Penumbral Filaments Research Articles

Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small δ Spot

In recent high-resolution observations of complex active regions, long-lasting and well-defined regions of strong flows were identified in major flares and associated with bright kernels of visible, near-infrared, and X-ray radiation. These flows, which occurred in the proximity of the magnetic neutral line, significantly contributed to the generation of magnetic shear. Signatures of these shear flows are strongly curved penumbral filaments, which are almost tangential to sunspot umbrae rather than exhibiting the typical radial filamentary structure. Solar active region NOAA 10756 was a moderately complex β δ sunspot group, which provided an opportunity to extend previous studies of such shear flows to quieter settings. We conclude that shear flows are a common phenomenon in complex active regions and δ spots. However, they are not necessarily a prerequisite condition for flaring. Indeed, in the present observations, the photospheric shear flows along the magnetic neutral line are not related to any change of the local magnetic shear. We present high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum reflector at Big Bear Solar Observatory (BBSO). Time series of speckle-reconstructed white-light images and two-dimensional spectroscopic data were combined to study the temporal evolution of the three-dimensional vector flow field in the β δ sunspot group. An hour-long data set of consistent high quality was obtained, which had a cadence of better than 30 seconds and subarcsecond spatial resolution.

Vector Spectropolarimetry of Dark-cored Penumbral Filaments with Hinode

We present spectropolarimetric measurements of dark-cored penumbral filaments taken with Hinode at a resolution of 0.3''. Our observations demonstrate that dark-cored filaments are more prominent in polarized light than in continuum intensity. Far from disk center, the Stokes profiles emerging from these structures are very asymmetric and show evidence for magnetic fields of different inclinations along the line of sight, together with strong Evershed flows of at least 6-7 km s-1. In sunspots closer to disk center, dark-cored penumbral filaments exhibit regular Stokes profiles with little asymmetries due to the vanishing line-of-sight component of the horizontal Evershed flow. An inversion of the observed spectra indicates that the magnetic field is weaker and more inclined in the dark cores as compared with the surrounding bright structures. This is compatible with the idea that dark-cored filaments are the manifestation of flux tubes carrying hot Evershed flows.

Modified p-modes in penumbral filaments?

Aims.The primary objective of this study is to search for and identify wave modes within a sunspot penumbra.

On the Moat-Penumbra Relation

Proper motions in a sunspot group with a δ-configuration and close to the solar disk center have been studied by employing local correlation tracking techniques. The analysis is based on a more than 1 hr time series of G-band images. Radial outflows with a mean speed of 0.67 km s-1 have been detected around the spots, the well-known sunspots moats. However, these outflows are not found in those umbral core sides without penumbra. Moreover, moat flows are only found in those sides of penumbrae located in the direction marked by the penumbral filaments. Penumbral sides perpendicular to them show no moat flow. These results strongly suggest a relation between the moat flow and the well-known, filament-aligned Evershed flow. The standard picture of a moat flow originating from a blocking of the upward propagation of heat is discussed in some detail.

Magnetic Solitons: Unified Mechanism for Moving Magnetic Features

In a highly dynamic environment with sources and sinks of energy, flux tubes do not in general obey local conservation laws, nor do the ensembles of flux tubes that exhibit collective phenomena. We use the approach of energetically open dissipative systems to study nonlinear waves in flux tubes and their role in the dynamics of the overlying atmosphere. We present results of theoretical and observational studies of the properties of moving magnetic features (MMFs) around sunspots and the response of the overlying atmosphere to various types of MMFs. We show that all types of MMFs, often having conflicting properties, can be described on a unified basis by employing the model of shocks and solitons propagating along the penumbral filaments co-aligned with Evershed flows. The model is also consistent with the response of the upper atmosphere to individual MMFs, which depends on their type. For example, soliton-type bipolar MMFs mainly participate in the formation of a moat and do not carry much energy into the upper atmosphere, whereas shock-like MMFs, with the appearance of single-polarity features, are often associated with chromospheric jets and microflares.

