Vacuum Solar Telescope Research Articles

Temporal Evolution of Fine Structures in and around Solar Pores

Time series of high-resolution white-light images of six solar pores, observed in 1993 and 1995 at the Swedish Vacuum Solar Telescope (La Palma), are analyzed. The pores constitute an almost ideal laboratory in which to study the interaction of a vertical magnetic field with surrounding convective motions, without the perturbation of the inclined magnetic field in the penumbra. Umbral dots observed in a large (D=89) pore are similar to those in mature umbrae, but they live longer, are brighter, and have a higher filling factor. Granular motions in the vicinity of pores are driven by mesogranular flows. Motions toward the pore dominate in the 2'' zone around the pore boundary, while at larger distances the granules move away from the pore. Pushed by these motions, small granules and granular fragments located close to the pore border sometimes penetrate into the pore, where they move inward as bright short-lived features very similar to umbral dots. The capture of bright features by the pore is probably a microscale manifestation of the turbulent erosion, which results in the decay of the pore. Formation of a transitory penumbra-like structure at the border of the large pore was observed simultaneously with the appearance of expanding elongated granules, separated by dark filaments, in an adjacent granular field. These effects can be interpreted as a consequence of emerging bipolar magnetic loops caused by a temporary protrusion of opposite magnetic polarity.

Dynamics of Magnetic Flux Elements in the Solar Photosphere

The interaction of magnetic -elds and convection is investigated in the context of the coronal heating problem. We study the motions of photospheric magnetic elements using a time series of high-resolution G-band and continuum -ltergrams obtained at the Swedish Vacuum Solar Telescope at La Palma. The G-band images show bright points arranged in linear structures ( -ligree II) located in the lanes between neighboring granule cells. We measure the motions of these bright points using an object tracking tech- nique, and we determine the autocorrelation function describing the temporal variation of the bright point velocity. The correlation time of the velocity is about 100 s. To understand the processes that determine the spatial distribution of the bright points, we perform simulations of horizontal motions of magnetic Nux elements in response to solar granulation Nows. Models of the granulation Now are derived from the observed granulation intensity images using a simple two-dimensional model that includes both inertia and horizontal temperature gradients; the magnetic Nux elements are assumed to be passively advected by this granulation Now. The results suggest that this passive advection model is in reasonable agreement with the observations, indicating that on a timescale of 1 hr the Nux tubes are not strongly a†ected by their anchoring at large depth. Finally, we use potential--eld modeling to extrapo- late the magnetic and velocity -elds to larger height. We -nd that the velocity in the chromosphere can be locally enhanced at the separatrix surfaces between neighboring Nux tubes. The predicted velocities are several km s~1, signi-cantly larger than those of the photospheric Nux tubes. The implications of these results for coronal heating are discussed. Subject headings: convection E Sun: corona E Sun: granulation E Sun: magnetic -elds

Measurements of Solar Magnetic Element Dispersal

The dispersal of elements in the solar photospheric flow field is studied by tracking individual magnetic bright points (MBPs) identified in a G-band 4305 A filtergram time series obtained at the 50 cm Swedish Vacuum Solar Telescope on La Palma, Spain. The time series spans approximately 70 minutes with a field of view of 29'' × 29'' near disk center. All images in the time series are restored to near the telescope diffraction limit (~02 in the G band) using partitioned phase diverse speckle techniques. Regions of enhanced network and quiet Sun are examined. In the network region, automated tracking of individual MBPs reveals approximately Gaussian diffusion, with indications for slightly superdiffusive dispersal. The inferred Gaussian diffusion coefficient is 60.4 ± 10.9 km2 s-1. In the quiet-Sun region, local correlation tracking velocity measurements show the dispersal of artificial tracers to be non-Gaussian over most of our data set with indications of an asymptotic approach to a 285 km2 s-1 Gaussian diffusion.

