Author name code: brun
ADS astronomy entries on 2022-09-14
author:"Brun, Allan Sacha"
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Title: Stochastic excitation of internal gravity waves in rotating
late F-type stars: A 3D simulation approach
Authors: Breton, Sylvain N.; Brun, Allan Sacha; García, Rafael A.
Bibcode: 2022arXiv220814759B
Altcode:
There are no strong constraints placed thus far on the amplitude
of internal gravity waves (IGWs) that are stochastically excited in
the radiative interiors of solar-type stars. Late F-type stars have
relatively thin convective envelopes with fast convective flows and
tend to be fast rotators compared to solar-type stars of later spectral
types. These two elements are expected to directly impact the IGW
excitation rates and properties. We want to estimate the amplitude
of stochastically excited gravity modes (g-modes) in F-type stars
for different rotational regimes. We used the ASH code to perform 3D
simulations of deep-shell models of 1.3 $M_\odot$ F-type solar-type
stars, including the radiative interior and the shallow convective
envelope. The IGWs are excited by interface interactions between
convective plumes and the top of the radiative interior. We were
able to characterise the IGWs and g-mode properties in the radiative
interior, and we compared these properties using the computation from
the 1D oscillation code GYRE. The amplitude of low-frequency modes is
significantly higher in fast-rotating models and the evolution of the
period spacing of consecutive modes exhibits evidence of a behaviour
that is modified by the influence of the Coriolis force. For our
fastest rotating model, we were able to detect the intermediate degree
g-mode signature near the top of the simulation domain. Nevertheless,
the predicted luminosity perturbations from individual modes still
remain at small amplitudes. We obtained mode amplitudes that are
several orders of magnitude higher than those of prior 3D simulations
of solar models. Our simulations suggest that g-mode signatures could
be detectable in late F-type stars. [abridged]
Title: Hunting for anti-solar differentially rotating stars using
the Rossby number -- An application to the Kepler field
Authors: Noraz, Quentin; Breton, Sylvain N.; Brun, Allan Sacha;
García, Rafael A.; Strugarek, Antoine; Santos, Angela R. G.; Mathur,
Savita; Amard, Louis
Bibcode: 2022arXiv220812297N
Altcode:
Anti-solar differential rotation profiles have been found for decades
in numerical simulations of convective envelopes of solar-type
stars. These profiles are characterized by a slow equator and fast
poles (i.e., reversed with respect to the Sun) and have been found
in simulations for high Rossby numbers (slow rotators). Rotation
profiles like this have been reported observationally in evolved
stars, but have never been unambiguously observed for cool solar-type
stars on the main sequence. In this context, detecting this regime
in main-sequence solar-type stars would improve our understanding
of their magnetorotational evolution. The goal of this study is to
identify the most promising cool main-sequence stellar candidates for
anti-solar differential rotation in the \textit{Kepler} sample. First,
we introduce a new theoretical formula to estimate fluid Rossby numbers,
$Ro_{\rm f}$, of main-sequence solar-type stars, from observational
quantities, and taking the influences of the internal structure and
metallicity into account. We obtain a list of the most promising stars
that are likely to show anti-solar differential rotation. We identify
two samples: one at solar metallicity, including 14 targets, and another
for other metallicities, including 8 targets. We find that the targets
with the highest $Ro_{\rm f}$ are likely to be early-G or late-F stars
at about log$_{10}g=4.37$~dex. We conclude that cool main-sequence
stellar candidates for anti-solar differential rotation exist in the
\textit{Kepler} sample. The most promising candidate is KIC~10907436,
and two other particularly interesting candidates are the solar analog
KIC~7189915 and the seismic target KIC~12117868. Future characterization
of these 22 stars is expected to help us understand how dynamics can
impact magnetic and rotational evolution of old solar-type stars at
high Rossby number.
Title: Solar wind speed and rotation: sources of shearing and impacts
on the corona and heliosphere
Authors: Pinto, Rui; Kouloumvakos, Athanasios; Brun, . Allan Sacha;
Lavraud, Benoit; Rouillard, Alexis; Finley, Adam; Griton, Léa;
Kieokaew, Rungployphan; Poirier, Nicolas; Fargette, Naïs
Bibcode: 2022cosp...44.1079P
Altcode:
The rotation of the solar corona and of the solar wind play a
fundamental role in a wide range of solar phenomena. However, the exact
configuration of azimuthal speeds in the solar atmosphere is much less
well known than that of its photospheric counterpart. Parker Solar
Probe has revealed that surprisingly large variations of solar wind
rotation rates can occurs across neighbouring solar wind streams. We
show by means of of global MHD simulations that coronal rotation is
highly structured in some regions of the solar corona, especially in
proximity to streamer/coronal hole boundary regions (in agreement with
preceding SoHO/UVCS observations, and potentially with future SO/Metis
campaigns). Enhanced poloidal and toroidal flow shear and magnetic
field gradients also develop there. Some of these regions develop with
field-aligned and/or transverse vorticity signatures that are driven
through large radial extensions (noticeable several tens of solar radii
away from the surface). Our simulations furthermore indicate that the
spatial structure of the solar wind shear will become more complex as
the solar cycle progresses, with strong and extended shears appearing
at heliographic latitudes that will be probed by Solar Orbiter in the
near future.
Title: MOVES - V. Modelling star-planet magnetic interactions of
HD 189733
Authors: Strugarek, A.; Fares, R.; Bourrier, V.; Brun, A. S.; Réville,
V.; Amari, T.; Helling, Ch; Jardine, M.; Llama, J.; Moutou, C.;
Vidotto, A. A.; Wheatley, P. J.; Zarka, P.
Bibcode: 2022MNRAS.512.4556S
Altcode: 2022arXiv220310956S; 2022MNRAS.tmp..872S
Magnetic interactions between stars and close-in planets may lead to
a detectable signal on the stellar disc. HD 189733 is one of the key
exosystems thought to harbour magnetic interactions, which may have
been detected in 2013 August. We present a set of 12 wind models at that
period, covering the possible coronal states and coronal topologies of
HD 189733 at that time. We assess the power available for the magnetic
interaction and predict its temporal modulation. By comparing the
predicted signal with the observed signal, we find that some models
could be compatible with an interpretation based on star-planet
magnetic interactions. We also find that the observed signal can be
explained only with a stretch-and-break interaction mechanism, while
that the Alfvén wings scenario cannot deliver enough power. We finally
demonstrate that the past observational cadence of HD 189733 leads
to a detection rate of only between 12 and 23 per cent, which could
explain why star-planet interactions have been hard to detect in past
campaigns. We conclude that the firm confirmation of their detection
will require dedicated spectroscopic observations covering densely the
orbital and rotation period, combined with scarcer spectropolarimetric
observations to assess the concomitant large-scale magnetic topology
of the star.
Title: Validation of a Wave Heated 3D MHD Coronal-wind Model using
Polarized Brightness and EUV Observations
Authors: Parenti, Susanna; Réville, Victor; Brun, Allan Sacha;
Pinto, Rui F.; Auchère, Frédéric; Buchlin, Éric; Perri, Barbara;
Strugarek, Antoine
Bibcode: 2022ApJ...929...75P
Altcode: 2022arXiv220310876P
The physical properties responsible for the formation and evolution
of the corona and heliosphere are still not completely understood. 3D
MHD global modeling is a powerful tool to investigate all the possible
candidate processes. To fully understand the role of each of them,
we need a validation process where the output from the simulations
is quantitatively compared to the observational data. In this work,
we present the results from our validation process applied to the
wave turbulence driven 3D MHD corona-wind model WindPredict-AW. At
this stage of the model development, we focus the work to the coronal
regime in quiescent condition. We analyze three simulation results,
which differ by the boundary values. We use the 3D distributions of
density and temperature, output from the simulations at the time of
around the first Parker Solar Probe perihelion (during minimum of
the solar activity), to synthesize both extreme ultraviolet (EUV)
and white-light-polarized (WL pB) images to reproduce the observed
solar corona. For these tests, we selected AIA 193 Å, 211 Å, and
171 Å EUV emissions, MLSO K-Cor, and LASCO C2 pB images obtained on
2018 November 6 and 7. We then make quantitative comparisons of the
disk and off limb corona. We show that our model is able to produce
synthetic images comparable to those of the observed corona.
Title: Two-dimensional simulations of solar-like models with
artificially enhanced luminosity. II. Impact on internal gravity waves
Authors: Le Saux, A.; Guillet, T.; Baraffe, I.; Vlaykov, D. G.;
Constantino, T.; Pratt, J.; Goffrey, T.; Sylvain, M.; Réville, V.;
Brun, A. S.
Bibcode: 2022A&A...660A..51L
Altcode: 2022arXiv220200801L
Artificially increasing the luminosity and the thermal diffusivity
of a model is a common tactic adopted in hydrodynamical simulations
of stellar convection. In this work, we analyse the impact of these
artificial modifications on the physical properties of stellar interiors
and specifically on internal gravity waves. We perform two-dimensional
simulations of solar-like stars with the MUSIC code. We compare three
models with different luminosity enhancement factors to a reference
model. The results confirm that properties of the waves are impacted by
the artificial enhancement of the luminosity and thermal diffusivity. We
find that an increase in the stellar luminosity yields a decrease
in the bulk convective turnover timescale and an increase in the
characteristic frequency of excitation of the internal waves. We also
show that a higher energy input in a model, corresponding to a larger
luminosity, results in higher energy in high frequency waves. Across
our tests with the luminosity and thermal diffusivity enhanced together
by up to a factor of 104, our results are consistent with
theoretical predictions of radiative damping. Increasing the luminosity
also has an impact on the amplitude of oscillatory motions across the
convective boundary. One must use caution when interpreting studies
of internal gravity waves based on hydrodynamical simulations with
artificially enhanced luminosity.
Title: MHD study of the planetary magnetospheric response during
extreme solar wind conditions: Earth and exoplanet magnetospheres
applications
Authors: Varela, J.; Brun, A. S.; Strugarek, A.; Réville, V.; Zarka,
P.; Pantellini, F.
Bibcode: 2022A&A...659A..10V
Altcode: 2022arXiv220302324V
Context. The stellar wind and the interplanetary magnetic field
modify the topology of planetary magnetospheres. Consequently, the
hazardous effect of the direct exposition to the stellar wind, for
example, regarding the integrity of satellites orbiting the Earth
or the habitability of exoplanets, depends upon the space weather
conditions.
Aims: The aim of the study is to analyze the
response of an Earth-like magnetosphere for various space weather
conditions and interplanetary coronal mass ejections. The magnetopause
standoff distance, the open-close field line boundary, and plasma
flows toward the planet surface are calculated.
Methods: We
used the magnetohydrodynamics code PLUTO in spherical coordinates to
perform a parametric study of the dynamic pressure and temperature
of the stellar wind as well as of the interplanetary magnetic field
intensity and orientation. The range of the parameters we analyzed
extends from regular to extreme space weather conditions, which is
consistent with coronal mass ejections at the Earth orbit for the
present and early periods of the solar main sequence. In addition,
implications of sub-Afvénic solar wind configurations for the
Earth and exoplanet magnetospheres were analyzed.
Results:
The direct precipitation of the solar wind at the Earth dayside in
equatorial latitudes is extremely unlikely even during super coronal
mass ejections. On the other hand, for early evolution phases during
the solar main sequence, when the solar rotation rate was at least five
times faster (<440 Myr), the Earth surface was directly exposed to
the solar wind during coronal mass ejections. Today, satellites at
high, geosynchronous, and medium orbits are directly exposed to the
solar wind during coronal mass ejections because part of the orbit at
the Earth dayside is beyond the nose of the bow shock.
Title: Flux rope and dynamics of the heliospheric current sheet. Study
of the Parker Solar Probe and Solar Orbiter conjunction of June 2020
Authors: Réville, V.; Fargette, N.; Rouillard, A. P.; Lavraud,
B.; Velli, M.; Strugarek, A.; Parenti, S.; Brun, A. S.; Shi, C.;
Kouloumvakos, A.; Poirier, N.; Pinto, R. F.; Louarn, P.; Fedorov,
A.; Owen, C. J.; Génot, V.; Horbury, T. S.; Laker, R.; O'Brien, H.;
Angelini, V.; Fauchon-Jones, E.; Kasper, J. C.
Bibcode: 2022A&A...659A.110R
Altcode: 2021arXiv211207445R
Context. Solar Orbiter and Parker Solar Probe jointly observed the
solar wind for the first time in June 2020, capturing data from very
different solar wind streams: calm, Alfvénic wind and also highly
dynamic large-scale structures. Context. Our aim is to understand the
origin and characteristics of the highly dynamic solar wind observed by
the two probes, particularly in the vicinity of the heliospheric current
sheet (HCS).
Methods: We analyzed the plasma data obtained
by Parker Solar Probe and Solar Orbiter in situ during the month of
June 2020. We used the Alfvén-wave turbulence magnetohydrodynamic
solar wind model WindPredict-AW and we performed two 3D simulations
based on ADAPT solar magnetograms for this period.
Results:
We show that the dynamic regions measured by both spacecraft are
pervaded by flux ropes close to the HCS. These flux ropes are also
present in the simulations, forming at the tip of helmet streamers,
that is, at the base of the heliospheric current sheet. The formation
mechanism involves a pressure-driven instability followed by a fast
tearing reconnection process. We further characterize the 3D spatial
structure of helmet streamer born flux ropes, which appears in the
simulations to be related to the network of quasi-separatrices.
Title: Impact of anti-solar differential rotation in mean-field
solar-type dynamos. Exploring possible magnetic cycles in slowly
rotating stars
Authors: Noraz, Q.; Brun, A. S.; Strugarek, A.; Depambour, G.
