Temporal evolution of the Evershed flow in sunspots. I. Observational characterization of Evershed clouds

[Abridged] The magnetic and kinematic properties of the photospheric Evershed flow are relatively well known, but we are still far from a complete understanding of its nature. The evolution of the flow with time, which is mainly due to appearance of velocity packets called Evershed clouds (ECs), may provide information to further constrain its origin. Here we undertake a detailed analysis of the evolution of the Evershed flow by studying the properties of ECs. In this first paper we determine the sizes, proper motions, location in the penumbra, and frequency of appearance of ECs, as well as their typical Doppler velocities, linear and circular polarization signals, Stokes V area asymmetries, and continuum intensities. High-cadence, high-resolution, full vector spectropolarimetric measurements in visible and infrared lines are used to derive these parameters. We find that ECs appear in the mid penumbra and propage outward along filaments with large linear polarization signals and enhanced Evershed flows. The frequency of appearance of ECs varies between 15 and 40 minutes in different filaments. ECs exhibit the largest Doppler velocities and linear-to-circular polarization ratios of the whole penumbra. In addition, lines formed deeper in the atmosphere show larger Doppler velocities, much in the same way as the ''quiescent'' Evershed flow. According to our observations, ECs can be classified in two groups: type I ECs, which vanish in the outer penumbra, and type II ECs, which cross the outer penumbral boundary and enter the sunspot moat. Most of the observed ECs belong to type I. On average, type II ECs can be detected as velocity structures outside of the spot for only about 14 min. Their proper motions in the moat are significantly reduced with respect to the ones they had in the penumbra.Comment: Accepted for publication in A&

Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds

Context: Evershed clouds (ECs) represent the most conspicuous variation of the Evershed flow in sunspot penumbrae. Aims: We determine the physical properties of ECs from high spatial and temporal resolution spectropolarimetric measurements. Methods: The Stokes profiles of four visible and three infrared spectral lines are subject to inversions based on simple one-component models as well as more sophisticated realizations of penumbral flux tubes embedded in a static ambient field (uncombed models). Results: According to the one-component inversions, the EC phenomenon can be understood as a perturbation of the magnetic and dynamic configuration of the penumbral filaments along which these structures move. The uncombed inversions, on the other hand, suggest that ECs are the result of enhancements in the visibility of penumbral flux tubes. We conjecture that the enhancements are caused by a perturbation of the thermodynamic properties of the tubes, rather than by changes in the vector magnetic field. The feasibility of this mechanism is investigated performing numerical experiments of thick penumbral tubes in mechanical equilibrium with a background field. Conclusions: While the one-component inversions confirm many of the properties indicated by a simple line parameter analysis (Paper I of this series), we tend to give more credit to the results of the uncombed inversions because they take into account, at least in an approximate manner, the fine structure of the penumbra.Comment: Accepted for publication in A&

Fine structure, magnetic field and heating of sunspot penumbrae

We interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above. This solves the classical discrepancy between the large heat flux and the low vertical velocities observed in the penumbra. The presence of the gaps causes strong small-scale fluctuations in inclination, azimuth angle and field strength, but without strong forces acting on the gas. The field is nearly horizontal in a region around the cusp-shaped top of the gap, thereby providing an environment for Evershed flows. We identify this region with the recently discovered dark penumbral cores. Its darkness has the same cause as the dark lanes in umbral light-bridges, reproduced in numerical simulations by Nordlund and Stein (2005). We predict that the large vertical and horizontal gradients of the magnetic field inclination and azimuth in the potential field model will produce the net circular polarization seen in observations. The model also explains the significant elevation of bright filaments above their surroundings. It predicts that dark areas in the penumbra are of two different kinds: dark filament cores containing the most inclined (horizontal) fields, and regions between bright filaments, containing the least inclined field lines.Comment: submitted to A&

A 3-D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating

I deduced a 3-D sunspot model that is in agreement with spectropolarimetric observations, to address the question of penumbral heating by the repetitive rise of flow channels. I performed inversions of data taken simultaneously in infrared and visible spectral lines. I used two independent magnetic components to reproduce the irregular Stokes profiles in the penumbra. I studied the averaged and individual properties of the two components. By integrating the field inclination to the surface, I developed a 3-D model of the spot from inversion results without intrinsic height information. I find that the Evershed flow is harbored by the weaker of the two field components. This component forms flow channels that show upstreams in the inner and mid penumbra, continue horizontally as slightly elevated loops throughout the penumbra, and finally bend down in the outer penumbra. I find several examples, where two or more flow channels are found along a radial cut from the umbra to the outer boundary of the spot. I find that a model of horizontal flow channels in a static background field is in good agreement with the observed spectra. The properties of the flow channels correspond very well to the simulations of Schlichenmaier et al. (1998). From the temporal evolution in intensity images and the properties of the flow channels in the inversion, I conclude that interchange convection of rising hot flux tubes in a thick penumbra still seems a possible mechanism for maintaining the penumbral energy balance.Comment: 17 pages, 21 figures, accepted by A&

