P. N. Brandt

On the umbra-penumbra area ratio of sunspots

The area ratio of umbra to penumbra (Au/Ap) was investigated for a series of 126 sunspots observed around the maximum of solar activity in 1980. Plotting log (Au) as function of log (Ap) a linear relation is obtained with a slope of 1.10, yielding an average ratio Au/Ap of 0.24 for small spots and of 0.32 for large spots. This deviates noticeably from the commonly adopted constant value of Au/Ap = 0.21 (Allen, 1973) used e.g. for modelling the sunspot irradiance deficit (cf. Willson et al., 1981) from the Solar Geophysical Data.

Sunspot photometry and the total solar irradiance deficit measured in 1980 by ACRIM

Until now a simple Photometric Sunspot Index (PSI) model was used (e.g. Willsonet al., 1981) to describe the contribution of sunspots to the solar irradiance deficit measurement by ACRIM. In this work we replace this model by a photometry of sunspot pictures for the period of 19 August to 4 September, 1980 taking into account the individual features, like lightbridges or umbral dots, of each spot. The main results of this preliminary analysis are: (1) theAu/Ap ratios and alsos the α values vary in a wide range and are by no means constant as in the PSI model; (2) the general trend of the irradiance deficit from our analysis agrees well with the ACRIM measurements; (3) on some days there are differences of more than 50% between the deficits derived from our measurements and from the PSI model.

On Novel Methods to Determine Areas of Sunspots from Photoheliograms

Two novel methods of measuring umbral and penumbral areas of sunspots and of complex sunspot groups are described. Both methods comprise the digitization of photoheliograms by a frame grabber and the computation of intensity histograms of selected areas of activity. The first method, called ‘cumulative histogram method’, in principle determines the intensity boundaries umbra–penumbra and penumbra–photosphere from the intersections of linear fits into the corresponding parts of the cumulative histograms of sunspots. The second method, called ‘maximum gradient method’, marks image pixels of a given intensity level ±2 units wide as a white isophote on a display. Interactive variation of this level makes it easy to visually select the contour line fitting the boundary penumbra–photosphere (or umbra–penumbra) best. At the same level usually the width of the contour line is smallest. In both cases the summation of the pixel numbers above the corresponding intensity levels yields the umbral and the total sunspot areas, respectively. Some limitations of the two methods are discussed.

On the centre-to-limb variation and latitude dependence of the asymmetry and wavelength shift of the solar line λ 5576

Low noise photoelectric measurements of the line profile of the g = 0 Fe line gl 5576.097 combined with determinations of the wavelength shift of its centre calibrated by use of an I2 absorption tube are reported. Measurements taken at various limb distances (1.0 ≤ cos ϑ ≤ 0.2) and along 4 different diameters of the Sun are used to investigate the behaviour of the line asymmetry (C-shape) and wavelength shift of the line centre as functions of cos ϑ and of latitude and to search for possible pole-equator differences.An accuracy of approx. 0.8 mÅ r.m.s. is achieved for the determination of the centre of the solar line relative to the iodine lines and of 0.3 mÅ to 1 mÅ r.m.s. for the relative variations of the C-shape. The analysis shows a significant difference between the limb-effect curves along polar and equatorial diameters for cos ϑ ≤ 0.4 and changes of the C-shape for 0.9 ≥ cos ϑ ≥ 0.6 with a rather strong indication of a latitude dependence of the C-shape. This latitude dependence may account for the so-called ‘ears’ observed by Howard et al. (1980) who used the well-known Doppler compensator method which integrates the line asymmetry from the line wings to the core.

Large-Scale Photospheric Motions: First Results from an Extraordinary Eleven-Hour Granulation Observation

We present a preliminary report of a unique 11-hr observation of solar granulation obtained at the Swedish Vacuum Solar Telescope (SVST) on La Palma, Canary Islands, on 5 June 1993. The high quality of the granulation images provided correlation tracking data during every observing minute of this run. A supergranular outflow and a sink (inflow) with associated vorticity lasted for the entire time.

Solar site testing for the Advanced Technology Solar Telescope

The location of the Advanced Technology Solar Telescope (ATST) is a critical factor in the overall performance of the telescope. We have developed a set of instrumentation to measure daytime seeing, sky brightness, cloud cover, water vapor, dust levels, and weather. The instruments have been located at six sites for periods of one to two years. Here we describe the sites and instrumentation, discuss the data reduction, and present some preliminary results. We demonstrate that it is possible to estimate seeing as a function of height near the ground with an array of scintillometers, and that there is a distinct qualitative difference in daytime seeing between sites with or without a nearby lake.

