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Author name code: ulmschneider
ADS astronomy entries on 2022-09-14
author:"Ulmschneider, Peter"
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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: Intelligent Life in the Universe
Authors: Ulmschneider, Peter
2006ilu..book.....U Altcode:
No abstract at ADS
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Title: On the Validity of Acoustically Heated Chromosphere Models
Authors: Ulmschneider, P.; Rammacher, W.; Musielak, Z. E.; Kalkofen, W.
2005ApJ...631L.155U Altcode:
Theoretical models of solar and stellar chromospheres heated by
acoustic waves have so far been constructed by using time-dependent,
one-dimensional, radiation-hydrodynamic numerical codes that are
based on the approximation of plane-parallel geometry. The approach
seems to be justified by the fact that the chromospheres of most
stars extend over very narrow height ranges compared to the stellar
radius. It is demonstrated that this commonly used assumption may lead
to unrealistic shock mergings, to the artificial formation of unusually
strong shocks and the artificial destruction of high-frequency acoustic
wave power. Comparing one-dimensional calculations with observations may
lead to severe misjudgment about the nature of chromospheric heating.
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Title: Fast Method for Calculating Chromospheric Ca II and Mg II
Radiative Losses
Authors: Rammacher, W.; Fawzy, D.; Ulmschneider, P.; Musielak, Z. E.
2005ApJ...631.1113R Altcode:
A fast and reasonably accurate method for calculating the total
radiative losses by Ca II and Mg II ions for time-dependent
chromospheric wave calculations has been developed. The method is
based on a two-level atom procedure with pseudo-partial frequency
redistribution (pseudo-PRD). The speed of the method is due to
scaling of the total losses from single-line results. Acceleration of
computation speeds by factors of roughly 10<SUP>2</SUP>-10<SUP>3</SUP>
can be achieved. The method is tested against the results from a
modified version of the multilevel atom code MULTI.
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Title: Book Review: Cox & Giuli's principles of stellar structure
/ A. Weiss W. Hillebrandt, H.-C. Thomas, H. Ritter (eds.). 2nd
enl. ed. Cambridge Scientific Publishers, Cambridge, 2004, 770 pp.,
ISBN 1-09044868-20-7.
Authors: Ulmschneider, Peter
2005S&W....44f..88U Altcode:
No abstract at ADS
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Title: Erratum: Dynamics and heating of the magnetic network on the
Sun. Efficiency of mode transformation
Authors: Hasan, S. S.; Ulmschneider, P.
2004A&A...428.1017H Altcode:
No abstract at ADS
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Title: Dynamics and heating of the magnetic network on the
Sun. Efficiency of mode transformation
Authors: Hasan, S. S.; Ulmschneider, P.
2004A&A...422.1085H Altcode: 2004astro.ph..6626H
We aim to identify the physical processes which occur in the magnetic
network of the chromosphere and which contribute to its dynamics
and heating. Specifically, we study the propagation of transverse
(kink) MHD waves which are impulsively excited in flux tubes through
footpoint motions. When these waves travel upwards, they get partially
converted to longitudinal waves through nonlinear effects (mode
coupling). By solving the nonlinear, time-dependent MHD equations
we find that significant longitudinal wave generation occurs in the
photosphere typically for Mach numbers as low as 0.2 and that the
onset of shock formation occurs at heights of about 600 km above the
photospheric base. We also investigate the compressional heating due
to longitudinal waves and the efficiency of mode coupling for various
values of the plasma β, that parameterises the magnetic field strength
in the network. We find that this efficiency is maximum for field
strengths corresponding to β≈ 0.2, when the kink and tube wave
speeds are almost identical. This can have interesting observational
implications. Furthermore, we find that even when the two speeds are
different, once shock formation occurs, the longitudinal and transverse
shocks exhibit strong mode coupling.
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Title: Torsional magnetic tube waves in stellar convection
zones. I. Analysis of wave generation and application to the Sun
Authors: Noble, M. W.; Musielak, Z. E.; Ulmschneider, P.
2003A&A...409.1085N Altcode:
An analytic approach to the generation of torsional magnetic tube waves
in stellar convection zones is presented. The waves are produced in
a thin, vertically oriented magnetic flux tube embedded in a magnetic
field-free, turbulent and compressible external medium and are excited
by external turbulent flows. A theory for this interaction is developed
and used to compute the wave energy spectra and fluxes carried by
torsional tube waves in the solar atmosphere. We find that these tube
waves have a characteristic cutoff frequency.
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Title: Atmospheric oscillations in solar magnetic flux
tubes. II. Excitation by transverse tube waves and random pulses
Authors: Musielak, Z. E.; Ulmschneider, P.
2003A&A...406..725M Altcode:
The response of an exponentially diverging magnetic flux tube embedded
in an isothermal solar atmosphere to the propagation of transverse tube
waves and random transverse pulses generated in the solar convection
zone is studied analytically. General solutions are presented and
applied to solar flux tubes located in the interior region and at
the boundary of supergranulation cells. It is shown that the period
of the free oscillations driven by transverse waves and pulses ranges
from 7 to 10 min for the considered values of the tube magnetic field,
and that these oscillations decay in time as t<SUP>-3/2</SUP>. Since
the observational signatures of these transverse oscillations are hard
to detect, we also consider the generation of longitudinal tube waves
by nonlinear mode coupling and the excitation of free atmospheric
oscillations by longitudinal waves. Our results show that the basic
properties of oscillations driven by transverse and longitudinal tube
waves are different. While transverse waves excite oscillations with
7-10 min periods, oscillations by longitudinal waves have periods near
3 min. This is consistent with the observed 3-min oscillations inside
the supergranule cells but inconsistent with the 7-min oscillations
observed in the chromospheric network. We suggest that an explanation of
the observed 7-min oscillations might be found by taking into account a
more realistic structure of flux tubes located in the magnetic network.
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Title: Time-dependent Ionization in Dynamic Solar and Stellar
Atmospheres. I. Methods
Authors: Rammacher, W.; Ulmschneider, P.
2003ApJ...589..988R Altcode:
We propose a new numerical method to compute one-dimensional
time-dependent wave propagation in stellar atmospheres that incorporates
the time-dependent treatment of hydrogen ionization together with an
evaluation of radiation losses under departures from local thermodynamic
equilibrium (NLTE). The method permits us to calculate acoustic
waves and longitudinal magnetohydrodynamic (MHD) tube waves. We
have tested the method for the solar atmosphere by calculating the
propagation of three types of waves, namely, a monochromatic acoustic
wave, a stochastic acoustic wave, and a stochastic longitudinal tube
wave. It was found that with a time-dependent treatment of the hydrogen
ionization (as well as the Mg ionizations) the degree of ionization
(H<SUP>+</SUP>/H) and the Mg II/Mg ratio become insensitive to the
temperature fluctuations, even in the presence of weak and moderately
strong shocks. Only when strong shocks appear do the transition rates
become large enough to cause a high correlation between the degree
of ionization and the high postshock temperatures. Our calculations
show that a mean degree of ionization gets established that increases
with height and is very little perturbed by the local temperature
fluctuations of the wave. In stochastic calculations, strong shocks
appeared periodically (roughly every 3 minutes), which in their
postshock regions carried a zone of high or complete ionization. Tests
with different numbers of frequency and height points, as well as of
the rate of convergence of the Λ-iteration, were performed.
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Title: Atmospheric oscillations in solar magnetic flux
tubes. I. Excitation by longitudinal tube waves and random pulses
Authors: Musielak, Z. E.; Ulmschneider, P.
2003A&A...400.1057M Altcode:
The response of exponentially spreading magnetic flux tubes embedded in
an isothermal solar atmosphere to the propagation of longitudinal tube
waves and random pulses produced in the solar convection zone is studied
analytically. General solutions are presented and applied to solar tube
models. It is shown that free atmospheric oscillations inside these
flux tubes are generated with oscillation periods near 3 min, which
are essentially identical to the oscillation periods observed in the
interior regions of supergranulation cells. The observed oscillations
are therefore consistent with processes in magnetic tubes as well as
in nonmagnetic regions. Stochastic perpetual excitation is expected
to keep these flux tube oscillations present at all times. These
oscillations are inconsistent with the observed 7-min oscillations in
the chromospheric network.
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Title: Kink and Longitudinal Oscillations in the Magnetic Network
on the Sun: Nonlinear Effects and Mode Transformation
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.;
Ulmschneider, P.
2003ApJ...585.1138H Altcode:
We examine the propagation of kink and longitudinal waves in the solar
magnetic network. Previously, we investigated the excitation of network
oscillations in vertical magnetic flux tubes through buffeting by
granules and found that footpoint motions of the tubes can generate
sufficient wave energy for chromospheric heating. We assumed that
the kink and longitudinal waves are decoupled and linear. We overcome
these limitations by treating the nonlinear MHD equations for coupled
kink and longitudinal waves in a thin flux tube. For the parameters
we have chosen, the thin tube approximation is valid up to the layers
of formation of the emission features in the H and K lines of Ca II,
at a height of about 1 Mm. By solving the nonlinear, time-dependent MHD
equations we are able to study the onset of wave coupling, which occurs
when the Mach number of the kink waves is of the order of 0.3. We also
investigate the transfer of energy from the kink to the longitudinal
waves, which is important for the dissipation of the wave energy in
shocks. We find that kink waves excited by footpoint motions of a
flux tube generate longitudinal modes by mode coupling. For subsonic
velocities, the amplitude of a longitudinal wave increases as the square
of the amplitude of the transverse wave, and for amplitudes near Mach
number unity, the coupling saturates and becomes linear when the energy
is nearly evenly divided between the two modes.
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Title: Lectures on Solar Physics
Authors: Antia, H. M.; Bhatnagar, A.; Ulmschneider, P.
2003LNP...619.....A Altcode: 2003lsp..conf.....A
No abstract at ADS
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Title: The Physics of Chromospheres and Coronae
Authors: Ulmschneider, P.
2003LNP...619..232U Altcode: 2003lsp..conf..232U
Towards the goal to unravel the physical reasons for the existence
of chromospheres and coronae significant progress has been
made. Chromospheres and coronae are layers which are dominated by
mechanical heating and usually by magnetic fields. The heating of
chromospheres can be explained by an ordered sequence of different
processes which systematically vary as function of height in the star
and with the speed of its rotation. It seems now pretty certain that
acoustic waves heat the low and middle chromosphere, and MHD waves the
magnetic regions up to the high chromosphere. With faster rotation,
the magnetic regions become more dominant. It seems that the highest
chromosphere needs additional non-wave heating mechanisms and that
there possibly reconnective microflare heating comes into play. For
the corona many different heating processes occur which work in the
various field geometries. Here more study is needed to identify the
relevance of these various processes.
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Title: Mechanisms of Chromospheric and Coronal Heating (Invited
review)
Authors: Ulmschneider, P.; Musielak, Z.
2003ASPC..286..363U Altcode: 2003ctmf.conf..363U
No abstract at ADS
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Title: The dynamics of the quiet solar chromosphere
Authors: Kalkofen, W.; Hasan, S. S.; Ulmschneider, P.
2003dysu.book..165K Altcode:
Wave propagation in the nonmagnetic chromosphere is described for
plane and spherical waves, and excitation by means of impulses in small
source regions in the photosphere; excitation for flux tube waves in
the magnetic network is described for large, single impulses and for a
fluctuating velocity field. Observational signatures of the various wave
types and their effect on chromospheric heating are considered. It is
concluded that calcium bright points in the nonmagnetic chromosphere
are due to spherical acoustic waves, and that for the oscillations
in the magnetic network, transverse waves are more important than
longitudinal waves; they may penetrate into the corona, giving rise
to some coronal heating.
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Title: Intelligent life in the universe : from common origins to
the future of humanity
Authors: Ulmschneider, Peter
2003ilu..book.....U Altcode:
No abstract at ADS
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Title: Heating of the solar chromosphere
Authors: Ulmschneider, P.; Kalkofen, W.
2003dysu.book..181U Altcode:
Overlying the photosphere is the chromosphere, a layer that is dominated
by mechanical and magnetic heating. By simulating the chromospheric
line and continuum emission, empirical models can be constructed that
allow the energy balance to be evaluated. Several possible heating
processes are discussed as well as the search is made for the actual
heating mechanisms. It is found that dissipation by acoustic waves is
the basic heating mechanism for nonmagnetic regions of the chromosphere,
and MHD tube waves for magnetic regions.
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Title: On the Generation of Flux-Tube Waves in Stellar Convection
Zones. IV. Longitudinal Wave Energy Spectra and Fluxes for Stars
with Nonsolar Metallicities
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
2002ApJ...573..418M Altcode:
In the previous papers of this series, we developed an analytical
method describing the generation of longitudinal tube waves in
stellar convection zones and used it to compute the wave energy
spectra and fluxes for late-type stars with the solar metal abundance
(Population I). We now extend these calculations to Population II
stars with effective temperatures ranging from T<SUB>eff</SUB>=2500
to 10,000 K, gravities logg=3-5, and with three different metal
abundances: 1/10, 1/100, and 1/1000 of solar metallicity. The obtained
results are valid for a single magnetic flux, and they show that
the effects of metallicity are important only for cool stars with
T<SUB>eff</SUB><6000 K and that the amount of the generated wave
energy decreases roughly by an order of magnitude for every decrease
of the metallicity by an order of magnitude. The maximum wave energy
flux generated in Population II stars is 7×10<SUP>8</SUP> ergs
cm<SUP>-2</SUP> s<SUP>-1</SUP>, and it is practically the same for
stars of different gravities and metallicities. The computed spectra
and fluxes can be used to construct theoretical models of magnetic
regions in chromospheres of Population II stars.
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Title: Acoustic and magnetic wave heating in stars . I. Theoretical
chromospheric models and emerging radiative fluxes
Authors: Fawzy, D.; Rammacher, W.; Ulmschneider, P.; Musielak, Z. E.;
Stȩpień, K.
2002A&A...386..971F Altcode:
We describe a method to construct theoretical, time-dependent,
two-component and wave heated chromosphere models for late-type
dwarfs. The models depend only on four basic stellar parameters:
effective temperature, gravity, metallicity and filling factor, which
determines the coverage of these stars by surface magnetic fields. They
consist of non-magnetic regions heated by acoustic waves and vertically
oriented magnetic flux tubes heated by longitudinal tube waves with
contributions from transverse tube waves. Acoustic, longitudinal
and transverse wave energy spectra and fluxes generated in stellar
convection zones are computed and used as input parameters for the
theoretical models. The waves are allowed to propagate and heat both
components by shock dissipation. We compute the time-dependent energy
balance between the dissipated wave energy and the most prominent
chromospheric radiative losses as function of height in the stellar
atmosphere. For the flux tube covered stars, the emerging radiative
fluxes in the Ca II and Mg II lines are computed by using a newly
developed multi-ray radiative transfer method.
