Photospheric Granulation Research Articles

Magnetic Fields beneath Active Region Coronal Loops

We examine the hypothesis that multipolar magnetic fields advected by photospheric granules can contribute to heating the active chromosphere and corona. On 2020 September 28 the Gregor Infrared Spectrograph (GRIS) and HiFI+ instruments at the GREGOR telescope obtained data of NOAA 12773. We analyze Stokes profiles of spectral lines of Si i and He i, to study magnetic fields from the photosphere to the upper chromosphere. Magnetogram and EUV data from the Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly instruments on the Solar Dynamics Observatory spacecraft are coaligned and studied in relation to the GRIS data. At coronal loop footpoints, minor polarity fields comprise just 0.2% and 0.02% of the flux measured over the 40″ × 60″ area observed in the photosphere and upper chromosphere, centered 320″ from the disk center. Significantly, the minority fields are situated ≳12″ from bright footpoints. We use physical arguments to show that any unresolved minority flux cannot reach coronal footpoints adjacent to the upper chromosphere. Even if it did, the most optimistic estimate of the energy released through chromospheric reconnection is barely sufficient to account for the coronal energy losses. Further, dynamical changes accompanying reconnection between uni- and multipolar fields are seen neither in the He i data nor in narrowband movies of the Hα line core. We conclude that the hypothesis must be rejected. Bright chromospheric, transition region, and coronal loop plasmas must be heated by mechanisms involving unipolar fields.

Interface region imaging spectrograph (IRIS) observations of the fractal dimension in the solar atmosphere

We focus here on impulsive phenomena and Quiet-Sun features in the solar transition region, observed with the Interface Region Imaging Spectrograph (IRIS) at 1,400 Å (at formation temperatures of Te ≈ 104–106 K). Summarizing additional literature values we find the following fractal dimensions (in increasing order): DA = 1.23 ± 0.09 for photospheric granulation, DA =1.40 ± 0.09 for chromospheric (network) patterns, DA = 1.54 ± 0.04 for plages in the transition region, DA = 1.56 ± 0.08 for extreme ultra-violet (EUV) nanoflares, DA = 1.59 ± 0.20 for active regions in photospheric magnetograms, and DA = 1.76 ± 0.14 for large solar flares. We interpret low values of the fractal dimension (1.0 ≲ DA ≲ 1.5) in terms of sparse curvi-linear flow patterns, while high values of the fractal dimension (1.5 ≲ DA ≲ 2.0) indicate quasi-space-filling transport processes, such as chromospheric evaporation in flares. Phenomena in the solar transition region appear to be consistent with self-organized criticality (SOC) models, based on their fractality and their size distributions of fractal areas A and (radiative) energies E, which show power law slopes of αAobs=2.51±0.21 (with αAtheo=2.33 predicted), and αEobs=2.03±0.18 (with αEtheo=1.80 predicted). This agreement suggests that brightenings detected with IRIS at 1,400 Å reveal the same nonlinear SOC statistics as their coronal counterparts in EUV.

Length Scale of Photospheric Granules in Solar Active Regions

Investigating the length scales of granules could help understand the dynamics of granules in the photosphere. In this work, we detected and identified granules in an active region near disk center observed at wavelength of TiO (7057 Å) by the 1.6 m Goode Solar Telescope (GST). By a detailed analysis of the size distribution and flatness of granules, we found a critical size that divides the granules in motions into two regimes: convection and turbulence. The length scales of granules with sizes larger than 600 km follow Gauss function and demonstrate “flat” in flatness, which reveal that these granules are dominated by convection. Those with sizes smaller than 600 km follow power-law function and behave power-law tendency in flatness, which indicate that the small granules are dominated by turbulence. Hence, for the granules in active regions, they are originally convective in large length scale, and directly become turbulent once their sizes turn to small, likely below the critical size of 600 km. Comparing with the granules in quiet regions, they evolve with the absence of the mixing motions of convection and turbulence. Such a difference is probably caused by the interaction between fluid motions and strong magnetic fields in active regions. The strong magnetic fields make high magnetic pressure which creates pressure walls and slows down the evolution of convective granules. Such walls cause convective granules extending to smaller sizes on one hand, and cause wide intergranular lanes on the other hand. The small granules isolated in such wide intergranular lanes are continually sheared, rotated by strong downflows in surroundings and hereby become turbulent.

