explanation blue bibcodes open ADS page with paths to full text
Author name code: rammacher
ADS astronomy entries on 2022-09-14
author:"Rammacher, Wolfgang"
---------------------------------------------------------
Title: Solar magnetic flux tube simulations with time-dependent
ionization
Authors: Fawzy, D. E.; Cuntz, M.; Rammacher, W.
2012MNRAS.426.1916F Altcode: 2012arXiv1208.1490F
In the present work we expand the study of time-dependent ionization
previously identified to be of pivotal importance for acoustic waves
in solar magnetic flux tube simulations. We focus on longitudinal tube
waves (LTW) known to be an important heating agent of solar magnetic
regions. Our models also consider new results of wave energy generation
as well as an updated determination of the mixing length of convection
now identified as 1.8 scale heights in the upper solar convective
layers. We present 1D wave simulations for the solar chromosphere by
studying tubes of different spreading as a function of height aimed
at representing tubes in environments of different magnetic filling
factors. Multilevel radiative transfer has been applied to correctly
represent the total chromospheric emission function. The effects of
time-dependent ionization are significant in all models studied. They
are most pronounced behind strong shocks and in low-density regions,
i.e. the middle and high chromosphere. Concerning our models of
different tube spreading, we attained pronounced differences between
the various types of models, which were largely initiated by different
degrees of dilution of the wave energy flux as well as the density
structure partially shaped by strong shocks, if existing. Models
showing a quasi-steady rise of temperature with height are obtained
via monochromatic waves akin to previous acoustic simulations. However,
longitudinal flux tube waves are identified as insufficient to heat the
solar transition region and corona in agreement with previous studies.
---------------------------------------------------------
Title: Linear wavelength correlation matrices of photospheric and
chromospheric spectral lines. I. Observations vs. modeling
Authors: Beck, C. A. R.; Rammacher, W.
2010A&A...510A..66B Altcode: 2009arXiv0909.2224B
Context. The process that heats the solar chromosphere is a difficult
target for observational studies because the assumption of local
thermal equilibrium (LTE) is not valid in the upper solar atmosphere,
which complicates the analysis of spectra. <BR /> Aims: We investigate
the linear correlation coefficient between the intensities at different
wavelengths in photospheric and chromospheric spectral lines because the
correlation can be determined directly for any spectra from observations
or modeling. Waves which propagate vertically through the stratified
solar atmosphere affect different wavelengths at different times when
the contribution functions for each wavelength peak in different
layers. This leads to a characteristic pattern of (non-)coherence
of the intensity at various wavelengths with respect to each other
which carries information on the physical processes. <BR /> Methods:
We derived the correlation matrices for several photospheric and
chromospheric spectral lines from observations. We separated locations
with a significant photospheric polarization signal and thus magnetic
fields from those without a polarization signal. For comparison with
the observations, we calculated correlation matrices for spectra from
simplified LTE modeling approaches, 1-D NLTE simulations, and a 3-D MHD
simulation run. We applied the correlation method also to temperature
maps at different optical depth layers derived from a LTE inversion
of Ca II H spectra. <BR /> Results: We find that all photospheric
spectral lines show a similar pattern: a pronounced asymmetry of
the correlation between line core and red or blue wing. The pattern
cannot be reproduced with a simulation of the granulation pattern, but
with waves that travel upwards through the formation heights of the
lines. The correct asymmetry between red and blue wing only appears
when a temperature enhancement occurs simultaneously with a downflow
velocity in the wave simulation. All chromospheric spectral lines show
a more complex pattern. The 1-D NLTE simulations of monochromatic
waves produce a correlation matrix that qualitatively matches the
observations near the very core of the Ca II H line. The photospheric
signature is well reproduced in the correlation matrix derived from
the 3-D MHD simulation. <BR /> Conclusions: The correlation matrices
of observed photospheric and chromospheric spectral lines are highly
structured with characteristic and different patterns in every spectral
line. The comparison with matrices derived from simulations and simple
modeling suggests that the main driver of the detected patterns are
upwards propagating waves. Application of the correlation method to
3-D temperature cubes seems to be a promising tool for a detailed
comparison of simulation results and observations in future studies.
