Dynamic phenomena in the chromospheric layer of a sunspot

We have studied running penumbral waves, umbral oscillations, umbral flashes and their interrelations from H observations of a large isolated sunspot. Using a subtraction image processing technique we removed the sharp intensity gradient between the umbra and the penumbra and enhanced the low contrast, fine features. We observed running penumbral waves which started in umbral elements with a size of a few arcseconds, covered the umbra and subsequently propagated through the penumbra. The period of the waves was 190 s and the mean propagation velocity was about 15 km s^–1. We detected intense brightenings, located between umbral elements from where waves started, which had the characteristics of umbral flashes. There are indications that umbral flashes are related to the propagation of the waves through the umbra and their coupling. The subtraction images also show considerable fine structure in the chromospheric umbra, with size between 0.3 and 0.8.     

High-resolution photography of the solar chromosphere

High-quality H photographs of the solar chromosphere reveal the presence around isolated sunspots of a pattern of elongated dark elements (fibrils) bearing a strong resemblance to a greatly enlarged version of the white-light penumbra. Individual fibrils have a representative length of some 25 of arc (18000 km) and a typical separation of 2–3. Comparison of pairs of photographs separated by intervals ranging from 0.5 min to 42 min shows that the fibrils undergo continual changes in brightness, size, and shape; their average lifetime is about 17 min. The question is raised whether the fibril structure around a spot is related in any way to the observed inflow of material from the surrounding chromosphere (Evershed effect).     

Periodic and non-periodic phenomena in a sunspot region

We have studied running penumbral waves, the homogeneous Evershed effect, and the spatial relation between intensity and Doppler velocity penumbral features of a chromospheric sunspot. The observations were obtained with the multichannel subtractive double-pass spectrograph (MSDP) operating in H at the Vacuum Tower Telescope (VTT) installed at Tenerife (Canary Islands). We derived intensity and Doppler velocity maps at H ± 0.3 Å over a two-dimensional field of view. We have computed the components of the velocity vector (radial, azimuthal, vertical) as a function of distance from the center of the spot under the assumption of axial symmetry. The results show the well-known, from previous observations, general large-scale characteristics of the chromospheric Evershed flow. Our measurements show that the axes along the discrete structures, where the Evershed flow is confined, are not spatially related to the axes along H ± 0.3 Å intensity features, and we suggest that either the flow is confined in flow channels or that it takes place along sheared magnetic field lines. We also detected, for the first time in velocity images, running penumbral waves, which started in the outer 0.3 of the umbral radius and propagated through the penumbra with propagation velocities 13–24 km s^–1. The propagation velocity, as well as the velocity amplitude, is greater for the waves closer to the center of the spot and diminishes as one moves outward.     

The effect of two-dimensional macroscopic velocity fields on models of the lower solar chromosphere

Two-dimensional macroscopic velocity fields are featured in the calculation of two-dimensional models of the lower solar chromosphere. Relative rms line centre intensity fluctuation data and mean limb darkening data obtained in Mg b and Na D are used together with values of the cross-correlation between line centre brightness and line of sight velocities. It is found that the large scale fluctuation data can be explained by models of the lower solar chromosphere in which the inhomogeneous effects arise only from horizontal, two-dimensional macroscopic velocity fields. It is also shown, however, that the corresponding small scale fluctuation data cannot be explained in a similar manner.The cross-correlation data is found to be a powerful constraint in the computation of two-dimensional models of these regions.     

Velocity Field of a Complex Sunspot with Light Bridges

For the leading part of sunspot group NOAA 8323, which rapidly changed its complex structure, a time series of the line-of-sight (LOS) component of the velocity field was obtained. With a two-dimensional Fabry–Pérot spectrometer, the magnetically insensitive line Fei 557.6 nm was scanned. The inclination of the LOS (heliographic angle) to the vertical was =28.5°. The umbra of the observed spot was divided by a system of light bridges into several parts. The spatial and temporal velocity field also exhibits a considerable complexity: in one extended umbral area there is a downward flow of 1 km s^–1 relative to other dark sub-umbrae. At the center-side penumbra, with a line-of-sight Evershed outflow of 1.5 km s^–1, a persistent patch, somewhat darker than the average penumbra, has a LOS velocity of 1.3 km s^–1 in opposite direction, probably a downflow. At the limb-side penumbra, a photosphere-like area is interspersed, interrupting the Evershed flow which resumes with typical strength beyond this feature towards the outer penumbral boundary. Most interesting is the behavior of the light bridges, which have a slight blue shift, interrupted by short events of strong blue or red shifts which – within the time resolution of 35 s – instantly affect a considerable part of a light bridge.     

