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Author name code: charbonneau
ADS astronomy entries on 2022-09-14
author:"Charbonneau, Paul"
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Title: Variability from thermo-resistive instability in the
atmospheres of hot jupiters
Authors: Hardy, Raphaël; Cumming, Andrew; Charbonneau, Paul
2022arXiv220803387H Altcode:
The atmosphere of a hot jupiter may be subject to a thermo-resistive
instability, in which the increasing electrical conductivity with
temperature leads to runaway Ohmic heating. We introduce a simplified
model of the local dynamics in the equatorial region of a hot jupiter
that incorporates the back reaction on the atmospheric flow as the
increasing electrical conductivity leads to flux freezing, which in
turn quenches the flow and therefore the Ohmic heating. We demonstrate
a new time-dependent solution that emerges for a temperature-dependent
electrical conductivity (whereas a temperature-independent conductivity
always evolves to a steady-state). The periodic cycle consists of
bursts of Alfven oscillations separated by quiescent intervals, with
the magnetic Reynolds number alternating between values smaller than
and larger than unity, maintaining the oscillation. We investigate
the regions of pressure and temperature in which the instability
operates. For the typical equatorial accelerations seen in atmospheric
models, we find instability at pressures $\sim 0.1$--$1\ {\rm bar}$
and temperatures $\approx 1300$--$1800\ {\rm K}$ for magnetic fields
$\sim 10\ {\rm G}$. Unlike previous studies based on a constant wind
velocity, we find that the instability is stronger for weaker magnetic
fields. Our results add support to the idea that variability should
be a feature of magnetized hot jupiter atmospheres, particularly at
intermediate temperatures. The temperature-dependence of the electrical
conductivity is an important ingredient that should be included in
MHD models of hot jupiter atmospheric dynamics.
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Title: Forecasting Solar Flares by Data Assimilation in Sandpile
Models
Authors: Thibeault, Christian; Strugarek, Antoine; Charbonneau, Paul;
Tremblay, Benoit
2022arXiv220613583T Altcode:
The prediction of solar flares is still a significant challenge in space
weather research, with no techniques currently capable of producing
reliable forecasts performing significantly above climatology. In this
paper, we present a flare forecasting technique using data assimilation
coupled with computationally inexpensive cellular automata called
sandpile models. Our data assimilation algorithm uses the simulated
annealing method to find an optimal initial condition that reproduces
well an energy-release time series. We present and empirically analyze
the predictive capabilities of three sandpile models, namely the Lu and
Hamilton model (LH) and two deterministically-driven models (D). Despite
their stochastic elements, we show that deterministically-driven
models display temporal correlations between simulated events, a needed
condition for data assimilation. We present our new data assimilation
algorithm and demonstrate its success in assimilating synthetic
observations produced by the avalanche models themselves. We then
apply our method to GOES X-Ray time series for 11 active regions having
generated multiple X-class flares in the course of their lifetime. We
demonstrate that for such large flares, our data assimilation scheme
substantially increases the success of “All-Clear” forecasts, as
compared to model climatology.
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Title: External Forcing of the Solar Dynamo
Authors: Charbonneau, Paul
2022FrASS...953676C Altcode:
In this paper I examine whether external forcing of the solar dynamo
on long timescales can produce detectable signal in the form of
long term modulation of the magnetic cycle. This task is motivated
in part by some recent proposals (Abreu et al., 2012; Astron. Ap.,
548, A88; Stefani et al., 2021; Solar Phys., 296, 88), whereby
modulation of the solar activity cycle on centennial and millennial
timescales, as recovered from the cosmogenic radioisotope record,
is attributed to perturbation of the tachocline driven by planetary
orbital motions. Working with a two-dimensional mean-field-like
kinematic dynamo model of the Babcock-Leighton variety, I show that
such an external forcing signal may be detectable in principle but is
likely to be obliterated by other internal sources of fluctuations,
in particular stochastic perturbations of the dynamo associated with
convective turbulence, unless a very efficient amplification mechanism
is at play. I also examine the ability of external tidal forcing to
synchronize an otherwise autonomous, internal dynamo operating at a
nearby frequency. Synchronization is readily achieved, and turns out to
be very robust to the introduction of stochastic noise, but requires
very high forcing amplitudes, again highlighting the critical need
for a powerful amplification mechanism.
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Title: Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos
of Sun-like Stars
Authors: Brun, Allan Sacha; Strugarek, Antoine; Noraz, Quentin;
Perri, Barbara; Varela, Jacobo; Augustson, Kyle; Charbonneau, Paul;
Toomre, Juri
2022ApJ...926...21B Altcode: 2022arXiv220113218B
We use the anelastic spherical harmonic code to model the convective
dynamo of solar-type stars. Based on a series of 15 3D MHD simulations
spanning four bins in rotation and mass, we show what mechanisms are
at work in these stellar dynamos with and without magnetic cycles
and how global stellar parameters affect the outcome. We also derive
scaling laws for the differential rotation and magnetic field based
on these simulations. We find a weaker trend between differential
rotation and stellar rotation rate, ( ${\rm{\Delta }}{\rm{\Omega
}}\propto {(| {\rm{\Omega }}| /{{\rm{\Omega }}}_{\odot })}^{0.46}$ )
in the MHD solutions than in their HD counterpart ${\left(| {\rm{\Omega
}}| /{{\rm{\Omega }}}_{\odot }\right)}^{0.66}$ ), yielding a better
agreement with the observational trends based on power laws. We find
that for a fluid Rossby number between 0.15 ≲ Ro <SUB>f</SUB> ≲
0.65, the solutions possess long magnetic cycle, if Ro <SUB>f</SUB>
≲ 0.42 a short cycle and if Ro <SUB>f</SUB> ≳ 1 (antisolar-like
differential rotation), a statistically steady state. We show that
short-cycle dynamos follow the classical Parker-Yoshimura rule
whereas the long-cycle period ones do not. We also find efficient
energy transfer between reservoirs, leading to the conversion of
several percent of the star's luminosity into magnetic energy that
could provide enough free energy to sustain intense eruptive behavior
at the star's surface. We further demonstrate that the Rossby number
dependency of the large-scale surface magnetic field in the simulation
( ${B}_{{\rm{L}},\mathrm{surf}}\sim {{Ro}}_{{\rm{f}}}^{-1.26}$ ) agrees
better with observations ( ${B}_{V}\sim {{Ro}}_{{\rm{s}}}^{-1.4\pm 0.1}$
) and differs from dynamo scaling based on the global magnetic energy
( ${B}_{\mathrm{bulk}}\sim {{Ro}}_{{\rm{f}}}^{-0.5}$ ).
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Title: Inferring depth-dependent plasma motions from surface
observations using the DeepVel neural network
Authors: Tremblay, Benoit; Cossette, Jean-François; Kazachenko,
Maria D.; Charbonneau, Paul; Vincent, Alain
2021JSWSC..11....9T Altcode:
Coverage of plasma motions is limited to the line-of-sight component
at the Sun's surface. Multiple tracking and inversion methods
were developed to infer the transverse motions from observational
data. Recently, the DeepVel neural network was trained with computations
performed by numerical simulations of the solar photosphere to
recover the missing transverse component at the surface and at two
additional optical depths simultaneously from the surface white light
intensity in the Quiet Sun. We argue that deep learning could provide
additional spatial coverage to existing observations in the form
of depth-dependent synthetic observations, i.e. estimates generated
through the emulation of numerical simulations. We trained different
versions of DeepVel using slices from numerical simulations of both
the Quiet Sun and Active Region at various optical and geometrical
depths in the solar atmosphere, photosphere and upper convection zone
to establish the upper and lower limits at which the neural network
can generate reliable synthetic observations of plasma motions from
surface intensitygrams. Flow fields inferred in the photosphere
and low chromosphere τ ∈ [0.1, 1) are comparable to inversions
performed at the surface (τ ≈ 1) and are deemed to be suitable
for use as synthetic estimates in data assimilation processes and
data-driven simulations. This upper limit extends closer to the
transition region (τ ≈ 0.01) in the Quiet Sun, but not for Active
Regions. Subsurface flows inferred from surface intensitygrams fail
to capture the small-scale features of turbulent convective motions
as depth crosses a few hundred kilometers. We suggest that these
reconstructions could be used as first estimates of a model's velocity
vector in data assimilation processes to nowcast and forecast short
term solar activity and space weather.
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Title: The Sun's Polar Magnetic Field as a Key Constraint on Dynamo
Models of the Solar Cycle
Authors: Charbonneau, P.
2020AGUFMSH014..05C Altcode:
The strength of the sun's dipole moment at solar minimum is long
known to be a good precursor for the forecasting of the amplitude of
the following solar activity cycle. The buildup of the polar fields
through the photospheric dispersal of magnetic flux liberated by the
decay of active regions is now being modelled very accurately using a
variety of surface flux transport models. Less clear is the role played
by the polar fields in the dynamo loop. I some classes of dynamo models
it is crucial, while in others it is a mere side-effect of the dynamo
operating in the interior. In this presentation I will use a variety
of current solar dynamo models to illustrate this diversity of roles
played by the polar fields. I will also argue that at this point, the
primary uncertainties lie not with the buildup of the polar fields,
but rather with its submergence into the solar interior, the essential
prerequisite for participating in the dynamo loop. Clarifying this
key issue observationally requires an accurate accounting of magnetic
flux balance for the polar caps (transport, emergence, submergence
and in situ production) for which out-of-the-ecliptic observations
are likely essential.
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Title: Impact of nonlinear surface inflows into activity belts on
the solar dynamo
Authors: Nagy, Melinda; Lemerle, Alexandre; Charbonneau, Paul
2020JSWSC..10...62N Altcode:
We examine the impact of surface inflows into activity belts on the
operation of solar cycle models based on the Babcock-Leighton mechanism
of poloidal field regeneration. Towards this end we introduce in the
solar cycle model of Lemerle & Charbonneau (2017. ApJ 834: 133) a
magnetic flux-dependent variation of the surface meridional flow based
on the axisymmetric inflow parameterization developped by Jiang et
al. (2010. ApJ 717: 597). The inflow dependence on emerging magnetic
flux thus introduces a bona fide nonlinear backreaction mechanism
in the dynamo loop. For solar-like inflow speeds, our simulation
results indicate a decrease of 10-20% in the strength of the global
dipole building up at the end of an activity cycle, in agreement with
earlier simulations based on linear surface flux transport models. Our
simulations also indicate a significant stabilizing effect on cycle
characteristics, in that individual cycle amplitudes in simulations
including inflows show less scatter about their mean than in the
absence of inflows. Our simulations also demonstrate an enhancement
of cross-hemispheric coupling, leading to a significant decrease in
hemispheric cycle amplitude asymmetries and temporal lag in hemispheric
cycle onset. Analysis of temporally extended simulations also indicate
that the presence of inflows increases the probability of cycle shutdown
following an unfavorable sequence of emergence events. This results
ultimately from the lower threshold nonlinearity built into our solar
cycle model, and presumably operating in the sun as well.
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Title: Towards an algebraic method of solar cycle
prediction. II. Reducing the need for detailed input data with ARDoR
Authors: Nagy, Melinda; Petrovay, Kristóf; Lemerle, Alexandre;
Charbonneau, Paul
2020JSWSC..10...46N Altcode: 2020arXiv200902300N
An algebraic method for the reconstruction and potentially prediction
of the solar dipole moment value at sunspot minimum (known to be a
good predictor of the amplitude of the next solar cycle) was suggested
in the first paper in this series. The method sums up the ultimate
dipole moment contributions of individual active regions in a solar
cycle: for this, detailed and reliable input data would in principle
be needed for thousands of active regions in a solar cycle. To reduce
the need for detailed input data, here we propose a new active region
descriptor called ARDoR (Active Region Degree of Rogueness). In a
detailed statistical analysis of a large number of activity cycles
simulated with the 2 × 2D dynamo model we demonstrate that ranking
active regions by decreasing ARDoR, for a good reproduction of the solar
dipole moment at the end of the cycle it is sufficient to consider
the top N regions on this list explicitly, where N is a relatively
low number, while for the other regions the ARDoR value may be set
to zero. For example, with N = 5 the fraction of cycles where the
dipole moment is reproduced with an error exceeding ±30% is only 12%,
significantly reduced with respect to the case N = 0, i.e. ARDoR set to
zero for all active regions, where this fraction is 26%. This indicates
that stochastic effects on the intercycle variations of solar activity
are dominated by the effect of a low number of large "rogue" active
regions, rather than the combined effect of numerous small ARs. The
method has a potential for future use in solar cycle prediction.
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Title: Dynamo models of the solar cycle
Authors: Charbonneau, Paul
2020LRSP...17....4C Altcode:
This paper reviews recent advances and current debates in modeling the
solar cycle as a hydromagnetic dynamo process. Emphasis is placed on
(relatively) simple dynamo models that are nonetheless detailed enough
to be comparable to solar cycle observations. After a brief overview
of the dynamo problem and of key observational constraints, I begin
by reviewing the various magnetic field regeneration mechanisms that
have been proposed in the solar context. I move on to a presentation
and critical discussion of extant solar cycle models based on these
mechanisms, followed by a discussion of recent magnetohydrodynamical
simulations of solar convection generating solar-like large-scale
magnetic cycles. I then turn to the origin and consequences of
fluctuations in these models and simulations, including amplitude
and parity modulation, chaotic behavior, and intermittency. The paper
concludes with a discussion of our current state of ignorance regarding
various key questions relating to the explanatory framework offered
by dynamo models of the solar cycle.
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Title: Grand Minima in a spherical non-kinematic α<SUP>2</SUP>Ω
mean-field dynamo model
Authors: Simard, Corinne; Charbonneau, Paul
2020JSWSC..10....9S Altcode:
We present a non-kinematic axisymetric α<SUP>2</SUP>Ω mean-field
dynamo model in which the complete α-tensor and mean differential
rotation profile are both extracted from a global magnetohydrodynamical
simulation of solar convection producing cycling large-scale magnetic
fields. The nonlinear backreaction of the Lorentz force on differential
rotation is the only amplitude-limiting mechanism introduced in the
mean-field model. We compare and contrast the amplitude modulation
patterns characterizing this mean-field dynamo, to those already
well-studied in the context of non-kinematic αΩ models using a
scalar α-effect. As in the latter, we find that large quasi-periodic
modulation of the primary cycle are produced at low magnetic Prandtl
number (Pm), with the ratio of modulation period to the primary cycle
period scaling inversely with Pm. The variations of differential
rotation remain well within the bounds set by observed solar torsional
oscillations. In this low-Pm regime, moderately supercritical solutions
can also exhibit aperiodic Maunder Minimum-like periods of strongly
reduced cycle amplitude. The inter-event waiting time distribution
is approximately exponential, in agreement with solar activity
reconstructions based on cosmogenic radioisotopes. Secular variations
in low-latitude surface differential rotation during Grand Minima,
as compared to epochs of normal cyclic behavior, are commensurate in
amplitude with historical inferences based on sunspot drawings. Our
modeling results suggest that the low levels of observed variations
in the solar differential rotation in the course of the activity cycle
may nonetheless contribute to, or perhaps even dominate, the regulation
of the magnetic cycle amplitude.
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Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
2019arXiv191014022S Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.
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Title: Grand Activity Minima and Maxima via Dual Dynamos
Authors: Ölçek, Deniz; Charbonneau, Paul; Lemerle, Alexandre;
Longpré, Gabriel; Boileau, Florence
2019SoPh..294...99O Altcode:
Reconstructions of past solar activity based on cosmogenic radioisotopes
have reavealed that the Sun spends a significant fraction (≈20 %) of
its time in aperiodically recurring states of so-called Grand Minima
or Grand Maxima, namely epochs of strongly supressed and markedly
above-average levels of magnetic activity, respectively. The physical
origin of these episodes is not yet understood. In this article we
present a dual-dynamo model of the solar cycle, combining a dominant
dynamo based on differential-rotation shear and surface decay of bipolar
active regions, and a weak, deep-seated turbulent dynamo. The resulting
dynamo simulations are found to exhibit the equivalent of observed
Grand Minima and Maxima. By adjusting the magnitude and saturation
level of the secondary turbulent dynamo, we can reproduce well the
duration and waiting-time distributions of Grand Minima and Maxima
inferred from the cosmogenic-isotope record. The exit from Grand Minima
episodes is typically characterized by strong hemispheric asymmetries,
in agreement with sunspot observations during the 1645 - 1715 Maunder
Minimum. In these simulations, Grand Maxima can be unambiguously
identified as a distinct dual-dynamo state resulting from constructive
interference between the two dynamos mechanisms operating within the
simulation. This interaction leads to the autonomous production of
long quasi-periodicities in the millennial range, commensurate with
the Halstatt cycle. Such a quasi-periodic modulation, readily produced
through dynamical backreaction on large-scale flows in non-kinematic
dynamo models, is quite uncommon in a purely kinematic solar-cycle model
such as the one developed herein. We argue that these long periodicities
are set by the long diffusion time of magnetic field accumulating in
the stable layers underlying the turbulent convection zone.
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Title: A Dynamo-based Forecast of Solar Cycle 25
Authors: Labonville, Francois; Charbonneau, Paul; Lemerle, Alexandre
2019SoPh..294...82L Altcode:
We present a data-driven version of the solar cycle model of Lemerle
and Charbonneau (Astrophys. J.834, 133; 2017), which we use to forecast
properties of the upcoming sunspot Cycle 25. The two free parameters of
the model are fixed by requiring the model to reproduce Cycle 24 upon
being driven by active region data for Cycle 23. Our forecasting model
incorporates self-consistently the expected fluctuations associated
with stochastic variations in properties of emerging active regions,
most notably the scatter in the tilt angle of the line segment
joining the opposite polarity focii of bipolar magnetic regions,
as embodied in Joy's law. By carrying out ensemble forecasts with
statistically independent realizations of active region parameters,
we can produce error bars that capture the impact of this physical
source of fluctuations. We forecast a smoothed monthly international
sunspot number (version 2.0) peaking at 89<SUB>−14</SUB><SUP>+29</SUP>
in year 2025.3<SUB>−1.05</SUB><SUP>+0.89</SUP>, with a 6 month onset
delay in the northern hemisphere, but a peak amplitude 20% higher than
in the southern hemisphere.
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Title: Impact of rogue active regions on hemispheric asymmetry
Authors: Nagy, Melinda; Lemerle, Alexandre; Charbonneau, Paul
2019AdSpR..63.1425N Altcode: 2019arXiv190907672N
The solar dipole moment at activity minimum is a good predictor of the
strength of the subsequent solar cycle. Through a systematic analysis
using a state-of-the-art 2 × 2 D solar dynamo model, we found that
bipolar magnetic regions (BMR) with atypical characteristics can modify
the strength of the next cycle via their impact on the buildup of the
dipole moment as a sunspot cycle unfolds. In addition to summarizing
these results, we present further effects of such "rogue" BMRs. These
have the ability to generate hemispheric asymmetry in the subsequent
sunspot cycle, since they modify the polar cap flux asymmetry of the
ongoing cycle. We found strong correlation between the polar cap flux
asymmetry of cycle i and the total pseudo sunspot number asymmetry of
cycle i + 1 . Good correlation also appears in the case of the time
lag of the hemispheres of cycle i + 1 .
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Title: Sandpile Models and Solar Flares: Eigenfunction Decomposition
for Data Assimilation
Authors: Strugarek, Antoine; Brun, Allan S.; Charbonneau, Paul;
Vilmer, Nicole
2018IAUS..335..250S Altcode:
The largest solar flares, of class X and above, are often associated
with strong energetic particle acceleration. Based on the self-similar
distribution of solar flares, self-organized criticality models
such as sandpiles can be used to successfully reproduce their
statistics. However, predicting strong (and rare) solar flares turns
out to be a significant challenge. We build here on an original idea
based on the combination of minimalistic flare models (sandpiles)
and modern data assimilation techniques (4DVar) to predict large
solar flares. We discuss how to represent a sandpile model over a
reduced set of eigenfunctions to improve the efficiency of the data
assimilation technique. This improvement is model-independent and
continues to pave the way towards efficient near real-time solutions
for predicting solar flares.
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Title: On the Sensitivity of Magnetic Cycles in Global Simulations
of Solar-like Stars
Authors: Strugarek, A.; Beaudoin, P.; Charbonneau, P.; Brun, A. S.
2018ApJ...863...35S Altcode: 2018arXiv180609484S
The periods of magnetic activity cycles in the Sun and solar-type
stars do not exhibit a simple or even single trend with respect to
rotation rate or luminosity. Dynamo models can be used to interpret
this diversity and can ultimately help us understand why some
solar-like stars do not exhibit a magnetic cycle, whereas some do,
and for the latter what physical mechanisms set their magnetic
cycle period. Three-dimensional nonlinear MHD simulations present
the advantage of having only a small number of tunable parameters,
and produce in a dynamically self-consistent manner the flows and the
dynamo magnetic fields pervading stellar interiors. We conduct a series
of such simulations within the EULAG-MHD framework, varying the rotation
rate and luminosity of the modeled solar-like convective envelopes. We
find decadal magnetic cycles when the Rossby number near the base of the
convection zone is moderate (typically between 0.25 and 1). Secondary,
shorter cycles located at the top of the convective envelope close to
the equator are also observed in our numerical experiments, when the
local Rossby number is lower than 1. The deep-seated dynamo sustained
in these numerical experiments is fundamentally nonlinear, in that it
is the feedback of the large-scale magnetic field on the large-scale
differential rotation that sets the magnetic cycle period. The cycle
period is found to decrease with the Rossby number, which offers an
alternative theoretical explanation to the variety of activity cycles
observed in solar-like stars.
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Title: Sandpile Model and Machine Learning for the Prediction of
Solar Flares
Authors: Tremblay, Benoit; Strugarek, Antoine; Charbonneau, Paul
2018shin.confE.143T Altcode:
X-class (and above) solar flares are amongst the largest (and rarest)
eruptive phenomena of the Sun. They are often accompanied by the
acceleration of energetic particles which can have significant impacts
on Earth's environment. The statistical features of large eruptive
events can be reproduced by self-organized criticality models such as
sandpile models. We previously developed a minimalistic sandpile model
which, coupled with a modern data assimilation technique (e.g. 4D-Var),
holds promising predictive capabilities. Our recent efforts focused on
training a neural network using time sequences of synthetic X-ray flux
emissions generated by the sandpile model to infer a set of initial
conditions of the sandpile model compatible with this sequence of
emissions (given as input). The training process is only carried out
once and replaces the computationally expensive minimization step of
the data assimilation procedure. Past sequences of GOES X-ray flux
measurements are then fed to the neural network to obtain a sandpile
model representative of a given epoch of the Sun. The inferred
initial conditions can be used in an assimilated sandpile model for
the prediction of upcoming flaring events.
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Title: Differential Rotation in Solar-like Convective Envelopes:
Influence of Overshoot and Magnetism
Authors: Beaudoin, Patrice; Strugarek, Antoine; Charbonneau, Paul
2018ApJ...859...61B Altcode:
We present a set of four global Eulerian/semi-Lagrangian fluid
solver (EULAG) hydrodynamical (HD) and magnetohydrodynamical (MHD)
simulations of solar convection, two of which are restricted to the
nominal convection zone, and the other two include an underlying stably
stratified fluid layer. While all four simulations generate reasonably
solar-like latitudinal differential rotation profiles where the
equatorial region rotates faster than the polar regions, the rotational
isocontours vary significantly among them. In particular, the purely
HD simulation with a stable layer alone can break the Taylor-Proudman
theorem and produce approximately radially oriented rotational
isocontours at medium to high latitudes. We trace this effect to the
buildup of a significant latitudinal temperature gradient in the stable
fluid immediately beneath the convection zone, which imprints itself
on the lower convection zone. It develops naturally in our simulations
as a consequence of convective overshoot and rotational influence of
rotation on convective energy fluxes. This favors the establishment
of a thermal wind balance that allows evading the Taylor-Proudman
constraint. A much smaller latitudinal temperature gradient develops
in the companion MHD simulation that includes a stable fluid layer,
reflecting the tapering of deep convective overshoot that occurs at
medium to high latitudes, which is caused by the strong magnetic fields
that accumulate across the base of the convection zone. The stable fluid
layer also has a profound impact on the large-scale magnetic cycles
developing in the two MHD simulations. Even though both simulations
operate in the same convective parameter regime, the simulation that
includes a stable layer eventually loses cyclicity and transits to a
non-solar, steady quadrupolar state.
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Title: The Effect of "Rogue" Active Regions on the Solar Cycle
Authors: Nagy, Melinda; Lemerle, Alexandre; Labonville, François;
Petrovay, Kristóf; Charbonneau, Paul
2017SoPh..292..167N Altcode: 2017arXiv171202185N
The origin of cycle-to-cycle variations in solar activity is currently
the focus of much interest. It has recently been pointed out that
large individual active regions with atypical properties can have a
significant impact on the long-term behavior of solar activity. We
investigate this possibility in more detail using a recently developed
2 ×2 D dynamo model of the solar magnetic cycle. We find that even
a single "rogue" bipolar magnetic region (BMR) in the simulations
can have a major effect on the further development of solar activity
cycles, boosting or suppressing the amplitude of subsequent cycles. In
extreme cases, an individual BMR can completely halt the dynamo,
triggering a grand minimum. Rogue BMRs also have the potential to
induce significant hemispheric asymmetries in the solar cycle. To
study the effect of rogue BMRs in a more systematic manner, a series
of dynamo simulations were conducted, in which a large test BMR
was manually introduced in the model at various phases of cycles of
different amplitudes. BMRs emerging in the rising phase of a cycle
can modify the amplitude of the ongoing cycle, while BMRs emerging
in later phases will only affect subsequent cycles. In this model,
the strongest effect on the subsequent cycle occurs when the rogue
BMR emerges around cycle maximum at low latitudes, but the BMR does
not need to be strictly cross-equatorial. Active regions emerging as
far as 20<SUP>∘</SUP> from the equator can still have a significant
effect. We demonstrate that the combined effect of the magnetic flux,
tilt angle, and polarity separation of the BMR on the dynamo is via
their contribution to the dipole moment, δ D<SUB>BMR</SUB>. Our
results indicate that prediction of the amplitude, starting epoch,
and duration of a cycle requires an accurate accounting of a broad
range of active regions emerging in the previous cycle.
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Title: Meridional circulation dynamics in a cyclic convective dynamo
Authors: Passos, D.; Miesch, M.; Guerrero, G.; Charbonneau, P.
2017A&A...607A.120P Altcode: 2017arXiv170202421P
Surface observations indicate that the speed of the solar meridional
circulation in the photosphere varies in anti-phase with the solar
cycle. The current explanation for the source of this variation is
that inflows into active regions alter the global surface pattern
of the meridional circulation. When these localized inflows are
integrated over a full hemisphere, they contribute to slowing down
the axisymmetric poleward horizontal component. The behavior of this
large-scale flow deep inside the convection zone remains largely
unknown. Present helioseismic techniques are not sensitive enough to
capture the dynamics of this weak large-scale flow. Moreover, the large
time of integration needed to map the meridional circulation inside the
convection zone, also masks some of the possible dynamics on shorter
timescales. In this work we examine the dynamics of the meridional
circulation that emerges from a 3D MHD global simulation of the solar
convection zone. Our aim is to assess and quantify the behavior of
meridional circulation deep inside the convection zone where the cyclic
large-scale magnetic field can reach considerable strength. Our analyses
indicate that the meridional circulation morphology and amplitude are
both highly influenced by the magnetic field via the impact of magnetic
torques on the global angular momentum distribution. A dynamic feature
induced by these magnetic torques is the development of a prominent
upward flow at mid-latitudes in the lower convection zone that occurs
near the equatorward edge of the toroidal bands and that peaks during
cycle maximum. Globally, the dynamo-generated large-scale magnetic
field drives variations in the meridional flow, in stark contrast to
the conventional kinematic flux transport view of the magnetic field
being advected passively by the flow.