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Title: Excitation of transverse magnetic tube waves in stellar
convection zones. II. Wave energy spectra and fluxes
Authors: Musielak, Z. E.; Ulmschneider, P.
2002A&A...386..606M Altcode:
The wave energy spectra and fluxes for transverse magnetic tube
waves generated in stellar convection zones are computed by using the
analytical method developed in the previous paper of this series. The
main physical process responsible for the generation of these waves is
shaking of a thin and vertically oriented magnetic flux tube by the
external turbulent convection. The approach includes the correlation
effects, which occur when the tube is shaken over a significant fraction
of its length, but is limited to linear waves. The calculations are
performed for population I stars with effective temperatures ranging
from T<SUB>eff</SUB> = 2000 K to 10 000 K, and with gravities log g =
3-5. It is shown that the fluxes carried by linear transverse waves
along a single flux tube are approximately one order of magnitude
higher than those carried by linear longitudinal tube waves. The
obtained results can be used to construct theoretical models of stellar
chromospheres and winds.
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Title: Excitation of transverse magnetic tube waves in stellar
convection zones . III. Effects of metallicity on wave energy spectra
and fluxes
Authors: Musielak, Z. E.; Ulmschneider, P.
2002A&A...386..615M Altcode:
In the previous two papers of this series, we developed an analytical
method describing the generation of transverse tube waves in stellar
convection zones and used it to compute the wave energy spectra and
fluxes for late-type stars with the solar metal abundance (population
I stars). We now extend these calculations to population II stars
with effective temperatures ranging from T<SUB>eff</SUB> = 2500 K
to 10 000 K, gravities log g = 3 - 5, and with three different metal
abundances: 1/10, 1/100 and 1/1000 of solar metallicity. The obtained
results are valid only for a single magnetic flux tube and they show
that the effects of metallicity are important only for cool stars with
T<SUB>eff</SUB> < 6000 K and that the amount of the generated wave
energy decreases roughly by an order of magnitude for every decrease
of the metallicity by an order of magnitude. The maximum wave energy
flux generated in population II stars is 3 x 10<SUP>9</SUP> erg
cm<SUP>-2</SUP> s<SUP>-1</SUP> and it is practically the same for
stars of different gravities and metallicities.
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Title: Acoustic and magnetic wave heating in stars . II. On the
range of chromospheric activity
Authors: Fawzy, D.; Ulmschneider, P.; Stȩpień, K.; Musielak, Z. E.;
Rammacher, W.
2002A&A...386..983F Altcode:
In the first paper of this series we developed a method to construct
theoretical, time-dependent and two-component chromosphere models
for late-type main sequence stars. The models consist of non-magnetic
regions heated by acoustic waves and magnetic flux tube regions heated
by magnetic tube waves. By specifying the magnetic filling factor,
theoretical models of stellar atmospheres with different chromospheric
activity can be calculated. Here, these models are used to simulate the
emerging Ca II and Mg II chromospheric emission fluxes and compare them
with observations. The comparison shows that the wave heating alone
can explain most but not all of the observed range of chromospheric
activity. In addition, the results obtained clearly imply that the base
of stellar chromospheres is heated by acoustic waves, the heating of
the middle and upper chromospheric layers is dominated by magnetic
waves associated with magnetic flux tubes, and that other non-wave
(e.g., reconnective) heating mechanisms are required to explain the
structure of the highest layers of stellar chromospheres.
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Title: Acoustic and magnetic wave heating in stars . III. The
chromospheric emission-magnetic filling factor relation
Authors: Fawzy, D.; Stȩpień, K.; Ulmschneider, P.; Rammacher, W.;
Musielak, Z. E.
2002A&A...386..994F Altcode:
Theoretical chromospheric models described in the two previous
papers of this series are used to study the relationship between the
chromospheric emission and the filling factor. This theoretically
determined relationship shows that the chromospheric emission flux
in Ca II (H+K) is approximately proportional to the square root of
the magnetic filling factor at the stellar surface. To relate the
filling factor to stellar rotation rate, we compare the theoretical
fluxes with observations of stars with known rotation period. The
comparison shows that the Rossby number is probably a more appropriate
measure of the rotation influence on activity of main-sequence stars
than the rotation period. Our theoretical Mg II (h+k) and Ca II (H+K)
emission fluxes are also found to be well correlated, which is in a
good agreement with the observational data.
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Title: Intelligent life in the universe : from common origins to
the future of humanity
Authors: Ulmschneider, Peter
2002iliu.book.....U Altcode:
No abstract at ADS
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Title: Main Heating Mechanisms in Stellar Atmospheres
Authors: Musielak, Z. E.; Fawzy, D.; Ulmschneider, P.; Rammacher,
W.; Stepien, K.
2001AAS...19914302M Altcode: 2001BAAS...33.1522M
To identify the main heating mechanisms operating in atmospheres of
late-type stars, we have constructed purely theoretical, two-component
and time-dependent models of stellar chromospheres. Our models depend
only on four basic stellar parameters: effective temperature, gravity,
metallicity, and filling factor, which determines the coverage of
these stars by surface magnetic fields and is treated as a free
parameter. They consist of non-magnetic regions heated by acoustic
waves and magnetic flux tubes heated by longitudinal and transverse
tube waves. At each height in stellar atmospheres, the time-dependent
energy balance between the dissipated wave energy and the most prominent
radiative losses is calculated. By specifying the filling factor,
theoretical models of stellar atmospheres with different chromospheric
activity are computed. We have used these models to simulate the
emerging Ca II and Mg II chromospheric emission fluxes and compare them
with observations. The comparison shows that the wave heating alone
can explain most but not all of the observed range of chromospheric
activity. In addition, the obtained results clearly imply that the base
of stellar chromospheres is heated by acoustic waves, the heating of the
middle and upper chromospheric layers is dominated by magnetic waves
associated with magnetic flux tubes, and that other non-wave (e.g.,
reconnective) heating mechanisms are required to explain the structure
of the highest layers of stellar chromospheres. This work was supported
by NSF, NATO, DFG, KBN and The Alexander von Humboldt Foundation.
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Title: Wave Heating and Range of Stellar Activity in Late-Type Dwarfs
Authors: Ulmschneider, P.; Fawzy, D.; Musielak, Z. E.; Stępień, K.
2001ApJ...559L.167U Altcode:
Theoretical time-dependent and two-component chromospheric models for
late-type dwarfs are constructed based on acoustic and magnetic wave
heating mechanisms. The models are used to predict the theoretical range
of chromospheric activity for these stars. Comparison of this range with
the one established observationally shows that the wave heating alone
can explain most but not all of the observed range of stellar activity.
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Title: Magnetic wave energy fluxes for late-type
stars. I. Longitudinal tube waves
Authors: Ulmschneider, P.; Musielak, Z. E.; Fawzy, D. E.
2001A&A...374..662U Altcode:
The wave energy fluxes carried by longitudinal tube waves along thin
vertical magnetic flux tubes embedded in atmospheres of late-type
stars are computed. The main physical process responsible for the
generation of these waves is the nonlinear time-dependent response of
the flux tubes to continuous and impulsive external turbulent pressure
fluctuations, which are represented here by an extended Kolmogorov
spatial and modified Gaussian temporal energy spectrum. Both the wave
energy fluxes and spectra are calculated for population I stars with
effective temperatures ranging from T<SUB>eff</SUB> = 3500 K to 7000
K, and with gravities in the range log g = 3-5. The obtained results
show that the computed wave energy may significantly contribute to
the enhanced heating observed in magnetic regions of late-type stars.
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Title: Excitation of transverse magnetic tube waves in stellar
convection zones. I. Analytical approach
Authors: Musielak, Z. E.; Ulmschneider, P.
2001A&A...370..541M Altcode:
Analytical treatment of the excitation of transverse magnetic tube
waves in stellar convection zones is presented. The waves are produced
by the interaction between thin and vertically oriented magnetic flux
tubes embedded in stellar convection zones and the external turbulent
motions. A general theory describing this interaction is developed
and used to compute the wave energy spectra and fluxes for the Sun.
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Title: Coronal Heating by Kink Waves
Authors: Hasan, S. S.; Kalkofen, W.; Ulmschneider, P.
2001AGUSM..SH41B01H Altcode:
We examine the hypothesis that kink waves contribute to coronal
heating. In earlier work we demonstrated that the excitation of
kink oscillations flux tubes in the magnetic network of the Sun
through their footpoint motions can provide sufficient energy for
chromospheric heating. This calculation assumed that: (a) the waves
could be treated using the linear approximation, and (b) the kink
and longitudinal waves were decoupled. These approximations, although
valid in the lower atmosphere, break down in the upper chromosphere,
where the wave amplitude becomes comparable with the tube speed. We
overcome the earlier limitations by numerically solving the nonlinear
MHD equations for coupled kink and longitudinal waves. Using a specified
form of the footpoint motions, which is compatible with observations,
we solve the nonlinear time-dependent MHD equations for a thin flux
tube extending vertically from the sub-photosphere to the base of the
corona. Our code is able to resolve shocks and also self-consistently
treats mode transformation. We calculate the energy fluxes in vertically
propagating kink waves and show that there is in principle adequate
energy in the waves to heat the corona.
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Title: Self-Consistent Magnetic/Acoustic Chromosphere Models of
Late-Type Stars (CD-ROM Directory: contribs/cuntz1)
Authors: Cuntz, M.; Ulmschneider, P.; Rammacher, W.; Musielak, Z. E.;
Saar, S. H.
2001ASPC..223..913C Altcode: 2001csss...11..913C
No abstract at ADS
---------------------------------------------------------
Title: Chromosphere: Heating Mechanisms
Authors: Ulmschneider, P.
2000eaa..bookE2260U Altcode:
Mechanical heating...
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Title: Time-dependent analytical solutions for MHD surface waves
propagating in a compressible plasma
Authors: Musielak, Z. E.; Huang, P.; Ulmschneider, P.
2000A&A...362..359M Altcode:
The propagation of linear MHD surface waves in a compressible plasma
with a discontinuous interface in the magnetic field and temperature
is considered. The initial perturbation is applied only to a vorticity
line, which is located on the RHS of the discontinuity. Two different
surface waves exist in this model, one associated with the vorticity
line and the other confined to the discontinuity. Time-dependent
analytical solutions for the wave velocity perturbations are obtained
for both surface waves by applying Laplace transforms. The solutions
are used to investigate the effects caused by compressibility on the
propagation of these waves. It is shown that the compressibility effects
are most important in the vicinity of the vorticity line and at the
magnetic interface, and that they affect differently the behavior of
the surface waves.
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Title: On the Generation of Flux-Tube Waves in Stellar Convection
Zones. III. Longitudinal Tube Wave-Energy Spectra and Fluxes for
Late-Type Stars
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
2000ApJ...541..410M Altcode:
The wave-energy spectra and fluxes for longitudinal tube waves
generated in stellar convection zones are computed by using analytical
methods developed in the two previous papers of this series. The main
physical process responsible for the generation of these waves is the
interaction between a thin and vertically oriented magnetic flux tube
and the external turbulent convection. The spatial component of the
turbulent convection is represented by an extended Kolmogorov turbulent
energy spectrum, and its temporal component by a modified Gaussian
frequency factor. The calculations are performed for Population I
stars with effective temperatures ranging from T<SUB>eff</SUB>=2000 K
to 10,000 K, and with gravities logg=3-5. The obtained results can be
used to construct theoretical models of magnetic regions in stellar
chromospheres.
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Title: The dynamics and heating of the quiet solar chromosphere
Authors: Kalkofen, Wolfgang; Ulmschneider, Peter
1999CSci...77.1496K Altcode:
No abstract at ADS
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Title: Propagation of nonlinear longitudinal-transverse waves along
magnetic flux tubes in the solar atmosphere. III. Modified equation
of motion
Authors: Osin, A.; Volin, S.; Ulmschneider, P.
1999A&A...351..359O Altcode:
In this series of papers the time-dependent propagation of nonlinear
longitudinal-transverse waves in thin vertical magnetic flux tubes
embedded in the solar atmosphere is investigated numerically using the
(one-dimensional) thin tube approximation. As in the last decade the
particular form of the backreaction term in the transverse equation of
motion has been under considerable dispute we investigate this issue
once again and suggest a new expression for the backreaction term in the
local approximation. This new expression intends to avoid criticisms
leveled at the previous terms. In the present paper we numerically
compare the actual effects these different terms produce in a number
of cases including the situations where the reported discrepancies are
prominent. We find that noticeable discrepancies between the various
proposed backreaction terms occur only, when strong longitudinal
fluid flows are present simultaneously with the swaying of the tube
and that for weak flows these discrepancies disappear. If only weak
longitudinal flows occur it thus appears that the particular choice
of the backreaction term is not important.
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Title: Two-Component Theoretical Chromosphere Models for K Dwarfs
of Different Magnetic Activity: Exploring the Ca II Emission-Stellar
Rotation Relationship
Authors: Cuntz, M.; Rammacher, W.; Ulmschneider, P.; Musielak, Z. E.;
Saar, S. H.
1999ApJ...522.1053C Altcode:
We compute two-component theoretical chromosphere models for K2 V
stars with different levels of magnetic activity. The two components
are a nonmagnetic component heated by acoustic waves and a magnetic
component heated by longitudinal tube waves. The filling factor for the
magnetic component is determined from an observational relationship
between the measured magnetic area coverage and the stellar rotation
period. We consider stellar rotation periods between 10 and 40
days. We investigate two different geometrical distributions of
magnetic flux tubes: uniformly distributed tubes, and tubes arranged
as a chromospheric network embedded in the nonmagnetic region. The
chromosphere models are constructed by performing state-of-the-art
calculations for the generation of acoustic and magnetic energy in
stellar convection zones, the propagation and dissipation of this
energy at the different atmospheric heights, and the formation of
specific chromospheric emission lines that are then compared to the
observational data. In all these steps, the two-component structure of
stellar photospheres and chromospheres is fully taken into account. We
find that heating and chromospheric emission is significantly increased
in the magnetic component and is strongest in flux tubes that spread
the least with height, expected to occur on rapidly rotating stars with
high magnetic filling factors. For stars with very slow rotation, we
are able to reproduce the basal flux limit of chromospheric emission
previously identified with nonmagnetic regions. Most importantly,
however, we find that the relationship between the Ca II H+K emission
and the stellar rotation rate deduced from our models is consistent
with the relationship given by observations.
---------------------------------------------------------
Title: Does the Sun Have a Full-Time Chromosphere?
Authors: Kalkofen, Wolfgang; Ulmschneider, Peter; Avrett, Eugene H.
1999ApJ...521L.141K Altcode:
The successful modeling of the dynamics of H<SUB>2v</SUB> bright
points in the nonmagnetic chromosphere by Carlsson & Stein gave as
a by-product a part-time chromosphere lacking the persistent outward
temperature increase of time-average empirical models, which is needed
to explain observations of UV emission lines and continua. We discuss
the failure of the dynamical model to account for most of the observed
chromospheric emission, arguing that their model uses only about 1%
of the acoustic energy supplied to the medium. Chromospheric heating
requires an additional source of energy in the form of acoustic waves
of short period (P<2 minutes), which form shocks and produce the
persistent outward temperature increase that can account for the UV
emission lines and continua.
