Dynamics of the solar granulation: VIII. Time and space development
TL;DR: In this paper, the authors studied the evolution of the granulation dynamics from the observational point of view using spectrograms taken at the VTT, Observatorio del Teide (Tenerife), in 1999.
Abstract: We study the evolution of the granulation dynamics from the observational point of view. Based on series of excellent spectrograms taken at the VTT, Observatorio del Teide (Tenerife), in 1999, we calculated temporal - spatial maps of the Doppler velocity, line width, and intensity in order to track the dynamical behavior of these observables at different positions along the spectrograph slit. The Doppler velocity map reveals a granular dynamical time - the characteristic time associated with the decay of the Doppler velocity - of approximately 2 min, while the line width map does not show any characteristic time scale but rather a strong intermittence. The intensity map reveals the life time of the granulation as it is given in the literature. The granular dynamical time is practically equal to the value determined from spectrograms taken at the solar minimum 1994; so the dynamical time does not show any change over the solar cycle. The stochastic properties of the Doppler velocity and intensity data samples are studied (i) by means of their statistical moments and (ii) theoretically using presupposed model distributions. For the latter we estimated the distributions' parameters by means of the maximum likelihood method. The histograms of the Doppler velocity variations point to an asymmetric model distribution, while the histograms of the intensity variations infer a symmetric one. The intensity variations can be described well by a Gaussian probability density function, while the Doppler velocity variations are described by the double exponential (Gumbel) distribution, an asymmetric probability function. A remarkable result of the statistical analysis based on both series of observations in 1994 and 1999 is the unambiguous lack of flows with large velocity amplitudes within the intergranular space.
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TL;DR: Radiative-hydrodynamical simulations of solar surface convection can be used as 2D/3D time-dependent models of the solar atmosphere to predict the emergent spectrum, and the resulting detailed spectral line profiles agree spectacularly well with observations without invoking any micro- and macroturbulence parameters.
Abstract: We review the properties of solar convection that are directly observable at the solar surface, and discuss the relevant underlying physics, concentrating mostly on a range of depths from the temperature minimum down to about 20 Mm below the visible solar surface. The properties of convection at the main energy carrying (granular) scales are tightly constrained by observations, in particular by the detailed shapes of photospheric spectral lines and the topology (time- and length-scales, flow velocities, etc.) of the up- and downflows. Current supercomputer models match these constraints very closely, which lends credence to the models, and allows robust conclusions to be drawn from analysis of the model properties. At larger scales the properties of the convective velocity field at the solar surface are strongly influenced by constraints from mass conservation, with amplitudes of larger scale horizontal motions decreasing roughly in inverse proportion to the scale of the motion. To a large extent, the apparent presence of distinct (meso- and supergranulation) scales is a result of the folding of this spectrum with the effective “filters” corresponding to various observational techniques. Convective motions on successively larger scales advect patterns created by convection on smaller scales; this includes patterns of magnetic field, which thus have an approximately self-similar structure at scales larger than granulation. Radiative-hydrodynamical simulations of solar surface convection can be used as 2D/3D time-dependent models of the solar atmosphere to predict the emergent spectrum. In general, the resulting detailed spectral line profiles agree spectacularly well with observations without invoking any micro- and macroturbulence parameters due to the presence of convective velocities and atmosphere inhomogeneities. One of the most noteworthy results has been a significant reduction in recent years in the derived solar C, N, and O abundances with far-reaching consequences, not the least for helioseismology. Convection in the solar surface layers is also of great importance for helioseismology in other ways; excitation of the wave spectrum occurs primarily in these layers, and convection influences the size of global wave cavity and, hence, the mode frequencies. On local scales convection modulates wave propagation, and supercomputer convection simulations may thus be used to test and calibrate local helioseismic methods. We also discuss the importance of near solar surface convection for the structure and evolution of magnetic patterns: faculae, pores, and sunspots, and briefly address the question of the importance or not of local dynamo action near the solar surface. Finally, we discuss the importance of near solar surface convection as a driver for chromospheric and coronal heating.
363 citations
TL;DR: In this paper, the authors used realistic 3D radiative hydrodynamical (RHD) simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet.