The Wind Temperature and Mass-loss Rate of Arcturus (K1.5 III)

In this paper we aim to constrain the wind temperature, outflow and turbulent velocities, ionization state, and mass-loss rate of the single red giant Arcturus (α Boo K1.5 III) using high spectral resolution Hubble Space Telescope Space Telescope Imaging Spectrograph profiles of Si iii 1206.5 Å , O i 1304 Å and 1306 Å, C ii 1334 Å and 1335 Å, and Mg ii h 2802 Å. The use of the E140-H setting for α Boo allows the Si iii 1206.5 Å line to be cleanly extracted from the echelle format for the first time. The ratios of the wind optical depths of lines from different species constrain the temperature at the base of the wind to T wind ∼ 15,400 K. The mass-loss rate derived is 2.5 × 10−11 for Epoch 2018–2019, smaller than previous semiempirical estimates. These results can be reconciled with multiwavelength Very Large Array radio continuum fluxes for Epoch 2011–2012 by increasing the temperature to T wind ∼ 18,000 K, or increasing the mass-loss rate to 4.0 × 10−11 . Interpreting the wind acceleration and turbulence in terms of a steady WKB Alfvén wave–driven wind reveals that the wave energy damping length increases with increasing radius, opposite to the trend expected for ion-neutral damping of monochromatic waves, confirming a previous result by Kuin and Ahmad derived for ζ Aur binaries. This implies that a spectrum of waves is required in this framework with wave periods in the range of hours to days, consistent with the photospheric granulation timescale. Constraints on a radial magnetic field (B) at 1.2 R * are an upper limit of B ≤ 2 G from the implied wave heating, and B ≥ 0.3 G to avoid excessive wave amplitudes.

Investigations of Sizes and Dynamical Motions of Solar Photospheric Granules by a Novel Granular Segmenting Algorithm

Abstract Granules observed in the solar photosphere are believed to be convective and turbulent, but the physical picture of the granular dynamical process remains unclear. Here we performed an investigation of granular dynamical motions of full length scales based on data obtained by the 1 m New Vacuum Solar Telescope and the 1.6 m Goode Solar Telescope. We developed a new granule segmenting method, which can detect both small faint and large bright granules. A large number of granules were detected, and two critical sizes, 265 and 1420 km, were found to separate the granules into three length ranges. The granules with sizes above 1420 km follow Gaussian distribution, and demonstrate flat in flatness function, which shows that they are non-intermittent and thus are dominated by convective motions. Small granules with sizes between 265 and 1420 km are fitted by a combination of power-law function and Gauss function, and exhibit nonlinearity in flatness function, which reveals that they are in the mixing motions of convection and turbulence. Mini granules with sizes below 265 km follow the power-law distribution and demonstrate linearity in flatness function, indicating that they are intermittent and strongly turbulent. These results suggest that a cascade process occurs: large granules break down due to convective instability, which transports energy into small ones; then turbulence is induced and grows, which competes with convection and further causes the small granules to continuously split. Eventually, the motions in even smaller scales enter in a turbulence-dominated regime.