---------------------------------------------------------
Title: Acoustic wave energy fluxes for late-type stars. II. Nonsolar
metallicities
Authors: Ulmschneider, P.; Theurer, J.; Musielak, Z. E.; Kurucz, R.
1999A&A...347..243U Altcode:
Using the Lighthill-Stein theory with modifications described by
Musielak et al. (1994), the acoustic wave energy fluxes were computed
for late-type stars with the solar metal abundance (population I stars)
by Ulmschneider et al. (1996). We now extend these computations to
stars with considerably lower metal content (population II stars
with 1/10 to 1/1000 of solar metallicity) and find that the acoustic
fluxes calculated for stars of different spectral types and different
luminosities are affected differently by the metallicity. It is found
that the Hertzsprung-Russel diagram can be subdivided into three domains
(labeled I, II and III) representing a different dependence of the
generated acoustic fluxes on the stellar metal abundance. For the
high T_eff stars of domain I there is no dependence of the generated
acoustic fluxes on metallicity. In domain III are stars with low
T_eff. Here the generated acoustic fluxes are lowered roughly by an
order of magnitude for every decrease of the metal content by an order
of magnitude. Finally, domain II represents the transition between the
other two domains and the generated acoustic fluxes strongly depend
on T_eff. The boundaries between the domains I and II, and II and
III can be defined by simple relationships between stellar effective
temperatures and gravities.
---------------------------------------------------------
Title: Numerical simulations of nonlinear MHD body and surface waves
in magnetic slabs
Authors: Huang, P.; Musielak, Z. E.; Ulmschneider, P.
1999A&A...342..300H Altcode:
The behavior of MHD waves propagating in magnetically structured plasmas
has been extensively investigated in the literature. In most of these
studies, the wave treatment was restricted to the linear regime. This
paper presents the results of time-dependent and nonlinear numerical
simulations of MHD body and surface waves propagating along magnetic
slabs. Both longitudinal and transverse waves are computed, and the wave
behavior in the linear and nonlinear regime is compared. Two physical
processes are investigated in detail. The first is the energy leakage
from the magnetic slab to the field-free external medium. It is found
that the energy leakage is 62% for transverse slab waves, which means
that the efficiency of energy transfer along the slab by these waves
is significantly reduced. The second process is the excitation of MHD
waves in two adjacent magnetic slabs by large amplitude acoustic waves
from the external medium. The slabs have physical parameters typical
for photospheric magnetic flux tubes. It is shown that only 1-3%
of the energy carried by these acoustic waves is transferred to the
slabs, and that the efficiency of this process strongly depends on the
location of the slabs relative to the source of acoustic waves and on
the amplitude of these waves. Both physical processes are important
for the problem of heating of magnetically structured regions in the
solar and stellar atmospheres.
---------------------------------------------------------
Title: Die heißen äußeren Schichten der Sonne.
Authors: Ulmschneider, P.
1999A&R....36...13U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theoretical Chromosphere Models
Authors: Ulmschneider, P.
1999ASPC..158..260U Altcode: 1999ssa..conf..260U
No abstract at ADS
---------------------------------------------------------
Title: The Generation of Longitudinal Tube Waves in Late Type Stars
Authors: Fawzy, D. E.; Musielak, Z. E.; Ulmschneider, P.
1999RoAJ....9S.149F Altcode:
The aim of the present work is to compute the generated nonlinear
tube wave energy fluxes carried by longitudinal waves as a result of
the interaction between a vertically directed thin magnetic flux tube
and the turbulent medium in the stellar convection zone of late type
stars. The computations are based on work by Ulmschneider and Musielak
(1998). The current computations are for stars of gravities log g =
3,4,5 and temperature range from Teff = 3500 to 7000 K.
---------------------------------------------------------
Title: Acoustic wave propagation in the solar
atmosphere. III. Analytic solutions for adiabatic wave excitations
Authors: Sutmann, G.; Musielak, Z. E.; Ulmschneider, P.
1998A&A...340..556S Altcode:
The response of an isothermal solar type atmosphere to adiabatic
excitations by various small amplitude acoustic disturbances is studied
analytically. Both continuous and pulse excitations are discussed. It
is shown that wavetrains of random pulses may be responsible for the
excitation of the 3-min solar oscillations.
---------------------------------------------------------
Title: Theoretical Models of Stellar Chromospheres
Authors: Musielak, Z. E.; Cuntz, M.; Ulmschneider, P.
1998AAS...193.2204M Altcode: 1998BAAS...30.1283M
To identify the basic physical processes that underlie stellar
chromospheric activity, we have taken a novel theoretical approach and
constructed first purely theoretical, two-component and time-dependent
models of stellar chromospheres. Our models require specifying only four
basic stellar parameters, namely, the effective temperature, gravity,
metallicity and rotation rate, and they take into account non-magnetic
and magnetic regions in stellar chromospheres. The non-magnetic regions
are heated by acoustic waves generated by the turbulent convection in
the stellar subphotospheric layers. The magnetic regions are identified
with magnetic flux tubes uniformly distributed over the entire
stellar surface and are heated by longitudinal tube waves generated by
turbulent motions in the subphotospheric and photospheric layers. The
coverage of stellar surface by magnetic regions (the so-called filling
factor) is estimated for a given rotation rate from an observational
relationship. The constructed models are based on the energy balance
between the amount of mechanical energy supplied by waves and radiative
losses in strong Ca II and Mg II emission lines. We have already
used our chromospheric models to predict the level of “basal flux”
and the decrease of chromospheric activity with stellar rotation in
selected late-type dwarfs. We present these new results and discuss how
to include stellar transition regions, coronae and winds in our models.
---------------------------------------------------------
Title: Two-Component Chromosphere Models for K Dwarf Stars: The
Chromospheric Emission --- Stellar Rotation Relationship
Authors: Cuntz, M.; Musielak, Z. E.; Ulmschneider, P.; Rammacher,
W.; Saar, S. H.
1998AAS...193.4402C Altcode: 1998BAAS...30.1315C
We present two-component theoretical chromosphere models for K dwarf
stars with different levels of magnetic activity. The two components
are: a nonmagnetic component heated by acoustic waves, and a magnetic
component heated by longitudinal tube waves. The filling factor for
the magnetic component is determined from an observational relationship
between the stellar rotation rate and the measured coverage of stellar
surface by magnetic fields. The chromosphere models are constructed by
performing state-of-the-art calculations of the generation of acoustic
and magnetic energy in stellar convection zones, the propagation and
dissipation of this energy at the different atmospheric heights,
and the formation of specific chromospheric emission lines, which
are then compared to the observational data. In all these steps, the
two-component structure of stellar photospheres and chromospheres is
fully taken into account. We find that due to the presence of magnetic
flux tubes, the heating and chromospheric emission is significantly
increased in the magnetic component. The heating and chromospheric
emission is found to be the strongest in flux tubes with small
spreading factors which are expected to be present in fast rotating
stars. For stars with very slow rotation we are able to reproduce
the basal flux limit of chromospheric emission previously identified
as due to pure acoustic heating. Most importantly, however, we find
that the relationship between the Ca II H+K emission and the stellar
rotation rate deduced from our models is consistent with the empirical
relationship given by observations.
---------------------------------------------------------
Title: On the generation of nonlinear magnetic tube waves in the
solar atmosphere. II. Longitudinal tube waves
Authors: Ulmschneider, P.; Musielak, Z. E.
1998A&A...338..311U Altcode:
The nonlinear time-dependent response to external pressure fluctuations
acting on a thin vertical magnetic flux tube embedded in the solar
atmosphere is investigated numerically. The continuous and impulsive
fluctuations are imposed on the tube at different atmospheric heights
and the resulting longitudinal tube wave energy fluxes are calculated
for an observationally established range of velocity amplitudes and
tube magnetic fields. The obtained results show that typical wave
energy fluxes carried by nonlinear longitudinal tube waves are of the
order of 2*10(8) erg/cm(2) s, which is roughly a factor of 30 less than
the flux for transverse waves. In contrast to our linear analytical
results the generated nonlinear longitudinal tube wave fluxes can be
up to an order of magnitude higher.
---------------------------------------------------------
Title: The heating of solar magnetic flux tubes. I. Adiabatic
longitudinal tube waves
Authors: Fawzy, Diaa E.; Ulmschneider, P.; Cuntz, M.
1998A&A...336.1029F Altcode:
We study the formation of shocks and shock heating by adiabatic
longitudinal tube waves in solar magnetic flux tubes of different
shape. Monochromatic waves with periods between 20 and 160 s and
energy fluxes ranging from 1* 10(7) to 1* 10(9) erg cm(-2) s(-1) were
considered. It is found that the tube shape is of critical importance
for the heating of flux tubes. Constant cross-section tubes show
large heating, whereas exponentially spreading tubes show little or no
heating at all. In tubes of intermediate shapes (“wine-glass tubes"),
the heating is essentially restricted to those regions, where the tube
has attained its maximum diameter. This finding is in good agreement
with the observation that the chromospheric network can still be seen
well above the canopy height. In tubes of lower field strength, the
shock formation is delayed and heating is reduced.
---------------------------------------------------------
Title: Basal Heating in Main-Sequence Stars and Giants: Results from
Monochromatic Acoustic Wave Models
Authors: Buchholz, Bernd; Ulmschneider, Peter; Cuntz, Manfred
1998ApJ...494..700B Altcode:
We calculate time-dependent models of acoustically heated chromospheres
for main-sequence stars between spectral type F0 V and M0 V and for
two giants of spectral type K0 III and K5 III assuming monochromatic
waves. The hydrodynamic equations are solved together with the radiative
transfer and statistical equilibrium equations to investigate the
propagation of acoustic waves into the chromospheric regions. The
emergent radiation in Mg II h + k and Ca II H + K is calculated and
compared with observations. We find good agreement, over nearly 2
orders of magnitude, between the time-averaged emission in these
lines and the observed basal flux emission, which had been suspected
to be due to nonmagnetic (i.e., acoustic) heating operating in all
late-type stars. The height dependence of the acoustic energy flux
can be explained by the limiting strength property of the acoustic
shocks and is consistent with that found in models of quiet solar
regions. We also confirm the validity of the Ayres scaling law,
which has originally been derived for semiempirical chromosphere
models and is thus independent of assumptions about the chromospheric
heating mechanism. Our results strongly support the idea that the
“basal heating” of chromospheres of late-type stars as revealed
by the frequency-integrated Mg II and Ca II line emission is fully
attributable to the dissipation of acoustic wave energy.
---------------------------------------------------------
Title: Self-Consistent and Time-Dependent Magnetohydrodynamic
Chromosphere Models for Magnetically Active Stars
Authors: Cuntz, Manfred; Ulmschneider, Peter; Musielak, Zdzislaw E.
1998ApJ...493L.117C Altcode:
We present self-consistent and time-dependent MHD heating models
for chromospheres of magnetically active stars. We investigate the
propagation and dissipation of longitudinal flux-tube waves in K2 V
stars with different rotation rates implying different photospheric
and chromospheric magnetic filling factors. These filling factors
are critical for determining the number of flux tubes on the stellar
surface and the spreading of the tubes with height, which is relevant
for the propagation and dissipation of the magnetic energy as well as
the generated radiative emission losses. The filling factors used in
this Letter are estimated using a relationship between the photospheric
values for B<SUB>0</SUB>f<SUB>0</SUB> and P<SUB>rot</SUB> in accord
with very recent magnetic field measurements by Rüedi et al. We also
consider revised computations of magnetic energy fluxes by Ulmschneider
& Musielak generated by turbulent motions. Our models show increased
shock strengths and energy dissipation rates in stars of faster rotation
because of the narrower spreading of the tubes. This also leads to
increased chromospheric emission, particularly in Mg II in stars of
faster rotation. We consider these results as a first step toward a
theoretical derivation of chromospheric emission--stellar rotation
relationships for stars of different masses and evolutionary status.
---------------------------------------------------------
Title: Heating of Chromospheres and Coronae
Authors: Ulmschneider, P.
1998HiA....11..831U Altcode:
Almost all nondegenerate statrs have chromospheres and coronae. These
hot outer layers are produced by mechanical heating. The heating
mechanisms of chromospheres and coronae, classified as hydrodynamic
and magnetic mechanism, are reviewed here. Both types of mechanisms
can be further subdivided on basis of the fluctuation frequency
into acoustic and pulsational waves for hydrodynamic and into AC-
and DC-mechanisms for magnetic heating. Intense heating is usually
associated with the formation of very small spatial scales, which
are difficult to observe. Yet, global stellar observations, because
of the dependence of the mechanical energy generation on the basic
stellar parameters (T<SUB>eff</SUB>, gravity, rotation, metallicity)
can be extremely important to identify the dominant heating mechanisms.
---------------------------------------------------------
Title: Acoustic and MHD Wave Energy Fluxes for Late-Type Stars
Authors: Musielak, Z. E.; Cuntz, M.; Ulmschneider, P.; Theurer, J.;
Kurucz, R.
1997AAS...191.1206M Altcode: 1997BAAS...29.1228M
The vast amount of observational data collected at wavelengths
ranging from X-rays to radio waves have indicated the ubiquity of
stellar chromospheres among late-type stars. In addition, there is
growing observational evidence for inhomogeneous and locally strong
magnetic fields in stellar atmospheres. It is reasonable to assume that
stellar magnetic inhomogeneities may be similar to the `flux tube'
structures observed in the solar atmosphere outside sunspots. If so,
two distinct components of stellar chromospheres must be recognized,
namely, non-magnetic component, where acoustic waves are responsible
for the heating, and magnetic component, where MHD tube waves supply
energy for the heating. To construct theoretical models of stellar
chromospheres (see paper by Cuntz et al. presented at this meeting),
it is necessary to know the amount of non-radiative energy generated
in stellar convective zones and carried by acoustic and MHD tube
waves through stellar photospheres. In this paper, we discuss the
correct status of computing acoustic and MHD wave energy fluxes for
the Sun and late-type dwarfs. Our calculations are based on grey LTE
mixing-length convection zone models and both linear and non-linear
theories of wave generation are used. New acoustic and MHD wave
energy fluxes are presented for stars of population I and II in the
range of effective temperatures T_eff 2000 - 10000 K and gravities
log g = 1 - 8. The turbulent flow field is represented by an extended
Kolmogorov spatial and modified Gaussian temporal energy spectrum. The
mixing-length parameter is varied in the range alpha = 1 - 2. We find
that the obtained acoustic wave energy strongly depend on stellar
chemical composition and that MHD fluxes show wide variations for a
given spectral type, variations which can be attributed to changes
in the stellar flux tube filling factor. We discuss the range of the
filling factor for which the calculated MHD fluxes may account for
the observed levels of chromospheric activity.