Abstract: Context. Stellar activity and convection-related surface structures might cause bias in planet detection and characterization that use these transits. Surface convection simulations help to quantify the granulation signal.Aims. We used realistic three-dimensional (3D) radiative hydrodynamical (RHD) simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet.Methods. We computed intensity maps from RHD simulations of the Sun and a K-dwarf star at different wavelength bands from optical to far-infrared that cover the range of several ground- and space-based telescopes which observe exoplanet transits. We modeled the transit using synthetic stellar-disk images obtained with a spherical-tile imaging method and emulated the temporal variation of the granulation intensity generating random images covering a granulation time-series of 13.3 h. We measured the contribution of the stellar granulation on the light curves during the planet transit.Results. We identified two types of granulation noise that act simultaneously during the planet transit: (i) the intrinsic change in the granulation pattern with timescale (e.g., 10 min for solar-type stars assumed in this work) is smaller than the usual planet transit (~hours as in our prototype cases); and (ii) the fact that the transiting planet occults isolated regions of the photosphere that differ in local surface brightness as a result of convective-related surface structures. First, we showed that our modeling approach returns granulation timescale fluctuations that are comparable with what has been observed for the Sun. Then, our statistical approach shows that the granulation pattern of solar and K-dwarf-type stars have a non-negligible effect of the light curve depth during the transit, and, consequentially on the determination of the planet transit parameters such as the planet radius (up to 0.90% and ~0.47% for terrestrial and gaseous planets, respectively). We also showed that larger (or smaller) orbital inclination angles with respect to values corresponding to transit at the stellar center display a shallower transit depth and longer ingress and egress times, but also granulation fluctuations that are correlated to the center-to-limb variation: they increase (or decrease) the value of the inclination, which amplifies the fluctuations. The granulation noise appears to be correlated among the different wavelength ranges either in the visible or in the infrared regions.Conclusions. The prospects for planet detection and characterization with transiting methods are excellent with access to large amounts of data for stars. The granulation has to be considered as an intrinsic uncertainty (as a result of stellar variability) on the precise measurements of exoplanet transits of planets. The full characterization of the granulation is essential for determining the degree of uncertainty on the planet parameters. In this context, the use of 3D RHD simulations is important to measure the convection-related fluctuations. This can be achieved by performing precise and continuous observations of stellar photometry and radial velocity, as we explained with RHD simulations, before, after, and during the transit periods.
46 citations
Cites background or methods from "Dynamics of the solar granulation: ..."
...We emulated the temporal variation of the granulation intensity, which is∼10 minutes (Nesis et al. 2002) for the Sun, generating random images that cover a granulation time-series of 13.3 hours....
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...We aim to emulate the temporal variation of the granulation in- tensity, which shows a timescale on the order of∼10 minutes (Nesis et al. 2002) for the Sun....
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...The granulation images were averaged over the Nterrestrial = 42 different realizations, whereN = ∆tσSun, ∆t is the transit duration andσSun is the observed granulation fluctuation timescale for the Sun, which is∼10 minutes (Nesis et al. 2002)....
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TL;DR: In this article, a correlation analysis of the brightness and velocity in the solar photosphere between the levels of formation of the continuum radiation and the temperature minimum was performed using the German Vacuum Tower Telescope in Izana, Spain.
Abstract: The granulation brightnesses and convective velocities in the solar photosphere between the levels of formation of the continuum radiation and the temperature minimum are examined. Spectral images of the granulation observed in lines of neutral and ionized iron with high spatial (0.5″) and temporal (9 s) resolutions were obtained using the German Vacuum Tower Telescope in Izana (Tenerife, Spain). A correlation analysis shows that the granules and intergranules change their relative brightness at a height near 250 km, and a general reversal of the velocity occurs near a height of 490 km, where the material above granules begins to predominantly descend, and the material above intergranules, to ascend. The maximum correlation coefficient between the velocity and the line brightnesdoesnot exceed 0.75. The properties of the brightness and velocity are analyzed in a sixteen-column model. Four sorts of motions are most typical and efficient. In the first two, only the sign of the relative contrast of the material changes (an efficiency of 46%). This occurs, on average, at a height of 270 km. In the last two motions, both the sign of the contrast and the direction of the motion are reversed near a height of 350 km (an efficiency of 28%). All the observed dependences are compared with theoretical relations obtained in a three-dimensional hydrodynamical model, with deviations from local thermodynamic equilibrium included in the calculation of the spectral-line profiles. This model can satisfactorily reproduce all the basic features of the convective velocities and intensities. It is concluded that the convective motions maintain their column structure throughout the photosphere, right to the level of the temperature minimum. This makes a separation of the photosphere into two regions with different granulation brightnesses and convective motions unjustified.