---------------------------------------------------------
Title: The Puzzling Dynamos of Stars: Recent Progress With Global
Numerical Simulations
Authors: Strugarek, Antoine; Beaudoin, Patrice; Charbonneau, Paul;
Brun, Allan S.
2017IAUS..328....1S Altcode:
The origin of magnetic cycles in the Sun and other cool stars is one
of the great theoretical challenge in stellar astrophysics that still
resists our understanding. Ab-initio numerical simulations are today
required to explore the extreme turbulent regime in which stars operate
and sustain their large-scale, cyclic magnetic field. We report in
this work on recent progresses made with high performance numerical
simulations of global turbulent convective envelopes. We rapidly
review previous prominent results from numerical simulations, and
present for the first time a series of turbulent, global simulations
producing regular magnetic cycles whose period varies systematically
with the convective envelope parameters (rotation rate, convective
luminosity). We find that the fundamentally non-linear character of
the dynamo simulated in this work leads the magnetic cycle period to
be inversely proportional to the Rossby number. These results promote
an original interpretation of stellar magnetic cycles, and could help
reconcile the cyclic behaviour of the Sun and other solar-type stars.
---------------------------------------------------------
Title: Challenges of Solar Cycle Prediction Introduced by 'Rogue'
Active Region Emergences
Authors: Nagy, Melinda; Charbonneau, Paul
2017shin.confE..48N Altcode:
The building up process of the polar magnetic field is still debated,
however, the peak value of that is the most promising solar cycle
predictor. According to earlier results, the tilt angles of active
regions (AR) emerging close to or across the equator have a crucial
role in this question. Besides this, it is pointed out that the flux
of an individual AR can be commensurable to the polar cap flux. In
the case of strong, cross-equatorial emergences, the contribution
to the solar dipole moment is huge. <P />In order to investigate in
detail the effect of such peculiar AR emergences on the amplitudes
of following cycles, test regions were inserted into sunspot cycles
simulated by a coupled 2 × 2D Babcock-Leighton kinematic solar dynamo
model (Lemerle et al., 2015, ApJ 801; Lemerle & Charbonneau, 2017,
ApJ 834). Several series of simulation runs were done while we changed
the emergence epoch; the latitude of the emergence; the flux and the
tilt angle of the AR and the angular separation between the leading
and trailing polarities. <P />It was found that ARs emerging close to
the equator during the rising phase of a cycle affects the amplitude
of the ongoing cycle itself. The peak value of the following cycle
is effected the most when the AR appears near cycle maximum. If the
flux, tilt angle or the separation was changed, the amplitude of the
next cycle changed accordingly. By changing the emergence latitude we
found that an AR emerging >20° far from the equator still can have
significant effect. Interestingly, the duration of the ongoing cycle
is affected as well, despite the constant meridional circulation speed
used within the dynamo model used for the analysis.
---------------------------------------------------------
Title: Reconciling solar and stellar magnetic cycles with nonlinear
dynamo simulations
Authors: Strugarek, A.; Beaudoin, P.; Charbonneau, P.; Brun, A. S.;
do Nascimento, J. -D.
2017Sci...357..185S Altcode: 2017arXiv170704335S
The Sun's activity, including sun-spot activity, varies on an 11-year
cycle driven by changes in its magnetic field. Other nearby solar-type
stars have their own cycles, but the Sun does not seem to match their
behavior. Strugarek et al. used magnetohydrodynamic simulations to
show that stellar activity periods should depend on the star's Rossby
number, the ratio between the inertial and Coriolis forces. Turning
to observations, they found that solar-type stars, including the Sun,
follow this relation. The results advance our understanding of how
stars generate their magnetic fields and confirm that the Sun is indeed
a solar-type star.
---------------------------------------------------------
Title: Can the solar cycle be predicted ?
Authors: Charbonneau, Paul
2017shin.confE.171C Altcode:
Because the solar magnetic activity cycle modulates the sun's radiative
output and solar wind properties, as well as the frequency of all
geoeffective solar eruptive phenomena, predicting its characteristics
--amplitude, duration, timing of maxima and polarity reversals--
remains a cornerstone of space weather research. Secular variations on
supra-cycle timescale are also now considered an important component
of solar forcing in climate simulations. Working through specific
examples, I will show that various classes of solar dynamo models
have very different predictive potential, and consequently that the
primary obstacle facing current prediction methods based on sch models
is the identification of the precise inductive mechanisms powering
the solar dynamo, and of the nonlinear feedback mechanism regulating
cycle amplitude. The response of these various models to stochastic
forcing, and its consequence for prediction, will also be addressed,
again through specific modeling examples.
---------------------------------------------------------
Title: Magnetically Modulated Heat Transport in a Global Simulation
of Solar Magneto-convection
Authors: Cossette, Jean-Francois; Charbonneau, Paul; Smolarkiewicz,
Piotr K.; Rast, Mark P.
2017ApJ...841...65C Altcode:
We present results from a global MHD simulation of solar convection
in which the heat transported by convective flows varies in-phase
with the total magnetic energy. The purely random initial magnetic
field specified in this experiment develops into a well-organized
large-scale antisymmetric component undergoing hemispherically
synchronized polarity reversals on a 40 year period. A key feature of
the simulation is the use of a Newtonian cooling term in the entropy
equation to maintain a convectively unstable stratification and drive
convection, as opposed to the specification of heating and cooling terms
at the bottom and top boundaries. When taken together, the solar-like
magnetic cycle and the convective heat flux signature suggest that a
cyclic modulation of the large-scale heat-carrying convective flows
could be operating inside the real Sun. We carry out an analysis of
the entropy and momentum equations to uncover the physical mechanism
responsible for the enhanced heat transport. The analysis suggests
that the modulation is caused by a magnetic tension imbalance inside
upflows and downflows, which perturbs their respective contributions to
heat transport in such a way as to enhance the total convective heat
flux at cycle maximum. Potential consequences of the heat transport
modulation for solar irradiance variability are briefly discussed.
---------------------------------------------------------
Title: Confinement of the solar tachocline by a cyclic dynamo
magnetic field
Authors: Barnabé, Roxane; Strugarek, Antoine; Charbonneau, Paul;
Brun, Allan Sacha; Zahn, Jean-Paul
2017A&A...601A..47B Altcode: 2017arXiv170302374B
Context. The surprising thinness of the solar tachocline is still not
understood with certainty today. Among the numerous possible scenarios
suggested to explain its radial confinement, one hypothesis is based on
Maxwell stresses that are exerted by the cyclic dynamo magnetic field of
the Sun penetrating over a skin depth below the turbulent convection
zone. <BR /> Aims: Our goal is to assess under which conditions
(turbulence level in the tachocline, strength of the dynamo-generated
field, spreading mechanism) this scenario can be realized in the
solar tachocline. <BR /> Methods: We develop a simplified 1D model of
the upper tachocline under the influence of an oscillating magnetic
field imposed from above. The turbulent transport is parametrized with
enhanced turbulent diffusion (or anti-diffusion) coefficients. Two main
processes that thicken the tachocline are considered; either turbulent
viscous spreading or radiative spreading. An extensive parameter study
is carried out to establish the physical parameter regimes under which
magnetic confinement of the tachocline that is due to a surface dynamo
field can be realized. <BR /> Results: We have explored a large range
of magnetic field amplitudes, viscosities, ohmic diffusivities and
thermal diffusivities. We find that, for large but still realistic
magnetic field strengths, the differential rotation can be suppressed
in the upper radiative zone (and hence the tachocline confined)
if weak turbulence is present (with an enhanced ohmic diffusivity
of η> 10<SUP>7-8</SUP> cm<SUP>2</SUP>/ s), even in the presence
of radiative spreading. <BR /> Conclusions: Our results show that a
dynamo magnetic field can, in the presence of weak turbulence, prevent
the inward burrowing of a tachocline subject to viscous diffusion or
radiative spreading.
---------------------------------------------------------
Title: A Coupled 2 × 2D Babcock-Leighton Solar Dynamo
Model. II. Reference Dynamo Solutions
Authors: Lemerle, Alexandre; Charbonneau, Paul
2017ApJ...834..133L Altcode: 2016arXiv160607375L
In this paper we complete the presentation of a new hybrid 2 × 2D
flux transport dynamo (FTD) model of the solar cycle based on the
Babcock-Leighton mechanism of poloidal magnetic field regeneration
via the surface decay of bipolar magnetic regions (BMRs). This hybrid
model is constructed by allowing the surface flux transport (SFT)
simulation described in Lemerle et al. to provide the poloidal source
term to an axisymmetric FTD simulation defined in a meridional plane,
which in turn generates the BMRs required by the SFT. A key aspect of
this coupling is the definition of an emergence function describing the
probability of BMR emergence as a function of the spatial distribution
of the internal axisymmetric magnetic field. We use a genetic algorithm
to calibrate this function, together with other model parameters,
against observed cycle 21 emergence data. We present a reference dynamo
solution reproducing many solar cycle characteristics, including good
hemispheric coupling, phase relationship between the surface dipole
and the BMR-generating internal field, and correlation between dipole
strength at cycle maximum and peak amplitude of the next cycle. The
saturation of the cycle amplitude takes place through the quenching
of the BMR tilt as a function of the internal field. The observed
statistical scatter about the mean BMR tilt, built into the model, acts
as a source of stochasticity which dominates amplitude fluctuations. The
model thus can produce Dalton-like epochs of strongly suppressed cycle
amplitude lasting a few cycles and can even shut off entirely following
an unfavorable sequence of emergence events.
---------------------------------------------------------
Title: Editorial: 50 Years of Solar Physics
Authors: Charbonneau, Paul; Leibacher, John; Mandrini, Cristina;
van Driel-Gesztelyi, Lidia; Wheatland, Michael S.
2016SoPh..291.3461C Altcode: 2016SoPh..tmp..189C
No abstract at ADS
---------------------------------------------------------
Title: Characterisation of the turbulent electromotive force and its
magnetically-mediated quenching in a global EULAG-MHD simulation of
solar convection
Authors: Simard, Corinne; Charbonneau, Paul; Dubé, Caroline
2016AdSpR..58.1522S Altcode: 2016arXiv160401533S
We perform a mean-field analysis of the EULAG-MHD millenium simulation
of global magnetohydrodynamical convection presented in Passos and
Charbonneau (2014). The turbulent electromotive force (emf) operating
in the simulation is assumed to be linearly related to the cyclic
axisymmetric mean magnetic field and its first spatial derivatives. At
every grid point in the simulation's meridional plane, this assumed
relationship involves 27 independent tensorial coefficients. Expanding
on Racine et al. (2011), we extract these coefficients from the
simulation data through a least-squares minimization procedure based
on singular value decomposition. The reconstructed α -tensor shows
good agreement with that obtained by Racine et al. (2011), who did
not include derivatives of the mean-field in their fit, as well as
with the α -tensor extracted by Augustson et al. (2015) from a
distinct ASH MHD simulation. The isotropic part of the turbulent
magnetic diffusivity tensor β is positive definite and reaches
values of 5.0 ×10<SUP>7</SUP> m<SUP>2</SUP> s<SUP>-1</SUP> in the
middle of the convecting fluid layers. The spatial variations of both
α<SUB>ϕϕ</SUB> and β<SUB>ϕϕ</SUB> component are well reproduced by
expressions obtained under the Second Order Correlation Approximation,
with a good matching of amplitude requiring a turbulent correlation
time about five times smaller than the estimated turnover time of
the small-scale turbulent flow. By segmenting the simulation data
into epochs of magnetic cycle minima and maxima, we also measure α -
and β -quenching. We find the magnetic quenching of the α -effect to
be driven primarily by a reduction of the small-scale flow's kinetic
helicity, with variations of the current helicity playing a lesser
role in most locations in the simulation domain. Our measurements of
turbulent diffusivity quenching are restricted to the β<SUB>ϕϕ</SUB>
component, but indicate a weaker quenching, by a factor of ≃ 1.36,
than of the α -effect, which in our simulation drops by a factor of
three between the minimum and maximum phases of the magnetic cycle.
---------------------------------------------------------
Title: Modeling turbulent stellar convection zones: Sub-grid scales
effects
Authors: Strugarek, A.; Beaudoin, P.; Brun, A. S.; Charbonneau, P.;
Mathis, S.; Smolarkiewicz, P. K.
2016AdSpR..58.1538S Altcode: 2016arXiv160508685S
The impressive development of global numerical simulations
of turbulent stellar interiors unveiled a variety of possible
differential rotation (solar or anti-solar), meridional circulation
(single or multi-cellular), and dynamo states (stable large scale
toroidal field or periodically reversing magnetic fields). Various
numerical schemes, based on the so-called anelastic set of equations,
were used to obtain these results. It appears today mandatory to assess
their robustness with respect to the details of the numerics, and in
particular to the treatment of turbulent sub-grid scales. We report
on an ongoing comparison between two global models, the ASH and EULAG
codes. In EULAG the sub-grid scales are treated implicitly by the
numerical scheme, while in ASH their effect is generally modeled by
using enhanced dissipation coefficients. We characterize the sub-grid
scales effect in a turbulent convection simulation with EULAG. We
assess their effect at each resolved scale with a detailed energy
budget. We derive equivalent eddy-diffusion coefficients and use the
derived diffusivities in twin ASH numerical simulations. We find a good
agreement between the large-scale flows developing in the two codes
in the hydrodynamic regime, which encourages further investigation in
the magnetohydrodynamic regime for various dynamo solutions.
---------------------------------------------------------
Title: Double Dynamo Signatures in a Global MHD Simulation and
Mean-field Dynamos
Authors: Beaudoin, Patrice; Simard, Corinne; Cossette, Jean-François;
Charbonneau, Paul
2016ApJ...826..138B Altcode:
The 11 year solar activity cycle is the most prominent periodic
manifestation of the magnetohydrodynamical (MHD) large-scale dynamo
operating in the solar interior, yet longer and shorter (quasi-)
periodicities are also present. The so-called “quasi-biennial”
signal appearing in many proxies of solar activity has been gaining
increasing attention since its detection in p-mode frequency shifts,
which suggests a subphotospheric origin. A number of candidate
mechanisms have been proposed, including beating between co-existing
global dynamo modes, dual dynamos operating in spatially separated
regions of the solar interior, and Rossby waves driving short-period
oscillations in the large-scale solar magnetic field produced by
the 11 year activity cycle. In this article, we analyze a global MHD
simulation of solar convection producing regular large-scale magnetic
cycles, and detect and characterize shorter periodicities developing
therein. By constructing kinematic mean-field α <SUP>2</SUP>Ω
dynamo models incorporating the turbulent electromotive force (emf)
extracted from that same simulation, we find that dual-dynamo behavior
materializes in fairly wide regions of the model’s parameters
space. This suggests that the origin of the similar behavior detected
in the MHD simulation lies with the joint complexity of the turbulent
emf and differential rotation profile, rather that with dynamical
interactions such as those mediated by Rossby waves. Analysis of the
simulation also reveals that the dual dynamo operating therein leaves
a double-period signature in the temperature field, consistent with a
dual-period helioseismic signature. Order-of-magnitude estimates for
the magnitude of the expected frequency shifts are commensurate with
helioseismic measurements. Taken together, our results support the
hypothesis that the solar quasi-biennial oscillations are associated
with a secondary dynamo process operating in the outer reaches of the
solar convection zone.
---------------------------------------------------------
Title: Solar physics: Dynamo theory questioned
Authors: Charbonneau, Paul
2016Natur.535..500C Altcode:
Observations of X-ray emission -- a diagnostic tool for the mechanisms
driving stellar magnetic fields -- from four cool stars call into
question accepted models of magnetic-field generation in the Sun and
stars. See Letter p.526
---------------------------------------------------------
Title: New Insights about Meridional Circulation Dynamics from 3D
MHD Global Simulations of Solar Convection and Dynamo Action
Authors: Passos, D.; Charbonneau, P.; Miesch, M. S.
2016ASPC..504..179P Altcode:
The solar meridional circulation is a "slow", large scale flow that
transports magnetic field and plasma throughout the convection zone in
the (r,θ) plane and plays a crucial role in controlling the magnetic
cycle solutions presented by flux transport dynamo models. Observations
indicate that this flow speed varies in anti-phase with the solar
cycle at the solar surface. A possible explanation for the source of
this variation is based on the fact that inflows into active regions
alter the global surface pattern of the meridional circulation. In
this work we examine the meridional circulation profile that emerges
from a 3D global simulation of the solar convection zone, and its
associated dynamics. We find that at the bottom of the convection
zone, in the region where the toroidal magnetic field accumulates,
the meridional circulation is highly modulated through the action of
a magnetic torques and thus provides evidence for a new mechanism to
explain the observed cyclic variations.
---------------------------------------------------------
Title: Division E Commission 10: Solar Activity
Authors: Schrijver, Carolus J.; Fletcher, Lyndsay; van Driel-Gesztelyi,
Lidia; Asai, Ayumi; Cally, Paul S.; Charbonneau, Paul; Gibson, Sarah
E.; Gomez, Daniel; Hasan, Siraj S.; Veronig, Astrid M.; Yan, Yihua
2016IAUTA..29..245S Altcode: 2015arXiv151003348S
After more than half a century of community support related to the
science of “solar activity”, IAU's Commission 10 was formally
discontinued in 2015, to be succeeded by C.E2 with the same area
of responsibility. On this occasion, we look back at the growth of
the scientific disciplines involved around the world over almost a
full century. Solar activity and fields of research looking into the
related physics of the heliosphere continue to be vibrant and growing,
with currently over 2,000 refereed publications appearing per year from
over 4,000 unique authors, publishing in dozens of distinct journals
and meeting in dozens of workshops and conferences each year. The
size of the rapidly growing community and of the observational and
computational data volumes, along with the multitude of connections
into other branches of astrophysics, pose significant challenges;
aspects of these challenges are beginning to be addressed through,
among others, the development of new systems of literature reviews,
machine-searchable archives for data and publications, and virtual
observatories. As customary in these reports, we highlight some
of the research topics that have seen particular interest over the
most recent triennium, specifically active-region magnetic fields,
coronal thermal structure, coronal seismology, flares and eruptions,
and the variability of solar activity on long time scales. We close
with a collection of developments, discoveries, and surprises that
illustrate the range and dynamics of the discipline.
---------------------------------------------------------
Title: Predicting large solar flares with data assimilation
Authors: Strugarek, Antoine; Charbonneau, Paul
2016IAUFM..29B.734S Altcode:
We propose to use a deterministically-driven class of self-organized
criticality sandpile models to carry out predictions of the largest,
most dangerous, and hardest to predict solar flares.
---------------------------------------------------------
Title: Evidence of Active MHD Instability in EULAG-MHD Simulations
of Solar Convection
Authors: Lawson, Nicolas; Strugarek, Antoine; Charbonneau, Paul
2015ApJ...813...95L Altcode: 2015arXiv150907447L
We investigate the possible development of magnetohydrodynamical
instabilities in the EULAG-MHD “millennium simulation” of Passos
& Charbonneau. This simulation sustains a large-scale magnetic
cycle characterized by solar-like polarity reversals taking place on
a regular multidecadal cadence, and in which zonally oriented bands
of strong magnetic fields accumulate below the convective layers,
in response to turbulent pumping from above in successive magnetic
half-cycles. Key aspects of this simulation include low numerical
dissipation and a strongly sub-adiabatic fluid layer underlying
the convectively unstable layers corresponding to the modeled solar
convection zone. These properties are conducive to the growth and
development of two-dimensional instabilities that are otherwise
suppressed by stronger dissipation. We find evidence for the action
of a non-axisymmetric magnetoshear instability operating in the upper
portions of the stably stratified fluid layers. We also investigate
the possibility that the Tayler instability may be contributing to
the destabilization of the large-scale axisymmetric magnetic component
at high latitudes. On the basis of our analyses, we propose a global
dynamo scenario whereby the magnetic cycle is driven primarily by
turbulent dynamo action in the convecting layers, but MHD instabilities
accelerate the dissipation of the magnetic field pumped down into the
overshoot and stable layers, thus perhaps significantly influencing
the magnetic cycle period. Support for this scenario is found in the
distinct global dynamo behaviors observed in an otherwise identical
EULAG-MHD simulations, using a different degree of sub-adiabaticity
in the stable fluid layers underlying the convection zone.
---------------------------------------------------------
Title: A comparison of stratospheric photochemical response to
different reconstructions of solar ultraviolet radiative variability
Authors: Bolduc, Cassandra; Bourqui, Michel S.; Charbonneau, Paul
2015JASTP.132...22B Altcode:
We present calculations of stratospheric chemical abundances
variations between different levels of solar activity using a simple
photochemistry model in transient chemistry mode. Different models
for the reconstruction of the solar spectrum, as well as observations
from the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) and
Spectral Irradiance Monitor (SIM) on the SOlar Radiation and Climate
Experiment (SORCE) satellite, are used as inputs to the calculations. We
put the emphasis on the MOnte CArlo Spectral Solar Irradiance Model
(MOCASSIM) reconstructions, which cover the spectral interval from
150 to 400 nm and extend from 1610 to present. We compare our results
with those obtained with the Naval Research Laboratory Solar Spectral
Irradiance (NRLSSI) model as well as with the Magnesium-Neutron Monitor
(MGNM) model over a period of time spanning the ascending phase of Cycle
22. We also perform the calculations using SORCE composite spectra for
the descending phase of Cycle 23 and with the reconstructed MOCASSIM,
NRLSSI and MGNM spectra for the same period for comparison. Finally,
we compare the chemical abundances obtained for the Maunder Minimum
with those obtained for the Cycle 23 minimum (in March 2009) and find
that stratospheric ozone concentration was slightly higher during the
recent minimum, consequent to the small positive variability between
the MOCASSIM spectra for both epochs, especially below 260 nm. We
find that the response in stratospheric ozone is not only dependent
on the variability amplitude in the solar spectrum (especially in
the 200-240 nm band), but also significantly on the base level of the
minimum solar spectrum.
---------------------------------------------------------
Title: A Coupled 2 × 2D Babcock-Leighton Solar Dynamo
Model. I. Surface Magnetic Flux Evolution
Authors: Lemerle, Alexandre; Charbonneau, Paul; Carignan-Dugas, Arnaud
2015ApJ...810...78L Altcode: 2015arXiv151108548L
The need for reliable predictions of the solar activity cycle motivates
the development of dynamo models incorporating a representation of
surface processes sufficiently detailed to allow assimilation of
magnetographic data. In this series of papers we present one such
dynamo model, and document its behavior and properties. This first
paper focuses on one of the model’s key components, namely surface
magnetic flux evolution. Using a genetic algorithm, we obtain best-fit
parameters of the transport model by least-squares minimization of the
differences between the associated synthetic synoptic magnetogram and
real magnetographic data for activity cycle 21. Our fitting procedure
also returns Monte Carlo-like error estimates. We show that the range
of acceptable surface meridional flow profiles is in good agreement
with Doppler measurements, even though the latter are not used in
the fitting process. Using a synthetic database of bipolar magnetic
region (BMR) emergences reproducing the statistical properties of
observed emergences, we also ascertain the sensitivity of global cycle
properties, such as the strength of the dipole moment and timing of
polarity reversal, to distinct realizations of BMR emergence, and on
this basis argue that this stochasticity represents a primary source
of uncertainty for predicting solar cycle characteristics.
---------------------------------------------------------
Title: Predicting large solar flares with data assimilation
Authors: Strugarek, Antoine; Charbonneau, Paul
2015IAUGA..2247834S Altcode:
Solar and stellar flares are magnetically-driven, scale-invariant
energy release events spanning over 8 orders of magnitude in
energy. The prediction of the largest solar flares, of class X,
is a particularly hard task due the scarcity of such events. The
detailed 3D modelling of flaring active regions still requires today
too much numerical resources to be routinely used for near real-time
predictions. Alternative, empirical models hence have to be designed to
perform such predictions. Among the models that adequately reproduce
the power-law distribution in flare sizes, avalanche models have
the advantage of being numerically cheap to operate. However, they
usually rely on a stochastic driver, which can be expected to degrade
their predictive capabilities. Building on the pioneering work of Lu
and Hamilton, we develop a class of avalanche models which succeed
in minimizing the built-in stochastic ingredients while retaining
the solar flares power-law distribution. We show that the largest
avalanches occurring in these models are robust with respect to the
stochastic realization, which opens new perspectives for the prediction
of the largest (and most dangerous) solar flares.We further combine
data assimilation of the GOES X-ray flux with our avalanche models
to carry out actual predictions. The GOES X-ray flux is transformed
into a series of peaks that is fed to the model, which automatically
finds an initial condition that is compatible with the observed series
of events. We then test our prediction model against past GOES large
events and discuss the possibility to use our data assimilation package
in near real-time applications.
---------------------------------------------------------
Title: Deep-seated dynamo-driven modulation of solar and stellar
luminosities
Authors: Charbonneau, Paul; Cossette, Jean-François; Smolarkiewicz,
Piotr
2015IAUGA..2255942C Altcode:
Cyclic photometric variations observed on solar-type stars are usually
ascribed to dynamo-driven magnetic cycles producing variations in the
photospheric coverage of magnetic structures having a photometric
contrast different from the quiet, unmagnetized photosphere. This
idea is well-supported by solar observations and attendant modelling,
which have shown that over 95% of the observed irradiance variability
on short to mid-timescales (hours to months) can be reproduced by
models. Yet another possible source of irradiance variability on
longer timescales resides with the interference of the dynamo magnetic
field with convective energy transport. This idea is supported by
helioseismology, which detect subphotospheric sound speed (temperature)
changes varying in phase with the magnetic cycle. In this talk I will
present recent result from magnetohydrodynamical numerical simulations
of solar convection in which a regular magnetic cycle develops, and
drives modulation of convective energy transport. Analysis of the
simulation indicates that this modulation is associated with changes
in the tails of the convective flux distribution, i.e., “hotspots”
associated with persistent upflow and downflow structures spanning
a significant fraction of the domain. The resulting non-local energy
transport cannot be captured by mixing-length-type formulations based
on the diffusion approximation.
---------------------------------------------------------
Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2015IAUTB..28..106V Altcode:
The Business Meeting of Commission 10 was held as part of the Business
Meeting of Division II (Sun and Heliosphere), chaired by Valentin
Martínez-Pillet, the President of the Division. The President of
Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
the chair for the business meeting of C10. She summarised the activities
of C10 over the triennium and the election of the incoming OC.
---------------------------------------------------------
Title: Inferring the Structure of the Solar Corona and Inner
Heliosphere During the Maunder Minimum Using Global Thermodynamic
Magnetohydrodynamic Simulations
Authors: Riley, Pete; Lionello, Roberto; Linker, Jon A.; Cliver,
Ed; Balogh, Andre; Beer, Jürg; Charbonneau, Paul; Crooker, Nancy;
DeRosa, Marc; Lockwood, Mike; Owens, Matt; McCracken, Ken; Usoskin,
Ilya; Koutchmy, S.
