Library

Notes: Abstract Modifications to a Zeiss 1/4 Å filter are described which allow high spatial resolution observations of the line-of-sight velocities and magnetic fields in the photosphere and in sunspots. First results show: (1) the granular velocity field to be very strong; differences in upward motions in the granules and downward motions in between are as much as 6 km/sec; (2) the Evershed effect in sunspots to originate primarily in the dark regions between bright penumbral filaments.

Notes: Abstract 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.

Notes: Abstract The principles of operation of photoelectric solar magnetographs are described in terms of the Poincaré sphere. The performance of photographic methods for measuring solar magnetic fields is compared with that of photoelectric magnetographs.

Notes: Abstract Observations of the intensity distribution near the solar limb at 2.43 and 22.5 μ, show the absence of limb brightening to within 1 or 2 arc sec of the limb. Observations at 1.2 mm indicate limb brightening at this wavelength. These results are compared with the Utrecht Reference Photosphere and with existing data on the solar flux in the millimeter range, and suggest that the temperature minimum is broad and extends above τ 5000 = 2 × 10−3. A sharp rise of temperature is required above τ 5000 = 10−5.

Notes: Abstract From simultaneous filtergrams obtained in the blue and red wings of a Fraunhofer line we analyzed the velocity and intensity field at the center of the solar disk. Results are as follows: (a) Cross correlation between velocity and intensity is 0.6. It increases somewhat when long wavelengths (〉5000 km) are eliminated. (b) An rms velocity and intensity variation of 0.45 km/sec and 3.9%. The velocity power spectrum and the turbulent velocities measured from line widths and curve of growth are consistent with a power distribution 39-01 of the form 39-02 where k is the wave number.

Notes: Abstract From an investigation of spectra in a magnetically sensitive (λ 6173, g = 2.5) and insensitive line (λ 5576, g = 0), we derived the following properties for a symmetrical sunspot: (a) The magnetic field strength varies with the distance ρ(ρ ⩽ 1) from the sunspot center like H(ρ) = H(0) (1 + ρ 2)-1 (b) The zenith angle of the magnetic field varies like 90°ρ. From this and from H(ρ) we find a height gradient of 0.5 gs/km at ρ = 0. (c) The equivalent width and the half width of λ 5576 show an increase in penumbral regions of maximum Evershed flow. Most likely this is due to a combination of inhomogeneities in the Evershed flow and ‘microturbulence’. (d) We find the magnetic field strength to be larger in the dark interfilamentary regions of the penumbra. These regions move downwards with respect to the bright filaments and probably have a more horizontal magnetic field. (e) In a weak light bridge and in extensions of bright penumbral filaments into the umbra, we find a decrease of the magnetic field strength, and a more horizontal field direction with respect to the umbral surrounding. li(f)

Notes: 7. Conclusion The author of a review article is undoubtedly the one who benefits most from it. Only by reviewing an entire subject does it become clear how much is known of it and in what areas more information is desired. In the past 10 years, spicules have probably been the best studied fine structures on the sun. A substantial amount of observational information on spicules is available as seen in Section 3. Some of the most important questions that remain to be answered in greater detail than available now are, in my opinion, the following: (1) Do spicules indeed diffuse after they reach their maximum growth and brightness? If so, is this an expansion of the spicule magnetic field, a change of the properties of the gases surrounding the spicule, or an actual diffusion of spicular matter across the spicular magnetic fields? (2) Is the group behaviour of spicules as that described by Lippincott (1957)? If true, it would imply that the spicule mechanism extends over a large area of the sun (100000 km), which conflicts with the spicule theories as given in Section 4. (3) Is the ‘tilt’ of spicule-emission lines real? If so, is it caused by spicule rotation and how does it differ between the various spicule-emission lines? (4) Is the spicule diameter different in the different spicule-emission lines? (5) Is it possible to measure the spicular magnetic field How large is it? (6) Do the physical conditions vary from spicule to spicule? How do they vary with height and time within one spicule? For this, one needs simultaneous spectra of spicules in many lines which have a different temperature behaviour. (7) Is it true that both the bright and dark elongated fine mottles seen in disk spectroheliograms are spicules? This requires renewed study of the solar disk. (8) After a definite identification of spicules with disk structure is made, what can one learn about spicule properties from the disk study? One can, for example, try to find a direct link with the solar granulation. (9) Are there spicules in active regions of the sun? How do they differ from spicules in quiescent regions? (10) Are chromospheric grains perhaps spicules which do not grow upwards because of a lack of magnetic fields? Are they perhaps related to granules? (11) What are the implications of a breakdown of the ‘statistically steady-state’ assumption in the spicule-intensity calculations? The answers to many of these questions are of great importance in the precise understanding of a spicule, and in the derivation of a magnetohydrodynamic model for it.

Notes: Abstract 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.