Relative umbral intensity of a large sunspot

Simultaneous observations of relative umbral intensities in four wavelength regions are presented. In the visual wavelength region the umbral intensities show lower values than given by most authors. By observing the same spot during different seeing conditions the method of correction for stray light is found to be consistent within the accuracy of the method. In addition, a new simple correction method is suggested.     

Intensity profiles of a sunspot near the solar limb

Photo-electric observations of the intensity profile of a sunspot at a heliocentric angle of 76° at four wavelengths are reported. An intensity peak at the centre-side penumbra-photosphere border predicted by Jensen et al. (1969) has been observed.     

Infrared photometry of a sunspot near the disk center

A time series of broadband images of a sunspot near the disk center was obtained simultaneously in two wavelength bands at 0.56 μm and 1.55 μm at the German VTT on June 17, 1998. We computed intensity difference images of the best frame pairs which reveal information about the facular distribution in the present field of view. Faculae are found around pores, in the quiet granulation and as well around the sunspot penumbra.     

The rotational temperature of the FeH molecules in a large sunspot

Equivalent widths for four FeH bands (0–0), (1–0), (2–0), and (2–1) versus rotational quantum number J have been used to determine the rotational temperature of the molecule. The average value obtained from the first three bands is $T_{\mathit{rot}}=2900\mathrm{K}\pm 300\mathrm{K}$. This result agrees well with the temperatures derived for other molecules in sunspot umbrae. The current value is notably higher than that obtained earlier by (Mulchaey, J.S., 1989. Pub. Astron. Soc. Pacific 101, 211) for the same molecule.     

The variation of the mean size of the photospheric granules close to and away from a sunspot

We study the mean size of granules as a function of distance from the boundaries of the sunspot penumbra. We use for the determination of the mean size two different methods, a visual and a photometric. In all cases the mean diameter of the granules away from the spot was greater than the mean diameter of the granules in the neighbourhood of the penumbra. Our study is based on an excellent sequence of photos, taken at the Pic-du-Midi Observatory on May 11, 1979.     

The intensity,velocity and magnetic structure of a sunspot region

High resolution sunspot photographs in the blue, red and infrared continuum exposed on various days were used to derive the center-to-limb variation of the intensity ratio = I _sp / I _ph. Special care was taken to correct for image blurring, scattered light and the influence of line absorption.The observed increase of specific umbral intensities _u towards the limb leads to an extremely small temperature gradient in the umbra. From geometrical changes of the profiles (Wilson effect), we derived an umbral depression of about 650 km and a density scale height of about 450 km when H ^- is assumed to be the predominant source of absorption. The penumbral depression was found to be 50 km or less. The density scale height of the umbra as computed from the observed temperature distribution is 80 km in the case of hydrostatic equilibrium. We conclude that either magnetic pressure components produce deviations from hydrostatic equilibrium or that another source of absorption, dominating in the outer layers, has to be taken into account.     

The intensity,velocity and magnetic structure of a sunspot region

A time sequence of high-resolution sunspot photographs, exposed almost simultaneously in two continuum wavelengths (4680 Å and 6400 Å), was used to study some properties of umbral fine structures (umbral dots). The lifetime of the umbral dots is found to be 1500 sec. Photometry of some bright dots leads to an observed intensity excess of 0.129 I _phot and 0.134 I _phot in the blue and red respectively. The observed mean diameter of the dots is found to be 420 km. These values still include the action of image blurring. From the color index the true intensity and diameter of the dots are estimated. It appears that the umbral dots are in reality of photospheric brightness having true diameters of 150–200 km. The spatial distribution of the dots in sunspot umbrae is discussed. Some peculiarities in recent sunspot magnetic-field observations may be explained by magnetic inhomogeneities associated with umbral dots.Presently guest investigator at the Göttingen Observatory.Previously member of the High Altitude Observatory solar project at Sacramento Peak (Contract Nr. AF (628) - 4078).     

