The temperature variation across the boundary of dark spots on the solar surface

We suppose the transport of energy in a sunspot (or pore) is described by a diffusion process. The thermal conductivities in the spot and its surroundings are assumed to be constant and isotropic, but with a reduced conductivity in the spot. The sunspot and the ambient medium are represented by semi-infinite strips of variable depth, with one common boundary. This interface is a plane inclined at an arbitrary angle with respect to the vertical in order to simulate the inclined magnetic field at the umbral/penumbral, penumbral/photospheric or pore/photospheric boundary.We show that the region with high conductivity below the interface produces a thermal disturbance in the surface layers of the umbra which manifests itself as a temperature enhancement at the umbral surface in a region near the boundary, resulting in a decreased temperature contrast across the surface. The thermal disturbance in the neighboring medium is confined to a very small region.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The temperature of penumbral filaments

The intensity of individual penumbral filaments has recently been measured at the Pic-du-Midi Observatory as well as from observations obtained during the third flight of the Soviet Stratospheric Solar Station. We have used the results of these measurements to calculate the corresponding average penumbral intensity as function of wavelength. The calculated average intensity is compared with the average intensity observed at the Oslo Solar Observatory. The Pic-du-Midi observations are supported by this comparison. The run of temperature versus optical depth is given for bright and dark penumbral filaments.The variation of gas pressure with geometrical depth is discussed. It is suggested that the magnetic field direction has a different variation with depth in bright and dark filaments.     

High‐resolution observations of extremely bright penumbral grains

We observed a cluster of extremely bright penumbral grains located at the inner limb‐side penumbra of the leading sunspot in active region NOAA 10892. The penumbral grains in the cluster showed a typical peak intensity of 1.58 times the intensity I₀ of the granulation surrounding the sunspot. The brightest specimen even reached values of 1.8–2.0 I₀, thus, exceeding the temperatures of the brightest granules in the immediate surroundings of the sunspot. We find that the observed sample of extremely bright penumbral grains is an intermittent phenomenon, that disappears on time scales of hours. Horizontal flow maps indicating proper motions reveal that the cluster leaves a distinct imprint on the penumbral flow field. We find that the divergence line co‐located with the cluster is displaced from the middle penumbra closer towards the umbra and that the radial outflow velocities are significantly increased to speeds in excess of 2 km s^–1. The extremely bright penumbral grains, which are located at the inner limb‐side penumbra, are also discernible in offband Hα images down to Hα ± 0.045 nm. We interpret the observations in the context of the moving flux tube model arguing that hotter than normal material is rapidly ascending along the inner footpoint of the embedded flux tube, i.e., the ascending hot material is the cause of the extremely bright penumbral grains. This study is based on speckle‐reconstructed broad‐band images taken at 600 nm and chromospheric Hα observations obtained with two‐dimensional spectroscopy. All data were taken with adaptive optics under very good seeing conditions at the Dunn Solar Telescope, National Solar Observatory/Sacramento Peak, New Mexico on 2006 June 10. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sunspot Bright Points

We used the flux-calibrated images from the Broad-band Filter Imager and Stokes Polarimeter data obtained with the Solar Optical Telescope onboard the Hinode spacecraft to study the properties of bright points in and around sunspots. The selected bright points are smaller in diameter than 150 km with contrasts exceeding about 3 % in the ratio of sunspot images obtained with the G-band (430.5 nm) and Ca ii H (396.85 nm) filters. The bright points are classified as umbral dot, peripheral umbral dot, penumbral grains, and G-band bright point depending on their location. The bright points are preferentially located around the penumbral boundary and in the fast decaying parts of the umbra. The color temperature of the bright points is in the range of 4600 K to 6600 K with cooler ones located in the central part of the umbra. The temperature increases as a function of distance from the center outward. The G-band, CN-band (388.35 nm), and Ca ii H fluxes of the bright points as a function of their blue-band (450.55 nm) brightness increase continuously in a nonlinear fashion unlike their red (668.4 nm) and green (555.05 nm) counterparts. This is consistent with a model in which the localized heating of the flux tube depletes the molecular concentration, resulting in the reduced opacity that leads to the exposition of deeper and hotter layers. The light curve of the bright points shows that the enhanced brightness at these locations lasts for about 15 to 60 min with the least contrast for the points outside the sunspot. The umbral dots near the penumbral boundary are associated with elongated filamentary structures. The spectropolarimeter observations show that the filling factor decreases as the G-band brightness increases. We discuss the results using the model in which the G-band bright points are produced in the cluster of flux tubes that a sunspot consists of.     

The Evershed motion in sunspots

The Evershed motion is postulated as a steady, laminar flow of material along a limiting field line which separates the umbral magnetic field from the penumbral. Assuming that the Evershed flow starts from the spot-base with a velocity which is adequate to carry the convective flux at that level, the velocity at the surface comes out to be of the order of 1 km/sec, in good agreement with the observed Evershed velocity.Supported in part by the National Science Foundation [GP-5391] and the Office of Naval Research [Nonr-220 (47)].     