Active-Region Magnetic Structure Observed in the Photosphere and Chromosphere

The full magnetic vector has been measured in both the photosphere and chromosphere across sunspots and plage in NOAA Active Region 8299. We investigate the vertical magnetic structure above the umbral, penumbral and plage regions using quantitative statistical comparisons of the photospheric and chromospheric magnetic data. The results include: (1) a general decrease in average magnetic flux density with height; (2) the direct detection of the superpenumbral canopy in the chromosphere; (3) values for dB/dz which are consistent with earlier investigations when derived from a straight difference between the two measurements, but which are somewhat small when derived from the B=0 condition, (4) a monolithic structure in the umbrae which extends well into the upper chromosphere, with a very complex and varied structure in penumbrae and plage, as evidenced by (5) a uniform magnetic scale height in the umbrae with an abrupt jump to widely varying scale heights in penumbral and plage regions. Further, we find (6) evidence that field extrapolations using the photospheric flux as the boundary may not agree with expectations or with observed coronal structures as well as those which use the chromospheric magnetic flux as the extrapolation starting point.     

Two-dimensional observations of the velocity field in and around sunspots

Doppler spectroheliograms of sunspots and their surroundings have been obtained with a spatial resolution approaching one second of arc and a time resolution of 20 s per frame. Observations of 5 sunspots, located 18°, 45°, 56°, 60° and 72° from the disk center respectively, showed considerable long-lived fine structure and, in particular, indicated the following:

1. The Evershed outflow terminated in spoke-like structures that constitute the ragged outer boundary of the penumbra. Some of these spokes extended more than 8000 km beyond the average outer boundary.
2. Although there was considerable long-lived fine structure of both Doppler polarities in the extra-penumbral photosphere, the spatially-averaged horizontal flow was outward for roughly 10000 km beyond the outer boundary of the penumbra. This extra-penumbral velocity field was distinct from the Evershed flow, and, in particular, did not represent its extension beyond the end of the penumbral spokes.

Although these results are based on observations of relatively few sunspots, they do suggest that if magnetic fields are carried outward from sunspots by material motions, then these motions are more like the supergranulation than an extension of the Evershed velocity.     

Fine structures of chromospheric magnetic field and material flow in a solar active region

In this paper, we analyze the relations between photospheric vector magnetic fields, chromospheric longitudinal magnetic fields and velocity fields in a solar active region. Agreements between the photospheric and chromospheric magnetograms can be found in large-scale structures or in the stronger magnetic structures, but differences also can be found in the fine structures or in other places, which reflect the variation of the magnetic force lines from the photosphere to the chromosphere. The chromospheric superpenumbral magnetic field, measured by the Hline, presents a spoke-like structure. It consists of thick magnetic fibrils which are different from photospheric penumbral magnetic fibrils. The outer superpenumbral magnetic field is almost horizontal. The direction of the chromospheric magnetic fibrils is generally parallel to the transverse components of the photospheric vector magnetic fields. The chromospheric material flow is coupled with the magnetic field structure. The structures of the H chromospheric magnetic fibrils in the network are similar to H dark fibrils, and the feet of the magnetic fibrils are located at the photospheric magnetic elements.     

Vector magnetic field measurements and penumbral structure

The alignment of penumbral fibrils along the direction of the transverse magnetic field is good in the center of the solar disk, but deteriorates near the limb. This effect was studied on the basis of 15 vector magnetograms from various observatories in the period 1966–1984 for 5 sunspot groups. The results can be described with a simple geometrical model, where the magnetic field vectors and the penumbral fibrils lie in the same vertical plane, the inclination of the penumbra to the solar surface is 0–5°, and the elevation angle of the magnetic field vector is 40–50°. An adequate fit to observations was achieved only when an assumed uncertainty with 25° r.m.s. standard error was introduced in the angle measurements. The results are similar to earlier measurements for the chromosphere, although in the chromosphere the alignment of structures along the transverse magnetic field is better.     

