Author name code: hammer ADS astronomy entries on 2022-09-14 author:"Hammer, Reiner" ------------------------------------------------------------------------ Title: Where does the Thermospheric Ionospheric GEospheric Research (TIGER) Program go? Authors: Schmidtke, G.; Avakyan, S. V.; Berdermann, J.; Bothmer, V.; Cessateur, G.; Ciraolo, L.; Didkovsky, L.; Dudok de Wit, T.; Eparvier, F. G.; Gottwald, A.; Haberreiter, M.; Hammer, R.; Jacobi, Ch.; Jakowski, N.; Kretzschmar, M.; Lilensten, J.; Pfeifer, M.; Radicella, S. M.; Schäfer, R.; Schmidt, W.; Solomon, S. C.; Thuillier, G.; Tobiska, W. K.; Wieman, S.; Woods, T. N. Bibcode: 2015AdSpR..56.1547S Altcode: At the 10th Thermospheric Ionospheric GEospheric Research (TIGER/COSPAR) symposium held in Moscow in 2014 the achievements from the start of TIGER in 1998 were summarized. During that period, great progress was made in measuring, understanding, and modeling the highly variable UV-Soft X-ray (XUV) solar spectral irradiance (SSI), and its effects on the upper atmosphere. However, after more than 50 years of work the radiometric accuracy of SSI observation is still an issue and requires further improvement. Based on the extreme ultraviolet (EUV) data from the SOLAR/SolACES, and SDO/EVE instruments, we present a combined data set for the spectral range from 16.5 to 105.5 nm covering a period of 3.5 years from 2011 through mid of 2014. This data set is used in ionospheric modeling of the global Total Electron Content (TEC), and in validating EUV SSI modeling. For further investigations the period of 3.5 years is being extended to about 12 years by including data from SOHO/SEM and TIMED/SEE instruments. Similarly, UV data are used in modeling activities. After summarizing the results, concepts are proposed for future real-time SSI measurements with in-flight calibration as experienced with the ISS SOLAR payload, for the development of a space weather camera for observing and investigating space weather phenomena in real-time, and for providing data sets for SSI and climate modeling. Other planned topics are the investigation of the relationship between solar EUV/UV and visible/near-infrared emissions, the impact of X-rays on the upper atmosphere, the development of solar EUV/UV indices for different applications, and establishing a shared TIGER data system for EUV/UV SSI data distribution and real-time streaming, also taking into account the achievements of the FP7 SOLID (First European SOLar Irradiance Data Exploitation) project. For further progress it is imperative that coordinating activities in this special field of solar-terrestrial relations and solar physics is emphasized. Title: Global and Local Cutoff Frequencies for Transverse Waves Propagating along Solar Magnetic Flux Tubes Authors: Routh, S.; Musielak, Z. E.; Hammer, R. Bibcode: 2013ApJ...763...44R Altcode: 2013arXiv1301.2736R It is a well-established result that the propagation of linear transverse waves along a thin but isothermal magnetic flux tube is affected by the existence of the global cutoff frequency, which separates the propagating and non-propagating waves. In this paper, the wave propagation along a thin and non-isothermal flux tube is considered and a local cutoff frequency is derived. The effects of different temperature profiles on this local cutoff frequency are studied by considering different power-law temperature distributions, as well as the semi-empirical VAL C model of the solar atmosphere. The obtained results show that the conditions for wave propagation strongly depend on the temperature gradients. Moreover, the local cutoff frequency calculated for the VAL C model gives constraints on the range of wave frequencies that are propagating in different parts of the solar atmosphere. These theoretically predicted constraints are compared to observational data and are used to discuss the role played by transverse tube waves in the atmospheric heating and dynamics, and in the excitation of solar atmospheric oscillations. 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: The origin of cutoff frequencies for torsional tube waves propagating in the solar atmosphere Authors: Hammer, R.; Musielak, Z. E.; Routh, S. Bibcode: 2010AN....331..593H Altcode: Torsional waves supported by magnetic flux tubes have long been thought to bear a high potential for supplying energy and momentum to the upper solar atmosphere, thereby contributing to its heating and to the driving of dynamic events like spicules. This hope rested on the belief that their propagation is not impeded by cutoff restrictions, unlike longitudinal and kink waves. We point out that this applies only to thin, isothermal tubes. When they widen in the chromosphere, and as a result of temperature gradients, cutoff restrictions arise. We compare them to recent observational reports of such waves and of vortex motions and find that their long period components are already affected by cutoff restrictions. An observational strategy is proposed that should permit the derivation of better information on vortex flows from off-center observations with next generation telescopes. Title: Temperature Gradients in the Solar Atmosphere and the Origin of Cutoff Frequency for Torsional Tube Waves Authors: Routh, S.; Musielak, Z. E.; Hammer, R. Bibcode: 2010ApJ...709.1297R Altcode: Fundamental modes supported by a thin magnetic flux tube embedded in the solar atmosphere are typically classified