A new spectroscopic method for measuring the temperature gradient in the solar photosphere

Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Certain solar-cycle variations of the quiet-Sun’s physical structure, such as the temperature gradient, might affect the irradiance. Accurate measurements of this quantity over the course of the activity cycle would improve our understanding of long-term irradiance variations. Aims. In a previous work, we introduced and successfully tested a new spectroscopic method for measuring the photospheric temperature gradient directly on a geometric scale in the case of non-magnetic regions. In this paper, we generalize this method for moderately magnetized regions that may be encountered in the quiet solar photosphere. Methods. To simulate spectroscopic observations, we used synthetic Stokes profiles I and V of the magnetic FeI 630.15 nm line and intensity profiles of the non-magnetic FeI 709 nm line computed from realistic three-dimensional magneto-hydrodynamical simulations of the photospheric granulation and line radiative transfer under local thermodynamical equilibrium conditions. We then obtained maps at different levels in the line-wings by convolution with the instrumental point spread function (PSF) under various conditions of atmospheric turbulence – with and without correction by an adaptive optics (AO) system. The PSF were obtained with the PAOLA software and the AO performance is inspired by the system that will be operating on the Daniel K. Inouye Solar Telescope. Results. We considered different conditions of atmospheric turbulence and photospheric regions with different mean magnetic strengths of 100 G and 200 G. As in non-magnetic cases studied in our previous work, the image correction by the AO system is mandatory for obtaining accurate measurements of the temperature gradient. We show that the non-magnetic line at 709 nm may be safely used in all the cases we have investigated. However, the intensity profile of the magnetic-sensitive line is broadened by the Zeeman effect, which would bias our temperature-gradient measurement. We thus implemented a correction procedure of the line profile for this magnetic broadening in the case of weakly magnetized regions. In doing so, we remarked that in the weak-field regime, the right- and left-hand (I + V and I − V) components have similar shapes, however, they are shifted in opposite directions due to the Zeeman effect. We thus reconstructed the intensity profile by shifting back the I + V and I − V profiles and by adding the re-centered profiles. The measurement then proceeds as in the non-magnetic case. We find that this correction procedure is efficient in regions where the mean magnetic strength is smaller or on the order of 100 G. Conclusions. The new method we implement here may be used to measure the temperature gradient in the quiet Sun from ground-based telescopes equipped with an efficient AO system. We stress that we derive the gradient on a geometrical scale and not on an optical-depth scale as we would do with other standard methods. This allows us to avoid any confusion due to the effect of temperature variations on the continuum opacity in the solar photosphere.

Spectral Diagnostics of Solar Photospheric Bright Points

Abstract Through the use of the high-resolution spectral data and the broadband imaging obtained with the Goode Solar Telescope at the Big Bear Solar Observatory on 2013 June 6, the spectra of three typical photospheric bright points (PBPs) have been analyzed. Based on the Hα and Ca ii 8542 Å line profiles, as well as the TiO continuum emission, for the first time, the non-LTE semiempirical atmospheric models for the PBPs are computed. The attractive characteristic is the temperature enhancement in the lower photosphere. The temperature enhancement is about 200–500 K at the same column mass density as in the atmospheric model of the quiet-Sun. The total excess radiative energy of a typical PBP is estimated to be 1 × 1027–2 × 1027 erg, which can be regarded as the lower limit energy of the PBPs. The radiation flux in the visible continuum for the PBPs is about 5.5 × 1010 erg cm−2 s−1. Our result also indicates that the temperature in the atmosphere above PBPs is close to that of a plage. It gives clear evidence that PBPs may contribute significantly to the heating of the plage atmosphere. Using our semiempirical atmospheric models, we estimate self-consistently the average magnetic flux density B in the PBPs. It is shown that the maximum value is about 1 kG, and it decreases toward both higher and lower layers, reminding us of the structure of a flux tube between photospheric granules.

Inferring Plasma Flows at Granular and Supergranular Scales With a New Architecture for the DeepVel Neural Network

The wealth of observational data available has been instrumental in investigating physical features relevant to solar granulation, supergranulation and Active Regions. Meanwhile, numerical models have attempted to bridge the gap between the physics of the solar interior and such observations. However, there are relevant physical quantities that can be modelled but that cannot be directly measured and must be inferred. For example, direct measurements of plasma motions at the photosphere are limited to the line-of-sight component. Methods have consequently been developed to infer the transverse plasma motions from continuum images in the case of the Quiet Sun and magnetograms in the case of Active Regions. Correlation-based tracking methods calculate the optical flows by correlating series of images locally while other methods like “Coherent Structure Tracking” or "Balltracking” exploit the coherency of photospheric granules to track them and use the group motions of the granules as a proxy of the average plasma flows advecting them. Recently, neural network computing has been used in conjunction with numerical models of the Sun to be able to recover the full velocity vector in photospheric plasma from continuum images. We experiment with a new architecture for the DeepVel neural network which takes inspiration from the U-Net architecture. Simulation data of the Quiet Sun and Active Regions are then used to evaluate the response at granular and supergranular scales of the aforementioned method.