---------------------------------------------------------
Title: Self-Consistent and Time-Dependent MHD Heating Models for
Chromospheres of Magnetically Active Stars
Authors: Cuntz, M.; Musielak, Z. E.; Ulmschneider, P.
1997AAS...191.1205C Altcode: 1997BAAS...29.1228C
We present self-consistent and time-dependent MHD heating models
for chromospheres of magnetically active stars. We investigate the
propagation and dissipation of longitudinal flux tube waves in K2V
stars with different rotation rates implying different photospheric
and chromospheric magnetic filling factors. These filling factors are
critical for determining the number of flux tubes on the stellar surface
and the spreading of the tubes with height, which is relevant for the
propagation and dissipation of the magnetic energy. The photospheric
filling factors are estimated using a relationship between the magnetic
field strength B_o multiplied by the photospheric magnetic filling
factor f_o and the stellar rotation P_rot in accord with very recent
magnetic field measurements. We also consider revised computations
for the initial magnetic energy fluxes generated by turbulent motions
(Ulmschneider & Musielak 1997, A&A, submitted). Our models
show increased shock strengths and energy dissipation rates in stars of
faster rotation due to the narrower spreading of the tubes. This also
leads to increased chromospheric emission particularly in Mg II in stars
of faster rotation. We consider these results as a first step toward a
theoretical derivation of chromospheric emission --- stellar rotation
relations for stars of different masses and evolutionary status.
---------------------------------------------------------
Title: Dynamical response of magnetic tubes to transverse
perturbations. II. Towards thin flux tubes.
Authors: Ziegler, U.; Ulmschneider, P.
1997A&A...327..854Z Altcode:
The dynamical response of magnetic flux tubes due to local transverse
periodic perturbations is investigated by numerical means. Our aim
was to check the applicability of the thin flux tube approximation
for vertically oriented flux tubes with moderate magnetic field
strength (plasma β=1) and diameters 100km, 50km and 25km immersed
in an otherwise homogeneous nonmagnetic environment. All tubes
has been subject to the same driver. To resolve the flux tube in
a 2000x2000x1000km^3^ computational domain, a multiple nested grid
version of a 3D MHD code is used. We find that a description as ideally
thin flux tube becomes more and more questionable if the diameter
of the tube decreases. For the thinnest tube we found eg. an almost
complete split up into a fork-like geometry with two counterrotating
legs. This can be explained by a more rigorous interaction of the
tube with the ambient medium: Geometrically thinner flux tubes are
less inert than corresponding thick tubes and therefore experience
stronger backreaction forces because thinner tubes are easier to
displace horizontally. As a consequence of this, their cross sections
are significantly deformed contradictory to the assumptions made in
the thin flux tube approximation. The energy loss from the internal
tube motions to the surroundings by acoustic radiation is found to be
anticorrelated with the tube radius ie. thinner tubes loose more wave
energy than thicker ones.
---------------------------------------------------------
Title: Dynamical response of magnetic tubes to transverse
perturbations. I. Thick flux tubes.
Authors: Ziegler, U.; Ulmschneider, P.
1997A&A...324..417Z Altcode:
By means of a 3D numerical code we investigate the response of
magnetic flux tubes to transverse perturbations. Various tubes with
plasma β ranging from 0.1 to 10 embedded in a uniform nonmagnetic
atmosphere are considered. High spatial resolution was obtained by
the application of a multiple nested grid strategy. Various kinds of
internal longitudinal and transverse body waves as well as surface waves
were found in addition to the external sound wave. A great advantage
of our 3D treatment is that it allows to treat energy leakage and mode
conversion. We investigated the efficiency of wave energy leakage from
the magnetic tube to the external medium and found leakage rates ranging
from 0.07 for the β=10 tube to 0.43 for the β=0.1 tube. This shows
that leakage is an important process particularly for low β tubes and
should not be ignored in studies of transverse wave propagation. As
already found in 1D calculations, the purely transverse excitation
generates longitudinal body waves of twice the frequency. This mode
conversion process is not very efficient. A very important result of our
computations, however, is the efficient generation of a non-axisymmetric
longitudinal body wave which does not derive from magnetic tension
forces, but is due to an inertial pile-up effect inside the tube
produced by the transverse motions. Particularly in low β-tubes,
the mode conversion rate for the longitudinal waves was found to be
as large as 90% of the total kinetic tube energy most of it is going
into the surface wave. This may be very significant for the heating
of flux tubes and thus for the chromospheric and coronal heating.
---------------------------------------------------------
Title: Acoustic wave propagation in the solar
atmosphere. IV. Nonadiabatic wave excitation with frequency spectra.
Authors: Theurer, J.; Ulmschneider, P.; Cuntz, M.
1997A&A...324..587T Altcode: 1997astro.ph..3106T
We study the response of the solar atmosphere to excitations
by large amplitude acoustic waves with radiation damping now
included. Monochromatic adiabatic waves, due to unbalanced heating,
generate continuously rising chromospheric temperature plateaus in
which the low frequency resonances quickly die out. All non-adiabatic
calculations lead to stable mean chromospheric temperature distributions
determined by shock dissipation and radiative cooling. For non-adiabatic
monochromatic wave excitation, a critical frequency ν_cr_~1/25Hz
is confirmed, which separates domains of different resonance
behaviour. For waves of ν<ν_cr_, the resonances decay, while for
waves of ν>ν_cr_ persistent resonance oscillations occur, which
are perpetuated by shock merging. Excitation with acoustic frequency
spectra produces distinct dynamical mean chromosphere models where the
detailed temperature distributions depend on the shape of the assumed
spectra. The stochasticity of the spectra and the ongoing shock merging
lead to a persistent resonance behaviour of the atmosphere. The acoustic
spectra show a distinct shape evolution with height such that at great
height a pure 3min band becomes increasingly dominant. With our Eulerian
code we did not find appreciable mass flows at the top boundary.
---------------------------------------------------------
Title: Acoustic wave propagation in the solar
atmosphere. V. Observations versus simulations.
Authors: Theurer, J.; Ulmschneider, P.; Kalkofen, W.
1997A&A...324..717T Altcode: 1997astro.ph..4067T
We study the evolution of spectra of acoustic waves that are generated
in the convection zone and propagate upward into the photosphere,
where we compare the simulated acoustic spectra with the spectrum
observed in an Fe I line. Although there is no pronounced 3-min
component in the spectra generated in the convection zone, there
are dominant 3-min features in the theoretical spectra, in agreement
with the observed spectrum. We interpret the occurrence of the 3-min
features as the response of the solar atmosphere to the acoustic waves
which shifts high frequency wave energy to low frequencies. We also
find qualitative agreement for the acoustic power between the wave
simulations and the observations.
---------------------------------------------------------
Title: Chromospheric and coronal heating mechanisms.
Authors: Ulmschneider, P.
1997ASIC..494...95U Altcode: 1997topr.conf...95U
The present work discusses the basic physical mechanisms which produce
the heating of stellar chromospheres and coronae and concentrates on
the physics of the heating processes.
---------------------------------------------------------
Title: Dynamics of Flux Tubes in the Solar Atmosphere: Theory
Authors: Roberts, B.; Ulmschneider, P.
1997LNP...489...75R Altcode: 1997shpp.conf...75R
The modes of oscillation of a photospheric magnetic flux tube
are reviewed, taking into account both linear and nonlinear
aspects. Analytical and computational developments are discussed,
beginning with the basic characteristics of linear wave propagation
and progressing to a consideration of nonlinearity and the question
of the generation of tube waves and the energy flux they transport.
---------------------------------------------------------
Title: Acoustic wave energy fluxes for late-type stars.
Authors: Ulmschneider, P.; Theurer, J.; Musielak, Z. E.
1996A&A...315..212U Altcode:
We revisit the problem of acoustic wave generation by turbulent
convection in late-type stellar atmospheres. Using the Lighthill-Stein
theory with modifications described in our recent paper Musielak
et al. (1994ApJ...423..474M), we compute both acoustic frequency
spectra and total acoustic fluxes on basis of grey LTE mixing-length
convection zone models for population I stars, in the range of effective
temperatures T_eff_=2000-10000K and gravities logg=0-8. The turbulent
flow field is represented by an extended Kolmogorov spatial and modified
Gaussian temporal energy spectrum. The mixing-length parameter was
varied in the range α=1-2. Particularly for M-dwarf stars we find large
discrepancies between our fluxes and those of Bohn (1981 (Ph.D. Thesis,
Univ. Wuerzburg, Germany), 1984A&A...130..202B). Except for very
cool dwarf stars our total fluxes are well reproduced by the simple
Lighthill-Proudmann formula already developed in the 1950's.
---------------------------------------------------------
Title: On the interaction of longitudinal and transversal waves in
thin magnetic flux tubes.
Authors: Nakariakov, V. M.; Zhugzhda, Y. D.; Ulmschneider, P.
1996A&A...312..691N Altcode:
An analytical investigation of the nonlinear interaction of longitudinal
and transversal waves in thin magnetic flux tubes is presented and
the nonlinear terms which give rise to wave generation of other modes
and to shock formation are isolated. The nonlinear resonant three-wave
interaction of longitudinal and transversal waves is studied together
with the growth and decay behaviour of these waves. This analytical
study clarifies our previous numerical computations of nonlinear wave
generation and of the steepening of longitudinal as well as transversal
wave profiles.
---------------------------------------------------------
Title: Chromospheric and Coronal Heating Mechanisms II
Authors: Narain, U.; Ulmschneider, P.
1996SSRv...75..453N Altcode:
We review the mechanisms which are thought to provide steady heating
of chromospheres and coronae. It appears now fairly well established
that nonmagnetic chromospheric regions of latetype stars are heated
by shock dissipation of acoustic waves which are generated in the
stellar surface convection zones. In the case of late-type giants there
is additional heating by shocks from pulsational waves. For slowly
rotating stars, which have weak or no magnetic fields, these two are
the dominant chromospheric heating mechanisms. Except for F-stars,
the chromospheric heating of rapidly rotating late-type stars is
dominated by magnetic heating either through MHD wave dissipation (AC
mechanisms) or through magnetic field dissipation (DC mechanisms). The
MHD wave and magnetic field energy comes from fluid motions in the
stellar convection zones. Waves are also generated by reconnective
events at chromospheric and coronal heights. The high-frequency part
of the motion spectrum leads to AC heating, the low frequency part
to DC heating. The coronae are almost exclusively heated by magnetic
mechanisms. It is not possible to say at the moment whether AC or DC
mechanisms are dominant, although presently the DC mechanisms (e.g.,
nanoflares) appear to be the more important. Only a more detailed study
of the formation of and the dissipation in small-scale structures can
answer this question. The X-ray emission in early-type stars shows the
presence of coronal structures which are very different from those in
late-type stars. This emission apparently arises in the hot post-shock
regions of gas blobs which are accelerated in the stellar wind by the
intense radiation field of these stars.
---------------------------------------------------------
Title: Chromosphere and coronal heating mechanisms
Authors: Ulmschneider, P.
1996ASPC..109...71U Altcode: 1996csss....9...71U
No abstract at ADS
---------------------------------------------------------
Title: New acoustic wave energy computations for late-type stars
Authors: Theurer, J.; Ulmschneider, P.; Musielak, Z.
1996ASPC..109..169T Altcode: 1996csss....9..169T
No abstract at ADS
---------------------------------------------------------
Title: Temporal Variations in Solar Chromospheric Modeling.
Authors: Avrett, E.; Hoeflich, P.; Uitenbroek, H.; Ulmschneider, P.
1996ASPC..109..105A Altcode: 1996csss....9..105A
No abstract at ADS
---------------------------------------------------------
Title: Generation of Linear and Nonlinear Magnetic Tube Waves in
the Solar Atmosphere
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1996mpsa.conf..427M Altcode: 1996IAUCo.153..427M
No abstract at ADS
---------------------------------------------------------
Title: Propagation of nonlinear longitudinal-transverse waves along
magnetic flux tubes in the solar atmosphere. II. The treatment
of shocks.
Authors: Zhugzhda, Y. D.; Bromm, V.; Ulmschneider, P.
1995A&A...300..302Z Altcode:
Equations were derived permitting the treatment of shocks which occur
in longitudinal-transverse waves propagating in thin magnetic flux
tubes. Such waves usually lead to kinks which may grow into oblique
shocks. The kink angles between the shock and the tube directions are
found to be symmetrical with respect to the shock. Purely longitudinal
shocks occur only in special cases of high symmetry or of well defined
propagation conditions. Wave calculations were performed which show
the formation of oblique shocks. We also discuss recent suggestions
that the basic equations for the treatment of longitudinal-transverse
waves are incomplete.
---------------------------------------------------------
Title: On the Generation of Flux-Tube Waves in Stellar Convection
Zones. II. Improved Treatment of Longitudinal Tube Wave Generation
Authors: Musielak, Z. E.; Rosner, R.; Gail, H. P.; Ulmschneider, P.
1995ApJ...448..865M Altcode:
We have previously considered the generation of purely longitudinal
magnetohydrodynamic tube waves by external turbulent convection and
derived general formulas for the source function and for the wave
energy fluxes. In this paper, we present an improved treatment of the
generation of such tube waves, based on a more sophisticated description
of the turbulence and more refined calculations. These improvements
allow us to compute and discuss in greater detail the spectra and fluxes
of longitudinal tube waves generated in the solar convective zone.
---------------------------------------------------------
Title: On the generation of nonlinear magnetic tube waves in the
solar atmosphere.
Authors: Huang, P.; Musielak, Z. E.; Ulmschneider, P.
1995A&A...297..579H Altcode:
The nonlinear time-dependent response to purely transverse shaking of
a thin vertical magnetic flux tube embedded in the solar atmosphere
is investigated numerically. The shaking is imposed on the tube
at different heights in the solar atmosphere and the resulting
magnetic wave energy fluxes are calculated for the observationally
established range of velocity amplitudes and tube magnetic fields. The
obtained results clearly demonstrate that typical wave energy fluxes
carried by nonlinear transverse tube waves are of the order of
10^9^erg/cm2/s. This, in contrast to previous analytical studies,
seems to indicate that there is enough wave energy to account for
the enhanced heating observed in the chromospheric network, and that
magnetic tube waves may also play some role in the heating of other
regions of the solar atmosphere.
---------------------------------------------------------
Title: Constraints on Wave Heating in Chromospheric Bright Points
Authors: Kalkofen, W.; Höflich, P.; Ulmschneider, P.
1995SPD....26..706K Altcode: 1995BAAS...27R.966K
No abstract at ADS
---------------------------------------------------------
Title: Acoustic wave propagation in the solar
atmosphere. II. Nonlinear response to adiabatic wave excitation.
Authors: Sutmann, G.; Ulmschneider, P.
