Rotation inversions of artificial solarFLAG Sun-as-a-star data (original) (raw)
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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.
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.