2015ApJ...802..105R Altcode:
Observations of the Sun’s corona during the space era have led to
a picture of relatively constant, but cyclically varying solar output
and structure. Longer-term, more indirect measurements, such as from
<SUP>10</SUP>Be, coupled by other albeit less reliable contemporaneous
reports, however, suggest periods of significant departure from this
standard. The Maunder Minimum was one such epoch where: (1) sunspots
effectively disappeared for long intervals during a 70 yr period; (2)
eclipse observations suggested the distinct lack of a visible K-corona
but possible appearance of the F-corona; (3) reports of aurora were
notably reduced; and (4) cosmic ray intensities at Earth were inferred
to be substantially higher. Using a global thermodynamic MHD model,
we have constructed a range of possible coronal configurations for the
Maunder Minimum period and compared their predictions with these limited
observational constraints. We conclude that the most likely state of the
corona during—at least—the later portion of the Maunder Minimum was
not merely that of the 2008/2009 solar minimum, as has been suggested
recently, but rather a state devoid of any large-scale structure,
driven by a photospheric field composed of only ephemeral regions,
and likely substantially reduced in strength. Moreover, we suggest
that the Sun evolved from a 2008/2009-like configuration at the start
of the Maunder Minimum toward an ephemeral-only configuration by the
end of it, supporting a prediction that we may be on the cusp of a
new grand solar minimum.
---------------------------------------------------------
Title: Meridional Circulation Dynamics from 3D Magnetohydrodynamic
Global Simulations of Solar Convection
Authors: Passos, Dário; Charbonneau, Paul; Miesch, Mark
2015ApJ...800L..18P Altcode: 2015arXiv150201154P
The form of solar meridional circulation is a very important ingredient
for mean field flux transport dynamo models. However, a shroud
of mystery still surrounds this large-scale flow, given that its
measurement using current helioseismic techniques is challenging. In
this work, we use results from three-dimensional global simulations of
solar convection to infer the dynamical behavior of the established
meridional circulation. We make a direct comparison between the
meridional circulation that arises in these simulations and the latest
observations. Based on our results, we argue that there should be an
equatorward flow at the base of the convection zone at mid-latitudes,
below the current maximum depth helioseismic measures can probe
(0.75 {{R}<SUB>⊙ </SUB>}). We also provide physical arguments to
justify this behavior. The simulations indicate that the meridional
circulation undergoes substantial changes in morphology as the magnetic
cycle unfolds. We close by discussing the importance of these dynamical
changes for current methods of observation which involve long averaging
periods of helioseismic data. Also noteworthy is the fact that these
topological changes indicate a rich interaction between magnetic fields
and plasma flows, which challenges the ubiquitous kinematic approach
used in the vast majority of mean field dynamo simulations.
---------------------------------------------------------
Title: Hemispheric Coupling: Comparing Dynamo Simulations and
Observations
Authors: Norton, A. A.; Charbonneau, P.; Passos, D.
2015sac..book..251N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flux Transport Dynamos: From Kinematics to Dynamics
Authors: Karak, Bidya Binay; Jiang, Jie; Miesch, Mark S.; Charbonneau,
Paul; Choudhuri, Arnab Rai
2015sac..book..561K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamo Action and Meridional Circulation Dynamics in Eulag-MHD
Global 3D MHD Simulations of Solar Convection
Authors: Passos, D. M. D. C.; Charbonneau, P.
2014AGUFMSH44A..04P Altcode:
The steady advance in computer power has finally enabled us to explore
the solar dynamo problem by means of 3D global magnetohydrodynamical
(MHD) simulations of the convection zone.Using the EULAG-MHD code,
we have succeeded in producing simulations of the Sun's magnetic
activity cycles that resemble the observed evolutionary patterns
of the large-scale solar magnetic field. In these simulations the
anelastic ideal MHD equations are solved in a thick, rotating shell
of electrically conducting fluid, under solar-like stratification and
thermal forcing. Since these simulations are fully dynamical in all
time and spatial resolved scales, they achieve highly turbulent regimes
and naturally produce variable amplitude solutions.We have recently
been able to produce a simulation that spans for 1650 years and that
produced 40 complete sunspot like cycles, the longest of its kind so
far.This allows to perform statistical studies and establish direct
comparisons with the observed solar cycle. Some of the main similarities
and differences between the statistical properties of simulated and
observed cycles are presented here (e.g. evidence for Gnevyshev-Ohl
patterns, Gleissberg modulation or hemispheric coupling). Additionally,
by studying the behaviour of the large scale flows in the simulation
(differential rotation and meridional circulation) we also find
evidence for solar cycle modulation of the deep equatorward flow in
the meridional circulation. This result is briefly discussed as well as
its implications for current helioseismic measurement methodologies and
for classical kinematic mean-field flux transport dynamo simulations.
---------------------------------------------------------
Title: Solar Spectral Irradiance Variability in November/December
2012: Comparison of Observations by Instruments on the International
Space Station and Models
Authors: Thuillier, G.; Schmidtke, G.; Erhardt, C.; Nikutowski, B.;
Shapiro, A. I.; Bolduc, C.; Lean, J.; Krivova, N.; Charbonneau, P.;
Cessateur, G.; Haberreiter, M.; Melo, S.; Delouille, V.; Mampaey,
B.; Yeo, K. L.; Schmutz, W.
2014SoPh..289.4433T Altcode: 2014SoPh..tmp..120T
Onboard the International Space Station (ISS), two instruments
are observing the solar spectral irradiance (SSI) at wavelengths
from 16 to 2900 nm. Although the ISS platform orientation generally
precludes pointing at the Sun more than 10 - 14 days per month, in
November/December 2012 a continuous period of measurements was obtained
by implementing an ISS `bridging' maneuver. This enabled observations to
be made of the solar spectral irradiance (SSI) during a complete solar
rotation. We present these measurements, which quantify the impact
of active regions on SSI, and compare them with data simultaneously
gathered from other platforms, and with models of spectral irradiance
variability. Our analysis demonstrates that the instruments onboard the
ISS have the capability to measure SSI variations consistent with other
instruments in space. A comparison among all available SSI measurements
during November-December 2012 in absolute units with reconstructions
using solar proxies and observed solar activity features is presented
and discussed in terms of accuracy.
---------------------------------------------------------
Title: Flux Transport Dynamos: From Kinematics to Dynamics
Authors: Karak, Bidya Binay; Jiang, Jie; Miesch, Mark S.; Charbonneau,
Paul; Choudhuri, Arnab Rai
2014SSRv..186..561K Altcode: 2014SSRv..tmp...55K
Over the past several decades, Flux-Transport Dynamo (FTD) models
have emerged as a popular paradigm for explaining the cyclic nature of
solar magnetic activity. Their defining characteristic is the key role
played by the mean meridional circulation in transporting magnetic
flux and thereby regulating the cycle period. Most FTD models also
incorporate the so-called Babcock-Leighton (BL) mechanism in which
the mean poloidal field is produced by the emergence and subsequent
dispersal of bipolar active regions. This feature is well grounded
in solar observations and provides a means for assimilating observed
surface flows and fields into the models in order to forecast future
solar activity, to identify model biases, and to clarify the underlying
physical processes. Furthermore, interpreting historical sunspot records
within the context of FTD models can potentially provide insight into
why cycle features such as amplitude and duration vary and what causes
extreme events such as Grand Minima. Though they are generally robust
in a modeling sense and make good contact with observed cycle features,
FTD models rely on input physics that is only partially constrained by
observation and that neglects the subtleties of convective transport,
convective field generation, and nonlinear feedbacks. Here we review
the formulation and application of FTD models and assess our current
understanding of the input physics based largely on complementary 3D
MHD simulations of solar convection, dynamo action, and flux emergence.
---------------------------------------------------------
Title: Hemispheric Coupling: Comparing Dynamo Simulations and
Observations
Authors: Norton, A. A.; Charbonneau, P.; Passos, D.
2014SSRv..186..251N Altcode: 2014arXiv1411.7052N; 2014SSRv..tmp...51N
Numerical simulations that reproduce solar-like magnetic cycles can be
used to generate long-term statistics. The variations in north-south
hemispheric solar cycle synchronicity and amplitude produced
in simulations has not been widely compared to observations. The
observed limits on solar cycle amplitude and phase asymmetry show that
hemispheric sunspot area production is no more than 20 % asymmetric for
cycles 17-23 and that phase lags do not exceed 20 % (or two years) of
the total cycle period, as determined from Royal Greenwich Observatory
sunspot data. Several independent studies have found a long-term trend
in phase values as one hemisphere leads the other for, on average, four
cycles. Such persistence in phase is not indicative of a stochastic
phenomenon. We compare these observational findings to the magnetic
cycle found in a numerical simulation of solar convection recently
produced with the EULAG-MHD model. This long "millennium simulation"
spans more than 1600 years and generated 40 regular, sunspot-like
cycles. While the simulated cycle length is too long (∼40 yrs) and
the toroidal bands remain at too high of latitudes (>30°), some
solar-like aspects of hemispheric asymmetry are reproduced. The model
is successful at reproducing the synchrony of polarity inversions and
onset of cycle as the simulated phase lags do not exceed 20 % of the
cycle period. The simulated amplitude variations between the north and
south hemispheres are larger than those observed in the Sun, some up
to 40 %. An interesting note is that the simulations also show that
one hemisphere can persistently lead the other for several successive
cycles, placing an upper bound on the efficiency of transequatorial
magnetic coupling mechanisms. These include magnetic diffusion,
cross-equatorial mixing within latitudinally-elongated convective
rolls (a.k.a. "banana cells") and transequatorial meridional flow
cells. One or more of these processes may lead to magnetic flux
cancellation whereby the oppositely directed fields come in close
proximity and cancel each other across the magnetic equator late
in the solar cycle. We discuss the discrepancies between model and
observations and the constraints they pose on possible mechanisms of
hemispheric coupling.
---------------------------------------------------------
Title: Predictive Capabilities of Avalanche Models for Solar Flares
Authors: Strugarek, A.; Charbonneau, P.
2014SoPh..289.4137S Altcode: 2014arXiv1406.6523S; 2014SoPh..tmp..110S
We assess the predictive capabilities of various classes of
avalanche models for solar flares. We demonstrate that avalanche
models cannot generally be used to predict specific events because
of their high sensitivity to the embedded stochastic process. We show
that deterministically driven models can nevertheless alleviate this
caveat and be efficiently used for predictions of large events. Our
results suggest a new approach for predictions of large (typically
X-class) solar flares based on simple and computationally inexpensive
avalanche models.
---------------------------------------------------------
Title: Solar Photospheric Network Properties and Their Cycle Variation
Authors: Thibault, K.; Charbonneau, P.; Béland, M.
2014ApJ...796...19T Altcode:
We present a numerical simulation of the formation and evolution
of the solar photospheric magnetic network over a full solar
cycle. The model exhibits realistic behavior as it produces large,
unipolar concentrations of flux in the polar caps, a power-law flux
distribution with index -1.69, a flux replacement timescale of 19.3 hr,
and supergranule diameters of 20 Mm. The polar behavior is especially
telling of model accuracy, as it results from lower-latitude activity,
and accumulates the residues of any potential modeling inaccuracy
and oversimplification. In this case, the main oversimplification
is the absence of a polar sink for the flux, causing an amount of
polar cap unsigned flux larger than expected by almost one order of
magnitude. Nonetheless, our simulated polar caps carry the proper signed
flux and dipole moment, and also show a spatial distribution of flux
in good qualitative agreement with recent high-latitude magnetographic
observations by Hinode. After the last cycle emergence, the simulation
is extended until the network has recovered its quiet Sun initial
condition. This permits an estimate of the network relaxation time
toward the baseline state characterizing extended periods of suppressed
activity, such as the Maunder Grand Minimum. Our simulation results
indicate a network relaxation time of 2.9 yr, setting 2011 October
as the soonest the time after which the last solar activity minimum
could have qualified as a Maunder-type Minimum. This suggests that
photospheric magnetism did not reach its baseline state during the
recent extended minimum between cycles 23 and 24.
---------------------------------------------------------
Title: Deterministically Driven Avalanche Models of Solar Flares
Authors: Strugarek, Antoine; Charbonneau, Paul; Joseph, Richard;
Pirot, Dorian
2014SoPh..289.2993S Altcode: 2014arXiv1402.4730S; 2014SoPh..tmp...43S
We develop and discuss the properties of a new class of lattice-based
avalanche models of solar flares. These models are readily amenable to a
relatively unambiguous physical interpretation in terms of slow twisting
of a coronal loop. They share similarities with other avalanche models,
such as the classical stick-slip self-organized critical model of
earthquakes, in that they are driven globally by a fully deterministic
energy-loading process. The model design leads to a systematic deficit
of small-scale avalanches. In some portions of model space, mid-size and
large avalanching behavior is scale-free, being characterized by event
size distributions that have the form of power-laws with index values,
which, in some parameter regimes, compare favorably to those inferred
from solar EUV and X-ray flare data. For models using conservative
or near-conservative redistribution rules, a population of large,
quasiperiodic avalanches can also appear. Although without direct
counterparts in the observational global statistics of flare energy
release, this latter behavior may be relevant to recurrent flaring in
individual coronal loops. This class of models could provide a basis
for the prediction of large solar flares.
---------------------------------------------------------
Title: A Reconstruction of Ultraviolet Spectral Irradiance During
the Maunder Minimum
Authors: Bolduc, C.; Charbonneau, P.; Barnabé, R.; Bourqui, M. S.
2014SoPh..289.2891B Altcode: 2014SoPh..tmp...41B
We present a reconstruction of the solar spectrum in the near and
mid-ultraviolet spectral range during the Maunder Minimum, a period of
strongly suppressed magnetic activity spanning the second half of the
17th century. This spectral reconstruction is based on an extension
of the Monte Carlo Solar Spectral Irradiance Model (MOCASSIM). The
new version of the model, documented in this paper, extends its
spectral range down to 150 nm, its temporal range back to 1610,
includes a secular modulation of the quiet-Sun emissivity based on
a total solar irradiance reconstruction, and uses the Atmospheric
Laboratory for Applications and Science-3 (ATLAS-3) spectrum as a
reconstruction baseline. The model is validated against the ATLAS-1
spectrum for 29 March 1992, showing a general agreement varying
from ∼ 1 % in the 300 - 400 nm range, up to 3 - 5 % below 200 nm,
the largest discrepancies occurring in emission lines formed in the
chromosphere and transition region. We also reconstruct ultraviolet
spectra for May 2008 and March 2009, spanning the extended phase of
low activity separating Cycles 23 and 24. Our results suggest that
despite the unusually long temporal extent of this activity minimum,
the ultraviolet emission still remained slightly higher than during the
Maunder Minimum, due to the lingering presence of decay products from
active regions having emerged in the late descending phase of Cycle 23.
---------------------------------------------------------
Title: Solar Dynamo Theory
Authors: Charbonneau, Paul
2014ARA&A..52..251C Altcode:
The Sun's magnetic field is the engine and energy source driving all
phenomena collectively defining solar activity, which in turn structures
the whole heliosphere and significantly impacts Earth's atmosphere down
at least to the stratosphere. The solar magnetic field is believed to
originate through the action of a hydromagnetic dynamo process operating
in the Sun's interior, where the strongly turbulent environment of
the convection zone leads to flow-field interactions taking place on
an extremely wide range of spatial and temporal scales. Following a
necessarily brief observational overview of the solar magnetic field and
its cycle, this review on solar dynamo theory is structured around three
areas in which significant advances have been made in recent years:
(a) global magnetohydrodynamical simulations of convection and magnetic
cycles, (b) the turbulent electromotive force and the dynamo saturation
problem, and (c) flux transport dynamos, and their application to model
cycle fluctuations and grand minima and to carry out cycle prediction.
---------------------------------------------------------
Title: Latest results from global 3D MHD simulations of solar
convection and dynamo action
Authors: Passos, Dário; Charbonneau, Paul
2014pnaa.conf...37P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Characteristics of magnetic solar-like cycles in a 3D MHD
simulation of solar convection
Authors: Passos, D.; Charbonneau, P.
2014A&A...568A.113P Altcode:
We analyse the statistical properties of the stable magnetic cycle
unfolding in an extended 3D magnetohydrodynamic simulation of solar
convection produced with the EULAG-MHD code. The millennium simulation
spans over 1650 years, in the course of which forty polarity reversals
take place on a regular ~40 yr cadence, remaining well-synchronized
across solar hemispheres. In order to characterize this cycle and
facilitate its comparison with measures typically used to represent
solar activity, we build two proxies for the magnetic field in
the simulation mimicking the solar toroidal field and the polar
radial field. Several quantities that characterize the cycle are
measured (period, amplitudes, etc.) and correlations between them are
computed. These are then compared with their observational analogs. From
the typical Gnevyshev-Ohl pattern, to hints of Gleissberg modulation,
the simulated cycles share many of the characteristics of their
observational analogs even though the simulation lacks poloidal field
regeneration through active region decay, a mechanism nowadays often
considered an essential component of the solar dynamo. Some significant
discrepancies are also identified, most notably the in-phase variation
of the simulated poloidal and toroidal large-scale magnetic components,
and the low degree of hemispheric coupling at the level of hemispheric
cycle amplitudes. Possible causes underlying these discrepancies
are discussed. <P />Appendix is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423700/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Analysis of Different Solar Spectral Irradiance Reconstructions
and Their Impact on Solar Heating Rates
Authors: Thuillier, G.; Melo, S. M. L.; Lean, J.; Krivova, N. A.;
Bolduc, C.; Fomichev, V. I.; Charbonneau, P.; Shapiro, A. I.; Schmutz,
W.; Bolsée, D.
2014SoPh..289.1115T Altcode:
Proper numerical simulation of the Earth's climate change requires
reliable knowledge of solar irradiance and its variability on
different time scales, as well as the wavelength dependence of this
variability. As new measurements of the solar spectral irradiance have
become available, so too have new reconstructions of historical solar
irradiance variations, based on different approaches. However, these
various solar spectral irradiance reconstructions have not yet been
compared in detail to quantify differences in their absolute values,
variability, and implications for climate and atmospheric studies. In
this paper we quantitatively compare five different reconstructions
of solar spectral irradiance changes during the past four centuries,
in order to document and analyze their differences. The impact on
atmosphere and climate studies is discussed in terms of the calculation
of short wave solar heating rates.
---------------------------------------------------------
Title: Spörer, Friedrich Wilhelm Gustav
Authors: Charbonneau, Paul
2014bea..book.2043C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Fabricius, Johann
Authors: Charbonneau, Paul
2014bea..book..690C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Wolf, Johann Rudolf
Authors: Charbonneau, Paul
2014bea..book.2366C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stewart, Balfour
Authors: Charbonneau, Paul
2014bea..book.2057C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Wassenius, Birger
Authors: Charbonneau, Paul
2014bea..book.2293C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Kirchhoff, Gustav Robert
Authors: Charbonneau, Paul
2014bea..book.1220C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Cyclic Thermal Signature in a Global MHD Simulation of Solar
Convection
Authors: Cossette, Jean-Francois; Charbonneau, Paul; Smolarkiewicz,
Piotr K.
2013ApJ...777L..29C Altcode:
Global magnetohydrodynamical simulations of the solar convection
zone have recently achieved cyclic large-scale axisymmetric magnetic
fields undergoing polarity reversals on a decadal time scale. In
this Letter, we show that these simulations also display a thermal
convective luminosity that varies in-phase with the magnetic cycle,
and trace this modulation to deep-seated magnetically mediated changes
in convective flow patterns. Within the context of the ongoing debate
on the physical origin of the observed 11 yr variations in total solar
irradiance, such a signature supports the thesis according to which all,
or part, of the variations on decadal time scales and longer could
be attributed to a global modulation of the Sun's internal thermal
structure by magnetic activity.
---------------------------------------------------------
Title: Stellar Dynamos and Cycles from Numerical Simulations of
Convection
Authors: Dubé, Caroline; Charbonneau, Paul
2013ApJ...775...69D Altcode:
We present a series of kinematic axisymmetric mean-field αΩ dynamo
models applicable to solar-type stars, for 20 distinct combinations of
rotation rates and luminosities. The internal differential rotation
and kinetic helicity profiles required to calculate source terms
in these dynamo models are extracted from a corresponding series of
global three-dimensional hydrodynamical simulations of solar/stellar
convection, so that the resulting dynamo models end up involving only
one free parameter, namely, the turbulent magnetic diffusivity in
the convecting layers. Even though the αΩ dynamo solutions exhibit
a broad range of morphologies, and sometimes even double cycles,
these models manage to reproduce relatively well the observationally
inferred relationship between cycle period and rotation rate. On the
other hand, they fail in capturing the observed increase of magnetic
activity levels with rotation rate. This failure is due to our use of a
simple algebraic α-quenching formula as the sole amplitude-limiting
nonlinearity. This suggests that α-quenching is not the primary
mechanism setting the amplitude of stellar magnetic cycles, with
magnetic reaction on large-scale flows emerging as the more likely
candidate. This inference is coherent with analyses of various recent
global magnetohydrodynamical simulations of solar/stellar convection.
---------------------------------------------------------
Title: Where is the solar dynamo?
Authors: Charbonneau, Paul
2013JPhCS.440a2014C Altcode:
In this paper I review results from recent global magnetohydrodynamical
numerical simulations of solar convection, as a springboard to
address the question "Where is the solar dynamo". I first describe
and contrast similarities and differences in the large-scale flows and
magnetic fields such simulations can produce, with emphasis on polarity
reversals (or lack thereof) in the large-scale magnetic components they
generate. On the basis of these simulation results, I argue that some
of the significant differences in the spatiotemporal evolution of the
large-scale magnetic field can be traced to the competing effects of
turbulent electromotive forces and induction by large-scale flows,
whose mutual near-cancellation in the nonlinearly saturated regime
leads to a high sensitivity to the numerical/physical treatment of
small scales. Some of these recent simulation results also reopen
the possibility that dynamo action driving the solar activity cycle
may reside entirely within the convection zone, with the tachocline
perhaps playing a lesser role than has been assumed in the last two
decades. On the other hand, other subsets of simulations suggest that
magnetohydrodynamical processes taking place within the tachocline
may have a significant impact on timescales comparable to or longer
than the primary cycle.
---------------------------------------------------------
Title: Magnetohydrodynamic Simulation-driven Kinematic Mean Field
Model of the Solar Cycle
Authors: Simard, Corinne; Charbonneau, Paul; Bouchat, Amélie
2013ApJ...768...16S Altcode:
We construct a series of kinematic axisymmetric mean-field dynamo models
operating in the αΩ, α<SUP>2</SUP>Ω and α<SUP>2</SUP> regimes, all
using the full α-tensor extracted from a global magnetohydrodynamical
simulation of solar convection producing large-scale magnetic fields
undergoing solar-like cyclic polarity reversals. We also include
an internal differential rotation profile produced in a purely
hydrodynamical parent simulation of solar convection, and a simple
meridional flow profile described by a single cell per meridional
quadrant. An α<SUP>2</SUP>Ω mean-field model, presumably closest to
the mode of dynamo action characterizing the MHD simulation, produces
a spatiotemporal evolution of magnetic fields that share some striking
similarities with the zonally-averaged toroidal component extracted
from the simulation. Comparison with α<SUP>2</SUP> and αΩ mean-field
models operating in the same parameter regimes indicates that much
of the complexity observed in the spatiotemporal evolution of the
large-scale magnetic field in the simulation can be traced to the
turbulent electromotive force. Oscillating α<SUP>2</SUP> solutions
are readily produced, and show some similarities with the observed
solar cycle, including a deep-seated toroidal component concentrated
at low latitudes and migrating equatorward in the course of the solar
cycle. Various numerical experiments performed using the mean-field
models reveal that turbulent pumping plays an important role in setting
the global characteristics of the magnetic cycles.
---------------------------------------------------------
Title: Torsional Oscillations in a Global Solar Dynamo
Authors: Beaudoin, P.; Charbonneau, P.; Racine, E.; Smolarkiewicz,
P. K.
2013SoPh..282..335B Altcode: 2012arXiv1210.1209B
We characterize and analyze rotational torsional oscillations developing
in a large-eddy magnetohydrodynamical simulation of solar convection
(Ghizaru, Charbonneau, and Smolarkiewicz, Astrophys. J. Lett.715,
L133, 2010; Racine et al., Astrophys. J.735, 46, 2011) producing an
axisymmetric, large-scale, magnetic field undergoing periodic polarity
reversals. Motivated by the many solar-like features exhibited by
these oscillations, we carry out an analysis of the large-scale zonal
dynamics. We demonstrate that simulated torsional oscillations are
not driven primarily by the periodically varying large-scale magnetic
torque, as one might have expected, but rather via the magnetic
modulation of angular-momentum transport by the large-scale meridional
flow. This result is confirmed by a straightforward energy analysis. We
also detect a fairly sharp transition in rotational dynamics taking
place as one moves from the base of the convecting layers to the
base of the thin tachocline-like shear layer formed in the stably
stratified fluid layers immediately below. We conclude by discussing
the implications of our analyses with regard to the mechanism of
amplitude saturation in the global dynamo operating in the simulation,
and speculate on the possible precursor value of torsional oscillations
for the forecast of solar-cycle characteristics.
---------------------------------------------------------
Title: Stellar Dynamos
Authors: Charbonneau, Paul
2013SAAS...39..187C Altcode: 2013SASS...39..187C
This chapter steps finally away from the sun and towards the stars, the
idea being to apply the physical insight gained so far to see how much
of stellar magnetism can be understood in terms of dynamo action. Dynamo
action in the convective core of massive main-sequence stars is first
considered and shown viable. For intermediate-mass main-sequence stars
the fossil field hypothesis will carry the day, although possible dynamo
alternatives are also briefly discussed. The extension of the solar
dynamo models investigated in Chap. 3 (10.1007/978-3-642-32093-4_3)
to other solar-type stars will first take us through an important
detour in first having to understand rotational evolution in response
to angular momentum loss in a magnetized wind. Dynamo action in fully
convective stars comes next, and the chapter closes with an overview
of the situation for pre- and post-main-sequence stars and compact
objects, leading finally to the magnetic fields of galaxies and beyond.
---------------------------------------------------------
Title: Magnetohydrodynamics
Authors: Charbonneau, Paul
2013SAAS...39....1C Altcode: 2013SASS...39....1C
This chapters establishes the physical and mathematical bases
of magnetohydrodynamics, the theory describing the dynamical
interactions of the flow of an electrically conducting fluid with
a magnetic field. After an overview of classical hydrodynamics, the
magnetohydrodynamical induction equation is derived from Maxwell's
equations, together with the volumetric form of the magnetic Lorentz
force applicable to a fluid. Various useful concepts such as magnetic
energy, helicity, vector potential, flux-freezing and force-free
magnetic fields are also introduced and discussed. The chapter closes
with a simple example of a mechanical dynamo, and a discussion of the
astrophysical dynamo problem in general terms.
---------------------------------------------------------
Title: Decay and Amplification of Magnetic Fields
Authors: Charbonneau, Paul
2013SAAS...39...37C Altcode: 2013SASS...39...37C
This chapter presents a series of very simple flows that can, or cannot,
act as dynamos. The journey begins with magnetic field decay by Ohmic
dissipation in the absence of flows, followed by magnetic amplication
by stretching and shearing in the absence of dissipation. The two
processes are then merged in discussing a series of ever more complex
flows, some acting as dynamo and others not, the distinction being
eventually understood through the help of anti-dynamo theorems. The
last two flows considered, the Roberts cell and CP flow, are used
to establish the distinction between fast and slow dynamo, and as
interpretative tool f or a brief look at dynamo action in numerical
simulation of MHD turbulence.