Measurements of Solar Magnetic Element Motion from High‐Resolution Filtergrams

Solar photospheric flowfield properties on sub-05 scales are measured using local correlation tracking (LCT) and object tracking of bright points (MBPs: photospheric bright points associated with elements). The dataset is a 70 minute time series of cospatial and cotemporal G-band 4305 A and wideband 4686 A filtergrams obtained with the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma, Spain. We examine a 29'' × 70'' field of view (FOV) near disk center and compare a 29'' × 29'' network subfield and a 27'' × 27'' apparently nonmagnetic quiet-Sun subfield. The mean time between frames is 23.75 s. Each frame is created by partitioned phase-diverse speckle restoration of three image pairs acquired rapidly in sequence. Angular resolution is ~04 or less in all frames of the restored data set. From LCT on a 04 grid with 083 FWHM apodization, we find the flow speeds to be Rayleigh distributed with a mode of 550 m s-1 and an average of 950 m s-1 in the network FOV; in the quiet FOV the modal speed is 700 m s-1 with a mean of 1100 m s-1. Within the network FOV, a magnetic defined by the loci of tracked MBPs exhibit even greater alteration: rms contrast of the region is 8% higher in the G band compared to areas outside, LCT speeds are reduced by a factor of 1.6, and the convective flow structures are smaller and much more chaotic. Mesogranulation is entirely absent in the region. The modal and mean speeds of 534 tracked MBPs are 300 m s-1 and 1470 m s-1, respectively. MBPs split and merge with mean times of 320 and 404 s respectively. The mean lifetime of MBPs in the data set is 9.33 minutes although approximately 5% of the objects can be tracked for the entire 70 minute duration of the time series.

Preparation of a Dual Wavelength Sequence of High‐Resolution Solar Photospheric Images Using Phase Diversity

The collection, seeing compensation, and temporal filtering of a high-resolution time-sequence of solar photospheric images is described. A 70 minute time series of cospatial and cotemporal G band 4305 A and wideband 4686 A filtergrams was obtained with the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma, Spain. The 29'' × 70'' field-of-view near disk center contains both an enhanced network region and an (apparently) nonmagnetic quiet region of granulation. The mean time between frames is 23.5 s. Each frame is created with partitioned phase-diverse speckle restoration of three realizations of the atmospheric turbulence acquired rapidly in sequence. The result is high-resolution movies of the solar photosphere, good enough to allow detection, tracking, and analysis of ~02 bright points. This analysis is the subject of a companion paper.

The Postflare Loops and the Nearby Active Chromosphere of 1992 June 26: Addendum

Video segments are presented that were processed from a quantitative study of the dynamics of the evolution of Hα postflare loops developed after a large solar flare. The high spatial resolution of the Swedish Vacuum Solar Telescope (SVST) at La Palma provided a unique set of data for such an event.

Centre-To-Limb Variation of the Solar Granulation

An excellent series of images of solar granulation was taken during the partial solar eclipse of 1994 May 10 at the Swedish Vacuum Solar Telescope at Roque de los Muchachos Observatory. Making use of the lunar limb profile, images at different heliocentric positions were corrected for instrumental and atmospheric effects. The centre-to-limb variation of the granulation contrast was calculated and compared with previous determinations.

Kinematic Modeling of Vortices in the Solar Photosphere

The application of local correlation tracking to the proper motions of granules yields the horizontal velocity field at the solar surface. The divergence of the velocity reveals a pattern of sources and sinks. The vorticity is concentrated at sinks to form local swirls (with either sense of motion). A simple kinematic model of such a vortex, in which the radial inflow is balanced by an eddy viscosity, predicts that the vorticity should have a Gaussian profile. This prediction is confirmed by comparison with three sets of high-resolution observations, obtained from Spacelab 2 and from the Swedish Vacuum Solar Telescope on La Palma. Finally, we develop a more precise version of the model and provide an estimate of the eddy viscosity due to small-scale granulation.

The Postflare Loops and the Nearby Active Chromosphere of 1992 June 26

The main objective of this paper is to present a quantitative study of the dynamics of the evolution of Hα postflare loops developed after a large solar flare and the processed videos produced from the observations. The high spatial resolution of the Swedish Vacuum Solar Telescope (SVST) at La Palma has provided a unique set of data for such an event. A Gaussian fitting method is proposed to derive intensities and Doppler shifts from observations taken at three wavelengths (Hα center and Hα ± 0.07 nm). Moving condensations or blobs of cold material provide transverse velocities, which, together with the radial component, enable us to derive their velocity vector magnitude. Plasma velocities are around free-fall velocities near the top of the loops but are significantly smaller close to their footpoints, suggesting a deceleration mechanism. The loops are anchored in the chromosphere, which shows tremendous activity in the active region and in the nearby quiet Sun. Spicules and ejection of plasmoids are also observed at the limb.