Bibcode: 2022A&A...658A.144N
Altcode: 2021arXiv211112722N
Context. Over the course of their lifetimes, the rotation of solar-type
stars goes through different phases. Once they reach the zero-age
main sequence, their global rotation rate decreases during the main
sequence until at least the solar age, approximately following the
empirical Skumanich's law and enabling gyrochronology. Older solar-type
stars might then reach a point of transition when they stop braking,
according to recent results of asteroseismology. Additionally, recent
3D numerical simulations of solar-type stars show that different regimes
of differential rotation can be characterized with the Rossby number. In
particular, anti-solar differential rotation (fast poles, slow equator)
may exist for high Rossby number (slow rotators). If this regime occurs
during the main sequence and, in general, for slow rotators, we may
consider how magnetic generation through the dynamo process might be
impacted. In particular, we consider whether slowly rotating stars are
indeed subject to magnetic cycles.
Aims: We aim to understand the
magnetic field generation of solar-type stars possessing an anti-solar
differential rotation and we focus on the possible existence of
magnetic cycles in such stars.
Methods: We modeled mean-field
kinematic dynamos in solar (fast equator, slow poles) and anti-solar
(slow equator, fast poles) differential rotation, using the STELEM
code. We consider two types of mean field dynamo mechanisms along with
the Ω-effect: the standard α-effect distributed at various locations
in the convective envelope and the Babcock-Leighton effect.
Results: We find that kinematic αΩ dynamos allow for the presence
of magnetic cycles and global polarity reversals for both rotation
regimes, but only if the α-effect is saddled on the tachocline. If it
is distributed in the convection zone, solar-type cases still possess
a cycle and anti-solar cases do not. Conversely, we have not found
any possibility for sustaining a magnetic cycle with the traditional
Babcock-Leighton flux-transport dynamos in the anti-solar differential
rotation regime due to flux addition. Graphic interpretations are
proposed in order to illustrate these cases. However, we find that
hybrid models containing both prescriptions can still sustain local
polarity reversals at some latitudes.
Conclusions: We conclude
that stars in the anti-solar differential rotation regime can sustain
magnetic cycles only for very specific dynamo processes. The detection
of a magnetic cycle for such a star would therefore be a particularly
interesting constraint in working to decipher what type of dynamo is
actually at work in solar-type stars.
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
Bibcode: 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 f ≲
0.65, the solutions possess long magnetic cycle, if Ro f
≲ 0.42 a short cycle and if Ro f ≳ 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
( ${B}_{\mathrm{bulk}}\sim {{Ro}}_{{\rm{f}}}^{-0.5}$ ).
Title: Adding a transition region in global MHD models of the
solar corona
Authors: Réville, V.; Parenti, S.; Brun, A. S.; Strugarek, A.;
Rouillard, A. P.; Velli, M.; Perri, B.; Pinto, R. F.
Bibcode: 2021sf2a.conf..230R
Altcode:
Global MHD simulations of the solar corona are an essential tool
to investigate long standing problems, such as finding the source
of coronal heating and the mechanisms responsible for the onset and
propagation of coronal mass ejections. The very low atmospheric layers
of the corona, are however, very difficult to model as they imply very
steep gradients of density and temperature over only a few thousand
kilometers. In this proceedings, we illustrate some of the benefits
of including a very simple transition region in global MHD models and
the differences in the plasma properties, comparing with in situ data
of the Parker Solar Probe.
Title: Rotational and orbital evolution of star-planet systems. Impact
of tidal and magnetic torques.
Authors: Ahuir, J.; Strugarek, A.; Brun, A. S.; Mathis, S.
Bibcode: 2021sf2a.conf..359A
Altcode:
The discovery of more than 4000 exoplanets during the last two
decades has shed light on the importance of characterizing star-planet
interactions. Indeed, a large fraction of these planets have short
orbital periods and are consequently strongly interacting with their
host star. In particular, several planetary systems are likely to host
exoplanets undergoing a migration due to tidal and magnetic torques. We
consider here the joint influence of stellar wind, tidal and magnetic
star-planet interactions on the star's rotation rate and planetary
orbital evolution. To this end, we have developed a numerical model
of a circular and coplanar star-planet system taking into account
stellar structural changes, wind braking and star-planet interactions,
called ESPEM (Evolution of Planetary Systems and Magnetism). We
present synthetic populations of star-planet systems and compare
their distribution in orbital period and in stellar rotation period
to the Kepler satellite data. We find that star-planet magnetic
interactions significantly modify the distribution of super-Earths
around slowly rotating stars, which improves the agreement between
synthetic populations and observations. Tidal effects, on the other
hand, shape the distribution of giant planets.
Title: How magnetism of solar-type stars evolves ?
Authors: Noraz, Quentin; Brun, Allan Sacha; Strugarek, Antoine
Bibcode: 2021plat.confE..82N
Altcode:
The solar magnetic field is generated and sustained through an internal
dynamo. In stars, this process is determined by the combined action of
turbulent convective motions and the differential rotation profile. It
can sometimes lead to magnetic cyclic variabilities, like in the Sun
with the 11 years cycle. Traces of magnetic cycles have been detected
for other solar-like stars as well, ranging from a few years to a
few tens of years. How are these cycles controlled? During their
life, the rotation of stars is subject to complex evolution. Recent
3D numerical simulations of solar-like stars show that different
regimes of differential rotation can be characterized with the Rossby
number. In particular, anti-solar differential rotation (fast poles,
slow equator) may exist for high Rossby number (slow rotators). If this
regime occurs during the stellar spin-down of the main sequence, and in
general for slow rotators, we may wonder how the magnetic generation
through dynamo process will be impacted. In particular, can slowly
rotating stars have magnetic cycles? We present a numerical multi-D
study with the STELEM and ASH codes to understand the magnetic field
generation of solar-like stars under various differential rotation
regimes, and focus on the existence of magnetic cycles. We find
in self-consistent 3D simulations that short cycles are favoured for
small Rossby numbers (fast rotators), and long cycles for intermediate
(solar-like) Rossby numbers. Slow rotators (high Rossby numbers) are
found to produce only steady dynamo with no cyclic activity. However we
find that specific mean-field models can produce magnetic cycles with
anti-solar differential rotation only if the alpha effect is fine tuned
for this purpose. It is still unclear today whether this latter regime
can be achieved self-consistently in global 3D simulations. We
then conclude that slow rotating stars in the anti-solar differential
rotation regime can sustain magnetic cycles only for very specific
dynamo processes. A detection of magnetic cycles for such stars would
therefore be a tremendous constrain on deciphering what type of dynamo
is actually acting in solar-like stars, and thus on how their magnetism
can evolve. This problematic is particularly relevant in the context of
the PLATO mission, which will provide new constraints, in particular
on the differential rotation and the magnetic activity taking place
in these stars.
Title: Solar wind rotation rate and shear at coronal hole
boundaries. Possible consequences for magnetic field inversions
Authors: Pinto, R. F.; Poirier, N.; Rouillard, A. P.; Kouloumvakos,
A.; Griton, L.; Fargette, N.; Kieokaew, R.; Lavraud, B.; Brun, A. S.
Bibcode: 2021A&A...653A..92P
Altcode: 2021arXiv210408393P
Context. In situ measurements by several spacecraft have revealed
that the solar wind is frequently perturbed by transient structures
that have been interpreted as magnetic folds, jets, waves, and flux
ropes that propagate rapidly away from the Sun over a large range
of heliocentric distances. Parker Solar Probe (PSP), in particular,
has detected very frequent rotations of the magnetic field vector at
small heliocentric radial distances, accompanied by surprisingly large
solar wind rotation rates. The physical origin of such magnetic field
bends and switchbacks, the conditions for their survival across the
interplanetary space, and their relation to solar wind rotation are
yet to be clearly understood.
Aims: We aim to characterise the
global properties of the solar wind flows crossed by PSP, to relate
those flows to the rotational state of the low solar corona, and to
identify regions of the solar surface and corona that are likely to be
sources of switchbacks and bends.
Methods: We traced measured
solar wind flows from the spacecraft position down to the surface of
the Sun to identify their potential source regions, and used a global
magneto-hydrodynamic model of the corona and solar wind to analyse the
dynamical properties of those regions. We identify regions of the solar
corona for which solar wind speed and rotational shear are important
and long-lived that can be favourable to the development of magnetic
deflections and to their propagation across extended heights in the
solar wind.
Results: We show that coronal rotation is highly
structured, and that enhanced flow shear and magnetic field gradients
develop near the boundaries between coronal holes and streamers,
and around and above pseudo-streamers, even when such boundaries are
aligned with the direction of solar rotation. The exact properties
and amplitudes of the shears are a combined effect of the forces
exerted by the rotation of the corona and of its magnetic topology. A
large fraction of the switchbacks identified by PSP map back to these
regions, both in terms of instantaneous magnetic field connectivity
and of the trajectories of wind streams that reach the spacecraft.
Conclusions: We conclude that these regions of strong shears are
likely to leave an imprint on the solar wind over large distances
and to increase the transverse speed variability in the slow solar
wind. The simulations and connectivity analysis suggest they could be
a source of the switchbacks and spikes observed by PSP.
Title: Solar inertial modes: Observations, identification, and
diagnostic promise
Authors: Gizon, Laurent; Cameron, Robert H.; Bekki, Yuto; Birch,
Aaron C.; Bogart, Richard S.; Brun, Allan Sacha; Damiani, Cilia;
Fournier, Damien; Hyest, Laura; Jain, Kiran; Lekshmi, B.; Liang,
Zhi-Chao; Proxauf, Bastian
Bibcode: 2021A&A...652L...6G
Altcode: 2021arXiv210709499G
The oscillations of a slowly rotating star have long been classified
into spheroidal and toroidal modes. The spheroidal modes include
the well-known 5-min acoustic modes used in helioseismology. Here
we report observations of the Sun's toroidal modes, for which the
restoring force is the Coriolis force and whose periods are on the
order of the solar rotation period. By comparing the observations
with the normal modes of a differentially rotating spherical shell,
we are able to identify many of the observed modes. These are the
high-latitude inertial modes, the critical-latitude inertial modes,
and the equatorial Rossby modes. In the model, the high-latitude
and critical-latitude modes have maximum kinetic energy density at
the base of the convection zone, and the high-latitude modes are
baroclinically unstable due to the latitudinal entropy gradient. As
a first application of inertial-mode helioseismology, we constrain
the superadiabaticity and the turbulent viscosity in the deep
convection zone. Movie associated to Fig. 2 is available at https://www.aanda.org
Title: Magnetic and tidal migration of close-in planets. Influence
of secular evolution on their population
Authors: Ahuir, J.; Strugarek, A.; Brun, A. -S.; Mathis, S.
Bibcode: 2021A&A...650A.126A
Altcode: 2021arXiv210401004A
Context. Over the last two decades, a large population of close-in
planets has been detected around a wide variety of host stars. Such
exoplanets are likely to undergo planetary migration through magnetic
and tidal interactions.
Aims: We aim to follow the orbital
evolution of a planet along the structural and rotational evolution of
its host star, simultaneously taking into account tidal and magnetic
torques, in order to explain some properties of the distribution of
observed close-in planets.
Methods: We rely on a numerical
model of a coplanar circular star-planet system called ESPEM,
which takes into account stellar structural changes, wind braking,
and star-planet interactions. We browse the parameter space of the
star-planet system configurations and assess the relative influence
of magnetic and tidal torques on its secular evolution. We then
synthesize star-planet populations and compare their distribution in
orbital and stellar rotation periods to Kepler satellite data.
Results: Magnetic and tidal interactions act together on planetary
migration and stellar rotation. Furthermore, both interactions can
dominate secular evolution depending on the initial configuration
of the system and the evolutionary phase considered. Indeed, tidal
effects tend to dominate for high stellar and planetary masses as well
as low semi-major axis; they also govern the evolution of planets
orbiting fast rotators while slower rotators evolve essentially
through magnetic interactions. Moreover, three populations of
star-planet systems emerge from the combined action of both kinds of
interactions. First, systems undergoing negligible migration define
an area of influence of star-planet interactions. For sufficiently
large planetary magnetic fields, the magnetic torque determines the
extension of this region. Next, planets close to fast rotators migrate
efficiently during the pre-main sequence, which engenders a depleted
region at low rotation and orbital periods. Then, the migration
of planets close to slower rotators, which happens during the main
sequence, may lead to a break in gyrochronology for high stellar and
planetary masses. This also creates a region at high rotation periods
and low orbital periods not populated by star-planet systems. We also
find that star-planet interactions significantly impact the global
distribution in orbital periods by depleting more planets for higher
planetary masses and planetary magnetic fields. However, the global
distribution in stellar rotation periods is marginally affected,
as around 0.5% of G-type stars and 0.1% of K-type stars may spin up
because of planetary engulfment. More precisely, star-planet magnetic
interactions significantly affect the distribution of super-Earths
around stars with a rotation period higher than around 5 days, which
improves the agreement between synthetic populations and observations
at orbital periods of less than 1 day. Tidal effects for their part
shape the distribution of giant planets.
Title: Energetic particles and the solar cycle: Impact of solar
magnetic field amplitude and geometry on SEPs and GCRs diffusion
coefficients
Authors: Perri, Barbara; Brun, Allan Sacha; Strugarek, Antoine;
Réville, Victor
Bibcode: 2021EGUGA..23.6394P
Altcode:
SEPs are correlated with the 11-year solar cycle due to their production
by flares and interaction with the inner heliosphere, while GCRs are
anti-correlated with it due to the modulation of the heliospheric
magnetic field. The solar magnetic field along the cycle varies in
amplitude but also in geometry, causing diffusion of the particles
along and across the field lines; the solar wind distribution also
evolves, and its turbulence affects particle trajectories.We combine
3D MHD compressible numerical simulations to compute the configuration
of the magnetic field and the associated polytropic solar wind up to
1 AU, with analytical prescriptions of the corresponding parallel
and perpendicular diffusion coefficients for SEPs and GCRs. First,
we analyze separately the impact of the magnetic field amplitude and
geometry for a 100 MeV proton. By varying the amplitude, we change
the amplitude of the diffusion by the same factor, and the radial
gradients by changing the spread of the current sheet. By varying the
geometry, we change the latitudinal gradients of diffusion by changing
the position of the current sheets. We also vary the energy, and show
that the statistical distribution of parallel diffusion is different
for SEPs and GCRs. Then, we use realistic solar configurations, showing
that diffusion is highly non-axisymmetric due to the configuration
of the current sheets, and that the distribution varies a lot with
the distance to the Sun, especially at minimum of activity. With this
model, we are thus able to study the direct influence of the Sun on
Earth spatial environment in terms of energetic particles.
