Local sunspot oscillations and umbral dots

Data analysis of sunspot oscillations based on a 6-hr SDO run of an observation showed that low frequency(0.2 ω 1 m Hz) oscillations are locally similar to three and five minute oscillations.The oscillations in the sunspot are concentrated in cells of a few arcsec, each of which has its own oscillation spectrum. The analysis of two scenarios for sunspot oscillations leads to a conclusion that local sunspot oscillations occur due to a subphotospheric resonator for slow MHD waves. Empirical models of a sunspot atmosphere and the theory of slow waves in thin magnetic flux tubes are applied to modeling the subphotospheric resonator. The spectrum of local oscillations consists of a great number of lines. This kind of spectrum can occur only if the subphotospheric resonator is a magnetic tube with a rather weak magnetic field. Magnetic tubes of this sort are umbral dots that appear due to the convective tongues in monolithic sunspots. The interrelation of local oscillations with umbral dots and wavefronts of traveling waves in sunspots is discussed.     

Observations of sunspot umbral velocity oscillations

Sunspot umbral molecular lines have been used to look for the oscillatory velocities in the umbra. Power spectrum analysis showed conspicuous power for periods in the range between 448 and 310 s. The maximum peak-to-peak amplitude of the umbral oscillatory velocity component is observed to be in the order of 0.5 km s^–1.Visiting Astronomer, Solar Division, Kitt Peak National ObservatoryOperated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Photospheric and chromospheric umbral dots in a decaying sunspot

Time-sequenced H filtergrams and narrow-band blue filtergrams (₀ = 4308 Å, = 10 Å) of umbral dots in a decaying sunspot were studied. The results are: (a) Photospheric umbral dots have lifetimes of about 40 min. (b) Two types of proper motion were found for photospheric umbral dots. Umbral dots born in the umbra or in the light bridge show virtually no proper motion. On the other hand, umbral dots of penumbral origin move inward to the umbra with speeds of about 0.4 km s ^–1. (c) Chromospheric umbral dots, which have dimensions of 0.6 × 1.2 in the mean, were more numerously found than photospheric umbral dots. (d) Photospheric umbral dots were observed to be associated with chromospheric umbral dots. Thus umbral dots are not phenomena confined to photospheric levels but also extend to chromospheric levels. (e) Some of the chromospheric umbral dots are unrelated to the photospheric umbral dots. They may be excited by the infalling matter from the umbral corona.Contribution from the Kwasan and Hida Observatories, University of Kyoto, No. 266.     

On umbral flashes in different sunspot groups

The results of a statistical investigation of the occurrence of umbral flashes for 40 sunspot groups are reported for the period 1966–1983. The following characteristics were chosen for the analysis: (a) position on the solar disk; (b) group area; (c) sunspot area; (d) maximum magnetic field strength of a sunspot; (e) modified Zürich class; (f) sunspot age; (g) magnetic structure; and (h) flare activity of a group. The dependence of umbral flashes on magnetic structure of a sunspot is the most essential feature. The absence of umbral flashes in the umbrae of main sunspots perhaps may be used as one of the predictors of flare activity.     

About the foreshortening effect on sunspot umbral dots

Using high-resolution pictures of the core of a unipolar sunspot observed with several cos values, we studied the center limb effect on the form of the bright umbral dots. The ratio of the apparent sizes in radial and tangential direction do not show the foreshortening effect typically observed in granular structures.On leave from University of Tabriz, Iran.Observational part of this work was done during a stay of S.K. at Sacramento Peak Observatory, U.S.A., as NRC Resident Research Associate.     

The spectrum of umbral oscillations of the Scheuer-Thomas sunspot model

Proper periods are calculated for oscillations of an isothermal gaseous layer with a homogeneous vertical magnetic field, limited by two rigid horizontal boundaries. The obtained results are in good agreement with the observed umbral oscillations in a sunspot.     