32 citations
TL;DR: In this paper, the authors used realistic three-dimensional radiative hydrodynamical simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune and a terrestrial planet.
Abstract: Stellar activity and convection-related surface structures might cause bias in planet detection and characterization that use these transits. Surface convection simulations help to quantify the granulation signal. We used realistic three-dimensional radiative hydrodynamical simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet. We computed intensity maps from RHD simulations of the Sun and a K-dwarf star at different wavelength bands from optical to far-infrared. We modeled the transit using synthetic stellar-disk images and emulated the temporal variation of the granulation intensity. We identified two types of granulation noise that act simultaneously during the planet transit: (i) the intrinsic change in the granulation pattern with timescales smaller than the usual planet transit, and (ii) the fact that the transiting planet occults isolated regions of the photosphere that differ in local surface brightness. Our modeling approach shows that the granulation pattern has a non-negligible effect on the light curve depth during the transit, and, consequentially on the determination of the radius of the planet transiting. The granulation noise appears to be correlated among the different wavelength ranges either in the visible or in the infrared regions. The granulation has to be considered as an intrinsic uncertainty (as a result of stellar variability) on the precise measurements of exoplanet transits of planets. The full characterization of the granulation is essential for determining the degree of uncertainty on the planet parameters. In this context, the use of 3D RHD simulations is important to measure the convection-related fluctuations.
24 citations
TL;DR: In this paper, the authors used the extensive solar observations obtained with SoHO/VIRGO to characterize the statistical properties of the flicker noise and quantify its impact on the inferred transit parameters.
Abstract: Context. In photometry, the short-timescale stellar variability (“flicker”), such as that caused by granulation and solar-like oscillations, can reach amplitudes comparable to the transit depth of Earth-sized planets and is correlated over the typical transit timescales. It can introduce systematic errors on the inferred planetary parameters when a small number of transits are observed.Aims. The objective of this paper is to characterize the statistical properties of the flicker noise and quantify its impact on the inferred transit parameters.Methods. We used the extensive solar observations obtained with SoHO/VIRGO to characterize flicker noise. We simulated realistic transits across the solar disk using SDO/HMI data and used these to obtain transit light curves, which we used to estimate the errors made on the transit parameters due to the presence of real solar noise. We make these light curves publicly available. To extend the study to a wider parameter range, we derived the properties of flicker noise using Kepler observations and studied their dependence on stellar parameters. Finally, we predicted the limiting stellar apparent magnitude for which the properties of the flicker noise can be extracted using high-precision CHEOPS and PLATO observations.Results. Stellar granulation is a stochastic colored noise, and is stationary with respect to the stellar magnetic cycle. Both the flicker correlation timescales and amplitudes increase with the stellar mass and radius. If these correlations are not taken into account when fitting for the parameters of transiting exoplanets, this can bias the inferred parameters. In particular, we find errors of up to 10% on the ratio between the planetary and stellar radius (R p ∕R s ) for an Earth-sized planet orbiting a Sun-like star.Conclusions. Flicker will significantly affect the inferred parameters of transits observed at high precision with CHEOPS and PLATO for F and G stars. Dedicated modeling strategies need to be developed to accurately characterize both the star and the transiting exoplanets.
21 citations
References
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01 Jan 1999TL;DR: The author of the book Statistical Distributions in Engineering fulfils its objectives as a reference but the number of included distributions could be extended and the material is highly condensed but it is informative and comprehensive.