1995A&A...294..241S Altcode:
We study the response of the solar atmosphere to excitations with
adiabatic large amplitude acoustic waves. Both monochromatic waves
and acoustic spectra are considered. For monochromatic excitation a
critical frequency ν_cr_ is found which separates different domains of
resonance behaviour. Upon excitation with frequencies ν<ν_cr_ the
atmospheric resonance decays rapidly with time as in the small amplitude
wave case, while for ν>ν_cr_, persistent resonance oscillations
occur which are caused by shock overtakings. Excitation by acoustic
spectra always leads to the ν>ν_cr_ behaviour. Independent of
the spectral shape and energy, acoustic spectra generate oscillations
mainly at frequencies ν=6-7mHz at the top of the chromosphere. The
photospheric 5-min oscillation does not influence our chromospheric
results. Shock heating by acoustic spectra is roughly similar to that of
a monochromatic wave of period 35s. Irrespective of the initial spectral
shape and energy and even of the gravity and effective temperature of
the star, a universal average shock strength M_S_=1.5 is found. Due
to our adiabatic treatment, the atmospheric slabs suffer extension
which grows with wave energy.
---------------------------------------------------------
Title: Acoustic wave propagation in the solar atmosphere. I. Linear
response to adiabatic wave excitation.
Authors: Sutmann, G.; Ulmschneider, P.
1995A&A...294..232S Altcode:
We study the response of various solar atmosphere models to excitation
by adiabatic small amplitude acoustic waves. Both monochromatic waves
and acoustic spectra are considered. We find that upon excitation,
strong resonance oscillations of a single frequency develop which
are superposed over the excitation signal. These resonances decay
exponentially with time; the decay rate varies strongly with the
temperature gradient of the model. In realistic and positive gradient
atmospheres the decay rate is much faster than in isothermal models,
while in negative gradient models it is even slower. Independent of
whether the atmosphere is excited by a pulse or by continuous wave
action a similar decay behaviour is found. The response to a stochastic
wave spectrum is a series of uncorrelated transient events, each with
the associated exponential decay of the resonance.
---------------------------------------------------------
Title: Line radiation cooling with partial redistribution in
atmospheres with shocks.
Authors: Huenerth, G.; Ulmschneider, P.
1995A&A...293..166H Altcode:
We investigate the energy loss by spectral line radiation assuming
partial redistribution PRD in atmospheres permeated by acoustic shock
waves. It is found that with PRD the emission is much reduced and much
more concentrated in a narrow region behind the shocks compared to cases
which assume complete redistribution CRD. A fast method to approximate
the radiation losses for time-dependent calculations is proposed.
---------------------------------------------------------
Title: Chromospheric Heating and Metal Deficiency in Cool Giants:
Theoretical Results versus Observations
Authors: Cuntz, M.; Rammacher, W.; Ulmschneider, P.
1994ApJ...432..690C Altcode:
We compute acoustic shock wave-heated chromosphere models for moderately
cool giant stars which differ greatly in metallicity. Subsequently, we
simulate the emerging Mg II k lines assuming partial redistribution. The
initial acoustic energy fluxes and the wave periods are taken from
acoustic wave generation calculations based on traditional convection
zone models. We find that the Mg II and Ca II core emissions are
close to the observed basal flux limits which are common for giants
and dwarfs. In addition, we find that the Mg II core emission is
independent of the metallicity, in agreement with observations. We
argue that these results should be considered as further evidence that
the basal flux limits are indeed due to acoustic shock heating. The
acoustic heating mechanism seems to be dominant in all nonmagnetic
nonpulsating late-type stars.
---------------------------------------------------------
Title: An operator splitting method for line radiation with partial
redistribution in atmospheres with shocks
Authors: Ulmschneider, P.
1994A&A...288.1021U Altcode:
An operator splitting method for the calculation of spectral line
radiation assuming partial redistribution is discussed which is suitable
for atmospheres with shock discontinuities.
---------------------------------------------------------
Title: An operator splitting method for atmospheres with shocks
Authors: Buchholz, B.; Hauschildt, P. H.; Rammacher, W.; Ulmschneider,
P.
1994A&A...285..987B Altcode:
We develop a fast operator splitting (OS) method to solve spectral line
radiative transfer problems in time-dependent hydrodynamic computations
with shock discontinuities, assuming complete redistribution. The
convergence properties and the results obtained with our method are
compared with results obtained using a modified core-saturation method
and with the {LAMBDA}-iteration. We find that our operator splitting
method is robust, accurate and fast.
---------------------------------------------------------
Title: On Sound Generation by Turbulent Convection: A New Look at
Old Results
Authors: Musielak, Z. E.; Rosner, R.; Stein, R. F.; Ulmschneider, P.
1994ApJ...423..474M Altcode:
We have revisited the problem of acoustic wave generation by turbulent
convection in stellar atmospheres. The theory of aerodynamically
generated sound, originally developed by Lighthill and later modified
by Stein to include the effects of stratification, has been used
to estimate the acoustic wave energy flux generated in solar and
stellar convection zones. We correct the earlier computations by
incorporating an improved description of the spatial and temporal
spectrum of the turbulent convection. We show the dependence of the
resulting wave fluxes on the nature of the turbulence, and compute the
wave energy spectra and wave energy fluxes generated in the Sun on
the basis of a mixing-length model of the solar convection zone. In
contrast to the previous results, we show that the acoustic energy
generation does not depend very sensitively on the turbulent energy
spectrum. However, typical total acoustic fluxes of order F<SUB>A</SUB>
= 5 x 10<SUP>7</SUP> ergs/sq cm/s with a peak of the acoustic frequency
spectrum near omega = 100 mHz are found to be comparable to those
previously calculated. The acoustic flux turns out to be strongly
dependent on the solar model, scaling with the mixing-length parameter
alpha as alpha<SUP>3.8</SUP>. The computed fluxes most likely constitute
a lower limit on the acoustic energy produced in the solar convection
zone if recent convection simulations suggesting the presence of shocks
near the upper layers of the convection zone apply to the Sun.
---------------------------------------------------------
Title: Theoretical Basal Flux Limits from Acoustic Wave Calculations
Authors: Buchholz, B.; Ulmschneider, P.
1994ASPC...64..363B Altcode: 1994csss....8..363B
No abstract at ADS
---------------------------------------------------------
Title: Effects of Metallicity on Chromospheric Emission in Cool
Giants: Results from Acoustic Wave Models
Authors: Cuntz, M.; Ulmschneider, P.
1994ASPC...64..368C Altcode: 1994csss....8..368C
No abstract at ADS
---------------------------------------------------------
Title: Oblique shocks in longitudinal-transverse MHD tube waves
Authors: Zhugzhda, Y.; Ulmschneider, P.; Bromm, V.
1994smf..conf..402Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Excitation of Nonlinear Magnetic Flux Tube Waves in the
Solar Atmosphere
Authors: Huang, P.; Musielak, Z. E.; Ulmschneider, P.
1993BAAS...25.1213H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Heating of Solar Coronal Holes by Means of Non-Linear
Alfven Wave Coupling
Authors: Stark, B.; Musielak, Z. E.; Suess, S. T.; Ulmschneider, P.
1993BAAS...25R1212S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Heating
Authors: Ulmschneider, P.
1993ASSL..183..533U Altcode: 1993pssc.symp..533U
No abstract at ADS
---------------------------------------------------------
Title: On Sound Generation by Turbulent Convection: A New Look at
Old Results
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.; Gail, P.;
Wang, A.
1992AAS...181.9403M Altcode: 1992BAAS...24.1269M
No abstract at ADS
---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere. IX - Three minute
pulsations driven by shock overtaking
Authors: Rammacher, Wolfgang; Ulmschneider, Peter
1992A&A...253..586R Altcode:
Computations are presented showing that short-period acoustic
waves with periods less than 40 sec can generate, by the process of
shock overtaking, 3-min type first-overtone pulsations of the solar
chromosphere. It is suggested that 3-min pulsations generated by
acoustic waves may be an explanation for the observed Ca II K(2V) cell
grains. It is shown that shock overtaking occurs only if the wavelength
of the shock wave is small enough, such that the excess speed of the
faster moving shock is able to catch up with the slower shock within
the chromosphere. It is suggested that the shock overtaking pulsation
process, by feeding energy into long-period waves may be very important
for the driving of mass loss in red giant stars.
---------------------------------------------------------
Title: Atmospheric Pulsations; Mass Loss Driven by Overtaking
Acoustic Shocks
Authors: Ulmschneider, P.; Rammacher, W.; Gail, H. -P.
1992ASPC...26..471U Altcode: 1992csss....7..471U
No abstract at ADS
---------------------------------------------------------
Title: On the Generation of Magnetic Tube Waves in the Solar
Convection Zone
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.; Gail, P.
1991BAAS...23.1442M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the generation of magnetic tube waves in the solar
convection zone.
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.; Gail, P.
1991BAAS...23.1037M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Generation of Magnetic Tube Waves in the Solar
Convection Zone
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.; Gail, P.
1991BAAS...23Q1037M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Propagation of nonlinear longitudinal-transverse waves along
magnetic flux tubes in the solar atmosphere. I - Adiabatic waves
Authors: Ulmschneider, P.; Zaehringer, K.; Musielak, Z. E.
1991A&A...241..625U Altcode:
The nonlinear time-dependent response to purely transverse foot point
shaking of a vertical magnetic flux tube in the solar atmosphere
was studied. The adiabatic calculations show the generation of a
longitudinal wave mode which has twice the frequency of the transverse
wave. The amplitude of the longitudinal wave increases with the wave
period and with the magnitude of the shaking. Due to the action of
centrifugal forces significant lifting of the tube gas was found. A
forced oscillator type resonance occurs which depends on the tube
length.
---------------------------------------------------------
Title: Heating of the Solar Atmosphere by Spicules (With 4 Figures)
Authors: Cheng, Q. Q.; Ulmschneider, P.; Korevaar, P.
1991mcch.conf..350C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mechanisms of Chromospheric and Coronal Heating
Authors: Ulmschneider, Peter; Priest, Eric R.; Rosner, Robert
1991mcch.conf.....U Altcode: 1991QB809.M43......
One of the great problems of astrophysics is the unanswered
question about the origin and mechanism of chromospheric and coronal
heating. Just how these outer stellar envelopes are heated is of
fundamental importance, since all stars have hot chromospheric and
coronal shells where the temperature rises to millions of degrees,
comparable to the temperatures in the stars' cores. Here for the first
time is a comprehensive inventory of the proposed chromospheric and
coronal heating theories. The proposed heating processes are critically
compared, and the observational evidence for the various mechanisms
is reviewed. This is essential reading for all those working in such
fields as stellar activity, radio and XUV emission, rotation, and mass
loss, for whom a detailed and consistent presentation of our knowledge
of chromospheric and coronal heating mechanisms is urgently needed.
---------------------------------------------------------
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: Acoustic Heating (With 9 Figures)
Authors: Ulmschneider, P.
1991mcch.conf..328U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric and Coronal Heating Mechanisms
Authors: Narain, U.; Ulmschneider, P.
1990SSRv...54..377N Altcode:
We review the mechanisms which have been proposed for the heating of
stellar chromospheres and coronae. These consist of heating by acoustic
waves, by slow and fast mhd waves, by body and surface Alfvén waves,
by current or magnetic field dissipation, by microflare heating and by
heating due to bulk flows and magnetic flux emergence. Some relevant
observational evidence has also been discussed.
---------------------------------------------------------
Title: Wave pressure in stellar atmospheres due to shock wave trains.
Authors: Gail, H. P.; Cuntz, M.; Ulmschneider, P.
1990A&A...234..359G Altcode:
Analytic expressions for the wave pressure of propagating shock wave
trains in stellar atmospheres or winds are derived. Applications to
weak shocks and stronger shocks with sawtooth profiles are discussed
in detail. The shocks are treated as discontinuities. The results
provide insight in the momentum balance of time-dependent stellar wind
flows. The analytic expressions can be used as an independent test of
hydrodynamic codes.
---------------------------------------------------------
Title: Chromospheric and Coronal Heating Mechanisms
Authors: Ulmschneider, P.; Narain, U.
1990IAUS..142...97U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Acoustic Heating of Stellar Chromospheres and Coronae
Authors: Ulmschneider, P.
1990ASPC....9....3U Altcode: 1990csss....6....3U
Acoustic wave dissipation is a viable mechanism for the heating of
chromospheres and coronae. In absence of magnetic fields in the center
of supergranulation cells on the sun and on very slowly rotating
stars the acoustic heating mechanism appears to dominate. In the
solar chromospheric network and on moderately or rapidly rotating
stars acoustic heating is a weak background effect. The theoretical
limiting shock strength behavior and the direct solar observation of
acoustic waves are both consistent with the empirical chromospheric
energy requirements of the sun.
---------------------------------------------------------
Title: Chromospheric phenomena in late-type stars
Authors: Ulmschneider, P.
1990nwus.book...45U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Generation of Transverse Magnetic Tube Waves and X-Ray
Emissions from Late-Type Dwarfs
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1990ASPC....9...79M Altcode: 1990csss....6...79M
The X-ray emissions observed in late-type stars are shown to be
associated with transverse magnetic tube waves generated in stellar
convective zones. The heating theory is examined to insure that it
accounts for the mechanical energy flux associated with the wide range
of X-ray emissions for each spectral type, and the inhomogeneous and
locally strong magnetic fields in stellar atmospheres. The values of
the free parameters from the wave-heating model developed agree with
observational data, and the tube waves can account for the observed
X-ray emissions of F, G, and K dwarfs.
---------------------------------------------------------
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: Longitudinal-Transverse Magnetic Tube Waves in the Solar
Atmosphere
Authors: Ulmschneider, P.; Musielak, Z.
1990ASPC....9..116U Altcode: 1990csss....6..116U
The propagation of nonlinear adiabatic magnetohydrodynamic waves in
a thin magnetic flux tube is studied. The waves are excited by purely
transverse shaking. Due to nonlinear coupling there is a significant
energy transfer to the longitudinal wave. This transfer is largest for
long period waves and increases with the shaking amplitude. Lifting
of the tube mass is found due to the increased swaying with height.
---------------------------------------------------------
Title: Recent Advances in Acoustic Heating
Authors: Ulmschneider, P.
1990IAUS..142..231U Altcode:
Turbulent surface convection zones of stars generate acoustic waves
which contribute to the heating of chromospheres and coronae. The
dissipation of limiting strength acoustic shock waves agrees well with
the empirically determined chromospheric radiation loss rates. Acoustic
waves with frequency and energy required for the chromospheric heating
are observed in the solar atmosphere. Acoustic heating can explain the
basal chromospheric emission of slowly rotating stars and constitutes
a weak background in faster rotating stars; it cannot explain the
emission-rotation correlation and the surface variation of emission
which are due to magnetic heating.
---------------------------------------------------------
Title: The chromospheric emission from acoustically heated stellar
atmospheres
Authors: Ulmschneider, P.