Title: Solar wind speed and rotational shear at coronal hole
boundaries, impacts on magnetic field inversions
Authors: Pinto, Rui; Poirier, Nicolas; Kouloumvakos, Athanasis;
Rouillard, Alexis; Griton, Léa; Fargette, Naïs; Kieokaew,
Rungployphan; Lavraud, Benoît; Brun, Allan Sacha
Bibcode: 2021EGUGA..2313552P
Altcode:
The solar wind is frequently perturbed by transient structures such
as magnetic folds, jets, waves and flux-ropes that propagate rapidly
away from the Sun over a large range of heliocentric distances. Parker
Solar Probe has revealed that rotations of the magnetic field vector
occur repeatedly at small heliocentric distances, on regions that also
display surprisingly large solar wind rotation rates. Sun-to-spacecraft
connectivity analysis shows that a large fraction of the solar wind
flows probed so far by Parker Solar Probe were formed and accelerated
in the vicinity of coronal hole boundaries.We show by means of of
global MHD simulations that coronal rotation is highly structured
in proximity to those boundary regions (in agreement with preceding
SoHO/UVCS observations), and that enhanced poloidal and toroidal flow
shear and magnetic field gradients also develop there. We identified
regions of the solar corona for which solar wind speed and rotational
shear are significant, that can be associated with field-aligned and/or
transverse vorticity, and that can be favourable to the development
of magnetic deflections. Some of these wind flow shears are driven
through large radial extensions, being noticeable tens of solar radii
away from the surface, and therefore have a potential impact on the
propagation of such magnetic perturbations across extended heights in
the solar wind. We conclude that these regions of persistent shears
are undoubtedly sources of complex solar wind structures, and suggest
that they can trigger instabilities capable of creating magnetic
field reversals detected in-situ in the heliosphere.Our simulations
furthermore indicate that the spatial structure of the solar wind
shear will become more complex as the solar cycle progresses, with
strong and extended shears appearing at heliographic latitudes that
will be probed by Solar Orbiter in the near future.
Title: Modeling Solar Wind Variations over an 11 Year Cycle with
Alfvén Wave Dissipation: A Parameter Study
Authors: Hazra, Soumitra; Réville, Victor; Perri, Barbara; Strugarek,
Antoine; Brun, Allan Sacha; Buchlin, Eric
Bibcode: 2021ApJ...910...90H
Altcode: 2021arXiv210111511H
We study the behavior and properties of the solar wind using a
2.5D Alfvén wave (AW)-driven wind model. We first systematically
compare the results of an AW-driven wind model with a polytropic
approach. Polytropic magnetohydrodynamic wind models are thermally
driven, while AWs act as additional acceleration and heating mechanisms
in the AW-driven model. We confirm that an AW-driven model is required
to reproduce the observed bimodality of slow and fast solar winds. We
are also able to reproduce the observed anticorrelation between the
terminal wind velocity and the coronal source temperature with the
AW-driven wind model. We also show that the wind properties along an 11
yr cycle differ significantly from one model to the other. The AW-driven
model again shows the best agreement with observational data. Indeed,
solar surface magnetic field topology plays an important role in the
AW-driven wind model, as it enters directly into the input energy
sources via the Poynting flux. On the other hand, the polytropic wind
model is driven by an assumed pressure gradient; thus, it is relatively
less sensitive to the surface magnetic field topology. Finally, we note
that the net torque spinning down the Sun exhibits the same trends in
the two models, showing that the polytropic approach still correctly
captures the essence of stellar winds.
Title: Dynamical Coupling of a Mean-field Dynamo and Its Wind:
Feedback Loop over a Stellar Activity Cycle
Authors: Perri, Barbara; Brun, Allan Sacha; Strugarek, Antoine;
Réville, Victor
Bibcode: 2021ApJ...910...50P
Altcode: 2021arXiv210201416P
We focus on the connection between the internal dynamo magnetic field
and the stellar wind. If the star has a cyclic dynamo, the modulations
of the magnetic field can affect the wind, which, in turn, can
back-react on the boundary conditions of the star, creating a feedback
loop. We have developed a 2.5D numerical setup to model this essential
coupling. We have implemented an alpha-omega mean-field dynamo in the
PLUTO code and then coupled it to a spherical polytropic wind model
via an interface composed of four grid layers with dedicated boundary
conditions. We present here a dynamo model close to a young Sun with
cyclic magnetic activity. First, we show how this model allows one to
track the influence of the dynamo activity on the corona by displaying
the correlation between the activity cycle, the coronal structure,
and the time evolution of integrated quantities. Then we add the
feedback of the wind on the dynamo and discuss the changes observed
in the dynamo symmetry and wind variations. We explain these changes
in terms of dynamo modes; in this parameter regime, the feedback
loop leads to a coupling between the dynamo families via a preferred
growth of the quadrupolar mode. We also study our interface in terms
of magnetic helicity and show that it leads to a small injection in
the dynamo. This model confirms the importance of coupling physically
internal and external stellar layers, as it has a direct impact on
both the dynamo and the wind.
Title: Can slowly rotating stars sustain magnetic cycles?
Authors: Noraz, Quentin; Brun, Allan Sacha; Strugarek, Antoine
Bibcode: 2021csss.confE.216N
Altcode:
The solar magnetic field is generated and sustained through an
internal dynamo. In stars, this process is determined by the combined
action of turbulent convective motions and the differential rotation
profile. It can sometimes lead to magnetic cyclic variabilities,
like in the Sun with the 11 years cycle. Traces of magnetic cycles
have been detected for other stars as well, ranging from a few years
to a few tens of years. How are these cycles controlled? During their
life, the rotation of stars is subject to complex evolution. Recent 3D
numerical simulations of solar-like stars show that different regimes
of differential rotation can be characterized with the Rossby number. In
particular, anti-solar differential rotation (fast poles, slow equator)
may exist for a high Rossby number (slow rotators). If this regime
occurs during the main sequence, and in general for slow rotators,
we may wonder how the magnetic generation through dynamo process will
be impacted. In particular, can slowly rotating stars have magnetic
cycles?We present a numerical multi-D study with the STELEM and ASH
codes to understand the magnetic field generation of solar-like stars
under various differential rotation regimes, and focus on the existence
of magnetic cycles.We find that short cycles are favoured for small
Rossby numbers (fast rotators), and long cycles for intermediate
(solar-like) Rossby numbers. Slow rotators (high Rossby numbers) are
found to produce only steady dynamo with no cyclic activity in most
cases considered. Magnetic cycles can be produced with anti-solar
differential rotation only if the alpha effect is fine tuned for
this purpose.We conclude that slow rotating stars in the anti-solar
differential rotation regime can sustain magnetic cycles only for
very specific dynamo processes. A detection of magnetic cycles for
such stars would therefore be a tremendous constrain on deciphering
what type of dynamo is actually acting in solar-like stars.
Title: Does the mean-field α effect have any impact on the memory
of the solar cycle?
Authors: Hazra, Soumitra; Brun, Allan Sacha; Nandy, Dibyendu
Bibcode: 2020A&A...642A..51H
Altcode: 2020arXiv200302776H
Context. Predictions of solar cycle 24 obtained from advection-dominated
and diffusion-dominated kinematic dynamo models are different if
the Babcock-Leighton mechanism is the only source of the poloidal
field. Some previous studies argue that the discrepancy arises
due to different memories of the solar dynamo for advection- and
diffusion-dominated solar convection zones.
Aims: We aim
to investigate the differences in solar cycle memory obtained from
advection-dominated and diffusion-dominated kinematic solar dynamo
models. Specifically, we explore whether inclusion of Parker's
mean-field α effect, in addition to the Babcock-Leighton mechanism,
has any impact on the memory of the solar cycle.
Methods: We
used a kinematic flux transport solar dynamo model where poloidal
field generation takes place due to both the Babcock-Leighton
mechanism and the mean-field α effect. We additionally considered
stochastic fluctuations in this model and explored cycle-to-cycle
correlations between the polar field at minima and toroidal field
at cycle maxima.
Results: Solar dynamo memory is always
limited to only one cycle in diffusion-dominated dynamo regimes
while in advection-dominated regimes the memory is distributed
over a few solar cycles. However, the addition of a mean-field α
effect reduces the memory of the solar dynamo to within one cycle in
the advection-dominated dynamo regime when there are no fluctuations
in the mean-field α effect. When fluctuations are introduced in the
mean-field poloidal source a more complex scenario is evident, with very
weak but significant correlations emerging across a few cycles.
Conclusions: Our results imply that inclusion of a mean-field α
effect in the framework of a flux transport Babcock-Leighton dynamo
model leads to additional complexities that may impact memory and
predictability of predictive dynamo models of the solar cycle.
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
Bibcode: 2020A&A...642A...3Z
Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
Title: Impact of solar magnetic field amplitude and geometry on
cosmic rays diffusion coefficients in the inner heliosphere
Authors: Perri, Barbara; Brun, Allan Sacha; Strugarek, Antoine;
Réville, Victor
Bibcode: 2020JSWSC..10...55P
Altcode: 2020arXiv201001880P
Cosmic rays are remarkable tracers of solar events when they are
associated with solar flares, but also galactic events such as supernova
remnants when they come from outside our solar system. Solar Energetic
Particles (SEPs) are correlated with the 11-year solar cycle while
Galactic Cosmic Rays (GCRs) are anti-correlated due to their interaction
with the heliospheric magnetic field and the solar wind. Our aim is
to quantify separately the impact of the amplitude and the geometry
of the magnetic field, both evolving during the solar cycle, on the
propagation of cosmic rays of various energies in the inner heliosphere
(within Earth orbit). We focus especially on the diffusion caused by
the magnetic field along and across the field lines. To do so, we use
the results of 3D magnetohydrodynamics (MHD) wind simulations running
from the lower corona up to 1 AU. This gives us the structure of the
wind and the corresponding magnetic field. The wind is modeled using a
polytropic approximation, and fits and power laws are used to account
for the turbulence. Using these results, we compute the parallel and
perpendicular diffusion coefficients of the Parker cosmic ray transport
equation, yielding 3D maps of the diffusion of cosmic rays in the
inner heliosphere. By varying the amplitude of the magnetic field, we
change the amplitude of the diffusion by the same factor, and the radial
gradients by changing the spread of the current sheet. By varying the
geometry of the magnetic field, we change the latitudinal gradients of
diffusion by changing the position of the current sheets. By varying
the energy, we show that the distribution of values for SEPs is more
peaked than GCRs. For realistic solar configurations, we show that
diffusion is highly non-axisymmetric due to the configuration of
the current sheets, and that the distribution varies a lot with the
distance to the Sun with a drift of the peak value. This study shows
that numerical simulations, combined with theory, can help quantify
better the influence of the various magnetic field parameters on
the propagation of cosmic rays. This study is a first step towards
the resolution of the complete Parker transport equation to generate
synthetic cosmic rays rates from numerical simulations.
Title: Models and data analysis tools for the Solar Orbiter mission
Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.;
Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.;
Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.;
Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi,
N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla,
T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.;
Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.;
Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.;
Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.;
Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot,
V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.;
Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.;
Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp,
E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud,
B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.;
Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.;
Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti,
S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.;
Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez,
L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca,
L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.;
Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov,
A. N.
Bibcode: 2020A&A...642A...2R
Altcode:
Context. The Solar Orbiter spacecraft will be equipped with a wide
range of remote-sensing (RS) and in situ (IS) instruments to record
novel and unprecedented measurements of the solar atmosphere and
the inner heliosphere. To take full advantage of these new datasets,
tools and techniques must be developed to ease multi-instrument and
multi-spacecraft studies. In particular the currently inaccessible
low solar corona below two solar radii can only be observed
remotely. Furthermore techniques must be used to retrieve coronal
plasma properties in time and in three dimensional (3D) space. Solar
Orbiter will run complex observation campaigns that provide interesting
opportunities to maximise the likelihood of linking IS data to their
source region near the Sun. Several RS instruments can be directed
to specific targets situated on the solar disk just days before
data acquisition. To compare IS and RS, data we must improve our
understanding of how heliospheric probes magnetically connect to the
solar disk.
Aims: The aim of the present paper is to briefly
review how the current modelling of the Sun and its atmosphere
can support Solar Orbiter science. We describe the results of a
community-led effort by European Space Agency's Modelling and Data
Analysis Working Group (MADAWG) to develop different models, tools,
and techniques deemed necessary to test different theories for the
physical processes that may occur in the solar plasma. The focus here
is on the large scales and little is described with regards to kinetic
processes. To exploit future IS and RS data fully, many techniques have
been adapted to model the evolving 3D solar magneto-plasma from the
solar interior to the solar wind. A particular focus in the paper is
placed on techniques that can estimate how Solar Orbiter will connect
magnetically through the complex coronal magnetic fields to various
photospheric and coronal features in support of spacecraft operations
and future scientific studies.
Methods: Recent missions such as
STEREO, provided great opportunities for RS, IS, and multi-spacecraft
studies. We summarise the achievements and highlight the challenges
faced during these investigations, many of which motivated the Solar
Orbiter mission. We present the new tools and techniques developed
by the MADAWG to support the science operations and the analysis of
the data from the many instruments on Solar Orbiter.
Results:
This article reviews current modelling and tool developments that ease
the comparison of model results with RS and IS data made available
by current and upcoming missions. It also describes the modelling
strategy to support the science operations and subsequent exploitation
of Solar Orbiter data in order to maximise the scientific output
of the mission.
Conclusions: The on-going community effort
presented in this paper has provided new models and tools necessary
to support mission operations as well as the science exploitation of
the Solar Orbiter data. The tools and techniques will no doubt evolve
significantly as we refine our procedure and methodology during the
first year of operations of this highly promising mission.
