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.     

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。     

Nonlinear Evolution of Axisymmetric Twisted Flux Tubes in the Solar Tachocline

We study the periodicity of twisting motions in sunspot penumbral filaments, which were recently discovered from space (Hinode) and ground-based (SST) observations. A sunspot was well observed for 97 minutes by Hinode/SOT in the G-band (4305 Å) on 12 November 2006. By the use of the time?–?space gradient applied to intensity space?–?time plots, twisting structures can be identified in the penumbral filaments. Consistent with previous findings, we find that the twisting is oriented from the solar limb to disk center. Some of them show a periodicity. The typical period is about ≈?four minutes, and the twisting velocity is roughly 6 km s^?1. However, the penumbral filaments do not always show periodic twisting motions during the time interval of the observations. Such behavior seems to start and stop randomly with various penumbral filaments displaying periodic twisting during different intervals. The maximum number of periodic twists is 20 in our observations. Studying this periodicity can help us to understand the physical nature of the twisting motions. The present results enable us to determine observational constraints on the twisting mechanism.     

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)     

Radiation transfer in prominences with filamentary structure

Observations concerning the structure of sunspots, obtained during the fourth flight of the Soviet Stratospheric Observatory (SSO), are discussed. Objects brighter than the mean photospheric background inside the sunspot penumbra retaining the stable position sometimes vary within time intervals of a few minutes. The brightness change in pores can be explained by their different location at highest levels of the photosphere. The same mechanism can cause the brightness difference of the penumbra filaments. The gradient of the brightness variation inside the pores is determined. The value of this gradient was found to be practically the same for all dark objects. Most penumbral filaments show no magnetic expansion with growing distance from the spot center.     

The color temperature of a sunspot penumbra

The new Multi-Diode Array and the recently modified Universal Birefringent Filter were used at the Vacuum Tower Telescope at the Sacramento Peak National Observatory to measure the continuum color temperature of a sunspot penumbra between 428.4 and 667.6 nm. The results show that the color temperatures within the penumbral structures closely follow a measure of the wavelength average of the brightness temperature. These observations suggest that, if the dark penumbral filaments overlie a normal quiet photosphere, they are opaque to the radiation from below.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the National Science Foundation.     

The structure of a sunspot

White-light photographs of a fairly regular sunspot have been obtained for all but one day of its passage across the disk. From microphotometer tracings across these photographs, intensity profiles across the spot have been obtained at several heliocentric angles, θ. Apparent sunspot, umbral and penumbral widths, have been obtained from these profiles, and an examination of these reveals that the well-known Wilson effect (Wilson, 1774) is a rather complex phenomenon comprising four main features:

1. The intensity profiles become increasingly asymmetric at large θ. The penumbra remote from the limb is poorly defined while the penumbral intensity plateau nearer the limb is well defined and sometimes enhanced by an intensity maximum near the umbra-penumbra boundary.
2. A gradual decrease in the apparent width of the disk-side penumbra may occur but this effect is barely significant compared with the rms errors of the observations.
3. The apparent width of the limb-side penumbra is independent of θ for θ < 60° but at larger heliocentric angles it increases sharply and by a significant amount.
4. The apparent umbral diameter also shows no θ-dependence for θ < 60° but beyond this it decreases in an almost complementary manner.

A general model for the structure of a sunspot is put forward which readily explains these results in a qualitative manner but it is emphasised that an adequate analysis of sunspot structure based on these observations requires solutions of the three-dimensional equation of radiative transfer.     

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     

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.     

