It is now well established (McIntosh 1979; Makarov and Sivaraman 1983) that the filament and filament channels seen in the H-alpha spectroheliograms (or filtergrams) can be used as reliable tracers for studying the time evolution of large-scale magnetic fields on the Sun. These features represent the neutral lines between the unipolar regions of opposite polarity. Comparison of the synoptic charts compiled from H-alpha pictures with those from full-disc magnetograms for the same period shows very good agreement and hence the former can be used with confidence for time evolution studies of large-scale unipolar regions for those periods when the magnetographs did not even exist. In this paper we shall present one of the results of our study (Makarov and Sivaraman 1983, 1989) on the migration of H-alpha filaments, namely, the existence of the meridional flow on the Sun. We shall extend it further to show the participation of this meridional flow in the solar cycle variation.

A study of the daily motions of individual sunspots and of sunspot groups has been undertaken using Kodaikanal and Mount Wilson white-light observations. The Mount Wilson measurements were completed several years ago, and the Kodaikanal measurements have recently been started, using a technique identical to that used for the Mount Wilson data. Both data sets started early in this century, and both have continued to the present time. Preliminary results of a comparison of the two data sets are presented which show a good agreement between them in spot areas and motions. Analysis of the combined data set for the years that are available so far indicates that many more spots are available in such a reduction because the 12-hour time difference between the two sites allows for a more certain identification of individual spots from one observation to the next than in the case when data from only one site are used. This greatly increases the number of sunspots available for motion studies, particularly the smaller sunspots. Preliminary rotation and latitude drift reduction from the combined data set confirm earlier results from the Mount Wilson data alone.

The German solar facilities at the Obsrvatorio del Teide are described first. Then, a few examples of recent results from magnetic features are given: spatial variation and velocity fluctuation of small-scale magnetic fluxtubes in the quiet Sun, Evershed flow and magnetic field in connection with penumbral fine structure, and magnetic field variation in sunspot umbrae.

The peak at ˜ 3/22 y−1 seen for all odd-degree axisymmetric modes in the SHF analysis of ‘nominal magnetic field’ seems to be an artifact of some non-linearity between the field magnitude and the sunspot occurence probability.

Abstract Non-linear interactions between small fluid elements, magnetized or otherwise, in an energetically open nonlinear system facilitate the formation of large coherent stable structures. This is knwon as self-organization. We interpret solar granulation on all scales and the formation and evolution of some structures in solar active regions to be the result of self-organization processes occuring in a turbulent medium.

The effect of horizontal magnetic field on the onset of three-dimensional convection in a horizontal fluid layer is studied. It is found that the two-dimensional solutions are unstable to three-dimensional disturbances. A detailed bifurcation study is reported.

Recent numerical investigations of fully compressible nonlinear magnetoconvection have clarified the nature of convection in sunspot umbrae. In a shallow layer with a strong vertical magnetic field oscillations give way to travelling waves as the Rayleigh number is increased but in a deep stratified layer oscillatory behaviour only appears after secondary bifurcations. This behaviour leads to a model that explains the formation of umbral dots. Penumbral structure is more difficult to explain owing to the apparent presence of adjacent horizontal and inclined fields in dark and bright filaments. The inner penumbra lies above a transition zone where volume currents are needed to maintain an overall static equilibrium; instabilities in this region may be responsible for filamentary structure in the penumbra as well as for fine structure at the umbral-penumbral boundary.

Recent observations have demonstrated that sunspots can absorb significant p-mode wave power, and that this absorption has a well defined wavelength dependence. These observations open, for the first time, the potential to deduce semi-empirically the subsurface structure of sunspots and active regions. Several physical mechanisms have been proposed to explain the absorption. In this paper, the proposed mechanisms are reviewed and their viability in view of the current knowledge of the scattering process is discussed.

The basic aspects of wave propagation in a magnetic flux tube are reviewed, with particular emphasis on the types of flux tube that occur in the solar atmosphere. Two fundamental speeds arise naturally in a description of wave propagation in a flux tube: the slow magnetoacoustic (cusp) speed cT, which is both subsonic and sub-Alfvénic, and a mean Alfvén speed ck. Both surface and body modes are supported by a tube. It is stressed that a flux tube may act as a wave guide, similar to the guidance of light by a fibre optic, or sound in an ocean layer, or seismic waves in the Earth's crust.

Some results of nonlinear studies of flux tube oscillations are summarized showing that each step made towards the real conditions of the solar atmosphere opens the existence of qualitatively new effects which form the basis of strongly nonlinear dynamics. Among these effects are, for example: explosive instability in the presence of negative energy waves which leads to an efficient and fast energy release; the generation of secondary plasma flows and current drive which strongly influence the evolution of magnetic flux tubes and their stability conditions; formation of shocks and solitary waves, temporal brightening of flux tubes and others.

A substantial amount (≲50%) of solar acoustic (p-mode) absorption in sunspot regions has been reported by Braun, Duvall and LaBonte (1988). A number of suggestions have been advanced to account for the observed p-mode absorption in sunspots (see review by Davila in these proceedings). These include processes involving resonance absorption, radiation damping, and non-dissipative mechanisms. The main purpose of the present work is to demonstrate that any dissipative process like radiative, viscous, or resistive dissipation leads to the resonant absorption of acoustic waves incident on the sunspot tube and that the resultant heating rate can be shown to be consistent with the observed absorption of the p-mode power impinging on an isolated inhomogeneously structured sunspot.