---------------------------------------------------------
Title: Fluctuations, Intermittency and Predictivity
Authors: Charbonneau, Paul
2013SAAS...39..153C Altcode: 2013SASS...39..153C
This chapter considers the various mechanisms capable of producing
amplitude and duration variations in the various dynamo models
introduced in Chap. 3 (10.1007/978-3-642-32093-4_3). After a survey
of observed and inferred fluctuation patterns of the solar cycle,
the effects on the basic cycle of stochastic forcing, dynamical
nonlinearities and time delay are considered in turn. The occurrence
of intermittency in a subset of these models is then investigated,
with an eye on explaining Grand Minima observed in the solar activity
record. The chapter closes with a brief discussion of solar cycle
prediction schemes based on dynamo models.
---------------------------------------------------------
Title: Solar physics: The planetary hypothesis revived
Authors: Charbonneau, Paul
2013Natur.493..613C Altcode:
The Sun's magnetic activity varies cyclically over a period of about
11 years. An analysis of a new, temporally extended proxy record of
this activity hints at a possible planetary influence on the amplitude
of the cycle.
---------------------------------------------------------
Title: Dynamo Models of the Solar Cycle
Authors: Charbonneau, Paul
2013SAAS...39...87C Altcode: 2013SASS...39...87C
This chapter details a series of dynamo models applicable to the sun
and solar-type stars. After introducing the theoretical framework
known as mean-field electrodynamics, a series of increasingly complex
dynamo models are constructed, with the primary aim of reproducing the
various basic observed characteristics of the solar magnetic activity
cycle. Global and local magnetohydrodynamcial simulations of solar
convection, and dynamo action therein, are also considered, and the
resulting magnetic cycles compared and contrasted to those obtained in
the simpler dynamo models. The focus throughout the chapter is on the
sun, simply because the amount of available observational material on
the solar magnetic field and its cycle dwarfs anything else in the
astrophysical realm, in terms of spatial and temporal resolution,
sensitivity, and time span.
---------------------------------------------------------
Title: Solar and Stellar Dynamos
Authors: Charbonneau, Paul
2013SAAS...39.....C Altcode: 2013SASS...39.....C
No abstract at ADS
---------------------------------------------------------
Title: The Buildup of a Scale-free Photospheric Magnetic Network
Authors: Thibault, K.; Charbonneau, P.; Crouch, A. D.
2012ApJ...757..187T Altcode:
We use a global Monte Carlo simulation of the formation of the solar
photospheric magnetic network to investigate the origin of the scale
invariance characterizing magnetic flux concentrations visible on
high-resolution magnetograms. The simulations include spatially and
temporally homogeneous injection of small-scale magnetic elements
over the whole photosphere, as well as localized episodic injection
associated with the emergence and decay of active regions. Network
elements form in response to cumulative pairwise aggregation or
cancellation of magnetic elements, undergoing a random walk on the
sphere and advected on large spatial scales by differential rotation
and a poleward meridional flow. The resulting size distribution of
simulated network elements is in very good agreement with observational
inferences. We find that the fractal index and size distribution of
network elements are determined primarily by these post-emergence
surface mechanisms, and carry little or no memory of the scales at
which magnetic flux is injected in the simulation. Implications for
models of dynamo action in the Sun are briefly discussed.
---------------------------------------------------------
Title: A Fast Model for the Reconstruction of Spectral Solar
Irradiance in the Near- and Mid-Ultraviolet
Authors: Bolduc, C.; Charbonneau, P.; Dumoulin, V.; Bourqui, M. S.;
Crouch, A. D.
2012SoPh..279..383B Altcode:
We present a model for the reconstruction of spectral solar irradiance
between 200 and 400 nm. This model is an extension of the total solar
irradiance (TSI) model of Crouch et al. (Astrophys. J.677, 723, 2008)
which is based on a data-driven Monte Carlo simulation of sunspot
emergence, fragmentation, and erosion. The resulting time-evolving
daily area distribution of magnetic structures of all sizes is used as
input to a four-component irradiance model including contributions from
the quiet Sun, sunspots, faculae, and network. In extending the model
to spectral irradiance in the near- and mid-ultraviolet, the quiet
Sun and sunspot emissivities are calculated from synthetic spectra at
T<SUB>eff</SUB>=5750 K and 5250 K, respectively. Facular emissivities
are calculated using a simple synthesis procedure proposed by Solanki
and Unruh (Astron. Astrophys.329, 747, 1998). The resulting time series
of ultraviolet flux is calibrated against the data from the SOLSTICE
instrument on the Upper Atmospheric Research Satellite (UARS). Using a
genetic algorithm, we invert quiet Sun corrections, profile of facular
temperature variations with height, and network model parameters which
yield the best fit to these data. The resulting best-fit time series
reproduces quite well the solar-cycle timescale variations of UARS
ultraviolet observations, as well as the short-timescale fluctuations
about the 81 day running mean. We synthesize full spectra between
200 and 400 nm, and validate these against the spectra obtained by the
ATLAS-1 and ATLAS-3 missions, finding good agreement, to better than 3 %
at most wavelengths. We also compare the UV variability predicted by our
reconstructions in the descending phase of sunspot cycle 23 to SORCE/SIM
data as well as to other reconstructions. Finally, we use the model to
reconstruct the time series of spectral irradiance starting in 1874,
and investigate temporal correlations between pairs of wavelengths in
the bands of interest for stratospheric chemistry and dynamics.
---------------------------------------------------------
Title: An Exploration of Non-kinematic Effects in Flux Transport
Dynamos
Authors: Passos, Dário; Charbonneau, Paul; Beaudoin, Patrice
2012SoPh..279....1P Altcode: 2012SoPh..tmp...71P
Recent global magnetohydrodynamical simulations of solar convection
producing a large-scale magnetic field undergoing regular, solar-like
polarity reversals also present related cyclic modulations of
large-scale flows developing in the convecting layers. Examination
of these simulations reveal that the meridional flow, a crucial
element in flux transport dynamos, is driven at least in part by
the Lorentz force associated with the cycling large-scale magnetic
field. This suggests that the backreaction of the field onto the flow
may have a pronounced influence on the long-term evolution of the
dynamo. We explore some of the associated dynamics using a low-order
dynamo model that includes this Lorentz force feedback. We identify
several characteristic solutions which include single period cycles,
period doubling and chaos. To emulate the role of turbulence in the
backreaction process we subject the model to stochastic fluctuations in
the parameter that controls the Lorentz force amplitude. We find that
short term fluctuations produce long-term modulations of the solar
cycle and, in some cases, grand minima episodes where the amplitude
of the magnetic field decays to near zero. The chain of events that
triggers these quiescent phases is identified. A subsequent analysis of
the energy transfer between large-scale fields and flows in the global
magnetohydrodynamical simulation of solar convection shows that the
magnetic field extracts energy from the solar differential rotation and
deposits part of that energy into the meridional flow. The potential
consequences of this marked departure from the kinematic regime are
discussed in the context of current solar cycle modeling efforts based
on flux transport dynamos.
---------------------------------------------------------
Title: Numerical Simulation of a Solar Active Region. I: Bastille
Day Flare
Authors: Vincent, Alain; Charbonneau, Paul; Dubé, Caroline
2012SoPh..278..367V Altcode:
We present three-dimensional unsteady modeling and numerical
simulations of a coronal active region, carried out within the
compressible single-fluid MHD approximation. We focus on AR 9077
on 14 July 2000, and the triggering of the X5.7 GOES X-ray class
"Bastille Day" flare. We simulate only the lower corona, although we
include a virtual photosphere and chromosphere below. The boundary
conditions at the base of this layer are set using temperature maps
from line intensities and line-of-sight magnetograms (SOHO/MDI). From
the latter, we generate vector magnetograms using the force-free
approximation; these vector magnetograms are then used to produce
the boundary condition on the velocity field using a minimum energy
principle (Longcope, Astrophys. J.612, 1181, 2004). The reconnection
process is modeled through a dynamical hyper-resistivity which is
activated when the current exceeds a critical value (Klimas et al.,
J. Geophys. Res.109, 2218, 2004). Comparing the time series of X-ray
fluxes recorded by GOES with modeled time series of various mean
physical variables such as current density, Poynting energy flux, or
radiative loss inside the active region, we can demonstrate that the
model properly captures the evolution of an active region over a day
and, in particular, is able to explain the initiation of the flare at
the observed time.
---------------------------------------------------------
Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2012IAUTA..28...69V Altcode:
Commission 10 of the International Astronomical Union has more than
650 members who study a wide range of activity phenomena produced by
our nearest star, the Sun. Solar activity is intrinsically related
to solar magnetic fields and encompasses events from the smallest
energy releases (nano- or even picoflares) to the largest eruptions
in the Solar System, coronal mass ejections (CMEs), which propagate
into the Heliosphere reaching the Earth and beyond. Solar activity is
manifested in the appearance of sunspot groups or active regions, which
are the principal sources of activity phenomena from the emergence of
their magnetic flux through their dispersion and decay. The period
2008-2009 saw an unanticipated extended solar cycle minimum and
unprecedentedly weak polar-cap and heliospheric field. Associated with
that was the 2009 historical maximum in galactic cosmic rays flux since
measurements begun in the middle of the 20th Century. Since then Cycle
24 has re-started solar activity producing some spectacular eruptions
observed with a fleet of spacecraft and ground-based facilities. In
the last triennium major advances in our knowledge and understanding
of solar activity were due to continuing success of space missions as
SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
by the breathtaking images of the solar atmosphere produced by the
Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
framework of NASA's Living with a Star program. In August 2012, at the
time of the IAU General Assembly in Beijing when the mandate of this
Commission ends, we will be in the unique position to have for the
first time a full 3-D view of the Sun and solar activity phenomena
provided by the twin STEREO missions about 120 degrees behind and
ahead of Earth and other spacecraft around the Earth and ground-based
observatories. These new observational insights are continuously
posing new questions, inspiring and advancing theoretical analysis
and modelling, improving our understanding of the physics underlying
magnetic activity phenomena. Commission 10 reports on a vigorously
evolving field of research produced by a large community. The number
of refereed publications containing `Sun', `heliosphere', or a synonym
in their abstracts continued the steady growth seen over the preceding
decades, reaching about 2000 in the years 2008-2010, with a total of
close to 4000 unique authors. This report, however, has its limitations
and it is inherently incomplete, as it was prepared jointly by the
members of the Organising Committee of Commission 10 (see the names
of the primary contributors to the sections indicated in parentheses)
reflecting their fields of expertise and interest. Nevertheless, we
believe that it is a representative sample of significant new results
obtained during the last triennium in the field of solar activity.
---------------------------------------------------------
Title: Magnetic reconnection in numerical simulations of the Bastille
day flare
Authors: Vincent, A. P.; Charbonneau, P.
2011AGUFMSH43A1925V Altcode:
If neither waves nor adiabatic heating due to compression are taken
into account, coronal heating may be obtained in numerical simulations
from current dissipation inside solar flares. To increase Joule heating
locally we used a model for hyper resistivity (Klimas et al., 2004:
Journal of Geophysical Research, 109, 2218-2231). Here the change in
resistivity is due to small scale (less than 1Mm in our simulations)
current density fluctuations. Whenever the current exceeds a cut-off
value, magnetic resistivity jumps sharply to reach a maximum locally
thus increasing magnetic gradients at the border of the flare. In
this way, not only the current increases but also the maximum is
slowly displaced and simulations of the full set of 3-D MHD equations
show a progression westward as can be seen in SOHO-EIT images of the
”slinky”. In our simulations of the Bastille day flare, most of
the reconnection events take place just above the transition and
mostly follow the neutral line but it is Spitzer thermal diffusivity
together with radiative cooling that illuminates magnetic arcades in
a way similar to what can be seen in extreme ultra-violet animations
of the slinky.
---------------------------------------------------------
Title: Using data assimilation to reconstruct convection patterns
below an active region of solar corona from observed magnetograms
Authors: Pirot, D.; Vincent, A. P.; Charbonneau, P.; Solar Physics
Research Group of University of Montreal
2011AGUFMSH54A..07P Altcode:
Solar magnetic field originates deep inside the convection zone and
rises through it to produce active regions. Detailled simulations of
solar convection including granulation and radiation that have been
performed in the past are important both to understand the physics
of magnetic flux tube evolution as well as the algorithms used for
simulations. A challenging problem is the reconstruction of the
effective patterns of convection below an observed active region as
given by magnetograms and temperature maps at photospheric levels. Since
convection in the sun is strongly stratified in density it can be
regarded as being anelastic, therefore we used ANMHD software. Here
we chosed AR9077-20000714 also known to have produced the ”Bastille
day” flare a region of area 175 Mm<SUP>2</SUP>. <P />To this purpose
we used an anelastic convection model that we modified to include
the Nudging Back and Forth, a Newtonian relaxation technique for the
data assimilation of SOHO/MDI temperature and magnetograms. Vector
magnetograms are first choice for the upper boundary condition to
be data assimilated. However they have been computed from SOHO line
of sight magnetograms using the force free hypothesis as if we would
be just above photosphere. We found that velocity shears between slow
diverging upflows and fast turbulent downflows produce Ω and U-shaped
magnetic field loops. The coronal arcade system of AR9077-20000714
(the “slinky”) is here understood as the emerging part of the magneto
convective pattern below.
---------------------------------------------------------
Title: Numerical simulations of the Sun's photospheric magnetic
network and its evolution over the solar cycle
Authors: Thibault, K.; Charbonneau, P.
2011AGUFMSH43B1942T Altcode:
We model, through 2D Monte-Carlo simulations on a spherical shell, the
spatiotemporal evolution of the photospheric magnetic network over solar
cycle timescales. Sources of magnetic flux in the simulations include
both injection of small-scale magnetic structures all over the quiet
sun, as well as the emergence and subsequent disintegration of sunspots
in active latitudes. We simulate the emergence, random walk, interaction
(aggregation or cancellation) and submergence of the smallest magnetic
elements and aggregates thereof, also taking into account advection by
differential rotation and meridional circulation. Although magnetic
flux is injected only at the two extreme ends of the flux scale
(elementary flux tubes and sunspots), the simulations produce a range
of intermediate scales through aggregation and cancellation of the
small-scale magnetic structures, whether injected all over the solar
surface (Quiet Sun) or re- leased by the decaying sunspots. Moreover,
the modeled flux is distributed in the form of a power law, as observed
by Parnell et al. (2009). We in- vestigate how this distribution varies
as a function fo solar cycle phase.
---------------------------------------------------------
Title: Magnetic cycles in global magnetohydrodynamical simulations
of solar convection
Authors: Charbonneau, P.
2011AGUFMSH52B..01C Altcode:
In this talk I will review some recent advances in our understanding
of the solar magnetic cycle through global magnetohydrodynamical
simulations of thermally-driven convection in a thick, stratified
spherical shell of electrically conducting fluid. I will focus on three
related issues: (1) the nature of the turbulent dynamo mechanism; (2)
the nature of the mechanism(s) controlling the cycle amplitude; and
(3) epochs of strongly suppressed cycle amplitudes, and the existence
of possible precursor to such events to be found in the patterns
of magnetically-driven torsional oscillations and meridional flow
variations arising in the simulations.
---------------------------------------------------------
Title: On the Mode of Dynamo Action in a Global Large-eddy Simulation
of Solar Convection
Authors: Racine, Étienne; Charbonneau, Paul; Ghizaru, Mihai; Bouchat,
Amélie; Smolarkiewicz, Piotr K.
2011ApJ...735...46R Altcode:
In this paper, we examine the mode of dynamo action in the implicit
large-eddy magnetohydrodynamical simulation of solar convection
reported upon in Ghizaru et al. Motivated by the presence of a strong
and well-defined large-scale axisymmetric magnetic component undergoing
regular polarity reversals, we define the fluctuating component of
the magnetic field as the difference between the total field and its
zonal average. The subsequent analysis follows the physical logic
and mathematical formulation of mean-field electrodynamics, whereby
a turbulent electromotive force (EMF) is computed by the suitable
averaging of cross-correlations between fluctuating flow and field
components and expressed in terms of the mean field via a linear
truncated tensorial expansion. We use singular value decomposition
to perform a linear least-squares fit of the temporal variation of
the EMF to that of the large-scale magnetic component, which yields
the components of the full α-tensor. Its antisymmetric component,
describing general turbulent pumping, is also extracted. The α-tensor
so calculated reproduces a number of features already identified
in local, Cartesian simulations of magnetohydrodynamical rotating
convection, including an α<SUB>phiphi</SUB> component positive in the
northern solar hemisphere, peaking at high latitudes, and reversing
sign near the bottom of the convection zone; downward turbulent
pumping throughout the convecting layer; and significant equatorward
turbulent pumping at mid latitudes, and poleward at high latitudes in
subsurface layers. We also find that the EMF contributes significantly
to the regeneration of the large-scale toroidal magnetic component,
which from the point of view of mean-field dynamo models would
imply that the simulation operates as an α<SUP>2</SUP>Ω dynamo. We
find little significant evidence of α-quenching by the large-scale
magnetic field. The amplitude of the magnetic cycle appears instead
to be regulated primarily by a magnetically driven reduction of the
differential rotation.
---------------------------------------------------------
Title: Formation and disruption of current filaments in a flow-driven
turbulent magnetosphere
Authors: Liu, W. W.; Morales, L. F.; Uritsky, V. M.; Charbonneau, P.
2011JGRA..116.3213L Altcode: 2010arXiv1008.2938L
Recent observations have established that the magnetosphere is a system
of natural complexity. The coexistence of multiscale structures such as
auroral arcs, turbulent convective flows, and scale-free distributions
of energy perturbations has lacked a unified explanation, although there
is strong reason to believe that they all stem from a common base of
physics. In this paper we show that a slow but turbulent convection
leads to the formation of multiscale current filaments reminiscent of
auroral arcs. The process involves an interplay between random shuffling
of field lines and dissipation of magnetic energy on sub-MHD scales. As
the filament system reaches a critical level of complexity, local
current disruption can trigger avalanches of energy release of varying
sizes, leading to scale-free distributions over energy perturbation,
power, and event duration. A long-term memory effect is observed whereby
the filament system replicates itself after each avalanche. The results
support the view that that the classical and inverse cascades operate
simultaneously in the magnetosphere. In the former, the high Reynolds
number plasma flow disintegrates into turbulence through successive
breakdowns; in the latter, the interactions of small-scale flow eddies
with the magnetic field can self-organize into elongated current
filaments and large-scale energy avalanches mimicking the substorm.
---------------------------------------------------------
Title: The dynamo basis of solar cycle precursor schemes
Authors: Charbonneau, Paul; Barlet, Guillaume
2011JASTP..73..198C Altcode:
We investigate the dynamo underpinning of solar cycle precursor
schemes based on direct or indirect measures of the solar surface
magnetic field. We do so for various types of mean-field-like kinematic
axisymmetric dynamo models, where amplitude fluctuations are driven
by zero-mean stochastic forcing of the dynamo number controlling the
strength of the poloidal source term. In all stochastically forced
models considered, the surface poloidal magnetic field is found to
have precursor value only if it feeds back into the dynamo loop, which
suggests that accurate determination of the magnetic flux budget of
the solar polar fields may hold the key to dynamo model-based cycle
forecasting.
---------------------------------------------------------
Title: Dual scaling for self-organized critical models of the
magnetosphere
Authors: VallièRes-Nollet, M. -A.; Charbonneau, P.; Uritsky, V.;
Donovan, E.; Liu, W.
2010JGRA..11512217V Altcode:
The central plasma sheet is a complex magnetized plasma structure
located in the equatorial plane of the magnetotail from where
substorms are believed to originate. Dynamically, it may behave
like a self-organized critical (SOC) system, driven by the slow
energy input of the solar wind. The power law distributions for the
sizes, energies, and durations of substorms that are reflected in
observations can be reproduced using such SOC models. However, the
expected scale invariance does not seem to hold for all scale ranges
and observables. Recent observations of all-sky auroras have suggested
a dual regime, where small and large events scale as different power
laws, the smaller events having a steeper slope. On the other hand,
scale-dependent substorm behavior can materialize as a consequence of
an energy loading-unloading cycle. Accordingly, we designed a 2-D SOC
model subject to global deterministic driving and a nonconservative
redistribution law. This model can reproduce the coexistence of two
scaling regimes, with the second regime appearing as a consequence
of the enhanced spatial development of avalanches caused by a higher
spatial intermittency in the energy gradients. Thresholded interevent
waiting time statistics showed a well-defined peak with an exponential
tail, consistent with observations and the expected dynamics of a
loading-unloading cycle. Finally, we show that the coherency index
extracted from the simulations decreases prior to large avalanches, as
is in fact observed in auroral arcs. This suggests that the coherency
index may be a useful substorm predictor.
---------------------------------------------------------
Title: Modeling solar and stellar dynamos
Authors: Charbonneau, Paul
2010hesa.book..141C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamo Models of the Solar Cycle
Authors: Charbonneau, Paul
2010LRSP....7....3C Altcode:
This paper reviews recent advances and current debates in modeling the
solar cycle as a hydromagnetic dynamo process. Emphasis is placed on
(relatively) simple dynamo models that are nonetheless detailed enough
to be comparable to solar cycle observations. After a brief overview
of the dynamo problem and of key observational constraints, we begin
by reviewing the various magnetic field regeneration mechanisms that
have been proposed in the solar context. We move on to a presentation
and critical discussion of extant solar cycle models based on these
mechanisms. We then turn to the origin and consequences of fluctuations
in these models, including amplitude and parity modulation, chaotic
behavior, intermittency, and predictability. The paper concludes with
a discussion of our current state of ignorance regarding various key
questions relating to the explanatory framework offered by dynamo
models of the solar cycle.
---------------------------------------------------------
Title: Magnetic Cycles in Global Large-eddy Simulations of Solar
Convection
Authors: Ghizaru, Mihai; Charbonneau, Paul; Smolarkiewicz, Piotr K.
2010ApJ...715L.133G Altcode:
We report on a global magnetohydrodynamical simulation of the solar
convection zone, which succeeds in generating a large-scale axisymmetric
magnetic component, antisymmetric about the equatorial plane and
undergoing regular polarity reversals on decadal timescales. We focus on
a specific simulation run covering 255 years, during which 8 polarity
reversals are observed, with a mean period of 30 years. Time-latitude
slices of the zonally averaged toroidal magnetic component at the base
of the convecting envelope show a well-organized toroidal flux system
building up in each solar hemisphere, peaking at mid-latitudes and
migrating toward the equator in the course of each cycle, in remarkable
agreement with inferences based on the sunspot butterfly diagram. The
simulation also produces a large-scale dipole moment, varying in phase
with the internal toroidal component, suggesting that the simulation may
be operating as what is known in mean-field theory as an αΩ dynamo.
---------------------------------------------------------
Title: Abnormal cycles from normal dynamos
Authors: Racine, Etienne; Charbonneau, Paul; Ghizaru, Mihai
2010cosp...38.1732R Altcode: 2010cosp.meet.1732R
The primary aim of this talk is to illustrate how simple yet
reasonably realistic dynamo models of the solar cycle can exhibit
"abnormal" features, including extended periods of minimum ac-tivity
as currently observed. We will show that dynamos including even very
simple amplitude quenching nonlinearities can, in the presence of
low-amplitude stochastic forcing, produce fluc-tuations that may
take the form of extended periods of strongly reduced activity. We
will also examine the occurrence of similar behavior in global MHD
simulations of the solar convection zones producing solar-like cycles
in the large-scale magnetic field.
---------------------------------------------------------
Title: When the rubber hits the road: turbulence and intermittencies
in Earth’s magnetosphere (Invited)
Authors: Liu, W.; Uritsky, V. M.; Charbonneau, P.; Valliere-Nollet,
M.; Morales, L. F.
2009AGUFMSM41C..02L Altcode:
The theory of MHD turbulence has a long history of development, but its
application to actual physical systems has not been as widespread as
the omnipresence of turbulent phenomena in these systems. The reasons
can be many. For example, the geometry, constitution, coupling, and
boundary conditions in an actual system may place it well beyond the
limit of the current theory. Or our measurement capability has not
progressed to the point that the salient theoretical predictions
can be reliably tested. In our view, however, the most pertinent
reason is the specialist’s unfamiliarity with and reluctance to
use the ‘generalist’ approach of complexity and turbulence, and
the generalist’s lack of attention to the particulars of specific
systems. Clearly, a closer dialogue between the two communities holds
significant promise for further advances. In this talk, complexity
and turbulence observed in Earth’s magnetosphere are reviewed. A
distinguishing characteristic of magnetospheric processes is strong
intermittency in global episodes and ever-present turbulence on
local scales. We explore the suggestion that avalanches of localized
turbulence are responsible for global dynamics. Some new theoretical
advances in elaborating this concept are discussed.
---------------------------------------------------------
Title: Abnormal cycles from normal dynamos (Invited)
Authors: Charbonneau, P.
2009AGUFMSH13C..01C Altcode:
The primary aim of this talk is to illustrate how simple yet
reasonably realistic dynamo models of the solar cycle can exhibit
"abnormal" features, including extended periods of minimum activity
as currently observed. I will show that dynamo including even very
simple amplitude quenching nonlinearities can, in the presence of
low-amplitude stochastic forcing, produce fluctuations that may be
mistaken for switches between distinct dynamical behaviors, and will
examine to what degree such behavior can be distinguished from true
intermittency, given extant data.
---------------------------------------------------------
Title: Abundances anomalies and meridional circulation in horizontal
branch stars
Authors: Quievy, D.; Charbonneau, P.; Michaud, G.; Richer, J.
2009A&A...500.1163Q Altcode:
Context: Photospheric chemical abundances on the horizontal branch (HB)
show some striking variations with effective temperature (T_eff). The
most straightforward explanation is that these anomalies develop
through diffusion processes, in particular gravitational settling
and radiative levitation. However, the abrupt disappearance of
strong abundance anomalies as one moves below about 11 000 K on
the HB suggests that another factor plays an important role. <BR
/>Aims: We test an extension to the HB of the diffusion model for
main-sequence HgMn stars, where strong anomalies can only develop in
the slower rotators. In these rotators the gravitational settling
of helium leads to the disappearance of its superficial convection
zone, so that chemical separation by radiative levitation can occur
all the way to the photosphere. <BR />Methods: More specifically,
we calculate the critical rotational velocity at which He settling is
prevented by rotationally-induced meridional circulation, in a suite
of stellar models spanning the zero-age HB. Helium settling serves
as the measure of the atomic diffusion of all species. <BR />Results:
Our abundance evolution calculations show that, for models with T_eff
less than about 11 500 K, corresponding to stars typically observed
with the same metal composition as giants, meridional circulation is
efficient enough to suppress He settling for rotational velocities, in
good agreement with observed values. Once the meridional circulation
profile of a star rotating as a near rigid body has been adopted,
no adjustable parameter is involved. <BR />Conclusions: The T_eff
dependence of abundance anomalies observed on the HB can be explained
by atomic diffusion transport if one introduces the competition of
meridional circulation with the observed T_eff dependence of rotation
velocity of HB stars.