as longitudinal, transverse, and torsional waves. If the tube is isothermal, then the propagation of longitudinal and transverse tube waves is restricted to frequencies that are higher than the corresponding global cutoff frequency for each wave. However, no such global cutoff frequency exists for torsional tube waves, which means that a thin and isothermal flux tube supports torsional tube waves of any frequency. In this paper, we consider a thin and non-isothermal magnetic flux tube and demonstrate that temperature gradients inside this tube are responsible for the origin of a cutoff frequency for torsional tube waves. The cutoff frequency is used to determine conditions for the wave propagation in the solar atmosphere, and the obtained results are compared to the recent observational data that support the existence of torsional tube waves in the Sun. Title: Generation and Propagation of Torsional Tube Waves in the Solar Atmosphere Authors: Musielak, Zdzislaw E.; Routh, S.; Hammer, R. Bibcode: 2010AAS...21532204M Altcode: 2010BAAS...42..324M Recent observational evidence for the existence of torsional tube waves in the solar atmosphere (e.g., Bonet et al. 2008; Jess et al. 2009) has triggered theoretical work on the generation and propagation of these waves. We modify our previous theory of generation of torsional tubes waves by the solar turbulent convection and compute the resulting the wave energy spectra and fluxes. We also demonstrate that tube thickness and temperature gradients lead to the origin of cutoff frequencies for torsional tube waves. Using the cutoffs, we calculate a range of frequencies corresponding to the propagating waves and compare it to that given by the observations. We use the obtained result to discuss the role of torsional tube waves in heating of the solar atmosphere. This work is supported by NSF under grant ATM-0538278. Title: Propagation of Waves in Solar Non-Isothermal Atmosphere Authors: Routh, Swati; Musielak, Z.; Hammer, R. Bibcode: 2009SPD....40.1001R Altcode: Fundamental modes supported by a thin and isothermal magnetic flux tube embedded in the solar atmosphere are typically identified as longitudinal, transverse and torsional waves. The propagation of longitudinal and transverse tube waves is restricted to frequencies that are higher than the corresponding global cutoff frequency for each wave. No such global cutoff frequency exists for torsional tube waves, which means that the flux tube supports torsional tube waves of any frequency. The situation becomes different when a 'wide’ and isothermal flux tube is concerned; in this case, a cutoff frequency for torsional tube waves arises naturally and it becomes a local quantity. Since the solar atmosphere is not isothermal, the effects of different temperature gradients on the cutoff are investigated. A method to determine these effects is presented and applied to specific models of solar magnetic flux tubes. An interesting result is that the resulting cutoff frequency is a local quantity in the model, and that its value at a given height determines the frequency that transverse and torsional tube waves must have in order to be propagating at this height. The obtained results are applied to the VAL C model of the solar atmosphere. Title: Simultaneous Maps of the Chromosphere for Ca II H and Ca II 8662 Authors: Rammacher, W.; Schmidt, W.; Hammer, R. Bibcode: 2008ESPM...12.2.40R Altcode: We study chromospheric dynamics by analyzing long (up to 3200 s), high spatial resolution time series of slit spectra of the Ca II H line and the Ca II infrared line at 8662 Angstrom recorded simultaneously near disk center of the Sun with the Echelle Spectrograph of the VTT on Tenerife in June 2007. Fast scans with 6 steps (0.5" per step) were done resulting in small stripes 3.5" wide and 160" long. A scan repetition time of 8 s was reached. These small 2D maps allow us to discover bright points, to follow their horizontal motion, and to associate photospheric objects. Horizontal variations of the intensity with time can be found for both lines and all wavelengths, including cases in which patches of enhanced intensity move rapidly over the entire map, with horizontal velocities of up to 30 km/s.

We study also a series of large x-y-maps (size 160" x 120"), simultaneously recorded for the Ca II H and Ca II 8662 lines. Because of the high spectral resolution we get for both lines 975 wavelength points covering spectral ranges of nearly 4.7 (H) and 10.3 (8662) Angstrom and therefore also 975 large maps for Ca II H as well as Ca II 8662: one X-Y map for each resolved wavelength. Our quasi-monochromatic maps have much narrower contribution functions than the usual filtergrams taken in rather broad spectral regions of the Ca II H & 8662 lines; thus they allow a more precise mapping of features to atmospheric regions of limited height extension. 