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Structure and evolution of solar supergranulation using SDO/HMI data
摘要： Context. Studying the motions on the solar surface is fundamental for understanding how turbulent convection transports energy and how magnetic fields are distributed across the solar surface. Aims. From horizontal velocity measurements all over the visible disc of the Sun and using data from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI), we investigate the structure and evolution of solar supergranulation. Methods. Horizontal velocity fields were measured by following the proper motions of solar granules using a newly developed version of the coherent structure tracking code. With this tool, maps of horizontal divergence were computed. We then segmented and identified supergranular cells and followed their histories by using spatio-temporal labelling. With this data set we derived the fundamental properties of supergranulation, including their motion. Results. We find values of the fundamental parameters of supergranulation similar to previous studies: a mean lifetime of 1.5 days and a mean diameter of 25 Mm. The tracking of individual supergranular cells reveals the solar differential rotation and a poleward circulation trend of the meridional flow. The shape of the derived differential rotation and meridional flow does not depend on the cell size. If there is a background magnetic field, the diverging flows in supergranules are weaker. Conclusions. This study confirms that supergranules are suitable tracers that may be used to investigate the large-scale flows of the solar convection as long as they are detectable enough on the surface.
关键词： Sun: granulation,Sun: photosphere,Sun: general
更新于2025-09-23 15:23:52
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Characterizing daytime wind profiles with the wide-field Shack–Hartmann wavefront sensor
摘要： Knowledge of the vertical spatio-temporal distribution of the wind speed and direction is essential for optimizing the performance of adaptive optics systems. An extension of SLODAR (slope detection and ranging) can be used for wind profiling by tracking the time-delayed cross-correlation peaks obtained from the slope of the Shack–Hartmann wavefront sensor. This wind-profiling method works well in situations where the cross-correlation peaks can be individualized, but in many cases the isolated cross-correlation peaks for each turbulent layer are difficult to find in the 2D cross-correlation maps, which can result in difficulties when estimating the wind profiles. To address this problem, we propose a method to measure the wind speed and direction of each layer. The method distinguishes isolated peaks more easily and is valid for multiple peaks, even for a weak response. Instead of tracking peaks in 2D cross-correlation maps, the proposed method traces a series of cross-correlation peaks in the curves of 1D slices through the maps from different directions, and finally determines the wind speed and direction according to the frozen-flow hypothesis. This method is verified with simulations, in which the input wind profiles are recovered accurately. Furthermore, this method is applied to 47 data sequences from the 1-m New Vacuum Solar Telescope at Fuxian Solar Observatory. The wind profiles are estimated from the ground up to 12 km, and the experimental results show that all layers move with a relatively low speed.
关键词： Sun: granulation,atmospheric effects,instrumentation: adaptive optics
更新于2025-09-23 15:23:52
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Latitude dependence of the solar granulation during the minimum of activity in 2009
摘要： Context. Knowledge of the latitude variation of the solar granulation properties (contrast and scale) is useful to better understand interactions between magnetic field, convection, differential rotation, and meridional circulation in the solar atmosphere. Aims. We investigated the latitude dependence of the contrast and scale of the solar granulation, with the help of HINODE/SOT blue continuum images taken in the frame of the HOP 79 program, along the central meridian and along the equator on a monthly basis in 2009 during the last solar minimum of activity. Methods. We selected the sharpest images in latitude and longitude intervals. The selected images in all the N-S and E-W scans taken in 2009 were combined to get statistically reliable results. Results. The contrast of the solar granulation decreases towards the poles and the scale increases, but not regularly since a perturbation occurs at around 60? where both quantities return close to their values at the disk center. Conclusions. Such a latitude variation in a period of minimum of activity (2009), is probably not due to magnetic field, neither the quiet magnetic field at the surface, nor the strong magnetic flux tubes associated with active regions, which could be embedded more or less deeply in the convection zone before they reach the surface. The decrease in contrast and increase in scale towards the pole seem to be related to the differential rotation and the perturbation around 60? to the meridional circulation.
关键词： Sun: evolution,Sun: granulation,Sun: rotation,Sun: interior,Sun: magnetic fields
更新于2025-09-23 15:19:57
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Using the Sun to estimate Earth-like planet detection capabilities
摘要： Context. Stellar variability, at a variety of timescales, can strongly affect the ability to detect exoplanets, in particular when using radial velocity (RV) techniques. Accurately characterized solar variations are precious in this context to study the impact of stellar variations on planet detectability. Here we focus on the impact of small timescale variability. Aims. The objective of this paper is to model realistic RV time series due to granulation and supergranulation and to study in greater detail the impact of granulation and supergranulation on RV times series in the solar case. Methods. We have simulated a collection of granules and supergranules evolving in time to reproduce solar photometric and RV time series. Synthetic time series are built over the full hemisphere over one solar cycle. Results. We obtain intensity and RV rms due to solar granulation of respectively 0.8 m/s and 67 ppm, with a strong variability at timescales up to more than 1 h. The rms RV due to supergranulation is between 0.28 and 1.12 m/s. Conclusions. To minimize the effect of granulation, the best strategy is to split the observing time during the night into several periods instead of observing over a consecutive duration. However, the best strategy depends on the precise nature of the signal. The granulation RV remains large after even an hour of smoothing (about 0.4 m/s) while the supergranulation signal cannot be significantly reduced on such timescales: a reduction of a factor 2 in rms RV can for example be obtained over 7 nights (with 26 min/night). The activity RV variability dominates at larger timescales. Detection limits can easily be as high as 1 MEarth or above for periods of tens or hundreds of days. The impact on detection limits is therefore important and may prevent the detection of 1 MEarth planets for long orbital periods, while the impact is much smaller at small orbital periods. These results do not take the presence of pulsations into account.
