Oscillatory behaviour in sunspot umbrae

Using a set of series of spectra of high spectral resoltion corresponding to the same umbra on several days, we have studied the oscillations related to evolution of the two main dark cores of it.We have detected differences in the behaviour of each core and an evolution in the power spectra not related with the external appearance of the spot. A quite constant value of the ratio of power at five minutes over three minutes has been found.Paper presented at the 11 th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

Resonant oscillations in sunspot umbrae

The Uchida and Sakurai (1975), Thomas and Scheuer (1982), and Scheuer and Thomas (1981) theory of umbral oscillations as resonant modes of magneto-acoustic-gravity waves is re-examined. For an isothermal atmosphere it is found that the quasi-Alfvén approximation is not a good approximation to the complete linearized wave equations. The new results presented here show that 3 min umbral oscillation periods are fairly insensitive to magnetic field strength above some critical value. For a detailed model umbra (Thomas and Scheuer, 1982) the calculations presented here show that 3 min umbral oscillations do not depend to any great extent on the level of forcing of the oscillations for those magnetic field strengths which are observed in sunspot umbras. Modes outside the 3 min range appear, as the lowest mode, as the level of forcing is placed at deeper and deeper levels in the solar atmosphere.     

Models for different sunspot umbrae

An empirical approach to interpret the time evolution of the high spatial resolution Ca ii K line is presented. We specify the physical parameters, such as electron temperature, hydrogen density, and velocity (microturbulent and systematic) as functions of height. The electron density is obtained from scaled non-LTE solutions for hydrogen ionization. The population indices, and thus the Ca ii source functions, for a 5-level Ca ii atom are computed by using the generalized Newton-Raphson method. K line profiles are then synthesized for different evolutionary stages and are compared with the observed ones. The explanation of the ‘peculiar’ type profile is also attempted.     

Hα oscillations in sunspot umbrae

Direct measurement of intensity and photographic subtractions of simultaneous Doppler filtergrams at Hα±0.3 Å in a time-lapse sequence of McMath 11482 has shown velocity oscillations in sunspot umbrae. These oscillations exhibit the following characteristics: (a) A characteristic oscillation frequency for each umbra, with the periods ranging from 145±10 s to 180±10 s. (b) A duration of at least 8 cycles. (c) A Doppler shift of 10 km s^-1. (d) An oscillating element size of 3″ or less. (e) A visibility to a radius vector of about 0.9.     

The interpretation of oscillations in sunspot umbrae

We review possibilities for an interpretation of oscillations observed in several period bands (3 min., 5 min., 20 min.) and at different heights in sunspot umbrae. At subphotospheric depths two independent resonators are acting: A resonator for slow, quasi-transverse waves can explain the lifetimes of bright umbral dots (≥20 sec.), while a resonator for fast (acoustic), quasi-longitudinal waves could result in the 5-min. oscillations. The acoustic resonator strongly couples with the slow-mode longitudinal resonator at photospheric and chromospheric heights, the latter produces the resonance peaks in the 3-min. period band. The whole scheme of resonance levels generalizes and corroborates a chromospheric resonator model earlier proposed by the present authors. Comparisons with alternative models and recent measurements show that the present model most naturally explains the majority of observed data.     

On the temperature structure of sunspot umbrae

From a high-resolution spectrum of a sunspot umbra (1.1 < < 2.3 m) we derive models of the temperature stratification in the deep layers of the umbra. The observed spectrum is corrected for straylight using the Hi Paschen line at gl = 1.282 m. A method is described for the iterative fitting of empirical temperature models to spectral information, and the method is applied to the present data. We find that the observed profiles of 3 high-excitation lines of Sii and the observed continuum contrast between umbra and photosphere cannot be reproduced with a single one-component model of the umbral atmosphere: the Si i lines require a model that is 460 K hotter at gt _0.5 = 3 than the continuum model. This indicates that hot and cool components coexist within the umbra. A temperature model derived from the relative intensity in the wings of 3 low-excitation lines of Mgi, Ali, and Sii is not significantly different from the continuum model.Based on observations obtained at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under contract with the National Science Foundation.     

The Mg ii h line in sunspot umbrae

Observations of the Mgii h line in the umbrae of five sunspots show a spread of a factor of 3 in peak intensity. This range is larger than, and brackets, the range found in a single umbra by Lites and Skumanich (1982).     

On physical conditions in the chromosphere above sunspot umbrae

By means of an inversion of H and K Ca ii line profiles the temperature and electron density in the chromosphere above the umbrae of two sunspots have been estimated. The temperature gradient 5 K km^–1 exceeds the corresponding values in both quiet regions and plages. At a height of about 1500 km the umbra becomes hotter than the quiet region. At a temperature of about 10000 K the temperature gradient increases sharply. The electron density at 1500 km is approximately the same as that in the quiet chromosphere at the same height.     