Title: Alfvén-wave-driven Magnetic Rotator Winds from Low-mass
Stars. I. Rotation Dependences of Magnetic Braking and Mass-loss Rate
Authors: Shoda, Munehito; Suzuki, Takeru K.; Matt, Sean P.; Cranmer,
Steven R.; Vidotto, Aline A.; Strugarek, Antoine; See, Victor;
Réville, Victor; Finley, Adam J.; Brun, Allan Sacha
Bibcode: 2020ApJ...896..123S
Altcode: 2020arXiv200509817S
Observations of stellar rotation show that low-mass stars lose angular
momentum during the main sequence. We simulate the winds of sunlike
stars with a range of rotation rates, covering the fast and slow
magneto-rotator regimes, including the transition between the two. We
generalize an Alfvén-wave-driven solar wind model that builds on
previous works by including the magneto-centrifugal force explicitly. In
this model, the surface-averaged open magnetic flux is assumed to scale
as ${B}_{* }{f}_{* }^{\mathrm{open}}\propto {\mathrm{Ro}}^{-1.2}$ ,
where ${f}_{* }^{\mathrm{open}}$ and Ro are the surface open-flux
filling factor and Rossby number, respectively. We find that, (1)
the angular-momentum loss rate (torque) of the wind is described
as ${\tau }_{{\rm{w}}}\approx 2.59\times {10}^{30}\ \mathrm{erg}\
{\left({{\rm{\Omega }}}_{* }/{{\rm{\Omega }}}_{\odot }\right)}^{2.82}$
, yielding a spin-down law ${{\rm{\Omega }}}_{* }\propto {t}^{-0.55}$
. (2) The mass-loss rate saturates at ${\dot{M}}_{{\rm{w}}}\sim
3.4\times {10}^{-14}{M}_{\odot }\ {\mathrm{yr}}^{-1}$ , due to
the strong reflection and dissipation of Alfvén waves in the
chromosphere. This indicates that the chromosphere has a strong impact
in connecting the stellar surface and stellar wind. Meanwhile, the
wind ram pressure scales as ${P}_{{\rm{w}}}\propto {{\rm{\Omega }}}_{*
}^{0.57}$ , which is able to explain the lower envelope of the observed
stellar winds by Wood et al. (3) The location of the Alfvén radius
is shown to scale in a way that is consistent with one-dimensional
analytic theory. Additionally, the precise scaling of the Alfvén
radius matches previous works, which used thermally driven winds. Our
results suggest that the Alfvén-wave-driven magnetic rotator wind
plays a dominant role in the stellar spin-down during the main sequence.
Title: Assessment of Critical Convection and Associated Rotation
States in Models of Sun-like Stars Including a Stable Layer
Authors: Takehiro, Shin-ichi; Brun, Allan Sacha; Yamada, Michio
Bibcode: 2020ApJ...893...83T
Altcode:
Recent numerical simulations of rotating stellar convection have
suggested the possible existence of retrograde (slow equator, fast
poles) or so-called antisolar differential rotation states in slowly
rotating stars possessing a large Rossby number. We aim to understand
whether such rotational states exist from the onset of convective
instability or are the outcome of complex nonlinear interactions in the
turbulent convective envelope. To this end, we have made a systematic
linear analysis of the critical state of convection in a series of
15 numerical simulations published in Brun et al. We have assessed
their degree of supercriticality and most-unstable mode properties,
and computed the second-order mean zonal flow response. We find that
none of the linear critical cases show a retrograde state at the onset
of convection even when their nonlinear counterparts do. We also
find that the presence of a stably stratified layer coupled to the
convectively unstable upper layer leads to interesting gravity-wave
excitation and angular momentum transport. We conclude that retrograde
states of differential rotation are probably the outcome of complex
mode-mode interactions in the turbulent convection layer and are,
as a consequence, likely to exist in real stars.
Title: From stellar coronae to gyrochronology: A theoretical and
observational exploration
Authors: Ahuir, J.; Brun, A. S.; Strugarek, A.
Bibcode: 2020A&A...635A.170A
Altcode: 2020arXiv200200696A
Context. Stellar spin down is the result of a complex process
involving rotation, dynamo, wind, and magnetism. Multiwavelength
surveys of solar-like stars have revealed the likely existence of
relationships between their rotation, X-ray luminosity, mass losses,
and magnetism. They impose strong constraints on the corona and wind
of cool stars.
Aims: We aim to provide power-law prescriptions
of the mass loss of stars, of their magnetic field, and of their
base coronal density and temperature that are compatible with their
observationally-constrained spin down.
Methods: We link the
magnetic field and the mass-loss rate from a wind torque formulation,
which is in agreement with the distribution of stellar rotation periods
in open clusters and the Skumanich law. Given a wind model and an
expression of the X-ray luminosity from radiative losses, we constrained
the coronal properties by assuming different physical scenarios linking
closed loops to coronal holes.
Results: We find that the magnetic
field and the mass loss are involved in a one-to-one correspondence that
is constrained from spin down considerations. We show that a magnetic
field, depending on both the Rossby number and the stellar mass, is
required to keep a consistent spin down model. The estimates of the
magnetic field and the mass-loss rate obtained from our formalism are
consistent with statistical studies as well as individual observations
and they give new leads to constrain the magnetic field-rotation
relation. The set of scaling-laws we derived can be broadly applied to
cool stars from the pre-main sequence to the end of the main sequence
(MS), and they allow for stellar wind modeling that is consistent with
all of the observational constraints available to date.
Title: Could star-planet magnetic interactions lead to planet
migration and influence stellar rotation?
Authors: Ahuir, Jérémy; Strugarek, Antoine; Brun, Allan Sacha;
Mathis, Stéphane; Bolmont, Emeline; Benbakoura, Mansour; Réville,
Victor; Le Poncin-Lafitte, Christophe
Bibcode: 2020IAUS..354..295A
Altcode: 2019arXiv191206867A
The distribution of hot Jupiters, for which star-planet interactions can
be significant, questions the evolution of exosystems. We aim to follow
the orbital evolution of a planet along the rotational and structural
evolution of the host star by taking into account the coupled effects
of tidal and magnetic torques from ab initio prescriptions. It allows
us to better understand the evolution of star-planet systems and
to explain some properties of the distribution of observed close-in
planets. To this end we use a numerical model of a coplanar circular
star-planet system taking into account stellar structural changes,
wind braking and star-planet interactions, called ESPEM (Benbakoura et
al. (<xref rid="r4" ref-type="bibr">2019</xref>)). We find
that depending on the initial configuration of the system, magnetic
effects can dominate tidal effects during the various phases of the
evolution, leading to an important migration of the planet and to
significant changes on the rotational evolution of the star. Both
kinds of interactions thus have to be taken into account to predict
the evolution of compact star-planet systems.
Title: On Solar and Solar-Like Stars Convection, Rotation and
Magnetism
Authors: Brun, Allan Sacha
Bibcode: 2020ASSP...57...75B
Altcode:
We honor Mike J. Thompson's legacy on solar and stellar convection,
rotation and magnetism and their seismic probing by discussing how
his major contributions have impacted or challenged the current state
of our understanding and guided the development of advanced numerical
simulations of the magnetohydrodynamics (MHD) of the Sun and Sun-like
stars.
Title: The impact of magnetism on tidal dynamics in the convective
envelope of low-mass stars
Authors: Astoul, A.; Mathis, S.; Baruteau, C.; Gallet, F.; Strugarek,
A.; Augustson, K. C.; Brun, A. S.; Bolmont, E.
Bibcode: 2020IAUS..354..195A
Altcode:
For the shortest period exoplanets, star-planet tidal interactions are
likely to have played a major role in the ultimate orbital evolution
of the planets and on the spin evolution of the host stars. Although
low-mass stars are magnetically active objects, the question of how
the star's magnetic field impacts the excitation, propagation and
dissipation of tidal waves remains open. We have derived the magnetic
contribution to the tidal interaction and estimated its amplitude
throughout the structural and rotational evolution of low-mass stars
(from K to F-type). We find that the star's magnetic field has little
influence on the excitation of tidal waves in nearly circular and
coplanar Hot-Jupiter systems, but that it has a major impact on the
way waves are dissipated.
Title: Stellar magnetism: bridging dynamos and winds
Authors: Brun, Allan Sacha; Strugarek, Antoine
Bibcode: 2020mdps.conf..171B
Altcode:
In this lecture on stellar magnetism we discuss how the dynamo generated
magnetic field shapes the extended hot atmosphere and how the feedback
loop between rotation, convection, turbulence, dynamo action and
braking by stellar wind influences the secular evolution and the
rotational history of solarlike stars. We discuss each key physical
mechanism such as dynamo action and wind dynamics and discuss angular
momentum transport inside and outside the star. In order to illustrate
these complex processes and their nonlinear interaction we use both
pedagogical exercises and discuss more advanced agnetohydrodynamics
numerical simulations. We propose seven problems and their solution to
help getting a good first understanding of stellar magnetohydrodynamics.
Title: A solar cycle 25 prediction based on 4D-var data assimilation
approach
Authors: Brun, Allan Sacha; Pui Hung, Ching; Fournier, Alexandre;
Jouve, Laurène; Talagrand, Olivier; Strugarek, Antoine; Hazra,
Soumitra
Bibcode: 2020IAUS..354..138B
Altcode: 2020IAUS..354..138S
Based on our modern 4D-var data assimilation pipeline Solar Predict
we present in this short proceeding paper our prediction for the next
solar cycle 25. As requested by the Solar Cycle 25 panel call issued
on January 2019 by NOAA/SWPC and NASA, we predict the timing of next
minimum and maximum as well as their amplitude. Our results are the
following: the minimum should have occured within the first semester of
year 2019. The maximum should occur in year 2024.4 ± 6 months, with
a value of the sunspot number equal to 92±10. This is in agreement
with the NOAA/NASA consensus published in April 2019. Note that our
prediction errors are based on 1-σ measure and do not consider all
the systematics, so they are likely underestimated. We will update our
prediction and error analysis regularly as more data becomes available
and we improve our prediction pipeline.
Title: Exoplanet host-star properties: the active environment of
exoplanets
Authors: Pye, John P.; Barrado, David; García, Rafael A.;
Güdel, Manuel; Nichols, Jonathan; Joyce, Simon; Huélamo, Nuria;
Morales-Calderón, María; López, Mauro; Solano, Enrique; Lagage,
Pierre-Olivier; Johnstone, Colin P.; Brun, Allan Sacha; Strugarek,
Antoine; Ahuir, Jérémy; Exoplanets-A Consortium
Bibcode: 2020IAUS..345..202P
Altcode: 2019arXiv190300234P
The primary objectives of the ExoplANETS-A project are to: establish new
knowledge on exoplanet atmospheres; establish new insight on influence
of the host star on the planet atmosphere; disseminate knowledge,
using online, web-based platforms. The project, funded under the EU's
Horizon-2020 programme, started in January 2018 and has a duration
∼3 years. We present an overview of the project, the activities
concerning the host stars and some early results on the host stars.
Title: Magnetic Hide & Seek in the Kepler-78 System: wind
modelling and star-planet magnetic interactions
Authors: Strugarek, A.; Ahuir, J.; Brun, A. S.; Donati, J. F.; Moutou,
C.; Réville, V.
Bibcode: 2019sf2a.conf..377S
Altcode:
Observational evidences for star-planet magnetic interactions (SPMIs)
in compact exosystems have been looked for in the past decades. Their
theoretical description has significantly progressed in the past
years. Nevertheless, their complete description requires a detailed
knowledge of the host star, and in particular its coronal magnetic and
plasma characteristics. We explore here the robustness of SPMIs models
with respect to the basic coronal properties commonly assumed for cool
stars, in the particular context of the Kepler-78 system. We show that
the amplitude of SPMIs is constrained only within one to two orders
of magnitude as of today. However, the temporal signature of SPMIs
can be robustly predicted from models, paving the road toward their
future detection in compact exosystems through dedicated observational
strategies.
Title: Does magnetic field impact tidal dynamics inside the convective
zone of low-mass stars along their evolution?
Authors: Astoul, A.; Mathis, S.; Baruteau, C.; Gallet, F.; Strugarek,
A.; Augustson, K. C.; Brun, A. S.; Bolmont, E.
Bibcode: 2019A&A...631A.111A
Altcode: 2019arXiv190910490A
Context. The dissipation of the kinetic energy of wave-like tidal flows
within the convective envelope of low-mass stars is one of the key
physical mechanisms that shapes the orbital and rotational dynamics
of short-period exoplanetary systems. Although low-mass stars are
magnetically active objects, the question of how the star's magnetic
field impacts large-scale tidal flows and the excitation, propagation
and dissipation of tidal waves still remains open.
Aims: Our
goal is to investigate the impact of stellar magnetism on the forcing
of tidal waves, and their propagation and dissipation in the convective
envelope of low-mass stars as they evolve.
Methods: We have
estimated the amplitude of the magnetic contribution to the forcing
and dissipation of tidally induced magneto-inertial waves throughout
the structural and rotational evolution of low-mass stars (from M to
F-type). For this purpose, we have used detailed grids of rotating
stellar models computed with the stellar evolution code STAREVOL. The
amplitude of dynamo-generated magnetic fields is estimated via physical
scaling laws at the base and the top of the convective envelope.
Results: We find that the large-scale magnetic field of the star
has little influence on the excitation of tidal waves in the case of
nearly-circular orbits and coplanar hot-Jupiter planetary systems, but
that it has a major impact on the way waves are dissipated. Our results
therefore indicate that a full magneto-hydrodynamical treatment of the
propagation and dissipation of tidal waves is needed to properly assess
the impact of star-planet tidal interactions throughout the evolutionary
history of low-mass stars hosting short-period massive planets.
Title: Magnetic games in compact exo-planetary systems
Authors: Strugarek, Antoine; Brun, Allan Sacha; François Donati,
Jean; Moutou, Claire; Réville, Victor
Bibcode: 2019EPSC...13..133S
Altcode:
I will present our current understanding of magnetic star-planet
interactions in compact exo-systems. In particular, I will give estimate
of the energetics of such interaction. I will also show that we can
predict the phase and amplitude of such interactions for well-observed
compact exo-systems, which opens new avenues to observationnally
constrain the magnetospheric characteristics of theses planets.
