explanation blue bibcodes open ADS page with paths to full text
Author name code: zacharias
ADS astronomy entries on 2022-09-14
author:"Zacharias, Pia"
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Title: High-resolution observations of the solar photosphere,
chromosphere, and transition region. A database of coordinated IRIS
and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
2020A&A...641A.146R Altcode: 2020arXiv200514175R
NASA's Interface Region Imaging Spectrograph (IRIS) provides
high-resolution observations of the solar atmosphere through ultraviolet
spectroscopy and imaging. Since the launch of IRIS in June 2013, we
have conducted systematic observation campaigns in coordination with
the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
complementary high-resolution observations of the photosphere and
chromosphere. The SST observations include spectropolarimetric imaging
in photospheric Fe I lines and spectrally resolved imaging in the
chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
a database of co-aligned IRIS and SST datasets that is open for
analysis to the scientific community. The database covers a variety
of targets including active regions, sunspots, plages, the quiet Sun,
and coronal holes.
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Title: Disentangling flows in the solar transition region
Authors: Zacharias, P.; Hansteen, V. H.; Leenaarts, J.; Carlsson,
M.; Gudiksen, B. V.
2018A&A...614A.110Z Altcode: 2018arXiv180407513Z
Context. The measured average velocities in solar and stellar spectral
lines formed at transition region temperatures have been difficult
to interpret. The dominant redshifts observed in the lower transition
region naturally leads to the question of how the upper layers of the
solar (and stellar) atmosphere can be maintained. Likewise, no ready
explanation has been made for the average blueshifts often found in
upper transition region lines. However, realistic three-dimensional
radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere
are able to reproduce the observed dominant line shifts and may thus
hold the key to resolve these issues. <BR /> Aims: These new 3D rMHD
simulations aim to shed light on how mass flows between the chromosphere
and corona and on how the coronal mass is maintained. These simulations
give new insights into the coupling of various atmospheric layers
and the origin of Doppler shifts in the solar transition region and
corona. <BR /> Methods: The passive tracer particles, so-called corks,
allow the tracking of parcels of plasma over time and thus the study of
changes in plasma temperature and velocity not only locally, but also
in a co-moving frame. By following the trajectories of the corks, we
can investigate mass and energy flows and understand the composition
of the observed velocities. <BR /> Results: Our findings show that
most of the transition region mass is cooling. The preponderance of
transition region redshifts in the model can be explained by the higher
percentage of downflowing mass in the lower and middle transition
region. The average upflows in the upper transition region can be
explained by a combination of both stronger upflows than downflows
and a higher percentage of upflowing mass. The most common combination
at lower and middle transition region temperatures are corks that are
cooling and traveling downward. For these corks, a strong correlation
between the pressure gradient along the magnetic field line and the
velocity along the magnetic field line has been observed, indicating a
formation mechanism that is related to downward propagating pressure
disturbances. Corks at upper transition region temperatures are
subject to a rather slow and highly variable but continuous heating
process. <BR /> Conclusions: Corks are shown to be an essential tool
in 3D rMHD models in order to study mass and energy flows. We have
shown that most transition region plasma is cooling after having been
heated slowly to upper transition region temperatures several minutes
before. Downward propagating pressure disturbances are identified as
one of the main mechanisms responsible for the observed redshifts at
transition region temperatures. <P />The movie associated to Fig. 3
is available at <A href="https://www.aanda.org">http://www.aanda.org</A>
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Title: New insights on mass flows in and out of the solar transition
region
Authors: Zacharias, Pia
2017psio.confE..56Z Altcode:
No abstract at ADS
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Title: Tracing Flows in the Solar Atmosphere Using the Bifrost
Stellar Atmosphere Code
Authors: Zacharias, P.
2015AGUFMSH31B2409Z Altcode:
All energy that heats the solar corona must pass through the
transition region between the chromosphere and corona. Observations
of transition region emission lines reveal the presence of redshifts
up to temperatures of about 200,000 K and blueshifts for higher
temperatures. The apparent large downward flows in the lower transition
region would lead to an emptying of the corona, in contrast to what is
being observed, thus some mechanism must be responsible for maintaining
the mass balance between the photosphere and corona. We use the Bifrost
stellar atmosphere code to perform 3D radiation MHD simulations of the
solar atmosphere. The focus of this study is on the temporal evolution
of mass flows into and out of the solar corona. By adding tracer
particles to the simulations and analyzing their paths over time,
we can provide new insights on the physical processes driving these
mass flows and on their role in the chromosphere-corona mass cycle.
