Context. The dynamical and topological properties of a fluid define its hydrodynamical state and energy transfer. By means of two-dimensional (2D) spectroscopy and 2D power and coherence analyses we study these properties in the solar photosphere.Aims. To obtain insight into the change of the velocity field with height in the solar photosphere we analyze 2D spectroscopic observations.Methods. Maps of the vertical velocity at four different photospheric heights are studied by means of 2D power and coherence analyses, in order to characterize the dynamical and topological properties of the velocity field in the 2D wave number domain (k(x), k(y)). (i) The power analysis shows the power amplitude and its distribution over the (kx, ky) domain for each velocity map and thus height level. We use the mean azimuthal presentation to provide a quick 1D overview. (ii) The cross-amplitude spectrum shows interrelationships between two velocity maps. We use the cross-amplitude spectrum to visualize and quantify changes of the velocity patterns with height in the photosphere. (iii) The square coherence is the normalized cross power spectrum; it represents the correlation in the (kx, ky) domain. The degree of isotropy of this quantity signifies the existence of velocity patterns with different shapes. To facilitate the visualization of the 2D power and coherence maps we calculate their 1D mean azimuthal values.Results. The 2D power and coherence analyses reveal that the velocity fields of the higher photospheric layers are different from the deeper granular layers. The loss of similarity is found to occur in the mid photosphere. The highest photospheric layers are characterized by (i) a diminution of the velocity power; (ii) a disappearance of the small velocity structures; and (iii) a tendency for larger upflow velocity structures to become asymmetric.

Based on a series of spectrograms taken with the German Vacuum Tower Telescope (VTT) at the Observatorio del Teide (Tenerife), we study the temporal evolution of granular dynamics and energy transport in the photospheric layers. We consider the ensemble of the granules cut by the spectrograph slit, modulated by wave motion, as a complex system. We describe this ensemble by the rms of the fluctuations of the observables along the slit: continuum intensity I, gas velocity v measured from line center Doppler shifts with respect to the mean profile, and line width w. The history of the rms of the observables v and w reflects the dynamical change of the system over the 20 min observation time. We find a burst-like change for both observables. However, the cross-correlation between I and v remains virtually constant, with the exception of two gaps. Using six lines of different strength we measure the rms of v in the deep photospheric layers. On the basis of this v variation we derive an upper limit of the kinetic energy flux as a function of height in the photosphere for different times during the observation. The shape of the variation with height is constant over time. A limit for the convective enthalpy flux is calculated using the temperature variations of our earlier models. Its shape remains the same over time. Taken together, these results quantify the different roles that the lower and higher photospheric layers play in the energetics of convective overshoot.

We present a 2D spectroscopic time series of an abnormal granulation region and describe the formation and decay of structures, in particular the gradual restitution of a granulation-like pattern. This behavior is discussed in relation to magnetoconvection. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spicules dominate the solar coronal mass balance. Their driving mechanisms have not yet been identified, although numerous theories have been put forward that appear to be able to explain the most basic spicule properties, i.e. their velocities and heights. We point out that some observational facts, in particular the spicular group behavior and bipolar flows, axe hardly consistent with a unique driving mechanism. Thus it may well be that different driving mechanisms axe operating at different times and places on the Sun, depending on the dynamics and magnetic topology. The basic reason why all these mechanisms produce spicules with the same observed speed must be sought in the equipartition between kinetic energy density and specific enthalpy, and also the magnetic energy density in the case of magnetic mechanisms. Equipartition between enthalpy and kinetic energy is a natural consequence of all mechanisms that axe based on a steep pressure gradient in the chromosphere, which results from either a pressure increase in the lower, or a decrease in the upper atmosphere.