Dust morphology of the inner coma of C/1995 O1 (Hale-Bopp)

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST)on La Palma in April, 1997. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalization, rotational derivative, temporal derivative and unsharp masking processing.

On the Dynamics of Small-Scale Solar Magnetic Elements

We report on the dynamics of the small-scale solar magnetic field, based on analysis of very high resolution images of the solar photosphere obtained at the Swedish Vacuum Solar Telescope. The data sets are movies from 1 to 4 hr in length, taken in several wavelength bands with a typical time between frames of 20 s. The primary method of tracking small-scale magnetic elements is with very high contrast images of photospheric bright points, taken through a 12 A bandpass filter centered at 4305 A in the Fraunhofer 'G band.' Previous studies have established that such bright points are unambiguously associated with sites of small-scale magnetic flux in the photosphere, although the details of the mechanism responsible for the brightening of the flux elements remain uncertain. The G band bright points move in the intergranular lanes at speeds from 0.5 to 5 km/s. The motions appear to be constrained to the intergranular lanes and are primarily driven by the evolution of the local granular convection flow field. Continual fragmentation and merging of flux is the fundamental evolutionary mode of small-scale magnetic structures in the solar photosphere. Rotation and folding of chains or groups of bright points are also observed. The timescale for magnetic flux evolution in active region plage is on the order of the correlation time of granulation (typically 6-8 minutes), but significant morphological changes can occur on timescales as short as 100 S. Smaller fragments are occasionally seen to fade beyond observable contrast. The concept of a stable, isolated subarcsecond magnetic 'flux tube' in the solar photosphere is inconsistent with the observations presented here.

Solar flares and coupling of active regions caused by spot motion: Liu Qing-zhong, Hu Fu-min % Song Mu-taoPurple Mountain Observatory, Nanjing 210008

Using the Ha-filtergrams obtained with our 26cm solar vacuum telescope, we have studied the evolution of magnetic field structure and the flares and flows in Active Region NoAA 6361, 1990 November 6 -13. We find that the flares are mostly triggered by rapid westward motion of the preceding spot p1, and all flares occur around spot p1. Also, the successive invasion motion of spot p1 causes this active region coupled with an old and dying active region to the southwest on November 11. At the separatrix, a lot of compact flares are observed, and the fibrils linking the two active regions changed their form very quickly indeed. The active phenomena do not cease until the end of the coupling. The large-scale mass motion on November 12 was due to extremely high shear and gradient in the magnetic field caused by spot p1 moving into the opposite polarity field. When spot p1 stopped moving on November 13, all activities mentioned above also disappeared completely.

An interpretation of the rapid frequency drift and polarization reversals in solar radio in terms of wave propagation: Yao Jin-xing & Yu Xing-fengPurple Mountain Observatory, Nanjing 210008

Using the Ha-filtergrams obtained with our 26cm solar vacuum telescope, we have studied the evolution of magnetic field structure and the flares and flows in Active Region NoAA 6361, 1990 November 6 -13. We find that the flares are mostly triggered by rapid westward motion of the preceding spot p1, and all flares occur around spot p1. Also, the successive invasion motion of spot p1 causes this active region coupled with an old and dying active region to the southwest on November 11. At the separatrix, a lot of compact flares are observed, and the fibrils linking the two active regions changed their form very quickly indeed. The active phenomena do not cease until the end of the coupling. The large-scale mass motion on November 12 was due to extremely high shear and gradient in the magnetic field caused by spot p1 moving into the opposite polarity field. When spot p1 stopped moving on November 13, all activities mentioned above also disappeared completely.

