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Author name code: hammer
ADS astronomy entries on 2022-09-14
author:"Hammer, Reiner"
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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.
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.
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Title: Global and Local Cutoff Frequencies for Transverse Waves
Propagating along Solar Magnetic Flux Tubes
Authors: Routh, S.; Musielak, Z. E.; Hammer, R.
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.
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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.
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Title: The origin of cutoff frequencies for torsional tube waves
propagating in the solar atmosphere
Authors: Hammer, R.; Musielak, Z. E.; Routh, S.
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.
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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.
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.
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Title: Generation and Propagation of Torsional Tube Waves in the
Solar Atmosphere
Authors: Musielak, Zdzislaw E.; Routh, S.; Hammer, R.
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.
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Title: Propagation of Waves in Solar Non-Isothermal Atmosphere
Authors: Routh, Swati; Musielak, Z.; Hammer, R.
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.
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Title: Simultaneous Maps of the Chromosphere for Ca II H and Ca
II 8662
Authors: Rammacher, W.; Schmidt, W.; Hammer, R.
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. <P />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.
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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.
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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.
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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.
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.
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Title: Waves in Inclined Solar Magnetic Flux Tubes
Authors: Routh, Swati; Musielak, Z.; Hammer, R.
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. <P />This work
is supported by NSF grant ATM 05-38278
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Title: Conditions for Propagation of Torsional Waves in Solar Magnetic
Flux Tubes
Authors: Routh, S.; Musielak, Z. E.; Hammer, R.
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.
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Title: Velocity Pattern Evolution in the Solar Photosphere
Authors: Nesis, Anastasios; Hammer, Reiner; Schleicher, Helmold
2007AN....328..702N Altcode:
No abstract at ADS
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Title: Propagation Characteristics of Waves in Solar Magnetic
Flux Tubes
Authors: Hammer, Reiner; Musielak, Zdzislaw E.; Routh, Swati
2007AN....328..704H Altcode:
No abstract at ADS
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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
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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
2007AN....328..657R Altcode:
No abstract at ADS
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Title: Chromospheric Dynamics and Line Formation
Authors: Hammer, R.; Ulmschneider, P.
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.
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Title: Observations and Simulations of Ca II H and Ca II 8662
Authors: Rammacher, W.; Schmidt, W.; Hammer, R.
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.
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Title: Cutoff-free Propagation of Torsional Alfvén Waves along Thin
Magnetic Flux Tubes
Authors: Musielak, Z. E.; Routh, S.; Hammer, R.
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.
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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.
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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.
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Title: A metatheory about spicules
Authors: Hammer, R.; Nesis, A.
2005ESASP.560..619H Altcode: 2005csss...13..619H
No abstract at ADS
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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.
2004sf2a.conf..531F Altcode:
No abstract at ADS
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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
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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
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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.
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Title: Equipartition in Spicules
Authors: Hammer, Reiner; Nesis, Anatasios
2003ANS...324..100H Altcode: 2003ANS...324..P06H
No abstract at ADS
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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
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Title: The Filling Factor of Solar Internetwork Grains
Authors: Kalkofen, Wolfgang; Hammer, Reiner
2003ANS...324..101K Altcode: 2003ANS...324..P07K
No abstract at ADS
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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
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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
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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
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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
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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.
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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: The solar atmosphere
Authors: Solanki, S. K.; Hammer, R.
2002css1.book.1065S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Issues of Rural Light Pollution
Authors: Osborn, W. H.; Hammer, R. L.; Hammer, A.
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.
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: 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.
2000IAUTA..24...67A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Connections Between X, EUV/UV and VIS Solar Activity
Authors: Hammer, R.; Schleicher, H.
2000PCEC...25..409H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Are Helium Line Intensities Affected By Abundance Variations?
Authors: Hammer, R.
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.
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: Working Group 4: The Quiet Corona and Regular Solar Wind
Authors: Hammer, R.
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.
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: Thermal Conduction in the Transitino Region and its Effects
on the Energy Balance of Open Coronal Regions
Authors: Hammer, R.
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.
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.
1994smf..conf..347H Altcode:
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: 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: On the Heating Mechanism of Coronal Holes
Authors: Hammer, R.; Moore, R. L.; Musielak, Z. E.; Suess, S. T.
1993ASSL..183..587H Altcode: 1993pssc.symp..587H
No abstract at ADS
---------------------------------------------------------
Title: The Coronal Energy Balance in Active Regions: Theoretical
Aspects
Authors: Hammer, R.
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.
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.
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.
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.
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.
1992ASPC...26..181N Altcode: 1992csss....7..181N
No abstract at ADS
---------------------------------------------------------
Title: On the Global Properties of Active Regions
Authors: Hammer, R.
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.
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.
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.
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.
1991mcch.conf..344H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Minimum Coronal Energy Requirements: Constraints for Heating
Mechanisms (With 2 Figures)
Authors: Hammer, R.
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.
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.
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: Some thoughts about the nonlinearity of flux-flux relations.
Authors: Hammer, R.
1990AGAb....5...37H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar wind and the winds from cool stars
Authors: Hammer, R.
1990nwus.book...77H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Possibility of Purely Acoustically Heated Coronae
Authors: Hammer, R.; Ulmschneider, P.
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.
1989AGAb....3...16H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Minimum Flux Coronae Revisited
Authors: Hammer, R.
1989AGAb....3...15H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Winds from Cool Stars Including the Sun
Authors: Hammer, R.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1984BAAS...16R.491H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Koronaschalen und ihre Anhängigkeit vom Heizungsgesetz
Authors: Hammer, R.
1984MitAG..62R.220H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Confrontation of Theory by Data
Authors: Hammer, R.
1984rcch.conf..355H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Zur Nichtlinearität des Zusammenhanges zwischen
chromosphärischer und koronaler Emission bei kühlen Sternen
Authors: Hammer, R.
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.
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.
1982BAAS...14..866H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Applicability of Solar Coronal Models to Stars
Authors: Hammer, R.
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.
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.
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<SUP>-2/7</SUP> for r → ∞). For large Land small
heating fluxes φ<SUB>M0</SUB>, 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 φ<SUB>M0</SUB> 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<SUP>-4/3</SUP> for r → ∞).
---------------------------------------------------------
Title: Dependence of open stellar coronal regions on coronal heating.
Authors: Hammer, R.
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 phi<SUB>Mo</SUB>
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 phi<SUB>Mo.</SUB> 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 phi<SUB>Mo</SUB> and/or outward thermal conduction (for
small phi sub Mo).
---------------------------------------------------------
Title: Energy balance and stability
Authors: Hammer, R.
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.
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.
1981BAAS...13..792H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Structure, energy balance, and mass loss of stellar coronae
Authors: Hammer, Reiner
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.
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.
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.
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.