The stellar variability noise floor for transiting exoplanet photometry with PLATO

ABSTRACT One of the main science motivations for the ESA PLAnetary Transit and Oscillations (PLATO) mission is to measure exoplanet transit radii with 3 per cent precision. In addition to flares and starspots, stellar oscillations and granulation will enforce fundamental noise floors for transiting exoplanet radius measurements. We simulate light curves of Earth-sized exoplanets transiting continuum intensity images of the Sun taken by the Helioseismic and Magnetic Imager (HMI) instrument aboard the Solar Dynamics Observatory (SDO) to investigate the uncertainties introduced on the exoplanet radius measurements by stellar granulation and oscillations. After modelling the solar variability with a Gaussian process, we find that the amplitude of solar oscillations and granulation is of order 100 ppm – similar to the depth of an Earth transit – and introduces a fractional uncertainty on the depth of transit of 0.73 per cent assuming four transits are observed over the mission duration. However, when we translate the depth measurement into a radius measurement of the planet, we find a much larger radius uncertainty of 3.6 per cent. This is due to a degeneracy between the transit radius ratio, the limb darkening, and the impact parameter caused by the inability to constrain the transit impact parameter in the presence of stellar variability. We find that surface brightness inhomogeneity due to photospheric granulation contributes a lower limit of only 2 ppm to the photometry in-transit. The radius uncertainty due to granulation and oscillations, combined with the degeneracy with the transit impact parameter, accounts for a significant fraction of the error budget of the PLATO mission, before detector or observational noise is introduced to the light curve. If it is possible to constrain the impact parameter or to obtain follow-up observations at longer wavelengths where limb darkening is less significant, this may enable higher precision radius measurements.

Temperature gradient in the solar photosphere. Test of a new spectroscopic method and study of its feasibility for ground-based telescopes

Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Some solar-cycle variations of the quiet-Sun physical structure, such as the temperature gradient, might affect the irradiance. The synoptic measurement of this quantity along the activity cycle would improve our understanding of long-term irradiance variations. Aims. We intend to test a method previously introduced for measuring the photospheric temperature gradient from high-resolution spectroscopic observation and to study its feasibility with ground-based instruments with and without adaptative optics. Methods. We used synthetic profiles of the FeI 630.15 nm obtained from realistic three-dimensional hydrodynamical simulations of the photospheric granulation and line radiative transfer computations under local thermodynamical equilibrium conditions. Synthetic granulation images at different levels in the line are obtained by convolution with the instrumental point spread function (PSF) under various conditions of atmospheric turbulence, with and without correction by an adaptative optics (AO) system. The PSF are obtained with the PAOLA software, and the AO performances are inspired by the system that will be operating on the Daniel K. Inouye Solar Telescope. Results. We consider two different conditions of atmospheric turbulence, with Fried parameters of 7 cm and 5 cm, respectively. We show that the degraded images lead to both a bias and a loss of precision in the temperature-gradient measurement, and that the correction with the AO system allows us to drastically improve the measurement quality. Conclusions. Long-term synoptic observations of the temperature gradient in the solar photosphere can be undertaken by implementing this method on ground-based solar telescopes that are equipped with an AO correction system.

The Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom?

