J. Jurčák

THREE-DIMENSIONAL VIEW OF TRANSIENT HORIZONTAL MAGNETIC FIELDS IN THE PHOTOSPHERE

We infer the three-dimensional magnetic structure of a transient horizontal magnetic field (THMF) during its evolution through the photosphere using SIRGAUS inversion code. The SIRGAUS code is a modified version of SIR (Stokes Inversion based on Response function), and allows for retrieval of information on the magnetic and thermodynamic parameters of the flux tube embedded in the atmosphere from the observed Stokes profiles. Spectropolarimetric observations of the quiet Sun at the disk center were performed with the Solar Optical Telescope on board Hinode with Fe I 630.2 nm lines. Using repetitive scans with a cadence of 130 s, we first detect the horizontal field that appears inside a granule, near its edge. On the second scan, vertical fields with positive and negative polarities appear at both ends of the horizontal field. Then, the horizontal field disappears leaving the bipolar vertical magnetic fields. The results from the inversion of the Stokes spectra clearly point to the existence of a flux tube with magnetic field strength of ~400 G rising through the line-forming layer of the Fe I 630.2 nm lines. The flux tube is located at around log τ500 ~ 0 at Δt = 0 s and around log τ500 ~ –1.7 at Δt = 130 s. At Δt = 260 s, the horizontal part is already above the line-forming region of the analyzed lines. The observed Doppler velocity is maximally 3 km s–1, consistent with the upward motion of the structure as retrieved from the SIRGAUS code. The vertical size of the tube is smaller than the thickness of the line-forming layer. The THMF has a clear Ω-shaped loop structure with the apex located near the edge of a granular cell. The magnetic flux carried by this THMF is estimated to be 3.1 × 1017 Mx.

The magnetic canopy above light bridges

An analysis of high-resolution Stokes observations of two light bridges in active region NOAA 8990 is presented. The observations were recorded with the La Palma Stokes Polarimeter attached to the Swedish Vacuum Solar Telescope. The stratification over the solar atmosphere of different physical parameters is retrieved from these data using the Stokes inversion based on response functions (SIR). Our results confirm previous observations of features such as the decrease in magnetic field strength and the increase in inclination in the light bridges. We also confirm a temperature increase in these structures with respect to the surrounding umbrae. The maps of the magnetic field strength and of the orientation of the magnetic field vector indicate the presence of a canopy structure above the light bridges. We derive the vertical component of electric current density (Jz) from the configuration of the magnetic field. The increased temperature found in the upper layers is studied in the context of the proposed canopy topology and could also explain the recently observed chromospheric heating processes found above light bridges.

EVOLUTION OF PHYSICAL CHARACTERISTICS OF UMBRAL DOTS AND PENUMBRAL GRAINS

A time series of full-Stokes spectropolarimetric observations of the sunspot NOAA 10944, acquired with HINODE/SOT in 2007 February, is analyzed. The data were inverted using the code SIR into a series of 34 maps covering 3 hr of umbra and penumbra evolution. The retrieved maps of plasma parameters show the spatial distribution of temperature, line-of-sight velocity, magnetic field strength, and inclination in two different ranges of optical depths corresponding to the low and high photosphere. In these maps, the evolution of central and peripheral umbral dots (CUDs and PUDs) and penumbral grains (PGs) was traced. While CUDs do not show any excess of line-of-sight velocity and magnetic field inclination with respect to the surrounding umbra, upflows of 400 m s–1 and a more horizontal magnetic field are detected in the low photospheric layers of PUDs. PGs have even stronger upflows and magnetic field inclination in the low photosphere than PUDs. The absolute values of these parameters decrease when PGs evolve into PUDs. It seems that PGs and PUDs are of a similar physical nature. Both classes of features appear in regions with a weaker and more horizontal magnetic field and their formation height reaches the low photosphere. On the other hand, CUDs appear in regions with a stronger and more vertical magnetic field and they are formed too deep to detect upflows and changes in magnetic field inclination.

