Author name code: nesis ADS astronomy entries on 2022-09-14 author:"Nesis, Anastasios" ------------------------------------------------------------------------ Title: Anisotropy and dynamics of photospheric velocity patterns: 2D power and coherence analyses Authors: Nesis, A.; Hammer, R.; Schleicher, H.; Roth, M. Bibcode: 2012A&A...542A..85N Altcode: 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 (kx,ky). (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. Title: Spicules: Energetics and the Role of Magnetic Waves Authors: Hammer, R.; Musielak, Z. E.; Routh, S.; Nesis, A. Bibcode: 2008ESPM...12.3.11H Altcode: The class of spicule-like processes comprises a variety of phenomena that were given different names, depending on how and where on the Sun they can be observed - including (classical limb) spicules, macrospicules, FUV spicules, mottles, fibrils, and the "type II spicules" recently discovered with Hinode. The relationships between these phenomena are not fully clarified. We point out that the reported observed properties cannot all be reconciled with a single driving mechanism. Indeed, many different mechanisms have been suggested in the past, all with some success. We argue that this was ultimately possible because the observed properties are consistent with an approximate equipartition between enthalpy, kinetic energy, and presumably also magnetic energy. One of the most popular driving mechanisms of the last few years invokes the guiding of global p-mode oscillations along magnetic field lines that are sufficiently inclined against the vertical, so that the cutoff frequency for longitudinal waves drops below the excitation frequency. We show that in such a situation transverse, and perhaps also torsional, waves are much more promising, because of their weaker cutoff restrictions even for the small inclinations that are typical for limb spicules. Indeed there is now growing observational evidence for transverse waves. We point out the need to understand better the cutoff properties of various kinds of magnetic waves. Title: Velocity Pattern Evolution Within the Photosphere Authors: Nesis, A.; Hammer, R.; Schleicher, H.; Roth, M. Bibcode: 2008ESPM...12.2.34N Altcode: The solar photosphere is the dynamical interface between the convection zone and the chromosphere. It is compressible, convectively stable, and affected by the overshooting granular flow. The photospheric dynamics must thus be investigated as the continuation of the granular dynamics as it spills over into the stable layers.

We investigate empirically the non-oscillatory small-scale velocity field of the photosphere. We are particularly interested in the temporal and height variations of the dynamics and its topological behavior, i.e. in the evolution of velocity patterns in comparison to the granular intensity patterns.

Our analysis is based on time series of 2D spectra taken with the triple etalon spectrograph TESOS at the VTT on Tenerife. Oscillations were filtered out in the Fourier domain. In a 2D time-series analysis, power spectra demonstrate the rapid decay of the vertical overshoot velocities with height by a factor 2 within less than 300 km above the surface, thus implying a decay of the associated kinetic energy flux density by nearly two orders of magnitude over the same height interval. As expected, this decay of the energy flux is accompanied by a change of the scales in the wavenumber domain. 2D coherence maps quantify the drastic change of the pattern of the velocity field with height: While the continuum layers are still governed by the typical granular-like structuring with small-scale isotropy, the higher layers show elongated patterns of upflow and downflow regions with short fragmentation and reorganization time scales. According to a cross-correlation analysis the extension of the granular upflows into the upper photosphere is a strongly local process, suggesting a burst-like nature of the granular velocity.

Over the scale of the field of view, the velocity field loses its horizontal isotropy with height. This suggests the action of a structural instability of the deeper layers. It is an open question which dynamical processes in the overshoot layers cause these effects. The fragmentation and immediate reorganization of the velocity field of the upper photosphere merit further study. Title: Velocity Pattern Evolution in the Solar Photosphere Authors: Nesis, Anastasios; Hammer, Reiner; Schleicher, Helmold Bibcode: 2007AN....328..702N Altcode: No abstract at ADS Title: The Multiple Time Scales of Solar Granulation Authors: Hammer, Reiner; Musielak, Zdzislaw E.; Nesis, Anastasios; Routh, Swati; Schleicher, Helmold Bibcode: 2007AN....328..703H Altcode: No abstract at ADS Title: Dynamics of the solar granulation. IX. A global approach Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H. Bibcode: 2006A&A...451.1081N Altcode: 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. Title: Topology and dynamics of abnormal granulation Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 2005AN....326..305N Altcode: 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. Title: A metatheory about spicules Authors: Hammer, R.; Nesis, A. Bibcode: 2005ESASP.560..619H Altcode: 2005csss...13..619H No abstract at ADS Title: Are there multiple spicule driving mechanisms? Authors: Hammer, R.; Nesis, A. Bibcode: 2004ANS...325...78H Altcode: 2004ANS...325..P02H; 2004ANS...325a..78H No abstract at ADS Title: Topological changes of abnormal solar granulation surrounded by pores. Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 2004ANS...325...77N Altcode: 2004ANS...325..P01N; 2004ANS...325a..77N No abstract at ADS Title: What Controls Spicule Velocities and Heights? Authors: Hammer, R.; Nesis, A. Bibcode: 2003csss...12..613H Altcode: Numerous mechanisms have been suggested to drive spicules. Many of them need a careful fine-tuning of free parameters in order to achieve the basic characteristics, like velocity and height, of observed spicules. There might, however, be general physical mechanisms that control these properties. We show that whenever upper chromospheric plasma is exposed to a significantly non-hydrostatic pressure gradient, it starts moving upward at the observed speeds. The plasma can reach significant heights, at least if it receives some net chromospheric heating during the rising phase. Therefore, such a hydrodynamic mechanism might help other (magnetic) drivers to control the basic properties of spicules. We suggest therefore to consider a new class of spicule driving mechanisms, in which the plasma is not only accelerated by wave or magnetic forces from below, but also by the generation of a low pressure region above the chromosphere. Such a situation could arise e.g. due to an instability in magnetic loops or as a result of the reconfiguration of open field lines. Title: Time Variation of Statistical Properties of the Solar Granulation Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 2003ANS...324Q..55N Altcode: 2003ANS...324Q.P08N No abstract at ADS Title: Evolution of the Solar Granulation Dynamics Authors: Nesis, Anastasios; Hammer, Reiner; Schleicher, Helmhold Bibcode: 2003ANS...324..103N Altcode: 2003ANS...324R.P09N No abstract at ADS Title: Dynamical Dichotomy of Granules Smaller and Larger than 1200 km Authors: Nesis, Anastasios; Hammer, Reiner; Schleicher, Helmhold Bibcode: 2003ANS...324..102N Altcode: 2003ANS...324R.P08N No abstract at ADS Title: A New Class of Driving Mechanisms for Solar Spicules Authors: Hammer, R.; Nesis, A. Bibcode: 2003ANS...324...56H Altcode: 2003ANS...324b..56H; 2003ANS...324..P10H No abstract at ADS Title: Merging and Splitting Phenomena in the Solar Granulation: A Spectroscopic Investigation Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 2003ANS...324R..55N Altcode: 2003ANS...324Q.P09N No abstract at ADS Title: Evolution of the Granular Dynamics and Energy Transport Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 2003SPD....34.0702N Altcode: 2003BAAS...35..820N Based on series of excellent spectrograms taken at the German Vacuum Tower Telescope (VTT) at the Observatorio del Teide (Tenerife), we study the temporal evolution of the granular dynamics and the energy transport in the photospheric layers. We consider the ensemble of the granules cut by the spectrograph slit as a complex system. We describe this ensemble by the rms of the fluctuations of the granular observables along the slit: continuum intensity I, Doppler velocity v, 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 minutes observation time. We find for both observables a quasi-periodical change. However, the history of the cross-correlation between I and v remains virtually constant, with the exception of two gaps. We measure the rms of v in the deep photospheric layers for six lines of different strength included in the spectrograms. Using a model velocity variation based on our previous publications, we assign photospheric heights to the velocity measurements. These heights agree with those calculated by other means. On the basis of this v variation we calculate the kinetic energy flux as a function of the height in the photosphere for different times during the observation. The form of the variation with height turns out to be constant in time. The convective energy flux, finally, is calculated from the measured velocity and the temperature variations of our earlier models. Again we find practically the same variation form over the time of the observation. Taken together, these results quantify the different roles that the lower and higher photospheric layers play for the energetics of the convective overshoot at the upper boundary of the superadiabatic region of the Sun.