Identification of Zirconium Oxide molecular lines in the sunspot umbral spectra

The ZrO molecule has been detected in sunspot umbrae through the identification of following laboratory molecular transitions: ¹Σ⁺ ? X¹Σ⁺ (0, 0), A³Φ₂ ? X²Δ₁ (0, 0), A³Φ₃ ? X²Δ₂ (0, 0), A³Φ₄ ? X²Δ₃ (0, 0), B³Π₂ ? X³Δ₃ (0, 0), B³Π₁ ? X³Δ₂ (0, 0) and B³Π₀ ? X³Δ₁ (0, 0) in red – infrared region using high resolution, visible range Fourier Transform Spectrum of sunspot umbra observed at the National Solar Observatory in Kitt Peak (NSO/KP). Much new identification has been made in the searched spectral wavenumber region from 16650 cm^?1 to 18007 cm^?1 of sunspot spectrum. Equivalent widths of well resolved lines, versus rotational quantum number J have been used to determine the effective rotational temperature for seven bands of the ZrO molecule. This result agrees well with the temperatures derived for other molecules’ presence in sunspot umbrae. It is evident that ZrO molecular lines are formed in higher layers of the atmosphere of relatively “cold” sunspots.     

An attempt of identification of barium hydride molecular lines in sunspot umbral spectra

A high-resolution sunspot umbra spectrum recorded in National Solar Observatory, Kitt Peak in the visible and infrared wave number range 13, 600 – 25, 000cm⁻¹ was taken in the present study for identifying the rotational lines of barium hydride (BaH) molecule. Number of chance coincidences was evaluated for the A ²Π_1/2 – X ²Σ ((0,0), (1,1), (2,2), (1,0), (2,1)), A ²Π_3/2 – X ²Σ ((0,0), (1,1), (2,2)), B ²Π_1/2 – X ²Σ (0,0), B ²Π_3/2 – X ²Σ (0,0), C ²Σ – X ²Σ ((1,1), (1,0), (2,2), (2,1), (3,2)) and D ²Σ – X ²Σ ((1,0), (2,0), (3,0), (4,0), (5,0), (8,0), (9,0)) band systems of BaH using line identification procedure. The obtained number of chance of coincidences was compared with I- parameter values. The highly resolved rotational lines were chosen to evaluate equivalent widths using triangle approximation method. The effective rotational temperatures were calculated for the bands (0,0), (1,1), (2,2) and (2,1) of A ²Π_1/2 – X ²Σ, (0,0) (1,1) and (2,2) of A ²Π_3/2 – X ²Σ, B ²Π_1/2 – X ²Σ (0,0) and B ²Π_3/2 – X ²Σ (0,0) of BaH molecule. The rotational temperature values calculated for these bands were found to be in the range 1185 – 3514 K. They were also compared with the already reported sunspot temperatures.     

Solar cycle variation of sunspot intensity

Broad band pinhole photometer intensity observations of 15 large sunspots covering the spectral region 0.387–2.35 m are presented. The data are based on measurements on approximately 500 days during the period June, 1967 to December, 1979.We have found real and significant intensity differences between large sunspots. These differences may be explained by a systematic variation in the umbral temperature throughout the solar cycle. A connection between umbra intensity and heliographic latitude is discussed.No center-limb variation in the umbra/photosphere intensity ratio is detected. We have searched for possible connections between umbra intensity and a number of other sunspot parameters, like the spot size, without detecting any significant correlation. We conclude that the umbra/photosphere intensity ratio seems to be a unique function of epoch for large sunspots.     

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).     

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).     

An upper limit of the swan band intensity in a sunspot spectrum

There is no evidence for the C₂(0,1) band in an investigated sunspot spectrum. Upper limits are given for the band intensity.     

On the dependence of sunspot minimum intensity on area

Highly resolved photographs of 25 sunspots and pores of different areas in 2 continuous wavelengths, obtained in summer 1966 at the Sacramento Peak Observatory, have been used to reinvestigate the dependence of sunspot minimum intensity on area. Special care was taken to correct for parasitic light caused by blurring or image motion and by light scattering in the instrument and atmosphere. We find a very small, if any, dependence ofI _min on sunspot area in contradiction to older measurements but in agreement with Sitnik's and Zwaan's statements.     