---------------------------------------------------------
Title: Simultaneous Maps of the Chromosphere for Ca II H and Ca
II 8662
Authors: Rammacher, W.; Schmidt, W.; Hammer, R.
2008ESPM...12.2.40R Altcode:
We study chromospheric dynamics by analyzing long (up to 3200 s), high
spatial resolution time series of slit spectra of the Ca II H line
and the Ca II infrared line at 8662 Angstrom recorded simultaneously
near disk center of the Sun with the Echelle Spectrograph of the VTT on
Tenerife in June 2007. Fast scans with 6 steps (0.5" per step) were done
resulting in small stripes 3.5" wide and 160" long. A scan repetition
time of 8 s was reached. These small 2D maps allow us to discover
bright points, to follow their horizontal motion, and to associate
photospheric objects. Horizontal variations of the intensity with time
can be found for both lines and all wavelengths, including cases in
which patches of enhanced intensity move rapidly over the entire map,
with horizontal velocities of up to 30 km/s. <P />We study also a series
of large x-y-maps (size 160" x 120"), simultaneously recorded for the
Ca II H and Ca II 8662 lines. Because of the high spectral resolution
we get for both lines 975 wavelength points covering spectral ranges of
nearly 4.7 (H) and 10.3 (8662) Angstrom and therefore also 975 large
maps for Ca II H as well as Ca II 8662: one X-Y map for each resolved
wavelength. Our quasi-monochromatic maps have much narrower contribution
functions than the usual filtergrams taken in rather broad spectral
regions of the Ca II H & 8662 lines; thus they allow a more precise
mapping of features to atmospheric regions of limited height extension.
---------------------------------------------------------
Title: CaII H+K & MgII h+k line Fluxes from Basal Flux Stars
Authors: Gadelmavla, D.; Diaa, F.; Stepien, K.; Rammacher, W.
2008ESPM...12..4.2G Altcode:
The heating mechanisms of the solar chromosphere are still under
discussion. Helpful for this topic is to look for basal flux stars
that are similar to the Sun. <P />The minimum observed CaII H+K &
MgII h+k line fluxes from stellar Chromospheres as function of effective
temperature Teff are called basal flux lines. It is widely thought that
these fluxes are caused by non-magnetic heating of the outer stellar
atmospheres. We present numerical calculations of basal fluxes for a
variety of stars (G0 - K5) of different metallicity and gravity based
on self-consistent time-dependent chromospheric models: Assuming purely
acoustic heating we theoretically investigate the differences between
the basal flux lines of dwarfs, giants and low metallicity stars and
compare them with observations. Since basal flux stars rotate very
slowly, they are not supposed to be dominated by magnetic fields and
therefore their chromospheres should depend only on the three parameters
Teff, gravity g and metallicity Z. <P />Since our acoustic models also
depend only on these three parameters, this Comparison could turn out to
be a powerful argument for the reality of the acoustic heating picture.
---------------------------------------------------------
Title: The signature of chromospheric heating in Ca II H spectra
Authors: Beck, C.; Schmidt, W.; Rezaei, R.; Rammacher, W.