Abstract: The author of the book Statistical Distributions in Engineering has stated two basic objectives: text and reference. The book fulfils its objectives as a reference but the number of included distributions could be extended. As a reference, the book could serve a great variety of readers, not limited to engineers only, but to all kind of readers involved in the application of statistics and statistical modelling. I cannot see the book as a text, especially as a primary text. As a supplementary text it could serve as a good and comprehensive reference. I highly appreciate the style, structure and format of the material presentation. The material is highly condensed but it is informative and comprehensive. Its structure allows relevant topics to be searched easily and the uniform format of presentation allows comparision between the properties of different distributions. The large number of well-selected examples illustrates the use of the given probability distribution and also some techniques of building statistical models. I found the examples highly useful. D Christozov
259 citations
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...…method is based on the intuitive notion that the point estimate θ̂ of an unknown parameter θ should be chosen so that the likelihood L(θ) of a given sample x is maximized, i.e. the slope of the function is zero (see Bury 1999): ∂L(θ) ∂θ ∣∣∣∣∣ θ̂ = 0, (1) or equivalently, ∂ ln L(θ) ∂θ ∣∣∣∣∣ θ̂ = 0 ....
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TL;DR: In this article, the properties of the evolution of solar granulation have been studied using an 80 minute time series of high-resolution white-light images obtained with the Swedish Vacuum Solar Telescope at the Observatorio del Roque de los Muchachos, La Palma.
Abstract: The properties of the evolution of solar granulation have been studied using an 80 minute time series of high spatial resolution white-light images obtained with the Swedish Vacuum Solar Telescope at the Observatorio del Roque de los Muchachos, La Palma. An automatic tracking algorithm has been developed to follow the evolution of individual granules, and a sample of 2643 granules has been analyzed. To check the reliability of this automatic procedure, we have manually tracked a sample of 481 solar granules and compared the results of both procedures. An exponential law gives a good fit to the distribution of granular lifetimes, T. Our estimated mean lifetime is about 6 minutes, which is at the lower limit of the ample range of values reported in the literature. We note a linear increase in the time-averaged granular sizes and intensities with the lifetime. T=12 minutes marks a sizeable change in the slopes of these linear trends. Regarding the location of granules with respect to the meso- and supergranular flow field, we find only a small excess of long-lived granules in the upflows. Fragmentation, merging, and emergence from (or dissolution into) the background are the birth and death mechanisms detected, resulting in nine granular families from the combination of these six possibilities. A comparative study of these families leads to the following conclusions: (1) fragmentation is the most frequent birth mechanism, while merging is the most frequent death mechanism; (2) spontaneous emergence from the background occurs very rarely, but dissolution into the background is much more frequent; and (3) different granular mean lifetimes are determined for each of these families; the granules that are born and die by fragmentation have the longest mean lifetime (9.23 minutes). From a comparison of the evolution of granules belonging to the most populated families, two critical values appear for the initial area in a granular evolution: 0.8 Mm2 (dg=139) and 1.3 Mm2 (dg=177). These values mark limits characterizing the birth mechanism of a granule, and predict its evolution to some extent. The findings of the present work complement the earlier results presented in this series of papers and reinforce with new inputs, as far as the evolutionary aspects are concerned, the conclusion stated there that granules can be classified into two populations with different underlying physics. The boundary between these two classes could be established at the scale of dg=14.
80 citations
"Dynamics of the solar granulation: ..." refers background or result in this paper
...Hirzberger et al. (1999), studying the evolution properties of granulation patterns, found that such spontaneous emergence of individual granules is a rare phenomenon....
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...- Bright granule structures (yellow to red color) exhibit a life time of 7– 9 min, in agreement with previous results (cf. Mehltretter 1978, Hirzberger et al. 1999)....
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01 Jan 2001
TL;DR: In this paper, the theories of solar convection, rotation, and activity were discussed, and the authors proposed a method for measuring near-surface flow fields using correlation tracking and time-distance analysis.