1989A&A...222..171U Altcode:
General properties of acoustically heated chromospheres of late-type
stars are derived. For a giant and a dwarf star of T(eff) = 5012 K,
detailed acoustically heated chromosphere models are constructed and
the theoretical Mg II and Ca II emission line fluxes are evaluated. The
initial acoustic wave flux in the giant is assumed to be eight times
larger than that of the dwarf. The computations show that due to
photospheric radiation damping and the limiting shock strength behavior,
the acoustic flux decreases much more rapidly in the giant than in the
dwarf star such that roughly the same theoretical emission line fluxes
are produced in both stars. This agrees with observations and removes
a major argument against the acoustic heating theory for chromospheres
of slowly rotating stars.
---------------------------------------------------------
Title: X-ray emission from acoustically heated coronae.
Authors: Stepien, K.; Ulmschneider, P.
1989A&A...216..139S Altcode:
Based on the limiting shock strength behavior of acoustic waves,
the expected X-ray emission from purely acoustically heated coronae
is estimated. The gas pressure at the base of the transition layer
of selected stars is first estimated using available empirical
chromospheric models. X-ray fluxes derived from the predicted limiting
acoustic fluxes are found to be much smaller than the observed
fluxes. The results support the argument that the observed coronae
are generated by magnetic heating.
---------------------------------------------------------
Title: Can Magnetic Tube Waves Account for X-ray Emissions Observed
from Late-Type Dwarfs
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1989BAAS...21..796M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Subphotospheric Excitation of Alfven Waves and Their Role in
the Solar Atmosphere
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.; Moore, R. L.
1989BAAS...21R.830M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Propagation of Nonlinear Longitudinal-Transverse Waves Along
Magnetic Flux Tubes in the Solar Atmosphere
Authors: Ulmschneider, P.; Zahringer, K.; Musielak, Z. E.
1989BAAS...21..844U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interaction Between Forced Turbulent Flow Field and Intense
Magnetic Flux Tubes
Authors: Rosner, R.; Musielak, Z. E.; Ulmschneider, P.
1989BAAS...21..844R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Generation of Flux Tube Waves in Stellar Convection
Zones. I. Longitudinal Tube Waves Driven by External Turbulence
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1989ApJ...337..470M Altcode:
The source functions and the energy fluxes for wave generation in
magnetic flux tubes embedded in an otherwise magnetic field-free,
turbulent, and compressible fluid are derived. The calculations
presented here assume that the tube interior is not itself turbulent,
e.g., that motions within the flux tube are due simply to external
excitation. Specific results for the generation of longitudinal tube
waves are presented.
---------------------------------------------------------
Title: Theoretical Limits on Stellar Coronae Heated by Compressive
Waves
Authors: Hammer, R.; Ulmschneider, P.
1989AGAb....3...16H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: What Do the Mg II Lines Tell Us About Waves and Magnetic
Fields?
Authors: Rammacher, W.; Ulmschneider, P.
1989ASIC..263..589R Altcode: 1989ssg..conf..589R
No abstract at ADS
---------------------------------------------------------
Title: Effects of CO molecules on the outer solar atmosphere -
Dynamical models with opacity distribution functions
Authors: Muchmore, D.; Ulmschneider, P.; Kurucz, R. L.
1988A&A...201..138M Altcode:
Carbon monoxide can be an important cooling agent in late-type
stars. This paper expands previous theoretical work by extending the
frequency set used in the radiation calculations, employing opacity
distribution functions for the infrared bands of CO and 19 frequency
points for the H(-) continuum. It is found that the net cooling rate
due to CO decreases by a factor of about 3 due to the large optical
depths in the line cores. The influence of the 2.2-micron CO band is
small compared to the effect of the 4.6-micron band. An atmospheric
structure with a sharp drop of the temperature in the outer photosphere,
where CO cooling sets in, was found again. This temperature drop occurs
higher in the atmosphere and is less steep than in the simpler models
but is nevertheless steep enough to be convectively unstable. When CO
cooling sets in, surface temperatures drop to very low values (T less
than 3000 K) for radiative equilibrium models, even without including
the effects of other molecules.
---------------------------------------------------------
Title: Radiation damping in acoustically heated atmospheres of
late-type stars
Authors: Ulmschneider, P.
1988A&A...197..223U Altcode:
It is shown that radiation damping of acoustic waves propagating through
a stellar photosphere is much larger in giant stars than in dwarfs. This
could remove a major argument against acoustic waves as possible
heating mechanism for the chromospheres of slowly rotating stars.
---------------------------------------------------------
Title: A hydrodynamic code for the treatment of late-type stellar
wind flowsbased on the method of characteristics.
Authors: Cuntz, M.; Ulmschneider, P.
1988A&A...193..119C Altcode:
The authors describe a time-dependent eulerian hydrodynamic code
based on the method of characteristics which allows the computation
of radiating stellar wind flows in tube-like structures on late-type
stars. The treatment of boundaries under sub- and supersonic
conditions is discussed as is the introduction of shock waves into the
atmosphere. Ionization is taken into account. For test and application
the authors study the behaviour to approach limiting shock strength.
---------------------------------------------------------
Title: Acoustic tube waves in the solar atmosphere. I - Magnesium
and calcium line emission with complete redistribution
Authors: Ulmschneider, P.; Muchmore, D.; Kalkofen, W.
1987A&A...177..292U Altcode:
The propagation of acoustic waves through vertical magnetic flux tubes
in the solar atmosphere was computed with radiation damping by (non-LTE)
emission from H(-) and the Mg II k line. Waves of various amplitudes and
periods were investigated in flux tubes with three different spreading
rates characterized by the radius as a: (1) constant, (2) linear or
(3) exponential function of height. The geometry of the flux tubes
greatly influences the behavior of the waves. Large wave amplitudes
and intense postshock radiative relaxation zones are found in constant
tubes while small amplitudes and weak radiation zones were found in
exponential tubes. In all calculations following Mg II ionization,
transition-layer-like rapid temperature rises formed and transient
mass flows were initiated. In constant tubes the rapid temperature
rises occurred at low heights while in exponential tubes these layers
were found much higher. Waves with longer periods produced steep
temperature rises at lower height and led to more mass motion. The
heights of the rapid temperature rises did not depend much on the
initial wave energy flux.
---------------------------------------------------------
Title: A Computational Code for Two-dimensional Unsteady
Magnetohydrodynamics by the Method of Characteristics
Authors: Lou, Y. Q.; Rosner, R.; Ulmschneider, P.
1987ApJ...315..349L Altcode:
The authors present a computational code for solving two-dimensional,
time-dependent magnetohydrodynamic (MHD) equations by the method
of characteristics. The physical system under consideration is
axisymmetric, with v<SUB>φ</SUB> = 0 and B<SUB>φ</SUB> = 0. The
numerical scheme is described in detail, and the results of calculations
are compared with two analytic solutions of the MHD equations: (1)
linearized, standing MHD wave motions in a magnetized cylindrical
plasma; (2) nonlinear self-similar expansion of a magnetized plasma
ball. In addition, the authors have studied the nonlinear development
of standing MHD wave solutions in a cylindrical plasma.
---------------------------------------------------------
Title: Acoustic wave driven mass loss in late-type giant stars.
Authors: Cuntz, M.; Hartmann, L.; Ulmschneider, P.
1987IAUS..122..325C Altcode:
Mass loss generated by radiatively damped acoustic waves is
investigated. The authors find that a persistent wave energy flux leads
to extended chromospheres. Mass loss is quite likely produced if the
wave field retains a transient character and if large wave periods
are used.
---------------------------------------------------------
Title: Magnetic Flux Tubes as Sources of Wave Generation
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1987LNP...291...66M Altcode: 1987csss....5...66M
Because solar (and, most likely, stellar) surface magnetic fields
are highly inhomogeneous, and show concentration into `flux tube'
structures, the wave energy generated in stellar convection zones
may be largely carried away by flux tube waves, which then become
important sources for the heating of the outer atmospheric layers. We
report calculations for longitudinal tube waves generated in magnetic
flux tubes embedded in an otherwise magnetic field-free, turbulent,
and stratified medium; we find that such waves are generated by dipole
emission, and that the generation efficiency is a strong function of
the magnetic field strength. We also present wave flux calculations
for magnetic flux tubes embedded in the solar convective zone; the
main result is that the longitudinal tube wave fluxes are at least 2
orders of magnitude too low to play a significant role in the heating
of the solar chromosphere.
---------------------------------------------------------
Title: Adiabatic Longitudinal-Transverse Magnetohydrodynamic Tube
Waves
Authors: Zähringer, Kurt; Ulmschneider, Peter
1987rfsm.conf..243Z Altcode:
The authors compute the propagation of adiabatic magnetohydrodynamic
waves along thin flux tubes in the solar atmosphere. The time-dependent
development and amplitude growth of the waves, excited at the foot of
the photosphere as pure transverse waves, was followed into the low
chromosphere. Strong mode-coupling to longitudinal waves was found. The
swaying of the tube which increases with height resulted in a lifting
of the entire tube mass which is attributed to centrifugal forces. This
lifting resulted in adiabatic cooling of the tube.
---------------------------------------------------------
Title: The Dynamics of Solar Magnetic Flux Tubes Subjected to Resonant
Foot Point Shaking
Authors: Ulmschneider, Peter; Zähringer, Kurt
1987LNP...291...63U Altcode: 1987csss....5...63U
Using local correlation tracking techniques on data obtained with the
SOUP instrument on Spacelab 2, Title (1987) has measured correlated
horizontal displacements of solar surface regions of much larger than
granular size with a periodicity of the five minute oscillation. We
investigate the time-dependent development of waves in thin magnetic
flux tubes generated by this purely transverse foot point shaking. The
resulting magnetic field and velocity variations, the pressure and
temperature fluctuations generated by nonlinear coupling to the
longitudinal wave mode, as well as the expected resonance effects
are discussed.
---------------------------------------------------------
Title: Generation of flux tube waves in stellar convection zones. 1:
Longitudinal tube waves
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.
1987STIN...8824561M Altcode:
The source functions and the energy fluxes are derived for wave
generation in magnetic flux tubes embedded in an otherwise magnetic-
field free, turbulent, and compressible fluid. Specific results for
the generation of longitudinal tube waves are presented.
---------------------------------------------------------
Title: Radiation pressure in acoustic wave calculations of early
type stars
Authors: Wolf, B. E.; Ulmschneider, P.
1987ilet.work..263W Altcode:
A new method to treat radiation pressure in spectral lines has been
developed which avoids the Sobolev-approximation. This method has been
used for time-dependent acoustic wave calculations in early-type stars.
---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere. VIII. Extrapolation
of the solution in time.
Authors: Ulmschneider, P.
1986A&A...168..308U Altcode:
An extrapolation method which follows the slowly varying evolution
of individual phases of the wave is described and its accuracy is
evaluated. The use of the extrapolation method for a transmitting
boundary condition is examined. It is noted that linear extrapolation
along the C(+) characteristics is useful for predicting the physical
variables of the wave to an accuracy of a few percent.
---------------------------------------------------------
Title: Efficiency of Flux Tube Wave Generation in Late Type Stars
Authors: Musielak, Z. E.; Rosner, R.; Ulmschneider, P.; Bohn, H. U.
1986BAAS...18.1002M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The present state of wave-heating theories of stellar
chromospheres
Authors: Ulmschneider, P.
1986AdSpR...6h..39U Altcode: 1986AdSpR...6...39U
Observational constraints are summarized and the present state of the
acoustic and mhd-wave heating theories for chromospheres in early
and late-type stars are discussed. It is found that the slow-mode
mhd-wave heating theory looks most promising but that mode-coupling
from transverse or torsional Alfven waves may be significant for the
upper layers.
---------------------------------------------------------
Title: Fully Dynamic Caluculations of (Magneto-) Hydrodynamic Wave
Propagation in Stellar Atmospheres
Authors: Ulmschneider, P.; Muchmore, D.
1986ssmf.conf..191U Altcode:
The authors review calculations of time-dependent (magneto-)
hydrodynamic wave propagation in stellar atmospheres in which the
fully dynamic coupling between the (magneto-) hydrodynamics, the
thermodynamics and the radiation are consistently treated. The methods
employed to handle the non-planar geometry, the time-dependent particle
conservation equations, the departures from LTE and the radiation
field are outlined. They discuss the various possible approximations in
photospheric, chromospheric and coronal loop applications in late-type
stars as well as the radiation pressure treatment in early-type stars.
---------------------------------------------------------
Title: Propagation of Magneto-Acoustic Waves Along Solar Flux Tubes
Authors: Ulmschneider, P.
1985tphr.conf..150U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere. VII - Non-grey,
non-LTE H(-) models
Authors: Schmitz, F.; Ulmschneider, P.; Kalkofen, W.
1985A&A...148..217S Altcode:
The propagation and shock formation of radiatively damped acoustic waves
in the solar chromosphere are studied under the assumption that H(-)
is the only absorber; the opacity is non-grey. Deviations from local
thermodynamic equilibrium (LTE) are permitted. The results of numerical
simulations show the depth dependence of the heating by the acoustic
waves to be insensitive to the mean state of the atmosphere. After the
waves have developed into shocks, their energy flux decays exponentially
with a constant damping length of about 1.4 times the pressure scale
height, independent of initial flux and wave period. Departures from
LTE have a strong influence on the mean temperature structure in
dynamical chromosphere models; this is even more pronounced in models
with reduced particle density - simulating conditions in magnetic flux
tubes - which show significantly increased temperatures in response
to mechanical heating. When the energy dissipation of the waves is
sufficiently large to dissociate most of the H(-) ions, a strong
temperature rise is found that is reminiscent of the temperature
structure in the transition zone between chromosphere and corona;
the energy flux remaining in the waves then drives mass motions.
---------------------------------------------------------
Title: Propagation of nonlinear, radiatively damped longitudinal
waves along magnetic flux tubes in the solar atmosphere
Authors: Herbold, G.; Ulmschneider, P.; Spruit, H. C.; Rosner, R.
1985A&A...145..157H Altcode:
For solar magnetic flux tubes the authors compare three types of
waves: longitudinal MHD tube waves, acoustic tube waves propagating
in the same tube geometry but with rigid walls and ordinary acoustic
waves in plane geometry. They find that the effect of distensibility
of the tube is small and that longitudinal waves are essentially
acoustic tube waves. Due to the tube geometry there is considerable
difference between longitudinal waves or acoustic tube waves and
ordinary acoustic waves. Longitudinal waves as well as acoustic tube
waves show a smaller amplitude growth, larger shock formation heights,
smaller mean chromospheric temperature but a steeper dependence of
the temperature gradient on wave period.
---------------------------------------------------------
Title: Effects of CO molecules on the outer solar atmosphere -
A time-dependent approach
Authors: Muchmore, D.; Ulmschneider, P.