---------------------------------------------------------
Title: Geometrical Properties of Avalanches in a Pseudo-3D Coronal
Loop
Authors: Morales, Laura; Charbonneau, Paul
2009ApJ...698.1893M Altcode:
We investigate the geometrical properties of energy release of
synthetic coronal loops constructed using a recently published
self-organized critical avalanche model of solar flares. The model is
based on an idealized representation of a coronal loop as a bundle
of closely packed magnetic flux strands wrapping around one another
in response to photospheric fluid motions, much as in Parker's
nanoflare model. Simulations are performed with a two-dimensional
cellular automaton that satisfies the constraint ∇ · B = 0 by
design. We transform the avalanching nodes produced by simulations
into synthetic flare images by converting the two-dimensional lattice
into a bent cylindrical loop that is projected onto the plane of the
sky. We study the statistical properties of avalanches peak snapshots
and time-integrated avalanches occurring in these synthetic coronal
loops. We find that the frequency distribution of avalanche peak areas
A assumes a power-law form f(A) ∝ A^{-α_{A}} with an index α<SUB>
A </SUB> sime 2.37, in excellent agreement with observationally inferred
values and reducing error bars from previous works. We also measure
the area fractal dimension D of avalanches produced by our simulations
using the box counting method, which yields 1.17 <= D <= 1.24, a
result falling nicely within the range of observational determinations.
---------------------------------------------------------
Title: Coexistence of distinct power-law regimes in Self Organized
Model for the Magnetosphere
Authors: Vallieres-Nollet, M.; Charbonneau, P.; Uritsky, V.; Liu, W.
2009AGUSMSM22A..06V Altcode:
It is now argued that the Central Pasma Sheet (CPS) may behave like a
Self-Organized Critical (SOC) system, driven by the the solar wind. The
power law distributions for the sizes, energy and durations of substorms
that are reflected in observations can be reproduced using such SOC
models. However, recent observations made with the POLAR-UVI instrument
showed that there is in fact two distinct regimes in substorms energies
: small and big events scales as different power laws, the smaller
events having a steeper slope. We used a 2D-SOC model subject to a
deterministic driving, with conservative redistributions laws. We where
able, with a slow driving together with a small dissipation in energy
redistribution, to reproduce the coexistence of these two scaling
regimes. The computation of the waiting-times, under the imposition
of a threshold, showed truncated exponentials distributions, which
is consistent with observations. Finally, we computed statistics
of substorms depending on their onset position, and found that the
southward mapping events tends to exhibit the dual power-law scaling,
while a single slope statistic was found for northward mapping
substorms, which is again consistent with recent observations.
---------------------------------------------------------
Title: Self-organized Critical Model of Energy Release in an Idealized
Coronal Loop
Authors: Morales, Laura; Charbonneau, Paul
2008ApJ...682..654M Altcode:
We present and discuss a new avalanche model for solar flares, based on
an idealized representation of a coronal loop as a bundle of magnetic
flux strands wrapping around one another. The model is based on a
two-dimensional cellular automaton with anisotropic connectivity,
where linear ensembles of interconnected nodes define the individual
strands collectively making up the coronal loop. The system is driven
by random deformation of the strands, and a form of reconnection is
assumed to take place when the angle subtended by two strands crossing
at the same lattice site exceed some preset threshold. Driven in this
manner, the cellular automaton produces avalanches of reconnection
events characterized by scale-free size distributions that compare
favorably with the corresponding size distribution of solar flares,
as inferred observationally. Although lattice-based and highly
idealized, the model satisfies the constraints Δ centerdot B = 0 by
design and is defined in such a way as to be readily mapped back onto
coronal loops with set physical dimensions. Carrying this exercise
for a generic coronal loop of length 10<SUP>10</SUP> cm and diameter
10<SUP>8</SUP> cm yields flare energies ranging from 10<SUP>23</SUP>
to 10<SUP>29</SUP> erg, for an instability threshold angle of 11°
between contiguous magnetic flux strands. These figures square well
with both observational determinations and theoretical estimates.
---------------------------------------------------------
Title: A Model for the Total Solar Irradiance Based on Active
Region Decay
Authors: Crouch, A. D.; Charbonneau, P.; Beaubien, G.; Paquin-Ricard,
D.
2008ApJ...677..723C Altcode:
We present a model for the total solar irradiance. The model takes
the observed location, timing, and area of emerging active regions
as input and produces a time-evolving size distribution of magnetic
structures over the solar surface. We assume that the bright magnetic
structures (faculae), which counteract the irradiance deficit caused
by sunspots, consist of the products of active region decay. We
simulate the decay process as a combination of fragmentation and
boundary erosion of large-scale magnetic structures. The model has
several adjustable parameters that control the decay processes and
the irradiance contribution from the quiet Sun and the small-scale
magnetic elements that are produced during the decay process. We
use a genetic algorithm to estimate these parameters by fitting to
the observed irradiance and daily sunspot area time series over the
1978-2007 time interval. Given the simplifications associated with
the model, the resultant parameter values are well constrained within
the imposed ranges. In addition, the irradiance and daily sunspot area
time series produced by the best-fit models agree very well with the
observations, although the sunspot area fits tend to perform better
than the irradiance fits. However, it is evident that the model is
neglecting a significant source of excess brightness, which manifests
itself in two ways. First, the optimal values for the lifetime and
intensity contrast of the bright, small-scale flux elements are both
larger than expected. Second, the synthetic irradiance consistently
underestimates the observations during the ascending phase of a cycle,
despite the daily sunspot area fitting the observations quite well
during these times. We also show that this genetic forward modeling
approach can be used to detect a long-term trend of decadal timescale in
the quiet-Sun irradiance. Assuming a constant quiet-Sun irradiance, we
reconstruct the total solar irradiance over the 1874-1978 time interval,
for which observational data of emerging active regions are available.
---------------------------------------------------------
Title: Scaling laws and frequency distributions of avalanche areas
in a self-organized criticality model of solar flares
Authors: Morales, Laura F.; Charbonneau, Paul
2008GeoRL..35.4108M Altcode:
We calculate the spreading exponents and some geometrical properties of
avalanches in a novel avalanche model of solar flares, closely built
on Parker's physical picture of coronal heating by nanoflares. The
model is based on an idealized representation of a coronal loop
as a bundle of magnetic flux strands wrapping around one another,
numerically implemented as an anisotropic cellular automaton. We
demonstrate that the growth of avalanches in this model exhibits
power-laws correlations that are numerically consistent with the
behavior of a general class of statistical physical systems in the
vicinity of a stationary critical point. This demonstrates that the
model indeed operates in a self-organized critical regime. Moreover,
we find that the frequency distribution of avalanche peak areas A
assumes a power-law form f(A) $\propto$ A <SUP>-α<SUB>A</SUB> </SUP>
with an index α <SUB> A </SUB> $\simeq$ 2.45, in excellent agreement
with observationally-inferred values.
---------------------------------------------------------
Title: Large scale numerical experiments on solar convection zone
Authors: Ghizaru, Mihai; Smolarkiewicz, Piotr; Charbonneau, Paul
2008cosp...37.1006G Altcode: 2008cosp.meet.1006G
Large scale dynamics under the influence of developing magnetic fields
inside the solar convection zone is investigated using the MHD version
of EULAG, a global numerical model designed to work in the spirit of
implicit large eddy simulations, using a higher-order upwind type
advection scheme, in the framework of the anelastic approximation
for MHD equations. Differential rotation profiles, turbulence
characteristics and magnetic field evolution are analyzed under various
parameters settings in an attempt to gain a better understanding of
different factors contributions to the solar activity characteristics.
---------------------------------------------------------
Title: Solar Magnetic Activity and Total Irradiance Since the
Maunder Minimum
Authors: Tapping, K. F.; Boteler, D.; Charbonneau, P.; Crouch, A.;
Manson, A.; Paquette, H.
2007SoPh..246..309T Altcode:
We develop a model for estimating solar total irradiance since 1600
AD using the sunspot number record as input, since this is the only
intrinsic record of solar activity extending back far enough in
time. Sunspot number is strongly correlated, albeit nonlinearly with
the 10.7-cm radio flux (F<SUB>10.7</SUB>), which forms a continuous
record back to 1947. This enables the nonlinear relationship to be
estimated with usable accuracy and shows that relationship to be
consistent over multiple solar activity cycles. From the sunspot
number record we estimate F<SUB>10.7</SUB> values back to 1600
AD. F<SUB>10.7</SUB> is linearly correlated with the total amount of
magnetic flux in active regions, and we use it as input to a simple
cascade model for the other magnetic flux components. The irradiance
record is estimated by using these magnetic flux components plus
a very rudimentary model for the modulation of energy flow to the
photosphere by the subphotospheric magnetic flux reservoir feeding the
photospheric magnetic structures. Including a Monte Carlo analysis of
the consequences of measurement and fitting errors, the model indicates
the mean irradiance during the Maunder Minimum was about 1 ± 0.4 W
m<SUP>−2</SUP> lower than the mean irradiance over the last solar
activity cycle.
---------------------------------------------------------
Title: Multiscale magnetospheric physics from a simple model of
self-organized dynamics
Authors: Liu, W.; Charbonneau, P.
2007AGUFMSM54A..05L Altcode:
Distributions of geomagnetic indices and aurora have been used to show
the nonlinear characteristics of magnetospheric dynamics. However,
the physics underlying these distributions is often not clear. Self
organization of micro-scale perturbations has been suggested as a
possibility whereby robust power-law distributions of magnetospheric
dynamics indices can be produced. Earlier we proposed that some aspects
of observed substorm distributions can be explained qualitatively by
a model featuring interactive discrete flux tubes which are used to
simulate the central plasma sheet dynamics driven by a constant energy
input. A one- dimensional simulation of the model yielded scale-free
distributions of auroral activity and quasiperiodic injection. This
result raised the question what dynamical category the substorm belongs
to, scale-free or with definite scales. In this talk, we present the
latest two-dimensional simulation and auroral observational results,
in an attempt to elucidate the dynamical nature of the substorm.
---------------------------------------------------------
Title: Predicting Solar Flares by Data Assimilation in Avalanche
Models. I. Model Design and Validation
Authors: Bélanger, Eric; Vincent, Alain; Charbonneau, Paul
2007SoPh..245..141B Altcode: 2007arXiv0708.1941B
Data assimilation techniques, developed in the past two decades mainly
for weather prediction, produce better forecasts by taking advantage
of both theoretical/numerical models and real-time observations. In
this paper, we explore the possibility of applying the four-dimensional
variational data assimilation (4D-VAR) technique to the prediction of
solar flares. We do so in the context of a continuous version of the
classical cellular-automaton-based self-organized critical avalanche
models of solar flares introduced by Lu and Hamilton (Astrophys. J.380,
L89, 1991). Such models, although a priori far removed from the
physics of magnetic reconnection and magnetohydrodynamical evolution
of coronal structures, nonetheless reproduce quite well the observed
statistical distribution of flare characteristics. We report here on
a large set of data assimilation runs on synthetic energy release
time series. Our results indicate that, despite the unpredictable
(and unobservable) stochastic nature of the driving/triggering
mechanism within the avalanche model, 4D-VAR succeeds in producing
optimal initial conditions that reproduce adequately the time series
of energy released by avalanches and flares. This is an essential
first step toward forecasting real flares.
---------------------------------------------------------
Title: Supergranulation as an Emergent Length Scale
Authors: Crouch, A. D.; Charbonneau, P.; Thibault, K.
2007ApJ...662..715C Altcode:
We have developed an n-body diffusion-limited aggregation model
to simulate the dispersal and interaction of small-scale magnetic
elements at the solar surface. The model is highly simplified and
is based on the observation that small-scale magnetic elements are
passively advected by the granular flow, which we approximate as
a random walk. With a great many magnetic elements executing this
random walk simultaneously, collisions are inevitable. We assume that
these collisions lead to the aggregation of the colliding magnetic
elements (if they have the same polarity) or mutual cancellation
(if opposite polarity). In a similar fashion, the resulting clusters
can subsequently interact with more magnetic elements or with other
clusters. The clusters also undergo a random walk. However, the step
size is reduced and the lifetime is increased in order to mimic the
observation that larger magnetic flux concentrations move slower and
live longer than smaller ones. The essential finding is that this
process can produce a spatial distribution of clusters comparable to
the supergranule cell pattern (depending on model parameters). The
characteristic length scale associated with the spatial distribution
of the clusters is quite sensitively dependent on the injection rate of
fresh magnetic elements-when the injection rate is high (low) the length
scale is small (large). This property provides a natural explanation
for the observation that supergranule cells tend to be smaller when
and where the level of magnetic activity is higher. We also find that
at length scales similar to supergranulation the dominance of a given
polarity tends to be enhanced, in comparison to the case where the same
clusters are situated randomly in space. This is potentially testable
by observation.
---------------------------------------------------------
Title: Fluctuations in Babcock-Leighton Dynamos. II. Revisiting the
Gnevyshev-Ohl Rule
Authors: Charbonneau, Paul; Beaubien, Geneviève; St-Jean, Cédric
2007ApJ...658..657C Altcode:
The Gnevyshev-Ohl rule refers to a pattern of alternating higher and
lower than average solar cycle amplitudes observed in the sunspot number
record. In this paper, we show that such a pattern arises naturally in
Babcock-Leighton models of the solar cycle as a consequence of the long
time delay built into the dynamo regenerative loop. This is investigated
using a simple but well-validated iterative map formulation, as well
as a seasoned two-dimensional axisymmetric kinematic dynamo model. The
good agreement between the results obtained via these two very different
modeling approaches offers confidence that Gnevyshev-Ohl-like patterns
of cycle amplitude fluctuations are a robust feature of this class of
solar cycle models.
---------------------------------------------------------
Title: Babcock Leighton models of the solar cycle: Questions and
issues
Authors: Charbonneau, Paul
2007AdSpR..39.1661C Altcode:
This paper is a review of our current state of understanding of dynamo
models of the solar cycle based on the Babcock-Leighton mechanism of
poloidal field regeneration by the decay of bipolar active regions. It
is organized in the form of "point and counterpoint" discussion
of ten issues or topics of contention to be found in the recent
literature on these dynamo models. These go from similarities and
differences with dynamo models based on mean-field electrodynamics,
the role of meridional circulation in setting the predicted form of
the sunspot butterfly diagram, constraints brought about by light
element abundances, non-linear magnetic backreaction on the driving
flows, up to the use of Babcock-Leighton models for predicting solar
cycle amplitudes.
---------------------------------------------------------
Title: Cross-hemispheric coupling in a Babcock Leighton model of
the solar cycle
Authors: Charbonneau, Paul
2007AdSpR..40..899C Altcode:
In this paper, I present some exploratory dynamo calculations
illustrating the influence of cross-hemispheric coupling in solar
cycle models based on the Babcock-Leighton mechanism of poloidal field
regeneration. Using two very distinct formulations of Babcock-Leighton
dynamos, I examine the issues of cross-hemispheric phase lag,
interference, and synchronization of Maunder-Minimum-like episodes
of suppressed activity. Although covering only a small fraction of
the model's relevant parameter space, results presented here suggest
that additional effects, such as transequatorial flows for example,
may be required to achieve sufficient cross-hemispheric coupling in
this class of solar cycle models.
---------------------------------------------------------
Title: The Role of MHD Mode Conversion in Sunspot Seismology
Authors: Crouch, A. P.; Cally, P. S.; Charbonneau, P.; Braun, D. C.;
Desjardins, M.
2006ASPC..354..161C Altcode:
Sunspots absorb energy from and shift the phase of f and p modes
incident upon them. Understanding the mechanism causing each of these
effects is vital to the local helioseismology of sunspots (and magnetic
flux concentrations in general). Because the beta-equals-unity layer
typically lies in the near surface layers below the photospheres
of sunspot umbrae, MHD mode conversion can occur. Mode conversion
provides a promising absorption mechanism because the slow
magnetoacoustic-gravity waves and Alfvén waves guide energy along
the magnetic field away from the acoustic cavity. Our previous mode
conversion calculations have shown that simple sunspot models with
non-vertical magnetic fields can produce ample absorption to explain the
Hankel analysis measurements, along with phase shift predictions that
agree well with the observations. Those calculations only considered
the possibility of MHD waves propagating down the magnetic field into
the interior. In this contribution, we consider a second additional
possibility -- waves propagating up into the atmosphere overlying
sunspots.
---------------------------------------------------------
Title: Energy avalanches in the central plasma sheet
Authors: Liu, W. W.; Charbonneau, P.; Thibault, K.; Morales, L.
2006GeoRL..3319106L Altcode:
The central plasma sheet (CPS) is simulated as a 1D cellular
automaton. The system is driven deterministically and globally by
a spatially non-uniform energy loading (convection). Each node (a
flux tube) evolves until one of two local instability criteria is
exceeded. The unstable node releases a small amount of energy to the
ionosphere and another small amount is distributed to its neighboring
nodes. The partition between the two modes of energy distribution
is the only random factor in the model. The energy redistribution
relaxes the node deterministically to a stable state. The simulation
suggests that a central plasma sheet driven in the above manner is
in a self-organized critical state, with energy avalanches obeying
a scale-free distribution. The avalanches, however, co-exist with
quasi-periodic intermittencies manifested in ring-current injection,
which is correlated with strong CPS avalanches, and tailward energy
ejection, which shows no apparent correlation in this aspect.
---------------------------------------------------------
Title: Supergranulation As An Emergent Length Scale
Authors: Crouch, Ashley D.; Charbonneau, P.; Thibault, K.
2006SPD....37.3004C Altcode: 2006BAAS...38..257C
We have developed an n-body simulation to model the motion and mutual
interaction of small scale magnetic elements at the solar surface. The
model is highly simplified and is based on the observation that small
magnetic elements are passively advected by the granular flow (which
involves granules fragmenting, merging, disappearing, and reforming). We
approximate this complicated motion as a random walk. With a great many
magnetic elements executing this random walk simultaneously, collisions
are inevitable. We assume that these collisions lead to the coalescence
of the colliding magnetic elements (if they have the same polarity)
or mutual cancellation (opposite polarity). In a similar fashion,
the resulting aggregates (or clusters) can subsequently interact with
more magnetic elements or with other clusters. The essential finding
is that this process (depending on the model parameters) can produce
structures on a spatial scale comparable to supergranulation (15-35Mm).
---------------------------------------------------------
Title: The origin of solar cycle fluctuations
Authors: Charbonneau, P.
2006cosp...36.2418C Altcode: 2006cosp.meet.2418C
A proper understanding of the physical origin of fluctuations in the
amplitude and duration of the solar magnetic activity cycle is essential
for our understanding of solar influences on Earth s climate and space
environment as well as for long-range forecasting of overall magnetic
activity levels on timescales commensurate with the planning of future
space missions In this talk I will argue that solar cycle fluctuations
can be traced at least in part to deterministic effects associated with
long time delays in the dynamo process I will present recent modelling
results supporting this conjecture and show that Maunder-minimum-like
episodes of strongly suppressed activity can arise naturally from
perturbation of this time-delay dynamics by low-amplitude magnetic
noise as produced for example by small-scale turbulent dynamo action
due to convection
---------------------------------------------------------
Title: Genetic magnetohelioseismology with Hankel analysis data
Authors: Crouch, A. D.; Cally, P. S.; Charbonneau, P.; Braun, D. C.;
Desjardins, M.
2005MNRAS.363.1188C Altcode: 2005MNRAS.tmp..856C
Hankel analysis determined that sunspots absorb energy from and
shift the phase of f- and p-modes incident upon them. One promising
mechanism that can explain the absorption is partial conversion
to slow magnetoacoustic-gravity (MAG) waves and Alfvén waves,
which guide energy along the magnetic field away from the acoustic
cavity. Our recent mode conversion calculations demonstrated that
simple sunspot models, which roughly account for the radial variation
of the magnetic field strength and inclination, can produce ample
absorption to explain the observations, along with phase shifts that
agree remarkably well with the Hankel analysis data. In this paper,
we follow the same approach, but adopt a more realistic model for the
solar convection zone that includes the thermal perturbation associated
with a sunspot's magnetic field. Consistent with our earlier findings,
we show that a moderately inclined, uniform magnetic field exhibits
significantly enhanced absorption (mode conversion) in comparison
to a vertical field (depending on the frequency and radial order of
the mode). A genetic algorithm is employed to adjust the parameters
that control the radial structure of our sunspot models, in order
to minimize the discrepancy between the theoretical predictions and
the Hankel analysis measurements. For models that best fit the phase
shifts, the agreement with the Hankel analysis data is excellent, and
the corresponding absorption coefficients are generally in excess of the
observed levels. On the other hand, for models that best fit the phase
shift and absorption data simultaneously, the overall agreement is very
good but the phase shifts agree less well. This is most likely caused
by the different sizes of the regions responsible for the absorption and
phase shift. Typically, the field strengths required by such models lie
in the range 1-3kG, compatible with observations for sunspots and active
regions. While there remain some uncertainties, our results provide
further evidence that mode conversion is the predominant mechanism
responsible for the observed absorption in sunspots; and that field
inclination away from vertical is a necessary ingredient for any model
that aims to simultaneously explain the phase shift and absorption data.
---------------------------------------------------------
Title: A New Avalanche Model for Solar Flares
Authors: Morales, L.; Charbonneau, P.
2005ESASP.592..507M Altcode: 2005soho...16E..93M; 2005ESASP.592E..93M
No abstract at ADS
---------------------------------------------------------
Title: Solar Flares: Avalanche Models and Data Assimilation
Authors: Bélanger, Eric; Charbonneau, Paul; Vincent, Alain
2005JRASC..99R.133B Altcode:
Solar flares play an important part in space meteorology because they
can eject charged particles that are the source of the geomagnetic
storms on Earth. These storms can interfere with communication
satellites and overload electric transformers. We need first to
understand the physical mechanisms at the origin of flares and to
be able to predict them sufficiently in advance. Several models
to explain the mechanism of solar flares were suggested. We have
investigated the self-organized criticality (or avalanche) model where
an instability related to the reconnection of the magnetic field lines
is propagated. The techniques of data assimilation were applied to
a 2-D avalanche model. Data assimilation generates better forecasts
by taking advantage of both the theoretical/numerical models and the
observations. With the increase in computational power and the numerous
satellites (SOHO, TRACE) observing the Sun with an improved spatial
and temporal resolution, these methods will surely give us a better
understanding of solar flares.
---------------------------------------------------------
Title: A Maunder Minimum Scenario Based on Cross-Hemispheric Coupling
and Intermittency
Authors: Charbonneau, Paul
2005SoPh..229..345C Altcode:
A novel scenario for Maunder minimum-like grand minima epochs of
reduced solar activity is proposed, based on diffusive coupling between
both solar hemispheres, each susceptible to stochastically-driven
intermittent behavior. After introducing cross-hemispheric coupling
into a well-validated reduced model of the solar cycle based on the
Babcock-Leighton mechanism for poloidal field regeneration, simulations
are presented demonstrating that even weak coupling can lead to a
high degree of synchronicity between the two hemispheres. This is in
qualitative agreement with the similar onset and recovery times of
sunspot activity at and around the Maunder minimum. Moreover, even
weak coupling manages to greatly reduce the frequency and duration of
quiescent episodes, again in qualitative agreement with the relative
paucity of grand minima in the sunspot and radioisotope records.
---------------------------------------------------------
Title: Dynamo Models of the Solar Cycle
Authors: Charbonneau, Paul
2005LRSP....2....2C Altcode:
This paper reviews recent advances and current debates in modeling the
solar cycle as a hydromagnetic dynamo process. Emphasis is placed on
(relatively) simple dynamo models that are nonetheless detailed enough
to be comparable to solar cycle observations. After a brief overview
of the dynamo problem and of key observational constraints, we begin
by reviewing the various magnetic field regeneration mechanisms that
have been proposed in the solar context. We move on to a presentation
and critical discussion of extant solar cycle models based on these
mechanisms. We then turn to the origin of fluctuations in these
models, including amplitude and parity modulation, chaotic behavior,
and intermittency. The paper concludes with a discussion of our
current state of ignorance regarding various key questions, the most
pressing perhaps being the identification of the physical mechanism(s)
responsible for the generation of the Sun's poloidal magnetic field
component.
---------------------------------------------------------
Title: Sunspot fragmentation and total solar irradiance modelling
Authors: Crouch, A. D.; Charbonneau, P.; Tapping, K. F.; Paquin-Ricard,
D.
2005AGUSMSH23B..03C Altcode:
Observational evidence suggests that sunspot decay is due, at least
in part, to a fragmentation process. We have developed a model for
this, where the sunspot fragmentation produces an ensemble of small
scale magnetic flux tubes, which themselves can remain at the surface
for many days. The number of sunspots emerging at the solar surface
varies over the 11 year solar activity cycle. Consequently, the size
distribution of magnetic structures is also modulated by the cycle. The
simplicity of our model allows us to track the evolution of this
distribution over very long time scales (many solar cycles). There are
several applications for such a model. For example, to the total solar
irradiance, which also varies over the solar cycle. Broadly speaking,
the irradiance contribution from different magnetic features depends
on their size (large features, such as sunspots, are dark, whereas
small flux tubes tend to be bright). By combining this property with
our sunspot fragmentation model, we show that the resultant system
can produce an irradiance contribution that behaves very much like the
observed total solar irradiance. Our model has several input parameters
(one crucial example is the actual relationship between the size of a
magnetic feature and its brightness). We have used a genetic algorithm
to adjust the parameters in order to optimize the agreement between
our model and the observations. Will we discuss those results and
also comment on the contribution our model makes to the weak field
component of solar magnetic flux budget.
---------------------------------------------------------
Title: Modelling the Interaction of p-modes With Sunspots
Authors: Crouch, A. D.; Cally, P. S.; Charbonneau, P.; Desjardins, M.
2005AGUSMSP23C..04C Altcode:
Sunspots absorb energy from and shift the phase of f- and p-modes
incident upon them. One promising absorption mechanism is partial
conversion to slow magnetoacoustic-gravity waves (and Alfvén waves),
which guide energy along the magnetic field away from the acoustic
cavity. Recent mode conversion calculations by Cally, Crouch, and
Braun have shown that simple sunspot models with non-vertical magnetic
fields can produce ample absorption to explain the observations,
along with phase shift predictions that agree remarkably well with the
Hankel analysis data. In this investigation, we further test the mode
conversion hypothesis. We use a realistic solar model that accounts for
both the magnetic and thermal influences associated with a sunspot. Our
model has several adjustable parameters - the field strength and
inclination can vary (crudely) across the spot. We employ a genetic
algorithm to adjust these parameters to optimize the agreement between
the model predictions and the observations. At this stage, our model
is too simple to perform quantitative forward modelling. However,
the genetic algorithm allows us to rigorously test the model. We
will discuss the results of this testing in detail. Broadly speaking,
our findings are consistent with those of Cally, Crouch, and Braun:
the predicted phase shifts are in excellent agreement with the
Hankel analysis data, and the corresponding absorption coefficients
are generally ample to explain the observations. While there remain
several uncertainties, our results further verify that mode conversion
is a significant process in sunspot acoustics.
---------------------------------------------------------
Title: Rotation and Magnetic Fields: the Evil Twins of Stellar
Evolution
Authors: Charbonneau, P.
