explanation blue bibcodes open ADS page with paths to full text
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.
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. <BR /> Aims: To obtain
insight into the change of the velocity field with height in the solar
photosphere we analyze 2D spectroscopic observations. <BR /> Methods:
Maps of the vertical velocity at four different photospheric heights
are studied by means of 2D power and coherence analyses, in order to
characterize the dynamical and topological properties of the velocity
field in the 2D wave number domain (k<SUB>x</SUB>,k<SUB>y</SUB>). (i)
The power analysis shows the power amplitude and its distribution
over the (k<SUB>x</SUB>,k<SUB>y</SUB>) 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 (k<SUB>x</SUB>,k<SUB>y</SUB>) 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. <BR />
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.
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.
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. <P />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. <P />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. <P />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
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
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.
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.
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.
2005ESASP.560..619H Altcode: 2005csss...13..619H
No abstract at ADS
---------------------------------------------------------
Title: Are there multiple spicule driving mechanisms?
Authors: Hammer, R.; Nesis, A.
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.
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.
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.
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
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
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.
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.
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.
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. <P />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.
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.
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.
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
τ<SUB>5000</SUB>=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.
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.
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.
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.
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.
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.
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 P<SUB>turb</SUB>, 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 P<SUB>turb</SUB>
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<SUP>-1</SUP>, both horizontally and
vertically (Nesis and Mattig 1989). This yields a contribution to the
ratio of turbulent to gas pressure of the order of P<SUB>turb</SUB>
/ P<SUB>gas</SUB> 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<SUP>-1</SUP>. The corresponding contribution
to the turbulent pressure near the solar surface is thus likewise
intermittent and amounts up to P<SUB>turb</SUB> = 10<SUP>4</SUP>
Pa. The ratio P<SUB>turb</SUB> / P<SUB>gas</SUB> 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1991BAAS...23R1048N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The small-scale velocity field in the solar photosphere
Authors: Komm, R.; Mattig, W.; Nesis, A.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
1989hsrs.conf..314H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Granulation Line Asymmetries
Authors: Mattig, W.; Hanslmeier, A.; Nesis, A.
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.
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.
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.
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.
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.
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.
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.
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
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.
1987MitAG..70..330N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Overshoot of the Solar Granulation
Authors: Nesis, A.
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.
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.
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.
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.