1985A&A...142..393M Altcode:
Carbon monoxide can be an important source of cooling near the solar
temperature minimum. The influence of radiation in the 4.6 micron
fundamental vibration-rotation band on the structure of the sun's
outer atmosphere is investigated. The model calculations employ
a 1-D hydrodynamical code in conjunction with a two-frequency
treatment of radiative transfer. Resulting models have a bistable
character. Radiative equilibrium atmospheres for a T(eff) less than
5800 K are dominated in outer layers by CO cooling which draws the
temperature down to T less than about 3000 K. For hooter models, CO
plays no important role. The chromosphere adjusts from one state to
another in a time of the order of one hour. Acoustic heating destroys
CO in the outermost parts of a model but for low and moderately high
acoustic fluxes CO continues to dominate the region of the temperature
minimum.
---------------------------------------------------------
Title: Propagation of magneto-acoustic waves along solar flux tubes.
Authors: Ulmschneider, P.
1985MPARp.212..150U Altcode:
Time-dependent calculations of longitudinal waves in magnetic flux
tubes are discussed. A comparison of adiabatic with non-adiabatic
calculations demonstrates that radiation processes determine decisively
the physical state in the tubes. Heating and mass motion produced by
the waves depends strongly on the tube geometry. Fast spreading tubes
are less heated. Long period waves show decreased heating and increased
mass motion.
---------------------------------------------------------
Title: Propagation of radiation damped acoustic waves in stellar
atmospheres
Authors: Ulmschneider, P.
1985cgd..conf..237U Altcode:
The propagation of radiatively damped acoustic waves along magnetic flux
tubes and its implication for the unknown chromospheric and coronal
heating mechanism is discussed. It is shown that the behaviour of the
radiative cooling strongly determines the structure of the chromosphere
and the transition-layer.
---------------------------------------------------------
Title: Apparent solar temperature enhancement due to large-amplitude
waves
Authors: Kalkofen, W.; Ulmschneider, P.; Schmitz, F.
1984ApJ...287..952K Altcode:
The effect of slow-mode acoustic-type MHD waves propagating outward
in the solar atmosphere on the temperature structures predicted by
empirical models is investigated analytically. A model is constructed,
and numerical results are presented for wing intensities, line profiles,
temperature enhancements, waves with higher energy flux, temperature
depression, and the Si continuum. The flux in the MgII and CaII UV
resonance lines is found to be increased relative to that in the IR
continuum, leading to model temperatures which depend systematically
on which observations are used in the computation. It is suggested
that mechanical heating may take place in smaller regions such as flux
tubes rather than uniformly over the surface.
---------------------------------------------------------
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: CO and the Thermal Stability of the Solar Chromosphere
Authors: Muchmore, D.; Ulmschneider, P.
1984BAAS...16R.450M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Core saturation in a moving medium.
Authors: Kalkofen, W.; Ulmschneider, P.
1984mrt..book..131K Altcode: 1984mrt..conf..131K
The authors discuss a method of solving line transfer problems in
moving media with plane symmetry. The method is an adaptation of the
core saturation method of Rybicki (1972) to a medium with internal
structure, such as shocks. Its speed follows from two simplifications:
the neglect of detailed transfer in the line core, and the neglect
of atmospheric regions that contribute negligibly to the formation of
the emergent spectrum of interest. The method has been tested with the
calculation of a line source function in a static, isothermal medium
and in a model solar atmosphere traversed by multiple shocks.
---------------------------------------------------------
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: Apparent Photospheric Temperature Enhancement Due to Large
Amplitude Waves
Authors: Kalkofen, W.; Ulmschneider, P.
1982BAAS...14..938K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Heating of stellar chromospheres when magnetic fields are
present.
Authors: Ulmschneider, P.; Stein, R. F.
1982A&A...106....9U Altcode:
Constraints on possible mechanisms of chromospheric heating are derived
from recent, semi-empirical solar models, OSO-8 observations, and the
stellar Mg II emission-line fluxes of Basri and Linsky (1979). It is
shown that the observational facts are best satisfied by a scenario
in which non-magnetic regions are heated by acoustic shock waves,
while magnetic regions are heated by slow-mode shock waves. For the
case of the high chromosphere, however, this mechanism must be either
supplemented or replaced by such alternatives as Alfven wave heating.
---------------------------------------------------------
Title: On the Acoustic and Magnetoacoustic Heating of the Outer
Atmospheres of Stars
Authors: Ulmschneider, P.
1981SSRv...29..355U Altcode:
Observational and theoretical evidence indicates that chromospheres are
very likely heated by acoustic and by slow mode magnetohydrodynamic
waves. The acoustic heating of coronae which has recently been
disclaimed still appears to be an important possibility.
---------------------------------------------------------
Title: Comments on the acoustic heating of stellar coronae
Authors: Ulmschneider, P.; Bohn, H. U.
1981A&A....99..173U Altcode:
Recent claims that UV and X-ray observations allow the rejection of the
acoustic heating theory of stellar coronae are shown to be unfounded. In
particular, the argument that in late type stars the acoustic flux is
insufficient to balance the observed X-ray flux is contradicted by
new calculations which use Lighthill's theory corrected for density
stratification and for strong magnetic fields.
---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere. VI - Feautrier type
radiation treatment
Authors: Wolf, B. E.; Schmitz, F.; Ulmschneider, P.
1981A&A....97..101W Altcode:
A differential equation method to solve the radiative transfer
equation in the presence of shocks is developed and tested against
exact solutions. The method appears sufficiently accurate for cases
where the shock is at relatively small optical depth. A comparison
with an integral method shows that the present method is 5 times faster.
---------------------------------------------------------
Title: Acoustic heating in late-type chromospheres.
Authors: Ulmschneider, P.
1981Obs...101...40U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theories of heating of solar and stellar chromospheres.
Authors: Ulmschneider, P.
1981ASIC...68..239U Altcode: 1981spss.conf..239U
Mechanisms for the mechanical heating of stellar and solar chromospheres
to temperatures above those provided by radiative heating alone are
examined. Classes of possible heating mechanisms are identified,
including explosive heating, quasi-steady heating and wave heating
processes, and evidence from empirical models which may allow a
selection among the various heating mechanisms are considered as they
relate to the total chromospheric radiation loss, the height of the
temperature minimum, the steepness of the chromospheric temperature
rise, and the variation of these parameters with effective temperature,
gravity and average magnetic field strength. Acoustic heating is then
discussed as the probable dominant heating mechanism. Theoretical
and empirical models developed in nonmagnetic cases for the sun and
late-type stars are compared, and possible mechanisms for chromospheric
heating in early-type stars are noted.
---------------------------------------------------------
Title: Theoretical stellar chromospheres of late type stars. V -
Temperature minimum in the grey LTE approach
Authors: Schmitz, F.; Ulmschneider, P.
1981A&A....93..178S Altcode:
With the computation of theoretical chromosphere models for a
number of stars with effective temperatures from 4000 to 7000 K
and logs of gravity 2 to 5, the monochromatic, gray LTE radiation
hydrodynamic approach to the acoustic heating theory is brought to
its conclusion. Theoretical predictions are compared with observations
and rather good agreement is found for the position of the temperature
minima, the total chromospheric radiation flux as well as the average
effective temperature slope of the Mg II emission. Both theoretical
and observed stellar chromospheres fall into two classes denoted S-
and R-types. Empirically found temperature enhancement is proposed to
be due to the large acoustic wave amplitude and the nonlinearity of
the Planck function. The shortcomings of the acoustic heating theory
are extensively discussed.
---------------------------------------------------------
Title: Theoretical stellar chromospheres of late type stars.
Authors: Schmitz, F.; Ulmschneider, P.
1980A&A....84...93S Altcode:
Theoretical chromosphere models for five late type dwarf stars
have been computed with the acoustic heating theory on the basis
of the Lighthill-Proudman and mixing length theories. For stars of
effective temperature greater than 5200 K good agreement is found
between empirical and theoretical positions of the temperature minima
as well as between the semiempirical chromospheric radiation flux
and the acoustic flux at the temperature minimum. This supports the
validity of the acoustic heating theory. A considerable discrepancy is
however found for late type dwarf stars where acoustic flux is clearly
missing. This however does not indicate that the acoustic heating
theory is wrong but that the theory of acoustic energy production is
insufficient for these stars.
---------------------------------------------------------
Title: Theoretical stellar chromospheres of late type stars. III.
Authors: Schmitz, F.; Ulmschneider, P.
1980A&A....84..191S Altcode:
Theoretical chromosphere models for six stars are computed on the basis
of the short period acoustic heating theory using acoustic fluxes
predicted by the Lighthill-Proudman and mixing length theories. A
comparison with semiempirical chromosphere models shows that the heights
of the temperature minima agree rather well. Independently the acoustic
flux at the temperature minimum is found to be roughly equal to the
semi- empirical chromospheric radiation flux. This supports claims
that short period acoustic waves are an important mechanism for the
heating of the lower chromospheres of stars. For late type stars with
high T<SUB>eff</SUB> or low gravity the temperature minimum is found
close to the outer limit of the radiative damping zone while for late
type dwarf stars the temperature minimum occurs close to the height
of shock formation.
---------------------------------------------------------
Title: Stellar Chromospheres
Authors: Ulmschneider, P.
1980MitAG..47...93U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar chromospheres
Authors: Ulmschneider, P.
1980AGAb...47...93U Altcode:
Observations indicating the presence of stellar chromospheres are
summarized. Undisputed indicators for hot stellar envelopes are
spectral lines of highly ionized atoms, Fe (2) emission lines, flares
in late type stars, and the presence of the He (1) 10830 A line. The
combined evidence to date shows that all nondegenerate type stars
have chromospheres except possibly the A stars. There are, however,
theoretical reasons for expecting chromospheres in A stars. Empirical
chromosphere models for a rapidly growing sample of stars was recently
constructed on the basis of Ca (2) and Mg (2) line observations. A
discussion of possible heating mechanisms is given and the relative
importance of these mechanisms is evaluated. For the low and middle
chromosphere the short period acoustics heating mechanisms seems to
be the dominant process, although there are still uncertainties. Both
steady and time dependent theoretical models of stellar chromospheres
based on the short period acoustics heating theory are discussed, and
predictions of these models are compared with results from empirical
models. This relatively favorable comparison shows that the explanation
of the Wilson-Bappu effect might be at hand.
---------------------------------------------------------
Title: Some effects of acoustic waves on spectral-line profiles.
Authors: Cram, L. E.; Keil, S. L.; Ulmschneider, P.
1979ApJ...234..768C Altcode:
The paper discusses the formation of spectral lines in the presence of
short-period, nonlinear, radiatively damped acoustic waves propagating
through a model of the solar atmosphere. The temperature and pressure
perturbations associated with the wave strongly influence the line
profile. Although their wavelength is less than the depth of the
velocity response function of photospheric spectral lines, the
acoustic waves produce large (greater than 100 m/s), short-period
line shifts. Acoustic waves of sufficient amplitude to account for
chromospheric heating do not significantly increase the equivalent
widths of photospheric lines and therefore are probably not responsible
for photospheric microturbulence.
---------------------------------------------------------
Title: Stellar Chromospheres
Authors: Ulmschneider, P.
1979SSRv...24...71U Altcode:
Observations indicating the presence of stellar chromospheres, that
is hot envelopes around stars are summarized. Undisputed indicators
(called type I) for hot stellar envelopes are spectral lines of
highly ionized atoms, Fe ii emission lines and flares in late type
stars and the presence of the He i10830 Å line. Whether indicators
(called type II) like emission cores in the Ca ii H and K and Mg ii h
and k lines or mass loss signify the presence of stellar chromospheres
is still somewhat debated, although the discussion points in favour
of the usefulness of these indicators. The combined evidence to date
shows that all non degenerate type stars have chromospheres except
possibly the A stars. There are however theoretical reasons for
expecting chromospheres in A stars. Empirical chromosphere models
for a rapidly growing sample of stars have recently been constructed
on the basis of Ca ii and Mg ii line observations. A discussion of
possible heating mechanisms is given and the relative importance of
these mechanisms is evaluated. For the low and middle chromosphere
the short period acoustic heating mechanism seems to be the dominant
process although there are still uncertainties. Both steady state and
time dependent theoretical models of stellar chromospheres, based on the
short period acoustic heating theory, are discussed, and predictions
of these models are compared with results from empirical models. This
relatively favourable comparison shows that the explanation of the
Wilson-Bappu effect might be at hand.
---------------------------------------------------------
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 dissipation and H<SUP>-</SUP> radiation in the solar
chromosphere. II.
Authors: Kalkofen, W.; Ulmschneider, P.
1979ApJ...227..655K Altcode:
The relation between mechanical heating and radiative cooling in
the solar chromosphere is investigated, and the dependence of the
temperature rise on the mechanical dissipation rate and on the
microscopic state of H(-) ions at the foot of the chromosphere is
studied. It is found that H(-) is very nearly in local thermodynamic
equilibrium (LTE) at the temperature minimum; but near the upper end of
the range where H(-) dominates the opacity, deviations from LTE should
be taken into account in estimating the mechanical energy input from
the empirical temperature distribution. The use of empirical models
for calculating the mechanical energy input into the chromosphere is
considered. The estimate of mechanical heating implied by the recent
work of Pradeire and Thomas (1976) is found to be too low by about an
order of magnitude.
---------------------------------------------------------
Title: Institut für Astronomie und Astrophysik, Lehrstuhl
Astronomie. Jahresbericht für 1978.
Authors: Isserstedt, J.; Ulmschneider, P.
1979MitAG..46..251I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Acoustic Dissipation and H Radiation in the Solar Chromosphere
I
Authors: Ulmschneider, P.; Kalkofen, W.
1978A&A....69..407U Altcode:
Summary. The radiative energy loss of the solar chromosphere due to H -
transitions in statistical equilibrium is estimated from the empirical
model of Vernazza et al. (1976), and the energy input due to dissipation
of acoustic sawtooth type shock waves is computed for various energy
fluxes and wave periods. The two energy rates show similar dependence
on height for waves with periods near 20 s only. This suggests that
the chromosphere is heated mainly by short period acoustic waves. Key
words: chromosphere - acoustic heating - H - radiation loss
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Institut für Astronomie und Astrophysik, Lehrstuhl
Astronomie. Jahresbericht für 1977.
Authors: Isserstedt, J.; Ulmschneider, P.
1978MitAG..44..201I Altcode: 1978MitAG..44..201.
No abstract at ADS
---------------------------------------------------------
Title: Observational tests of the shock heating theory for late-type
stellar chromospheres.
Authors: Cram, L. E.; Ulmschneider, P.
1978A&A....62..239C Altcode:
Recent predictions of the positions of stellar temperature minima
are combined with a theory of the formation of Ca(+) resonance lines,
and thus present observable tests of the shock wave heating theory of
stellar chromospheres. Although the trend in the predicted line widths
agrees with the trend in the observations, the quantitative agreement
is only satisfactory for a solar-type star with log g 4 and T(eff) 6000
K. The theoretical minima of giant stars are located much deeper and the
minima of cool dwarf stars are located much higher than the observations
suggest. Possible explanations of this disparity are discussed.