Abstract In this study, we analyze Atmospheric Imaging Assembly (AIA) and Hi-C images in order to investigate absolute limits for the finest loop strands. We develop a model of the occurrence–size distribution function of coronal loop widths, characterized by the lower limit of widths w min, the peak (or most frequent) width w p , the peak occurrence number n p , and a power-law slope a. Our data analysis includes automated tracing of curvilinear features with the OCCULT-2 code, automated sampling of the cross-sectional widths of coronal loops, and fitting of the theoretical size distribution to the observed distribution. With Monte Carlo simulations and variable pixel sizes , we derive a first diagnostic criterion to discriminate whether the loop widths are unresolved or fully resolved (if ). For images with resolved loop widths, we can apply a second diagnostic criterion that predicts the lower limit of loop widths as a function of the spatial resolution. We find that the loop widths are marginally resolved in AIA images but are fully resolved in Hi-C images, where our model predicts a most frequent (peak) value at , in agreement with recent results of Brooks et al. This result agrees with the statistics of photospheric granulation sizes and thus supports coronal heating mechanisms operating on the macroscopic scale of photospheric magneto-convection, rather than nanoflare braiding models on unresolved microscopic scales.

Diagnostics of turbulent and fractal properties of photospheric plasma outside active regions of the Sun

Results of analysis of multi-scale and turbulent properties of observed photospheric granulation patterns in undisturbed solar photosphere are presented. Data were obtained with the New Solar Telescope at Big Bear Solar observatory. Different types of magnetic environment were explored: a coronal hole (CH) area, a quiet sun (QS) intranetwork area, a QS/network area, and an area with small pores. The property of multifractality was revealed for granulation patterns in all environments on scales below 600 km. The degree of multifractality tends to be stronger as the magnetic environment becomes weaker. Analysis of turbulent diffusion on scales less than 1000–2000 km revealed the regime of super-diffusivity for all data sets. Super-diffusion becomes stronger from the QS/network to the QS/intranetwork to the CH. Both multifractality and super-diffusivity on very small scales are associated with the fast turbulent dynamo action. The results show that the most favorable conditions for the fast turbulent dynamo are met outside the network, inside vast areas of weakest magnetic fields, which supports the idea of nonlocal, deep turbulent dynamo.

Statistics of Local Seismic Emission from the Solar Granulation

We apply computational seismic holography to high-frequency helioseismic observations of the quiet Sun from SDO/HMI to locate predominant sources of seismic emission with respect to the structure of the solar granulation. The regions of greatest seismic emission are the edges of photospheric granules. Seismic emission from regions whose continuum brightnesses are 95–100% of the mean, as resolved by HMI, are about 2.5 times as emissive as regions whose brightnesses are 100-104% of the mean. The greater seismic emissivity from regions whose brightnesses are somewhat less than the mean is roughly in line with expectations from an understanding that attributes most seismic emission to cool plumes plummeting from the edges of granules. However, seismic emission from regions whose continuum brightnesses significantly exceed 104% of the mean is also remarkably high. This unexpected feature of high-frequency seismic emission from the solar granulation begs to be understood.

Empirical determination of the temperature stratification in the photosphere of the quiet Sun

Context. Detailed realistic 3D simulations of the photosphere of the Sun are now available, but 1D models of the average quiet-Sun photosphere are still widely used, in particular for spectro-polarimetric inversions. Aims. Here we present an empirical determination of the average radiation temperature variations as a function of the geometrical height above the continuum formation level in the solar photosphere.Methods. We used high resolution spectroscopic scans in the 630 nm Fe i line pair at varying heliocentric angles along the north-south polar axis of the Sun, made with SOT onboard Hinode. Implementing a new method for image reconstruction, we obtained images of the photospheric granulation at constant continuum opacity levels, from the upper photosphere seen at line centers to the low photosphere. The Fourier cross-spectra of images at different opacity levels were computed, and we derived the formation depths of images without referring to any atmospheric model, by measuring the slope of the cross-spectrum phase. Results. A modified Milne-Eddington model for the line formation was tested by comparing it with the average line-intensity profiles observed at solar disk center. It yields consistent results for the FeI 630.2 nm line, whereas the FeI line at 630.1 nm is not well reproduced by the model. We ascribe this discrepancy to non-LTE effects in the line formation processes. The average image intensities at the different FeI 630.2 nm levels were used to determine the depth-variation of the temperature for an average 1D model of the quiet photosphere. We compared our empirical temperature model with the widely used FALC model. Both models agree well for the temperature variations with the continuum optical depth. But in the low photosphere, the temperature gradient we measure with respect to the geometrical height is significantly softer than in Model C. We argue that some of the assumptions used to solve the pseudohydrostatic equilibrium in semi-empirical models are probably at fault. We also derived empirical values for the 630 nm continuum absorption coefficient as a function of the geometrical height in the low photosphere. Finally, we were able to measure the altitude of the base of the granulation contrast inversion layer, which is found at about 130 km above the base of the photosphere, in agreement with 3D MHD simulations.