Title: Chasing Star-Planet Magnetic Interactions: The Case of
Kepler-78
Authors: Strugarek, A.; Brun, A. S.; Donati, J. -F.; Moutou, C.;
Réville, V.
Bibcode: 2019ApJ...881..136S
Altcode: 2019arXiv190701020S
Observational evidence of star-planet magnetic interactions (SPMIs)
in compact exosystems have been looked for in the past decades. Indeed,
planets in close-in orbit can be magnetically connected to their host
star and can channel Alfvén waves carrying large amounts of energy
toward the central star. The strength and temporal modulation of SPMIs
are primarily set by the magnetic topology of the host star and the
orbital characteristics of the planet. As a result, SPMI signals can be
modulated over the rotational period of the star, the orbital period of
the planet, or a complex combination of the two. The detection of SPMIs
thus has to rely on multiple-epoch and multiple-wavelength observational
campaigns. We present a new method to characterize SPMIs and apply it to
Kepler-78, a late G star with a super-Earth on an 8.5 hr orbit. We model
the corona of Kepler-78 using the large-scale magnetic topology of the
star observed with Zeeman-Doppler imaging. We show that the closeness of
Kepler-78b allows the interaction with channel energy flux densities up
to a few kW m-2 toward the central star. We show that this
flux is large enough to be detectable in classical activity tracers
such as Hα. It is nonetheless too weak to explain the modulation
observed by Moutou et al. We furthermore demonstrate how to predict
the temporal modulation of SPMI signals in observed systems such as
Kepler-78. The methodology presented here thus paves the way toward
denser, more specific observational campaigns that would allow proper
identification of SPMIs in compact star-planet systems.
Title: Impact of Stellar Magnetism on Star-planet Tidal Interactions
Authors: Astoul, Aurélie; Mathis, Stéphane; Baruteau, Clément;
Gallet, Florian; Strugarek, Antoine; Augustson, Kyle; Brun, Allan
Sacha; Bolmont, Emeline
Bibcode: 2019ESS.....431908A
Altcode:
Over the last two decades, about 4000 exoplanets have been discovered
around low-mass stars. For the shortest period exoplanets, star-planet
tidal interactions are likely to have played a major role in the
ultimate orbital evolution and on the spin axis evolution of the
host stars. Although low-mass stars are magnetically active objects,
the question of how the star's magnetic field impacts the excitation,
propagation and dissipation of tidal waves remains open. In this
work, we have derived the magnetic contribution to the tidal force
and estimated its amplitude all along the structural and rotational
evolutions of low-mass stars (from M to F-type). For this purpose,
we have used detailed grids of rotating stellar models computed with
the stellar evolution code STAREVOL. The amplitude of dynamo-generated
magnetic fields is estimated via physical scaling laws at the base and
the top of the convective envelope. We find that the star's magnetic
field has little influence on the excitation of tidal waves in near
circular and coplanar Hot-Jupiter systems, but that it has a major
impact on the waves dissipation. Our results therefore indicate that a
full MHD treatment of the propagation and dissipation of tidal waves
is needed to assess the impact of star-planet tidal interactions for
all low-mass stars along their evolution.
Title: Detecting volcanically produced tori along orbits of exoplanets
using UV spectroscopy
Authors: Kislyakova, Kristina G.; Fossati, Luca; Shulyak, Denis;
Günther, Eike; Güdel, Manuel; Johnstone, Colin P.; Airapetian,
Vladimir; Boro Saikia, Sudeshna; Brun, Allan Sacha; Dobos, Vera;
France, Kevin; Gaidos, Eric; Khodachenko, Maxim L.; Lanza, Antonino
F.; Lammer, Helmut; Noack, Lena; Luger, Rodrigo; Strugarek, Antoine;
Vidotto, Aline; Youngblood, Allison
Bibcode: 2019arXiv190705088K
Altcode:
We suggest to use the Hubble Space Telescople (HST) follow-up
observations of the TESS targets for detecting possible plasma
tori along the orbits of exoplanets orbiting M dwarfs. The source
of the torus could be planetary volcanic activity due to tidal or
electromagnetic induction heating. Fast losses to space for planets
orbiting these active stars can lead to the lost material forming a
torus along the planetary orbit, similar to the Io plasma torus. We show
that such torus would be potentially detectable by the HST in the UV.
Title: Turbulence, magnetism, and transport inside stars
Authors: Brun, A. S.; Strugarek, A.
Bibcode: 2019EAS....82..311B
Altcode:
We present recent progress made in modelling stars and their
turbulent magnetized dynamics in 3-D. This work is inspired by many
years of discussion with Jean-Paul Zahn. I (ASB) first met him as a
professor of astrophysical fluid dynamics (AFD) at the Paris-Meudon
observatory's graduate school of astrophysics in 1994-1995. He made
me the honor of accepting to be my PhD's advisor (1995-1998). He
then supported me during my postdoc years in Boulder with his long
time friend Prof. Juri Toomre between January 1999 and December 2002
and through the difficult process of getting a tenure position, and
then since as a tenure researcher in Department of Astrophysics at
CEA Paris-Saclay. I have been fortunate and lucky to share so many
years discussing and doing scientific projects with Jean-Paul. As I
was getting more experienced and started supervising my own students,
he was always available, guiding us with his acute scientific vista
and encouraging them. Antoine Strugarek, who co-author this paper,
was like me fortunate to share Jean-Paul's knowledge. The three of
us published several papers together during Antoine's PhD (2009-2012)
addressing the dynamics of the solar tachocline and its interplay with
convection. We miss him greatly. In this paper, we discuss mainly two
topics that benefited from Jean-Paul's deep understanding of AFD: a)
the dynamics of the solar tachocline and angular momentum transport
in stellar interior and b) turbulent convection and dynamo action in
stellar convection zones.
Title: Spin evolution and saturation: new insights through 3D MHD
simulations of young solar analogs
Authors: Réville, V.; Brun, A. S.
Bibcode: 2019EAS....82..233R
Altcode:
We examine how 3D MHD simulations can deliver clues on the mechanisms
at the origin of angular momentum loss saturation of rapidly rotating
solar-like stars. Based on a study of six targets, whose magnetic field
has been observed by Zeeman Doppler Imaging (ZDI), we find that the
saturation could be explained by a extremely strong coverage of the
stellar surface of a large scale dipolar mode, in disagreement with
recent works.
Title: Rossby and Magnetic Prandtl Number Scaling of Stellar Dynamos
Authors: Augustson, K. C.; Brun, A. S.; Toomre, J.
Bibcode: 2019ApJ...876...83A
Altcode:
Rotational scaling relationships are examined for the degree of
equipartition between magnetic and kinetic energies in stellar
convection zones. These scaling relationships are approached from two
paradigms, with first a glance at scaling relationship built on an
energy-balance argument and second a look at a force-based scaling. The
latter implies a transition between a nearly constant inertial scaling
when in the asymptotic limit of minimal diffusion and magnetostrophy,
whereas the former implies a weaker scaling with convective Rossby
number. Both scaling relationships are then compared to a suite of 3D
convective dynamo simulations with a wide variety of domain geometries,
stratifications, and range of convective Rossby numbers.
Title: Evolution of star-planet systems under magnetic braking and
tidal interaction
Authors: Benbakoura, M.; Réville, V.; Brun, A. S.; Le Poncin-Lafitte,
C.; Mathis, S.
Bibcode: 2019A&A...621A.124B
Altcode: 2018arXiv181106354B
Context. With the discovery over the last two decades of a large
diversity of exoplanetary systems, it is now of prime importance to
characterize star-planet interactions and how such systems evolve.
Aims: We address this question by studying systems formed by
a solar-like star and a close-in planet. We focus on the stellar
wind spinning down the star along its main-sequence phase and tidal
interaction causing orbital evolution of the systems. Despite recent
significant advances in these fields, all current models use parametric
descriptions to study at least one of these effects. Our objective is
to introduce ab initio prescriptions of the tidal and braking torques
simultaneously, so as to improve our understanding of the underlying
physics.
Methods: We develop a one-dimensional (1D) numerical
model of coplanar circular star-planet systems taking into account
stellar structural changes, wind braking, and tidal interaction and
implement it in a code called ESPEM. We follow the secular evolution
of the stellar rotation and of the semi-major axis of the orbit,
assuming a bilayer internal structure for the former. After comparing
our predictions to recent observations and models, we perform tests
to emphasize the contribution of ab initio prescriptions. Finally,
we isolate four significant characteristics of star-planet systems:
stellar mass, initial stellar rotation period, planetary mass and
initial semi-major axis; and browse the parameter space to investigate
the influence of each of them on the fate of the system.
Results:
Our secular model of stellar wind braking accurately reproduces the
recent observations of stellar rotation in open clusters. Our results
show that a planet can affect the rotation of its host star and that
the resulting spin-up or spin-down depends on the orbital semi-major
axis and on the joint influence of magnetic and tidal effects. The ab
initio prescription for tidal dissipation that we used predicts fast
outward migration of massive planets orbiting fast-rotating young
stars. Finally, we provide the reader with a criterion based on the
characteristics of the system that allows us to assess whether or not
the planet will undergo orbital decay due to tidal interaction.
Title: Erratum: “The Mass-dependence of
Angular Momentum Evolution in Sun-like Stars” (2015, ApJL, 799,
L23)
Authors: Matt, Sean P.; Brun, A. Sacha; Baraffe, Isabelle; Bouvier,
Jérôme; Chabrier, Gilles
Bibcode: 2019ApJ...870L..27M
Altcode:
No abstract at ADS
Title: Does magnetic field modify tidal dynamics in the convective
envelope of Solar mass stars?
Authors: Astoul, A.; Mathis, S.; Baruteau, C.; Gallet, F.; Augustson,
K. C.; Bolmont, E.; Brun, A. S.; Strugarek, A.
Bibcode: 2018sf2a.conf..495A
Altcode: 2018arXiv181108649A
The energy dissipation of wave-like tidal flows in the convective
envelope of low-mass stars is one of the key physical mechanisms
that shape the orbital and rotational dynamics of short-period
planetary systems. Tidal flows, and the excitation, propagation,
and dissipation of tidally-induced inertial waves can be modified
by stellar magnetic fields (e.g., Wei 2016, 2018, Lin and Ogilvie
2018). It is thus important to assess for which stars, at which
location of their internal structure, and at which phase of their
evolution, one needs to take into account the effects of magnetic
fields on tidal waves. Using scaling laws that provide the amplitude
of dynamo-generated magnetic fields along the rotational evolution
of these stars (e.g., Christensen et al. 2009, Brun et al. 2015),
combined with detailed grids of stellar rotation models (e.g., Amard
et al. 2016), we examine the influence of a magnetic field on tidal
forcing and dissipation near the tachocline of solar-like stars. We
show that full consideration of magnetic fields is required to compute
tidal dissipation, but not necessarily for tidal forcing.