A.N. acknowledges travel support from the German science foundation DFG. Title: Dynamics of the solar granulation. VIII. Time and space development Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H. Bibcode: 2002A&A...396.1003N Altcode: We study the evolution of the granulation dynamics from the observational point of view. Based on series of excellent spectrograms taken at the VTT, Observatorio del Teide (Tenerife), in 1999, we calculated temporal - spatial maps of the Doppler velocity, line width, and intensity in order to track the dynamical behavior of these observables at different positions along the spectrograph slit. The Doppler velocity map reveals a granular dynamical time - the characteristic time associated with the decay of the Doppler velocity - of approximately 2 min, while the line width map does not show any characteristic time scale but rather a strong intermittence. The intensity map reveals the life time of the granulation as it is given in the literature. The granular dynamical time is practically equal to the value determined from spectrograms taken at the solar minimum 1994; so the dynamical time does not show any change over the solar cycle. The stochastic properties of the Doppler velocity and intensity data samples are studied (i) by means of their statistical moments and (ii) theoretically using presupposed model distributions. For the latter we estimated the distributions' parameters by means of the maximum likelihood method. The histograms of the Doppler velocity variations point to an asymmetric model distribution, while the histograms of the intensity variations infer a symmetric one. The intensity variations can be described well by a Gaussian probability density function, while the Doppler velocity variations are described by the double exponential (Gumbel) distribution, an asymmetric probability function. A remarkable result of the statistical analysis based on both series of observations in 1994 and 1999 is the unambiguous lack of flows with large velocity amplitudes within the intergranular space. Title: Dynamics of the solar granulation. VII. A nonlinear approach Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H. Bibcode: 2001A&A...373..307N Altcode: We investigate the attractor underlying the granular phenomenon by applying nonlinear methods to series of spectrograms from 1994 and 1999. In the three-dimensional phase space spanned by intensity, Doppler velocity, and turbulence (line broadening), the granulation attractor does not fill the entire phase space, as expected from the high Reynolds and Rayleigh numbers of the photospheric plasma, but rather shows a highly structured form. This could be due to the correlations between intensity, turbulence, and velocity, which represent also the Reynolds stress. To obtain insight into the dimensionality of the attractor, we use the time lag method, a nonlinear method that enables us to get information about the underlying attractor of a dynamical system (granulation) from the measurement of one physical quantity only. By applying this method to the observed Doppler velocities, we show that the granulation attractor can be described by three independent variables. The dimension of the granulation attractor seems to be independent of the appearance of big granules and shear flow. Furthermore, the power analysis of the Doppler velocity shows power down to the spatial resolution of the instrument (0.3 arcsec). In order to decide whether the power at the smallest scales is real or noise, we use again the time lag method in combination with either a high pass digital or wavelet filter, which filters out the large wave numbers. It appears that the power at the smallest scales represents a real signal. Title: Dynamics of the solar granulation - On the Time Variation of the Granular Flow Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H.; Soltau, D.; Staiger, J. Bibcode: 2001SoPh..200...11N Altcode: The emergence and evolution of large granules shows thegranular dynamics particularly well. We therefore investigate the time dependence of the convective flows within a regular and an exploding granule. The observational material for this study was taken at the center of the solar disk with the German VTT in Izaña (Tenerife, Spain) during an observing campaign in the year 1994. It consists of series of spectrograms of high spatial resolution, which were digitized and processed with wavelet techniques. Among other features, our data show the dynamical portrait of a regular and an exploding granule. We can follow their temporal evolution over more than 12 min. Using absorption lines of different strength we are able to see the dynamical change of both granules at several heights within the first 200 km above τ5000=1. The observations reveal significant changes of the convective flow of both granules over time as well as over height, which are discussed in detail. Title: Development of the Dynamics of Solar Granulation in Space and Time Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H. Bibcode: 2001AGUSM..SP41B03N Altcode: We address the evolutionary behavior of the granulation dynamics from an observational point of view by analyzing a series of excellent spectrograms taken at the VTT in Izaña/Tenerife (Spain) in 1999. We present temporal-spatial maps of the Doppler velocity, turbulent line broadening, and intensity variations, which allow us to study the dynamical behavior of these observables at different positions on the solar surface. The Doppler velocity maps reveal a granular dynamical time -- the time associated with the Doppler velocity decay -- of approximately 2 min, while the turbulence map does not show any characteristic time but a strong intermittency. The intensity map reveals the life time of the granulation as it is given in the literature. The granular dynamical time is practically equal to the value determined from spectrograms taken during the solar minimum in 1994; so the dynamical time does not show any change over half a solar cycle. The stochastical properties of the Doppler velocity and intensity data samples were studied (i) by means of their statistical moments and (ii) by using theoretically model distributions. For the latter we estimated the distributions' parameters by means of the maximum likelihood method. The histograms of the Doppler velocity variations point to an asymmetrical model distribution, while the histograms of the intensity variations infer a symmetrical one. Thus the intensity variations can be described well by a Gaussian probability density function, while the Doppler velocity variations can be described by a Gumbel distribution an asymmetrical probability function. A remarkable result of the statistical analysis of both the observations from 94 and 99 is the unambiguous lack of flows with large velocity amplitudes within the intergranular space. Title: Dynamics of the Granulation: A Non-Linear Approach Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H. Bibcode: 2000SPD....31.0101N Altcode: 2000BAAS...32..801N Observables like Doppler velocity, intensity, and turbulence (line broadening) can provide insight into the physics of the granulation -- i.e., into the physics of the upper solar convective layers. So far, measurements of these observables have been processed by means of a power and coherence analysis, which is actually connected with the physical concept of modes in linear theories. The upper solar convective layer, however, is a highly nonlinear dissipative system. According to theoretical considerations, such a system may approach a strange attractor in its phase space with time. Based on a series of spectrograms taken at the German VTT on Tenerife in the summer of 1999, we address the granulation attractor and its dimension from an observational point of view. In the three-dimensional phase space spanned by the observables intensity, Doppler velocity, and turbulence, the granulation attractor shows a high level of structuring. By means of the time-lag and correlation integral methods applied to the Doppler velocities we found (i) that the granulation attractor can indeed be described by only three variables and (ii) that its dimension