Search

Time series of simultaneous high resolution filtergram observations in white light and in CaK2 have been employed to study the dynamical interdependence of the ‘low’ and the ‘high’ layers of the solar atmosphere, the 3-D propagation of perturbations in the stratification, and the excitation of p-mode and other oscillations. Impulsive and oscillatory perturbations have been isolated in the wavenumber and frequency regime, and their mutual interaction was also studied.Several of our findings challenge interpretation and deserve further study. •The occurrence of a granule can be clearly recognized in the upper chromosphere.•A granule induces low brightness and low oscillation amplitudes at the CaK2 level, whereas the opposite happens at the intergranular border.•It is not always evident that the ‘source’ of the perturbation is located at the lower, i.e. the photospheric layer.•It is far from being clear which components of the various perturbations contribute to a net outward mechanical energy flux.

In continuation of a series of studies devoted to the dynamics of the solar photosphere and chromosphere we have attempted to further extend the range of heights in the atmosphere towards the transition region by including observations of the He I 10830 line. We have recorded simultaneous time series of He I 10830 and Mg I 8807 spectra in the quiet solar atmosphere using the echelle spectrograph at the German Vacuum Tower Telescope in Izaña, Tenerife. The velocity signal derived from the Doppler shifts of He 10830 clearly reveals oscillatory motions. The intensity of He 10830, on the other hand, is hardly affected by the oscillations. In the cell interior the 3-min oscillations prevail. Longer periods are found ain the cell boundaries of the chromospheric network where He absorption is enhanced. The V–V phase difference spectrum between the oscillations of He 10830 and those of Mg 8807 confirms previous observations of a non-propagating component that dominates the acoustic wave spectrum in the chromosphere.

Observations of a fairly regular sunspot penumbra near disk center have been carried out using a Stokes V polarimeter at the German Vacuum-Tower-Telescope in Izaña, Tenerife. We obtained two-dimensional spectral data by scanning the limb oriented sector of the penumbra in steps of 1” with the slit parallel to the solar limb. From Stokes V and Stokes I spectra of four magnetic sensitive lines (Fe I 5247.1, Cr I 5247.6, Fe I 5250.2, and Fe I 5250.7) obtained with 1024 × 1024 pixel CCDs we have determined Doppler shifts and several magnetic parameters. The results of a correlation analysis point out a clear relation between continuum brightness and magnetic field inclination in the sense of steeper field lines in the bright penumbral filaments. There is also a doubtless connection between intensity and Doppler shift. We did find indications for a correlation between field strength and brightness which is not uniform across the penumbra, though.

We study the oscillatory behaviour of small-scale magnetic flux-tube concentrations and their immediate surroundings by analysing high spatial resolution time series of spectra of four Zeeman sensitive lines (Fe I 5247.1, Cr I 5247.6, Fe I 5250.2, Fe I 5250.7) taken simultaneously in left-hand and right-hand circularly polarized light with the Echelle spectrograph at the Vacuum Tower Telescope in Izaña, Tenerife. The velocity field inside the magnetic elements is dominated by the 5-min oscillations which also show up as periodic variations in the relative amplitude asymmetry of the Stokes V profiles. In other parameters as for instance the Stokes V peak separation we cannot detect an oscillatory component. Neither the magnetic line ratio nor the thermal line ratio is affected by the 5-min oscillations, although the latter reveals remarkable spatiotemporal variations.

Recent observational studies of the spatial and temporal relations between impulsive and periodic perturbations (Deubner and Kleineisel, 1997; Espagnet et al., 1996; see also Goode et al., these proceedings) suggest that oscillations in the solar atmosphere are excited locally and stochastically by turbulent convection. Proper characterization of the dynamical processes involved requires careful analysis of the observations in the spatial as well as in the temporal domain. Wavelet transforms (Combes et al., 1984) allow the characterization of waveforms together with the temporal delays that occur between the observed fluctuations. We have employed Morlet wavelets (Daubechies, 1992) in the present analysis. In addition to the phase relation between convective eddies and the atmospheric waves we study in particular the 2-D spatial evolution of the perturbations, to better distinguish different modes of oscillation. The observational material used is the same as in Deubner and Kleineisel (1997).

The value of p-modes of high degree l as a diagnostic for the structure and dynamics of the solar envelope is reviewed.

The amount of energy carried in the solar atmosphere by short period acoustic waves is particularly uncertain in the range of frequencies (ν ≥ 10mHz) which is potentially relevant for acoustic heating.

With the Capri magnetograph of the Fraunhofer Institute simultaneous measurements of sightline velocities and longitudinal magnetic fields have been carried out, in order to investigate the dependence of the 300-sec oscillation of sightline velocities on the magnetic-field strengths, and the oscillatory behavior of the latter, if there is any.

Photoelectric polarization measurements in a stable sunspot (type H) with a particularly dark umbra, where ‘umbral dots’ were virtually lacking, have been carried out with the Capri magnetograph. The measurements were evaluated in terms of Unno's theory to give the value and direction of the magnetic field vector. The parameters η0 = 5, β0 = 2.5 and ΔλD = 40mÅ have been adopted for the Fe I 5250 line. Taking the configuration of the sunspot into account as well as simple conditions of steadiness of the distributions to be obtained, it is possible to derive the magnetic vector field from two-dimensional records of circular and linear polarization without ambiguities.