Multiwavelength Study of the M8.9/3B Solar Flare from AR NOAA 10960

We present a multiwavelength analysis of a long-duration, white-light solar flare (M8.9/3B) event that occurred on 04 June 2007 from AR NOAA 10960. The flare was observed by several spaceborne instruments, namely SOHO/MDI, Hinode/SOT, TRACE, and STEREO/SECCHI. The flare was initiated near a small, positive-polarity, satellite sunspot at the center of the active region, surrounded by opposite-polarity field regions. MDI images of the active region show a considerable amount of changes in the small positive-polarity sunspot of δ configuration during the flare event. SOT/G-band (4305 Å) images of the sunspot also suggest the rapid evolution of this positive-polarity sunspot with highly twisted penumbral filaments before the flare event, which were oriented in a counterclockwise direction. It shows the change in orientation, and also the remarkable disappearance of twisted penumbral filaments (≈35?–?40%) and enhancement in umbral area (≈45?–?50%) during the decay phase of the flare. TRACE and SECCHI observations reveal the successive activation of two helically-twisted structures associated with this sunspot, and the corresponding brightening in the chromosphere as observed by the time-sequence of SOT/Ca?ii H line (3968 Å) images. The secondary, helically-twisted structure is found to be associated with the M8.9 flare event. The brightening starts six?–?seven minutes prior to the flare maximum with the appearance of a secondary, helically-twisted structure. The flare intensity maximizes as the secondary, helically-twisted structure moves away from the active region. This twisted flux tube, associated with the flare triggering, did not launch a CME. The location of the flare activity is found to coincide with the activation site of the helically-twisted structures. We conclude that the activation of successive helical twists (especially the second one) in the magnetic-flux tubes/ropes plays a crucial role in the energy build-up process and the triggering of the M-class solar flare without a coronal mass ejection (CME).     

Enhanced p-Mode Absorption Seen Near the Sunspot Umbral – Penumbral Boundary

We investigate p-mode absorption in a sunspot using SOHO/MDI high-resolution Doppler images. The Doppler power computed from a 3.5-hour data set is used for studying the absorption in a sunspot. The result shows an enhancement in absorption near the umbral?–?penumbral boundary of the sunspot. We attempt to relate the observed absorption with the magnetic-field structure of the sunspot. The transverse component of the potential field is computed by using the observed SOHO/MDI line-of-sight magnetograms. A comparison of the power map and the computed potential field shows enhanced absorption near the umbral?–?penumbral boundary where the computed transverse field strength is higher.     

1986年2月4日太阳耀斑的演化研究

本文根据乌鲁木齐天文站的H_α耀斑及3.2cm射电流量观侧资料、云南天文台的黑子精细结构照相和Marshall Space Flight Center的向量磁场图,对1986年2月4日的六个耀斑的形态相关及演化联系,特别是0736UT 4B/3X大耀斑的发展过程进行了综合分析。主要结果是: 1.4日大耀斑的初始亮点和闪光相的主要形态演化,与活动区中沿中性线新浮现的强大电流/磁环系密切相关。后者的主要标志是沿中性线的长的剪切半影纤维及它两端的偶极旋涡黑子群(1_3F_3)。 2.上述大耀斑与1972年8月4日0624 UT大耀斑爆发的磁场背景及主要形态特征相似,表明两者的储能和触发机制可能相同。 3.大耀斑爆发的H_α初始亮点,双带出现,环系形成,亮物质抛射和吸收冕珥等现象同3.2cm射电流量的变化在时间上有较好的对应关系。 4.重复性的前期小耀斑爆发位置和发展趋势与大耀斑的主要形态及演化特征相似。它们相对于剪切的纵场中性线两侧的位置相近或相同。因而,可以看作上述强大电流/磁环系不稳性发展过程中的前置小爆发。     

黑子半影内偶极运动磁特征的Hinode观测研究

利用Hinode卫星观测的单色像和磁图,对出现在黑子半影内的35对偶极运动磁特征进行形态特征、运动速度以及低层太阳大气响应3方面的研究,得出以下结论:(1)偶极运动磁特征正负两极成对出现在黑子半影较垂直的磁场之间并向着半影外边界运动,间接验证了偶极运动磁特征起源于黑子半影水平磁场,在2-8小时的时间间隔内,同一位置上会反复出现形态特征和运动速度相似的偶极运动磁特征,为海蛇状磁力线模型提供了证据支持. (2)光球和色球在偶极运动磁特征向外运动过程中会出现增亮,说明偶极运动磁特征会加热中低层太阳大气.(3)偶极运动磁特征的出现位置和半影磁场结构分布符合非梳子状黑子半影结构特征.     