2005EAS....17..217C Altcode:
In this paper I give an overview of the numerous ways in which rotation
and magnetic fields can interact under stellar interior conditions. I
first provide “tutorial” examples of how magnetic fields can (1)
alter existing stellar internal flows, (2) generate internal flows,
and of how rotation can (3) amplify or (4) destroy magnetic fields. The
upshot of all this is that treating rotation or magnetic fields in
isolation of one another, as intermediate steps towards the “full
picture”, may yield a situation that can only be applied meaningfully
under very limited and specific astrophysical circumstances, if any.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Intermittency and Phase Persistence in a Babcock-Leighton
Model of the Solar Cycle
Authors: Charbonneau, Paul; Blais-Laurier, Guillaume; St-Jean, Cédric
2004ApJ...616L.183C Altcode:
We present and discuss a numerical simulation of the solar cycle based
on the Babcock-Leighton mechanism of poloidal field regeneration by
the surface decay of sunspots. The simulation includes low-amplitude
stochastic noise and exhibits intermittency, i.e., quiescent episodes
of strongly reduced amplitude irregularly interspersed between epochs
of “normal” cyclic behavior. We show that the phase of the cycle
can persist across these quiescent episodes, a feature normally not
expected from intermittency. We ascribe this behavior to the regulatory
influence of meridional circulation in the solar convective envelope,
which is known to be the primary determinant of cycle period in this
class of dynamo models. We also discuss similarities and differences
between these results and the behavior of the sunspot cycle during
the Maunder minimum of solar activity.
---------------------------------------------------------
Title: Three Single Stars (See How They Spin) (Invited Review)
Authors: Charbonneau, P.
2004IAUS..215..366C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Are surface magnetic fields responsible for the solar
irradiance variation?
Authors: Crouch, A. D.; Charbonneau, P.
2004AAS...204.2009C Altcode: 2004BAAS...36..687C
Observations over the last 25 years have shown that the total solar
irradiance varies on the same timescale as the solar magnetic activity
cycle (11 years). The irradiance fluctuates by about 0.1% and peaks
during solar maximum. The cause of the positive correlation between
magnetic activity and irradiance is still unclear. We investigate the
influence of surface magnetic fields on heat transport in the solar
convection zone and their role in the subsequent energy output at the
solar photosphere. We consider simplified two-dimensional models of heat
transport by convection based on the diffusion approximation. Modelling
the effect of magnetic fields on convection is very complicated. Our
approach is based on two observations. Broadly speaking, the presence
of strong surface magnetic fields tends to suppress convection and
block heat transport. Consequently, sunspots (large-scale magnetic flux
tubes) appear darker than the surrounding photosphere. In contrast,
small-scale magnetic flux tubes appear brighter. We account for
both of these effects by modifying the diffusion coefficient in
magnetised regions (according to their size and other parameter such
as temperature). We track the evolution over several solar cycles and
determine the correlation between the flux output at the surface and
the surface magnetic filling factor. We are then able to investigate
what conditions are necessary for heat to be stored in the subsurface
layers over timescales comparable to the solar cycle.
---------------------------------------------------------
Title: Erratum: Continuum analysis of an avalanche model for solar
flares [ Phys. Rev. E 66, 056111 (2002)]
Authors: Liu, Han-Li; Charbonneau, Paul; Pouquet, Annick; Bogdan,
Thomas; McIntosh, Scott
2004PhRvE..69e9904L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flares as Avalanches?
Authors: Charbonneau, P.
2003SPD....34.2101C Altcode: 2003BAAS...35..849C
In 1991, E.T. Lu and R. Hamilton (ApJ 380, L89) suggested that
flares could be interpreted as avalanches of reconnection events
in coronal magnetic structures driven to a self-organized critical
state. Physical underpinning for the simple cellular automaton model
they used to illustrate their idea can be readily found in the nanoflare
conjecture for coronal heating championed over the past two decades
by E.N. Parker (e.g., ApJ 330, 474 [1988]). <P />In this lecture I
will give a brief overview of Lu & Hamilton's avalanche model,
and describe how it can be physically interpreted in the context of
Parker's nanoflare conjecture. After discussing some illustrative model
results, I will focus on recent comparisons of the model's predictions
with flare observations. Finally, I will discuss some recent attempts
at quantitatively exploring the physical relationship between model
components and the physics of magnetic reconnection.
---------------------------------------------------------
Title: Stellar structure modeling using a parallel genetic algorithm
for objective global optimization
Authors: Metcalfe, Travis S.; Charbonneau, Paul
2003JCoPh.185..176M Altcode: 2002astro.ph..8315M
Genetic algorithms are a class of heuristic search techniques that
apply basic evolutionary operators in a computational setting. We
have designed a fully parallel and distributed hardware/software
implementation of the generalized optimization subroutine PIKAIA, which
utilizes a genetic algorithm to provide an objective determination
of the globally optimal parameters for a given model against an
observational data set. We have used this modeling tool in the context
of white dwarf asteroseismology, i.e., the art and science of extracting
physical and structural information about these stars from observations
of their oscillation frequencies. The efficient, parallel exploration
of parameter-space made possible by genetic-algorithm-based numerical
optimization led us to a number of interesting physical results:
(1) resolution of a hitherto puzzling discrepancy between stellar
evolution models and prior asteroseismic inferences of the surface
helium layer mass for a DBV white dwarf; (2) precise determination
of the central oxygen mass fraction in a white dwarf star; and (3) a
preliminary estimate of the astrophysically important but experimentally
uncertain rate for the <SUP>12</SUP>C(α,γ)<SUP>16</SUP>O nuclear
reaction. These successes suggest that a broad class of computationally
intensive modeling applications could also benefit from this approach.
---------------------------------------------------------
Title: The rise and fall of the first solar cycle model
Authors: Charbonneau, Paul
2002JHA....33..351C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Continuum analysis of an avalanche model for solar flares
Authors: Liu, Han-Li; Charbonneau, Paul; Pouquet, Annick; Bogdan,
Thomas; McIntosh, Scott
2002PhRvE..66e6111L Altcode:
We investigate the continuum limit of a class of self-organized
critical lattice models for solar flares. Such models differ from the
classical numerical sandpile model in their formulation of stability
criteria in terms of the curvature of the nodal field, and are known
to belong to a different universality class. A fourth-order nonlinear
hyperdiffusion equation is reverse engineered from the discrete model's
redistribution rule. A dynamical renormalization-group analysis of the
equation yields scaling exponents that compare favorably with those
measured in the discrete lattice model within the relevant spectral
range dictated by the sizes of the domain and the lattice grid. We
argue that the fourth-order nonlinear diffusion equation that models
the behavior of the discrete model in the continuum limit is, in fact,
compatible with magnetohydrodynamics (MHD) of the flaring phenomenon
in the regime of strong magnetic field and the effective magnetic
diffusivity characteristic of strong MHD turbulence.
---------------------------------------------------------
Title: Geometrical properties of avalanches in self-organized critical
models of solar flares
Authors: McIntosh, Scott W.; Charbonneau, Paul; Bogdan, Thomas J.;
Liu, Han-Li; Norman, James P.
2002PhRvE..65d6125M Altcode:
We investigate the geometrical properties of avalanches in
self-organized critical models of solar flares. Traditionally, such
models differ from the classical sandpile model in their formulation of
stability criteria in terms of the curvature of the nodal field, and
belong to a distinct universality class. With a view toward comparing
these properties to those inferred from spatially and temporally
resolved flare observations, we consider the properties of avalanche
peak snapshots, time-integrated avalanches in two and three dimensions,
and the two-dimensional projections of the latter. The nature of the
relationship between the avalanching volume and its projected area is
an issue of particular interest in the solar flare context. Using our
simulation results we investigate this relationship, and demonstrate
that proper accounting of the fractal nature of avalanches can bring
into agreement hitherto discrepant results of observational analyses
based on simple, nonfractal geometries for the flaring volume.
---------------------------------------------------------
Title: Effects of Temperature Bias on Nanoflare Statistics
Authors: Aschwanden, Markus J.; Charbonneau, Paul
2002ApJ...566L..59A Altcode:
Statistics of solar flares, microflares, and nanoflares have been
gathered over an energy range of some 8 orders of magnitude, over
E~10<SUP>24</SUP>-10<SUP>32</SUP> ergs. Frequency distributions
of flare energies are always determined in a limited temperature
range, e.g., at T~1-2 MK if the 171 and 195 Å filters are used
from an extreme ultraviolet telescope (the Solar and Heliospheric
Observatory/EUV Imaging Telescope or the Transitional Region and
Coronal Explorer). Because the electron temperature T<SUB>e</SUB>
and the thermal energy E=3n<SUB>e</SUB>k<SUB>B</SUB>T<SUB>e</SUB>V
are statistically correlated in flare processes, statistics in
a limited temperature range introduce a bias in the frequency
distribution of flare energies, N(E)~E<SUP>-a<SUB>E</SUB></SUP>. We
demonstrate in this Letter that the power-law slope of nanoflare
energies, e.g., a<SUB>E</SUB>~1.9, as determined in a temperature
range of T~1.1-1.6 MK (195 Å), corresponds to a corrected value of
a<SUP>'</SUP><SUB>E</SUB>~1.4 in an unbiased, complete sample. This
corrected value is in much better agreement with predictions from
avalanche models of solar flares. However, it also implies that all
previously published power-law slopes of EUV nanoflares, covering
a range of a<SUB>E</SUB>~1.8-2.3, correspond to unbiased values of
a<SUB>E</SUB><2, which then poses a serious challenge to Parker's
hypothesis of coronal heating by nanoflares.
---------------------------------------------------------
Title: Mixing in magnetized interiors of solar-type stars: frequently
asked questions
Authors: Charbonneau, Paul
2002HiA....12..301C Altcode:
This short paper discusses various issues pertaining to the
redistribution of angular momentum and mixing of chemical species in
the radiative interior of solar-type stars. Results obtained to date
indicate that models combining magnetically-mediated angular momentum
transport with turbulent mixing of chemical species offer the best
hope of explaining the observed rotation and light element evolution
in open clusters, without doing excess violence to seismic measurements
of the solar internal differential rotation.
---------------------------------------------------------
Title: Geometric Effects in Avalanche Models of Solar Flares:
Implications for Coronal Heating
Authors: McIntosh, S. W.; Charbonneau, P.
2001ApJ...563L.165M Altcode:
Observational inferences of the power-law frequency distribution of
energy release by solar flares, and in particular its logarithmic
slope α<SUB>E</SUB>, depend critically on the geometric relationship
assumed to relate the observed emitting area A and the underlying
emitting volume V. Recent results on the fractal nature of avalanches
in self-organized critical models for solar flares indicate that
this relationship is a power law V~A<SUP>γ</SUP> with index
γ=1.41(+/-0.04). We show that when proper account is made for
the fractal geometry of the flaring volume, hitherto discrepant
observational inferences of α<SUB>E</SUB> are brought in much closer
agreement. The resulting values of α<SUB>E</SUB> lie tantalizingly
close, but still below the critical value α<SUB>E</SUB>=2.0, beyond
which Parker's conjecture of coronal heating by nanoflares is tenable.
---------------------------------------------------------
Title: Avalanche models for solar flares (Invited Review)
Authors: Charbonneau, Paul; McIntosh, Scott W.; Liu, Han-Li; Bogdan,
Thomas J.
2001SoPh..203..321C Altcode:
This paper is a pedagogical introduction to avalanche models of solar
flares, including a comprehensive review of recent modeling efforts and
directions. This class of flare model is built on a recent paradigm in
statistical physics, known as self-organized criticality. The basic
idea is that flares are the result of an `avalanche' of small-scale
magnetic reconnection events cascading through a highly stressed coronal
magnetic structure, driven to a critical state by random photospheric
motions of its magnetic footpoints. Such models thus provide a natural
and convenient computational framework to examine Parker's hypothesis
of coronal heating by nanoflares.
---------------------------------------------------------
Title: Magnetic Fields in Massive Stars. I. Dynamo Models
Authors: Charbonneau, Paul; MacGregor, Keith B.
2001ApJ...559.1094C Altcode:
Motivated by mounting evidence for the presence of magnetic fields
in the atmospheres of “normal” early-type main-sequence stars,
we investigate the various possible modes of dynamo action in
their convective core. Working within the framework of mean field
electrodynamics, we compute α<SUP>2</SUP> and α<SUP>2</SUP>Ω
dynamo models and demonstrate that the transition from the former
class to the latter occurs smoothly as internal differential rotation
is increased. Our models also include a magnetic diffusivity contrast
between the core and radiative envelope. The primary challenge facing
such models is to somehow bring the magnetic field generated in the
deep interior to the stellar surface. We investigate the degree to which
thermally driven meridional circulation can act as a suitable transport
agent. In all models with strong core-to-envelope magnetic diffusivity
contrast-presumably closest to reality- whenever circulation is strong
enough to carry a significant magnetic flux, it is also strong enough
to prevent dynamo action. Estimates of typical meridional circulation
speeds indicate that this regime is likely not attained in the interior
of early-type main-sequence stars. Dynamo action then remains highly
probable, but an alternate mechanism must be sought to carry the
magnetic field to the surface.
---------------------------------------------------------
Title: Preliminary Constraints on <SUP>12</SUP>C(α,γ)<SUP>16</SUP>O
from White Dwarf Seismology
Authors: Metcalfe, T. S.; Winget, D. E.; Charbonneau, P.
2001ApJ...557.1021M Altcode: 2001astro.ph..4205M
For many years, astronomers have promised that the study of pulsating
white dwarfs would ultimately lead to useful information about
the physics of matter under extreme conditions of temperature and
pressure. In this paper, we finally make good on that promise. Using
observational data from the Whole Earth Telescope and a new analysis
method employing a genetic algorithm, we empirically determine that the
central oxygen abundance in the helium-atmosphere variable white dwarf
GD 358 is 84%+/-3%. We use this value to place preliminary constraints
on the <SUP>12</SUP>C(α,γ)<SUP>16</SUP>O nuclear reaction cross
section. More precise constraints will be possible with additional
detailed simulations. We also show that the pulsation modes of our
best-fit model probe down to the inner few percent of the stellar
mass. We demonstrate the feasibility of reconstructing the internal
chemical profiles of white dwarfs from asteroseismological data and
find an oxygen profile for GD 358 that is qualitatively similar to
recent theoretical calculations.
---------------------------------------------------------
Title: Waiting-Time Distributions in Lattice Models of Solar Flares
Authors: Norman, James P.; Charbonneau, Paul; McIntosh, Scott W.;
Liu, Han-Li
2001ApJ...557..891N Altcode:
It has recently been argued that the distribution of waiting times
between successive solar flares is incompatible with the prediction
of lattice models, which interpret flares as avalanches of magnetic
reconnection events within a stressed magnetic structure driven to
a state of self-organized criticality by stochastic motions of the
photospheric magnetic footpoints. Inspired by a suggestion recently
made by Wheatland, we construct modified lattice models driven by
a nonstationary random process. The resulting models have frequency
distributions of waiting times that include a power-law tail at long
waiting times, in agreement with observations. One model, based on a
random walk modulation of an otherwise stationary driver, yields an
exponent for the power-law tail equal to 2.51+/-0.16, in reasonable
agreement with observational inferences. This power-law tail survives
in the presence of noise and a detection threshold. These findings
lend further support to the avalanche model for solar flares.
---------------------------------------------------------
Title: Geometrical Aspects of SOC Flare Models
Authors: McIntosh, S. W.; Charbonneau, P.
2001AGUSM..SP52B04M Altcode:
In this paper we address the geometrical properties of SOC Flare Models
and possible connections to high spatial resolution observations of the
solar coronal plasma. We discuss the study of geometrical projection
effects and the fractal nature of avalanches in large two and three
dimensional Cartesian lattice models and of (nano-)flares observed by
the TRACE spacecraft. We examine the differences, and similarities,
between the behavior of the lattice model and the observational data.
---------------------------------------------------------
Title: Analysis of an avalanche model in the continuum limit
Authors: Liu, H.; Charbonneau, P.; Bogdan, T. J.; Pouquet, A.;
McIntosh, S. W.; Norman, J. P.
2001AGUSM..SP51C03L Altcode:
It is shown that in the continuum limit, the avalanche system postulated
by Lu and Hamilton (1991) (LH91) can be described by a hyper-diffusion
equation in regions where every lattice is in avalanche, and the
overall system can be approximated by a randomly forced system with
a anomalous hyper-diffusion term and a cubic nonlinear transport
term. The LH91 is equivalent to a finite difference approximation to
the the equation with 2nd order center differencing in space and simple
forward time integration, and is numerically unstable. The modified rule
by Lu et al. (1993) (LH93) actually overcame the numerical stability
problem by essentially reducing the diffusion coefficient. We apply a
dynamical renormalization group analysis to the continuum system. The
frequency power spectrum scaling behavior of the "dissipating energy"
and "falling-off energy" derived from this analysis is in reasonable
agreement with the results from the LH93 avalanche model.
---------------------------------------------------------
Title: Multiperiodicity, Chaos, and Intermittency in a Reduced Model
of the Solar Cycle
Authors: Charbonneau, Paul
2001SoPh..199..385C Altcode:
In a recent paper, Durney (2000) has discussed a physically plausible
procedure whereby the dynamo equations describing magnetic field
regeneration in Babcock-Leighton models of the solar cycle can be
reduced to a one-dimensional iterative map. This procedure is used here
to investigate the behavior of various dynamo-inspired maps. Durney's
explanation of the so-called odd-even effect in sunspot cycle peak
amplitudes, which he ascribed to a period-2 limit cycle, is found to
be robust with respect the choice of nonlinearity defining the map,
and to the action of strong stochastic forcing. In fact, even maps
without limit cycles are found to show a strong odd-even signal in the
presence of forcing. Some of the stochastically forced maps are found
to exhibit a form of on-off intermittency, with periods of activity
separated by quiescent phases of low cycle amplitudes. In one such
map, a strong odd-even signal is found to be a good precursor to the
transition from bursting to quiescent behavior.
---------------------------------------------------------
Title: Towards a Global Picture: Observable Effects Induced by or
Related to Magnetic Fields
Authors: Judge, P. G.; Charbonneau, P.
2001ASPC..248..659J Altcode: 2001mfah.conf..659J
No abstract at ADS
---------------------------------------------------------
Title: Stellar Dynamos: A Modeling Perspective
Authors: Charbonneau, P.; Saar, S. H.
2001ASPC..248..189C Altcode: 2001mfah.conf..189C
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Cycles and Activity in FGK Stars in the Framework
of Babcock-Leighton Dynamos
Authors: Dikpati, M.; Saar, S. H.; Brummell, N.; Charbonneau, P.
2001ASPC..248..235D Altcode: 2001mfah.conf..235D
No abstract at ADS
---------------------------------------------------------
Title: Stochastic Fluctuations in a Babcock-Leighton Model of the
Solar Cycle
Authors: Charbonneau, Paul; Dikpati, Mausumi
2000ApJ...543.1027C Altcode:
We investigate the effect of stochastic fluctuations on a flux
transport model of the solar cycle based on the Babcock-Leighton
mechanism. Specifically, we make use of our recent flux transport
model (Dikpati & Charbonneau) to investigate the consequences of
introducing large-amplitude stochastic fluctuations in either or both
the meridional flow and poloidal source term in the model. Solar
cycle-like oscillatory behavior persists even for fluctuation
amplitudes as high as 300%, thus demonstrating the inherent robustness
of this class of solar cycle models. We also find that high-amplitude
fluctuations lead to a spread of cycle amplitude and duration showing
a statistically significant anticorrelation, comparable to that
observed in sunspot data. This is a feature of the solar cycle that is
notoriously difficult to reproduce with dynamo models based on mean
field electrodynamics and relying only on nonlinearities associated
with the back-reaction of the Lorentz force to produce amplitude
modulation. Another noteworthy aspect of our flux transport model is the
fact that meridional circulation in the convective envelope acts as a
“clock” regulating the tempo of the solar cycle; shorter-than-average
cycles are typically soon followed by longer-than-average cycles. In
other words, the oscillation exhibits good phase locking, a property
that also characterizes the solar activity cycle. This shows up quite
clearly in our model, but we argue that it is in fact a generic property
of flux transport models based on the Babcock-Leighton mechanism,
and relies on meridional circulation as the primary magnetic field
transport agent.
---------------------------------------------------------
Title: Rotation & Turbulence in the Solar Radiative Core
Authors: Charbonneau, Paul
2000astu.confE...8C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Discussion on The Angular Momentum Distribution in the Sun:
Rotation, Turbulence, and Magnetic Fields
Authors: Charbonneau, Paul; Spiegel, Ed
2000astu.confE...9C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preconditioning the Differential Emission Measure
(T<SUB>e</SUB>) Inverse Problem
Authors: McIntosh, S. W.; Charbonneau, P.; Brown, J. C.
2000ApJ...529.1115M Altcode:
In an inverse problem of any kind, poor conditioning of the inverse
operator decreases the numerical stability of any unregularized
solution in the presence of data noise. In this paper we show that
the numerical stability of the differential emission measure (DEM)
inverse problem can be considerably improved by judicious choice of
the integral operator. Specifically, we formulate a combinatorial
optimization problem where, in a preconditioning step, a subset of
spectral lines is selected in such a way as to minimize explicitly the
condition number of the discretized integral operator. We tackle this
large combinatorial optimization problem using a genetic algorithm. We
apply this preconditioning technique to a synthetic data set comprising
of solar UV/EUV emission lines in the SOHO SUMER/CDS wavelength
range. Following which we test the same hypothesis on lines observed by
the Harvard S-055 EUV spectroheliometer. On performing the inversion
we see that the temperature distribution in the emitting region of
the solar atmosphere is recovered with considerably better stability
and smaller error bars when our preconditioning technique is used,
in both synthetic and “real” cases, even though this involves
the analysis of fewer spectral lines than in the “All-lines”
approach. The preconditioning step leads to regularized inversions
that compare favorably to inversions by singular value decomposition,
while providing greater flexibility in the incorporation of physically
and/or observationally based constraints in the line selection process.
---------------------------------------------------------
Title: Magnetic fields and light element depletion in the Sun
Authors: Charbonneau, P.; Barnes, G.; MacGregor, K. B.
2000IAUJD...5E..14C Altcode:
I will first briefly review some important similarities and differences
in models for the spin-down of solar-type stars, with or without
internal magnetic fields in their radiative interior. This will
be followed by a presentation of some simple calculations for
the main-sequence depletion of Lithium and Beryllium in the Sun,
in a regime where magnetic fields provide the chief mechanism for
the internal redistribution of angular momentum. In this model the
transport of light elements still occurs in response to shear-induced
small-scale turbulence, following various commonly used prescriptions
for the transport coefficients. For some (physically reasonable)
values of model parameters, both internal differential rotation and
light element abundances end up solar-like at 4.5Gyr. Within this
framework light element depletion is a sensitive function of the
strength of the assumed internal magnetic field.
---------------------------------------------------------
Title: Helioseismic Constraints on the Structure of the Solar
Tachocline
Authors: Charbonneau, P.; Christensen-Dalsgaard, J.; Henning, R.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Tomczyk, S.
1999ApJ...527..445C Altcode:
This paper presents a series of helioseismic inversions aimed at
determining with the highest possible confidence and accuracy the
structure of the rotational shear layer (the tachocline) located
beneath the base of the solar convective envelope. We are particularly
interested in identifying features of the inversions that are robust
properties of the data, in the sense of not being overly influenced
by the choice of analysis methods. Toward this aim we carry out
two types of two-dimensional linear inversions, namely Regularized
Least-Squares (RLS) and Subtractive Optimally Localized Averages
(SOLA), the latter formulated in terms of either the rotation rate or
its radial gradient. We also perform nonlinear parametric least-squares
fits using a genetic algorithm-based forward modeling technique. The
sensitivity of each method is thoroughly tested on synthetic data. The
three methods are then used on the LOWL 2 yr frequency-splitting
data set. The tachocline is found to have an equatorial thickness
of w/R<SUB>solar</SUB>=0.039+/-0.013 and equatorial central radius
r<SUB>c</SUB>/R<SUB>solar</SUB>=0.693+/-0.002. All three techniques
also indicate that the tachocline is prolate, with a difference in
central radius Δr<SUB>c</SUB>/R<SUB>solar</SUB>~=0.024+/-0.004 between
latitude 60° and the equator. Assuming uncorrelated and normally
distributed errors, a strictly spherical tachocline can be rejected
at the 99% confidence level. No statistically significant variation
in tachocline thickness with latitude is found. Implications of these
results for hydrodynamical and magnetohydrodynamical models of the
solar tachocline are discussed.
---------------------------------------------------------
Title: Stability of the Solar Latitudinal Differential Rotation
Inferred from Helioseismic Data
Authors: Charbonneau, Paul; Dikpati, Mausumi; Gilman, Peter A.
1999ApJ...526..523C Altcode:
We revisit the hydrodynamical stability problem posed by the
observed solar latitudinal differential rotation. Specifically,
we carry out stability analyses on a spherical shell for
solar-like two-dimensional inviscid shear flow profiles of the form
ν=s<SUB>0</SUB>-s<SUB>2</SUB>μ<SUP>2</SUP>-s<SUB>4</SUB>μ<SUP>4</SUP>,
where μ is the sine of latitude. We find that stability is remarkably
sensitive to the magnitude of the μ<SUP>4</SUP> term. This allows us
to reconcile apparently conflicting results found in the published
literature. We then use latitudinal differential rotation profiles
extracted from various helioseismic inversions of the solar internal
rotation and investigate their stability as a function of depth from the
base of the tachocline to the top of the convective envelope. In all
cases considered, we find that the latitudinal differential rotation
in the tachocline is stable while that in the bulk of the convective
envelope is unstable. Under the assumption that the instability is
not impeded by finite Reynolds number or three-dimensional effects
not accounted for in our analysis, we speculate on possible observable
consequences of the occurrence of the instability in the top half of
the convective envelope.
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones. IV. Ferraro's Theorem and the Solar Tachocline
Authors: MacGregor, K. B.; Charbonneau, P.
1999ApJ...519..911M Altcode:
We consider the circumstances under which the latitudinal differential
rotation of the solar convective envelope can (or cannot) be imprinted
on the underlying radiative core through the agency of a hypothetical
weak, large-scale poloidal magnetic field threading the solar
radiative interior. We do so by constructing steady, two-dimensional
axisymmetric solutions to the coupled momentum and induction equations
under the assumption of a purely zonal flow and time-independent
poloidal magnetic field. Our results show that the structure of the
interior solutions is entirely determined by the boundary conditions
imposed at the core-envelope interface. Specifically, in the high
Reynolds number regime a poloidal field having a nonzero component
normal to the core-envelope interface can lead to the transmission
of significant differential rotation into the radiative interior. In
contrast, for a poloidal field that is contained entirely within
the radiative core, any differential rotation is confined to a
thin magnetoviscous boundary layer located immediately beneath the
interface, as well as along the rotation/magnetic axis. We argue
that a magnetically decoupled configuration is more likely to be
realized in the solar interior. Consequently, the helioseismically
inferred lack of differential rotation in the radiative core does not
necessarily preclude the existence of a weak, large-scale poloidal
field therein. We suggest that such a field may well be dynamically
significant in determining the structure of the solar tachocline.