seems to depend on the appearance of enhanced shear flow (shear turbulence) at the granular borders. This means that the dynamical system underlying the large scale granulation is a low dimension attractor. The time-lag and correlation integral methods enable us also to decide between noise and signal: in the case of pure noise the method does not converge. We found that the residual velocity associated with the small sub-granular scales does converge, however, in higher than 10 embedding dimensions. This implies that for small scale variations the underlying attractor is not a low dimension one. Title: Dynamics of the solar granulation. VI. Time variation of the granular shear flow Authors: Nesis, A.; Hammer, R.; Kiefer, M.; Schleicher, H.; Sigwarth, M.; Staiger, J. Bibcode: 1999A&A...345..265N Altcode: Excellent spectrograms can yield observational insight in the dynamics of the solar surface not yet accessible to numerical simulations. We present results of the elaboration of a series of spectrograms taken at the center of the solar disk. Each of the spectrograms includes more than 250 granules, while the series covers a time of 12 min. Our main emphasis is to study the dynamics of the visible solar layers not only as a function of height but also as a function of time. We investigated the temporal and spatial behavior of the turbulent concentration at the granular borders and its spreading-out into the intergranular space. In the deep photosphere, enhanced turbulence is concentrated predominantly near granular borders, while at higher layers the turbulence spreads out over the entire intergranular space. Remarkable is the decay of the turbulence with the height in the photosphere. There was no significant variation of the turbulence over the 12 min. We also determined the rms turbulent pressure at the granulation layers near tau_ {5000}=1. The average ratio of turbulent to gas pressure is of the order of 0.1; values of this size are also discussed in recent theoretical works. In order to take the intermittency into account, we traced the peak to peak variations of the turbulent velocity, which turn out to be ~ 4 km sec(-1) . The corresponding ratio of turbulent to gas pressure may thus reach locally significant values up to about 0.3. We did not find either a correlation or an anticorrelation between turbulence and convective flow, although the turbulence is presumably generated by granular shear flow. We suggest that the intermittent turbulence in the visible layers and the convective flow constitutes a dynamical system. This turbulence-granulation-dynamical system exhibits a cyclic behavior corresponding to the dynamical time of the granules, i.e. the growth and decay of their velocity profile. The power spectra of the turbulent and granular velocity show a two-component character, which presumably reflects the action of two different processes determining the dynamics of the solar convective boundary layers and above. Title: Granulation and its variation with time Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H. Bibcode: 1999AGAb...15...89N Altcode: 1999AGM....15..P04N The velocity fields on the solar surface are influenced by large granules. They are stochastic events of convective origin which affect the dynamics of the solar layers in various ways, for example, by shear flow. Shear flows on the other hand produce turbulence, and thus turbulent pressure is capable of back reacting upon the convection. This leads to three principal questions: Is there a characteristic time associated with the decay of the granular velocity amplitudes? What is the nature of the attenuation of the velocity with time? What interactions take place with the local oscillations? These questions will be addressed through the discussion of spectrograph observations made at the VTT. Title: Time Variation of Granular Dynamics Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1999soho....9E..74N Altcode: We found observationally well determined changes of the convective flow of both regular and exploding granules over time as well as over height. The attenuation of the velocity amplitude of both types of granules must be attributed to different processes. Changes of granular flow take place within time intervals (dynamical time) of 3 to 5 mins, significantly shorter than the mean life time of a granule. Furthermore the attenulation process is different at different heights in the photosphere. The observational material for this study consists of series of spectrograms of high spatial resolution taken at the center of the solar disk with the German Vacuum Tower Telescope in Izana (Tenerife, Spain) in 1994 and 1998. The series shows the dynamical portrait of a regular and an exploding granule within the first 200 km above the continuum, which can be followed over 12 min, more than the life time of a granule. Title: Granular Shear Flows - Influence on Helioseismology Authors: Nesis, A.; Hammer, R.; Kiefer, M.; Schleicher, H. Bibcode: 1998ESASP.418..829N Altcode: 1998soho....6..829N Over the past few years, turbulence and the associated turbulent pressure in the upper convective layer have been recognized as potentially important for the physics of solar oscillations. The turbulent kinetic energy per volume, the turbulent pressure Pturb, could affect the thickness of the strongly superadiabatic layers and thus some of the results of helioseismology,(see Rosenthal 1998). According to Stein and Nordlund (1998), turbulent pressure is important in extending the mean atmosphere in the superadiabatic layers, which lowers the eigenfrequencies of medium and high ell modes. Our main emphasis in this paper is to provide observational support for this influence. Series of spectrograms of high spatial resolution, taken at the center of the solar disk with the German Vacuum Tower Telescope in Izana (Tenerife, Spain) in 1994 and 1997, represent the observational material for this study. The spectrograms were digitized and processed with wavelet techniques and regression analysis. The turbulent pressure Pturb is determined by the velocity in the granular layers. This velocity can be measured as Doppler shifts if it is spatially resolved, and as enhanced line widths otherwise. The resolved flow velocities are typically found to be ~1 km sec-1, both horizontally and vertically (Nesis and Mattig 1989). This yields a contribution to the ratio of turbulent to gas pressure of the order of Pturb / Pgas 0.12. Values of this size are also discussed in recent theoretical works (e.g., Stein and Nordlund 1998). Unresolved velocities are observed in terms of enhanced line broadening. We find these velocities to be highly intermittent along the slit, especially at locations with steep velocity gradients near the borders of large granules. In order to take this intermittency into account, we trace the peak to peak variations of the unresolved velocity, which turn out to be ~4km sec-1. The corresponding contribution to the turbulent pressure near the solar surface is thus likewise intermittent and amounts up to Pturb = 104 Pa. The ratio Pturb / Pgas may thus locally reach significant values up to about 0.5. Furthermore, we found that resolved and unresolved velocities in the granule/intergranule system cannot be related by a regression line, i.e. they are neither correlated nor anticorrelated globally. Rather the convective flow and the turbulence should be interpreted in terms of a turbulence-granulation dynamical system, which may indeed affect helioseismology. This system reveals a periodic cycle similar to the growth/decay models described by the Volterra-Lotka equations. The power spectra of the turbulent and granular velocity show a two-component character, which presumably reflects the action of two different processes determining the dynamics of the solar surface layers. Title: Dynamics of the deep solar photosphere at supergranular scales Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1998IAUS..185..451N Altcode: Extending our previous studies of the dynamics of solar granulation we investigated the relationship between granular flow and the emergence of turbulence in the deep photosphere. Our main emphasis