Rotation inversions of artificial solarFLAG Sun-as-a-star data (original) (raw)
Monthly Notices of the Royal Astronomical Society, 2006
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 dataset. The dataset 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 modeled 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 targets for asteroseismic campaigns.
Solar Physics, 2000
Observations of the Sun performed at 37 GHz with the 14-m radio telescope of the Metsähovi Radio Observatory were analyzed. Rotation velocities were determined, tracing Low Temperature Regions (LTRs) in the years Statistical weights were ascribed to the determined rotation velocities of LTRs, according to the number of tracing days. Measured changes of the rotation velocity during the solar activity cycle, as well as a north-south rotation asymmetry, are discussed. The results obtained with and without the statistical weights procedure are compared, and it was found that the statistical significance of the solar differential rotation parameters' changes is higher when the statistical weights procedure is applied. A selective application of the height correction on LTR's positions has not removed the cycle-related changes nor the north -south asymmetry of the solar rotation measured tracing LTRs. So, projection effects cannot explain these changes. The differential rotation of LTRs is more rigid than the differential rotation obtained tracing magnetic features and measuring Doppler shifts, which can be explained by the association rate of the LTRs' positions with rigidly rotating 'pivot points'. The observed cycle-related changes and the north -south asymmetry of the rotation velocity of LTRs are consistent with the cycle-related changes and the north -south asymmetry of the association rate between LTRs and pivot points.
The solar internal rotation from GOLF splittings
The low degree splittings obtained from one year of GOLF data analysis are combined with the MDI medium-l 144-day splittings in order to infer the solar internal rotation as a function of the radius down to 0.2Rodot0.2R_\odot0.2Rodot. Several inverse methods are applied to the same data and the uncertainties on the solution as well as the resolution reachable are discussed. The results are compared with the one obtained from the low degree splittings estimated from GONG network.
Astronomy & Astrophysics, 2015
Context. Coronal bright points (CBP) are ubiquitous small brightenings in the solar corona associated with small magnetic bipoles. Aims. We derive the solar differential rotation profile by tracing the motions of CBPs detected by the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). We also investigate problems related to detection of coronal bright points resulting from instrument and detection algorithm limitations.
Measurements of Solar Differential Rotation Using the Century Long Kodaikanal Sunspot Data
2021
The rotational profile of the Sun is considered to be one of the key inputs in a solar dynamo model. Hence, precise and long-term measurements of this quantity is important for our understanding of solar magnetism and its variability. In this study, we use the newly digitized, white light sunspot data (1923 – 2011) from Kodaikanal Solar Observatory (KoSO) to derive the solar rotation profile. An automated correlation based sunspot tracking algorithm is implemented to measure the rotation parameters, A, the equatorial rotation rate and B, the latitudinal gradient. Our measurements of A=14.381±0.004 and B=-2.72±0.04 compare well with previous studies. In our analysis, we find that the bigger sunspots (with area >400 μHem) rotate slower than the smaller ones. At the same time, we do not find any variation in the rotation rates between activity extremes, i.e solar maxima and minima. Lastly, we employ our tracking algorithm on the Michelson Doppler Imager (MDI) data and compare the MD...
Solar differential rotation derived from sunspot observations
Solar Physics, 1990
Sunspot drawings obtained at National Astronomical Observatory of Japan during the years 1954-1986 were used to determine the differential rotation of the Sun. From the limited data set of three solar cycles it was found that three factors (the level of cycle activity, the cycle phase, and sunspot type) affect the solar rotation rate. The differential rotation varies from cycle to cycle in such a way that the rotation velocity in the low activity cycle (cycle 20) is higher than in the high-activity cycle (cycle 19). The equatorial rotation rate shows a systematic variation within each cycle. The rate is higher at the beginning of the cycle and decreases subsequently. Although quite small, the variation of solar differential rotation with respect to Ztirich sunspot type was found. The H and J types show the slowest rotation among all the sunspot types.
Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum (dataset)
2019
The time-variable velocity fields of solar-type stars limit the precision of radialvelocity determinations of their planets' masses, obstructing detection of Earth twins. Since 2015 July we have been monitoring disc-integrated sunlight in daytime using a purpose-built solar telescope and fibre feed to the HARPS-N stellar radial-velocity spectrometer. We present and analyse the solar radial-velocity measurements and cross-correlation function (CCF) parameters obtained in the first 3 years of observation, interpreting them in the context of spatially-resolved solar observations. We describe a Bayesian mixture-model approach to automated data-quality monitoring. We provide dynamical and daily differential-extinction corrections to place the radial velocities in the heliocentric reference frame, and the CCF shape parameters in the sidereal frame. We achieve a photon-noise limited radial-velocity precision better than 0.43 m s -1 per 5-minute observation. The day-to-day precision is limited by zero-point calibration uncertainty with an RMS scatter of about 0.4 m s -1 . We find significant signals from granulation and solar activity. Within a day, granulation noise dominates, with an amplitude of about 0.4 m s -1 and an autocorrelation half-life of 15 minutes. On longer timescales, activity dominates. Sunspot groups broaden the CCF as they cross the solar disc. Facular regions temporarily reduce the intrinsic asymmetry of the CCF. The radial-velocity increase that accompanies an active-region passage has a typical amplitude of 5 m s -1 and is correlated with the line asymmetry, but leads it by 3 days. Spectral line-shape variability thus shows promise as a proxy for recovering the true radial velocity.