In this paper, wave propagation in sunspot umbrae is analysed. The stratification in a typical umbra is approximated by a model atmosphere, extending vertically from a depth of a few thousand kilometres below the photosphere, to the transition region. No boundaries are imposed in the horizontal direction. It is assumed that this atmosphere is embedded in a uniform vertical field. Using a Rayleigh-Ritz variational technique, the normal mode frequencies of the umbra are calculated for different values of the horizontal wave number k. A theoretical diagnostic diagram is generated. An interesting feature of the solutions is the existence of ‘avoided crossings’, similar to those found in the study of global oscillations. The nature of the wave modes is examined by decomposing the eigenvectors into longitudinal and transverse components. In general, it is found that the character of a mode changes with height. For large k, modes with periods in the 2-3 min range, correspond to low order modes in the present calculation. Above the photosphere, they resemble slow waves.

We present the results of our study of a long sequence of high spatial resolution filtergrams obtained with the Universal Birefringent filter (UBF) at the Vacuum Tower Telescope of the National Solar Observatory, Sacramento Peak on July 14, 1987. The UBF has a band pass of 0.25A and was set sequentially in the following wavelengths, and filtergrams were obtained at each of the settings.

The quiet solar chromosphere shows three distinct regions. Ordered according to the strength of the emission from the low and middle chromosphere they are (1) the magnetic elements on the boundary of supergranulation cells, (2) the bright points in the cell interior, and (3) the truly quiet chromosphere, also in the cell interior. The magnetic elements on the cell boundary are associated with intense magnetic fields and are heated by waves with very long periods, ranging from six to twelve minutes; the bright points are associated with magnetic elements of low field strength and are heated by (long-period) waves with periods near the acoustic cutoff period of three minutes; and the quiet cell interior, which is free of magnetic field, may be heated by short-period acoustic waves, with periods below one minute. This paper reviews mainly the heating of the bright points and concludes that the large-amplitude, long-period waves heating the bright points dissipate enough energy to account for their chromospheric temperature structure.

Present views on DC current coronal heating are presented. The relation to AC mechanisms, the importance of MHD turbulence in both processes, and the convergence of presently proposed ideas is outlined.

Driven, dissipative MHD fluids often seem to undergo relaxation processes. After a turbulent formation phase, a geometrically simpler and less disordered configuration emerges. The best known example is the laboratory reversed-field pinch (RFP); similar field topologies have been proposed for solar prominences and astrophysical “flux ropes.” In a transient situation, the more rapid decay of kinetic and magnetic energy relative to magnetic helicity provides a mechanism for generating an MHD configuration with several similarities to observed RFP states. (This is the Taylor hypothesis, not unrelated to turbulent inverse magnetic cascades.) For the driven steady state, however, all quantities are supplied at the same time-averaged rate at which they are dissipated, by definition; nothing decays relative to anything else. Some other unifying principle, beyond “minimum energy” or “selective decay,” seems necessary to describe the results of driven, steady-state MHD computations. We have been attempting to adapt the principle of minimum energy dissipation rate to MHD. It is a 19th century principle that achieved some success in hydrodynamics and separately in dissipative electrodynamics.

The resistive dissipation of Alfvén waves in magnetically structured media is examined within the framework of an analytically solvable model in plane geometry. A new class of rapidly oscillations solutions is found, for which the role of resistivity extends to the whole system.

Turbulent surface convection zones of stars generate acoustic waves which contribute to the heating of chromospheres and coronae. The dissipation of limiting strength acoustic shock waves agrees well with the empirically determined chromospheric radiation loss rates. Acoustic waves with frequency and energy required for the chromospheric heating are observed in the solar atmosphere. Acoustic heating can explain the basal chromospheric emission of slowly rotating stars and constitutes a weak background in faster rotating stars; it can not explain the emission-rotation correlation and the surface variation of emission which are due to magnetic heating.

The paper deals with the surface wave propagation in the solar atmosphere. The plasma motion is supposed to be described by magnetohydrodynamic equations. In the first part of the paper the surface wave propagation on a single magnetic interface in the solar corona is considered. The plasma is assumed to be cold. Using the reductive perturbation method we derive the equation governing the evolution of nonlinear small-amplitude surface waves as follows This equation is written in a coordinate system moving with the phase velocity of linear waves. Dimensionless variables are used. The symbol H denotes the Hilbert transform. The shape of the interface is defined by the equation z = h(t,x) in the Cartesian coordinates x, y, z. The Reynolds number R determines the relative contributions of nonlinearity and viscosity. We take a source radiating a sinusoidal wave. Then the wave evolution is calculated numerically. We get that at large R the wave steepening takes place. This steepening leads to a strong increase of wave damping.

The Landau damping of sound waves in a plasma consisting of ensemble of magnetic flux tubes is discussed. It is shown that sound waves cannot be Landau damped in general but under certain restricted conditions on plasma parameters the possibility of absorption of these waves can exist. The possibility of radiative damping of the acoustic waves along the magnetic filaments is also discussed. It appears that the most plausible mechanism of damping of sound waves in a plasma consisting of magnetic filaments can be due to scattering of a sound wave by the filaments.