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Title: Analysis of mass and energy flows in coronal loops using the
Bifrost stellar atmosphere code
Authors: Zacharias, Pia; Leenaarts, Jorritt
2014cosp...40E3750Z Altcode:
Identifying and quantifying processes that lead to the regulation of
the mass and energy supply from the chromosphere to the outer layers
of the solar atmosphere is key to advancing our understanding of the
coupling between the solar chromosphere and corona. The combination
of large-scale numerical simulations of the rapidly evolving
detailed physical processes occurring in the solar atmosphere and
high-resolution observations from instruments like IRIS, Hinode and
SDO makes this undertaking more and more feasible. Using the Bifrost
stellar atmosphere code, we have performed 3D radiation MHD simulations
of the solar atmosphere extending from the convection zone to the
corona to study the evolution of magnetic field structures and the
underlying dynamics. By adding tracer particles to the simulation,
we are able to follow the magnetic field lines properly. A detailed
analysis of the plasma properties along those field lines and of the
generation and propagation of waves travelling into the corona will
be presented. By tracking the evolution of the observed perturbations,
we provide new insights on the physical processes driving them and on
their role in the chromosphere-corona mass cycle.
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Title: Studies of the dynamics and energetics of cool plasma ejections
into the corona
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
2013enss.confE.142Z Altcode:
The corona is highly dynamic and shows transient events on various
scales in space and time. Most of these features are related to
changes in the magnetic field structure or impulsive heating caused
by the conversion of magnetic to thermal energy. We are applying
three-dimensional magnetohydrodynamic models in order to investigate
the structure and dynamics in the upper solar atmosphere above a small
active region. Emission line spectra are synthesized from the model
and compared to spectra and images observed by current space-based
instruments, such as the EUV Imaging Spectrometer (EIS) onboard the
Hinode satellite and the Atmospheric Imaging Assembly (AIA) onboard the
Solar Dynamics Observatory. We investigate mass and energy flows between
the solar chromosphere and corona and discuss possible scenarios for a
mass cycle between the lower and upper solar atmosphere. In particular,
we have studied the processes that lead to the formation and ejection
of a confined plasma ejection into the solar corona. A description of
the nature of this particular feature will be presented which is found
to be a hydrodynamic phenomenon triggered by a heating event above the
chromosphere. A detailed analysis of 1D coronal loop models has been
performed to understand how the plasma responds to a heating pulse. The
results confirm the formation mechanism of the blob observed in the 3D
model. This raises the question if other small-scale ejection features
seen on the Sun could also be based on hydrodynamic processes instead
of being plasmoid-type phenomena as it is usually assumed.
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Title: Mass flows between the chromosphere and corona - comparison
of 1D and 3D coronal loop models
Authors: Zacharias, P.; Bingert, S.; Peter, H.
2012AGUFMSH33B2239Z Altcode:
The ejection and return of cool transition region plasma into the
corona will be discussed in the context of our three-dimensional
magnetohydrodynamic (3D MHD) model of the solar corona. The dynamics
of the ejection are investigated in more detail in a 1D loop model
and are compared to observations from Hinode/EIS and SDO/AIA. Results
from the 3D model analysis serve as input for the 1D loop model. In the
3D case, a heating pulse is the main driver of the ejection. To mimic
the situation in 1D, a heating pulse is injected at different heights
along the loop with varying amplitude and width. As a consequence,
the heating rate is strongly increased in a localized area and leads
to enhanced evaporation that causes the material to rise. We present
results that show the successful reproduction of the ejection in a 1D
loop model following the injection of a heating pulse. In contrast to
earlier studies, where similar heating events lead to both redshifts
in transition region emission lines and blueshifts in coronal emission
lines, preliminary results of our parameter study show exclusively
upflows along the loop and almost no downflows during the heating
phase. We will discuss these findings in terms of the mass cycle
between the chromosphere and corona.