is to explore if such a relationship exists, and if so, to define it quantitatively. To this end we take advantage of the excellent signal approximation property of wavelets. Spectrograms of high spatial resolution taken at the center of the solar disk with the German Vacuum Tower Telescope in Izana (Tenerife, Spain) in 1994 and 1995 represent the material for this study. The spectrograms were digitized and processed with wavelet techniques and regression analysis. The latter was applied to granular convective flow and the apparently associated turbulence in order to investigate their mutual connection. We found that granular flow speed and turbulence cannot be related by a regression line; rather the convective flow and the turbulence appear to be related by an attractor in the convective flow speed--turbulence phase space. Thus, it is well possible that the convective flow and turbulence can be interpreted in terms of a dynamical system; and both quantities can now be described mathematically and not only phenomenologically as in the past. This will have consequences for our understanding of the p-mode excitation and provide better insight into the physics of the origin of the turbulence in the deep photosphere and its implications for helioseismology. Title: The Solar Intergranular Space: Time and Height Variability Authors: Nesis, A.; Hammer, R.; Kiefer, M.; Schleicher, H. Bibcode: 1998ASPC..154..658N Altcode: 1998csss...10..658N We investigate the turbulent velocity field and its interaction with the granular flow as a function of height in the photosphere and over one turn-over time of the granule. We are using a series of spectrograms which includes absorption lines of different strengths. The spectrograms were taken at the center of the solar disk with the German Vacuum Tower Telescope (VTT) in Izana (Tenerife, Spain) in 1994. The processing of the 7 best spectrograms from a series covering 12 min shows that the intergranular space is always turbulent whereas the granule reveals a practically pure laminar convective flow. In the deep photosphere, the turbulence in the intergranular space is concentrated predominantly near the granular border. At higher layers, however, the turbulence spreads out over the entire intergranular space. Remarkable is the decay of the turbulence with the height in the photosphere. Title: Dynamics of the solar granulation. IV. Granular shear flow. Authors: Nesis, A.; Hammer, R.; Hanslmeier, A.; Schleicher, H.; Sigwarth, M.; Staiger, J. Bibcode: 1997A&A...326..851N Altcode: Strong velocity gradients at granular borders appear to be the source of unresolved velocity fluctuations detectable as line broadening variations of magnetically and thermally insensitive absorption lines. Based on spectrograms of high spatial and spectral resolution taken with the German Vacuum Tower Telescope(VTT) in Izana (Tenerife) we study the strong velocity gradients and the unresolved velocity field as well as their mutual interaction. We also investigate the variation of these quantities with the height in the photosphere, for both a regular and an exploding granule. By means of a coherence analysis we study, furthermore, the extension of the convective and turbulent fluctuation field of the granulation layers into the overlying overshoot layers as a function of the wavenumber. The results of the coherence analysis are consistent with, and complementary to, those obtained from the investigation of regular and exploding granules. The small and large scales of the convective and unresolved velocity field behave clearly different as far as their penetration into the overlying photospheric layers is concerned. One pressure scale height above the continuum we find an unresolved velocity field that does not show any resemblance to the same velocity field at the continuum level. We find that the symmetry behavior of the unresolved velocity field with respect to the granular flow varies with the height in the photosphere. The unresolved velocity field could be of oscillatory, convective, or turbulent character. However, the fact that the unresolved velocity field is more prominent at the granular border, which is also the location of strong shear flow, favors its turbulent character. In this sense the granules can be seen as quasi-laminar convective flows emerging in the turbulent field of the overshoot layers. Title: Evolution of the Granular Shear Flow Authors: Nesis, A.; Hammer, R.; Kiefer, M.; Schleicher, H. Bibcode: 1997BAAS...29.1121N Altcode: No abstract at ADS Title: Turbulent and convective velocity fields in the solar photosphere. Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1996AGAb...12..164N Altcode: No abstract at ADS Title: Dynamics of the solar granulation. V. The intergranular space. Authors: Nesis, A.; Hammer, R.; Hanslmeier, A.; Schleicher, H.; Sigwarth, M.; Staiger, J. Bibcode: 1996A&A...310..973N Altcode: This investigation is based on a spectrogram of extraordinary spatial resolution selected from a series of 80 spectrograms taken with the vacuum tower telescope at Izana (Tenerife) in May 1994. The wavelength range was λλ: 491.00-491.40nm and includes both magnetically sensitive and insensitive spectral lines. The spectrograph slit intersected parts of the border and interior of CaII network cells, thus permitting a comparative study of the granular dynamics at varying, but moderate, levels of magnetic activity. As diagnostic tools we use the Doppler shift variation of line cores, which is associated with spatially resolved velocity structures, and the line broadening variation, which is a signature of unresolved velocity fluctuations. We discuss in particular the granular dynamics and the intermittency of the line broadening within the intergranular space as functions of height and position relative to network cells. Our results suggest that the magnetic field in the network is not only located preferentially in the intergranular space, but furthermore coincides with regions of enhanced line broadening. We confirm that the Doppler shift variation is reduced in regions of enhanced magnetic field, but we find that this reduction affects the entire range of granular scales. The slopes of the velocity power spectra are independent of the magnetic activity level. This result is surprising, since on the basis of classical MHD turbulence theory one would have expected shallower power spectra in magnetically active regions. The line broadening variation is much less sensitive to the magnetic field than the line shift variation. Title: On the Turbulence of the Solar Photosphere Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1996AAS...188.0202N Altcode: 1996BAAS...28..820N Velocity fields of convective origin and unresolved velocity fluctuations ("turbulence spots") are distributed on the solar surface in characteristic ways. The velocity field fluctuations (measured as Doppler shifts) show a pattern similar to that of the granulation, while the turbulence spots are concentrated in the intergranular space near the granular borders and are apparently connected with shear flows. Doppler velocity fields as well as turbulence spots are tightly connected with the dynamics of the granular layers but seem to influence the overlying layers. Emerging ordered laminar convective flows produce shear flows which subsequently generate turbulence, apparently a major controller of the atmospheric dynamics of the sun. A central issue is the extension of the granular dynamics into the overlying photospheric layers. In this investigation we address mainly the turbulence spots: the change of their distribution with height in the photosphere, their generation, and their relationship to the granular velocity. We are also interested in the granular velocity patterns and their extension into the photospheric layers. Our observational material consists of spectrograms of excellent spectral and spatial quality. Doppler velocity field and turbulence are measured simultaneously at various heights in the photosphere by means of absorption lines of different strength. To investigate the extension of the influence of the granular dynamics into the photospheric layers we use the