Field-Dependent Adaptive Optics Correction Derived with the Spectral Ratio Technique

In this empirical study, we compare high-resolution observations obtained with the 65-cm vacuum reflector at Big Bear Solar Observatory (BBSO) in 2005 and with the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak (NSO/SP) in 2006. We measure the correction of the high-order adaptive optics (AO) systems across the field of view (FOV) using the spectral ratio technique, which is commonly employed in speckle masking imaging, and differential image motion measurements. The AO correction is typically much larger (10′′ to 25′′) than the isoplanatic angle and can be described by a radially symmetric function with a central core and extended wings. The full-width at half-maximum (FWHM) of the core represents a measure of the AO correction. The average FWHM values for BBSO and NSO/SP are 23.5′′ and 18.2′′, respectively. The extended wings of the function show that the AO systems still contribute to an improved speckle reconstruction at the periphery of the 80′′×80′′ FOV. The major differences in the level of AO correction between BBSO and NSO/SP can be explained by different contributions of ground-layer- and free-atmosphere-dominated seeing, as well as different FOVs of the wavefront sensors. In addition, we find an anisotropic spectral ratio in sunspot penumbrae caused by the quasi-one-dimensional nature of penumbral filaments, which introduces a significant error in the estimation of the Fourier amplitudes during the image restoration process.

Observations of dark-cored filaments in sunspot penumbrae

Context. The recent discovery of dark-cored penumbral filaments suggests that we are resolving the building blocks of sunspot penumbrae. Their properties are largely unknown but provide important clues to understanding penumbral fine structure.

Evershed Clouds as Precursors of Moving Magnetic Features around Sunspots

The relation between the Evershed flow and moving magnetic features (MMFs) is studied using high-cadence, simultaneous spectropolarimetric measurements of a sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler velocities, magnetograms, and total linear polarization maps are calculated from the observed Stokes profiles. We follow the temporal evolution of two Evershed clouds that move radially outward along the same penumbral filament. Eventually, the clouds cross the visible border of the spot and enter the moat region, where they become MMFs. The flux patch farther from the sunspot has the same polarity of the spot, while the MMF closer to it has opposite polarity and exhibits abnormal circular polarization profiles. Our results provide strong evidence that at least some MMFs are the continuation of the penumbral Evershed flow into the moat. This, in turn, suggests that MMFs are magnetically connected to sunspots.

Moving Magnetic Features in and out of Penumbral Filaments

A time sequence of high-resolution SOHO/MDI magnetograms, Dopplergrams, and continuum images is used to study the moving magnetic features (MMFs) in and out of penumbral filaments. Precursors of MMFs have been observed inside the penumbral filaments. One hundred and fifteen out of 127 well-observed individual MMFs in the moat of two sunspots have been identified to have precursors at an average distance of 4″ inside the penumbral filaments. The velocity of these precursors is small inside the penumbral filaments and becomes large once the MMFs cross the outer penumbra. The paths followed by the MMFs exhibit large fluctuations in their magnetic field strength values, with an additional hike in the fluctuations near the outer penumbra. It is also observed that the path followed by the MMFs appear as a cluster of fibrils which could be traced back inside the penumbra. The appearance of MMFs and their azimuthal velocity is position and time dependent.

The Evershed Flow: Flow Geometry and Its Temporal Evolution

A diffraction-limited 120 minute time sequence of Evershed flows along penumbral filaments was obtained using high-order adaptive optics in conjunction with postprocessing. We observe individual Evershed flow channels and study their evolution in time. The vast majority of flow channels originate in bright, inner footpoints of size 02-04 with an upflow. The upflow turns into a horizontal outflow along a dark penumbral filament within fractions of 1'' (300-500 km). The time sequence clearly shows that both (bright) upflow and (dark) horizontal flow move around and evolve as a unit, indicating that they are part of the same feature. The inner footpoints are brighter than the average quiet photosphere and move inward at 0.5-1 km s-1. Our observations provide strong evidence that penumbral grains are the inner footpoints of Evershed flows where a hot upflow occurs. We observe an Evershed flow channel as it appears to emerge near the outer penumbra and track the flow over a period of about 100 minutes as it moves toward the penumbra-umbra edge, where it disappeared. We observe a steep decline (≤02) of the velocity at outer end of individual flow channels, even for flow channels that end well within the penumbra. This sharp outer edge of the flow channels is also observed to move inward toward the penumbra-umbra boundary. Flows in dark-cored penumbral filaments appear to be produced by the Evershed effect. We discuss our observational results in the context of models of the Evershed effect. Some aspects of our observations provide strong support for the moving tube model of the Evershed flow.