Development of structure in pores and sunspots: flows around axisymmetric magnetic flux tubes

Flux elements, pores and sunspots form a family of magnetic features observed at the solar surface. As a first step towards developing a fully non-linear model of the structure of these features and of the dynamics of their interaction with solar convection, we conduct numerical experiments on idealized axisymmetric flux tubes in a compressible convecting atmosphere in cylindrical boxes of radius up to 8 times their depth. We find that the magnetic field strength of the flux tubes is roughly independent of both distance from the centre and the total flux content of the flux tube, but that the angle of inclination from the vertical of the field at the edge of the tube increases with flux content. In all our calculations, fluid motion converges on the flux tube at the surface. The results compare favourably with observations of pores; in contrast, large sunspots lie at the centre of an out-flowing moat cell. We conjecture that there is an inflow hidden beneath the penumbrae of large spots, and that this inflow is responsible for the remarkable longevity of such features.     

The intensity,velocity and magnetic structure of a sunspot region

The observational set-up for a detailed study of the velocity, intensity and magnetic-field fine structure in and around a sunspot is described. On highly resolved spectra we detected in the vicinity of a sunspot a large number of points with strong magnetic fields (magnetic knots). The magnetic field in these knots causes a striking decrease of the line depth (or a line gap after Sheeley, 1967). The properties of the magnetic knots are: (1) magnetic fields up to 1400 gauss; (2) diameter 1100 km; (3) coincidence with dark intergranular spaces; (4) generally downward material motion; (5) lifetime>30min; (6) estimated total number around an unipolar spot 2000; (7) combined magnetic flux comparable to the sunspot flux; (8) coincidence with Ca⁺ plages.For the smallest sunspots (pores) we obtained magnetic fields >1500 gauss. Hence a magnetic field of about 1400–1500 gauss appears to be a rather critical level for pore and spot formation.We found a large number of small areas producing line gaps without measurable magnetic field. These non-magnetic gap-regions coincide with bright continuum structures.Some aspects arising from the occurrence of hundreds of magnetic knots in an active region are discussed in the last section.Presently guest investigator at the Göttingen Observatory.Previously member of the High Altitude Observatory solar project at Sacramento Peak (Contract Nr. AF (628) - 4078).     

Magnetic field and flares of the large sunspot group of April–May 1978

From magnetograms of the active region 15266 obtained at Yunnan observatory we found that 1) an inverted field configuration and a twisted neutral line are both closely correlated with high-energy flare eruptions. After the flares, the field configuration and the neutral line revert to more stable states. 2) Most of the flares of the present region occurred away from the neutral line; those near the line occurred either in regions of high field gradient or in the vicinity of Severny's “neutral points”.     

Large-scale structure of a sunspot and its surrounding photosphere

Preliminary results of a study of photographic and photometric properties of the large-scale ( 3) structure of a sunspot and its surrounding photosphere are given. Stratospheric direct frames of the solar photosphere were used in the study. Isophotes located immediately beyond the outer edge of the penumbra were of an irregular form and reflected bright and dark regions. No presence of either a sunspot bright outer ring or inner ring was detected. The photospheric structure and its behaviour with time were, in fact, unchanged up to the very boundary of the penumbra.A distribution of the smeared intensity in a sunspot has been derived. The mean brightness of the penumbra is I _PU 0.62 I and umbra I _U < 0.15 I . An analysis of the obtained results allowed us to make a conclusion that the area of the dark penumbral regions exceeds that of the bright penumbral regions, and the condition S _BR/S _DR < 1 should be fulfilled in the penumbrae of sunspots.     

Free convection and mass transfer effects on the magnetohydrodynamic flows near a moving plate in a rotating medium

An exact solution of the free-convection flow near an infinite vertical plate moving in a rotating medium in the presence of foreign mass and a transverse magnetic field is presented under a constant heating of the plate. It is apparent from this solution that the effects of the motion, the temperature, and the mass transfer are linear and, hence, can be studied independently. Three applications of physical interest are discussed. The non-magnetic case and non-rotating case are are also discussed.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, on 30 May 1988.     