solarFLAG hare and hounds: estimation of p-mode frequencies from Sun-as-star helioseismology data
Monthly Notices of the Royal Astronomical Society, 2008
We report on the results of the latest solarFLAG hare-and-hounds exercise, which was concerned with testing methods for extraction of frequencies of low-degree solar p modes from data collected by Sun-as-a-star observations. We have used the new solarFLAG simulator, which includes the effects of correlated mode excitation and correlations with background noise, to make artificial timeseries data that mimic Doppler velocity observations of the Sun as a star. The correlations give rise to asymmetry of mode peaks in the frequency power spectrum. Ten members of the group (the hounds) applied their "peak bagging" codes to a 3456-day dataset, and the estimated mode frequencies were returned to the hare (who was WJC) for comparison. Analysis of the results reveals a systematic bias in the estimated frequencies of modes above ≈ 1.8 mHz. The bias is negative, meaning the estimated frequencies systematically underestimate the input frequencies.
The Astrophysical Journal, 1998
The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale Ñows and rotation permits study of the variation of angular velocity) with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft positioned at the Lagrangian point in continuous sunlight, yield oscillation power spectra with very high signal-to-L 1 noise ratios that allow frequency splittings to be determined with exceptional accuracy. This paper reports on joint helioseismic analyses of solar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving modes of angular degree l extending to about 250) obtained from the Ðrst 144 day interval of SOI-MDI observations in 1996. Drawing inferences about the solar internal rotation from the splitting data is a subtle process. By applying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solutions. Here we have used seven di †erent inversion methods. To test the reliability and sensitivity of these methods, we have performed a set of controlled experiments utilizing artiÐcial data. This gives us some conÐdence in the inferences we can draw from the real solar data. The inversions of SOI-MDI data have conÐrmed that the decrease of) with latitude seen at the surface extends with little radial variation through much of the convection zone, at the base of which is an adjustment layer, called the tachocline, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which) increases just below the surface is discernible at low to mid latitudes. Using the new data, we have also been able to study the solar rotation closer to the poles than has been achieved in previous investigations. The data have revealed that the angular velocity is distinctly lower at high latitudes than the values previously extrapolated from measurements at lower latitudes based on surface Doppler observations and helioseismology. Furthermore, we have found some evidence near latitudes of 75¡ of a submerged polar jet which is rotating more rapidly than its immediate surroundings. Superposed on the relatively smooth latitudinal variation in) are alternating zonal bands of slightly faster and slower rotation, each extending some 10¡ to 15¡ in latitude. These relatively weak banded Ñows have been followed by inversion to a depth of about 5% of the solar radius and appear to coincide with the evolving pattern of "" torsional oscillations ÏÏ reported from earlier surface Doppler studies.
Astronomy and Astrophysics, 2004
Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse solar differential rotation by tracing coronal bright points for the period June 4, 1998 to May 22, 1999. A method for the simultaneous determination of the true solar synodic rotation velocity and the height of the tracers is applied to data sets analysed with interactive and automatic methods. The calculated height of coronal bright points is on average 8000-12000 km above the photosphere. Corrected rotation velocities are transformed into sidereal ones and compared with results from the literature, obtained with various methods and tracers. The differential rotation profile determined by coronal bright points with the interactive method corresponds roughly to the profile obtained by correlating photospheric magnetic fields and the profile obtained from the automatic method corresponds roughly to the rotation of sunspot groups. This result is interpreted in terms of the differences obtained in the latitudinal distribution of coronal bright points using the two methods.
2009
The Sun: A Slowly Rotating Star.- What Is Coming: Issues Raised from Observation of the Shape of the Sun.- Effects of Rotation on Stellar p-Mode Frequencies.- Approaching the Low-Frequency Spectrum of Rotating Stars.- The Rotation of the Solar Core.- Physics of Rotation in Stellar Models.- Long Baseline Interferometry of Rotating Stars Across the HR Diagram: Flattening, Gravity Darkening, Differential Rotation.- Is the Critical Rotation of Be Stars Really Critical for the Be Phenomenon?.- On the Rotation of A-Type Stars.- The Solar Magnetic Field: Surface and Upper Layers, Network and Internetwork Field.