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: Local Cutoff Frequencies for Transverse Waves Propagating Along Thin and Non-Isothermal Magnetic Flux Tubes Authors: Musielak, Z. E.; Subramaniam, S.; Routh, S.; Hammer, R. Bibcode: 2008ESPM...12.3.25M Altcode: Propagation of transverse waves along thin magnetic flux tubes embedded in an isothermal atmosphere is affected by the cutoff frequency, which is a global quantity that restricts the wave propagation to only those frequencies that are higher than the cutoff. Since the solar atmosphere is not isothermal, the effects of different temperature gradients on the cutoff are investigated. A method to determine these effects is presented and applied to specific models of solar magnetic flux tubes. An interesting result is that the resulting cutoff frequency is a local quantity in the model, and that its value at a given height determines the frequency that transverse tube waves must have in order to be propagating at this height. The obtained results are applied to the solar atmospheric oscillations. Title: Waves in Inclined Solar Magnetic Flux Tubes Authors: Routh, Swati; Musielak, Z.; Hammer, R. Bibcode: 2007AAS...211.8001R Altcode: 2007BAAS...39..870R The solar chromosphere shows numerous needle-shaped extensions into the overlying corona, in which plasma shoots up at high speeds. These phenomena are variously called spicules, mottles, or dynamic fibrils, depending on where and how they are observed on the Sun. Many different explanations have been suggested for how these phenomena might be generated in the dynamic solar atmosphere. The currently most popular suggestion is that global solar oscillations propagate as longitudinal waves along magnetic flux tubes. These long-period waves suffer from a cutoff restriction, but if the flux tubes are sufficiently inclined to the vertical they can propagate nevertheless. We will analyze the cutoff behavior of longitudinal and also other wave modes, namely kink flux tube waves inside these inclined tubes and show that they have better chances to transport energy upward. Such waves must therefore be taken into account in order to understand spicules.

This work is supported by NSF grant ATM 05-38278 Title: Conditions for Propagation of Torsional Waves in Solar Magnetic Flux Tubes Authors: Routh, S.; Musielak, Z. E.; Hammer, R. Bibcode: 2007SoPh..246..133R Altcode: Propagation of torsional waves along isothermal and initially-untwisted magnetic-flux tubes embedded in the solar atmosphere is studied analytically. Conditions for wave propagation along thin and wide magnetic-flux tubes are determined, and it is shown that the propagation along thin tubes is cutoff free; however, for wide tubes the propagation is affected by a cutoff frequency. A method to determine the cutoff frequency is presented and applied to a specific model of solar magnetic flux tubes. An interesting result is that the cutoff frequency is a local quantity in the model and that its value at a given height determines the frequency that torsional tube waves must have to propagate at this height. 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: Propagation Characteristics of Waves in Solar Magnetic Flux Tubes Authors: Hammer, Reiner; Musielak, Zdzislaw E.; Routh, Swati Bibcode: 2007AN....328..704H 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: Simultaneous Observations of Solar Ca II H and Ca II 8662 lines and Numerical Simulation of these lines Authors: Rammacher, Wolfgang; Schmidt, Wolfgang; Hammer, Reiner Bibcode: 2007AN....328..657R Altcode: No abstract at ADS Title: Chromospheric Dynamics and Line Formation Authors: Hammer, R.; Ulmschneider, P. Bibcode: 2007AIPC..919..138H Altcode: 2007arXiv0707.2166H The solar chromosphere is very dynamic, due to the presence of large amplitude hydrodynamic waves. Their propagation is affected by NLTE radiative transport in strong spectral lines, which can in turn be used to diagnose the dynamics of the chromosphere. We give a basic introduction into the equations of NLTE radiation hydrodynamics and describe how they are solved in current numerical simulations. The comparison with observation shows that one-dimensional codes can describe strong brightenings quite well, but the overall chromospheric dynamics appears to be governed by three-dimensional shock propagation. Title: Observations and Simulations of Ca II H and Ca II 8662 Authors: Rammacher, W.; Schmidt, W.; Hammer, R. Bibcode: 2007ASPC..368..147R Altcode: We study chromospheric dynamics by analyzing long high spatial resolution time series of spectra of the Ca II H line and the Ca II infrared line at 8662 Å, recorded simultaneously near disk center of the sun. The observations were made at the VTT, Tenerife. The time series have a temporal resolution of 3 (8662) and 6 s (H), respectively. After the statistical analysis of the observation results, we used 1-D chromosphere simulation codes to make a series of computations with purely acoustic waves to obtain a time series of synthetic line profiles for Ca II H. A comparison of observational and theoretical results shows profound differences between these model calculations and the observations. A more detailed description of this work is in preparation and will be published in a main astronomy journal. Title: Cutoff-free Propagation of Torsional Alfvén Waves along Thin Magnetic Flux Tubes Authors: Musielak, Z. E.; Routh, S.; Hammer, R. Bibcode: 2007ApJ...659..650M Altcode: 2007astro.ph..1720M Propagation of torsional Alfvén waves along magnetic flux tubes has been extensively studied for many years, but no conclusive results regarding the existence of a cutoff frequency for these waves have been obtained. The main purpose of this paper is to derive new wave equations that describe the propagation of linear torsional Alfvén waves along thin and isothermal magnetic flux tubes and to use these wave equations to demonstrate that the torsional wave propagation is not affected by any cutoff frequency. It is also shown that this cutoff-free propagation is independent of the different choices of coordinate system and wave variables adopted in previous studies. A brief discussion of the implications of this cutoff-free propagation of torsional tube waves on theories of wave heating of the solar and stellar atmospheres is also given. 