Context. Supergranulation is a pattern of the velocity field at the surface of the Sun, which has been known about for more than fifty years, however, no satisfactory explanation of its origin has ...been proposed. Aims. New observational constraints are therefore needed to guide theoretical approaches which hesitate between scenarios that either invoke a large-scale instability of the surface turbulent convection or a direct forcing by buoyancy. Methods. Using the 14-Mpixel CALAS camera at the Pic-du-Midi observatory, we obtained a 7.5 h-long sequence of high resolution images with unprecedented field size. Tracking granules, we have determined the velocity field at the Sun's surface in great detail from a scale of 2.5 Mm up to 250 Mm. Results. The kinetic energy density spectrum shows that supergranulation peaks at 36 Mm and spans on scales ranging between 20 Mm and 75 Mm. The decrease of supergranular flows in the small scales is close to a k-2-power law, steeper than the equipartition Kolmogorov one. The probability distribution function of the divergence field shows the signature of intermittency of the supergranulation and thus its turbulent nature. Conclusions.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context.Supergranules have long been believed to rotate faster than plasma and than magnetic structures. However, it has recently been shown that this could be due to a bias appearing when using ...Dopplergrams, which are strongly affected by projection effets. Aims.Our purpose is to perform new measurements of supergranule angular velocities using a technique that would not be significantly influenced by projection effets. Methods.We are therefore tracking horizontal divergence maps smoothed on the supergranular scale, either globally or on the scale of cells. Tracking the magnetic field using the same technique allows them to be used as a reference. Doppler tracking is performed for a direct comparison. Results.We confirm that the tracking of Doppler features is not reliable when computing the dynamics on the supergranular scale. However, we find that divergence features are still rotating faster than the magnetic field, when using two independent time series. Conclusions.We conclude that the long-standing puzzle of supergranular superrotation persists, and that interpreting it in terms of anchorage depth inside the convective zone may not be sufficient to explain it.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context.The variation of supergranule cell sizes with the magnetic environment is still controversial. Aims.We study this relation in detail to understand the discrepancies observed between previous ...results. Methods.We determine the cell size using divergence of horizontal flows derived from local correlation tracking of intensity maps (MDI/SOHO). We study the variation of the cell size as a function of the magnetic field inside the cell. We also consider which component of the magnetic field most influences the cell size. Results.Our main conclusion is that there are no large cells when the magnetic field (in absolute value) averaged over the cell is large. This is mostly due to the magnetic field inside the cell (intranetwork fields), while strong network magnetic fields (at the cell boundary) are associated with larger cells. Further studies of the evolution of the cells and of the flux imbalance suggest that a high level of weak fields may prevent the formation of large cells. This is compatible with the expectation that strong magnetic fields should prevent large-scale flows. Conclusions.The relation between the local activity level determined by the average magnetic field inside the cells and the supergranule size is not linear. Furthermore, it strongly depends on the definition of the activity level (magnetic field inside the cell or magnetic network) and on the magnetic sensitivity of the data. This last point probably explains at least partially the conflicting results obtained up to now.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Aims. We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. Methods. We used the spectropolarimeter NARVAL at the Telescope ...Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the Ca ii H and K lines, the Ca ii infrared triplet, and Hα line. Results. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context. 14 Ceti is a subgiant star of F spectral class that displays variations in the S-index of its Ca ii H & K lines and an X-ray emission that is stronger than the mean observed for its spectral ...class, which may be due to some magnetic activity. Aims. We attempt to Zeeman-detect and study the magnetic field of 14 Ceti and to infer its origin. Methods. We used the spectropolarimeter Narval at the Telescope Bernard Lyot, Pic du Midi Observatory, and the least squares deconvolution method to create high signal-to-noise ratio Stokes V profiles. We derived the surface-averaged longitudinal magnetic field Bl. We also measured the S-index, and the radial velocity for each observation. Results. 14 Ceti is Zeeman-detected for the 30 observed dates spanning from August 2007 to January 2012. The average longitudinal magnetic field does not reverse its sign, reaches about −35 G, and shows some month-long-timescale variations in our 2008 and 2011−2012 observations. The S-index follows the same long-term trend as Bl. 14 Ceti is confirmed as a single star without H-K emission cores. The strength of the observed surface magnetic field of 14 Ceti is one order of magnitude greater than the observed one for late F main-sequence stars, and is comparable to the values measured in the active late F pre-main-sequence star HR 1817. On the other hand, taking into account the post-main-sequence evolution of an Ap star, an oblique rotator model can explain the strength of the magnetic field of 14 Ceti. The variations with a timescale of months observed for both the Bl and S-index could be due to the rotation. Conclusions. The most probable scenario to explain our observations appears to be that 14 Ceti is the descendant of a cool Ap star.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
We investigate the properties of acoustic events (AEs), defined as spatially concentrated and short duration energy flux, in the quiet Sun, using observations of a 2D field of view (FOV) with high ...spatial and temporal resolution provided by the Solar Optical Telescope (SOT) onboard
Hinode
. Line profiles of Fe
i
557.6 nm were recorded by the Narrow-band Filter Imager (NFI) on a 82″×82″ FOV during 75 min with a time step of 28.75 s and 0.08″ pixel size. Vertical velocities were computed at three atmospheric levels (80, 130, and 180 km) using the bisector technique, allowing the determination of energy flux to be made in the range 3 – 10 mHz using two complementary methods (Hilbert transform and Fourier power spectrum). Horizontal velocities were computed using local correlation tracking (LCT) of continuum intensities providing divergences. We found that the net energy flux is upward. In the range 3 – 10 mHz, a full FOV space and time averaged flux of 2700 W m
−2
(lower layer 80 – 130 km) and 2000 W m
−2
(upper layer 130 – 180 km) is concentrated in less than 1 % of the solar surface in the form of narrow (0.3″) AE. Their total duration (including rise and decay) is of the order of 10
3
s. Inside each AE, the mean flux is 1.6×10
5
W m
−2
(lower layer) and 1.2×10
5
W m
−2
(upper). Each event carries an average energy (flux integrated over space and time) of 2.5×10
19
J (lower layer) to 1.9×10
19
J (upper). More than 10
6
events could exist permanently on the Sun, with a birth and decay rate of 3500 s
−1
. Most events occur in intergranular lanes, downward velocity regions, and areas of converging motions.
