters. Using a preliminary version of CoRoT light curves of HD 42618, p modes are detected around 3.2 mHz associated to ℓ 0, 1 and 2 modes with a large spacing of 142 μHz. Various methods are then used to derive the mass and radius of this star (scaling relations from solar values as well as comparison between theoretical and observationnal frequencies) giving values in the range of (0.80 − 1.02)M⊙ and (0.91 − 1.01)R⊙. A preliminary analysis of ℓ 0 and 1 modes allows us also to study the amount of penetrative convection at the base of the convective envelope.
Context.Photometry of Procyon obtained by the MOST satellite in 2004 has been searched for p modes by several groups, with sometimes contradictory interpretations. Aims.We explore two possible ...factors that complicate the analysis and may lead to erroneous reports of p modes in these data. Methods.Two methods are used to illustrate the role of subtle instrumental effects in the photometry: time-frequency analysis, and a search for regularly spaced peaks in a Fourier spectrum based on the echelle diagramme approach. Results.We find no convincing evidence of a p-mode signal in the MOST Procyon data. We can account for an apparent excess of power close to the p-mode frequency range and signs of structure in an echelle diagramme in terms of instrumental effects.
We report on results from the first solar Fitting at Low-Angular degree Group (solar FLAG) hare-and-hounds exercise. The group is concerned with the development of methods for extracting the ...parameters of low-l solar p-mode data (‘peak bagging’), collected by Sun-as-a-star observations. Accurate and precise estimation of the fundamental parameters of the p modes is a vital pre-requisite of all subsequent studies. Nine members of the FLAG (the ‘hounds’) fitted an artificial 3456-d data set. The data set was made by the ‘hare’ (WJC) to simulate full-disc Doppler velocity observations of the Sun. The rotational frequency splittings of the l= 1, 2 and 3 modes were the first parameter estimates chosen for scrutiny. Significant differences were uncovered at l= 2 and 3 between the fitted splittings of the hounds. Evidence is presented that suggests this unwanted bias had its origins in several effects. The most important came from the different way in which the hounds modelled the visibility ratio of the different rotationally split components. Our results suggest that accurate modelling of the ratios is vital to avoid the introduction of significant bias in the estimated splittings. This is of importance not only for studies of the Sun, but also of the solar analogues that will be targets for asteroseismic campaigns.
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