STELLAR SURFACE MAGNETO-CONVECTION AS A SOURCE OF ASTROPHYSICAL NOISE. I. MULTI-COMPONENT PARAMETERIZATION OF ABSORPTION LINE PROFILES

We outline our techniques to characterise photospheric granulation as an astrophysical noise source. A four component parameterisation of granulation is developed that can be used to reconstruct stellar line asymmetries and radial velocity shifts due to photospheric convective motions. The four components are made up of absorption line profiles calculated for granules, magnetic intergranular lanes, non-magnetic intergranular lanes, and magnetic bright points at disc centre. These components are constructed by averaging Fe I $6302 \mathrm{\AA}$ magnetically sensitive absorption line profiles output from detailed radiative transport calculations of the solar photosphere. Each of the four categories adopted are based on magnetic field and continuum intensity limits determined from examining three-dimensional magnetohydrodynamic simulations with an average magnetic flux of $200 \mathrm{G}$. Using these four component line profiles we accurately reconstruct granulation profiles, produced from modelling 12 x 12 Mm$^2$ areas on the solar surface, to within $\sim \pm$ 20 cm s$^{-1}$ on a $\sim$ 100 m s$^{-1}$ granulation signal. We have also successfully reconstructed granulation profiles from a $50 \mathrm{G}$ simulation using the parameterised line profiles from the $200 \mathrm{G}$ average magnetic field simulation. This test demonstrates applicability of the characterisation to a range of magnetic stellar activity levels.

DYNAMICS OF THE SOLAR MAGNETIC BRIGHT POINTS DERIVED FROM THEIR HORIZONTAL MOTIONS

The sub-arcsec bright points (BP) associated with the small scale magnetic fields in the lower solar atmosphere are advected by the evolution of the photospheric granules. We measure various quantities related to the horizontal motions of the BPs observed in two wavelengths, including the velocity auto-correlation function. A 1 hr time sequence of wideband H$\alpha$ observations conducted at the \textit{Swedish 1-m Solar Telescope} (\textit{SST}), and a 4 hr \textit{Hinode} \textit{G}-band time sequence observed with the Solar Optical telescope are used in this work. We follow 97 \textit{SST} and 212 \textit{Hinode} BPs with 3800 and 1950 individual velocity measurements respectively. For its high cadence of 5 s as compared to 30 s for \textit{Hinode} data, we emphasize more on the results from \textit{SST} data. The BP positional uncertainty achieved by \textit{SST} is as low as 3 km. The position errors contribute 0.75 km$^2$ s$^{-2}$ to the variance of the observed velocities. The \textit{raw} and \textit{corrected} velocity measurements in both directions, i.e., $(v_x,v_y)$, have Gaussian distributions with standard deviations of $(1.32,1.22)$ and $(1.00, 0.86)$ km s$^{-1}$ respectively. The BP motions have correlation times of about $22 - 30$ s. We construct the power spectrum of the horizontal motions as a function of frequency, a quantity that is useful and relevant to the studies of generation of Alfv\'en waves. Photospheric turbulent diffusion at time scales less than 200 s is found to satisfy a power law with an index of 1.59.