Interpretation of oscillations in UV lines observed above sunspot umbrae

Our theory of a resonator for slow magneto-atmospheric waves in the chromosphere of a sunspot umbra has been used to check different models of the structure of the chromosphere and transition region. Oscillations of velocity and intensity in Civ, Siiv, and Oiv lines observed by Gurman et al. (1982) on the SMM spacecraft have been compared with the calculated oscillations. The observed spectrum of resonant peaks could well be explained by a gradient model of the umbral chromosphere. Different assumptions concerning the structure of the transition region do not influence the calculated resonance periods, but the amplitudes and phases of oscillations are modified. There is strong evidence for a concentration of the observed oscillations in cold fine structure elements of the transition region, even if the filling factor of such elements is very small (some few percent). Isothermal rather than adiabatic oscillations in the cold elements should be assumed in order to explain the observed fluctuations of line intensity; the relative amplitudes of pressure oscillations in the hot main component with a steep gradient of temperature are too small to explain the observed intensity fluctuations.     

The rotational temperatures of Silicon hydride isotopomer lines in sunspot umbrae

The published sunspot spectrum obtained with National Solar Observatory/Kitt Peak laboratory’s high signal to noise ratio telescope and high resolution Fourier Transform Spectrometer were used for the study. A search was made for the prominent lines of the (0, 0) and (1, 1) A²Δ − X²Π for Silicon hydride isotopomers which lie between 23500 cm⁻¹ and 24500 cm⁻¹. The presence of lines from the (0, 0) and (1, 1) A²Δ − X²Π transition of SiH molecule coincided with the sunspot umbral lines suggest that Silicon hydride appears to be a non-negligible component of sunspot umbrae. However, the presence of A²Δ − X²Π (0, 0) and (1, 1) bands of SiD was found to be doubtful because of the lack of number of well identified lines in sunspot umbral spectra. Equivalent widths have been measured for well-resolved lines and, thereby, the rotational temperatures have been estimated for the band systems for which the presence is confirmed.     

Shapes and centre-to-limb variation of the H and K lines in sunspot umbrae

The shapes of the Ca ii H and K lines in sunspot umbral spectra vary from single asymmetric peaks near the centre of the disk to almost symmetric double peaks at the limb. In addition, there are other differences in the behaviour of both H and K lines in sunspots compared to the quiet Sun. The whole complex of the phenomena observed can not be explained by large scale chromosphere motions. Instead, a satisfactory model reproducing in detail peculiarities of the umbral emission reversals contains a cloud of emitting and absorbing gas located above the chromosphere, which flows into the sunspot. The radiation field parameters in such a cloud are consistent with the concept of weak quiescent prominences above the umbra.     

On the inhomogeneous structure of the chromosphere-corona transition region above sunspot umbrae

Multiple wavelength observations of sunspot umbrae can only be expalined by an inhomogeneous, two-component model for the structure of the umbral transition region and lower corona. The ‘Wroclaw-Ondrejov sunspot model’ was a first step in this direction. This working model has now been improved using analytic expressions for the atmospheric structure in each component and fitting the free parameters to recent sunspot observations, particularly in EUV lines. The main component has a shallow transition region and a deep-set corona. The second, ‘active’ component has a vast transition region in relatively cool fine structure elements embedded in the coronal main component. The spatial filling factor of this active component amounts to 5–10% in sunspots with bright EUV plumes, but is is more than ten times smaller in sunspot without such plumes. Observations with high spatial and temporal resolutions are necessary to understand in more detail the basic physical processes.     

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.     

Chromospheric granulation

Photographs obtained in the core of H, free of parasitic continuum radiation, reveal a granular structure in supergranule centres wherever not obscured by mottles or fibrils. Granulation is seen well in the wings out to ±0.5 Å from line centre, the contrasts being largely reversed in opposite wings. The granule diameters (from boundary to boundary) are observed down to 800 km, and are typically 1200 km, both probably upper limits imposed by telescope resolution. At 0.25 Å, the intensity fluctuates over a greater range (some 14%) in the blue wing than in the red (about 10%). The fluctuation is only 4% at line centre. This smaller value represents a real variation at the local line centre, whereas the appearance of granulation in the wings reflects to a large extent Doppler shifts due to vertical velocities. There is a tendency for the bright features at line centre to be moving downwards.While of similar sizes, there is no direct superposition of chromospheric over photospheric granules.     

Chromospheric rotation

The dependence of solar rotation on the size of the chromospheric tracers is considered. On the basis of an analysis of Ca ii K₃ daily filtergrams taken in the period 8 May–14 August, 1972, chromospheric features can be divided into two classes according to their size. Features with size falling into the range 24 000–110 000 km can be identified with network elements, while those falling into the range 120 000–300 000 km with active regions, or brightness features of comparable size present at high latitudes. The rotation rate is determined separately for the two families of chromospheric features by means of a cross-correlation technique which directly yields the average daily displacement of tracers due to rotation. Before computing the cross-correlation functions, chromospheric brightness data have been filtered with appropriate bandpass and highpass filters for separating spatial periodicities whose wavelengths fall into the two ranges of size, characteristic of the network pattern and of the activity centers. A difference less than 1% of the rotation rate of the two families of chromospheric features has been found. This is an indication for a substantial corotation at chromospheric levels of different short-lived features, both related to solar activity and controlled by the convective supergranular motions.