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: The DisGal database: A series of 3D tools Authors: Flores, H.; Puech, M.; Hammer, R.; Orieux, F.; Prugniel, P.; Theureau, G.; Royer, F.; Guibert, J.; Tajahmady, F.; Jegouzo, I.; Vétois, J. Bibcode: 2004sf2a.conf..531F Altcode: 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: Equipartition in Spicules Authors: Hammer, Reiner; Nesis, Anatasios Bibcode: 2003ANS...324..100H Altcode: 2003ANS...324..P06H No abstract at ADS 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: The Filling Factor of Solar Internetwork Grains Authors: Kalkofen, Wolfgang; Hammer, Reiner Bibcode: 2003ANS...324..101K Altcode: 2003ANS...324..P07K 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: The solar atmosphere Authors: Solanki, S. K.; Hammer, R. Bibcode: 2002css1.book.1065S Altcode: No abstract at ADS Title: Issues of Rural Light Pollution Authors: Osborn, W. H.; Hammer, R. L.; Hammer, A. Bibcode: 2001AAS...19910405O Altcode: 2001BAAS...33.1470O Light pollution is generally considered mostly an urban problem. Common sources of light pollution are poorly designed lighting of streets, parking lots, businesses and advertizing signs and for security. These sources, and the amount of light pollution generated, increase with population density. Nevertheless, light pollution can also be significant in rural areas. Rural light pollution differs from that in urban settings, both in the types of pollution and in the means that must be employed to control it. In the country the offending sources are often isolated lights such as from farm barns, vacation cottages, radio and cell phone towers, and road intersections. A culture of strong property rights and privacy rights affects attempts to control rural light pollution. We describe how some of these issues may be addressed based on the results from an Eagle Scout project carried out in central Michigan. 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: Commission 10: Solar Activity: (Activite Solaire) Authors: Ai, G.; Benz, A.; Dere, K. P.; Engvold, O.; Gopalswamy, N.; Hammer, R.; Hood, A.; Jackson, B. V.; Kim, I.; Marten, P. C.; Poletto, G.; Rozelot, J. P.; Sanchez, A. J.; Shibata, K.; van Driel-Geztelyi, L. Bibcode: 2000IAUTA..24...67A Altcode: No abstract at ADS Title: Connections Between X, EUV/UV and VIS Solar Activity Authors: Hammer, R.; Schleicher, H. Bibcode: 2000PCEC...25..409H Altcode: No abstract at ADS Title: Are Helium Line Intensities Affected By Abundance Variations? Authors: Hammer, R. Bibcode: 1999ESASP.446..347H Altcode: 1999soho....8..347H In situ solar wind measurements show that the helium abundance in the outer solar atmosphere is far from being constant. A considerable amount of theoretical research (e.g., Hansteen et al. 1997, ApJ 482, 498) has isolated the physical mechanisms that determine helium abundance variations along an open magnetic field line: gravitational settling in both the chromosphere and corona tends to transport helium downward, while the thermal force in the transition region and the drag force exerted by solar wind protons in the corona transport helium outward, and mixing processes work towards keeping everything in balance. The mixing efficiency is conceivably different in magnetically open and closed regions; and wind drag is missing altogether in the latter. This poster discusses if such differences might help to understand some of the features observed in the important helium lines accessible to several SoHO instruments. 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: Working Group 4: The Quiet Corona and Regular Solar Wind Authors: Hammer, R. Bibcode: 1997ESASP.404..141H Altcode: 1997cswn.conf..141H 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: Thermal Conduction in the Transitino Region and its Effects on the Energy Balance of Open Coronal Regions Authors: Hammer, R. Bibcode: 1994kofu.symp..335H Altcode: Thermal conductive energy flow into the transition region represents a considerable energy loss for the solar corona. Inclusion of this term lead to a new class of self-consistent models for magnetically open coronal regions. Recently a number of modifications of the thermal conductivity term were suggested, mainly in order to account for the emission measure reversal in the lower transition region. These suggestions include the contributions by ions and neutrals to thermal conduction, ambipolar diffusion, and turbulence. We show that diffusion, since it affects only the lowermost parts of the transition region, does not significantly change the global properties of the corona, unless the transition region base pressure is chosen unrealistically small. This behavior is explained in terms of the boundary layer character of the lower transition region in such models. By contrast, turbulence can enhance the conductivity more drastically and up to higher temperatures. For! large and intermediate pressures, turbulent conduction tends to flatten the lower, but to steepen the remainder of the transition region. The overall thickness of the transition region is reduced, but the total energy losses are enhanced compared to models based on the Spitzer conductivity. The minimum possible energy losses of the corona are also enhanced. The coronal temperature that minimizes the energy losses, however, does not depend much on the type of conductivity. 