Fast horizontal flows in a quiet sun MHD simulation and their spectroscopic signatures

Numerical simulations of solar surface convection have predicted the existence of supersonic horizontal flows in the photospheric granulation. Recently, the detection of such flows in data from the Hinode satellite was reported. We study supersonic granular flows in detail to understand their signatures in spectral lines and to test the observational detection method used to identify these flows in the Hinode observations. We perform time-dependent 3D radiative MHD numerical simulations and synthesize the Fe i 6302 A spectral lines at the resolution of the Hinode data for different viewing angles covering the center-limb variation. There is very large variation in the detailed shape of the emergent line profiles depending on the viewing angle and the particular flow properties and orientation. At the full simulation resolution the supersonic flows can even produce distinct satellite lines. After smearing to the Hinode resolution sufficient signature of supersonic motion remains. Our analysis shows that the detection method used to analyze the Hinode data is indeed applicable. However, the detection is very sensitive to ad hoc parameter choices and can also misidentify supersonic flows.

UPFLOWS IN THE CENTRAL DARK LANE OF SUNSPOT LIGHT BRIDGES

We use high spatial and spectral resolution observations obtained with the CRisp Imaging SpectroPolarimeter at the Swedish 1-m Solar Telescope to analyze the velocity profile of granular light bridges in a sunspot. We find upflows associated with the central dark lanes of the light bridges. From bisectors in the Fe I 630.15 nm line we find that the magnitude of the upflows varies with height with the strongest upflows being deeper in the atmosphere. Typical upflow velocities measured from the 70% bisector are around 500 m/s with peaks above 1 km/s. The upflows in the central dark lane are surrounded by downflows of weaker magnitude, sometimes concentrated in patches with enhanced velocities reaching up to 1.1 km/s. A small spatial offset between the upflows and the continuum dark lane is interpreted as a line-of-sight effect due to the elevated nature of the dark lane and the light bridge above the umbral surroundings. Our observations show that the central dark lane in granular light bridges is not equivalent to the intergranular lanes of normal photospheric granulation that host convective downflows. These results support recent MHD simulations of magneto-convection in sunspot atmospheres.

Helical Motion of Magnetic Flux Tubes in the Solar Atmosphere

Photospheric granulation may excite transverse kink pulses in anchored vertical magnetic flux tubes. The pulses propagate upward along the tubes with the kink speed, while oscillating wakes are formed behind the wave front in a stratified atmosphere. The wakes oscillate at the kink cutoff frequency of stratified medium and gradually decay in time. When two or more consecutive kink pulses with different polarizations propagate in the same thin tube, then the wakes corresponding to different pulses may be superimposed. The superposition sets up helical motions of magnetic flux tubes in the photosphere/chromosphere as seen in recent Hinode movies. The energy carried by the pulses is enough to heat the solar chromosphere/corona and accelerate the solar wind.

Properties of sunspot moats derived from horizontal motions

Context: Sunspots in late phases of evolution are usually surrounded by annular moats, regions where systematic horizontal flows are observed to be directed radially away from the spot. These flows are considered to be a manifestation of the sub-photospheric convection. Aims: The characteristics of moats are derived at two different heights in the solar atmosphere from horizontal motions around sunspots of different sizes, shapes, and phases of evolution. We also study the temporal evolution of moats. Methods: Local correlation tracking is applied to approximately 70-min long time series of white-light and 1600 A images, acquired by the satellite TRACE, to analyse the horizontal motions of photospheric granules and C IV emission structures in the vicinity of 32 sunspots. Moat regions are defined by means of radially-oriented, outward velocities. Results: Relations between sunspot types and the occurrence, areas, and horizontal velocities of moats in the photosphere and transition region are described. Moats do not show substantial changes during the period of about 12 h. Observed asymmetries in moat shapes and velocities are related to the height in the atmosphere, to sunspot age, and to proper motion. It is suggested that the sub-photospheric convective flows around sunspots may be influenced by the spots' proper motion through the convection zone.