Observations of the Solar Limb Shape Distortions
2003
Two new campaigns devoted to the observation of the solar limb distortions were made at the Pic-du-Midi Observatory, in September 2000 and September 2001, by means of the scanning heliometer. This apparatus can be used now routinely to accurately determine solar limb profiles (at two wavelengths), at any heliographic latitudes. Each measurement is made within 44 milliseconds (of time) which permits to record a limb profile together with the seeing. Scans are automatically rejected for seeing larger than 1.3 arc sec. Such conditions are essential to perform high-quality observations necessary to obtain the quadrupole term (l = 2) in the polynomial expansion of the radius contour R(ψ) | ρ=constant = R 0 1 + l c l P l (ψ). Exceptional meteorological conditions in September 2001 (seeing of the order of 18 cm, for a 50 cm clear aperture of the refractor) enabled us to determine c 2 and c 4 (see Table I) with an accuracy of a few milli-arc-sec. Results indicate a distorted solar shape, the departures from a pure spherical body not exceeding 20 milli-arc-sec. We propose a model to interpret such results (the combination of a nearly uniform rotating core with a prolate solar tachocline and an oblate surface), which is briefly discussed. Our results are confronted to those obtained from space. We conclude that measurements of the quadrupole term from the ground are possible, but of high difficulty and can be obtained only during excellent weather conditions. The hexadecapole term should be only obtained from space. We show that an astrometric satellite would be required, whose mission would be also to accurately determine the solar rotation profiles (both surface and in depth) in order to unambiguously determine the inertia moments of the Sun through the J n terms. Such values are also briefly discussed.
Novel Method for the Evaluation of the Rotation Velocity of Solar-like Stars
viXra, 2017
The evaluation of stars’ properties is classically performed by comparing the stars’ light spectra and fluxes to the known solar properties. Another method to find the stars’ rotation velocity will be discussed here: the extrapolation to the other solar-like stars of an empiric solar property, which consists of a direct relationship between the star’s frequency and its surface gravity, without the need of any other parameters.
About the rotation of the solar radiative interior
Solar Physics, 2004
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. * rgarcia@cea.fr Solar Physics 220: 269-285, 2004.
Rotation period distribution of CoRoT and Kepler Sun-like stars
Astronomy & Astrophysics, 2015
Aims. We study the distribution of the photometric rotation period (P rot), which is a direct measurement of the surface rotation at active latitudes, for three subsamples of Sun-like stars: one from CoRoT data and two from Kepler data. For this purpose, we identify the main populations of these samples and interpret their main biases specifically for a comparison with the solar P rot. Methods. P rot and variability amplitude (A) measurements were obtained from public CoRoT and Kepler catalogs, which were combined with public data of physical parameters. Because these samples are subject to selection effects, we computed synthetic samples with simulated biases to compare with observations, particularly around the location of the Sun in the Hertzsprung-Russel (HR) diagram. Publicly available theoretical grids and empirical relations were used to combine physical parameters with P rot and A. Biases were simulated by performing cutoffs on the physical and rotational parameters in the same way as in each observed sample. A crucial cutoff is related with the detectability of the rotational modulation, which strongly depends on A. Results. The synthetic samples explain the observed P rot distributions of Sun-like stars as having two main populations: one of young objects (group I, with ages younger than ∼ 1 Gyr) and another of main-sequence and evolved stars (group II, with ages older than ∼ 1 Gyr). The proportions of groups I and II in relation to the total number of stars range within 64-84% and 16-36%, respectively. Hence, young objects abound in the distributions, producing the effect of observing a high number of short periods around the location of the Sun in the HR diagram. Differences in the P rot distributions between the CoRoT and Kepler Sun-like samples may be associated with different Galactic populations. Overall, the synthetic distribution around the solar period agrees with observations, which suggests that the solar rotation is normal with respect to Sun-like stars within the accuracy of current data.
The Astrophysical Journal, 2005
Improving the accuracy and resolution of helioseismic inversions calls for more accurate modeling of the observational p-mode power spectra from which the solar oscillation frequencies are traditionally measured. We present a new technique of calculating the response function ( leakage matrix) for Doppler velocity measurements that is based largely on an analytical description of the relevant instrumental and physical effects. The computational efficiency of the new approach allows us to implement the response function in an adaptive manner: i.e., the compensation for instrumental or optical distortions of unknown magnitude can be performed as a part of the spectral fitting procedure.
Rotation of Solar Analogs Crossmatching Kepler and Gaia DR2
The Astrophysical Journal
A major obstacle to interpreting the rotation period distribution for main-sequence stars from Kepler mission data has been the lack of precise evolutionary status for these objects. We address this by investigating the evolutionary status based on Gaia Data Release 2 parallaxes and photometry for more than 30,000 Kepler stars with rotation period measurements. Many of these are subgiants, and should be excluded in future work on dwarfs. We particularly investigate a 193-star sample of solar analogs, and report newly-determined rotation periods for 125 of these. These include 54 stars from a prior sample, of which can confirm the periods for 50. The remainder are new, and 10 of them longer than solar rotation period, suggesting that sun-like stars continue to spin down on the main sequence past solar age. Our sample of solar analogs could potentially serve as a benchmark for future missions such as PLATO, and emphasizes the need for additional astrometric, photometric, and spectroscopic information before interpreting the stellar populations and results from time-series surveys.