Using a numerical simulation of compressible convection with radiative transfer mimicking the solar photosphere, we compare the velocity field derived from granule motions to the actual velocity ...field of the plasma. We thus test the idea that granules may be used to trace large-scale velocity fields at the sun's surface. Our results show that this is indeed the case provided the scale separation is sufficient. We thus estimate that neither velocity fields at scales less than 2500 km nor time evolution at scales shorter than 0.5 hr can be faithfully described by granules. At larger scales the granular motions correlate linearly with the underlying fluid motions with a slope of ≲2 reaching correlation coefficients up to ~0.9.
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We briefly review the observations of the solar photosphere and pinpoint some open questions related to the magnetohydrodynamics of this layer of the Sun. We then discuss the current modelling ...efforts, addressing among other problems, that of the origin of supergranulation.
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We analyse a 7 d (167 h) sequence of TRACE white-light images with 1´´ angular resolution taken at 1 min cadence. The TRACE resolution and the fast cadence allows us to produce maps of the horizontal ...flow fields with high angular (1´´) and temporal resolution (5 min). The field of view of $128\arcsec$ $\times$ $128\arcsec$ (≈ 93 Mm $ \times$ 93 Mm) covers approximately an area of 10 to 30 supergranules. This area was followed during solar rotation. Magnetic flux was artificially inserted into the successive flow maps in the form of ephemeral regions with positive and negative polarity. The emergence rate of 2 $\times$ 1022 Mx h-1 with an average flux per region of about 1.1 $\times$ 1019 Mx produces a good reproduction of the chromospheric network as observed in images taken simultaneously at 1600 Å. In addition, we show that the quiet network can be maintained only if field elements of both polarities are inserted into the flow fields. Our analysis suggests that the network is fully replenished on a time scale of a day and the lifetimes of the magnetic elements are of a similar duration.
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Context.
Large-scale flows in the Sun play an important role in the dynamo process linked to the solar cycle. The important large-scale flows are the differential rotation and the meridional ...circulation with an amplitude of km s
−1
and few m s
−1
, respectively. These flows also have a cycle-related components, namely the torsional oscillations.
Aim.
Our attempt is to determine large-scale plasma flows on the solar surface by deriving horizontal flow 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 influence of a large sunspot on these large-scale flows with a specific 7-day HMI/SDO sequence has been also studied.
Results.
The large-scale flows measured by the CST on the solar surface and the same flow determined from the same data with the helioseismology in the first 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 flows such as the meridional circulation and torsional waves. The results of our methods are in good agreement with helioseismic measurements.
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Aims. We compare surface maps of the chemically peculiar star HD 50773 produced with a Bayesian technique and based on high quality CoRoT photometry with those derived from rotation phase resolved ...spectropolarimetry. The goal is to investigate the correlation of surface brightness with surface chemical abundance distribution and the stellar magnetic surface field. Methods. The rotational period of the star was determined from a nearly 60 days long continuous light curve obtained during the initial run of CoRoT. Using a Bayesian approach to star-spot modelling, which in this work is applied for the first time for the photometric mapping of a CP star, we derived longitudes, latitudes and radii of four different spot areas. Additional parameters like stellar inclination and the spot's intensities were also determined. The CoRoT observations triggered an extensive ground-based spectroscopic and spectropolarimetric observing campaign and enabled us to obtain 19 different high resolution spectra in Stokes parameters I and V with NARVAL, ESPaDOnS, and SemelPol spectropolarimeters. Doppler and Magnetic Doppler imaging techniques allowed us to derive the magnetic field geometry of the star and the surface abundance distributions of Mg, Si, Ca, Ti, Cr, Fe, Ni, Y, and Cu. Results. We find a dominant dipolar structure of the surface magnetic field. The CoRoT light curve variations and abundances of most elements mapped are correlated with the aforementioned geometry: Cr, Fe, and Si are enhanced around the magnetic poles and coincide with the bright regions on the surface of HD 50773 as predicted by our light curve synthesis and confirmed by photometric imaging.
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The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation ...(one-Megameter horizontal scale), are well understood, the questions of the physical origin and dynamical organization of larger-scale flows, such as the 30-Megameters supergranulation and flows deep in the solar convection zone, remain largely open in spite of their importance for solar dynamics and magnetism. Here, we present a new critical global observational characterization of multiscale photospheric flows and subsequently formulate an anisotropic extension of the Bolgiano-Obukhov theory of hydrodynamic stratified turbulence that may explain several of their distinctive dynamical properties. Our combined analysis suggests that photospheric flows in the horizontal range of scales between supergranulation and granulation have a typical vertical correlation scale of 2.5 to 4 Megameters and operate in a strongly anisotropic, self-similar, nonlinear, buoyant dynamical regime. While the theory remains speculative at this stage, it lends itself to quantitative comparisons with future high-resolution acoustic tomography of subsurface layers and advanced numerical models. Such a validation exercise may also lead to new insights into the asymptotic dynamical regimes in which other, unresolved turbulent anisotropic astrophysical fluid systems supporting waves or instabilities operate.
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Photospheric flows measured with TRACE Krijger, J. M.; Roudier, T.; Rieutord, M.
Astronomy and astrophysics (Berlin),
05/2002, Volume:
387, Issue:
2
Journal Article
Peer reviewed
Open access
We analyse white-light image sequences taken with the Transition Region and Coronal Explorer (TRACE) using an optimised local correlation tracking (LCT) method to measure the horizontal flows in the ...quiet solar photosphere with high spatial (1 arcsec) and temporal (5 min) resolution. Simultaneously taken near-ultraviolet images from TRACE confirm that our LCT-determined flows recover the actual supergranulation pattern, thus proving that the topology of the horizontal flow distribution and network assembly may be studied from long-duration TRACE white-light sequences with our method.
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Based on time series of 2D MSDP spectrograms, taken at the Turret Dome in Pic du Midi, we present the temporal evolution of exploding granules in intensity and Doppler velocity through the solar ...photosphere. We describe the penetration of exploding granules in the solar photosphere during their lifes and the related phenomena like the "Bright Plumes"located in the downflowing plasma just on the edge of the granule. We suggest a possible scenario of the exploding granule evolution in the solar photosphere.
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We present first results of the magnetic survey of a sample of slow rotating giant stars for which an X-ray emission or variations of CaII H & K lines have been already detected.
Since 2004, at the Laboratoire d'Astrophysique de Toulouse Tarbes (LATT), we work with CMOS APS detectors, firstly to develop a large-field,high-resolution camera for the observation of the solar ...supergranulation, secondly to develop a fast camera for an adaptive optics test bench. In these two projects, we use detectors from FillFactory, now continued by Cypress Semiconductor Corporation: IBIS4-14000, IBIS-16000, LUPA-4000, 14 Mpixels, 16 Mpixels, 4 Mpixels respectively. The last one just reads in a 240 × 240 pixels window to obtain readout rate of 1000 Image/s. For these purposes we developed dedicated controllers to follow the high pixel rate and multi-output readout of this type of detectors. We also studied the characterization methods and measured the main parameters of these CMOS detectors to know their behaviour. Using these kinds of APS detectors in these two particular projects proves that we can already find niches to use CMOS detectors in astronomy taking advantage of their present specificities. Recent improvements like back illumination, noise reduction, should rapidly open news possibilities.
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