coherence analysis, which makes use of the characteristic dynamical patterns of the turbulence and Doppler velocity. We find that the small scale turbulence pattern changes rapidly with height over a scale of one pressure scale height. This result can be seen as a manifestation of lateral diffusion of turbulence in the intergranular space after its generation by the shear flow at granular borders. This explains the turbulent state of the intergranular space. Title: Effects of thermal conduction on the energy balance of open coronal regions Authors: Hammer, R.; Nesis, A.; Moore, R. L.; Suess, S. T.; Musielak, Z. M. Bibcode: 1996ASPC..109..525H Altcode: 1996csss....9..525H No abstract at ADS Title: Dynamics of the Solar Granulation: Its Interaction with the Magnetic Field Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1996mpsa.conf..617N Altcode: 1996IAUCo.153..617N No abstract at ADS Title: Evidence of shear flows in the solar granulation Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1996ASPC..109..143N Altcode: 1996csss....9..143N No abstract at ADS Title: Dynamics of the solar granulation. III. Fractional diffusion. Authors: Nesis, A.; Hammer, R.; Hanslmeier, A.; Staiger, J.; Westendorp Plaza, C.; Grabowski, U. Bibcode: 1995A&A...296..210N Altcode: In most papers dealing with random motions and diffusion of small magnetic elements in the photosphere, the convective flows and in particular the granulation are considered as drivers of these motions. The results of these works have been discussed in terms of the fractal dimension of the granulation as seen in intensity pictures. So far neither a fractal dimension associated with the granular velocity field nor the nature of the random walks in the granular intergranular space have been determined. Using spectrograms of high spatial resolution taken with the VTT at Izana (Tenerife, Spain) we investigated the granular velocity field in terms of its fractal nature and its diffusion properties. We applied the rescaled range analysis to both the velocity and intensity fields, thus enabling us to calculate a fractal dimension as well as a "diffusion" exponent which together characterize the diffusion properties of the granulation layers. We found a fractal dimension of the granular velocity of the same order as the fractal dimensions of the distribution of the magnetic elements in the photosphere, and the fractal dimension corresponding to the diffusion of the magnetic elements in a fractal geometry. The diffusion processes in the granulation layers show a subdiffusive nature characteristic of anomalous diffusion rather than the classical Fickian diffusion. Anomalous diffusion is often found in stochastic transport in spatially heterogeneous media. The velocity field of the granulation can be thought of as a heterogeneous turbulent medium: the granules show less turbulence than the intergranular space. Title: Self-Organization of Magnetic and Velocity Fields in Solar Intergranules Authors: Nesis, A.; Hammer, R.; Schleicher, H. Bibcode: 1995SPD....26..504N Altcode: 1995BAAS...27..957N No abstract at ADS Title: Dynamics of the solar granulation: bisector analysis Authors: Hanslmeier, A.; Nesis, A.; Mattig, W. Bibcode: 1994A&A...288..960H Altcode: Using four selected examples of bisector and line parameter variations due to granular/intergranular motions in the solar photosphere, we investigate in this paper the coherence between these parameters over single granular/intergranular areas. It is shown, that there is no definite correlation between intensity variations, velocity variations and the shape of the respective bisectors. This is in contradiction to some model calculations but is consistent with the results extracted from spatially highly resolved spectrograms in previous papers. We found enhancements of the line parameter full width at half maximum at the granular/intergranular border, non symmetric intensity and velocity variations around their maxima or minima values and a correlation between velocity variations on the horizontal scale with the full width at half maximum values (δfw). However, there is no correlation between δfw and continuum intensity, because the enhancement of δfw occurs at moderate values of continuum intensity. Since the full width at half maximum indicates enhanced non thermal motions, these areas are the location of post shock turbulence as it is described by newer hydrodynamical models. These examples are of course influenced by subjective selection but should be on the other hand also described by hydrodynamical model calculations. Title: Non linear dynamics of the solar granulation: a first approach Authors: Hanslmeier, A.; Nesis, A. Bibcode: 1994A&A...286..263H Altcode: The non turbulent or turbulent behaviour of overshooting convective motions in the solar photosphere is studied by analysing spatially highly resolved spectrograms. We calculate the variation of a function similar to the well known Liapunov exponent derived from several line parameters. When the data are filtered and the variation of the parameters is considered over subgranular scales (<1arcsec) the Liapunov like exponents show a random variation at positive values. Normally, positive values of Liapunov exponents indicate chaotic motions, and our results are a hint that at subgranular scales the physics change which is in agreement with results from a coherence analysis between line parameters at several photospheric heights, the enhancement of the full width at half maximum in the intergranulum and model calculations that give evidence for the existence of a turbulent component of the temperature and velocity field. Title: The Dynamics of the Solar Granulation Investigated by Fractal Statistics Authors: Nesis, A.; Hammer, R.; Hanslmeier, A. Bibcode: 1994ASPC...64..655N Altcode: 1994csss....8..655N No abstract at ADS Title: Fractal Behavior of the Solar Granular Velocity Authors: Nesis, A.; Hammer, R.; Hanslmeier, A. Bibcode: 1994smf..conf..288N Altcode: No abstract at ADS Title: Dynamics of the solar granulation. Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.; Staiger, J. Bibcode: 1993A&A...279..599N Altcode: This investigation is based on a series of spectrograms of extraordinary spatial resolution taken with the vacuum tower telescope (VTT) at Izana (Tenerife) in 1990. The quantitative analysis of these spectrograms reveals an asymmetrical character of the granular flow (non-Benard like convection). We suggest that a typical granule consists of a region of high intensity and low turbulence in its interior and a region of high turbulence and moderate intensity at its border. In other words, we surmise that reigons of enhanced turbulence outline the borders of granules. By means of power and coherence analyses we found two different scaling laws for the small scale range: both the velocity and intensity power as well as various cross-correlation functions change their behavior near log k approximately = 0.8. Title: Fractal Distributions of the Intensity and Velocity Variations of the Solar Granulation Authors: Nesis, A.; Hammer, R.; Hanslmeier, A. Bibcode: 1993BAAS...25.1184N Altcode: No abstract at ADS Title: Dynamics of the solar granulation - Coherence of line parameters and their variation with the height Authors: Hanslmeier, A.; Nesis, A.; Mattig, W. Bibcode: 1993A&A...270..516H Altcode: We give a coherence analysis of various line parameters deduced from spatially highly resolved solar photospheric spectra obtained with the VTT at Izana, Tenerife. The high quality of the spectra and the selection of the wavelength range containing lines of different strengths allow us to investigate the transition from coherent to noncoherent flow patterns in the photosphere which occurs about a height of 150 km. The low correlation values found here are explained by an enhanced resolution of random motions and phase shifts between the intensity-velocity coherence which therefore reduces the coefficients. Two data sets containing a different number of data samples are compared and show nearly an identical behavior. Title: Rapid Variations in the Intergranular Space Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.; Staiger, J. Bibcode: 1993ASPC...46..222N Altcode: 1993mvfs.conf..222N; 1993IAUCo.141..222N No abstract at ADS Title: Coherence Analysis of Photospheric Line Parameters in Active and Non-Active Solar Regions Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1993ASPC...46...36H Altcode: 1993IAUCo.141...36H; 1993mvfs.conf...36H No abstract at ADS Title: Evidence for Transonic Flows in the Solar Granulation Authors: Nesis, A.; Bogdan, T. J.; Cattaneo, F.; Hanslmeier, A.; Knoelker, M.; Malagoli, A. Bibcode: 1992ApJ...399L..99N Altcode: High-resolution observations of the solar granulation are interpreted in the light of recent numerical simulations of compressible convection. The observations show a negative correlation between the width of suitably chosen, nonmagnetic lines and the continuum intensity. This result is consistent with a model of granular convection where regions of supersonic horizontal flow form intermittently in the vicinity of the downflow lanes. We conjecture that the observed line broadening in the regions of low intensity is caused by enhanced turbulent fluctuations generated by the passage of shock fronts bounding the regions of supersonic motion. Title: What are the Boundaries of Solar Granules? Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.; Staiger, J. Bibcode: 1992AAS...180.5109N Altcode: 1992BAAS...24..814N This investigation is based on a series of spectrograms of extraordinary spatial resolution taken with the vacuum tower telescope(VTT) at Iza\ na (Tenerife) in 1990. The quantitative analysis of these spectrograms revealed an asymmetrical character of the granular flow (non-Benard like convection). The intensity maximum and the maximum of the upward line-of-sight velocity do not coincide. In most cases the maximum of the velocity lies near the border of the granule and falls rapidly to the adjacent intergranular lane(from 1.5 to 0.2kmsec(-1) over 200km), but moderately towards the other intergranular lane. In some granules the position with zero velocity coincides with the position of highest intensity, whereas maxima of velocities with different signs lie at their border, thus reflecting a typical velocity profile of a rotating eddy. The low correlation(of less than 0.5) between intensity and Doppler velocity fluctuations along the spectrograph slit reflects the asymmetric character of the solar granular flow. Concerning the border of granules we find that bright regions often exhibit downward, instead of the expected upward velocity. Moreover, by investigating the broadening of a non-magnetically sensitive line, we were able to localize regions with enhanced turbulence within the intergranular space. We find that these regions do not always cover the whole intergranular lane, but are concentrated at the border of the granules, especially where the steep decrease of the velocity takes place. On the basis of these findings we suggest that a typical granule consists of a region of high intensity and low turbulence in its interior and a region of high turbulence and moderate intensity at its border. In other words, we surmise that regions of enhanced turbulence outline the borders of granules. Using our time series of spectrograms, which were taken every 15sec over a total of 5min, we followed the dynamics of these properties and the evolution of the steep intensity and velocity changes along the slit. These changes are connected with shear instabilities and turbulence production. The findings from non-active regions will be compared with those from active regions based on magnetically sensitive lines. Title: Speckle observations of solar granulation. Authors: de Boer, C. R.; Kneer, F.; Nesis, A. Bibcode: 1992A&A...257L...4D Altcode: We present observations of solar granulation in a plage region near disc center obtained with the Vacuum Tower Telescope at Observatorio del Teide, Tenerife. Speckle methods were employed for data acquisition and data reduction. The images show small-scale structures of the size near the telescopic diffraction limit of 0.2 arcsec. We call attention to bright lanes at the borders between granules and intergranular areas. Conceivably, they are the intensity signature of strong upflows at the border of granules or of shocks in supersonic convection which are predicted by computer simulations of the granular phenomenon. Title: Solar Granulation Spectroscopy: Dynamics of the Intergranular Space Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Mattig, R. Komm W.; Staiger, J. Bibcode: 1992ASPC...26..181N Altcode: 1992csss....7..181N No abstract at ADS Title: Velocity Fluctuations; Energy Dissipation in the Solar Photosphere Authors: Komm, R.; Mattig, W.; Nesis, A. Bibcode: 1992ASPC...26..175K Altcode: 1992csss....7..175K No abstract at ADS Title: Dynamics of the solar granulation. I - A phenomenological approach Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.; Staiger, J. Bibcode: 1992A&A...253..561N Altcode: High-spatial-resolution spectrograms taken with the vacuum tower telescope in Tenerife were used to investigate the dynamics of the deep photospheric layers by tracing the motions of small-scale structures such as granulation. Based on a time series of these spectrograms, traces of line Doppler shifts were detected which show strong asymmetries within solar granules. The results are discussed within the framework of different granulation flow models. Title: Bisector; Line Parameter Variation Over a Single Solar Granulum Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1992ASPC...26..168H Altcode: 1992csss....7..168H No abstract at ADS Title: The decay of granular motions and the generation of gravity waves in the solar photosphere Authors: Komm, R.; Mattig, W.; Nesis, A. Bibcode: 1991A&A...252..827K Altcode: The solar photosphere was investigated using a coherence analysis of rms-velocities. Results confirm that there is a distiction between the granular structures of the lower photosphere and the secondary structures of the higher photosphere. It is shown that the conversion of motions occurs well below a height of 200 km. In the layers of the higher photosphere (above 170 km) structures are found in the wavenumber range from 2.5 M/m to 7.0 M/m with a dominant scale of about 4.0 M/m. It is concluded that secondary motions are generated by decaying granular motions. Title: The height dependence of velocity-intensity fluctuations and several non-dimensional parameters in the solar photosphere Authors: Komm, R.; Mattig, W.; Nesis, A. Bibcode: 1991A&A...252..812K Altcode: The quiet photosphere was studied using autocorrelation functions (ACFs) of intensity-velocity fluctuations. It is found that all ACFs get broader with height for the whole center-to-limb variation. For the length scale L, there is no significant center-to-limb variation but a general height dependence. In the lower photospheric layers (less than 150 km), L is of the order of 500 km, while in the higher layers L increases to 700 km. The vorticity is found to be 0.025/s in the lower photosphere and 0.001/s in the higher layers. The height dependence of several independent nondimensional parameters has been deduced to study the effect of dissipative processes on the granular motions. The Reynolds number is found to decrease from 5 x 10 exp 9 at the continuum layers to 10 exp 8 at the temperature minimum; the Peclet number decreases from 70 to 1; and the magnetic Reynolds number is of the order of 500,000. It is concluded that granules are buoyantly rising turbulent structures which disintegrate due to turbulent mixing with the environment. Title: The variation of the solar granulation structure in active and non-active regions Authors: Hanslmeier, A.; Nesis, A.; Mattig, W. Bibcode: 1991A&A...251..307H Altcode: With the aid of a coherence analysis between line-center velocities, continuum-brightness variations and residual intensities the height-dependent evolution of overshooting convective elements in the solar photosphere is studied in magnetically active and nonactive regions. Evidence of a structuring influence of the magnetic field on the height dependent evolution of temperature and velocity patterns is found. From a comparison of intensity and