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: The Magnetism of the Quiet Chromosphere and Open Corona (Invited Review) Authors: Hammer, R. Bibcode: 1994smf..conf..347H Altcode: 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: 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: On the Heating Mechanism of Coronal Holes Authors: Hammer, R.; Moore, R. L.; Musielak, Z. E.; Suess, S. T. Bibcode: 1993ASSL..183..587H Altcode: 1993pssc.symp..587H No abstract at ADS Title: The Coronal Energy Balance in Active Regions: Theoretical Aspects Authors: Hammer, R. Bibcode: 1993ASSL..183..591H Altcode: 1993pssc.symp..591H No abstract at ADS Title: A New Way to Convert Alfven Waves into Heat in Solar Coronal Holes: Intermittent Magnetic Levitation Authors: Moore, R. L.; Hammer, R.; Musielak, Z. E.; Suess, S. T.; An, C. -H. Bibcode: 1992ApJ...397L..55M Altcode: In our recent analysis of Alfven wave reflection in solar coronal holes, we found evidence that coronal holes are heated by reflected Alfven waves. This result suggests that the reflection is inherent to the process that dissipates these Alfven waves into heat. We propose a novel dissipation process that is driven by the reflection, and that plausibly dominates the heating in coronal holes. 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: Intermittent Magnetic Levitation and Heating by Alfven Waves in Solar Coronal Holes Authors: Moore, R. L.; Hammer, R.; Musielak, Z. E.; Suess, S. T.; An, C. -H. Bibcode: 1992AAS...180.5506M Altcode: 1992BAAS...24R.819M No abstract at ADS Title: Design Rationale of the Solar Ultraviolet Network / Sun Authors: Dame, L.; Acton, L.; Bruner, M. E.; Connes, P.; Cornwell, T. J.; Curdt, W.; Foing, B. H.; Hammer, R.; Harrison, R.; Heyvaerts, J.; Karabin, M.; Marsch, E.; Martic, M.; Mattic, W.; Muller, R.; Patchett, B.; Roca-Cortes, T.; Rutten, R. J.; Schmidt, W.; Title, A. M.; Tondello, G.; Vial, J. C.; Visser, H. Bibcode: 1992ESOC...39..995D Altcode: 1992hrii.conf..995D No abstract at ADS 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: On the Global Properties of Active Regions Authors: Hammer, R. Bibcode: 1992ASPC...26..528H Altcode: 1992csss....7..528H 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: Heating Times and Heating Mechanisms in the Quiet Solar Atmosphere Authors: Hammer, R.; Moore, R. L. Bibcode: 1991BAAS...23.1442H Altcode: No abstract at ADS 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: On the Intrinsic Difficulty of Producing Stellar Coronae With Acoustic Waves (With 1 Figure) Authors: Hammer, R.; Ulmschneider, P. Bibcode: 1991mcch.conf..344H Altcode: No abstract at ADS Title: Minimum Coronal Energy Requirements: Constraints for Heating Mechanisms (With 2 Figures) Authors: Hammer, R. Bibcode: 1991mcch.conf..316H Altcode: No abstract at ADS 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 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: Some thoughts about the nonlinearity of flux-flux relations. Authors: Hammer, R. Bibcode: 1990AGAb....5...37H Altcode: No abstract at ADS Title: The solar wind and the winds from cool stars Authors: Hammer, R. Bibcode: 1990nwus.book...77H Altcode: No abstract at ADS Title: On the Possibility of Purely Acoustically Heated Coronae Authors: Hammer, R.; Ulmschneider, P. Bibcode: 1990ASPC....9...51H Altcode: 1990csss....6...51H The notion of coronae heated by purely acoustic processes is developed theoretically based on the existence of chromospheres heated by acoustic means only. Acoustic wave energy flux is considered in light of the minimum energy requirements of coronae, and valid purely acoustic coronae occur when the coronal energy loss flux is balanced by the acoustic flux. Very low coronal pressures are related to purely acoustically heated coronae, but in slowly rotating stars, the observable coronae are probably not produced acoustically. Title: Theoretical Limits on Stellar Coronae Heated by Compressive Waves Authors: Hammer, R.; Ulmschneider, P. Bibcode: 1989AGAb....3...16H Altcode: No abstract at ADS Title: Minimum Flux Coronae Revisited Authors: Hammer, R. Bibcode: 1989AGAb....3...15H Altcode: No abstract at ADS Title: The Winds from Cool Stars Including the Sun Authors: Hammer, R. Bibcode: 1988ASSL..148...51H Altcode: 1988pmls.conf...51H The primary objective of this paper is to review similarities and differences between the winds from solar-like stars and from low gravity cool stars. Space limitations do not permit a detailed analysis of special observational and theoretical techniques. The main emphasis, therefore, lies in a broad overview of the whole subject, with the aim to provide some background and perspective for further discussions of these and other types of stars and of individual wind acceleration mechanisms. Title: Solar and Stellar Chromospheres Authors: Hammer, R. Bibcode: 1987LNP...292...77H Altcode: 1987ssp..conf...77H Global properties of stellar chromospheres and their variation among the stars, and the underlying fine structure that affects or determines these global properties are highlighted. Attention is given to the chromospheric geometric extent and its dependence