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)     

1986年2月初太阳活动区的形态研究

根据Marshall空间飞行中心(MSFC)太阳天文台的矢量磁场测量和云南天文台的黑子细节照相资料,作者们详细研究了1986年2月初太阳大活动区(AR4711)的形态和演化。主要结论如下: i)几乎在活动区中每处地方,相距五小时观测到横向磁场排列方向和黑子半影纤维形态之间存在良好的相似性。 ii)利用文[4]的方法,推断了本活动区强的垂直电流源和强的水平电流渠道。 iii)与1972年8月初著名的太阳活动区(McMath 11976)相类似,沿老活动区的中性线的新浮磁通管的两足点(偶极黑子)的分离运动导致了一个密集四极磁结构的形成。 iv)新浮磁通管似乎是本活动区最强的电流系统。 上述结论将为进一步研究本区电流/磁场环系的演化及其与耀斑活动的关系提供一个基础数据。     

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.     

Constructing Semi-Empirical Sunspot Models for Helioseismology

One goal of helioseismology is to determine the subsurface structure of sunspots. In order to do so, it is important to understand first the near-surface effects of sunspots on solar waves, which are dominant. Here we construct simplified, cylindrically-symmetric sunspot models that are designed to capture the magnetic and thermodynamics effects coming from about 500 km below the quiet-Sun τ ₅₀₀₀=1 level to the lower chromosphere. We use a combination of existing semi-empirical models of sunspot thermodynamic structure (density, temperature, pressure): the umbral model of Maltby et al. (1986, Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989, Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used to derive the sound-speed profile. We smoothly merge the near-surface properties to the quiet-Sun values about 1 Mm below the surface. The umbral and penumbral radii are free parameters. The magnetic field is added to the thermodynamic structure, without requiring magnetostatic equilibrium. The vertical component of the magnetic field is assumed to have a Gaussian horizontal profile, with a maximum surface field strength fixed by surface observations. The full magnetic-field vector is solenoidal and determined by the on-axis vertical field, which, at the surface, is chosen such that the field inclination is 45° at the umbral – penumbral boundary. We construct a particular sunspot model based on SOHO/MDI observations of the sunspot in active region NOAA 9787. The helioseismic signature of the model sunspot is studied using numerical simulations of the propagation of f, p ₁, and p ₂ wave packets. These simulations are compared against cross-covariances of the observed wave field. We find that the sunspot model gives a helioseismic signature that is similar to the observations.     

A Curious History of Sunspot Penumbrae

Daily records of sunspot group areas compiled by the Royal Observatory, Greenwich, from May of 1874 through 1976 indicate a curious history for the penumbral areas of the smaller sunspot groups. On average, the ratio of penumbral area to umbral area in a sunspot group increases from 5 to 6 as the total sunspot group area increases from 100 to 2000 μHem (a μHem is 10^?6 the area of a solar hemisphere). This relationship does not vary substantially with sunspot group latitude or with the phase of the sunspot cycle. However, for the sunspot groups with total areas <?100 μHem, this ratio changes dramatically and systematically through this historical record. The ratio for these smallest sunspots is near 5.5 from 1874 to 1900. After a rapid rise to more than 7 in 1905, it drops smoothly to less than 3 by 1930 and then rises smoothly back to more than 7 in 1961. It then returns to near 5.5 from 1965 to 1976. The smooth variation from 1905 to 1961 shows no indication of any step-like changes that might be attributed to changes in equipment or personnel. The overall level of solar activity was increasing monotonically during this time period when the penumbra-to-umbra area ratio dropped to less than half its peak value and then returned. If this history can be confirmed by other observations (e.g. Mt. Wilson or Kodaikanal), it may impact our understanding of penumbra formation, our dynamo models, and our estimates of historical changes in the solar irradiance.     

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.     

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.