Dynamics of the solar atmosphere above a pore with a light bridge

Context. Solar pores are small sunspots lacking a penumbra that have a prevailing vertical magnetic-field component. They can include light bridges at places with locally reduced magnetic field. Like sunspots, they exhibit a wide range of oscillatory phenomena. Aims. A large isolated pore with a light bridge (NOAA 11005) is studied to obtain characteristics of a chromospheric filamentary structure around the pore, to analyse oscillations and waves in and around the pore, and to understand the structure and brightness of the light bridge. Methods. Spectral imaging observations in the line Ca II 854.2 nm and complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS spectrometer and HINODE/SOT spectropolarimeter, were used to measure photospheric and chromospheric velocity fields, oscillations, waves, the magnetic field in the photosphere, and acoustic energy flux and radiative losses in the chromosphere. Results. The chromospheric filamentary structure around the pore has all important characteristics of a superpenumbra: it shows an inverse Evershed effect and running waves, and has a similar morphology and oscillation character. The granular structure of the light bridge in the upper photosphere can be explained by radiative heating. Acoustic waves leaking up from the photosphere along the inclined magnetic field in the light bridge transfer enough energy flux to balance the entire radiative losses of the light-bridge chromosphere. Conclusions. A penumbra is not a necessary condition for the formation of a superpenumbra. The light bridge is heated by radiation in the photosphere and by acoustic waves in the chromosphere.

The Analysis of Penumbral Fine Structure Using an Advanced Inversion Technique

We present a method to study the penumbral fine structure using data obtained by the spectropolarimeter on board Hinode. For the first time, the penumbral filaments can be considered as being resolved in spectropolarimetric measurements. This enables us to use inversion codes with only one-component model atmospheres, and thus to assign the obtained stratifications of the plasma parameters directly to the penumbral fine structure. This approach was applied to the limb-side part of the penumbra in the active region NOAA 10923. Preliminary results show a clear dependence of the plasma parameters on the continuum intensity in the inner penumbra, i.e., a weaker and horizontal magnetic field along with an increased line-of-sight velocity are found in the low layers of the bright filaments. The results in the mid penumbra are ambiguous, and future analyses are necessary to unveil the magnetic field structure and other plasma parameters there.

Magnetic field strength distribution of magnetic bright points inferred from filtergrams and spectro-polarimetric data

Small scale magnetic fields can be observed on the Sun in G-band filtergrams as MBPs (magnetic bright points) or identified in spectro-polarimetric measurements due to enhanced signals of Stokes profiles. These magnetic fields and their dynamics play a crucial role in understanding the coronal heating problem and also in surface dynamo models. MBPs can theoretically be described to evolve out of a patch of a solar photospheric magnetic field with values below the equipartition field strength by the so-called convective collapse model. After the collapse, the magnetic field of MBPs reaches a higher stable magnetic field level. The magnetic field strength distribution of small scale magnetic fields as seen by MBPs is inferred. Furthermore, we want to test the model of convective collapse and the theoretically predicted stable value of about 1300 G. We used four different data sets of high-resolution Hinode/SOT observations that were recorded simultaneously with the broadband filter device (G-band, Ca II-H) and the spectro-polarimeter. To derive the magnetic field strength distribution of these small scale features, the spectropolarimeter (SP) data sets were treated by the Merlin inversion code. The four data sets comprise different solar surface types: active regions (a sunspot group and a region with pores), as well as quiet Sun. In all four cases the obtained magnetic field strength distribution of MBPs is similar and shows peaks around 1300 G. This agrees well with the theoretical prediction of the convective collapse model. The resulting magnetic field strength distribution can be fitted in each case by a model consisting of log-normal components. The important parameters, such as geometrical mean value and multiplicative standard deviation, are similar in all data sets, only the relative weighting of the components is different.