Full text
Available for:
DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Context. The determination of horizontal velocity fields at the solar surface is crucial to understanding the dynamics and magnetism of the convection zone of the sun. These measurements can be done ...by tracking granules. Aims. Tracking granules from ground-based observations, however, suffers from the Earths atmospheric turbulence, which induces image distortion. The focus of this paper is to evaluate the influence of this noise on the maps of velocity fields. Methods. We use the coherent structure tracking algorithm developed recently and apply it to two independent series of images that contain the same solar signal. Results. We first show that a k-\omega filtering of the times series of images is highly recommended as a pre-processing to decrease the noise, while, in contrast, using destretching should be avoided. We also demonstrate that the lifetime of granules has a strong influence on the error bars of velocities and that a threshold on the lifetime should be imposed to minimize errors. Finally, although solar flow patterns are easily recognizable and image quality is very good, it turns out that a time sampling of two images every 21 s is not frequent enough, since image distortion still pollutes velocity fields at a 30% level on the 2500 km scale, i.e. the scale on which granules start to behave like passive scalars. Conclusions. The coherent structure tracking algorithm is a useful tool for noise control on the measurement of surface horizontal solar velocity fields when at least two independent series are available.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Acoustic Events in the Solar Atmosphere Malherbe, J.; Roudier, T.; Berger, T. ...
IEEE transactions on plasma science,
2011-Nov., 2011-11-00, 20111101, Volume:
39, Issue:
11
Journal Article
Peer reviewed
New observations have been obtained on September 4, 2010, by the Solar Optical Telescope on board the satellite HINODE, in the magnetically insensitive Fel 557.6-nm line formed in the solar ...atmosphere. Acoustic events (AEs, defined as spatially concentrated propagating waves) have been derived in the photosphere from amplitudes and phases of vertical Doppler velocities with outstanding spatial resolution (0.3") and field of view. AEs have a mean lifetime of 500 s and contain about 2 × 10 19 J each. Most AEs are upward and occur in intergranular lanes and converging regions associated to permanent plasma downflow. They partly contribute (10%) to energy transport involved in chromospheric heating and could be the drivers of solar oscillations. More than 106 events are simultaneously present on the Sun with a birth/decay rate of 2000 events per second.
Aims. We study the influence of large-scale photospheric motions on the destabilization of an eruptive filament, observed on October 6, 7, and 8, 2004, as part of an international observing campaign ...(JOP 178). Methods. Large-scale horizontal flows were investigated from a series of MDI full-disc Dopplergrams and magnetograms. From the Dopplergrams, we tracked supergranular flow patterns using the local correlation tracking (LCT) technique. We used both LCT and manual tracking of isolated magnetic elements to obtain horizontal velocities from magnetograms. Results. We find that the measured flow fields obtained by the different methods are well- correlated on large scales. The topology of the flow field changed significantly during the filament eruptive phase, suggesting a possible coupling between the surface flow field and the coronal magnetic field. We measured an increase in the shear below the point where the eruption starts and a decrease in shear after the eruption. We find a pattern in the large- scale horizontal flows at the solar surface that interact with differential rotation. Conclusions. We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptive filament.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context.
Detailed knowledge of surface dynamics is one of the key points in understanding magnetic solar activity. The motions of the solar surface, to which we have direct access via the ...observations, tell us about the interaction between the emerging magnetic field and the turbulent fields.
Aims.
The flows computed with the coherent structure tracking (CST) technique on the whole surface of the Sun allow for the texture of the velocity modulus to be analyzed and for one to locate the largest horizontal flows and determine their organization.
Methods.
The velocity modulus maps show structures more or less circular and closed which are visible at all latitudes; here they are referred to as donuts. They reflect the most active convective cells associated with supergranulation. These annular flows are not necessarily joined as would seem to indicate the divergence maps.
Results.
The donuts have identical properties (amplitude, shape, inclination, etc. ) regardless of their position on the Sun. The average donuts computed from all the donuts shows an asymmetry east-west of the amplitude which is related to previous works on the wave-like properties of supergranulation. A kinematic simulation of the donuts’ outflow applied to passive scalar (corks) indicates the preponderant action of the selected donuts which are, from our analysis, one of the major actors for the magnetic field diffusion on the quiet Sun.
Conclusions.
The absence of donuts in the magnetized areas (plages) indicates the action of the magnetic field on the strongest supergranular flows and thus modifies the diffusion of the magnetic field in that location. The detection of the donuts is a way to locate – in the quiet Sun – the vortex and the link with the jet, blinkers, coronal bright points (campfires), or other physical structures. Likewise, the study of the influence of donuts on the evolution of active events, such as the destruction of sunspots, filament eruptions, and their influences on upper layers via spicules and jets, could be done more efficiently via the detection of that structures.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
PDF