---------------------------------------------------------
Title: A Babcock-Leighton Flux Transport Dynamo with Solar-like
Differential Rotation
Authors: Dikpati, Mausumi; Charbonneau, Paul
1999ApJ...518..508D Altcode:
We investigate the properties of a kinematic flux transport solar
dynamo model. The model is characterized by a solar-like internal
differential rotation profile, a single-cell meridional flow in
the convective envelope that is directed poleward at the surface,
and a magnetic diffusivity that is constant within the envelope but
decreases sharply at the core-envelope interface. As in earlier flux
transport models of the Babcock-Leighton type, we assume that the
poloidal field is regenerated as a consequence of the emergence at the
surface, and subsequent decay, of bipolar active regions exhibiting
a systematic tilt with respect to the east-west direction. Inspired
by recent simulations of the rise of toroidal magnetic flux ropes
across the solar convective envelope, we model this poloidal field
regeneration mechanism as a nonlocal source term formulated in such
a way as to account for some of the properties of rising flux ropes
revealed by the simulations. For a broad range of parameter values the
model leads to solar cycle-like oscillatory solutions. Because of the
solar-like internal differential rotation profile used in the model,
solutions tend to be characterized by time-latitude (butterfly) diagrams
that exhibit both poleward- and equatorward-propagating branches. We
demonstrate that the latitudinal shear in the envelope, often omitted
in other flux transport models previously published in the literature,
actually has a dominant effect on the global morphology and period of
the solutions, while the radial shear near the core-envelope interface
leads to further intensification of the toroidal field. On the basis
of an extensive parameter space study, we establish a scaling law
between the time period of the cycle and the primary parameters of
the model, namely the meridional flow speed, source coefficient, and
turbulent diffusion coefficient. In the parameter regime expected
to characterize the Sun, we show that the time period of the cycle
is most significantly influenced by the circulation flow speed and,
unlike for conventional mean field αΩ dynamos, is little affected
by the magnitude of the source coefficient. Finally, we present one
specific solution that exhibits features that compare advantageously
with the observed properties of the solar cycle.
---------------------------------------------------------
Title: Preconditioning the DEM(T) inverse problem
Authors: Charbonneau, P.; McIntosh, S.
1999AAS...194.9313C Altcode: 1999BAAS...31..990C
In an inverse problem of any kind, poor conditioning of the inverse
operator decreases the numerical stability of any unregularized solution
in the presence of data noise. In this poster we show that the numerical
stability of the differential emission measure (DEM) inverse problem
can be considreably improved by judicious choice of the integral
operator. Specifically, we formulate a combinatorial optimization
problem where, in a preconditioning step, a subset of spectral
lines is selected in order to minimize the condition number of the
discretized integral operator. This turns out to be a hard combinatorial
optimization problem, which we tackle using a genetic algorithm. We
apply the technique to the dataset comprising the solar UV/EUV emission
lines in the SOHO SUMER/CDS wavelength range, and to the Harvard S-055
EUV spectroheliometer data. The temperature distribution in the emitting
region of the solar atmosphere is recovered with considerably better
stability and smaller error bars when our preconditioning technique is
used, even though this involves the analysis of fewer spectral lines
than in the conventional “all-lines” approach.
---------------------------------------------------------
Title: Intermittency in Solar Cycle Caused by Stochastic Fluctuation
in Meridional Circulation
Authors: Dikpati, M.; Charbonneau, P.
1999AAS...194.9204D Altcode: 1999BAAS...31..987D
We present here how the stochastic fluctuation in meridional circulation
can cause variation in amplitude and phase of the solar cycle as often
observed as a long-term variability in the butterfly diagram. It has
been shown by various authors that a kinematic, flux-transport dynamo
with meridional circulation is capable of reproducing a majority of
surface magnetic features. The scaling law, proposed on the basis of
detail parameter space survey, suggests that the time-period of cycle
is largely governed by flow speed. Since meridional circulation is a
weak flow, vigorous convections could perturb this flow randomly by
causing a stochastic fluctuation in its magnitude. Surface observations
suggest variation of flow speed with time, while simulations suggest
up to 100% fluctuation in amplitude of the flow in deep convection
zone also. Assuming a correlation length in the streamlines comparable
to dimension of granules and treating the correlation time (tau_c )
between two fluctuating flows and the amplitude of the fluctuating
component as parameters, we show that the long-term variability in the
amplitude of the solar cycle is most sensitive to the amplitude of the
stochastic fluctuation in meridional circulation, but not so sensitive
to tau_c . On the other hand, the longer tau_c affects the time-period,
though not violating the so-called "phase locking" of the solar cycle,
since we always find that whenever there occur a few short cycles,
a few compensating longer cycles also occur to adjust the solar clock
again. The National Center for Atmospheric Research is sponsored by
the National Science Foundation.
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones. III. The Solar Light-Element Abundances
Authors: Barnes, G.; Charbonneau, P.; MacGregor, K. B.
1999ApJ...511..466B Altcode:
We calculate the depletion of the trace elements lithium and beryllium
within a solar-mass star during the course of its evolution from
the zero-age main sequence to the age of the present-day Sun. In the
radiative layers beneath the convection zone, we assume that these
elements are transported by the turbulent fluid motions that result from
instability of the shear flow associated with internal differential
rotation. This turbulent mixing is modeled as a diffusion process,
using a diffusion coefficient that is taken to be proportional to the
gradient of the angular velocity distribution inside the star. We study
the evolution of the light-element abundances produced by rotational
mixing for models in which internal angular momentum redistribution
takes place either by hydrodynamic or by hydromagnetic means. Since
models based on these alternative mechanisms for angular-momentum
transport predict similar surface rotation rates late in the evolution,
we explore the extent to which light-element abundances make it possible
to distinguish between them. In the case of an internally magnetized
star, our computations indicate that both the details of the surface
abundance evolution and the magnitude of the depletion at solar age can
depend sensitively on the assumed strength and configuration of the
poloidal magnetic field inside the star. For a configuration with no
direct magnetic coupling between the radiative and convective portions
of the stellar interior, the depletion of lithium calibrated to the
solar lithium depletion at the solar age is similar at all ages to
the lithium depletion of a model in which angular-momentum transport
occurs solely by hydrodynamical processes. However, the two models can
be distinguished on the basis of their respective beryllium depletions,
with the depletion of the magnetic model being significantly smaller
than that of the nonmagnetic model.
---------------------------------------------------------
Title: Hélioseismologie de la tachocline solaire.
Authors: Charbonneau, P.
1998JRASC..92..311C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectral decomposition by genetic forward modelling
Authors: McIntosh, S. W.; Diver, D. A.; Judge, P. G.; Charbonneau,
P.; Ireland, J.; Brown, J. C.
1998A&AS..132..145M Altcode:
We discuss the analysis of real and simulated line spectra using
a genetic forward modelling technique. We show that this Genetic
Algorithm (GA) based technique experiences none of the user bias
or systematic problems that arise when faced with poorly sampled or
noisy data. An important feature of this technique is the ease with
which rigid a priori constraints can be applied to the data. These
constraints make the GA decomposition much more accurate and stable,
especially at the limit of instrumental resolution, than decomposition
algorithms commonly in use.
---------------------------------------------------------
Title: Empirical modeling of the solar corona using genetic algorithms
Authors: Gibson, S. E.; Charbonneau, P.
1998JGR...10314511G Altcode:
Many remote sensing applications encountered in astronomy and space
science involve the solution of nonlinear inverse problems. These
are often difficult to solve because of nonlinearities, ill-behaved
integration kernels, and amplification of data noise associated
with the inversion of the integral operator. In some cases these
difficulties are severe enough to warrant repeated evaluations of the
forward problem as an alternate approach to formal inversion. Because
a forward approach is intrinsically repetitive and time consuming, an
efficient and flexible forward technique is required for this avenue
to be practical. We show how a forward technique based on a genetic
algorithm allows us to fit magnetostatic models of the solar minimum
corona to observations in white light to a degree that would otherwise
have been computationally prohibitive. In addition, and perhaps equally
important, the method also allows the determination of global error
estimates on the model parameters defining the best fit solution.
---------------------------------------------------------
Title: Gravity Waves in a Magnetized Shear Layer
Authors: Barnes, G.; MacGregor, K. B.; Charbonneau, P.
1998ApJ...498L.169B Altcode:
We use the equations governing the propagation of a gravity wave
in the presence of a background flow and magnetic field to derive,
in the Boussinesq approximation, dispersion relations for plane wave
solutions in certain special cases. We show how, under conditions
typical of the interior of the Sun, the addition of a magnetic field
may prevent certain wavevectors from propagating and alter the existence
and position of any critical layer that might absorb the gravity wave.
---------------------------------------------------------
Title: The Rotation of the Solar Core Inferred by Genetic Forward
Modeling
Authors: Charbonneau, P.; Tomczyk, S.; Schou, J.; Thompson, M. J.
1998ApJ...496.1015C Altcode:
Genetic forward modeling is a genetic algorithm-based modeling technique
that can be used to perform helioseismic inversions of the Sun's
internal angular velocity profile. The method can easily accommodate
constraints such as positivity and monotonicity and readily lends itself
to the use of robust statistical goodness-of-fit estimators. After
briefly describing the technique, we ascertain its performance
by carrying out a series of inversions for artificial splitting
data generated from a set of synthetic internal rotation profiles
characterized by various small inward increases in angular velocity in
the deep solar core (r/R<SUB>⊙</SUB> <= 0.5). These experiments
indicate that the technique is accurate down to r/R<SUB>⊙</SUB>
~= 0.2, and retains useful sensitivity down to r/R<SUB>⊙</SUB> ~=
0.1. <P />We then use genetic forward modeling in conjunction with the
LOW degree L (LOWL) 2 year frequency-splitting data set to determine
the rotation rate in the deep solar core. We perform a large set
of one-dimensional and 1.5-dimensional inversions using regularized
least-squares minimization, conventional least-squares minimization
with a monotonicity constraint (∂Ω/∂r <= 0), and inversions
using robust statistical estimators. These calculations indicate that
the solar core rotates very nearly rigidly down to r/R<SUB>⊙</SUB> ~
0.1. More specifically, on spatial scales >~0.04 R<SUB>⊙</SUB> we
can rule out inward increases by more than 50% down to r/R<SUB>⊙</SUB>
= 0.2, and by more than a factor of 2 down to r/R<SUB>⊙</SUB> =
0.1. Thorough testing of various possible sources of bias associated
with our technique indicates that these results are robust with respect
to intrinsic modeling assumptions. Consequences of our results for
models of the rotational evolution of the Sun and solar-type stars
are discussed.
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones: The Solar Light Element Abundances
Authors: Barnes, G.; Charbonneau, P.; MacGregor, K. B.
1998ASPC..154..886B Altcode: 1998csss...10..886B
We calculate the depletion of the trace elements lithium and beryllium
within a solar mass star, during the course of its evolution from
the zero-age main sequence to the age of the present-day Sun. In the
radiative layers beneath the convection zone, we assume that these
elements are transported by the turbulent fluid motions that result from
the instability of the shear flow associated with internal differential
rotation. This turbulent mixing is modeled as a diffusive process,
using a diffusion coefficient that is taken to be proportional to the
gradient of the angular velocity distribution inside the star. We study
the evolution of the light element abundances produced by rotational
mixing for models in which internal angular momentum redistribution
takes place either by hydrodynamic or by hydromagnetic means. Since
models based on these alternative mechanisms for angular momentum
transport predict similar surface rotation rates late in the evolution,
we explore the extent to which light element abundances make it possible
to distinguish between them. In the case of an internally magnetized
star, our computations indicate that both the details of the surface
abundance evolution and the magnitude of the depletion at solar age can
depend sensitively on the assumed strength and configuration of the
poloidal magnetic field inside the star. For a configuration with no
direct magnetic coupling between the radiative and convective portions
of the stellar interior, the depletion of lithium as a function of
age is similar to that of a model in which angular momentum transport
occurs solely by hydrodynamical processes. However, the two models can
be distinguished on the basis of their respective beryllium depletions,
with the depletion of the magnetic model being significantly smaller
than that of the non-magnetic model.
---------------------------------------------------------
Title: Solar Interface Dynamos. I. Linear, Kinematic Models in
Cartesian Geometry
Authors: MacGregor, K. B.; Charbonneau, P.
1997ApJ...486..484M Altcode:
We describe a simple, kinematic model for a dynamo operating in
the vicinity of the interface between the convective and radiative
portions of the solar interior. The model dynamo resides within
a Cartesian domain, partioned into an upper, convective half and
lower, radiative half, with the magnetic diffusivity η of the
former region (η<SUB>2</SUB>) assumed to exceed that of the latter
(η<SUB>1</SUB>). The fluid motions that constitute the α-effect
are confined to a thin, horizontal layer located entirely within the
convective half of the domain; the vertical shear is nonzero only
within a second, nonoverlapping layer contained inside the radiative
half of the domain. We derive and solve a dispersion relation that
describes horizontally propagating dynamo waves. For sufficiently large
values of a parameter analogous to the dynamo number of conventional
models, growing modes can be found for any ratio of the upper and
lower magnetic diffusivities. However, unlike kinematic models in
which the shear and α-effect are uniformly distributed throughout
the same volume, the present model has wavelike solutions that grow
in time only for a finite range of horizontal wavenumbers. <P />An
additional consequence of the assumed dynamo spatial structure is that
the strength of the azimuthal magnetic field at the location of the
α-effect layer is reduced relative to the azimuthal field strength
at the shear layer. When the jump in η occurs close to the α-effect
layer, it is found that over one period of the dynamo's operation,
the ratio of the maximum strengths of the azimuthal fields at these
two positions can vary as the ratio (η<SUB>1</SUB>/η<SUB>2</SUB>)
of the magnetic diffusivities.
---------------------------------------------------------
Title: Solar Interface Dynamos. II. Linear, Kinematic Models in
Spherical Geometry
Authors: Charbonneau, P.; MacGregor, K. B.
1997ApJ...486..502C Altcode:
Numerical models of interface dynamos are constructed, and their
properties discussed in some detail. These models are extensions in
spherical geometry of the Cartesian interface models considered by
Parker and in the first paper of this series. The models are cast in
the framework of classical mean-field electrodynamics and make use of
a realistic solar-like internal differential rotation profile. The
magnetic diffusivity is assumed to vary discontinously by orders of
magnitude across the core-envelope interface. This allows the buildup of
very strong toroidal magnetic fields below the interface, as apparently
required by recent models of erupting bipolar magnetic regions. <P
/>Distinct dynamo modes powered either by the latitudinal or radial
shear can coexist and, under certain conditions, interfere destructively
with one another. Hybrid modes, relying on the latitudinal shear both
in the envelope and below it, are most easily excited in some portions
of parameter space, and represent a class of dynamo solutions distinct
from the true interface modes previously investigated in Cartesian
geometry. Which mode is preferentially excited depends primarily
on the assumed ratio of magnetic diffusivities on either side of
the core-envelope interface. For an α-effect having a simple cos θ
latitudinal dependency, the interface mode associated with the radial
shear below the polar regions of the interface is easier to excite than
its equatorial counterpart. In analogy with more conventional dynamo
models, interface modes propagate equatorward if the product of the
radial shear (∂Ω/∂r) and α-effect coefficient (C<SUB>α</SUB>)
is negative, and poleward if that product is positive. <P />Interface
dynamo modes powered by the positive radial shear localized below the
core-envelope interface in the equatorial regions can be produced
by artificially restricting the α-effect to low latitudes. For
negative dynamo number, those modes are globally dipolar, propagate
toward the equator, and are characterized by a phase relationship
between poloidal and toroidal magnetic field components that is in
agreement with observations. <P />While the models discussed in this
paper are linear and kinematic, and consequently rather limited in
their predictive power, results obtained so far certainly suggest
that interface dynamos represent a very attractive alternative to
conventional solar mean-field dynamo models.
---------------------------------------------------------
Title: A Search for large-scale Symmetries in the Emergence of
active Regions
Authors: Charbonneau, Paul; Bogdan, Thomas J.
1997SPD....28.0253C Altcode: 1997BAAS...29..902C
Recent models of the stability, destabilization and subsequent rise
of toroidal flux ropes stored immediately beneath the base of the
solar convective envelope indicate that the zonal order of the most
unstable mode is a function of storage latitude and magnetic field
strength. Taken at face value, this would suggest that large-scale
symmetries should be apparent in the distribution of longitudes of
emergence for active regions. We are using the Mt Wilson sunspot dataset
(coverage extending from 1917 to 1985) to establish observational
support (or lack thereof) for this conjecture. In this contribution
we discuss our method of analysis, and present preliminary results
for a few activity cycles.
---------------------------------------------------------
Title: Helioseismology by Genetic Forward Modeling
Authors: Charbonneau, P.; Tomczyk, S.
1997ASPC..123...49C Altcode: 1997taca.conf...49C
No abstract at ADS
---------------------------------------------------------
Title: Angular Momentum Evolution in Late-Type Stars
Authors: Charbonneau, P.; Schrijver, C. J.; MacGregor, K. B.
1997cwh..conf..677C Altcode: 2006mslp.conf..677C
No abstract at ADS
---------------------------------------------------------
Title: ALFVÉN Wave-Driven Winds
Authors: MacGregor, K. B.; Charbonneau, P.
1997cwh..conf..327M Altcode: 2006mslp.conf..327M
No abstract at ADS
---------------------------------------------------------
Title: On the Generation of Equipartition-Strength Magnetic Fields
by Turbulent Hydromagnetic Dynamos
Authors: Charbonneau, P.; MacGregor, K. B.
1996ApJ...473L..59C Altcode:
The generation of a mean magnetic field by the action of small-scale
turbulent fluid motions, the alpha -effect, is a fundamental ingredient
of mean-field dynamo theory. However, recent mathematical models and
numerical experiments are providing increasingly strong support to the
notion that at high magnetic Reynolds numbers, the alpha -effect is
strongly impeded long before the mean magnetic field has reached energy
equipartition with the driving fluid motions. Taken at face value,
this raises serious doubt as to whether the solar magnetic field is
produced by a turbulent hydromagnetic dynamo after all, since it is an
observed fact that the Sun does possess a structured, large-scale mean
magnetic field of strength comparable to equipartition. In this Letter
we demonstrate that the class of mean-field turbulent hydromagnetic
models known as interface dynamos can produce equipartition-strength
mean magnetic fields even in the presence of strong alpha -quenching.
---------------------------------------------------------
Title: The oscillation modes of θ^2^ Tauri. Results from the 1992
MUSICOS campaign.
Authors: Kennelly, E. J.; Walker, G. A. H.; Catala, C.; Foing, B. H.;
Huang, L.; Jiang, S.; Hao, J.; Zhai, D.; Zhao, F.; Neff, J. E.;
Houdebine, E. R.; Ghosh, K. K.; Charbonneau, P.
1996A&A...313..571K Altcode:
We have analyzed a series of 619 spectra of θ^2^ Tauri taken with
four telescopes over four consecutive nights during the 1992 global
MUSICOS campaign. Radial velocity variations provide information
about the oscillation frequencies of low degree (0<=l<=3),
and line-profile variations provide information on modes of higher
degree (3<=l<=10). The radial velocities were derived with
a cross-correlation technique. In addition to detecting several
frequencies found photometrically (e.g., Breger et al. 1989), we have
found two new frequencies, which implies that the oscillation spectrum
of θ^2^ Tau may not be stable. Variations within rotationally broadened
absorption lines were transformed by a Fourier-Doppler imaging analysis
into a map of apparent frequency versus apparent azimuthal order. From
this two-dimensional Fourier representation we identify some seven
oscillation modes using a genetic algorithm to explore parameter
space. While we find good agreement between the detected frequencies
and those predicted to be unstable based on the models of Dziembowski
(1990), it is still not clear why only certain modes are selected.
---------------------------------------------------------
Title: Absorption of p-Modes by Slender Magnetic Flux Tubes and
p-Mode Lifetimes
Authors: Bogdan, T. J.; Hindman, B. W.; Cally, P. S.; Charbonneau, P.
1996ApJ...465..406B Altcode:
The presence of a fibril magnetic field in the solar envelope not only
induces shifts in the p-mode resonant frequencies, but also contributes
to the line width of the modes. The augmentation of the line widths
results from two related physical processes: the excitation of tube
mode oscillations on the individual magnetic fibrils and the attendant
mode mixing between p-modes with identical oscillation frequencies. We
assay the magnitude of the contribution from the former physical
process based upon an idealized model consisting of vertical, slender,
magnetic flux tubes embedded in a plane-parallel isentropic polytrope
of index m. We restrict our attention to axisymmetric flux tubes
that are in mechanical and thermal equilibrium with their immediate
nonmagnetic surroundings. For low p-mode oscillation frequencies,
ω, this model predicts that the line width, F, varies as <P />Γ
∝ fωM<SUP>-½</SUP> ∝fω<SUP>m+2</SUP>, <P />where M the mode
mass, and f is the magnetic filling factor reckoned at the surface
of the polytrope. This scaling is in better overall agreement with
the observations (Γ ∝ ω<SUP>4.2</SUP>) than previous predictions
based on the excitation and damping of solar p-modes by turbulent
convection (which yields Γ ∝ γ<SUP>2</SUP> M<SUP>-1</SUP>
∝ω<SUP>2m+4</SUP>), or the scattering of p-modes by convective
eddies (which yields Γ ∝ ω<SUP>(4/3)m+3)</SUP>, and it suggests
that tube mode excitation on fibril magnetic fields may be a dominant
and detectable (through its solar cycle variation) component of the
low-frequency p-mode line widths.
---------------------------------------------------------
Title: A Numerical Study of the Pre-Ejection, Magnetically-Sheared
Corona as a Free Boundary Problem
Authors: Chou, Yung-Ping; Charbonneau, Paul
1996SoPh..166..333C Altcode:
A class of magnetostatic equilibria with axial symmetry outside a
unit sphere in the presence of plasma pressure and an r<SUP>−2</SUP>
gravitational field is constructed. The structure contains a localized
current-carrying region confined by a background bipolar potential
field, and the shape of the region changes subject to the variation
of the electric current. The continuity requirement for the magnetic
field and plasma pressures at the outer boundary of the cavity defines
a free boundary problem, which is solved numerically using a spectral
boundary scheme. The model is then used to study the expansion of the
current-carrying region, caused by the buildup of magnetic shear,
against the background confining field. The magnetic shear in our
model is induced by the loading of an azimuthal field, accompanied by
a depletion of plasma density.
---------------------------------------------------------
Title: Nonlinear interface dynamos with α-quenching.
Authors: Charbonneau, P.; MacGregor, K. B.
1996BAAS...28..935C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Corona as a Minimum Energy System?
Authors: Charbonneau, P.; Hundhausen, A. J.
1996SoPh..165..237C Altcode:
This paper is an exploration of the possibility that the large-scale
equilibrium of plasma and magnetic fields in the solar corona is
a minimum energy state. Support for this conjecture is sought by
considering the simplest form of that equilibrium in a dipole solar
field, as suggested by the observed structure of the corona at times
of minimum solar activity. Approximate, axisymmetric solutions to
the MHD equations are constructed to include both a magnetically
closed, hydrostatic region and a magnetically open region where plasma
flows along field lines in the form of a transonic, thermally-driven
wind. Sequences of such solutions are obtained for various degrees
of magnetic field opening, and the total energy of each solution is
computed, including contributions from both the plasma and magnetic
field. It is shown that along a sequence of increasingly closed coronal
magnetic field, the total energy curve is a non-monotonic function of
the parameter measuring the degree of magnetic field opening, with a
minimum occurring at moderate field opening.
---------------------------------------------------------
Title: Magnetic Fields and Rotation in the Interior of the Sun
and Stars
Authors: Charbonneau, Paul
1996APS..MAY.K1404C Altcode:
Magnetism is known to be the primary agent governing the various
phenomena grouped under the name of solar activity, and is most likely
responsible for the manifestations of solar-like activity observed
in stars other than the Sun. Rotation and magnetism are ubiquitous
among solar-type stars, as the two are closely linked through the
hydromagnetic dynamo believed to operate in stellar interiors. A proper
understanding of stellar and solar magnetism consequently requires
an understanding of rotational evolution. In the first part of this
talk I will briefly review the various ways in which magnetic fields
affect the rotational evolution of solar-type stars on and near the
main-sequence (the primary hydrogen burning phase of stellar evolution),
and will present some of the relevant observational material. I will
then discuss in more detail one specific topic, namely how magnetic
fields can mediate the redistribution of angular momentum throughout
stellar interiors, and in doing so influence the overall rotational
evolution of stars.
---------------------------------------------------------
Title: Applications of Genetic Algorithms to Solar Coronal Modeling
Authors: Gibson, S.; Charbonneau, P.
1996AAS...188.3622G Altcode: 1996BAAS...28..876G
Genetic algorithms are efficient and flexible means of attacking
optimization problems that would otherwise be computationally
prohibitive. Consider a model that represents an observable quantity in
terms of a few parameters, with an associated chi (2) measuring goodness
of fit with respect to data. If the modeled observable is non-linear
in the parameters, there can exist a degeneracy of minimum chi (2)
in parameter space. It is then essential to understand the location
and extent of this degeneracy in order to find the global optimum
and quantify the degeneracy error around it. Traditional methods of
spanning parameter space such as a grid search or a Monte Carlo approach
scale exponentially with the number of parameters, and waste a great
deal of computational time looking at “un-fit” solutions. Genetic
algorithms, on the other hand, converge rapidly onto regions of
minimum chi (2) while continuously generate “mutant solutions”,
allowing an efficient and comprehensive exploration of parameter
space. Our aim has been to develop an approach that simultaneously
yields a best fit solution and global error estimates, by modifying
and extending standard genetic algorithm-based techniques. We fit two
magnetostatic models of the solar minimum corona to observations in
white light. The first model allows horizontal bulk currents and the
second also allows sheet currents enclosing and extending out from the
equatorial helmet streamer. Using our genetic algorithm approach, we
map out the degeneracy of model parameters that reproduce observations
well. The flexibility of genetic algorithms facilitates incorporating
the additional observational constraint of photospheric magnetic
flux, reducing the degeneracy of solutions to a range represented
by reasonable error bars on the model predictions. By using genetic
algorithms we are able to identify and constrain the degeneracy inherent
to the models, a task, which, particularly for the more complex second
model, would be impractical using a traditional technique. The ultimate
result is a greater understanding of the large scale structure of the
solar corona, providing clues to the mechanisms heating the corona
and accelerating the solar wind.
---------------------------------------------------------
Title: Nonlinear interface dynamos with alpha -quenching
Authors: Charbonneau, P.; MacGregor, K. B.
1996AAS...188.6902C Altcode: 1996BAAS...28Q.935C
There exist various mechanisms capable of limiting the magnitude of the
(presumably) dynamo-generated, large-scale solar magnetic field. One
such mechanism is the so-called “alpha -quenching”. The underlying
idea is that the Lorentz force associated with the dynamo-generated
magnetic fields impedes the small scale, turbulent fluid motions
giving rise to the so-called “alpha -effect” (the production of
poloidal from toroidal fields in the framework of mean-field dynamo
theory). In mean-field models, a popular ---yet essentially ad hoc---
prescription for alpha -quenching consists in replacing the coefficient
(alpha ) of the alpha -effect source term in the dynamo equations by an
expression of the form alpha -> alpha (B) =alpha_0 /(1+(|B|/B_eq)(2))
, where alpha_0 is a measure of the strength of the alpha -effect in the
linear regime, and B_eq is the equipartition field strength, based on
the kinetic energy of the turbulent, convective fluid motions (B_eq ~
10(4) G at the base of the solar convection zone). In principle, such
“Weak Quenching” allows the production of magnetic fields of roughly
equipartition strength, as demonstrated by the numerous conventional
mean-field dynamo models making use of eq. (1), or some close variant,
published to date. Vainshtein & Cattaneo (1992, ApJ 393, 165)
and Gruzinov & Diamond (1995, Phys. Plasmas 2, 1941) have argued,
however, that alpha -quenching should be described by alpha ->
alpha (B) =alpha_0 /(R_m(|B|/B_eq)(2)) where R_m is a magnetic
Reynolds number based on the microscopic properties of the flow
(R_m>> 1 for solar interior conditions). This now describes
a much stronger form of alpha -quenching, and, with R_m>>
1, could be fatal to large-scale dynamo action, in the sense that
the dynamo could only produce magnetic fields of strength <<
B_eq. This is in marked contradiction with the demands set by recent
models of bipolar magnetic region emergence, which require field
strengths of order 10x B_eq ~ 10(5) G for the observed latitudes and
tilt of emergence to be adequately reproduced. In this contribution, we
investigate the circumstances under which interface dynamos can avoid
alpha -quenching, either in the “Weak” or “Strong” forms defined
above. In interface dynamos the alpha -effect is assumed to operate
within the solar convective envelope, while the strongest magnetic
fields are generated by shearing below the core-envelope interface
(Parker 1993, ApJ 408, 707; Charbonneau & MacGregor, submitted to
ApJ). This spatial segregation of the alpha -effect source region is
the key to avoiding alpha -quenching. This is illustrated using a few
nonlinear, kinematic interface dynamo solutions applicable to the Sun.