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Title: Ejection of cool plasma into the corona - comparison of
results from a 3D MHD model with results from AIA/SDO, EIS/Hinode
and a 1D loop model
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
2012decs.confE..48Z Altcode:
The formation and subsequent ejection of cool plasma into the
corona will be discussed, as observed in our three-dimensional
magnetohydrodynamic (3D MHD) model of the solar atmosphere extending
from the photosphere into the corona. The model accounts properly
for the energy balance, especially for heat conduction and radiative
losses, allowing us to reliably synthesize the profiles of optically
thin extreme ultraviolet emission lines and compare them to existing
observations. A detailed description of the nature of this particular
phenomenon will be provided. The analysis of the various forces
acting upon the plasma in the 3D model shows that the pressure
gradient which is driving the ejection is due to Ohmic dissipation of
currents resulting from the braiding of the magnetic field lines by
photospheric plasma motions. Preliminary results of a parameter study
on the reproduction of the phenomenon in a one-dimensional loop model
support the scenario of a heating event that leads to the ejection
of cool plasma into the corona in both, the 1D loop model and the 3D
model. In addition, results of the numerical model will be compared to
observations from the Extreme Ultraviolet Imaging Spectrometer (EIS)
onboard Hinode and the Atmospheric Imaging Assembly (AIA) onboard SDO,
where we have also found evidence of cool plasma ejecta that are moving
along magnetic field lines.
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Title: Ejection of cool plasma into the hot corona
Authors: Zacharias, P.; Peter, H.; Bingert, S.
2011A&A...532A.112Z Altcode: 2011arXiv1106.5972Z
Context. The corona is highly dynamic and shows transient events
on various scales in space and time. Most of these features are
related to changes in the magnetic field structure or impulsive
heating caused by the conversion of magnetic to thermal energy. <BR
/> Aims: We investigate the processes that lead to the formation,
ejection and fall of a confined plasma ejection that was observed in
a numerical experiment of the solar corona. By quantifying physical
parameters such as mass, velocity, and orientation of the plasma
ejection relative to the magnetic field, we provide a description of
the nature of this particular plasma ejection. <BR /> Methods: The
time-dependent three-dimensional magnetohydrodynamic (3D MHD) equations
are solved in a box extending from the chromosphere, which serves as
a reservoir for mass and energy, to the lower corona. The plasma is
heated by currents that are induced through field line braiding as
a consequence of photospheric motions included in the model. Spectra
of optically thin emission lines in the extreme ultraviolet range are
synthesized, and magnetic field lines are traced over time. We determine
the trajectory of the plasma ejection and identify anomalies in the
profiles of the plasma parameters. <BR /> Results: Following strong
heating just above the chromosphere, the pressure rapidly increases,
leading to a hydrodynamic explosion above the upper chromosphere in the
low transition region. The explosion drives the plasma, which needs
to follow the magnetic field lines. The ejection is then moving more
or less ballistically along the loop-like field lines and eventually
drops down onto the surface of the Sun. The speed of the ejection is in
the range of the sound speed, well below the Alfvén velocity. <BR />
Conclusions: The plasma ejection observed in a numerical experiment
of the solar corona is basically a hydrodynamic phenomenon, whereas
the rise of the heating rate is of magnetic nature. The granular
motions in the photosphere lead (by chance) to a strong braiding of
the magnetic field lines at the location of the explosion that in
turn is causing strong currents which are dissipated. Future studies
need to determine if this process is a ubiquitous phenomenon on the
Sun on small scales. Data from the Atmospheric Imaging Assembly on
the Solar Dynamics Observatory (AIA/SDO) might provide the relevant
information. <P />Appendix and movie are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
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Title: Investigation of mass flows in the transition region and
corona in a three-dimensional numerical model approach
Authors: Zacharias, P.; Peter, H.; Bingert, S.