Video image selection studies of granules,pores, and penumbral flows near a large sunspot

An excellent high-resolution movie in the green continuum was produced by shift-and-add treatment of two 60-min videotapes obtained at the Big Bear Solar Observatory. We have studied the digitized images by direct measurement, cross-correlation techniques, and correlation tracking. The seeing-limited resolution was about 0.3 arc sec.While the cross-correlation lifetime for granules is about five minutes, we find that actually tracking the growth and decay of a granule gives lifetimes from 10 to 22 minutes, the longest lifetimes pertaining to the largest granules. The longer lifetime comes from tracking the granule while it undergoes large changes in size and shape, while the cross-correlation lifetime is just the time in which it grows by a factor two. All the granules followed began as small elements, grew to some size, and either faded (88%), exploded (2%) or were hit by an exploding granule (10%). The major variation in granule structure appears to be due to substantial variations in the dark lanes, which often double in width.The granulation shows the typical exploding granule behavior; we find the probability that any granule will be affected by an exploding granule during its lifetime to be 10%. We also observed a larger scale explosion covering about 10 granules. This explosion was marked by rapid (1 km s^–1) outward flux of the granules.We tracked the development of six small pores, one of which could be followed for two hours. The latter showed four maxima of absorption separated by about 30 min each, virtually disappearing in between. Another was observed to form in about 20 min, but no changes occur in less than granule lifetime.We confirm the inflow in penumbral fibrils observed by Muller. The inflow velocity is about 0.5 km s^–1, and all bright spots disappear into the umbra. The inflow which affects bright and dark features in the penumbral fibrils, is also observed in the smaller spots. We surmise that the Evershed flow is limited to the areas between the bright fibrils. We confirm granular outflow outside the penumbra.     

Chromospheric inhomogeneities in sunspot umbrae

The properties of rapidly changing inhomogeneities visible in the H and K lines above sunspot umbrae are described. We find as properties for these ‘Umbral Flashes’:

1. A lifetime of 50 sec. The light curve is asymmetrical, the increase is faster than the decrease in brightness.
2. A diameter ranging from the resolution limit up to 2000 km.
3. A tendency to repeat every 145 sec.
4. A ‘proper motion’ of 40 km/sec generally directed towards the penumbra.
5. A Doppler shift of 6 km/sec.
6. A magnetic field of 2100 G.
7. A decrease in this field of 12 G/sec. This decrease is probably related to the flash motion.
8. At any instant an average of 3–5 flashes in a medium-sized umbra. A weak feature often persists in the umbra after the flash. This post-flash structure initially shows a blue shift, but 100–120 sec after the flash, it shows a rapid red shift just before the flash repeats.

     

On the sunspot structure

The fine structure of a sunspot is studied on a series of photographs obtained during the third flight of the Soviet Stratospheric Solar Station. The main results are as follows:

1. The micro-photometer tracings on the frames show extremely high Rayleigh resolution of small elements, the smallest distances being near to the theoretical limit. The half-widths of the brighter elements are given in Tables III and VI. The corrected brightness of umbral dots has large dispersion.
2. The dimensions of the smallest dots are equal to the diffraction image of bright points. So the real radii of these objects are smaller than 150km, which is consistent with opaque models of sunspot umbra.
3. The penumbra and umbra structure (dark and bright objects) is in good agreement with the picture of magnetic field splitting in a system of magnetic ropes giving rise to the magnetic arcs in the chromosphere and corona. Only in the umbra do we meet the large scale continuities.

     

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 properties of velocity oscillations in vicinities of sunspot penumbra

It is suggested that the problem of the extent of the outer penumbra could be considered by investigating the range over which the influence of the penumbra on the oscillation regime of the surrounding medium extends. In summer 1998, velocity oscillations were observed in the vicinities of the penumbra of the NOAA 8263 sunspot. The observations were carried out in the H line (chromosphere) and in the far wing of Nii 4857.2 Å (lower photosphere) simultaneously. The oscillation regime typical for sunspot penumbrae is observed within distances of about 15 from the visible outer boundary of the penumbra.     

Broad-band circular polarimetry of sunspots, 0.4–1.7 microns: Spatial scans with a 3.4 arc sec diameter aperture

Spatial scans with a resolution of 3.4 arc sec of the broad-band circular polarization of several sunspots have been made in five filter bands over the wavelength range 0.4–1.7µ with a sensitivity of 1 × 10^–6 fractional polarization. The scans, across a spot through the penumbra and umbra center, revealed two important features: (1) The broad-band circularly polarized fluxV reverses in sign, or diminishes to near zero, at the center of the umbral region relative to the outer penumbra. This effect was wavelength dependent and was most clearly detected as a definite reversal in a band at 1.2µ, although a reversal was also detected in a very broad band extending from 0.8 to 1.6µ. (2) There is a marked asymmetry: in all cases the limbward penumbral region exhibited strongerV values than did the disk-center (inward) side of the spot, at all observed wavelengths. Such previously unreported structure in the magnetic circular polarization of sunspots provides new clues for understanding the anomalous large broad-band polarization at short wavelengths and at the same time imposes new constraints on sunspot models. For example, the polarization reversal in the umbra relative to the penumbra can be naively explained by return-flux sunspot models; but this is not the only interpretation. Alternatively, it can relate to reversals in mass-flow velocities and/or vertical velocity gradients, as between the umbra and penumbra.     