Title: A chemical survey of exoplanets with ARIEL
Authors: Tinetti, Giovanna; Drossart, Pierre; Eccleston, Paul; Hartogh,
Paul; Heske, Astrid; Leconte, Jérémy; Micela, Giusi; Ollivier,
Marc; Pilbratt, Göran; Puig, Ludovic; Turrini, Diego; Vandenbussche,
Bart; Wolkenberg, Paulina; Beaulieu, Jean-Philippe; Buchave, Lars A.;
Ferus, Martin; Griffin, Matt; Guedel, Manuel; Justtanont, Kay; Lagage,
Pierre-Olivier; Machado, Pedro; Malaguti, Giuseppe; Min, Michiel;
Nørgaard-Nielsen, Hans Ulrik; Rataj, Mirek; Ray, Tom; Ribas, Ignasi;
Swain, Mark; Szabo, Robert; Werner, Stephanie; Barstow, Joanna;
Burleigh, Matt; Cho, James; du Foresto, Vincent Coudé; Coustenis,
Athena; Decin, Leen; Encrenaz, Therese; Galand, Marina; Gillon,
Michael; Helled, Ravit; Morales, Juan Carlos; Muñoz, Antonio García;
Moneti, Andrea; Pagano, Isabella; Pascale, Enzo; Piccioni, Giuseppe;
Pinfield, David; Sarkar, Subhajit; Selsis, Franck; Tennyson, Jonathan;
Triaud, Amaury; Venot, Olivia; Waldmann, Ingo; Waltham, David; Wright,
Gillian; Amiaux, Jerome; Auguères, Jean-Louis; Berthé, Michel;
Bezawada, Naidu; Bishop, Georgia; Bowles, Neil; Coffey, Deirdre;
Colomé, Josep; Crook, Martin; Crouzet, Pierre-Elie; Da Peppo, Vania;
Sanz, Isabel Escudero; Focardi, Mauro; Frericks, Martin; Hunt, Tom;
Kohley, Ralf; Middleton, Kevin; Morgante, Gianluca; Ottensamer,
Roland; Pace, Emanuele; Pearson, Chris; Stamper, Richard; Symonds,
Kate; Rengel, Miriam; Renotte, Etienne; Ade, Peter; Affer, Laura;
Alard, Christophe; Allard, Nicole; Altieri, Francesca; André, Yves;
Arena, Claudio; Argyriou, Ioannis; Aylward, Alan; Baccani, Cristian;
Bakos, Gaspar; Banaszkiewicz, Marek; Barlow, Mike; Batista, Virginie;
Bellucci, Giancarlo; Benatti, Serena; Bernardi, Pernelle; Bézard,
Bruno; Blecka, Maria; Bolmont, Emeline; Bonfond, Bertrand; Bonito,
Rosaria; Bonomo, Aldo S.; Brucato, John Robert; Brun, Allan Sacha;
Bryson, Ian; Bujwan, Waldemar; Casewell, Sarah; Charnay, Bejamin;
Pestellini, Cesare Cecchi; Chen, Guo; Ciaravella, Angela; Claudi,
Riccardo; Clédassou, Rodolphe; Damasso, Mario; Damiano, Mario;
Danielski, Camilla; Deroo, Pieter; Di Giorgio, Anna Maria; Dominik,
Carsten; Doublier, Vanessa; Doyle, Simon; Doyon, René; Drummond,
Benjamin; Duong, Bastien; Eales, Stephen; Edwards, Billy; Farina,
Maria; Flaccomio, Ettore; Fletcher, Leigh; Forget, François; Fossey,
Steve; Fränz, Markus; Fujii, Yuka; García-Piquer, Álvaro; Gear,
Walter; Geoffray, Hervé; Gérard, Jean Claude; Gesa, Lluis; Gomez,
H.; Graczyk, Rafał; Griffith, Caitlin; Grodent, Denis; Guarcello,
Mario Giuseppe; Gustin, Jacques; Hamano, Keiko; Hargrave, Peter;
Hello, Yann; Heng, Kevin; Herrero, Enrique; Hornstrup, Allan; Hubert,
Benoit; Ida, Shigeru; Ikoma, Masahiro; Iro, Nicolas; Irwin, Patrick;
Jarchow, Christopher; Jaubert, Jean; Jones, Hugh; Julien, Queyrel;
Kameda, Shingo; Kerschbaum, Franz; Kervella, Pierre; Koskinen, Tommi;
Krijger, Matthijs; Krupp, Norbert; Lafarga, Marina; Landini, Federico;
Lellouch, Emanuel; Leto, Giuseppe; Luntzer, A.; Rank-Lüftinger,
Theresa; Maggio, Antonio; Maldonado, Jesus; Maillard, Jean-Pierre;
Mall, Urs; Marquette, Jean-Baptiste; Mathis, Stephane; Maxted, Pierre;
Matsuo, Taro; Medvedev, Alexander; Miguel, Yamila; Minier, Vincent;
Morello, Giuseppe; Mura, Alessandro; Narita, Norio; Nascimbeni,
Valerio; Nguyen Tong, N.; Noce, Vladimiro; Oliva, Fabrizio; Palle,
Enric; Palmer, Paul; Pancrazzi, Maurizio; Papageorgiou, Andreas;
Parmentier, Vivien; Perger, Manuel; Petralia, Antonino; Pezzuto,
Stefano; Pierrehumbert, Ray; Pillitteri, Ignazio; Piotto, Giampaolo;
Pisano, Giampaolo; Prisinzano, Loredana; Radioti, Aikaterini; Réess,
Jean-Michel; Rezac, Ladislav; Rocchetto, Marco; Rosich, Albert;
Sanna, Nicoletta; Santerne, Alexandre; Savini, Giorgio; Scandariato,
Gaetano; Sicardy, Bruno; Sierra, Carles; Sindoni, Giuseppe; Skup,
Konrad; Snellen, Ignas; Sobiecki, Mateusz; Soret, Lauriane; Sozzetti,
Alessandro; Stiepen, A.; Strugarek, Antoine; Taylor, Jake; Taylor,
William; Terenzi, Luca; Tessenyi, Marcell; Tsiaras, Angelos; Tucker,
C.; Valencia, Diana; Vasisht, Gautam; Vazan, Allona; Vilardell,
Francesc; Vinatier, Sabrine; Viti, Serena; Waters, Rens; Wawer, Piotr;
Wawrzaszek, Anna; Whitworth, Anthony; Yung, Yuk L.; Yurchenko, Sergey
N.; Osorio, María Rosa Zapatero; Zellem, Robert; Zingales, Tiziano;
Zwart, Frans
Bibcode: 2018ExA....46..135T
Altcode: 2018ExA...tmp...53T
Thousands of exoplanets have now been discovered with a huge range
of masses, sizes and orbits: from rocky Earth-like planets to
large gas giants grazing the surface of their host star. However,
the essential nature of these exoplanets remains largely mysterious:
there is no known, discernible pattern linking the presence, size, or
orbital parameters of a planet to the nature of its parent star. We
have little idea whether the chemistry of a planet is linked to its
formation environment, or whether the type of host star drives the
physics and chemistry of the planet's birth, and evolution. ARIEL was
conceived to observe a large number ( 1000) of transiting planets
for statistical understanding, including gas giants, Neptunes,
super-Earths and Earth-size planets around a range of host star types
using transit spectroscopy in the 1.25-7.8 μm spectral range and
multiple narrow-band photometry in the optical. ARIEL will focus on
warm and hot planets to take advantage of their well-mixed atmospheres
which should show minimal condensation and sequestration of high-Z
materials compared to their colder Solar System siblings. Said warm
and hot atmospheres are expected to be more representative of the
planetary bulk composition. Observations of these warm/hot exoplanets,
and in particular of their elemental composition (especially C, O, N,
S, Si), will allow the understanding of the early stages of planetary
and atmospheric formation during the nebular phase and the following
few million years. ARIEL will thus provide a representative picture
of the chemical nature of the exoplanets and relate this directly to
the type and chemical environment of the host star. ARIEL is designed
as a dedicated survey mission for combined-light spectroscopy, capable
of observing a large and well-defined planet sample within its 4-year
mission lifetime. Transit, eclipse and phase-curve spectroscopy methods,
whereby the signal from the star and planet are differentiated using
knowledge of the planetary ephemerides, allow us to measure atmospheric
signals from the planet at levels of 10-100 part per million (ppm)
relative to the star and, given the bright nature of targets, also
allows more sophisticated techniques, such as eclipse mapping, to give
a deeper insight into the nature of the atmosphere. These types of
observations require a stable payload and satellite platform with broad,
instantaneous wavelength coverage to detect many molecular species,
probe the thermal structure, identify clouds and monitor the stellar
activity. The wavelength range proposed covers all the expected
major atmospheric gases from e.g. H2O, CO2,
CH4 NH3, HCN, H2S through to the
more exotic metallic compounds, such as TiO, VO, and condensed
species. Simulations of ARIEL performance in conducting exoplanet
surveys have been performed - using conservative estimates of mission
performance and a full model of all significant noise sources in the
measurement - using a list of potential ARIEL targets that incorporates
the latest available exoplanet statistics. The conclusion at the end
of the Phase A study, is that ARIEL - in line with the stated mission
objectives - will be able to observe about 1000 exoplanets depending
on the details of the adopted survey strategy, thus confirming the
feasibility of the main science objectives.
Title: Simulations of solar wind variations during an 11-year cycle
and the influence of north-south asymmetry
Authors: Perri, B.; Brun, A. S.; Réville, V.; Strugarek, A.
Bibcode: 2018JPlPh..84e7601P
Altcode: 2018arXiv180903205P
We want to study the connections between the magnetic field generated
inside the Sun and the solar wind impacting Earth, especially the
influence of north-south asymmetry on the magnetic and velocity
fields. We study a solar-like 11-year cycle in a quasi-static way:
an asymmetric dynamo field is generated through a 2.5-dimensional
(2.5-D) flux-transport model with the Babcock-Leighton mechanism,
and then is used as bottom boundary condition for compressible 2.5-D
simulations of the solar wind. We recover solar values for the mass
loss rate, the spin-down time scale and the Alfvén radius, and are
able to reproduce the observed delay in latitudinal variations of the
wind and the general wind structure observed for the Sun. We show that
the phase lag between the energy of the dipole component and the total
surface magnetic energy has a strong influence on the amplitude of
the variations of global quantities. We show in particular that the
magnetic torque variations can be linked to topological variations
during a magnetic cycle, while variations in the mass loss rate appear
to be driven by variations of the magnetic energy.
Title: Effect of the exoplanet magnetic field topology on its
magnetospheric radio emission
Authors: Varela, J.; Réville, V.; Brun, A. S.; Zarka, P.; Pantellini,
F.
Bibcode: 2018A&A...616A.182V
Altcode: 2018arXiv180704417V
Context. The magnetized wind from stars that impact exoplanets should
lead to radio emissions. According to the scaling laws derived in
the solar system, the radio emission should depend on the stellar
wind, interplanetary magnetic field, and topology of the exoplanet
magnetosphere.
Aims: The aim of this study is to calculate
the dissipated power and subsequent radio emission from exoplanet
magnetospheres with different topologies perturbed by the interplanetary
magnetic field and stellar wind, to refine the predictions from
scaling laws, and to prepare the interpretation of future radio
detections.
Methods: We use the magnetohydrodynamic (MHD) code
PLUTO in spherical coordinates to analyze the total radio emission
level resulting from the dissipation of the kinetic and magnetic
(Poynting flux) energies inside the exoplanet's magnetospheres. We
apply a formalism to infer the detailed contribution in the exoplanet
radio emission on the exoplanet's day side and magnetotail. The model
is based on Mercury-like conditions, although the study results are
extrapolated to exoplanets with stronger magnetic fields, providing
the lower bound of the radio emission.
Results: The predicted
dissipated powers and resulting radio emissions depend critically on
the exoplanet magnetosphere topology and interplanetary magnetic field
(IMF) orientation. The radio emission on the exoplanet's night and day
sides should thus contain information on the exoplanet magnetic field
topology. In addition, if the topology of an exoplanet magnetosphere is
known, the radio emission measurements can be used as a proxy of the
instantaneous dynamic pressure of the stellar wind, IMF orientation,
and intensity.
Title: Influence of Star-Planet Magnetic Torques on Orbital Secular
Evolution
Authors: Ahuir, Jérémy; Strugarek, Antoine; Benbakoura, Mansour;
Brun, Allan-Sacha; Mathis, Stéphane; Bolmont, Emeline; Le
Poncin-Lafitte, Christophe; Réville, Victor
Bibcode: 2018EPSC...12..641A
Altcode:
We develop a 1D numerical model of star-planet systems taking into
account stellar evolution, assuming a simplified two zones stellar
internal structure, wind braking, tidal and magnetic interactions
implemented in the ESPEM code (French acronym for Evolution of Planetary
Systems and Magnetism). We follow the secular evolution of the stellar
rotation and of the semi-major axis of the orbit. After comparing
our predictions to recent observations and models, we perform tests
to emphasize the contribution of ab-initio prescriptions. Finally,
we isolate the stellar mass, the initial stellar rotation period,
the planetary mass and the initial semi-major axis, which characterize
star-planet systems and browse the parameter space to investigate the
influence of each of them on the fate of the system. We find that
depending on the characteristics of the system, tidal or magnetic
effects can dominate. For very close-in planets, we find that both
torques can make a planet migrate on a timescale as small as 10-100
thousands of years. We also provide a criterion on the system's
characteristics, determining whether or not the planet will undergo
orbital decay due to tidal interaction and star-planet magnetic
interaction. Both effects thus have to be taken into account when
predicting the evolution and the architecture of compact systems.
Title: Impact of general differential rotation on gravity waves in
rapidly rotating stars
Authors: Prat, Vincent; Mathis, Stéphane; Augustson, Kyle; Lignières,
François; Ballot, Jérôme; Alvan, Lucie; Brun, Allan Sacha
Bibcode: 2018phos.confE..42P
Altcode: 2018arXiv181203101P
Differential rotation plays a key role in stellar evolution by
triggering hydrodynamical instabilities and large-scale motions that
induce transport of chemicals and angular momentum and by modifying the
propagation and the frequency spectrum of gravito-inertial waves. It
is thus crucial to investigate its effect on the propagation of
gravity waves to build reliable seismic diagnostic tools, especially
for fast rotating stars, where perturbative treatments of rotation
fail. Generalising a previous work done in the case of uniform
rotation, we derived a local dispersion relation for gravity waves in
a differentially rotating star, taking the full effect of rotation
(both Coriolis and centrifugal accelerations) into account. Then we
modelled the propagation of axisymmetric waves as the propagation of
rays. This allowed us to efficiently probe the properties of the waves
in various regimes of differential rotation.
Title: Sandpile Models and Solar Flares: Eigenfunction Decomposition
for Data Assimilation
Authors: Strugarek, Antoine; Brun, Allan S.; Charbonneau, Paul;
Vilmer, Nicole
Bibcode: 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.
Title: Towards Estimating the Solar Meridional Flow and Predicting the
11-yr Cycle Using Advanced Variational Data Assimilation Techniques
Authors: Hung, Ching Pui; Brun, Allan Sacha; Fournier, Alexandre;
Jouve, Laurène; Talagrand, Olivier; Zakari, Mustapha
Bibcode: 2018IAUS..335..183H
Altcode:
We present in this work the development of a solar data assimilation
method based on an axisymmetric mean field dynamo model and magnetic
surface data. Our mid-term goal is to predict the solar quasi cyclic
activity. We focus on the ability of our variational data assimilation
algorithm to constrain the deep meridional circulation of the Sun based
on solar magnetic observations. Within a given assimilation window, the
assimilation procedure minimizes the differences between data and the
forecast from the model, by finding an optimal meridional circulation
in the convection zone, and an optimal initial magnetic field, via a
quasi-Newton algorithm. We demonstrate the capability of the technique
to estimate the meridional flow by a closed-loop experiment involving
40 years of synthetic, solar-like data. We show that the method is
robust in estimating a (stochastic) time-varying flow fluctuating 30%
about the average, and that the horizon of predictability of the method
is ~ 1 cycle length.
Title: On the Sensitivity of Magnetic Cycles in Global Simulations
of Solar-like Stars
Authors: Strugarek, A.; Beaudoin, P.; Charbonneau, P.; Brun, A. S.
Bibcode: 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.
Title: Asymptotic theory of gravity modes in rotating
stars. II. Impact of general differential rotation
Authors: Prat, V.; Mathis, S.; Augustson, K.; Lignières, F.; Ballot,
J.; Alvan, L.; Brun, A. S.
Bibcode: 2018A&A...615A.106P
Altcode: 2018arXiv180304229P
Context. Differential rotation has a strong influence on stellar
internal dynamics and evolution, notably by triggering hydrodynamical
instabilities, by interacting with the magnetic field, and more
generally by inducing transport of angular momentum and chemical
elements. Moreover, it modifies the way waves propagate in
stellar interiors and thus the frequency spectrum of these waves,
the regions they probe, and the transport they generate.
Aims: We investigate the impact of a general differential rotation
(both in radius and latitude) on the propagation of axisymmetric
gravito-inertial waves.