---------------------------------------------------------
Title: Further thoughts on the solar corona as a minimum energy
system.
Authors: Charbonneau, P.; Hundhausen, A. J.
1996ASIC..481..249C Altcode:
The authors conjecture that the global, large-scale structure of the
solar corona represents a form of minimum energy state. They illustrate
this conjecture with the help of an approximate model applicable to
quiet solar minimum conditions. Possible implications and applications
of the conjecture are discussed in the context of coronal mass ejections
and of empirical modeling of the solar corona.
---------------------------------------------------------
Title: Genetic Algorithms in Astronomy and Astrophysics
Authors: Charbonneau, P.
1995ApJS..101..309C Altcode:
This paper aims at demonstrating, through examples, the applicability of
genetic algorithms to wide classes of problems encountered in astronomy
and astrophysics. Genetic algorithms are heuristic search techniques
that incorporate, in a computational setting, the biological notion
of evolution by means of natural selection. While increasingly in
use in the fields of computer science, artificial intelligence, and
computed-aided engineering design, genetic algorithms seem to have
attracted comparatively little attention in the physical sciences
thus far. The following three problems are treated: (1) modeling the
rotation curve of galaxies, (2) extracting pulsation periods from
Doppler velocities measurements in spectral lines of δ Scuti stars,
and (3) constructing spherically symmetric wind models for rotating,
magnetized solar-type stars. A listing of the genetic algorithm-based
general purpose optimization subroutine PIKAIA, used to solve these
problems, is given in the Appendix.
---------------------------------------------------------
Title: Stellar Winds with Non-WKB Alfven Waves. II. Wind Models for
Cool, Evolved Stars
Authors: Charbonneau, P.; MacGregor, K. B.
1995ApJ...454..901C Altcode:
We construct Alfvén wave-driven wind models for physical conditions
appropriate to the expanding envelopes of cool, evolved stars. To
derive wind properties, we assume steady, isothermal, spherically
symmetric flow, but do not use the WKB (i.e., short-wavelength)
approximation to calculate the wave amplitudes. Instead, we make
use of the formalism developed in the first paper of this series
(MacGregor & Charbonneau 1994), which describes wave reflection
and associated modifications to the wave force, and consistently
incorporates these effects into the treatment of wind dynamics. <P
/>For flows containing undamped Alfvén waves of arbitrarily long
wavelength we find that the occurrence of wave reflection has profound
consequences for wind acceleration and mass loss. Specifically, in all
of our computed models, the outward-directed wave force near the base
of the flow is significantly reduced relative to that in comparable
WKB models. As a result, the initial expansion speeds and mass flux
densities of model winds that include non-WKB effects are smaller than
those of corresponding WKB winds. Moreover, at large distances from
the star, wave reflection leads to an enhancement of the wave force
relative to models in which all waves are presumed to be outwardly
propagating. This tendency, when combined with the previously noted
reduction in mass flux, produces winds with higher asymptotic flow
speeds than those driven by high-frequency, short-wavelength Alfvén
waves. Given that the challenge of modeling winds from cool evolved
stars is to produce winds with high mass fluxes and low asymptotic
flow speeds, we argue that Alfvén waves provide an acceptable driving
mechanism only if their wavelengths are sufficiently short that minimal
reflection occurs near the base of the flow. For stellar parameters
characteristic of a supergiant star with spectral type ∼K5, this
translates into an upper bound on Alfvén wave periods of ∼1 day.
---------------------------------------------------------
Title: Constraining Solar Core Rotation with Genetic Forward Modelling
Authors: Tomczyk, S.; Charbonneau, P.; Schou, J.; Thompson, M. J.
1995ESASP.376b.271T Altcode: 1995soho....2..271T; 1995help.confP.271T
No abstract at ADS
---------------------------------------------------------
Title: On the evolution of rotational velocity distributions for
solar-type stars.
Authors: Keppens, R.; MacGregor, K. B.; Charbonneau, P.
1995A&A...294..469K Altcode:
We investigate how the distribution of rotational velocities for
late-type stars of a given mass evolves with age, both before
and during residence on the main sequence. Starting from an age
~10^6^years, an assumed pre-main sequence rotational velocity/period
distribution is evolved forward in time using the model described by
MacGregor & Brenner (1991) to trace the rotational histories of
single, constituent stars. This model treats: (i) stellar angular
momentum loss as a result of the torque applied to the convection
zone by a magnetically coupled wind; (ii) angular momentum transport
from the radiative interior to the convective envelope in response
to the rotational deceleration of the stellar surface layers; and
(iii), angular momentum redistribution associated with changes in
internal structure during the process of contraction to the main
sequence. We ascertain how the evolution of a specified, initial
rotational velocity/period distribution is affected by such things as:
(i) the dependence of the coronal magnetic field strength on rotation
rate through a prescribed, phenomenological dynamo relation; (ii) the
magnitude of the timescale τ_c_ characterizing the transfer of angular
momentum from the core to the envelope; (ii) differences in the details
and duration of pre-main sequence structural evolution for stars with
masses in the range 0.8<=M_*_/M<SUB>sun</SUB>_<=1.0 and (iv),
the exchange of angular momentum between a star and a surrounding,
magnetized accretion disk during the first few million years of
pre-main sequence evolution following the development of a radiative
core. The results of this extensive parameter study are compared with
the distributions derived from measurements of rotational velocities
of solar-type stars in open clusters with known ages. Starting from an
initial distribution compiled from observations of rotation among T
Tauri stars, we find that reasonable agreement with the distribution
evolution inferred from cluster observations is obtained for: (i)
a dynamo law in which the strength of the coronal field increases
linearly with surface angular velocity for rotation rates <=20
times the present solar rate, and becomes saturated for more rapid
rotation; (ii) a coupling timescale ~10^7^years; (iii) a mix of stellar
masses consisting of roughly equal numbers of 0.8M<SUB>sun</SUB>_ and
1.0M<SUB>sun</SUB>_ stars; and (iv), disk regulation of the surface
rotation up to an age ~6x10^6^years for stars with initial rotation
periods longer than 5days. A number of discrepancies remain, however:
even with the most favorable choice of model parameters, the present
calculations fail to produce a sufficiently large proportion of slow
(equatorial velocities less than 10km/s) rotators on the Zero-Age
Main Sequence.
---------------------------------------------------------
Title: Stellar Winds with Non-WKB Alfven Waves. I. Wind Models for
Solar Coronal Conditions
Authors: MacGregor, K. B.; Charbonneau, P.
1994ApJ...430..387M Altcode:
We have constructed numerical models for stationary, wind-type
outflows that include treatment of the force produced by propagating
Alfven waves. We make no assumptions regarding the relative sizes
of the wavelengths of such disturbances and the scale lengths that
characterize the variation of the physical properties of the expanding
stellar atmosphere. Consequently, our models take account the process of
Alfven wave reflection, and provide for dynamical effects arising from
the simultaneous presence of outward and inward traveling waves in the
wind. For physical conditions like those prevailing in the outer solar
corona and wind, we find that even relatively high frequency, short
wavelength waves can suffer some reflection from the gradient in Alfven
speed at the vase of the flow. Among the consequences of the interaction
between outward and inward directed perturbations in the sub-Alfvenic
portion of the wind is a reduction in the magnitude of the time-averaged
wave force relative to its value in the Wentzel-Kramer-Brillouin (WKB)
(i.e., short-wavelenght) limit. As a result, the flow velocities of
our models interior to the Alfven radius are smaller than those of
corresponding WKB models. For models containing very low frequency,
long wavelength waves, a substantial amount of wave reflection can also
take place in the super-Alvenic portion of the wind. The resulting
modifications to the spatial dependences of the eave magnetic and
velocity amplitudes can lead to a wave force whose magnitude at large
distances exceeds that of an equivalent WKB solution.
---------------------------------------------------------
Title: Angular Momentum Evolution of Late-Type Stars: A Theoretical
Perspective (Invited Review)
Authors: MacGregor, K. B.; Charbonneau, P.
1994ASPC...64..174M Altcode: 1994csss....8..174M
No abstract at ADS
---------------------------------------------------------
Title: Angular Momentum Loss from the Young Sun: Improved Wind and
Dynamo Models
Authors: Keppens, R.; Charbonneau, P.; MacGregor, K. B.; Brandenburg,
A.
1994ASPC...64..193K Altcode: 1994csss....8..193K
No abstract at ADS
---------------------------------------------------------
Title: Stellar Dynamos: The Rossby Number Dependence
Authors: Brandenburg, A.; Charbonneau, P.; Kitchatinov, L. L.;
Rudiger, G.
1994ASPC...64..354B Altcode: 1994csss....8..354B
No abstract at ADS
---------------------------------------------------------
Title: Solar wind with non-WKB Alfvén waves
Authors: Charbonneau, P.; MacGregor, K. B.
1994smf..conf..405C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modelling magnetoacoustic oscillations in sunspots: a
progress report
Authors: Charbonneau, P.; Cally, P. S.; Bogdan, T. J.
1994smf..conf..251C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones. II. The Solar Spin-down
Authors: Charbonneau, P.; MacGregor, K. B.
1993ApJ...417..762C Altcode:
We present a large set of numerical calculations describing the
rotational evolution of a solar-type star, in response to the torque
exerted on it by a magnetically coupled wind emanating from its
surface. We consider a situation where the internal redistribution
of angular momentum in the radiative part of the envelope is
dominated by magnetic stresses arising from the shearing of a
preexisting, large-scale, poloidal magnetic field. <P />By assuming
a time-independent poloidal magnetic field, neglecting fluid motions
in meridional planes, and restricting our attention to axisymmetric
systems, we reduce the spin-down problem to solving the (coupled)
ψ-components of the momentum and induction equations. Nevertheless, our
computations remain dynamical, in that they take into account both the
generation of a toroidal magnetic field by shearing of the preexisting
poloidal field, and the back-reaction of the resulting Lorentz force
on the differential rotation. It becomes possible to draw, for the
first time, a reasonably realistic and quantitative picture of the
effects of large-scale internal magnetic fields on the main-sequence
rotational evolution of solar-type stars. <P />We perform spin-down
calculations for a standard solar model, starting from the ZAMS and
extending all the way to the solar age. The wind-induced surface torque
is computed using the axisymmetric formulation of Weber & Davis
(1967). We consider a number of poloidal magnetic field configurations
which differ both in the degree of magnetic coupling between the
convective envelope and radiative core and in average strength. <P
/>The rotational evolution can be divided into three more or less
distinct phases: an initial phase of toroidal field buildup in the
radiative zone, lasting from a few times 10<SUP>4</SUP> to a few times
10<SUP>6</SUP> yr; a second period in which oscillations set up in the
radiative zone during the first phase are damped; and a third period,
lasting from an age of about 10<SUP>7</SUP> yr onward, characterized
by a state of dynamical balance between the total stresses (magnetic +
viscous) at the core-envelope interface and the wind-induced surface
torque, leading to a quasistatic internal magnetic and rotational
evolution. <P />Our results also demonstrate (1) the existence of
classes of large-scale internal magnetic fields that can accommodate
rapid spin-down near the ZAMS and yield a weak internal differential
rotation by the solar age, (2) the importance of phase mixing in
efficiently damping large-scale toroidal oscillations pervading the
radiative interior at early times, (3) the near-independence of the
present solar surface angular velocity on the strength and geometry
(past and present) of any internal large-scale magnetic field pervading
the radiative interior, and (4) the greater dependence of the present
solar internal differential rotation on the overall morphology (but
not on the strength) of the internal magnetic field.
---------------------------------------------------------
Title: Particle Transport and the lambda Bootis Phenomenon. II. an
Accretion/Diffusion Model
Authors: Turcotte, S.; Charbonneau, P.
1993ApJ...413..376T Altcode:
The surface and internal abundance evolution in a star accreting
metal-depleting material from the interstellar medium are described
using a set of numerical calculations. The transport model takes into
account contributions from chemical separation mechanisms, rotationally
induced meridional circulation, and accretion. Computations demonstrate
that the maintenance of the abundance signature of the accreted
materials is possible only for accretion rates larger than few 10
exp -14 solar masses/yr. Upon termination of the accretion episode,
chemical separation destroys any surface abundance pattern set up
earlier by accretion during the period of 10 exp 6 yr.
---------------------------------------------------------
Title: Particle Transport and the lambda Bootis Phenomenon. I. The
Diffusion/Mass-Loss Model Revisited
Authors: Charbonneau, P.
1993ApJ...405..720C Altcode:
In this paper, results of two-dimensional linear time-dependent particle
transport calculations in a Lambda Bootis star are presented for two
representative elements, titanium and calcium. These demonstrate
unambiguously that the inclusion of meridional circulation in the
original diffusion/mass-loss model of Lambda Bootis stars has profound
consequences on the abundance evolution. More specifically, circulation
prevents the appearance, at any epoch of main-sequence evolution, of
the underabundance patterns characteristic of Lambda Bootis stars. This
indicates that the diffusion/mass-loss model for these objects must
be either abandoned or significantly modified. Caveats and possible
alternatives are discussed, and a few observational tests are suggested.
---------------------------------------------------------
Title: Solar Spin-down with Internal Magnetic Fields: Erratum
Authors: Charbonneau, P.; MacGregor, K. B.
1993ApJ...403L..87C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spin down of solar-type stars with internal magnetic fields
Authors: Charbonneau, P.; MacGregor, K. B.
1993ASPC...40..464C Altcode: 1993IAUCo.137..464C; 1993ist..proc..464C
No abstract at ADS
---------------------------------------------------------
Title: Chemical Separation Versus Rotation in A-Stars and F-Stars
Authors: Charbonneau, P.
1993ASPC...44..474C Altcode: 1993pvnp.conf..474C; 1993IAUCo.138..474C
No abstract at ADS
---------------------------------------------------------
Title: Phase mixing and the solar spin-down.
Authors: Charbonneau, P.
1993wpst.conf..125C Altcode:
After placing the spin-down problem in its solar/astrophysical context
and briefly describing the simulations themselves, this paper focuses
on the phase mixing mechanism in the context of the solar spin-down.
---------------------------------------------------------
Title: Solar Spin-down with Internal Magnetic Fields
Authors: Charbonneau, P.; MacGregor, K. B.
1992ApJ...397L..63C Altcode:
We investigate the rotational evolution of a solar-type star containing
a large-scale poloidal magnetic field in its radiative core, in response
to the torque applied to it by magnetically coupled wind. Our model
takes into account both the generation of a toroidal magnetic component
via shearing of the existing poloidal component by differential
rotation, as well as the back-reaction on the differential rotation due
to Lorentz forces associated with the toroidal field. Our computations
demonstrate the existence of classes of large-scale poloidal magnetic
fields allowing rapid spin-down of the surface layers shortly after the
arrival on the zero-age main sequence, while producing weak internal
differential rotation in the radiative core by the solar age. This
indicates that the constraints brought about by rotational evolution
of solar analogs in young clusters and by helioseismology are not
incompatible with the existence of large-scale magnetic fields in
stellar radiative interiors. The present surface solar rotation rate
is also shown to be a poor indicator of the strength and geometry of
hypothetical poloidal magnetic fields pervading the solar radiative
interior.
---------------------------------------------------------
Title: Numerical experiments on the effects of horizontal turbulent
diffusion on transport by meridional circulation
Authors: Charbonneau, P.
1992A&A...259..134C Altcode:
A set of numerical experiments aimed at quantifying various aspects
of the horizontal turbulence (HT) ansatz of Chaboyer and Zahn
(1992) is presented. It is demonstrated that the transition to 1-D
behavior for both particle and angular momentum transport occurs for
horizontal Reynolds numbers R<SUB>H</SUB> not greater than about 1,
while the effective inhibition of meridional-circulation-mediated
particle transport occurs for R<SUB>H</SUB> not greater than about
0.01. Constraints on the HT hypothesis can also be inferred from the
diffusion models for FmAm and HgMn stars.
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones. I. Numerical Solutions to the Core Spin-up Model Problem
Authors: Charbonneau, P.; MacGregor, K. B.
1992ApJ...387..639C Altcode:
The present paper investigates the time evolution of the angular
momentum and induced toroidal magnetic field distribution in
an initially nonrotating radiative stellar envelope containing a
large-scale poloidal magnetic field, following the impulsive spin-up of
the underlying core. A large set of numerical calculations pertaining
to monopolar, dipolar, and quadrupolar magnetic configurations, with
and without density gradients across the envelope, as well as a set
of solutions for which the poloidal field is only partially anchored
on the core is presented. It is demonstrated that in moderate to
high Reynolds-number systems, any global magnetic dissipation time
scale constructed using length scales of order of the stellar radius
greatly overestimates the true dissipation time scale of the toroidal
magnetic component.
---------------------------------------------------------
Title: On the Evolving Role of Computational Astrophysics
Authors: Charbonneau, P.
1992JRASC..86...31C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Numerical Scheme for the Modeling of Electric Current Sheet
Formation in the Solar Atmosphere
Authors: Charbonneau, P.; Low, B. C.
1992ASPC...26..531C Altcode: 1992csss....7..531C
No abstract at ADS
---------------------------------------------------------
Title: Modeling Stellar Angular Momentum Evolution (Invited Review)
Authors: Charbonneau, P.
1992ASPC...26..416C Altcode: 1992csss....7..416C
No abstract at ADS
---------------------------------------------------------
Title: The Lithium Abundance in Stars
Authors: Michaud, Georges; Charbonneau, Paul
1991SSRv...57....1M Altcode: 1991SSRev..57....1M
The observations of Li abundances in pre-Main-Sequence, Main-Séquence
(Population I and II), sub-giant, and giant stars are reviewed
in order to show how Li can be used as a constraint on stellar
hydrodynamics and in particular on particle transport processes in
stars. Important observational results include the tight Li abundance
dependence on T <SUB>eff</SUB> in the Hyades, the time dependence of
the Li abundance below T <SUB>eff</SUB> = 6000 K, the presence of a
Li gap at T <SUB>eff</SUB> = 6700 K in young clusters and the large Li
abundance in some peculiar giants. The observed abundances are compared
to models which include progressively more physical processes. The
‘standard’ stellar evolution model is compatible with the upper
envelope of the observations in young clusters such as the Pleiades and
α Per. The observed Li underabundances is then caused by Li burning
on the pre-Main Sequence. The large abundance spread observed is not
understood. It does not appear to be simply related to rotation since
the Pleiades stars rotate more slowly but have larger Li abundances
than many stars of α Per. The Li abundance gap observed in clusters is
not explained by the ‘standard’ model. Models involving diffusion
seem to explain it in a natural way, though meridional circulation
could also be involved. Evolutionary effects and the interaction
between diffusion and meridional circulation should, however, be
taken more fully into account in those models. The Li abundances in
giants show that additional destruction processes are involved beyond
those included in the ‘standard’ evolutionary models. Meridional
circulation is compatible with most of those observations, without
any arbitrary parameter being adjusted. While turbulence is nearly
certainly present in stars, it is poorly understood and we suggest
that it should be invoked to explain only those phenomena that the
better understood processes cannot explain. Its description always
involves arbitrary parameters. Turbulence appears to be required to
explain the Li abundances in the Sun and in G stars of the Hyades and
older clusters. In halo stars, the observed Li abundance has probably
been reduced from the original by a factor of 2 so that the original
abundance was probably equal to log N(Li) = 2.5. More calculations
are needed to better establish this value. The large Li abundances
observed in some peculiar giants are not understood.
---------------------------------------------------------
Title: A Simple Accretion/Diffusion Model for lambda Bootis Stars
Authors: Charbonneau, Paul
1991ApJ...372L..33C Altcode:
It has recently been suggested that the peculiar abundance patterns
observed in the Lambda Bootis stars could be understood in terms of
accretion of gas previously depleted in metals by means of grain
formation in the interstellar medium. A simple analytical model
is presented, describing the evolution of elemental abundances in
these stars, under the combined influence of accretion and chemical
separation. The only arbitrary parameter involved is the accretion
rate. A rate of order 10 to the -13th solar mass/yr is found to
naturally reproduce many peculiar characteristics of Lambda Bootis
stars, in particular their restriction to the spectral type range
A0-F0. This lends quantitative support to the accretion hypothesis as
a key component toward an understanding of the Lambda Bootis phenomenon.
---------------------------------------------------------
Title: Meridional Circulation and Diffusion in A and Early F Stars
Authors: Charbonneau, Paul; Michaud, Georges
1991ApJ...370..693C Altcode:
Time-dependent two-dimensional calculations of diffusion in the
presence of meridional circulation are presented for stellar models
pertaining to FmAm stars. It is shown that, once the helium superficial
convection zone (HSCZ) has disappeared, the meridional circulation
has little influence on chemical separation. In stars rotating too
rapidly to become FmAm stars, chemical separation remains possible
under the HSCZ. Meridional circulation does not completely wipe out
chemical separation at these velocities, and cannot by itself lead
to the abundance patterns characteristic of Lambda Booti stars. Upper
limits to turbulence are set. In the presence of meridional circulation,
helium settling in stars rotating at the observed cutoff for FmAm stars
remains possible for values of vertical turbulent diffusion coefficients
of order 1000 sq cm/s under the helium convection zone. This sets
extremely tight constraints on turbulence in stars with equatorial
rotational velocities of 100 km/s or less.
---------------------------------------------------------
Title: Observational study of the spiral galaxy NGC 6946 . II. H I
kinematics and mass distribution.
Authors: Carignan, C.; Charbonneau, P.; Boulanger, F.; Viallefond, F.
1990A&A...234...43C Altcode:
A study of the kinematics and mass distribution of NGC 6946 is presented
from Westerbork H I line observations. We find that the H I distribution
is not symmetric but is more extended on the NE side compared to the
SW side by about 25% in radius. The overall velocity field is fairly
regular with some signs of non-circular motions associated with the
optical and H I spiral arms. A well defined rotation curve is derived
which is essentially flat from 4' to 10'. At larger radii, the H,
runs out on the SW side. The kinematical parameters are V_sys_ =
47 km s^-1^, i = 38^deg^, and PA = 240^deg^. An analysis of the mass
distribution yields a well defined ratio of dark-to-luminous matter
of ~0.75 at the Holmberg radius.
---------------------------------------------------------
Title: Turbulence and the Li Abundance in Main-Sequence and Giant
Stars
Authors: Charbonneau, Paul; Michaud, Georges
1990ApJ...352..681C Altcode:
Calculations of Li burning via turbulent transport are conducted to
determine the extent to which observed Li abundances in first ascent
giants constrain the various turbulence parameterizations used to model
the main-sequence surface Li abundance evolution. A full time-dependent
solution to the transport equation is performed, including nuclear
reaction terms and evolutionary effects. It is found that turbulence can
lead to the extreme Li underabundances observed in giants of M67 and NGC
752. Consideration is given to the possibility of using observations of
Li abundances to discriminate between turbulent particle transport and
meridional circulation transport. Numerical solutions of the turbulent
diffusion coefficient of Vauclair (1988) is used to model the Hyades
Li abundance gap. The astrophysical implications of the results for
main-sequence and giant stars are discussed.
---------------------------------------------------------
Title: Modélisation numérique des processus de transport dans les
enveloppes stellaires
---------------------------------------------------------
Title: Modélisation numérique des processus
de transport dans les enveloppes stellaires
---------------------------------------------------------
Title: Numerical modelling
of transport processes in stellar envelopes;
Authors: Charbonneau, Paul
1990PhDT........69C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Lithium Abundance in Cluster Giants: Constraints on Meridional
Circulation Transport on the Main Sequence
Authors: Charbonneau, Paul; Michaud, Georges; Proffitt, Charles R.
1989ApJ...347..821C Altcode:
The observed Li abundances in giants are used here to constrain
meridional circulation transport on the main sequence. It is shown how
meridional circulation, operating over the main-sequence lifetime, can
lead to Li depletion in the upper radiative envelope and eventually to
extreme Li underabundance in first-ascent giants, following convective
dilution on the lower giant branch. In the mass range 1.2-2.0 solar,
stars with equatorial rotational velocities greater than 30-35 km/s on
the ZAMS should destroy most of their Li. These result are compared to
recent Li abundance determination in three moderately old clusters,
NGC 7789, NGC 752, and M67. Reasonably good agreement is found with
data on M67 and NGC 752, but surprising disagreement with data on NGC
7789 is found. Possible explanations are considered.
---------------------------------------------------------
Title: Transport processes on the main sequence.
Authors: Charbonneau, P.; Michaud, G.
1989JRASC..83..296C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Meridional Circulation and the Lithium Abundance Gap in F Stars
Authors: Charbonneau, Paul; Michaud, Georges
1988ApJ...334..746C Altcode:
The effect of meridional circulation on the time evolution of
superficial abundances of helium, lithium, and beryllium in F stars
is calculated in detail. It is shown that, as long as the presence of
convection zones is assumed not to modify global meridional circulation
patterns, the maximum equatorial rotational velocity allowing the
settling of Li and He decreases rapidly with T(eff), going from 50 km/s
at 7250 K to only 5 km/s at 6400 K. It is also shown that, for stars
of the age of the Hyades with T(eff) less than 7000 K and rotational
velocities larger than about 25 km/s, meridional circulation is rapid
enough to bring to the surface matter that originally was deep enough
to have been depleted of its Li through nuclear burning. Observational
tests are suggested to distinguish between this scenario and the
gravitational settling model or turbulent diffusion model. It is shown
how the Li/Be ratio varies in the presence of transport by meridional
circulation.
---------------------------------------------------------
Title: Two-dimensional Particle Transport in HgMn and FmAm Stars
Authors: Charbonneau, Paul; Michaud, Georges
1988ApJ...327..809C Altcode:
Detailed two-dimensional diffusion calculations of helium are carried
out to determine the maximum equatorial rotational velocity allowing
the gravitational settling of He. Once the He abundance has decreased
sufficiently in the superficial convection zones, the He convection zone
disappears, abundance anomalies become large, and the HgMn and FmAm
phenomena appear. The limiting equatorial velocity is found to be 75
and 100 km s<SUP>-1</SUP>, respectively, for HgMn and FmAm stars. It
depends mainly on gravity, becoming much smaller as soon as gravity
goes down. The observed upper limit being ≡100 km s<SUP>-1</SUP>,
the agreement is quite satisfactory. These results are very similar
to those obtained with the one-dimensional approximation and justify
the use of those results.