Spectral observations of spicules at two heights in the solar chromosphere

An observational program at the Sacramento Peak Observatory in 1965 provided high-dispersion spectra of the solar chromosphere in several spectral regions simultaneously. These regions included various combinations of the spectral lines Hα, Hβ and H?, the D₃-line of Hei, the infrared triplet of Oi, and the H- and K-lines and the infrared triplet of Caii. With the use of an image slicer the observations were made simultaneously at two heights in the solar chromosphere separated by several thousand kilometers. From these data we draw the following conclusions:

1. Emission of different lines arises in the same chromospheric features. The intensity ratio of lines of different elements varies significantly from spicule to spicule. For the H- and K-lines of ionized calcium, this ratio remains constant, independent of wavelength throughout the line, overall intensity, and height in the chromosphere. Two rare-earth lines in the wing of the H-line show no spicular structure at all.
2. The line-of-sight velocities of many features reverse as a function of time, although most spicules show velocities in only one direction. The simultaneous spectra at two heights show most spicules to have the same line-of-sight velocity at both. There may be an additional class of features, mostly rapidly moving, whose members have line-of-sight velocities that increase with height. These features comprise perhaps 10% of the total. Velocity changes occur simultaneously, to within 20 sec, at two heights separated by 1800 km, indicating velocities of propagation of hundreds of km/sec. The velocity field of individual features is often quite complicated; many spectral features are inclined to the direction of dispersion, implying that differential mass motions are present.
3. The existence of anomalously broad H and K profiles is real. Even with high dispersion and the best seeing, such profiles are not resolved into smaller features. The central reversal in K, H and Hα appears to remain unshifted when the wings are displaced in wavelength, indicating that the reversal is non-spicular.

     

A model of a penumbral chromosphere

A semi-empirical model of a penumbral chromosphere is presented which represents a specific region of the penumbra located approximately one-fourth the distance outward from the umbra-penumbra interface. The model is based on simultaneous observations of high-resolution spectra of Caii K, H, and 8498 made over a sunspot penumbra (SPO 5007) with the Echelle Spectrograph at the Vacuum Tower Telescope at Sacramento Peak Observatory on 18 December, 1979.Spectral profiles were calculated using a non-LTE line formation procedure with various chromospheric models where the optimum model is determined by matching the synthesized profiles with the observational features. The best fit yields a model with overlying column mass m ₀ of 8 × 10^–6 g cm^–2 which also agrees with the observed K₃/H₃ intensity ratio of 1.22.This work was supported by the US-Republic of Korea Cooperative Science Program (K-53).     

Observations of the intensity of the penumbra of sunspots

Observations of the penumbral intensity of sunspots in 13 wavelength regions are presented. In 4 wavelength regions 54 sunspots are measured. In the other wavelength regions the number of sunspots considered ranges from 3–19.The penumbral intensity alters with position within the spot. This intensity variation is found to be comparable with the change in intensity from one spot to another. The penumbral intensity is found to be independent of spot size in the sample considered.The penumbra model of Kjeldseth Moe and Maltby (1969) with = 0.055 is supported by the measurements.     

The crossover effect in sunspots and the fine structure of penumbra

The peculiarities of magnetosensitive lines in the penumbral spectrum and the abnormal distribution of circular polarization in them are explained satisfactorily in terms of superposition of radiation originating in different elements of penumbral fine structure. Complicated asymmetric rv ^– contours can be represented as a sum of two components related to bright (BR) and dark (DR) penumbral regions. Crossover effect in sunspot penumbra appears, when there is considerable relative radial mass velocity in BR and DR, having the magnetic field of different polarities in them. Such conditions are supposed to exist in the penumbra of some sunspots, situated close to the solar limb.     

The internal motion of quiescent prominences

A study has been made of fine structure wavelength shift in the K line spectra from quiescent prominences. A persistent small scale motion is found in the prominence main body. In places where we see the characteristic thread like fine structure in the accompanying H filtergrams the average line-of-sight velocity amplitude is about 1 km s^–1. A higher velocity ( 4 km s^–1) is associated with a slightly coarser, mottled prominence fine structure. In the low lying regions, connecting the prominence body and the chromosphere, we do not detect any fine structure line shift (v 1/2 km s^–1).