2008A&A...479..213B Altcode: 2007arXiv0712.2538B
Context: The heating process that balances the solar chromospheric
energy losses has not yet been determined. Conflicting views exist on
the source of the energy and the influence of photospheric magnetic
fields on chromospheric heating. <BR />Aims: We analyze a 1-h time
series of cospatial Ca II H intensity spectra and photospheric
polarimetric spectra around 630 nm to derive the signature of the
chromospheric heating process in the spectra and to investigate its
relation to photospheric magnetic fields. The data were taken in a
quiet Sun area on disc center without strong magnetic activity. <BR
/>Methods: We have derived several characteristic quantities of Ca
II H to define the chromospheric atmosphere properties. We study the
power of the Fourier transform at different wavelengths and the phase
relations between them. We perform local thermodynamic equilibrium (LTE)
inversions of the spectropolarimetric data to obtain the photospheric
magnetic field, once including the Ca intensity spectra. <BR />Results:
We find that the emission in the Ca II H line core at locations
without detectable photospheric polarization signal is due to waves
that propagate in around 100 s from low forming continuum layers in
the line wing up to the line core. The phase differences of intensity
oscillations at different wavelengths indicate standing waves for ν
< 2 mHz and propagating waves for higher frequencies. The waves
steepen into shocks in the chromosphere. On average, shocks are both
preceded and followed by intensity reductions. In field-free regions,
the profiles show emission about half of the time. The correlation
between wavelengths and the decorrelation time is significantly higher
in the presence of magnetic fields than for field-free areas. The
average Ca II H profile in the presence of magnetic fields contains
emission features symmetric to the line core and an asymmetric
contribution, where mainly the blue H2V emission peak is increased
(shock signature). <BR />Conclusions: We find that acoustic waves
steepening into shocks are responsible for the emission in the Ca II H
line core for locations without photospheric magnetic fields. We suggest
using wavelengths in the line wing of Ca II H, where LTE still applies,
to compare theoretical heating models with observations. <P />Appendices
are only available in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Simultaneous Observations of Solar Ca II H and Ca II 8662
lines and Numerical Simulation of these lines
Authors: Rammacher, Wolfgang; Schmidt, Wolfgang; Hammer, Reiner
2007AN....328..657R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: What is Heating the Quiet-Sun Chromosphere?
Authors: Wedemeyer-Böhm, S.; Steiner, O.; Bruls, J.; Rammacher, W.
2007ASPC..368...93W Altcode: 2006astro.ph.12627W
It is widely believed that the heating of the chromosphere in quiet-Sun
internetwork regions is provided by dissipation of acoustic waves
that are excited by the convective motions close to the top of
the convection zone and in the photospheric overshoot layer. This
view lately became challenged by observations suggesting that the
acoustic energy flux into the chromosphere is too low, by a factor
of at least ten. Based on a comparison of TRACE data with synthetic
image sequences for a three-dimensional simulation extending from
the top layers of the convection zone to the middle chromosphere,
we come to the contradicting conclusion that the acoustic flux in the
model provides sufficient energy for heating the solar chromosphere of
internetwork regions. The role of a weak magnetic field and associated
electric current sheets is also discussed.
---------------------------------------------------------
Title: Observations and Simulations of Ca II H and Ca II 8662
Authors: Rammacher, W.; Schmidt, W.; Hammer, R.
2007ASPC..368..147R Altcode:
We study chromospheric dynamics by analyzing long high spatial
resolution time series of spectra of the Ca II H line and the Ca II
infrared line at 8662 Å, recorded simultaneously near disk center
of the sun. The observations were made at the VTT, Tenerife. The
time series have a temporal resolution of 3 (8662) and 6 s (H),
respectively. After the statistical analysis of the observation
results, we used 1-D chromosphere simulation codes to make a series
of computations with purely acoustic waves to obtain a time series of
synthetic line profiles for Ca II H. A comparison of observational and
theoretical results shows profound differences between these model
calculations and the observations. A more detailed description of
this work is in preparation and will be published in a main astronomy
journal.
---------------------------------------------------------
Title: Acoustic Heating of the Solar Chromosphere: Present Indeed
and Locally Dominant
Authors: Cuntz, M.; Rammacher, W.; Musielak, Z. E.
2007ApJ...657L..57C Altcode:
We investigate the physical reality of acoustic heating in the
solar chromosphere. Evidence is provided that contrary to previous
claims by Fossum & Carlsson, high-frequency acoustic waves are
indeed sufficient to heat the nonmagnetic solar chromosphere. This
assessment is based on three different lines of evidence, which are
(1) a discussion of the inherent problems of the limited sensitivity of
TRACE when assessing the three-dimensional solar chromospheric topology,
(2) a study of the acoustic chromospheric wave energy flux, and (3)
a new look at the heating and emission of chromospheric basal flux
stars such as τ Ceti.
---------------------------------------------------------
Title: How Strong is the Dependence of the Solar Chromosphere upon
the Convection Zone?
Authors: Rammacher, W.
2005ESASP.596E..60R Altcode: 2005ccmf.confE..60R
No abstract at ADS
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Definition and significance of average temperatures in
time-dependent solar chromosphere models
Authors: Rammacher, W.; Cuntz, M.