Timed solar eclipses in Chinese records and the secular changes in the Earth's rotation before the 8th century: Zhang Pei-yu & Han Yan-benPurple Mountain Observatory, Nanjing 210008

Observation of the main sunspot of AR11692 were carried out with the 1m New Vacuum Solar Telescope (NVST) located on the Fuxian Solar Observatory (FSO) with the Hα on 2013 march 13. The high cadence Hα intensity images show that running waves formed periodically within umbra, and propagate outwards with the shape of partial arcs. The running waves run across the umbra-penumbra boundary and disappear at the outer edge of the penumbra eventually. Confusingly, the period of the running waves within umbra is half of that within penumbra, i.e., the former is a typical 3-min oscillation with a period of 156s and the latter is typical 5-min oscillation with a period of 312s. To investigate the nature of the umbral and penumbral oscillations, we selected 300 sample points from the umbra and 150 sample points from the penumbra, respectively, of the sunspot to analyze the distributions of the extreme values related to the intensity profiles of the sample points. It is found that there are significant difference between the distributions of the extreme values related to umbral and penumbral oscillation intensity profiles. When a possible steady solar radiation, which was replaced by the average of troughs of the umbral oscillation intensity profile, was filtered from the umbral oscillation intensity, the distribution of the trough values is a normal distribution symmetrical about zero, the distribution of the crest values is a skewed distribution. When another possible steady solar radiation, which was replaced by the average of the penumbral oscillation intensity profile, was filtered from the penumbral oscillation intensity, while the distribution of the crest values and trough values were a pair of skewed distribution symmetrical about zero. According to the statistical characteristic of umbral and penumbral oscillations, an interpretation seemingly no having physical meaning was used to explain the statistical results. The possible interpretation for the phenomenon is that the pair of oscillations with different period in the intensity are produced by a same wave with a basis oscillation period of 312s. The basis wave can be expressed as Z(x,y,t)=A(x,y)cos(ωt+θ), where ω=(2π)/312 is the angular frequency of the wave. In the umbra, the intensity due to the wave is proportional to the square of the displacement of the wave, IU=a2Z2=a22A2(x,y)[1+cos(2ωt+2θ)], while the intensity in the penumbra due to the wave is proportional to the basis wave, IP=a1Z=A(x,y)cos(ωt+θ). Correspondingly, the period of the intensity within umbra is TU=156s, while the period of the intensity within penumbra is TP=312s.

Structure of the martian north polar cap and vernal hood system at L s 61‡–66‡ of the 1995 apparition

The boundary and internal structure of the north polar deposits and polar hood vernal remnant on Mars have been mapped at L s 61‡–66‡ on the hemisphere centered on longitude Ω = 0‡, using images obtained in Feb–Mar 1995 with the Swedish Vacuum Solar Telescope on La Palma. On red light images, several internal rifts, including the historically well documented Rima Tenuis and Rima Hyperborea, as well as an internal, long absent, annular rift were mapped. The ground cap was asymmetric with a mean boundary at 72‡ N for Ω = 270‡, increasing to 77‡ N at Ω = 90‡. Images in green light showed the locations of high opacity hood clouds, including an extensive outflow to 67‡ N at Ω ≈ 100‡. The state of the cap and hood is compared with the findings of previous studies and the historical significance of the annular rift structure is discussed. It is concluded, based on the structure of the deposited laminae, that the north polar climate was nearly, or possibly slightly milder than, normal at the northern hemisphere spring season studied.

New Observations of Subarcsecond Photospheric Bright Points

We have used an interference filter centered at 4305 A within the bandhead of the CH radical (the 'G band') and real-time image selection at the Swedish Vacuum Solar Telescope on La Palma to produce very high contrast images of subarcsecond photospheric bright points at all locations on the solar disk. During the 6 day period of 1993 September 15-20 we observed active region NOAA 7581 from its appearance on the East limb to a near-disk-center position on September 20. A total of 1804 bright points were selected for analysis from the disk center image using feature extraction image processing techniques. The measured Full Width at Half Maximum (FWHM) distribution of the bright points in the image is lognormal with a modal value of 220 km (0 sec .30) and an average value of 250 km (0 sec .35). The smallest measured bright point diameter is 120 km (0 sec .17) and the largest is 600 km (O sec .69). Approximately 60% of the measured bright points are circular (eccentricity approx. 1.0), the average eccentricity is 1.5, and the maximum eccentricity corresponding to filigree in the image is 6.5. The peak contrast of the measured bright points is normally distributed. The contrast distribution variance is much greater than the measurement accuracy, indicating a large spread in intrinsic bright-point contrast. When referenced to an averaged 'quiet-Sun' area in the image, the modal contrast is 29% and the maximum value is 75%; when referenced to an average intergranular lane brightness in the image, the distribution has a modal value of 61% and a maximum of 119%. The bin-averaged contrast of G-band bright points is constant across the entire measured size range. The measured area of the bright points, corrected for pixelation and selection effects, covers about 1.8% of the total image area. Large pores and micropores occupy an additional 2% of the image area, implying a total area fraction of magnetic proxy features in the image of 3.8%. We discuss the implications of this area fraction measurement in the context of previously published measurements which show that typical active region plage has a magnetic filling factor on the order of 10% or greater. The results suggest that in the active region analyzed here, less than 50% of the small-scale magnetic flux tubes are demarcated by visible proxies such as bright points or pores.