Modelling solar irradiance variations with an area dependent photometric sunspot index

The He 1083 nm line equivalent width and the 10.7 cm radio flux are employed to model the total solar irradiance corrected for sunspot deficit. A new “area dependent photometric sunspot index” (APSI) based on sunspot photometry by Steinegger et al. (1990) is used to correct the irradiance data for sunspot deficits. Two periods of time are investigated: firstly, the 1980–1989 period between the maxima of solar cycles 21 and 22; this period is covered by ACRIM I irradiance data. Secondly, the 1978–92 period which includes both maxima; here, the revised Nimbus-7 ERB data are used.For both He 1083 nm and 10.7 cm radio flux irradiance models as well as ACRIM I and ERB irradiance data, the APSI yields an improved fit compared to the one obtained with the standard “Photometric Sunspot Index” (PSI) which uses a constant bolometric spot contrastα. With APSI, the standard deviation calculated from daily values is 0.461 Wm−2 for the period 1980–89 modelling ACRIM I vs. He 1083 nm, as compared to 0.478 when PSI is used, and to 0.531 for the uncorrected ACRIM series. A similar improvement is obtained for the same period modelling ERB vs. He 1083 nm, while there is almost no improvement for the long period.As a general result the models provide a good fit with the spot-deficit.-corrected irradiance only during the period between the maxima. If both maxima are included (period 1978–92) the He 1083 nm and 10.7 cm radio flux models show appreciably larger discrepancies to the irradiances corrected for PSI or APSI.

The JOSO site testing campaigns in the Canary Islands

Abstract In 1968 an informal cooperation of European solar research institutions was established under the name of JOSO (Joint Organization for Solar Observations). A brief account is given of the site testing carried out by this organization to find an “ideal” solar observatory site. After having tested nearly 40 sites in the Mediterranean and Atlantic coastal areas, it was found that, notwithstanding the great homogeneity of the maritime airmasses, coastal sites were not suitable for high resolution solar observations. Consequently, in the search for mountain sites imbedded in highly homogeneous air, two sites were chosen in the Canary Islands: Izana (Tenerife) and Roque de los Muchachos (La Palma). Aircraft and radiosonde tests of the microthermal vertical profile in the atmosphere have shown the superior homogeneity of the Atlantic airmass above the Canary Islands. An extended test of 160 days (in 1979) of meteorological and solar observations performed simultaneously with two telescopes, both photographically and with photoelectric seeing monitors, has led to the conclusion that the Izana site is somewhat superior to the site at Roque de los Muchachos as far as daytime seeing is concerned. Measurements during and after the campaign have also shown that for certain periods during daytime the telescopes installed at both sites appear to be inside the large-scale airmass whose residual inhomogeneities limit the night-time seeing.

On Photospheric Flows and Chromospheric Corks

Proper motions of granules are measured by local correlation tracking on a 4.5 h image sequence obtained with the Swedish Vacuum Tower Telescope at La Palma. A 2 arcsec spatial low-pass filter is applied to obtain meso-scale flow patterns. We find that their characteristic lifetime (1/e value) has a lower limit of five to six hours. Comparison with a simultaneous co-spatial sequence of chromospheric Ca II K2V images shows that these flows sweep supergranulation cells clean in about the same period. A chromospheric “persistent flasher”, seen during three hours in Ca II K2V , migrates to the magnetic network as if it were a photospheric cork.

Turbulence, Fractals, and the Solar Granulation

We give a brief, mostly qualitative introduction into the topics of convection and turbulence, and their description, mainly referring to laboratory experiments. Following Mandelbrot (1967), the concept of the fractal dimension is introduced and some earlier results of measurements of the fractal dimension in laboratory turbulence are discussed. Next, we address the question whether hints of turbulence have been observed in the solar photosphere, and describe three independent methods of determining the fractal dimension of the solar granulation: the area-perimeter relation, the line and the plane intersection method. An analysis of a set of high resolution granulation photographs taken with the balloon-borne’ spektro-Stratoskop’ telescope yields a fractal dimension of d ≈ 1.9 for granules of diameters > 1.32 arcsec and of d ≈ 1.3 for smaller granules, analysing the area-perimeter relation of approx. 40 000 granules. At first sight we seem to confirm the results obtained by Roudier and Muller (1986), Darvann and Kusoffsky (1989), and by Karpinsky (1990), who claim to see a splitting of the granulation into two regimes of different fractal dimension. However, a more detailed investigation of the analysis technique reveals that: i) there exists a smooth transition of the fractal dimension from small to large granules, and ii) the fractal dimension of small granules seems to be dominated by the finite resolution, and therefore no positive statement concerning the turbulent origin of small granules seems possible with the present technique. The fractal dimensions determined from the same material with the other two methods (i.e. the line and the plane intersection method), applied to both the intensity pattern itself and the lane map, all range between 1.88 and 1.97.