Recurrent magnetic activity,sunspot number and its rate of decline

Power spectral densities computed from low-latitude horizontal intensity of the Earth's magnetic field over two-year periods of declining phases of solar cycles 16 to 19 show a close relationship with the maximum relative sunspot number of the following solar cycles. The maximum sunspot number shows an exponential rise with the power density near 1/27 cd^?1; maximum R _z,however, increases linearly with power density near 1/14 cd^?1. It is also shown that the rate of decline of sunspot number in a solar cycle is almost exactly related, linearly, to power spectral density for the preceding solar cycle. Power densities near 1/27 and 1/14 cd^?1 in declining phase of solar cycle appear to be satisfactory indices for the maximum relative sunspot number of the following cycle and its rate of decline thereafter.     

Comparison of the microwave and soft X-ray emission above a sunspot

The Westerbork Synthesis Radio Telescope (WSRT) 6 cm radio observations of the active region HL 16864 large spot (Strong, Alissandrakis, and Kundu, 1984) are compared with X-ray data obtained from the Flat Crystal Spectrometer (FCS) onboard the Solar Maximum Mission satellite on May 25, 1980. The X-ray data confirm the presence of a temperature depression above the spot umbra in agreement with suggestions obtained from radio data analysis. Significant differences in the spatial distribution of both kinds of emission observed in the corona above this spot are attributed mainly to the strong resonant character of the cyclotron radio radiation. Some differences are also caused by both the relatively low efficiency and the low spatial resolution of the FCS. Deconvolution of X-ray images allows to see the new structures and enhances the mutual correlation between X-ray and radio pictures.     

A comparison of the oscillations in sunspot umbrae,penumbrae, and the surrounding photosphere

Time series of the nonsplit Fei 7090 Å line have been observed in several sunspots with a 100 x 100 diode array corresponding to 48 arc sec times 1.39 Å. The spatial behaviour of Doppler motions along one fixed slit position has been studied as a function of time. Former results are confirmed, that the power in the five minute range decreases from the photosphere to the umbra, where, however, values still well above the noise level are measured. Regarding the penumbra, the power tends to exhibit a maximum at locations where the line-of-sight component of a radial horizontal field should be maximal. This indicates that the direction of the oscillatory velocities might be influenced by the magnetic field or the Evershed flow. No significant power is found in the 3 min range. An exception might be seen in a small patch at the limb of the umbra of one spot.     

The effect of partial ionisation of a gas on the electrical conductivity,with reference to sunspot umbrae

Partial ionisation of a gas allows there to be anisotropies in the electrical conductivity in the presence of a magnetic field. The components of the conductivity tensor are calculated using the data from three sunspot models. The largest component of electrical resistivity is found to be at least an order of magnitude smaller than the thermal diffusivity in the same region.     

Observations of the longitudinal magnetic field in the transition region and photosphere of a sunspot

The Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission spacecraft has observed for the first time the longitudinal component of the magnetic field by means of the Zeeman effect in the transition region above a sunspot. The data presented here were obtained on three days in one sunspot, have spatial resolutions of 10 arc sec and 3 arc sec, and yield maximum field strengths greater than 1000 G above the umbrae in the spot. The method of analysis, including a line-width calibration feature used during some of the observations, is described in some detail in an appendix; the line width is required for the determination of the longitudinal magnetic field from the observed circular polarization.The transition region data for one day are compared with photospheric magnetograms from the Marshall Space Flight Center. Vertical gradients of the magnetic field are computed from the two sets of data; the maximum gradients of 0.41 to 0.62 G km^–1 occur above the umbra and agree with or are smaller than values observed previously in the photosphere and low chromosphere.