On the generation of umbral flashes and running penumbral waves

From a review of the observed properties of umbral flashes and running penumbral waves it is proposed that the source of these periodic phenomena is the oscillatory convection which Danielson and Savage (1968) and Savage (1969) ave shown is likely to occur in the superadiabatic subphotospheric layers of sunspot umbras. Periods and growth rates are computed for oscillatory modes arising in a simple two-layer model umbra. The results suggest that umbral flashes result from disturbances produced by oscillatory convection occurring in the upper subphotospheric layer of the umbra where the superadiabatic temperature gradient is much enhanced over that in lower layers, while running penumbral waves are due to oscillations in a layer just below this upper layer.     

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. Electronic Supplementary Material Electronic Supplementary Material is available for this article at     

太阳黑子半影纤维尺度的一个解释

在黑子半影电流的磁场中存在扰动不稳定模式,本文认为黑子半影纤维是由这种不稳定扰动发展而形成的,利用短波近似,分别在黑子半径方向及围绕黑子方向上求解非绝热慢波色散方程。由不稳定条件可得到(1)纤维的长度与宽度的数值;(2)纤维模式在长度方向上是静止的,在宽度方向上几乎是不动的;(3)半影纤维是黑子在重力场中的磁流特征之一;(4)半影纤维的出现,表示着黑子扭转磁场的存在。     

The fine structure of light-bridges in sunspots

High resolution photographs obtained at the Pic du Midi Observatory show that there are three types of sunspot light bridges according to their morphological structures: the photospheric ones, the penumbral ones and the umbral ones. Consequently there are no specific structures in light bridges; it results that they should not be due to specific physical properties. Properties of the fine structure of a penumbral light bridge are described.     

Magnetic field strength and inclination in the penumbral fine-structure

The uncertainty about a possible correlation between magnetic field strength, inclination, and the continuum intensity of sunspot penumbral fine-structure has been removed from detailed analysis of a spatially very well-resolved spectrum: the darker, long penumbral lanes host a 10% stronger and 30° flatter magnetic field as compared to the field in bright penumbral locations. This finding is not only based on the high spatial resolution but also on the use of a spectral line, here Fe 6842.7 Å, obtaining its essential contribution from those deep layers where the penumbral structure is seen, i.e. the continuum intensity level. The almost perfect correlation establishes that the penumbral structure is formed by the two magnetic components mainly differing by the field inclination. The different results from other Zeeman lines, as, e.g., Fe 6302.5 Å, indicate a different field structure above the white-light penumbral layers.     

An explanation of the dimensions of penumbral filaments

Unstable pertubation modes exist in the magnetic field of penumbral electric current and I think the penumbral filaments are formed from the development of such modes. Under the short wave approximation the non-adiabatic dispersion equation is solved in the radial and transverse directions of the sunspot. From the condition of instability the length and width of the penumbral filament can be evaluated and it is found that the filament mode is static in the direction of the length and is non-moving in the direction of the width, that the penumbral filaments are a feature of the sunspot magnetic flow under gravity and that the presence of the filaments implies the existence of a twisted magnetic field.     

Imaging Spectropolarimetry of Ti I 2231 nm in a Sunspot

Spectro-polarimetric observations at 2231 nm were made of NOAA 10008 near the west solar limb on 29 June 2002 using the National Solar Observatory McMath–Pierce Telescope at Kitt Peak and the California State University Northridge – National Solar Observatory infrared camera. Scans of spectra in both Stokes I and Stokes V were collected; the intensity spectra were processed to remove strong telluric absorption lines, and the Stokes V umbral spectra were corrected for instrumental polarization. The sunspot temperature is computed using the continuum contrast and umbral temperatures down to about 3700 K are observed. A strong Tii line at 2231.0 nm is used to probe the magnetic and velocity fields in the spot umbra and penumbra. Measurements of the Tii equivalent width versus plasma temperature in the sunspot agree with model predictions. Zeeman splitting measurements of the Stokes I and Stokes V profiles show magnetic fields up to 3300 G in the umbra, and a dependence of the magnetic field on the plasma temperature similar to that which was seen using Fei 1565 nm observations of the same spot two days earlier. The umbral Doppler velocity measurements are averaged in 16 azimuthal bins, and no radial flows are revealed to a limit of ±200 m s^–1. A Stokes V magnetogram shows a reversal of the line-of-sight magnetic component between the limb and disk center sides of the penumbra. Because the Tii line is weak in the penumbra, individual spectra are averaged in azimuthal bins over the entire penumbral radial extent. The averaged Stokes V spectra show a magnetic reversal as a function of sunspot azimuthal angle. The mean penumbral magnetic field as measured with the Stokes V Zeeman component splitting is 1400 G. Several weak spectral lines are observed in the sunspot and the variation of the equivalent width versus temperature for four lines is examined. If these lines are from molecules, it is possible that lines at 2230.67, 2230.77, and 2231.70 nm originate from OH, while the line at 2232.21 nm may originate from CN.     