on the position of the star in the HR diagram. Main features of proposed chromospheric heating mechanisms and the role that magnetic fields play in the transport and dissipation of energy are discussed. The dependence of chromospheric magnetic fine structure on global stellar properties are explored. Title: Solar and stellar chromospheres Authors: Hammer, R. Bibcode: 1987LNP...292...75H Altcode: This review attempts to highlight two fundamental and complementary aspects of the chromospheric phenomenon; viz., global properties of stellar chromospheres and their variation among the stars, and the underlying fine structure that affects or determines these global properties. After an introductory discussion of the gross vertical structure of a stellar chromosphere, attention is given to the chromospheric geometric extent and its dependence on the position of the star in the HR diagram. This includes a critical review of various explanations of the dividing line that separates red giants from Solar-like stars. A subsequent chapter summarizes main features of proposed chromospheric heating mechanisms and discusses the role that magnetic fields play in the transport and dissipation of energy. The onset of stellar chromospheres and the magnitude of nonthermal motions can be probed by the Wilson-Bappu effect and by similar empirical results. The well-known insensitivity of the CaIIK line width to the stellar activity level might be largely due to a collective effect. The final chapter explores the dependence of chromospheric magnetic fine structure on global stellar properties. Title: On the Existence of Hot Coronae around Cool Star Authors: Hammer, R. Bibcode: 1987IAUS..122..361H Altcode: 1987IAUS..122..361R A star cannot have a solar-like corona if the available mechanical energy flux in the chromosphere is either too large or decreases outward more rapidly than the pressure. This result might be relevant for hybrid stars and cool giants. Title: Wave and Thermal Instabilities in Flux Tubes: Their Role for the Structure of the Chromosphere and Transition Region Authors: Hammer, R. Bibcode: 1987rfsm.conf..255H Altcode: The outer solar atmosphere is highly inhomogeneous and temporally variable. Many of the observed fluctuations in space and time are manifestations of magnetic field concentrations and of time dependent phenomena associated with them, such as waves and instabilities. The author discusses some of the basic properties of time dependent phenomena and their relevance to the chromosphere and transition region of the quiet Sun. Title: Confrontation of theory by data: the winds of cool stars. Authors: Hammer, R. Bibcode: 1985rbcc.conf..358H Altcode: The author first gives a very general introduction to the equations, boundary conditions, and parameters that describe the physics of stellar winds and of hot coronae. Then he briefly outlines some of the basic properties of the simplest possible case, a corona with a thermally driven wind. And finally, the author discusses to which stars this case can be applied. Title: Overheated open coronal regions. Authors: Hammer, R. Bibcode: 1985NASCP2358..125H Altcode: 1985onhm.rept..125H This paper studies the stability of stellar coronal shells when the coronal heating flux exceeds a certain limit, which depends on the damping length over which the energy is dissipated in the corona. Title: The scaling of coronal models from one star to another. Authors: Hammer, R. Bibcode: 1985NASCP2358..121H Altcode: 1985onhm.rept..121H The purpose of this paper is to discuss the requirements that must be met in order that stationary numerical corona models can be scaled from one star to another. Title: Corona models tested with IUE and Einstein observations. Authors: Hammer, R.; Linsky, J. L. Bibcode: 1984ESASP.218...25H Altcode: 1984iue..conf...25H Compilations of IUE and Einstein observations which show that the emissions from the outer layers of cool stars are nonlinearly correlated are discussed. This result can be used to test theoretical corona models as well as hypotheses on the mechanism that determines the location of the transition region. In stars in which most of the X-ray emission originates in small coronal loops it may be necessary that part of the emitting plasma is hotter than 20 million K or that the transition region is not only heated by thermal conduction, but also by downflows. Observational evidence for both these effects, and methods for analyzing the geometrical structure of outer stellar atmospheres are considered. Title: Energy balance of stellar coronae. III - Effect of stellar mass and radius Authors: Hammer, R. Bibcode: 1984ApJ...280..780H Altcode: A homologous transformation is derived which permits the application of the numerical coronal models of Hammer from a star with solar mass and radius to other stars. This scaling requires a few approximations concerning the lower boundary conditions and the temperature dependence of the conductivity and emissivity. These approximations are discussed and found to be surprisingly mild. Therefore, the scaling of the coronal models to other stars is rather accurate; it is found to be particularly accurate for main-sequence stars. The transformation is used to derive an equation that gives the maximum temperature of open coronal regions as a function of stellar mass and radius, the coronal heating flux, and the characteristic damping length over which the corona