Two-dimensional spectroscopy of a sunspot

We investigate the thermal and kinematic configuration of a sunspot penumbra using high spectral and spatial resolution intensity profiles of the non-magnetic 557.6 nm line. The data set was acquired with the 2D solar spectrometer TESOS. The profiles are inverted using a one-component model atmosphere with gradients of the physical quantities. From this inversion we obtain the stratification with depth of temperature, line-of-sight velocity, and microturbulence across the penumbra. Our results suggest that the physical mechanism(s) responsible for the penumbral filaments operate preferentially in the lower photosphere. The spot, located at an heliocentric angle of 23°, exhibits larger continuum intensities in the center-side penumbra as compared with the limb side, which translates into an average temperature difference of 100-150 K at . We investigate the nature of the bright ring that appears in the inner penumbra when sunspots are observed in the wing of spectral lines. It is suggested that the bright ring does not reflect a temperature enhancement in the mid photospheric layers. The line-of-sight velocities retrieved from the inversion are used to determine the flow geometry at different heights in the photosphere. Both the flow speed and flow angle increase with optical depth and radial distance. Downflows are detected in the mid and outer penumbra, but only in deep layers (). We demonstrate that the velocity stratifications retrieved from the inversion are consistent with the idea of penumbral flux tubes channeling the Evershed flow. Finally, we show that larger Evershed flows are associated with brighter continuum intensities in the inner center-side penumbra. Dark structures, however, are also associated with significant Evershed flows. This leads us to suggest that the bright and dark filaments seen at 05 resolution are not individual flow channels, but a collection of them. Our analysis highlights the importance of very high spatial resolution spectroscopic and spectropolarimetric measurements for a better understanding of sunspot penumbrae.

Thermal convection in a twisted horizontal magnetic field

The onset of convection in a layer of an electrically conducting fluid heated from below is considered in the case when the layer is permeated by a horizontal magnetic field of strength B 0 the orientation of which varies sinusoidally with height. The critical value of the Rayleigh number for the onset of convection is derived as a function of the Chandrasekhar number Q. With increasing Q the height of the convection rolls decreases, while their horizontal wavelength slowly increases. Potential applications to the penumbral filaments of sunspots are briefly discussed.

High‐Resolution Proper Motions in a Sunspot Penumbra

Local correlation tracking techniques are used to measure proper motions in a series of high angular resolution (~01) penumbra images. If these motions trace true plasma motions, then we have detected converging flows that arrange the plasma in long narrow filaments cospatial with dark penumbral filaments. Assuming that these flows are stationary, the vertical stratification of the atmosphere and the conservation of mass suggest downflows in the filaments on the order of 200 m s-1. The association between downflows and dark features may be a sign of convection, as it happens with the nonmagnetic granulation. Insufficient spatial resolution may explain why the estimated vertical velocities are not fast enough to supply the radiative losses of penumbrae.

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.

Multi-line spectroscopy of dark-cored penumbral filaments

Dark-cored filaments could be the basic building blocks of sunspot penumbrae. Yet, their nature and physical conditions are unknown. In an attempt to improve this situation, we present the first high-resolution spectra of dark-cored penumbral filaments. Several such filaments were observed near the umbra/penumbra boundary of a sunspot located at heliocentric angles of 5° and 20°. Our data reveal (a) significantly larger Doppler shifts in the dark cores as compared to their lateral brightenings; (b) Doppler shifts that increase with depth in the photosphere, up to 1.5 km s -1 ; and (c) Doppler shifts that increase with increasing heliocentric distance. The Doppler velocities measured in the dark cores are almost certainly produced by upflows. In addition, dark-cored penumbral filaments exhibit weaker fields than their surroundings (by 100-300 G). These results provide new constraints for models of dark-cored penumbral filaments.