velocity-frequency distributions in nonactive and active regions enhanced downward motions in active regions were found. The results are in agreement with small fluxtube concepts. Title: Selected examples of bisector and line parameter variation over a granular-intergranular region Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1991A&A...251..669H Altcode: Four examples of bisector and corresponding line parameter variations over a granular-intergranular region are discussed. These detailed case studies permit a direct comparison with hydrodynamical model calculations. Generally, the variation of the line parameters is found to be more similar than for data where granular and intergranular areas are averaged. However, the transition from granulum to intergranulum was found to be nonmonotonic and at the position where the velocity gradients reach a minimum, the slope of the line parameter curves also changes. Title: Granular and intergranular line profiles in solar active and quiet regions Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1991A&A...248..232H Altcode: Two spectra obtained with high spatial resolution in active and nonactive regions of the sun have been analyzed. The enhanced fluctuations at subgranular scales in active regions observed in the power spectra of intensity and velocity, provide evidence for the existence of small magnetic flux tubes. Using the brightest and darkest continuum, intensity as indicators for granulum-intergranulum, granular and intergranular line profiles have been determined. In Ca(+) active regions, the intergranular profiles are changed more than the granular profiles. Also the full width at half maximum of the intergranular line profiles is enhanced in the active region. These results have been obtained without any polarization equipment and suggest that the magnetic elements are predominantly located in the intergranular regions. Title: High spatial resolution solar photospheric line observations in Ca(+) active regions Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1991A&A...244..521H Altcode: Spatially highly resolved solar photospheric line profiles are analyzed by calculating bisectors, line center velocities, and line asymmetries in order to investigate the influence of magnetic fields on these parameters. A set of three spectrograms containing regions of different magnetic activity is used. In the active regions, a reduction of continuum intensity fluctuations as well as small-scale velocity fluctuations is confirmed; however, on subgranual scales, the continuum intensity fluctuations and line center velocities for the lines originating higher than 200 km in the troposphere are found to be increasing in active regions and becoming equal to or higher than those in nonactive regions. Significant changes are observed in the profiles: the mean line asymmetry is negative in the nonactive regions and positive with a reduction of the standard deviation in the active regions. Title: Granulation Spectroscopy: First Results from VTT-Tenerife Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.; Staiger, J. Bibcode: 1991BAAS...23R1048N Altcode: No abstract at ADS Title: The small-scale velocity field in the solar photosphere Authors: Komm, R.; Mattig, W.; Nesis, A. Bibcode: 1991A&A...243..251K Altcode: The center-to-limb variation of velocity fluctuations derived from several spectral lines is presented and, from these data, the height dependence of the vertical and horizontal components of the small-scale velocity field is deduced. A strong decrease in the lower photosphere and a flat gradient in the upper photosphere are observed for both the horizontal and vertical velocity. It is concluded that the convective motions decay in the middle photosphere up to a height of about 170 km and so-called secondary motions dominate the upper layers of the photosphere. A stability criterion is used to interpret this conversion of motions and, by utilizing the Richardson number and several length scales, it is shown that the stable stratification of the photosphere causes the decay of the granular convective motions. Title: On the Dynamics of Granulation in Active Regions and the Heating Problem (With 2 Figures) Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W. Bibcode: 1991mcch.conf...36N Altcode: No abstract at ADS Title: The height dependence of intensity structures in the solar photosphere Authors: Komm, R.; Mattig, W.; Nesis, A. Bibcode: 1990A&A...239..340K Altcode: The results are presented of a power and coherence analysis of intensity variations derived from the wings of the solar Mg b2 line. It is found that the power spectra can be represented by a power law function in the range of wavenumbers between 2.8/Mm. The deep photosphere shows the Kolmogorov (-5/3)-scaling law. The values of the exponent and also of the rms intensity itself decrease with height, attain a minimum, and increase again. While small structures are coherent up to higher photospheric layers, the coherence of the larger structures breaks down in the same layer where the rms intensity shows its minimum. It is concluded that the large intensity structures reflect the effect of convective overshoot, and the breakdown of the coherence reflects the disappearance of convective structures up to a certain height in the photosphere, while the small structures are of turbulent origin. Title: High spatial resolution observations of some solar photospheric line profiles Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1990A&A...238..354H Altcode: Results are presented of high resolution spectroscopic solar granulation observations with the Gregory Coude Telescope at Izana (Tenerife) in photospheric nonactive regions. Line asymmetries of four Fe I lines were analyzed depending on their origination in granular or intergranular regions. With the increased spatial resolution, instead of the classical C-shape, red line asymmetries are found in the intergranulum and blue line asymmetries in the granulum. Correlations between various line parameters such as continuum intensity, line center velocity, and equivalent width were examined. The results are in agreement with theoretical model calculations. Title: The vertical motion of the solar convective elements Authors: Banos, G.; Nesis, A. Bibcode: 1990A&A...232..231B Altcode: The ascent velocity of the solar convective elements (cells) and its variation with depth are deduced by using a temporal evolution of the apparent radius (size) of 13 granules and a simple morphological model for calculations. An attempt is made to 'see' somewhat deeper into the unstable zone and to deduce the acceleration of the convective motion. It is assumed that granules are spherical and practically do not expand as they rise. Thus, the upward convection velocity increases with decreasing depth below tau(5000) = 1; the rms vertical velocity changes from 0.5 km/s to 1.6 km/s between -880 km and -390 km. This is compatible with a verticle mass flux conservation. The acceleration rate is approximately 4 m/s sq. Larger convective elements have higher ascent velocities. The existence of horizontal velocities at tau(5000) = 1 driven by a pressure gradient and the subsequent fall of the material can be understood as a phenomenon related to the late evolution of the cell-granule element. Title: The decay process of the granulation and its influence on the absorption lines. Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W. Bibcode: 1990AGAb....5...33N Altcode: No abstract at ADS Title: The Upper Boundary of the Solar Convection Zone - Hydrodynamical Aspects Authors: Nesis, Anastasios; Hammer, Reiner; Mattig, Wolfgang Bibcode: 1990ASPC....9..113N Altcode: 1990csss....6..113N Using spectrograms of high spatial resolution, the horizontal rms velocity of the granulation is measured at different depths in the photosphere. A steep vertical gradient of the horizontal velocity is found, indicating strong dissipation in the first 100 km. Using the boundary layer concept the dissipation is estimated to be 10 percent of the total energy. Beyond 200 km, granulation triggers gravity waves. The turbulent viscosity is estimated to be 10 to the 11th/sq cm/sec. Title: The Influence of the Granulation on the Absorption Lines I. Nonactive Regions Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W. Bibcode: 1990PDHO....7..108N Altcode: 1990ESPM....6..108N; 1990dysu.conf..108N No abstract at ADS Title: The height dependence of vertical and horizontal velocities attributed to the convective overshoot in the solar atmosphere Authors: Nesis, A.; Mattig, W. Bibcode: 1989A&A...221..130N Altcode: The paper presents the results of an analysis of the variation of the granular velocity fluctuations with height in the photosphere. For the vertical and horizontal granular velocity fluctuations a steep gradient in the deep photosphere, a velocity minimum, and a rise of the velocity above this minimum were found. It is argued that within these convective overshoot layers the ordered convective motion is converted gradually into another type of motion thereby inducing the velocity field of the higher photospheric layers. Title: New results on the hydrodynamics of the overshoot layers in "active regions" Authors: Nesis, A.; Fleig, K. -H.; Mattig, W. Bibcode: 1989hsrs.conf..321N Altcode: No abstract at ADS Title: Line assymetries and parameters in spatially highly resolved spectra Authors: Hanslmaier, A.; Mattig, W.; Nesis, A. Bibcode: 1989hsrs.conf..314H Altcode: No abstract at ADS Title: Granulation Line Asymmetries Authors: Mattig, W.; Hanslmeier, A.; Nesis, A. Bibcode: 1989ASIC..263..187M Altcode: 1989ssg..conf..187M No abstract at ADS Title: RMS Velocities in Solar Active Regions Authors: Nesis, A.; Fleig, K. -H.; Mattig, W. Bibcode: 1989ASIC..263..289N Altcode: 1989ssg..conf..289N No abstract at ADS Title: Dependence of solar line bisectors on equivalent widths. Authors: Hanslmeier, A.; Mattig, W.; Nesis, A. Bibcode: 1989sasf.confP.251H Altcode: 1988sasf.conf..251H; 1989IAUCo.104P.251H Spectroscopic highly resolved solar granulation observations lead to intense line asymmetries for rising and sinking elements. In order to average several granules it is better to use equivalent widths than continuum intensities. Title: Dynamics of the overshoot layers and boundary conditions in helioseismology. Authors: Nesis, A. Bibcode: 1988ESASP.286...37N Altcode: 1988ssls.rept...37N The variation of both the horizontal and the vertical small scale rms velocity with height in the photosphere shows a minimum in photospheric layers about 150 - 200 km above the continuum. In the context of the non-equilibrium thermodynamics the author supposes that the small scale rms velocity variation reflects the variation of the entropy production in these layers. He proposes that the upper boundary of theoretical helioseismology models should be placed at that height where the entropy production shows its minimum. Title: Overshoot of horizontal and vertical velocities in the deep solar photosphere Authors: Nesis, A.; Mattig, W.; Durrant, C. J. Bibcode: 1988A&A...201..153N Altcode: The authors present the results of a coherence analysis of the centre-to-limb behaviour of the small-scale intensity and velocity fluctuations. The vertical velocity is coherent throughout the low-middle photosphere and is correlated with the continuum intensity variations. The horizontal velocity variations are not coherent with the intensity variations, and with the vertical velocity variations. The horizontal velocity is coherent only in the low atmosphere. Thus the horizontal motion is independent from the vertical motion and is not of convective nature. Title: Convective Overshoot and Upper Boundary Conditions Authors: Nesis, A. Bibcode: 1988IAUS..123..443N Altcode: Calculations of the frequency of solar oscillations are sensitive to the upper boundary conditions of the model. The author's investigations of the velocity fields of the overshoot layers (photosphere) have shown that there is a minimum of velocity at about 200 km above τ = 1: It is suggested that this minimum provides a natural upper boundary condition for the calculations of solar oscillations. The propagation of sound in these layers has to be regarded as a propagation in a turbulent medium. Title: The gradient of the small-scale velocity fluctuation in the solar atmosphere Authors: Nesis, A.; Fleig, K. H.; Mattig, W.; Wiehr, E. Bibcode: 1987A&A...182L...5N Altcode: The vertical small-scale velocity gradient in the photosphere is determined using spectrograms of high spatial resolution obtained with the Gregory-Coude telescope in Izana on August 1, 1986. The measured Doppler shifts along the spectrograph slits are analyzed using spatial Fourier analysis. The height dependence of the size of the velocity fluctuations measured at Doppler velocity is examined. It is observed that both the amplitude of the smallest velocity structures and the integrated rms small-scale velocity decrease with height in the photosphere. The derived rms small-scale velocity values are compared with the data of Canfield (1976) and Nesis (1985), and good correlation is observed. Title: The gradient of the small-scale velocity fluctuation in the solar atmosphere Authors: Nesis, A.; Mattig, W.; Fleig, K. H.; Wiehr, E. Bibcode: 1987BAAS...19..942N Altcode: No abstract at ADS Title: Velocity Variations of Small Scale Solar Structures, and Physical Problems Related to the Overshoot Layers Authors: Nesis, Anastasios; Severino, Giuseppe Bibcode: 1987LNP...291..154N Altcode: 1987csss....5..154N We compare our results about the variation of the vertical and horizontal velocity with height in the Solar photosphere with the theoretical granulation model by Nelson. The comparison shows, (i) that the mixing length derived by Nelson corresponds to the height of the overshoot-layers derived by Nesis, and (ii) that the large spatial structures with large horizontal velocities dominate the continuum layers. Title: Velocity Variations of Small Scale Solar Structures, and Physical Problems Related to the Overshoot Layers Authors: Nesis, A.; Severino, G. Bibcode: 1987MitAG..70..330N Altcode: No abstract at ADS Title: Overshoot of the Solar Granulation Authors: Nesis, A. Bibcode: 1987rfsm.conf..322N Altcode: The author found that the deepest layers which are just above the continuum, the velocity field is dominated by horizontal motions which are connected with structures larger than 2arcsec.6. The horizontal velocity could be due to gravity waves. Furthermore, the finding that the horizontal velocity decreases with the height in the overshoot layers could demonstrate the existence of large vortices with a high turn-over time. To decide whether gravity waves or vortices are involved is, however, difficult. Title: Granulare Overshoot-Schichten als Randbedingungen Authors: Nesis, A.; Komm, R.; Mattig, W. Bibcode: 1986MitAG..67..289N Altcode: No abstract at ADS Title: Oscillations of the sun's chromosphere. III - Simultaneous H-alpha observations from two sites Authors: von Uexkuell, M.; Kneer, F.; Mattig, W.; Nesis, A.; Schmidt, W. Bibcode: 1985A&A...146..192V Altcode: The authors analyze time sequences of Hα filtergrams taken simultaneously from two distant observatories, Capri and Izaña. By means of a coherence analysis the authors discriminate between instrumental effects including seeing and truly solar intensity fluctuations. Waves with periods as short as 60 s are present in the solar chromosphere; the lower limit is set by the time resolution of the observations. Title: A Model of the Run of the Horizontal and Vertical Velocities in the Deep Photosphere Authors: Nesis, A. Bibcode: 1985LNP...233..249N Altcode: 1985hrsp.proc..249N A correlation has been noted between intensity and velocity fields in the deeper photosphere which is not found at its middle and upper levels and which may be the basis of absorption line asymmetry. An attempt is presently made to determine the extension up to which the velocity field is correlated with the intensity field, giving attention to lower photosphere dynamics in light of horizontal and vertical small scale velocity variations with atmospheric height. A model run is developed which indicates that there is an overshoot up to a height of 150 km for the vertical velocity. Above this level, there is a secondary motion of nonconvective nature.