is heated. Title: Nonlinear two-dimensional dynamics of stellar atmospheres. I - A computational code Authors: Stefanik, R. P.; Ulmschneider, P.; Hammer, R.; Durrant, C. J. Bibcode: 1984A&A...134...77S Altcode: The authors present a computational code that allows the nonlinear equations of motion for a compressible fluid to be solved. Earlier work on one-dimensional problems using the method of characteristics is generalised to two dimensions employing cylindrical geometry. The scheme is described in detail and its effectiveness is demonstrated using analytic examples of small-amplitude motion in an isothermal, stably stratified, atmosphere. The code is designed specifically to handle the problem of the overshoot and decay of convective motion in stellar atmospheres and their coupling to acoustic and internal wave fields. Title: On the Properties of Coronal Shells Authors: Hammer, R. Bibcode: 1984BAAS...16R.491H Altcode: No abstract at ADS Title: Koronaschalen und ihre Anhängigkeit vom Heizungsgesetz Authors: Hammer, R. Bibcode: 1984MitAG..62R.220H Altcode: No abstract at ADS Title: Confrontation of Theory by Data Authors: Hammer, R. Bibcode: 1984rcch.conf..355H Altcode: No abstract at ADS Title: Zur Nichtlinearität des Zusammenhanges zwischen chromosphärischer und koronaler Emission bei kühlen Sternen Authors: Hammer, R. Bibcode: 1984MitAG..62Q.220H Altcode: No abstract at ADS Title: Stellar coronae - What can be predicted with minimum flux models? Authors: Hammer, R.; Endler, F.; Ulmschneider, P. Bibcode: 1983A&A...120..141H Altcode: In order to determine the possible errors of various minimum flux corona (MFC) predictions, MFC models are compared with a grid of detailed coronal models covering a range of two orders of magnitude in coronal heating and damping length values. The MFC concept is totally unreliable in the prediction of mass loss and the relative importance of various kinds of energy losses, and MFC predictions for the mass loss rate and energy losses due to stellar wind can be wrong by many orders of magnitude. It is suggested that for future applications, the unreliable MFC formulas should be replaced by a grid of related models accounting for the coronal dependence on damping length, such as the models underlying the present study. Title: Stellar Activity and the Connection Between Chromospheric and Coronal Energy Losses Authors: Hammer, R.; Linsky, J. L. Bibcode: 1982BAAS...14..866H Altcode: No abstract at ADS Title: Applicability of Solar Coronal Models to Stars Authors: Hammer, R. Bibcode: 1982BAAS...14..864H Altcode: No abstract at ADS Title: Energy balance of stellar coronae. I - Methods and examples. II - Effect of coronal heating Authors: Hammer, R. Bibcode: 1982ApJ...259..767H Altcode: Simplified models of magnetically open coronal regions are computed, with the aim of fulfilling appropriate boundary conditions at the base of the atmosphere, at the critical point, and at infinity. The models are determined by the stellar mass and radius and by the amount and location of coronal heating, and this dependence is analyzed in terms of pressure, temperature, characteristic heights, energy losses, mass loss, and asymptotic behavior. The results are used to classify the magnetically open coronal regions according to the energy loss mechanism that dominates in the region between the base and the critical point, and it is shown that more complicated heating mechanisms may be replaced by the presented exponential heating law, provided that the damping length is suitably chosen. Title: Energy Balance of Stellar Coronae - Part Two - Effect of Coronal Heating Authors: Hammer, R. Bibcode: 1982ApJ...259..779H Altcode: Models of magnetically open regions in hot stellar coronae are computed; and their dependence on the total amount and the location of coronal heating is investigated in terms of the following characteristic quantities: the pressure at the base of the transition region; the maximum coronal temperature and the temperature at the critical point; the heights of the temperature maximum and of the critical point; the coronal energy losses due to radiation, wind, and thermal conduction; the mass loss; and the type of the asymptotic behavior of the solution. The relations between these quantities are discussed. Depending on the heating parameters, there exist three typical kinds of coronal models. For dissipation scale lengths L small compared to the stellar radius, the corona is radiation dominated; that is, the wind and outward conductive energy losses at the critical point are small, the base pressure is high, the coronal temperature is low, and the asymptotic behavior of the solutions is of the Parker type (T ∼ r-2/7 for r → ∞). For large Land small heating fluxes φM0, the outward heat conduction contributes significantly to the energy losses; the coronal temperature is large, the base pressure is small, the mass loss rate is relatively small, and the asymptotic behavior is again of the Parker type. Finally, for both large L and large φM0 the corona is wind dominated; that is, the radiative and conductive energy losses as well as the base pressure are small, the coronal temperature and the mass loss rate are large, and the asymptotic behavior is of the Durney type (T ∼ r-4/3 for r → ∞). Title: Dependence of open stellar coronal regions on coronal heating. Authors: Hammer, R. Bibcode: 1982SAOSR.392A.121H Altcode: 1982csss....2..121H Models