Moving Magnetic Features as Prolongation of Penumbral Filaments

A sequence of 633 high spatial resolution magnetograms and continuum images from SOHO MDI of NOAA AR 0330 is used to study moving magnetic feature (MMF) activity in the moat surrounding a mature leader sunspot. The time-averaged frame shows that the moat region is covered by a magnetic field that exhibits the same polarity distribution as that observed in the penumbra. The moat field displays the true polarity of the spot in the sector where the penumbra displays it. Similarly, on the side where the penumbra shows a polarity opposite the true one (due to projection effects after the so-called apparent neutral line), the moat field also displays a polarity opposite the true one. This is only compatible with a moat field that is horizontal almost everywhere, as in the outer penumbra. Indeed, this horizontal moat field is seen to be physically connected with the penumbra. This connection is made evident when analyzing the individual structures detected in the averaged images, which we call moat filaments. The filaments stretch out for 12'' in the moat and can be traced back into the penumbra. The observed polarity distribution along them is only compatible with mean inclinations in the range of 80°-90°. Inside the spot, these filaments are linked to the more horizontal magnetic field component that is thought to carry a large part of the Evershed flow. Several bipolar MMFs are seen to originate inside the penumbra and cross the sunspot outer boundary to enter the moat region, following the paths outlined by the moat filaments. These results are discussed in the frame of our current theoretical understanding of the Evershed flow and MMF activity.

The structure of a penumbral connection between solar pores

High resolution 2D-spectro-polarimetric observations have been used to analyse the magnetic field and flow topologies of a penumbral connection between two opposite polarity solar pores. A filamentary structured Evershed-like material flow from one pore to the other along the magnetic field lines has been detected. The flow channels are co-spatial with bright penumbral filaments close to the pore which feeds the flow and the clear brightness-velocity relation vanishes close to the pore which represents the sink of the flow. The boundary between umbra and penumbra of the two pores show significant differences: bright comet-like penumbral grains represent endings of penumbral filaments at the flow sources whereas no such grains were found at the sinks of the flow. Furthermore, a systematic variation of the asymmetries of measured Stokes V profiles across the penumbral connection have been found. The obtained results are in accordance with the widely-accepted uncombed penumbra hypothesis and the moving flux tube model.

Inclination of magnetic fields and flows in sunspot penumbrae

An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0. �� 2 is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12 ◦ and 39 ◦ were analyzed. We find that the magnetic field at the level of the formation of the Fe -line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45 ◦ over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component - designated B - increases outwards from approximately 40 ◦ in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments - designated A - is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35 ◦ in the inner penumbra, increasing to about 60 ◦ towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubio et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993).

Asymmetrical appearance of dark-cored filaments in sunspot penumbrae

Recent sunspot observations at unprecedented resolution have led to the discovery of dark cores in the bright filaments that form the penumbra (Scharmer et al. 2002). The discovery paper considered spots at disk center only, so the properties of the dark-cored filaments remain largely unknown. Here we analyze a speckle-reconstructed time series of G-band and blue continuum images of a sunspot acquired with the Dutch Open Telescope. The target was located at an heliocentric angle of 27 deg. We confirm the existence of dark-cored penumbral filaments also in spots outside the disk center, and report on distinct differences between the center and limb-side penumbra. In the inner center-side penumbra, filaments are detected as two narrow bright streaks separated by a central obscuration. These structures move together as a single entity. On the limb side, dark cores are hardly seen. The time series is used to determine the sizes (∼200-250 km), proper motions (∼280 m s -1 ), and lifetimes (≤45 min) of typical dark-cored filaments.

On the origin of filamentary structure in sunspot penumbrae: non-linear results

As the magnetic flux in a pore increases, it grows in radius and the magnetic field at its outer edge becomes increasingly inclined to the vertical. It is believed that this increased field inclination causes filamentary convection to set in and that this new pattern of convection eventually develops into the highly fluted azimuthal structure of sunspots. The linear instability was investigated in a highly idealized Boussinesq model by Tildesley. A non-linear extension of this work is carried out here and the saturation of the instability is explored. These solutions are contrasted with those obtained when the upper-boundary condition is modified by matching the magnetic field to a potential field above the convecting layer. In the non-linear regime an alternating pattern of hot and cool stripes is produced at the upper surface. The results from these model calculations are related to the transition from pores to protospots and to the development of penumbral filaments in sunspots.