Models and Observations of Sunspot Penumbrae

The mysteries of sunspot penumbrae have been under an intense scrutiny for the past 10 years. During this time, some models have been proposed and refuted, while the surviving ones had to be modified, adapted and evolved to explain the ever-increasing array of observational constraints. In this contribution I will review two of the present models, emphasizing their contributions to this field, but also pinpointing some of their inadequacies to explain a number of recent observations at very high spatial resolution. To help explaining these new observations I propose some modifications to each of them. These modifications bring those two seemingly opposite models closer together into a general picture that agrees well with recent 3D magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism, Corona and Space Weather--Chinese Space Solar Telescope Scienc

The multi-component field topology of sunspot penumbrae - A diagnostic tool for spectropolarimetric measurements

Context: Sunspot penumbrae harbor highly structured magnetic fields and flows. The moving flux tube model offers an explanation for several observed phenomena, e.g. the Evershed effect and bright penumbral grains. Aims: A wealth of information can be extracted from spectropolarimetric observations. In order to deduce the structure of the magnetic field in sunspot penumbrae, detailed forward modeling is necessary. On the one hand, it gives insight into the sensitivity of various spectral lines to different physical scenarios. On the other hand, it is a very useful tool to guide inversion techniques. In this work, we present a generalized 3D geometrical model that embeds an arbitrarily shaped flux tube in a stratified magnetized atmosphere. Methods: The new semi-analytical geometric model serves as a frontend for a polarized radiative transfer code. The advantage of this model is that it preserves the discontinuities of the physical parameters across the flux tube boundaries. This is important for the detailed shape of the emerging Stokes Profiles and the resulting net circular polarization (NCP). Results: (a) The inclination of downflows in the outer penumbra must be shallower than approximately 15 degrees. (b) Observing the limb-side NCP of sunspots in the Fe I 1564.8 nm line offers a promising way to identify a reduced magnetic field strength in flow channels. (c) The choice of the background atmosphere can significantly influence the shape of the Stokes profiles, but does not change the global characteristics of the resulting NCP curves for the tested atmospheric models.Comment: 12 pages, 14 figure

Two-dimensional solar spectropolarimetry with the KIS/IAA Visible Imaging Polarimeter

Spectropolarimetry at high spatial and spectral resolution is a basic tool to characterize the magnetic properties of the solar atmosphere. We introduce the KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that upgrades the TESOS spectrometer at the German VTT into a full vector polarimeter. VIP is a collaboration between the KIS and the IAA. We describe the optical setup of VIP, the data acquisition procedure, and the calibration of the spectropolarimetric measurements. We show examples of data taken between 2005 and 2008 to illustrate the potential of the instrument. VIP is capable of measuring the four Stokes profiles of spectral lines in the range from 420 to 700 nm with a spatial resolution better than 0.5". Lines can be sampled at 40 wavelength positions in 60 s, achieving a noise level of about 2 x 10E-3 with exposure times of 300 ms and pixel sizes of 0.17" x 0.17" (2 x 2 binning). The polarization modulation is stable over periods of a few days, ensuring high polarimetric accuracy. The excellent spectral resolution of TESOS allows the use of sophisticated data analysis techniques such as Stokes inversions. One of the first scientific results of VIP presented here is that the ribbon-like magnetic structures of the network are associated with a distinct pattern of net circular polarization away from disk center. VIP performs spectropolarimetric measurements of solar magnetic fields at a spatial resolution that is only slightly worse than that of the Hinode spectropolarimeter, while providing a 2D field field of view and the possibility to observe up to four spectral regions sequentially with high cadence. VIP can be used as a stand-alone instrument or in combination with other spectropolarimeters and imaging systems of the VTT for extended wavelength coverage.Comment: 10 pages, 8 figures, accepted by Astronomy and Astrophysics v2: figures updated with improved qualit