On the Dynamics of Bright Features in Sunspot Umbrae

Time series of white-light pictures of the sunspot NOAA 7522, obtained at the Swedish Vacuum Solar Telescope (La Palma), were analyzed to study the proper motion of bright features in sunspots. For the first time, its relation with the dark nuclei present in the umbra is investigated. The bright features are visible in the penumbra as bright grains moving into the umbra. A few of them cross the penumbra/umbra boundary, becoming peripheral umbral dots, which move farther into the umbra until they meet dark nuclei, braking their motion and disappearing. In some cases the encounter with a dark nucleus produces a brightening of the central umbral dots placed on the opposite side of the nucleus. A similar phenomenon is observed in the grains of a faint light bridge, when bright penumbral grains collide with one of the edges of the bridge.

Impact L Observed at a Wavelength of 892 nm with the Solar Vacuum Telescope on Tenerife

During the impact of fragment L filtergrams of high spatial and temporal resolution were obtained in the methane band centered at 892 nm with the solar vacuum tower telescope (VTT) located at Izaña, Tenerife. The VTThas an aperture of 0.70 m and a focal length of 45 m. Images were taken every 20 s in the prime focus with a CCD camera operated in 512 х 512 pixel mode. One pixel corresponds to 0.175 arcsec. The spatial resolution varied between 0.5 and 1.5 arcsec due to seeing effects. The exposure time was 2.5 s. At 22:20:52 UT, four minutes after the impact time based on Galileo images, a first brightening above the dark southeast limb of Jupiter was recognized. A first maximum was reached after five minutes. The subsequent decline of brightness was interrupted at 22:31:52 by a sudden bright flare with a lifetime of 90 s. Within our resolution the two recorded emission features are cospatial. The flux of this flare was (3.4 ± 0.2) х 10-4 of Jupiter’s total flux at this wavelength. The feature became almost invisible 19 minutes after its first detection. At 22:43 UT, 26 minutes after the impact, a new brightness enhancement rotated into view which could be identified with the dark impact signature (debris cloud) observed in the visual wavelength range.

Filigree, magnetic fields, and flows in the photosphere

The interaction between small-scale magnetic fields and horizontal photospheric flows has been studied from observations of high angular resolution\(( \sim \tfrac{1}{4}arc\sec )\) obtained with the Lockheed narrowband filter in the Swedish Vacuum Solar Telescope at La Palma. The measured magnetic flux density (αB‖) is in the range from a detection limit of ≈ 10 G to about 500 G, showing a good correspondence with the filigree. The magnetic flux elements take part in the local flows towards downdrafts at the supergranulation cell boundaries. The measured flux density, as well as the associated filigree intensity, decrease as the structures approach the downdrafts, presumably as a result of increased tilting and possibly submergence of flux into the downdrafts.

Active Region Evolution Through Coordinated Observations

On May 7, 1991, several ground-based instruments have been coordinated to observe AR 6615 (S10-W26), before and during the flight of the SERTS rocket experiment (Schmieder et al.,1992):– Imaging spectroscopy (MSDP) at the VTT telescope of Tenerife (collaboration Paris-Meudon and KIS Freiburg). Hα and NaD1 are observed simultaneously in 2D fields, with 9 and 11 wavelengths respectively (Mein, 1991).– High resolution “white light” pictures (around 4686Å) at the Swedish Vacuum Solar Telescope (SVST) of La Palma (Scharmer et al., 1985).We present below some data from both instruments. Magnetic field maps are also available from the Huairou magnetograph (China), and from the GCT telescope (KIS/Tenerife).