We analyse time sequences of high spatial resolution filtergrams obtained simultaneously in Mg b(2) and Ca K-2 upsilon from quiet Sun disc centre with the Vacuum Tower Telescope at Observatorio del Teide, Tenerife. Special attention is paid to the temporal evolution of the bright points, or K grains, in the interior of the chromospheric network. These represent the intensity maxima of brightness oscillations in K-2 upsilon with large amplitude. The oscillations last 1 to 5 periods of 150-240 s duration and are strongly non-sinusoidal. They do not appear to be excited by pulses from below and then to decay. They rather exhibit the beat phenomenon of a horizontally extended wave field. The high amplitude oscillations occur only rarely, during 5-10% of the time, if we pose the limit that high amplitude means that the maximum intensity is a factor 1.5 larger than the average intensity. The power spectrum does not show any signature of a chromospheric ''3 min'' mode nor of a mode at the acoustic cutoff frequency (period 3.5 min), nor do we find a power ridge at constant frequency. Instead, as in our earlier finding, the chromospheric oscillations are ordered in modes in continuation of the 5 min modes. Modeling will thus need to adopt that the waves propagate in a three-dimensional medium with, at least partially, reflecting boundaries. The bright points of the chromospheric network behave very differently, more chaotic, like noise. They show most power at low frequencies which we interpret as the consequence of the stochastic intensity fluctuations.

We present observations of simultaneous filtergram time sequences in Mg b2, Ca K and Halpha obtained from quiet Sun disc centre with the Vacuum Tower Telescope at Observatorio del Teide, Tenerife. Fourier analyses are performed to obtain power, coherence and phase spectra in the k-omega plane. There, the dominant features are the wellknown ridges of the 5 min resonant modes. Yet in the chromosphere the ridges extend to high wavenumbers (wavelengths almost-equal-to 1.3 Mm) and to high frequencies (periods almost-equal-to 105 s). Neither the famous chromospheric ''3 min'' oscillations nor an oscillation at the acoustic cutoff frequency (period 210 s) appear exceptionally pronounced. The signature of gravity waves is indicated from phase relations.We distinguish between the behaviour in the interior of the chromospheric network and on the boundary. The network boundary behaves less oscillatory than the interior. In snapshots of chromospheric intensities the K grains (Beckers 1964), or, synonymously the bright cell points, appear in the cell interior. They represent the phases of high temperature of a wave field with partly resonant and coherent properties. (We take intensity fluctuations as proxies for temperature fluctuations.) The waves are only partly upward propagating p-modes with a multitude of eigenvalues in frequency and wavenumber, like the subphotospheric p-modes. We suggest that an excitation mechanism acts within the chromosphere itself to drive the waves. This could explain the phase relations between intensity and velocity oscillations.

A photographic spectrogram with high spatial and spectral resolution, taken from a plage region close to the disc centre of the Sun with the Gregory-Coude telescope at the Observatorio del Teide/Tenerife, is analysed. The spectral region around 6150 angstrom contains lines of neutral and singly ionised Iron, Vanadium, and Sodium which we select to extract information on the structure of line gap regions. The gaps are most pronounced in the Fe I line, not so much in the other lines. In the continuum intensity no signature of the line gaps is found at the spatial resolution limited by diffraction and seeing. Line gap profiles show redshifts of 6-8 m-angstrom compared to the average non-gap profiles, the fluctuation of their position in wavelength corresponds to velocities V(rms) almost-equal-to 200 m s-1. The asymmetry of the line gap profiles indicates increasing downflow with increasing depth.Theoretical line profile calculations for comparison with the observations are based on the following model assumptions: a hot, slender tube with the magnetic field perpendicular to the solar surface is embedded in a surrounding gas that is cooler than the average quiet solar atmosphere. Hydrostatic equilibrium is assumed for the magnetic tube and the surrounding gas to calculate pressure and density. The cool gas moves downward with a speed of several km s-1, while the gas in the tube is at rest. By suitable parameter choices and averaging it is possible to reproduce the observed intensity profiles. However, with this type of velocity field, we are unable to explain the reported asymmetries of the Stokes V profiles from network boundaries and plages.

Des spectrogrammes en H α a 2 dimensions du centre du disque solaire sont analyses pour determiner la structure spatio-temporelle du reseau chromospherique

Afin d'etudier la dynamique de la chromosphere solaire, on fait une analyse de Fourier des sequences dans le temps des filtregrammes photographiques Hα obtenus