关键词： Sun: granulation,Sun: activity,stars: solar-type,planetary systems,techniques: radial velocities,stars: activity
更新于2025-09-23 15:19:57
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Large-scale photospheric motions determined from granule tracking and helioseismology from SDO/HMI data
摘要： Context. Large-scale ?ows in the Sun play an important role in the dynamo process linked to the solar cycle. The important large-scale ?ows are the differential rotation and the meridional circulation with an amplitude of km s?1 and few m s?1, respectively. These ?ows also have a cycle-related components, namely the torsional oscillations. Aims. Our attempt is to determine large-scale plasma ?ows on the solar surface by deriving horizontal ?ow velocities using the techniques of solar granule tracking, dopplergrams, and time–distance helioseismology. Methods. Coherent structure tracking (CST) and time-distance helioseismology were used to investigate the solar differential rotation and meridional circulation at the solar surface on a 30-day HMI/SDO sequence. The in?uence of a large sunspot on these large-scale ?ows with a speci?c 7-day HMI/SDO sequence has been also studied. Results. The large-scale ?ows measured by the CST on the solar surface and the same ?ow determined from the same data with the helioseismology in the ?rst 1 Mm below the surface are in good agreement in amplitude and direction. The torsional waves are also located at the same latitudes with amplitude of the same order. We are able to measure the meridional circulation correctly using the CST method with only 3 days of data and after averaging between ±15? in longitude. Conclusions. We conclude that the combination of CST and Doppler velocities allows us to detect properly the differential solar rotation and also smaller amplitude ?ows such as the meridional circulation and torsional waves. The results of our methods are in good agreement with helioseismic measurements.
关键词： Sun: helioseismology,Sun: granulation,Sun: atmosphere,Sun: rotation
更新于2025-09-09 09:28:46
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Effects of granulation on the visibility of solar oscillations
摘要： Context. The interaction of solar oscillations with near surface convection is poorly understood. These interactions are likely the cause of several problems in helio- and astero-seismology, including the so-called surface effect and apparently unphysical travel time shifts as a function of center to limb distance. There is thus a clear need for further theoretical understanding and observational tests. Aims. The aim is to determine how the observed modes are affected by the convection. Methods. I used HMI velocity and intensity images to construct k-ω diagrams showing how the oscillation amplitude and phase depend on the local granulation intensity. Results. There is a clear and significant dependence of the observed properties of the oscillations on the local convection state.
关键词： Sun: oscillations,Sun: granulation
更新于2025-09-04 15:30:14
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Spectroscopy of the K0 Binary Giant <i>α</i> UMa
摘要： High-resolution spectroscopic observations of the K0 II–III star α UMa were taken at the Elgin?eld Observatory over 11 years. Radial velocities were measured for nine of these years. They do not cover enough of the 44.5 year orbital period to give de?nitive elements on their own, but combined with published visual orbits, the spectroscopic-orbit parameters are well constrained. The spectra show no evidence of the secondary star, which remains an unsolved puzzle. Line-depth ratios show that α UMa has temperature variations ～3 K, possibly periodic, over the 2001–2010 interval. Fourier analysis of the line broadening gives the projected rotation velocity of 2.66 ± 0.15 km s?1 and a radial-tangential macroturbulence dispersion of 4.97 ± 0.08 km s?1. The third-granulation signature shows the granulation velocities of α UMa to be essentially solar, with a scale factor of 0.98 ± 0.10. The absolute radial velocity of the star, with granulation blueshifts removed is ?10,035 ± 100 m s?1 at the mean time of the observations, 2005.2544. The line bisector of Fe I λ6253 is normal and shows the classic “C” shape with the blue-most point commensurate with its absolute magnitude. Mapping this bisector on to the third signature gives a ?ux de?cit similar to those of other giants, with a fractional area of 0.131, suggesting a temperature difference between granules and lanes of 127 K. The velocity position of the de?cit is slightly higher than that for previously analyzed giants, extending the correlation with absolute magnitude.
关键词： line: pro?les,stars: late-type,techniques: spectroscopic,Sun: granulation,stars: rotation,binaries: spectroscopic
更新于2025-09-04 15:30:14