---------------------------------------------------------
Title: Theoretical stellar chromospheres of late type
stars. II. Temperature minima.
Authors: Ulmschneider, P.; Schmitz, F.; Renzini, A.; Cacciari, C.;
Kalkofen, W.; Kurucz, R.
1977A&A....61..515U Altcode:
The theory of heating by short period acoustic waves is applied to
predict the height of shock formation and the acoustic flux at the
base of the chromosphere for stars with effective temperatures of
4000 to 6500 K and log g of 2 to 4. These predictions are compared
with heights of temperature minima and with chromospheric radiation
losses computed from semiempirical models.
---------------------------------------------------------
Title: Theoretical chromospheres of late type stars. I. Acoustic
energy generation.
Authors: Renzini, A.; Cacciari, C.; Ulmschneider, P.; Schmitz, F.
1977A&A....61...39R Altcode:
Acoustic-energy generation in stars is investigated, and results are
presented for computations concerning acoustic-wave propagation in
stellar atmospheres. Acoustic fluxes and wave periods are calculated
for the chemical composition X 0.7, Y 0.28, Z 0.2, as well as for log
g of -1 to 6, effective temperatures of 2800 to 10,000 K, and mixing
length/pressure scale height ratios of 0.5, 1, and 1.5. The Cox and
Stewart (1970) opacities are used together with a cubic interpolation
procedure, the standard mixing-length theory of convection, and the
Lighthill (1952) theory of sound generation. The variation of convective
velocity as a function of input parameters is examined, and acoustic
fluxes are obtained for several stellar envelope models. The period
at which a monochromatic acoustic flux has its maximum is evaluated
as a function of surface gravity. Uncertainties in the analysis are
briefly discussed.
---------------------------------------------------------
Title: Short period acoustic heating theory and its application to
the construction of model chromospheres
Authors: Ulmschneider, P.
1977MmSAI..48..439U Altcode:
Adiabatic linear waves in an isothermal gravitational atmosphere,
nonlinear isentropic waves, nonlinear nonadiabatic waves in a
nonisothermal gravitational atmosphere, and shocks are analyzed,
and a perturbation method for nonlinear radiatively damped waves in a
gravitational atmosphere is proposed. The time-independent approach to
the construction of model chromospheres, based on heating formulas,
is examined, showing that at heights below 1600 km, wave pressure,
thermal conduction, viscosity, and the solar-wind mass and energy
flow may be neglected. Time independent models of the chromosphere
are constructed by solving the hydrostatic and energy equations,
using a weak shock heating formula. Chromospheric heating mechanisms
(short period acoustic heating theory) are compared with such heating
mechanisms as gravity waves, five minute oscillations, Alfven waves,
and magnetic field annihilation.
---------------------------------------------------------
Title: Thermal conductivity in stellar atmospheres II, without
magnetic field.
Authors: Nowak, T.; Ulmschneider, P.
1977A&A....60..413N Altcode:
The Chapman-Enskog-Burnett theory is used to calculate the coefficient
of thermal conductivity for a gas mixture of stellar abundances,
assuming departures from local thermodynamic equilibrium (non-LTE). In
addition, using elementary kinetic theory, simple approximation
formulas for the coefficient of thermal conductivity are given that
allow evaluation under arbitrary ionization conditions and for an
arbitrary element abundance. The deviation of the approximation from
the rigorous calculation in both the LTE and the non-LTE cases is at
most ten percent.
---------------------------------------------------------
Title: On the observations of stellar temperature minima.
Authors: Schmitz, F.; Ulmschneider, P.
1977A&A....59..177S Altcode:
Minima of brightness temperature due to the presence of
stellar temperature minima are predicted in the UV, infrared, and
radio-frequency ranges. The theoretical temperature minima for stars in
the range of effective temperature from 4000 to 6500 K and log g from
2 to 4 used in this work are calculated on basis of the short-period
acoustic heating theory.
---------------------------------------------------------
Title: Acoustic Waves in the Solar Atmosphere. Ill. A Theoretical Tem-
perature Minimum
Authors: Ulmschneider, P.; Kalkofen, W.
1977A&A....57..199U Altcode:
Summary. In this third of a series ofpapers studying large amplitude
acoustic waves we use the methods developed in previous papers to
compute the propagation of acoustic waves until shock formation. A
perturbation approach is used to let the waves travel on top of
a prescribed atmosphere. From estimates of the fluxes and periods
of acoustic waves generated in the convection zone we predict the
position ofthe temperature minimum and the energy flux transferred to
the chromosphere. The agreement between predictions and observations
gives strong support to the short period acoustic heating theory of the
chromosphere. A scaling law was found for radiatively damped acoustic
waves. This law is used to compute acoustic frequency spectra at the
base of the chromosphere. Key words: acoustic waves shocks radiation
damping temperature minimum chromospheric heating
---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere. II. Radiative damping.
Authors: Kalkofen, W.; Ulmschneider, P.
1977A&A....57..193K Altcode:
A method is described for calculating radiative damping of
large-amplitude acoustic waves propagating in the solar photosphere
and low chromosphere. The basic radiation expressions to be used with
the time-dependent hydrodynamic equations are derived, the equation
of radiative transfer is solved numerically for an atmosphere in
which shock discontinuities may arise, and approximations valid in
different limiting cases are considered. The numerical solution is
tested against an exact solution, and the transfer equation is combined
with the hydrodynamic equations to construct a model atmosphere in
radiative equilibrium. The methods developed are then employed to
determine theoretically the location of the temperature minimum and
to investigate the validity of the short-period acoustic-heating
theory for the solar atmosphere. A perturbation approach is adopted
to compute radiative damping of the acoustic waves, and the time
behavior of acoustic waves with various periods is evaluated. The
results are taken as convincing evidence that short-period acoustic
waves constitute the main chromospheric heating mechanism.
---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere. I. The hydrodynamic
code.
Authors: Ulmschneider, P.; Kalkofen, W.; Nowak, T.; Bohn, U.
1977A&A....54...61U Altcode:
This paper studies large-amplitude radiatively damped acoustic waves in
the solar atmosphere. A modified method of characteristics is described
for the solution of the time-dependent hydrodynamic equations in a
gravitational atmosphere. A procedure for the detection of shocks is
outlined. Several tests of the accuracy of the method are described. The
evolution of the wave and the height of shock formation are computed
for several values of the period and the initial acoustic flux in
isothermal atmospheres with temperatures of 4000 and 5000 K as well
as in a model solar atmosphere.
---------------------------------------------------------
Title: Is the solar 5 min oscillation an important heating mechanism
for the chromosphere and the corona?
Authors: Ulmschneider, P.
1976SoPh...49..249U Altcode:
Missing power in the spectrum of intensity fluctuations of both XUV
and radio emission in the transition layer and inner corona as well
as the 90° phase shift between intensity and velocity fluctuations
in the chromosphere indicate that the 5 min oscillation transports
little energy and is not a significant mechanical heat source for the
chromosphere and possibly not even for the corona.
---------------------------------------------------------
Title: Chromosphärischer Temperaturanstieg in der Sonne aufgrund
akustischer Heizung
Authors: Bohn, H. U.; Ulmschneider, P.
1976MitAG..38..202B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theoretische Chromosphären später Sterne.
Authors: Schmitz, F.; Ulmschneider, P.
1976MitAG..38..174S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theoretical temperature minimum for the Sun
Authors: Ulmschneider, P.; Kalkofen, W.
1976pmas.conf..103U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Theoretical Temperature Minimum
Authors: Kalkofen, W.; Ulmschneider, P.
1975BAAS....7..363K Altcode:
No abstract at ADS
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Title: Zur Berechnung der Wärmeleitfähigkeiten in Sternatmosphären
Authors: Nowak, T.; Ulmschneider, P.
1975MitAG..36...72N Altcode:
It is pointed out that the currently known methods for the calculation
of the thermal conductivity of neutral, partly ionized, or completely
ionized gases at high temperatures require a relatively large
computational effort, not taking into account the method reported by
Spitzer and Haerm (1953) for completely ionized gases. An equation
is derived which provides an approach for an approximate calculation
involving a smaller amount of computational work.
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Title: Theoretisches Temperaturminimum der Sonne auf Grund der
Ausbreitung von strahlungsgedämpften akustischen Wellen
Authors: Ulmschneider, P.; Kalkofen, W.
1975MitAG..36..150U Altcode:
No abstract at ADS
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Title: Radiation Loss and Mechanical Heating in the Solar Chromosphere
Authors: Ulmschneider, P.
1974SoPh...39..327U Altcode:
The raditation loss of the solar chromosphere is evaluated on the basis
of the Harvard Smithsonian Reference Atmosphere. The total radiative
flux is found to be between 2.5 and 3.3 E6 erg cm<SUP>−2</SUP>
s<SUP>−1</SUP>. A discussion of possible heating mechanisms shows that
the short period acoustic wave theory is the only one able to balance
the chromospheric radiation loss and is consistent with observation.
---------------------------------------------------------
Title: Line Profiles and Turbulence Generated by Acoustic Waves in
the Solar Chromosphere. I. Absorption Profiles and Height Variation
of Velocity Amplitudes
Authors: Oster, L.; Ulmschneider, P.
1973A&A....29....1O Altcode:
Summary. Line profiles produced by Doppler effects due to acoustic waves
with velocity profiles ranging from purely sinusoidal to sawtooth-type
shock waves are computed in the temperature range typical for the solar
chromosphere. The results can be used as local absorption profiles in
the computation of the profiles of emerging chromospheric lines. The
height variation of velocity profiles and amplitudes is deduced from
previous model calculations by Ulmschneider. Key words: acoustic waves -
line profiles - microturbulence - solar chromosphere
---------------------------------------------------------
Title: Strahlungsdämpfung akustischer Wellen beim Austritt aus den
optischen dicken Schichten der Sonnenatmosphäre
Authors: Ulmschneider, P.
1973MitAG..34..111U Altcode:
No abstract at ADS
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Title: The Effect of Mechanical Waves on Empirical Solar Models
Authors: Ulmschneider, Peter; Kalkofen, Wolfgang
1973SoPh...28....3U Altcode:
Empirical solar models contain the effect of heating due to radiative
energy loss from acoustic waves. We estimate here the temperature
difference between the radiative equilibrium model and the empirical
model. At optical depth τ<SUB>5000</SUB> = 0.1 this difference
is small, but near the temperature minimum (τ<SUB>5000</SUB> =
10<SUP>−4</SUP>) it reaches between 53 and 83 K. The temperature
difference between the equator and the poles caused by a hypothetical
difference in the heating is estimated.
---------------------------------------------------------
Title: Line Profiles and Microturbulence Generated by Acoustic Waves
in the Solar Chromosphere
Authors: Oster, Ludwig; Ulmschneider, Peter
1972BAAS....4Q.389O Altcode:
No abstract at ADS
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Title: On the Propagation of a Spectrum of Acoustic Waves in the
Solar Atmosphere
Authors: Ulmschneider, P.
1971A&A....14..275U Altcode:
The propagation of acoustic waves, their transformation into shock
waves and their dissipation has been computed on basis of the Harvard
Smithsonian Reference Atmosphere (HRA) for the sun. Acoustic frequency
spectra of Stein (1968) were used and the effect of radiative damping
included. Good agreement was found between the heating produced by these
waves and the computed radiative losses in the chromosphere. Coronal
heating proved more difficult to explain. Key words: acoustic waves -
shock waves - chromosphere - corona
---------------------------------------------------------
Title: On the Computation of Shock Heated Models for the Solar
Chromosphere and Corona
Authors: Ulmschneider, P.
1971A&A....12..297U Altcode:
Theoretical models are computed for the chromosphere and corona starting
at a height of 800 km. The observational chromospheric models can
be reproduced very well up to 1400 km assuming weak shock waves with
periods of around 27s. At greater heights the Hcc and Lyman continuum
radiation loss regions can presently not be treated adequately. The
observed Lycc and XUv line fluxes are used to extend the theoretical
model computation into the transition region and lower corona. Acoustic
waves of periods between 100 and 200s are shown to develop into strong
shocks in the lower corona. The heating of these waves determines the
theoretical models up to the coronal temperature maximum. The notion
that short period waves of around 27s heat the chromosphere while long
period waves of around 100 to 200s heat the corona was found to be
consistent with the acoustic wave fluxes and frequencies of spectra
given by Stein (1968). Key words: shock heating - chromosphere - corona
---------------------------------------------------------
Title: On Frequency and Strength of Shock Waves in the Solar
Atmosphere
Authors: Ulmschneider, Peter
1970SoPh...12..403U Altcode:
Comparison of computed radiative energy losses of several new empirical
chromospheric models with heating by shock wave dissipation gives
information on the frequency and strength of shock waves in the
solar chromosphere. A mechanical flux of around 2.5 × 10<SUP>6</SUP>
erg/cm<SUP>2</SUP> sec is found for the base of the chromosphere. The
shocks are weak and the wave period is around 10 sec.
---------------------------------------------------------
Title: Thermal Conductivity in Stellar Atmospheres I. Without
Magnetic Field
Authors: Ulmschneider, P.
1970A&A.....4..144U Altcode:
The coefficient of thermal conductivity for pure hydrogen, pure
helium and a gas mixture appropriate for a stellar atmosphere has been
computed in the temperature range 3000 to 100000 0K and the range of gas
pressure from 10-8 to 10e dyn/cm8. The translational as well as reactive
contributions to the coefficient are given. The error is less than 10%
(except for pure He) in the neutral region but may be more than 35 %
in the ionizing region. In the fully ionized region there is complete
agreement with Spitzer and (1953).
---------------------------------------------------------
Title: Information on the Strength and Frequency of Shock Waves
Involved in the Heating of the Solar Atmosphere Deduced from
Empirical Models
Authors: Ulmschneider, P.
1969cctr.conf..151U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal Conductivity in Stellar Atmospheres I. Without
Magnetic Field
Authors: Ulmschneider, P.
1969cctr.conf....9U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Information on shock waves deduced from empirical solar models
and a theoretical continuation into the transition region
Authors: Ulmschneider, P.
1969MitAG..27..210U Altcode:
No abstract at ADS
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Title: Possible Explanation of the 300-SECOND Type Oscillation in
the Solar Chromosphere
Authors: Ulmschneider, P. H.
1968ApJ...152..349U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Schockwellenheizung von Chromosphäre und Korona der Sonne
und dreier Sterne
Authors: Ulmschneider, P.
1968MitAG..25..199U Altcode:
No abstract at ADS
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Title: The Structure of the Outer Atmosphere of the Sun and of
Cool Stars
Authors: Ulmschneider, P.
1967ZA.....67..193U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of the Outer Atmosphere of Cool Stars.
Authors: Ulmschneider, Peter Hermann
1966PhDT.........2U Altcode:
No abstract at ADS