2011A&A...531A..97Z Altcode: 2011arXiv1105.5491Z
Context. The origin of solar transition region redshifts is not
completely understood. Current research is addressing this issue
by investigating three-dimensional magneto-hydrodynamic models
that extend from the photosphere to the corona. <BR /> Aims: By
studying the average properties of emission line profiles synthesized
from the simulation runs and comparing them to observations with
present-day instrumentation, we investigate the origin of mass
flows in the solar transition region and corona. <BR /> Methods:
Doppler shifts were determined from the emission line profiles of
various extreme-ultraviolet emission lines formed in the range of T =
10<SUP>4</SUP>-10<SUP>6</SUP> K. Plasma velocities and mass flows
were investigated for their contribution to the observed Doppler
shifts in the model. In particular, the temporal evolution of plasma
flows along the magnetic field lines was analyzed. <BR /> Results:
Comparing observed vs. modeled Doppler shifts shows a good correlation
in the temperature range log (T/[K]) = 4.5-5.7, which is the basis of
our search for the origin of the line shifts. The vertical velocity
obtained when weighting the velocity by the density squared is shown
to be almost identical to the corresponding Doppler shift. Therefore,
a direct comparison between Doppler shifts and the model parameters
is allowed. A simple interpretation of Doppler shifts in terms of
mass flux leads to overestimating the mass flux. Upflows in the model
appear in the form of cool pockets of gas that heat up slowly as they
rise. Their low temperature means that these pockets are not observed
as blueshifts in the transition region and coronal lines. For a set of
magnetic field lines, two different flow phases could be identified. The
coronal part of the field line is intermittently connected to subjacent
layers of either strong or weak heating, leading either to mass flows
into the loop (observed as a blueshift) or to the draining of the loop
(observed as a redshift).
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Title: On the nature of coronal loops above the quiet sun network
Authors: Bingert, S.; Zacharias, P.; Peter, H.; Gudiksen, B. V.
2010AdSpR..45..310B Altcode:
The structure and dynamics of a box in a stellar corona can be
modeled employing a 3D MHD model for different levels of magnetic
activity. Depending on the magnetic flux through the surface the
nature of the resulting coronal structures can be quite different. We
investigate a model of an active region for two sunspots surrounded by
magnetic field patches comparable in magnetic flux to the sunspots. The
model results in emission from the model corona being concentrated in
loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV
and X-ray emission outline the magnetic field, following the general
paradigm. However, in our model, where the magnetic field is far from
a force-free state, the loops seen in X-ray emission do not follow
the magnetic field lines. This result is of interest especially for
loops as found in areas where the magnetic field emerging from active
regions interacts with the surrounding network.
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Title: Spectral analysis of 3D MHD models of coronal structures
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
2009AdSpR..43.1451Z Altcode: 2009arXiv0904.2312Z
We study extreme-ultraviolet emission line spectra derived from
three-dimensional magnetohydrodynamic models of structures in the
corona. In order to investigate the effects of increased magnetic
activity at photospheric levels in a numerical experiment, a much
higher magnetic flux density is applied at the photosphere as compared
to the Sun. Thus, we can expect our results to highlight the differences
between the Sun and more active, but still solar-like stars. We discuss
signatures seen in extreme-ultraviolet emission lines synthesized from
these models and compare them to observed signatures in the spatial
distribution and temporal evolution of Doppler shifts in lines formed
in the transition region and corona. This is of major interest to test
the quality of the underlying magnetohydrodynamic model to heat the
corona, i.e. currents in the corona driven by photospheric motions
(flux braiding).
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Title: Doppler shifts in the transition region and corona. Mass
cycle between the chromosphere and the corona
Authors: Zacharias, P.; Bingert, S.; Peter, H.
2009MmSAI..80..654Z Altcode:
Emission lines in the transition region and corona show persistent
line shifts. It is a major challenge to understand the dynamics in
the upper atmosphere and thus these line shifts, which are a signature
of the mass cycle between the chromosphere and the corona. We examine
EUV emission line profiles synthesized from a 3D MHD coronal model of
a solar-like corona, in particular of an active region surrounded by
strong chromospheric network. This allows us to investigate the physical
processes leading to the line Doppler shifts, since we have access to
both, the synthetic spectra and the physical parameters, i.e. magnetic
field, temperature and density in the simulation box. By analyzing the
evolution of the flows along field lines together with the changing
magnetic structure we can investigate the mass cycle. We find evidence
that loops are loaded with mass during a reconnection process, leading
to upflows. After the loops disconnect from the reconnection site,
they cool and drain which leads to the observed redshifts. Previous 1D
loop models (neglecting the 3D nature) assumed that heating leads to
evaporation and upflows followed by a cooling phase after the heating
stops. The scenario modeled here is quite different, as it shows
that the continuously changing three-dimensional magnetic structure
is of pivotal importance to understand the mass balance between the
chromosphere and the corona.
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Title: Spectral Analysis of 3D MHD Models of Quiet Sun and Active
Region Structures
Authors: Zacharias, P.; Bingert, S.; Peter, H.