The association between sunspot magnetic fields and superpenumbral fibrils

Spectropolarimetric observations of a sunspot were carried out with the Tenerife Infrared Polarimeter at Observatorio del Teide, Tenerife, Spain. Maps of the physical parameters were obtained from an inversion of the Stokes profiles observed in the infrared Fe I line at 15648 Å The regular sunspot consisted of a light bridge which separated the two umbral cores of the same polarity. One of the arms of the light bridge formed an extension of a penumbral filament which comprised weak and highly inclined magnetic fields. In addition, the Stokes V profiles in this filament had an opposite sign as the sunspot and some resembled Stokes Q or U. This penumbral filament terminated abruptly into another at the edge of the sunspot, where the latter was relatively vertical by about 30°. Chromospheric Hα and He II 304 Å filtergrams revealed three superpenumbral fibrils on the limb‐side of the sunspot, in which one fibril extended into the sunspot and was oriented along the highly inclined penumbral counterpart of the light bridge. An intense, elongated brightening was observed along this fibril that was co‐spatial with the intersecting penumbral filaments in the photosphere. Our results suggest that the disruption in the sunspot magnetic field at the location of the light bridge could be the source of reconnection that led to the intense chromospheric brightening and facilitated the supply of cool material in maintaining the overlying superpenumbral fibrils. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Umbral oscillations and penumbral waves in Hα

We present examples of umbral oscillations observed on Big Bear H filtergram movies and investigate the relation between umbral oscillations and running penumbral waves occurring in the same sunspot. Umbral oscillations near the center of the umbra are probably physically independent of the penumbral waves because the period of these umbral oscillations (150 s) is shorter than the penumbral wave period (270 s) but not a harmonic. We also report dark puffs which emerge from the edge of the umbra and move outward across the penumbra, and which have the same period as the running penumbral waves. We interpret these dark puffs to be the extension of chromospheric umbral oscillations at the edge of the umbra. It is suggested that the dark puffs and the running penumbral waves have a common source: photospheric oscillations just inside the umbra.     

Waves in the sunspot penumbra

The stability of a plane-parallel polytropic fluid layer in the presence of a uniform horizontal magnetic field is investigated to explore the possibility of identifying the running penumbral waves and the penumbral filaments with different types of instabilities.     

A search for sunspot canopies using a vector magnetograph

Using a magnetograph, we examine four sunspots for evidence of a magnetic canopy at the penumbra/photosphere boundary. The penumbral edge is determined from the photometric intensity and is defined to correspond to the value of the average intensity minus twice the standard deviation from the average. From a comparison of the location of this boundary with the location of contours of the vertical and horizontal components of the magnetic field, we conclude that the data are best represented by canopy-type fields close to all four sunspots. There is some evidence that the magnetic inclination in the canopies is 5°–15° with respect to the horizontal and that the canopy base height lies in the middle/upper photosphere. The observations further suggest that the magnetic canopy of a sunspot begins at its outer penumbral boundary.     

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.     

On the width distribution of penumbral filaments in sunspots

The mean width and distribution of penumbral filaments of a sunspot have been estimated, using white light photographs obtained with a vacuum, Newtonian type, telescope. Three areas corresponding to the penumbra of a sunspot have been analysed. Data were collected during the solar eclipse of June 1973. The photometric profiles of the Moon limb over the photosphere have been analysed to obtain useful information on both, atmospheric and instrumental perturbation on each exposure. The mean value of the width of penumbral filaments is 0.37 arc sec.Now at INTA-Villafranca, S.T.S., P.O. Box 54065, Madrid, Spain.     

Magnetoacoustic–Gravity Surface Waves with Evershed flow – A model for Running Penumbral Waves

The penumbral region of a sunspot is modelled as a two-layer plasma. The upper layer with magnetic field is taken with Evershed flow and the static lower layer is assumed to be field-free. The magnetoacoustic–gravity surface wave (MAGSW) propagation along this interface is studied. Our results show that the flow suppresses the fast MAGSW and allows only slow MAGSW. More importantly, we suggest that the running penumbral waves are more likely to be slow MAGSW.     

On the dynamic activity in sheared corridors of large delta spots

In the extraordinarily flare-prolific region of March 1989, NOAA region No. 5395, unusual dynamic activity in the photosphere was observed for the first time inside the large delta spot (Wanget al., 1991). Analyses of two additional large delta spots with sheared penumbral fibrils revealed that what occurred in the March 1989 delta spot is not an isolated case; similar complex dynamic activity was observed in the August and October 1989 delta spots. Both are flare-prolific regions as well, each producing 5 X-class flares. As in the March 1989 case, registered and highly time-compressed white-light movies were made from digital data obtained at Big Bear Solar Observatory. The new evidence confirmed the unusual activity: (1) penumbral motions in the directions of sheared penumbral fibrils near the inversion line as well as elsewhere in the delta complex, and (2) new spots emerging in the midst of penumbral motions. The manner and place of emergence are different from those in ordinary emerging flux regions, and often the spots are without observable opposite polarity flux. It is easy to see how the emergence of new spots in the midst of strong fields as well as the shear motions near the inversion line further enhance the flare productivity of the large delta spot regions. But we have yet to understand the origin of the dynamic activity observed.