of open regions in hot stellar coronae are presented. For a given star these regions depend on the total amount phiMo of coronal heating and on the characteristic length (L) over which this energy is dissipated. The height of the temperature maximum is mainly determined by L. The coronal temperature, the mass loss rate, and the relative fraction of wind energy losses increase strongly with L as long as L is much smaller than the stellar radius. For large L, however, these quantities are only weak functions of L, while they still increase with increasing phiMo. Thus, if the heating occurs close to the stellar surface, the open coronal regions are cool, and most of the energy is used for radiation. Extended coronal heating, on the other hand, leads to hot coronal regions with small base pressure and predominating energy losses due to stellar wind (for large phiMo and/or outward thermal conduction (for small phi sub Mo). Title: Energy balance and stability Authors: Hammer, R. Bibcode: 1982AdSpR...2i.261H Altcode: 1982AdSpR...2..261H Recent theoretical work pertaining to the energy balance and stability of stellar coronae is reviewed. While in magnetically confined coronal regions coronal heating balances only the radiation losses of the corona and transition region, open regions can also lose energy by means of stellar wind and outward thermal conduction. The relative importance of these kinds of energy losses, and the asymptotic properties of the stellar wind, depend sensitively on the radial distribution of the energy input. Solar coronal models, both for open and closed regions, can often be applied to other stars. The requirements for which this is possible are briefly discussed. Stability analyses that are not restricted to the corona alone, but rather include the transition region and upper chromosphere, are important for our understanding of the detailed energy balance of outer stellar atmospheres. The stability of the complete system chromosphere/transition region/corona is probably closely related to the recent observational result that with increasing stellar activity the coronal radiation increases faster than the chromospheric radiation. Title: On the correlation between chromospheric and coronal emission. Authors: Hammer, R.; Linsky, J. L.; Endler, F. Bibcode: 1982NASCP2238..268H Altcode: 1982NASCP2338..268H; 1982auva.nasa..268H It is shown that with increasing stellar activity the emission of the transition region and corona increases faster than the emission of the chromosphere. It is also explained why the pressure of the transition region increases with increasing stellar activity. Further, it is shown that this relation is a necessary requirement for the global stability of the chromosphere/transition region/corona system. Title: Invalidity of the Minimum Flux Corona Concept Authors: Hammer, R. Bibcode: 1981BAAS...13..792H Altcode: No abstract at ADS Title: Structure, energy balance, and mass loss of stellar coronae Authors: Hammer, Reiner Bibcode: 1981PhDT.......180H Altcode: No abstract at ADS Title: Theoretical temperature minima for Arcturus (K2 IIIp), a possible explanation of the Wilson-Bappu effect. Authors: Ulmschneider, P.; Schmitz, F.; Hammer, R. Bibcode: 1979A&A....74..229U Altcode: The temperature minimum and acoustic flux at the base of the chromosphere are determined theoretically for Arcturus. The results are combined with values previously derived for the sun, and a tentative explanation of the Wilson-Bappu effect is given in terms of the acoustic heating mechanism. Additional chromospheric heating mechanisms are considered which are suggested by the secular variability of Ca II K2 emission. Title: On the energybalance of stellar coronae. Authors: Endler, F.; Hammer, R.; Ulmschneider, P. Bibcode: 1979A&A....73..190E Altcode: From a survey of proposed coronal heating mechanisms, it is concluded that these processes not only provide a certain heating flux but also, through a damping length L, determine the mode of dissipation of this flux. Both for simplified and more elaborate models it is found that L determines both the magnitude and the position of the coronal temperature maximum. Such detailed determination of the corona model by the heating mechanism appears to contradict the concept of a minimum-flux corona. Title: Acoustic waves in the solar atmosphere. IV. On the efficiency of one-dimensional hydrodynamic codes. Authors: Hammer, R.; Ulmschneider, P. Bibcode: 1978A&A....65..273H Altcode: Various forms of modified characteristics methods (MCM) are compared with the leapfrog type finite difference method (FDM) for accuracy and efficiency in modelling acoustic waves in the solar atmosphere. Hydrodynamic codes are computed with FORTRAN IV software for all the methods discussed. Also considered is the case of a sinusoidal piston with gravity, as well as nonhydrodynamic criteria for the computation of acoustic waves. Results indicate that the MCM, with its three interpolations (by quadratic- and weighted quadratic parabolas, and by natural cubic spline functions) offers, in general, a slower but more accurate method of calculation than does the FDM. It is noted, however, that the FDM and the quadratic parabola interpolation of the MCM differ only to the extent that the former method requires a pseudoviscosity and a greater number of timesteps. The overall preferred method is the MCM with natural cubic spline interpolation.