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.     

Relative sunspot numbers in the first half of eighteenth century

We revised relative sunspot numbers in the time interval 1700–1748 for which Wolf derived their annual means. The frequency of daily observations, counting simultaneously the number of sunspots and the number of sunspot groups necessary for determinating Wolf's relative sunspot numbers, is in this time interval very low and covers, on average, 4.8% of the number of all days only. There also exist incomplete observations not convenient to determine relative sunspot numbers. To enlarge the number of daily relative sunspot numbers we used the nonlinear, two-step interpolation method derived earlier by Letfus (1996, 1999). After interpolation, the mean value increased to 13.8%. Waldmeier (1968) found that the scaling factor k can be derived directly from the observed number of spots f and from the number of sunspot groups g. From the observations made at Zürich (Wolf and his assistants, Wolfer), at Peckeloh, and at Moncalieri during the years 1861–1928, we derived a new, more correct empirical relation. The resulting annual relative sunspot numbers are given in Table II. However, only for 26 years (53.0%) from the total number of 49 years was it possible to derive annual relative sunspot numbers. The observations were missing for the other years. This corresponds with results of Wolf, which gives the annual relative sunspot numbers for all 49 years. For the years when the data were missing, he marked these values as interpolated or very uncertain ones. Most of the observations originate from two data series (Kirch, Plantade), for which Wolf derived a higher scaling factor (k=2.0) than followed from the newly derived relation (k=1.40). The investigated time interval covers four solar cycles. After our results, the height of the first cycle (No. –4), given by Wolf, should be lowered by about two-thirds, the following two cycles (Nos. –3 and –2) lowered by one-third, as given by Wolf, and only the height of the fourth one (No. –1) should be unchanged. The activity levels of the cycles, as represented by group sunspot numbers, are lower by about one-fourth and, in the case of the first one (No. –4) even by two-thirds of the levels derived by us. The group sunspot numbers, derived from a much greater number of observations, have also greater credibility than other estimates. The shapes of the cycles, as given by Wolf, can be considered only as their more or less idealized form.     

Structure of a sunspot

From enlargements of patrol photographs of the disk passage of the sunspot of July 20 – August 2, 1966, intensity profiles across the spot are obtained at several positions near the disk-center and at each limb. It is found that these profiles show asymmetric features near each limb (increasingly sharp limb-side penumbra and poorly resolved disk-side penumbra) which are similar to those reported in Paper III of this series. It is suggested that these profile asymmetries are the essential feature of the center-limb variations in the appearance of a sunspot which have become known as the Wilson effect.Conventionally the Wilson effect is described as the extreme foreshortening and eventual disappearance of the disk-side penumbra and, recently, Suzuki has referred to this as the occultation of the penumbra by the photosphere. We find no evidence at all for the disappearance of the disk-side penumbra at the limb in this spot. Defining half-height points on the profile curves as the umbral and penumbral boundaries, we find that, near the west limb where the spot is stable and regular, the limb-side penumbra increases by about 10% at the expense of the umbra. This result qualitatively supports the results reported in Paper III although it is smaller in magnitude.Other observations of sunspots which appear to exhibit the conventional Wilson effect are discussed and it is concluded that in no case yet published is the resolution and seeing of sufficient quality to demonstrate unambiguously the disappearance of the disk-side penumbra.     

The umbral flash as a magneto-acoustic wave phenomenon

The suggestion that an umbra flash may be caused by a magneto-acoustic wave phenomenon is examined. It is suggested that the flash in Ca ii lines is formed during the compressional stage in a magneto-acoustic wave. The compression which is assumed to be adiabatic will produce a rise in temperature and a corresponding increase in number of Ca ii atoms. The variations in line emission (absorption) coefficient of the Ca ii K-line are calculated on this assumption and are found to be in general agreement with the observed variations. Other observed quantities as proper motion, magnitude of line shift etc., also agree with the wave hypothesis. Further observations which may serve as tests on the wave hypothesis are suggested.