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Within the last few years significant progress has been made in our understanding of the small-scale structures in active regions. Here I present some of the newest findings obtained by using speckle interferometric techniques. There exist continuum bright points with a contrast of about 30% that are cospatial with strong magnetic fields. The observations are consistent with the assumption that some facular and network bright points are the white-light signature of magnetic fluxtubes with a diameter of about 200 km. Magnetic elements larger than about 300 km are mainly darker than the average quiet sun. Their properties are similar to what has been called magnetic knots or invisible sunspots. In highly magnetic areas there is no clear relationship between continuum intensity and magnetogram signal at the smallest spatial scales. The magnetic field of pores extends beyond the dark umbra. There radially elongated structures may appear that are similar to penumbral filaments in sunspots.
We analysed calibrated Stokes V magnetograms and simultaneously measured Stokes I spectra of high spatial and spectral resolution taken in a medium sized sunspot. We found a clear (anti-) correlation between the brightness variation of penumbral structures and the longitudinal component (B*cosγ) of the magnetic field. No azimuthal variation of the amount of the magnetic field strength (B) was observed across dark and bright structures. There the field is more vertical in bright filaments compared to dark ones.
Results of a study of longitudinal magnetic fields in active regions are presented. The observed magnetic field strength increases with height in the photosphere. The maximum of the magnetic field intensity coincides with the level where the central parts of λ5324,2 Å FeI and λ5269,5 FeI line profiles are formed. On the Hβ formation level the observed magnetic field intensity is smaller as compared with the potential one calculated on the basis of the observed field in FeI λ5253, 5Å line. The difference between the observed magnetic field and potential one is explained in terms of transverse electric currents. The current value can mount to 3×1011 A.
We present the preliminary results of photoelectric measurements of line-of-sight motions in the quiet and active photosphere. We show the topology, amplitudes and signs of Doppler motion distribution in active regions and their surroundings. We pinpoint the different characters of both topologies.
Examples of coherence functions between continuum intensity and line center velocity fluctuations are discussed for non active and active regions. It seems that the magnetic field leads to a different phase shift between intensity and velocity fluctuations than for non active regions.
Motions of large, stable sunspots – which are in some cases even recurrent – are determined using the Vacuum Tower Telescope (VTT) at Tenerife and sunspot drawings taken at the Kanzelhöhe Observatory in Austria. Around the sunspots fields of about 100 by 100 arcseconds are covered by a scanning spectrograph entrance slit in steps of one arcsecond every 15 seconds to determine the plasma velocities. The main aim is a determination of plasma and sunspot velocities within larger fields of activity and a search for special patterns of plasma flows, which reflect the well-known braking of the sunspot rotation velocity during aging.
Observations of a fairly regular sunspot penumbra near disk center have been carried out using a Stokes V polarimeter at the German Vacuum-Tower-Telescope in Izaña, Tenerife. We obtained two-dimensional spectral data by scanning the limb oriented sector of the penumbra in steps of 1” with the slit parallel to the solar limb. From Stokes V and Stokes I spectra of four magnetic sensitive lines (Fe I 5247.1, Cr I 5247.6, Fe I 5250.2, and Fe I 5250.7) obtained with 1024 × 1024 pixel CCDs we have determined Doppler shifts and several magnetic parameters. The results of a correlation analysis point out a clear relation between continuum brightness and magnetic field inclination in the sense of steeper field lines in the bright penumbral filaments. There is also a doubtless connection between intensity and Doppler shift. We did find indications for a correlation between field strength and brightness which is not uniform across the penumbra, though.
The relationship between the magnetic vector and the temperature of a large symmetric sunspot is studied on the basis of 1.56 µm spectra. From this relation we estimate the shape of the τ = 1 surface. i.e. the Wilson depression, as a function of radial distance in the sunspot. We find that the Wilson depression is relatively small throughout the penumbra and changes by 200–500 km at the umbral boundary. We also estimate the magnitude of magnetic gradients and curvature forces.
An analysis of Stokes I and V profiles of 1.56 µm lines of sunspots near the solar limb shows that the magnetic field continues outside the visible contours of sunspots in the form of a low-lying superpenumbral canopy. We also find that the V profiles formed in the canopy exhibit the Evershed effect (with line shifts of 1–2 km s−1), while the matter below it shows no sign of a flow. Therefore, the Evershed effect definitely is present beyond the visible sunspot boundary. However, if we interpret the line shifts in terms of stationary flows, then only a small fraction of the matter seen to be flowing outwards in the penumbra can be accounted for by the outward flow in the superpenumbral canopy. Therefore, although the Evershed “flow” does not stop at the boundary of the spot, most of the flowing matter stops or disappears there.
The local relation between the temperature and the magnetic field in sunspot umbrae is studied. Sunspots with different sizes have been studied and we found a linear relation between temperature and the square of the magnetic field. We have found also a dependence between the minimum continuum intensity and the size of the spot.
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).
The distribution of sunspot decay rates is studied using the Greenwich Photoheliographic Results (GPR). The decay rates are seen to be lognormally distributed. An analysis of the lognormal distributions associated to each sunspot group type and for isolated spots is presented. As a remarkable property of the decay process, we show that it happens at a constant total to umbral area ratio.
A sample of 221 active regions with δ magnetic configurations from 1986 to 1991 in Solar Cycle 22 is used to investigate the characteristics of δ-sunspots and the relationship between the δ-sunspots and X class X-ray flares.
In this paper the dynamics of the photospheric magnetic field near the sunspot of the active region McMath 15508 is considered on the basis of longitudinal magnetic field B11 magnetograms. The magnetograms were obtained at the panoramic magnetograph of the Sayan Observatory. The time taken to obtain one magnetogram was 5 min. The spatial resolution was 1.8“ × 3.6“. In the active region McMath 15508 a new bipolar spot group formed westward of the existing sunspot group on September 1; the next day the main sunspots had penumbras, and on September 3 the new sunspot group reached a maximum area and began decaying thereafter.