2005A&A...438..721R Altcode:
We assess different types of average temperatures in time-dependent
solar chromosphere models. They include the conventional definition
of mean and median temperature, and a formal definition related to the
model-dependent hydrogen ionization degree, referred to as ionization
temperature. It is found that the latter is always higher than the
mean and median temperatures, except in the photosphere, and that the
mean temperatures are always higher than the median temperatures,
especially in models with frequency spectra. The most dramatic
differences are attained in the topmost portion of one of our models
with the ionization temperatures up to a factor 150 higher than the
mean and median temperatures. The differences between the mean, median,
and ionization temperatures are a direct consequence of nonlinearities
(“spikyness”) of the temperatures in the models mostly due to strong
shocks. The main results hold for both acoustic and magnetic models
despite significant differences in the initial wave energy fluxes,
densities, and geometrical settings.
---------------------------------------------------------
Title: Time-dependent ionization in solar magnetic flux tubes
Authors: Rammacher, W.; Cuntz, M.
2005ESASP.560..891R Altcode: 2005csss...13..891R
No abstract at ADS
---------------------------------------------------------
Title: Acoustic Heating Models for the Basal Flux Star τ Ceti
Including Time-dependent Ionization: Results for Ca II and Mg
II Emission
Authors: Rammacher, W.; Cuntz, M.
2003ApJ...594L..51R Altcode:
We present new calculations of chromospheric heating for τ Ceti (G8
V), a star exhibiting an extremely low level of chromospheric activity,
thus also referred to as a basal flux star or a flat activity star. Our
simulations consider energy deposition by acoustic shocks and also
take into account time-dependent (i.e., noninstantaneous) ionization
processes of hydrogen, magnesium, and calcium, allowing us to attain a
new generation of chromospheric heating models previously obtained for
the Sun. We consider both monochromatic waves and acoustic frequency
spectra. The latter are calculated using new models of acoustic energy
generation based on an extended Kolmogorov spectrum with a modified
Gaussian frequency factor. Our models show that the theoretically
deduced emergent Ca II and Mg II emission very much agree with
observations, adding to the argument that the chromospheres of basal
flux stars are predominantly heated by acoustic shocks.
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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.
---------------------------------------------------------
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: 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: 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: 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 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: 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: Effects of Mg II and Ca II ionization on ab-initio solar
chromosphere models
Authors: Rammacher, W.; Cuntz, M.
1991A&A...250..212R Altcode:
Acoustically heated solar chromosphere models are computed considering
radiation damping by (non-LTE) emission from H(-) and by Mg II and Ca
II emission lines. The radiative transfer equations for the Mg II k
and Ca II K emission lines are solved using the core-saturation method
with complete redistribution. The Mg II k and Ca II K cooling rates
are compared with the VAL model C. Several substantial improvements
over the work of Ulmschneider et al. (1987) are included. It is
found that the rapid temperature rises caused by the ionization of
Mg II are not formed in the middle chromosphere, but occur at larger
atmospheric heights. These models represent the temperature structure
of the 'real' solar chromosphere much better. This result is a major
precondition for the study of ab-initio models for solar flux tubes
based on MHD wave propagation and also for ab-initio models for the
solar transition layer.
---------------------------------------------------------
Title: Ca- und Mg-Linienbildung in dynamischen, magnetischen
Sonnenregionen.
Authors: Rammacher, W.
1991cumd.book.....R Altcode:
Contents: 1. Einleitung. 2. Die Behandlung der
Hydrodynamik. 3. Die Strahlungsbehandlung. 4. Das
Verhalten der Mg II-k-Profile. 5. Diskussion der Ca
II-K-Profile und Vergleich mit Mg II-k. 6. Linienentstehung und
Blau-Rot-Asymmetrie. 7. Zwei-Komponent-Modelle. 8. Vergleich mit den
Beobachtungen.
---------------------------------------------------------
Title: Line Simulation of Solar Structures Permeated by Acoustic
and MHD-Waves (With 6 Figures)
Authors: Rammacher, W.
1991mcch.conf..414R 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