Methods: We use a small-wavelength
approximation to obtain a local dispersion relation for these waves. We
then describe the propagation of waves thanks to a ray model that
follows a Hamiltonian formalism. Finally, we numerically probe the
properties of these gravito-inertial rays for different regimes
of radial and latitudinal differential rotation.
Results:
We derive a local dispersion relation that includes the effect of a
general differential rotation. Subsequently, considering a polytropic
stellar model, we observe that differential rotation allows for a large
variety of resonant cavities that can be probed by gravito-inertial
waves. We identify that for some regimes of frequency and differential
rotation, the properties of gravito-inertial rays are similar to those
found in the uniformly rotating case. Furthermore, we also find new
regimes specific to differential rotation, where the dynamics of rays
is chaotic.
Conclusions: As a consequence, we expect modes to
follow the same trend. Some parts of oscillation spectra corresponding
to regimes similar to those of the uniformly rotating case would exhibit
regular patterns, while parts corresponding to the new regimes would be
mostly constituted of chaotic modes with a spectrum rather characterised
by a generic statistical distribution.
Title: Interactions of Twisted Ω-loops in a Model Solar Convection
Zone
Authors: Jouve, L.; Brun, A. S.; Aulanier, G.
Bibcode: 2018ApJ...857...83J
Altcode: 2018arXiv180304709J
This study aims at investigating the ability of strong interactions
between magnetic field concentrations during their rise through
the convection zone to produce complex active regions at the solar
surface. To do so, we perform numerical simulations of buoyant magnetic
structures evolving and interacting in a model solar convection
zone. We first produce a 3D model of rotating convection and then
introduce idealized magnetic structures close to the bottom of the
computational domain. These structures possess a certain degree of
field line twist and they are made buoyant on a particular extension
in longitude. The resulting twisted Ω-loops will thus evolve inside a
spherical convective shell possessing large-scale mean flows. We present
results on the interaction between two such loops with various initial
parameters (mainly buoyancy and twist) and on the complexity of the
emerging magnetic field. In agreement with analytical predictions, we
find that if the loops are introduced with opposite handedness and same
axial field direction or the same handedness but opposite axial field,
they bounce against each other. The emerging region is then constituted
of two separated bipolar structures. On the contrary, if the loops are
introduced with the same direction of axial and peripheral magnetic
fields and are sufficiently close, they merge while rising. This more
interesting case produces complex magnetic structures with a high
degree of non-neutralized currents, especially when the convective
motions act significantly on the magnetic field. This indicates that
those interactions could be good candidates to produce eruptive events
like flares or CMEs.
Title: The Influence of Metallicity on Stellar Differential Rotation
and Magnetic Activity
Authors: Karoff, Christoffer; Metcalfe, Travis S.; Santos, Ângela
R. G.; Montet, Benjamin T.; Isaacson, Howard; Witzke, Veronika;
Shapiro, Alexander I.; Mathur, Savita; Davies, Guy R.; Lund, Mikkel N.;
Garcia, Rafael A.; Brun, Allan S.; Salabert, David; Avelino, Pedro P.;
van Saders, Jennifer; Egeland, Ricky; Cunha, Margarida S.; Campante,
Tiago L.; Chaplin, William J.; Krivova, Natalie; Solanki, Sami K.;
Stritzinger, Maximilian; Knudsen, Mads F.
Bibcode: 2018ApJ...852...46K
Altcode: 2017arXiv171107716K
Observations of Sun-like stars over the past half-century have improved
our understanding of how magnetic dynamos, like that responsible for the
11 yr solar cycle, change with rotation, mass, and age. Here we show
for the first time how metallicity can affect a stellar dynamo. Using
the most complete set of observations of a stellar cycle ever obtained
for a Sun-like star, we show how the solar analog HD 173701 exhibits
solar-like differential rotation and a 7.4 yr activity cycle. While
the duration of the cycle is comparable to that generated by the solar
dynamo, the amplitude of the brightness variability is substantially
stronger. The only significant difference between HD 173701 and the
Sun is its metallicity, which is twice the solar value. Therefore,
this provides a unique opportunity to study the effect of the
higher metallicity on the dynamo acting in this star and to obtain a
comprehensive understanding of the physical mechanisms responsible
for the observed photometric variability. The observations can be
explained by the higher metallicity of the star, which is predicted to
foster a deeper outer convection zone and a higher facular contrast,
resulting in stronger variability.
Title: Variational Estimation of the Large-scale Time-dependent
Meridional Circulation in the Sun: Proofs of Concept with a Solar
Mean Field Dynamo Model
Authors: Hung, Ching Pui; Brun, Allan Sacha; Fournier, Alexandre;
Jouve, Laurène; Talagrand, Olivier; Zakari, Mustapha
Bibcode: 2017ApJ...849..160H
Altcode: 2017arXiv171002114H
We present in this work the development of a solar data assimilation
method based on an axisymmetric mean field dynamo model and magnetic
surface data. Our midterm goal is to predict quasi-cyclic solar
activity. Here we focus on the ability of our algorithm to constrain
the deep meridional circulation of the Sun based on solar magnetic
observations. To that end, we develop a variational data assimilation
technique. Within a given assimilation window, the assimilation
procedure minimizes the differences between the data and the forecast
from the model by finding an optimal meridional circulation in the
convection zone and an optimal initial magnetic field via a quasi-Newton
algorithm. We demonstrate the capability of the technique to estimate
the meridional flow through a closed-loop experiment involving 40
years of synthetic, solar-like data. By assimilating the synthetic
magnetic proxies, we are able to reconstruct a (stochastic) time-varying
meridional circulation that is also slightly equatorially asymmetric. We
show that the method is robust in estimating a flow whose level of
fluctuation can reach 30% about the average, and that the horizon of
predictive capability of the method is of the order of one cycle length.
Title: Global Solar Magnetic Field Organization in the Outer Corona:
Influence on the Solar Wind Speed and Mass Flux Over the Cycle
Authors: Réville, Victor; Brun, Allan Sacha
Bibcode: 2017ApJ...850...45R
Altcode: 2017arXiv171002908R
The dynamics of the solar wind depends intrinsically on the structure of
the global solar magnetic field, which undergoes fundamental changes
over the 11-year solar cycle. For instance, the wind terminal velocity
is thought to be anti-correlated with the expansion factor, a measure of
how the magnetic field varies with height in the solar corona, usually
computed at a fixed height (≈ 2.5 {R}⊙ , the source
surface radius that approximates the distance at which all magnetic
field lines become open). However, the magnetic field expansion affects
the solar wind in a more detailed way, its influence on the solar wind
properties remaining significant well beyond the source surface. We
demonstrate this using 3D global magnetohydrodynamic (MHD) simulations
of the solar corona, constrained by surface magnetograms over half a
solar cycle (1989-2001). A self-consistent expansion beyond the solar
wind critical point (even up to 10 {R}⊙ ) makes our model
comply with observed characteristics of the solar wind, namely, that the
radial magnetic field intensity becomes latitude independent at some
distance from the Sun, and that the mass flux is mostly independent
of the terminal wind speed. We also show that near activity minimum,
the expansion in the higher corona has more influence on the wind
speed than the expansion below 2.5 {R}⊙ .
Title: The Puzzling Dynamos of Stars: Recent Progress With Global
Numerical Simulations
Authors: Strugarek, Antoine; Beaudoin, Patrice; Charbonneau, Paul;
Brun, Allan S.
Bibcode: 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: The Fate of Close-in Planets: Tidal or Magnetic Migration?
Authors: Strugarek, A.; Bolmont, E.; Mathis, S.; Brun, A. S.; Réville,
V.; Gallet, F.; Charbonnel, C.
Bibcode: 2017ApJ...847L..16S
Altcode: 2017arXiv170905784S
Planets in close-in orbits interact magnetically and tidally with
their host stars. These interactions lead to a net torque that makes
close-in planets migrate inward or outward depending on their orbital
distance. We systematically compare the strength of magnetic and
tidal torques for typical observed star-planet systems (T-Tauri and
hot Jupiter, M-dwarf and Earth-like planet, K star and hot Jupiter)
based on state-of-the-art scaling laws. We find that depending on
the characteristics of the system, tidal or magnetic effects can
dominate. For very close-in planets, we find that both torques can
make a planet migrate on a timescale as small as 10-100 thousands of
years. Both effects thus have to be taken into account when predicting
the evolution of compact systems.
Title: Dynamo action and magnetic activity during the pre-main
sequence: Influence of rotation and structural changes
Authors: Emeriau-Viard, Constance; Brun, Allan Sacha
Bibcode: 2017IAUS..328...77E
Altcode:
During the PMS, structure and rotation rate of stars evolve
significantly. We wish to assess the consequences of these drastic
changes on stellar dynamo, internal magnetic field topology and
activity level by mean of HPC simulations with the ASH code. To answer
this question, we develop 3D MHD simulations that represent specific
stages of stellar evolution along the PMS. We choose five different
models characterized by the radius of their radiative zone following
an evolutionary track, from 1 Myr to 50 Myr, computed by a 1D stellar
evolution code. We introduce a seed magnetic field in the youngest model
and then we spread it through all simulations. First of all, we study
the consequences that the increase of rotation rate and the change of
geometry of the convective zone have on the dynamo field that exists in
the convective envelop. The magnetic energy increases, the topology of
the magnetic field becomes more complex and the axisymmetric magnetic
field becomes less predominant as the star ages. The computation of
the fully convective MHD model shows that a strong dynamo develops with
a ratio of magnetic to kinetic energy reaching equipartition and even
super-equipartition states in the faster rotating cases. Magnetic fields
resulting from our MHD simulations possess a mixed poloidal-toroidal
topology with no obvious dominant component. We also study the
relaxation of the vestige dynamo magnetic field within the radiative
core and found that it satisfies stability criteria. Hence it does
not experience a global reconfiguration and instead slowly relaxes by
retaining its mixed poloidal-toroidal topology.
Title: Magnetism, dynamo action and the solar-stellar connection
Authors: Brun, Allan Sacha; Browning, Matthew K.
Bibcode: 2017LRSP...14....4B
Altcode:
The Sun and other stars are magnetic: magnetism pervades their interiors
and affects their evolution in a variety of ways. In the Sun, both
the fields themselves and their influence on other phenomena can be
uncovered in exquisite detail, but these observations sample only a
moment in a single star's life. By turning to observations of other
stars, and to theory and simulation, we may infer other aspects of
the magnetism—e.g., its dependence on stellar age, mass, or rotation
rate—that would be invisible from close study of the Sun alone. Here,
we review observations and theory of magnetism in the Sun and other
stars, with a partial focus on the "Solar-stellar connection": i.e.,
ways in which studies of other stars have influenced our understanding
of the Sun and vice versa. We briefly review techniques by which
magnetic fields can be measured (or their presence otherwise inferred)
in stars, and then highlight some key observational findings uncovered
by such measurements, focusing (in many cases) on those that offer
particularly direct constraints on theories of how the fields are built
and maintained. We turn then to a discussion of how the fields arise
in different objects: first, we summarize some essential elements of
convection and dynamo theory, including a very brief discussion of
mean-field theory and related concepts. Next we turn to simulations
of convection and magnetism in stellar interiors, highlighting both
some peculiarities of field generation in different types of stars and
some unifying physical processes that likely influence dynamo action
in general. We conclude with a brief summary of what we have learned,
and a sampling of issues that remain uncertain or unsolved.
Title: Origin and Evolution of Magnetic Field in PMS Stars: Influence
of Rotation and Structural Changes
Authors: Emeriau-Viard, Constance; Brun, Allan Sacha
Bibcode: 2017ApJ...846....8E
Altcode: 2017arXiv170904667E
During stellar evolution, especially in the pre-main-sequence phase,
stellar structure and rotation evolve significantly, causing major
changes in the dynamics and global flows of the star. We wish to assess
the consequences of these changes on stellar dynamo, internal magnetic
field topology, and activity level. To do so, we have performed a series
of 3D HD and MHD simulations with the ASH code. We choose five different
models characterized by the radius of their radiative zone following an
evolutionary track computed by a 1D stellar evolution code. These models
characterized stellar evolution from 1 to 50 Myr. By introducing a seed
magnetic field in the fully convective model and spreading its evolved
state through all four remaining cases, we observe systematic variations
in the dynamical properties and magnetic field amplitude and topology
of the models. The five MHD simulations develop a strong dynamo field
that can reach an equipartition state between the kinetic and magnetic
energies and even superequipartition levels in the faster-rotating
cases. We find that the magnetic field amplitude increases as it
evolves toward the zero-age main sequence. Moreover, the magnetic
field topology becomes more complex, with a decreasing axisymmetric
component and a nonaxisymmetric one becoming predominant. The dipolar
components decrease as the rotation rate and the size of the radiative
core increase. The magnetic fields possess a mixed poloidal-toroidal
topology with no obvious dominant component. Moreover, the relaxation
of the vestige dynamo magnetic field within the radiative core is
found to satisfy MHD stability criteria. Hence, it does not experience
a global reconfiguration but slowly relaxes by retaining its mixed
stable poloidal-toroidal topology.
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.
Bibcode: 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: Confinement of the solar tachocline by a cyclic dynamo
magnetic field
Authors: Barnabé, Roxane; Strugarek, Antoine; Charbonneau, Paul;
Brun, Allan Sacha; Zahn, Jean-Paul
Bibcode: 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.
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.
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.
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> 107-8 cm2/ s), even in the presence
of radiative spreading.