Abstract: Preface. I: Theories of Solar Convection, Rotation, and Activity. Towards Understanding Solar Convection and Activity D. Gough. Fluid Dynamics and MHD of the Solar Convection Zone and Tachocline: Current Understanding and Unsolved Problems P.A. Gilman. Can We Get the Bottom B? A. Ruzmaikin. The Coupling of Solar Convection and Rotation M.S. Miesch. Realistic Solar Convection Simulations R.F. Stein, A. Nordlund. Solar Magnetoconvection N.E. Hurlburt, et al. The Solar Dynamo and Emerging Flux G.H. Fisher, et al. On the Twist of Emerging Flux Loops in the Solar Convection Zone Y. Fan, D. Gong. II: Helioseismic Tomography. Time-Distance Inversion Methods and Results A.G. Kosovichev, et al. Time-Distance Helioseismology with f Modes as a Method for Measurement of Near-Surface Flows T.S. Duvall Jr., L. Gizon. Travel Time Sensitivity Kernels A.C. Birch, A.G. Kosovichev. Calculation of the Sun's Acoustic Impulse Response by Multi-Dimensional Spectral Factorization J.E. Rickett, J.F. Claerbout. Ray Travel Time and Distance for the Planar Polytrope G.H. Price. Sensitivity Kernels for Time-Distance Inversion J.M. Jensen, et al. III: Acoustic Imaging and Holography. Acoustic Imaging of Solar Active Regions D.-Y. Chou. Basic Principles of Solar Acoustic Holography C. Lindsey, D.C. Braun. Helioseismic Holography of Active-Region Subphotospheres D.C. Braun, C. Lindsey. Phase-Sensitive Holography of Solar Activity D.C. Braun, C. Lindsey. Stochastic Seismic Emission from Acoustic Glories and the Quiet Sun A.-C. Donea, et al. IV: Ring-Diagram Analysis. Solar Shear Flow Deduced from Helioseismic Dense-Pack Samplings of RingDiagrams D.A. Haber, et al. Near-Surface Flow Fields Deduced Using Correlation Tracking and Time-Distance Analyses M. de Rosa, et al. Local Fractional Frequency Shifts Used as Tracers of Magnetic Activity B. Hindman, et al. V: Magnetic Fields and Oscillations. Sunspot Oscillations: A Review T.J. Bogdan. Modelling p-Mode Interaction with a Spreading Sunspot Field P.S. Cally. Measuring Magnetic Oscillations in the Solar Photosphere: Coordinated Observations with MDI, ASP and MWO A.A. Norton, R.K. Ulrich. Interaction between Network and Intranetwork Magnetic Fields J. Zhang, et al. VI: Solar-Cycle Variations of the Internal Structure and Rotation. Variations in Solar Sub-Surface Rotation from Gong. Data 1995-1998 R. Howe, et al. Time Variability of Rotation in Solar Convection Zone from SOI-MDI J. Toomre, et al. Possible Solar Cycle Variations in the Convection Zone S. Basu, H.M. Antia. Solar Cycle Variation in Solar f-Mode Frequencies and Radius H.M. Antia, et al. Solar Cycle Variations of Large-Scale Flows in the Sun S. Basu, H.M. Antia. Does the Tachocline Show Solar Cycle Related Changes? S. Basu, J. Schou. Empirical Estimate of p-Mode Frequency Shift for Solar Cycle 23 K. Jain, et al.
14 citations
TL;DR: In this article, the authors investigate the time dependence of the convective flows within a regular and an exploding granule and show the dynamical change of both granules at several heights within the first 200 km.
Abstract: The emergence and evolution of large granules shows thegranular dynamics particularly well. We therefore investigate the time dependence of the convective flows within a regular and an exploding granule. The observational material for this study was taken at the center of the solar disk with the German VTT in Izana (Tenerife, Spain) during an observing campaign in the year 1994. It consists of series of spectrograms of high spatial resolution, which were digitized and processed with wavelet techniques. Among other features, our data show the dynamical portrait of a regular and an exploding granule. We can follow their temporal evolution over more than 12 min. Using absorption lines of different strength we are able to see the dynamical change of both granules at several heights within the first 200 km above τ5000=1. The observations reveal significant changes of the convective flow of both granules over time as well as over height, which are discussed in detail.
8 citations
"Dynamics of the solar granulation: ..." refers background or result in this paper
...…evidence for a distinct time scale that characterizes the temporal behavior of the granular dynamics has been provided for the first time by Nesis et al. (2001); elaborating a spectrogram series of 12 min they found that the change of the granular velocity patterns within the granular life…...
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...This time scale is in agreement with the dynamical time of the granular velocity structures reported by Nesis et al. (2001)....
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