Magnetic and velocity fields of a solar pore

Context. Solar pores are intermediate-size magnetic flux features that emerge at the surface of the Sun. The absence of a filamentary penumbra indicates that there is a relatively simple magnetic structure with a prevailing vertical magnetic field. Aims. Relations between the magnetic field components, line-of-sight velocities, and horizontal motions in and around a large pore (Deff = 8. �� 5) are analysed to provide observational constraints on theoretical models and numerical simulations. Methods. Spectropolarimetric observations in Fe I 617.3 nm of the pore NOAA 11005 with the IBIS spectrometer attached to the Dunn Solar Telescope are inverted into series of maps of thermal, magnetic, and velocity parameters using the SIR code. Horizontal velocities are obtained from series of white-light images by means of local correlation tracking. Results. The magnetic field B extends from the visible pore border of more than 3. �� 5 and has a radial structure in a form of spines that are co-spatial with dark intergranular lanes. The horizontal component Bhor is more extended than the vertical component Bz. The temperature linearly decreases with increasing Bz, by about −300 K kG −1 in the photosphere and −800 K kG −1 in the umbra. The temperature contrast of granulation increases with increasing magnetic field strength and is then suppressed for Bz > 1200 G. Granular upflows dominate in regions with Bz 1000 G. Horizontal motions of granules start to be damped for Bz > 500 G and recurrently exploding granules appear only in magnetic fields comparable to or weaker than the equipartition field strength 400 G.

Penumbral models in the light of Hinode spectropolarimetric observations

Aims. The realism of current models of the penumbra is assessed by comparing their predictions with the plasma properties of penumbral filaments as retrieved from spectropolarimetric observations. Methods. The spectropolarimeter onboard Hinode allows us, for the first time, to distinguish the fine structure of the penumbra. Therefore, we can use one-component inversions to obtain the stratifications of plasma parameters in each pixel. The correlations between the plasma parameters and the continuum intensity are studied. Results. We find that, in the outer penumbra, the stronger flows and higher values of magnetic field inclination tend to be located in dark filaments. This finding does not seem to be compatible with the scenario of a field-free gappy penumbra.

The Formation and Disintegration of Magnetic Bright Points Observed by Sunrise/IMaX

The evolution of the physical parameters of magnetic bright points (MBPs) located in the quiet Sun (mainly in the interwork) during their lifetime is studied. First, we concentrate on the detailed description of the magnetic field evolution of three MBPs. This reveals that individual features follow different, generally complex, and rather dynamic scenarios of evolution. Next, we apply statistical methods on roughly 200 observed MBP evolutionary tracks. MBPs are found to be formed by the strengthening of an equipartition field patch, which initially exhibits a moderate downflow. During the evolution, strong downdrafts with an average velocity of 2.4?km?s?1 set in. These flows, taken together with the concurrent strengthening of the field, suggest that we are witnessing the occurrence of convective collapses in these features, although only 30% of them reach kG field strengths. This fraction might turn out to be larger when the new 4?m class solar telescopes are operational as observations of MBPs with current state of the art instrumentation could still be suffering from resolution limitations. Finally, when the bright point disappears (although the magnetic field often continues to exist) the magnetic field strength has dropped to the equipartition level and is generally somewhat weaker than at the beginning of the MBPs evolution. Also, only relatively weak downflows are found on average at this stage of the evolution. Only 16% of the features display upflows at the time that the field weakens, or the MBP disappears. This speaks either for a very fast evolving dynamic process at the end of the lifetime, which could not be temporally resolved, or against strong upflows as the cause of the weakening of the field of these magnetic elements, as has been proposed based on simulation results. It is noteworthy that in about 10% of the cases, we observe in the vicinity of the downflows small-scale strong (exceeding 2?km?s?1) intergranular upflows related spatially and temporally to these downflows. The paper is complemented by a detailed discussion of aspects regarding the applied methods, the complementary literature, and in depth analysis of parameters like magnetic field strength and velocity distributions. An important difference to magnetic elements and associated bright structures in active region plage is that most of the quiet Sun bright points display significant downflows over a large fraction of their lifetime (i.e., in more than 46% of time instances/measurements they show downflows exceeding 1?km?s?1).

A distinct magnetic property of the inner penumbral boundary - Formation of a stable umbra-penumbra boundary in a sunspot

A sunspot emanates from a growing pore or protospot. In order to trigger the formation of a penumbra, large inclinations at the outskirts of the protospot are necessary. The penumbra develops and establishes by colonising both umbral areas and granulation. Evidence for a unique stable boundary value for the vertical component of the magnetic field strength,