2008ESPM...12.3.39Z Altcode:
We study EUV emission line spectra derived from 3D MHD models
of structures in the corona, in particular of an active region
surrounded by a strong chromospheric network. The 3D MHD models account
properly for the energy balance, especially for heat conduction and
radiative losses. This allows us to reliably synthesize the profiles
of EUV emission lines observable with current EUV spectrometers,
e.g. SUMER/SOHO and EIS/Hinode. We investigate the temporal evolution
and spatial distribution of the Doppler shifts of the EUV emission
lines synthesized from these models. This is of major interest
for the underlying mechanism of the heating of the solar corona,
i.e. dissipation of currents in the corona driven by photospheric
motions (flux braiding). Based on the 3D MHD models we can also derive
intensity maps as they will be observed through the coronal channels
of the AIA-instrument onboard SDO and we can analyze how to process the
AIA maps in order to derive physical quantities such as temperatures and
densities. Since we have access not only to the synthetic spectra, but
also to the magnetic field in the box we can explore how the magnetic
field relates to the emission in various coronal lines. In the present
paper we will show results on the evolution of spectral properties
such as line shifts or widths in different modeled coronal structures
and compare these to results from SUMER/SOHO as well as EIS/Hinode data.
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Title: On the Nature of Coronal Loops
Authors: Bingert, S.; Zacharias, P.; Peter, H.; Gudiksen, B.
2008ESPM...12.3.29B Altcode:
The structure and dynamics of a box in a stellar corona can be
modeled employing a 3D MHD model for different levels of magnetic
activity. <P />Depending on the magnetic flux through the surface the
nature of the resulting coronal structures can be quite different. <P
/>We will compare two different models of an active region, one for
two basically isolated sunspots, and another one for two sunspots
surrounded by magnetic field patches similar to the chromospheric
network. <P />The current paradigm is that these loops follow magnetic
field lines as pearls on a string, and thus the majority of present
corona models describe structures following the field lines. <P />Our
study challenges this paradigm by showing through a three-dimensional
model that coronal structures in complex magnetic field geometries might
appear loop-like while they are not aligned with the magnetic field. <P
/>Using a forward model approach, both models result in emission from
the corona being concentrated in loop structures. <P />In the first case
the loops seen in EUV and X-ray emission are following the magnetic
field. <P />However, in the second case, where the magnetic field is
far from a force-free state, the loops seen in X-ray emission do not
follow the magnetic field, but are more related to the current sheets
formed in response to the footpoint motions of the magnetic field. <P
/>This result is of interest especially for loops as found in areas
where the magnetic field emerging from active regions interacts with
the surrounding network or in the complex magnetic structures within
chromospheric network patches.
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Title: On the nature of coronal loops
Authors: Bingert, Sven; Zacharias, Pia; Peter, Hardi
2008cosp...37..302B Altcode: 2008cosp.meet..302B
The structure and dynamics of a box in a stellar corona can be
modeled employing a 3D MHD model for different levels of magnetic
activity. In these models we account for the mass, momentum and energy
balance including heat conduction and radiative losses. The heating
is through current dissipation in the corona driven by photospheric
motions (flux braiding). Depending on the magnetic flux through
the surface the nature of the resulting coronal structures can be
quite different. We will compare two different models of an active
region, one for two basically isolated sunspots, and another one for
two sunspots surrounded by strong magnetic field patches mimicking a
strong chromospheric network. Both models result in emission from the
model corona being concentrated in loop structures. In the first case
the loops seen in EUV and X-ray emission are aligned with the magnetic
field, following the general paradigm. However, in the second case,
where the magnetic field is far from a force-free state, the loops seen
in X-ray emission do not follow the magnetic field, but are related to
the current sheets formed in response to the footpoint motions of the
magnetic field. For the Sun this result is of interest especially for
loops as found in areas where the magnetic field emerging from active
regions interacts with the surrounding network. These models are now
ready to face detailed comparisons with EUV spectroscopic observations
from Hinode and imaging from SDO/AIA.
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Title: 3D MHD models compared to EUV observations of quiet Sun and
active region structures
Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi
2008cosp...37.3579Z Altcode: 2008cosp.meet.3579Z
We study EUV emission line spectra derived from 3D MHD models of
structures in the corona, in particular of an active region surrounded
by a strong chromospheric network. The 3D MHD models account properly
for the energy balance, especially for heat conduction and radiative
losses, which allows us to reliably synthesize the profiles of
EUV emission lines observable with current EUV spectrometers,
i.e. SUMER/SOHO and EIS/Hinode. Thus we can directly compare these
synthesized spectra to real observations with these instruments. We
will discuss differences of models with different levels of magnetic
activity in terms of signatures seen in EUV lines synthesized from
these models and compare them to signatures found, e.g., in the spatial
distribution and temporal evolution of Doppler shifts of lines formed
in the transition region and corona. This is of major interest to test
the quality of the underlying model to heat the corona, i.e. currents
in the corona driven by photospheric motions (flux braiding). Based
on the 3D MHD model we can also derive intensity maps as they will
be observed through the coronal channels of the AIA-instrument on
SDO. Through this we can provide some guidance on how to process the
AIA maps in order to derive physical quantities such as temperature
and density through a combination of filters.
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Title: Asteroseismological Studies of Long-Period Variable Subdwarf
B Stars. II. Two-Color Photometry of PG 1338+481
Authors: Randall, S. K.; Green, E. M.; Fontaine, G.; Brassard, P.;
Terndrup, D. M.; Brown, N.; Fontaine, M.; Zacharias, P.; Chayer, P.
2006ApJ...645.1464R Altcode:
We present the results of an observational campaign for the long-period
variable subdwarf B star PG 1338+481. Seven continuous weeks of
observing time at the Steward Observatory 1.55 m Kuiper telescope
on Mount Bigelow, Arizona, and the 1.3 m MDM telescope at Kitt Peak
rendered ~250 hr of simultaneous U/R time series photometry, as well as
an extra ~70 hr of R-band-only data. The analysis of the combined light
curves resulted in the extraction of 13 convincing periodicities in the
2100-7200 s range, with amplitudes up to ~0.3% and ~0.2% in the U and
R, respectively. Comparing the ratios of amplitudes in the two wave
bands to those predicted from theory suggests the presence of dipole
modes, a notion that is further supported by the period spacing between
the highest amplitude peaks. If confirmed, this poses a challenge to
current nonadiabatic theory. At the quantitative level, we find that the
distribution of the observed period spectrum is highly nonuniform and
much sparser than that predicted from a representative model. We provide
a possible interpretation in the text. The asteroseismological analysis
attempted for PG 1338+481 on the basis of six observed periodicities
believed to constitute consecutive dipole modes renders encouraging
results. Fixing the effective temperature and surface gravity to
the spectroscopic estimates, we successfully isolate just one family
of optimal models that can reproduce the measured periods to better
than 1%. While the stellar parameters thus inferred must be regarded
as preliminary, the achieved fit bodes well for future asteroseismic
analyses of long-period variable subdwarf B stars. <P />Some of the
observations reported here were obtained at the MMT Observatory, a joint
facility of the University of Arizona and the Smithsonian Institution.
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Title: Fluctuations in Babcock-Leighton Dynamos. I. Period Doubling
and Transition to Chaos
Authors: Charbonneau, Paul; St-Jean, Cédric; Zacharias, Pia
2005ApJ...619..613C Altcode:
We present a large series of numerical simulations of the solar
magnetic activity cycle based on the Babcock-Leighton mechanism for the
regeneration of the solar poloidal magnetic field. While the primary
cycle period changes very little as the dynamo number is increased,
the model shows a well-defined transition to chaos through a sequence
of period-doubling bifurcations, i.e., the sequential appearance of
modulations of the primary cycle's amplitude, with associated periods
equal to twice the periods characterizing the amplitude variations prior
to a given bifurcation. This behavior arises through the unavoidable
time delay built into this type of solar dynamo model, rather than
through the effects of complex, nonlinear magnetic back-reaction on
the fluid motions driving the dynamo process. It is noteworthy that
a chaotic regime exists in this numerical model, given that the only
nonlinearity present is a simple algebraic amplitude-quenching factor in
one of the governing partial differential equations. The results also
represent a rare instance in which the complex dynamical behavior of a
spatially extended, diffusive solar dynamo model can be reproduced in
detail on the basis of the simplest of low-order dynamical systems,
namely a one-dimensional iterative map. The numerical results also
demonstrate the central role of meridional circulation in setting the
primary cycle period in this class of dynamo models; despite variations
by many orders of magnitude in the dynamo number and concomitant large
and sometimes even chaotic variations in amplitude, the cycle period
remains tightly locked to the meridional circulation turnover time.