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: On Differential Rotation and Overshooting in Solar-like Stars
Authors: Brun, Allan Sacha; Strugarek, Antoine; Varela, Jacobo; Matt,
Sean P.; Augustson, Kyle C.; Emeriau, Constance; DoCao, Olivier Long;
Brown, Benjamin; Toomre, Juri
Bibcode: 2017ApJ...836..192B
Altcode: 2017arXiv170206598B
We seek to characterize how the change of global rotation rate
influences the overall dynamics and large-scale flows arising in the
convective envelopes of stars covering stellar spectral types from
early G to late K. We do so through numerical simulations with the
ASH code, where we consider stellar convective envelopes coupled to
a radiative interior with various global properties. As solar-like
stars spin down over the course of their main sequence evolution,
such a change must have a direct impact on their dynamics and rotation
state. We indeed find that three main states of rotation may exist for
a given star: anti-solar-like (fast poles, slow equator), solar-like
(fast equator, slow poles), or a cylindrical rotation profile. Under
increasingly strict rotational constraints, the last profile can
further evolve into a Jupiter-like profile, with alternating prograde
and retrograde zonal jets. We have further assessed how far the
convection and meridional flows overshoot into the radiative zone
and investigated the morphology of the established tachocline. Using
simple mixing length arguments, we are able to construct a scaling of
the fluid Rossby number {R}{of}=\tilde{ω }/2{{{Ω }}}*
∼ \tilde{v}/2{{{Ω }}}* {R}* , which we
calibrate based on our 3D ASH simulations. We can use this scaling to
map the behavior of differential rotation versus the global parameters
of stellar mass and rotation rate. Finally, we isolate a region on
this map (R of ≳ 1.5-2) where we posit that stars with
an anti-solar differential rotation may exist in order to encourage
observers to hunt for such targets.
Title: Helioseismology and Dynamics of the Solar Interior
Authors: Thompson, M. J.; Brun, A. S.; Culhane, J. L.; Gizon, L.;
Roth, M.; Sekii, T.
Bibcode: 2017hdsi.book.....T
Altcode:
No abstract at ADS
Title: Recent Advances on Solar Global Magnetism and Variability
Authors: Brun, A. S.; Browning, M. K.; Dikpati, M.; Hotta, H.;
Strugarek, A.
Bibcode: 2017hdsi.book..107B
Altcode:
No abstract at ADS
Title: Preface: Helioseismology and Dynamics of the Solar Interior
Authors: Gizon, Laurent; Thompson, Michael J.; Brun, A. Sacha; Culhane,
J. Len; Roth, Markus; Sekii, Takashi
Bibcode: 2017hdsi.book....1G
Altcode:
No abstract at ADS
Title: The Solar-Stellar Connection
Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.;
Pinsonneault, M.
Bibcode: 2017hdsi.book..309B
Altcode:
No abstract at ADS
Title: Simple Scaling Relationships for Stellar Dynamos
Authors: Augustson, Kyle; Mathis, Stéphane; Brun, Allan Sacha
Bibcode: 2017arXiv170102582A
Altcode:
This paper provides a brief overview of dynamo scaling relationships for
the degree of equipartition between magnetic and kinetic energies. Three
basic approaches are adopted to explore these scaling relationships,
with a first look at two simple models: one assuming magnetostrophy
and another that includes the effects of inertia. Next, a third scaling
relationship is derived that utilizes the assumptions that the dynamo
possesses two integral spatial scales and that it is driven by the
balance of buoyancy work and ohmic dissipation as studied in Davidson
2013. The results of which are then compared to a suite of convective
dynamo simulations that possess a fully convective domain with a weak
density stratification and that captured the behavior of the resulting
dynamo for a range of convective Rossby numbers (Augustson et al. 2016).
Title: Age Dependence of Wind Properties for Solar-type Stars:
A 3D Study
Authors: Réville, Victor; Folsom, Colin P.; Strugarek, Antoine;
Brun, Allan Sacha
Bibcode: 2016ApJ...832..145R
Altcode: 2016arXiv160906602R
Young and rapidly rotating stars are known for intense, dynamo-generated
magnetic fields. Spectropolarimetric observations of those stars
in precisely aged clusters are key input for gyrochronology and
magnetochronology. We use Zeeman Doppler imaging maps of several
young K-type stars of similar mass and radius but with various ages
and rotational periods to perform three-dimensional (3D) numerical
MHD simulations of their coronae and follow the evolution of their
magnetic properties with age. Those simulations yield the coronal
structure as well as the instant torque exerted by the magnetized,
rotating wind on the star. As stars get older, we find that the angular
momentum loss decreases with {{{Ω }}}\star 3,
which is the reason for the convergence on the Skumanich law. For the
youngest stars of our sample, the angular momentum loss shows signs of
saturation around 8{{{Ω }}}⊙ , which is a common value
used in spin evolution models for K-type stars. We compare these results
to semianalytical models and existing braking laws. We observe a complex
wind-speed distribution for the youngest stars with slow, intermediate,
and fast wind components, which are the result of interaction with
intense and nonaxisymmetric magnetic fields. Consequently, in our
simulations, the stellar wind structure in the equatorial plane of
young stars varies significantly from a solar configuration, delivering
insight about the past of the solar system interplanetary medium.
Title: Simple Scaling Relationships For Stellar Dynamos
Authors: Augustson, Kyle; Mathis, Stéphane; Brun, Allan Sacha
Bibcode: 2016csss.confE.152A
Altcode:
This paper provides a brief overview of dynamo scaling relationships for
the degree of equipartition between magnetic and kinetic energies. Three
basic approaches are adopted to explore these scaling relationships,
with a first look at two simple models: one assuming magnetostrophy
and another that includes the effects of inertia. Next, a third scaling
relationship is derived that utilizes the assumptions that the dynamo
possesses two integral spatial scales and that it is driven by the
balance of buoyancy work and ohmic dissipation as studied in Davidson
2013. The results of which are then compared to a suite of convective
dynamo simulations that possess a fully convective domain with a weak
density stratification and that captured the behavior of the resulting
dynamo for a range of convective Rossby numbers (Augustson et al. 2016).
Title: Space-weather assets developed by the French space-physics
community
Authors: Rouillard, A. P.; Pinto, R. F.; Brun, A. S.; Briand, C.;
Bourdarie, S.; Dudok De Wit, T.; Amari, T.; Blelly, P. -L.; Buchlin,
E.; Chambodut, A.; Claret, A.; Corbard, T.; Génot, V.; Guennou, C.;
Klein, K. L.; Koechlin, L.; Lavarra, M.; Lavraud, B.; Leblanc, F.;
Lemorton, J.; Lilensten, J.; Lopez-Ariste, A.; Marchaudon, A.; Masson,
S.; Pariat, E.; Reville, V.; Turc, L.; Vilmer, N.; Zucarello, F. P.
Bibcode: 2016sf2a.conf..297R
Altcode:
We present a short review of space-weather tools and services developed
and maintained by the French space-physics community. They include
unique data from ground-based observatories, advanced numerical
models, automated identification and tracking tools, a range of space
instrumentation and interconnected virtual observatories. The aim of
the article is to highlight some advances achieved in this field of
research at the national level over the last decade and how certain
assets could be combined to produce better space-weather tools
exploitable by space-weather centres and customers worldwide. This
review illustrates the wide range of expertise developed nationally
but is not a systematic review of all assets developed in France.
Title: Characterizing the feedback of magnetic field on the
differential rotation of solar-like stars
Authors: Varela, J.; Strugarek, A.; Brun, A. S.
Bibcode: 2016AdSpR..58.1507V
Altcode: 2016arXiv160802920V
The aim of this article is to study how the differential rotation of
solar-like stars is influenced by rotation rate and mass in presence of
magnetic fields generated by a convective dynamo. We use the ASH code
to model the convective dynamo of solar-like stars at various rotation
rates and masses, hence different effective Rossby numbers. We obtained
models with either prograde (solar-like) or retrograde (anti-solar-like)
differential rotation. The trends of differential rotation versus
stellar rotation rate obtained for simulations including the effect of
the magnetic field are weaker compared with hydro simulations (ΔΩ
∝(Ω /Ω⊙) 0.44 in the MHD case and ΔΩ
∝(Ω /Ω⊙) 0.89 in the hydro case), hence
showing a better agreement with the observations. Analysis of angular
momentum transport revealed that the simulations with retrograde
and prograde differential rotation have opposite distribution of
the viscous, turbulent Reynolds stresses and meridional circulation
contributions. The thermal wind balance is achieved in the prograde
cases. However, in retrograde cases Reynolds stresses are dominant for
high latitudes and near the top of the convective layer. Baroclinic
effects are stronger for faster rotating models.
Title: Radio emission in Mercury magnetosphere
Authors: Varela, J.; Reville, V.; Brun, A. S.; Pantellini, F.;
Zarka, P.
Bibcode: 2016A&A...595A..69V
Altcode: 2016arXiv160803571V
Context. Active stars possess magnetized wind that has a direct impact
on planets that can lead to radio emission. Mercury is a good test case
to study the effect of the solar wind and interplanetary magnetic field
(IMF) on radio emission driven in the planet magnetosphere. Such studies
could be used as proxies to characterize the magnetic field topology
and intensity of exoplanets.
Aims: The aim of this study is
to quantify the radio emission in the Hermean magnetosphere.
Methods: We use the magnetohydrodynamic code PLUTO in spherical
coordinates with an axisymmetric multipolar expansion for the Hermean
magnetic field, to analyze the effect of the IMF orientation and
intensity, as well as the hydrodynamic parameters of the solar wind
(velocity, density and temperature), on the net power dissipated on the
Hermean day and night side. We apply the formalism derived by Zarka et
al. (2001, Astrophys. Space Sci., 277, 293), Zarka (2007, Planet. Space
Sci., 55, 598) to infer the radio emission level from the net dissipated
power. We perform a set of simulations with different hydrodynamic
parameters of the solar wind, IMF orientations and intensities,
that allow us to calculate the dissipated power distribution and
infer the existence of radio emission hot spots on the planet day
side, and to calculate the integrated radio emission of the Hermean
magnetosphere.
Results: The obtained radio emission distribution
of dissipated power is determined by the IMF orientation (associated
with the reconnection regions in the magnetosphere), although the radio
emission strength is dependent on the IMF intensity and solar wind hydro
parameters. The calculated total radio emission level is in agreement
with the one estimated in Zarka et al. (2001, Astrophys. Space Sci.,
277, 293) , between 5 × 105 and 2 × 106 W.
Title: Planet migration and magnetic torques
Authors: Strugarek, A.; Brun, A. S.; Matt, S. P.; Reville, V.
Bibcode: 2016IAUFM..29A..14S
Altcode:
The possibility that magnetic torques may participate in close-in planet
migration has recently been postulated. We develop three dimensional
global models of magnetic star-planet interaction under the ideal
magnetohydrodynamic (MHD) approximation to explore the impact of
magnetic topology on the development of magnetic torques. We conduct
twin numerical experiments in which only the magnetic topology of
the interaction is altered. We find that magnetic torques can vary by
roughly an order of magnitude when varying the magnetic topology from
an aligned case to an anti-aligned case. Provided that the stellar
magnetic field is strong enough, we find that magnetic migration time
scales can be as fast as ~100 Myr. Hence, our model supports the idea
that magnetic torques may participate in planet migration for some
close-in star-planet systems.
Title: The Magnetic Furnace: Intense Core Dynamos in B Stars
Authors: Augustson, Kyle C.; Brun, Allan Sacha; Toomre, Juri
Bibcode: 2016ApJ...829...92A
Altcode: 2016arXiv160303659A
The dynamo action achieved in the convective cores of main-sequence
massive stars is explored here through three-dimensional (3D) global
simulations of convective core dynamos operating within a young
10 {M}⊙ B-type star, using the anelastic spherical
harmonic code. These simulations capture the inner 65% of this star by
radius, encompassing the convective nuclear-burning core (about 23%
by radius) and a portion of the overlying radiative envelope. Eight
rotation rates are considered, ranging from 0.05% to 16% of the surface
breakup velocity, thereby capturing both convection that barely senses
the effects of rotation and other situations in which the Coriolis
forces are prominent. The vigorous dynamo action realized within all
of these turbulent convective cores builds magnetic fields with peak
strengths exceeding a megagauss, with the overall magnetic energy (ME)
in the faster rotators reaching super-equipartition levels compared
to the convective kinetic energy (KE). The core convection typically
involves turbulent columnar velocity structures roughly aligned with
the rotation axis, with magnetic fields threading through these rolls
and possessing complex linkages throughout the core. The very strong
fields are able to coexist with the flows without quenching them
through Lorentz forces. The velocity and magnetic fields achieve such
a state by being nearly co-aligned, and with peak magnetic islands
being somewhat displaced from the fastest flows as the intricate
evolution proceeds. As the rotation rate is increased, the primary
force balance shifts from nonlinear advection balancing Lorentz forces
to a magnetostrophic balance between Coriolis and Lorentz forces.
Title: Superradially Expanding Flux Tubes Of Young Star'S Coronae
Authors: Réville, Victor; Folsom, Colin P.; Strugarek, Antoine;
Brun, Allan Sacha
Bibcode: 2016csss.confE..33R
Altcode:
We discuss the reasons for extremely high wind speed observed in 3D
MHD simulations of fast rotating young stars with intense magnetic
fields. We find that superradially expanding flux tubes in latitude
and in longitude are responsible for a significant acceleration in
our simulations. We extend here the analysis presented in Reville et
al. (2016) thanks to an analytical model introduced by Kopp & Holzer
(1976). We find that the expansion factor observed in the simulations
is coherent with the fastest speeds we observe. This phenomena needs
to be accounted for to model speed distribution of young stars' winds.
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.
Bibcode: 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: The Magnetic Furnace: Examining Fully Convective Dynamos And
The Influence Of Rotation
Authors: Augustson, Kyle; Mathis, S.; Brun, A. S.; Toomre, J.
Bibcode: 2016csss.confE..29A
Altcode:
The dynamo action likely present within fully convective regions
is explored through global-scale 3-D simulations. These simulations
provide a contextual analog for the convective dynamos that are likely
operating deep within the interiors of fully convective low mass
stars. A logarithmic range of rotation rates is considered, thereby
capturing both convection barely sensing the effects of rotation
to others in which the Coriolis forces are prominent. The vigorous
dynamo action realized within all of these turbulent convective cores
builds magnetic fields with peak strengths exceeding a megagauss,
with the overall magnetic energy (ME) in the faster rotators reaching
super-equipartition levels compared to the convective kinetic energy
(KE). Such strong fields are able to coexist with the flows without
quenching them through Lorentz forces. This state is achieved due to
the velocity and magnetic fields being nearly co-aligned, and with
peak magnetic islands being somewhat displaced from the fastest flows
as the intricate evolution of these MHD structures proceeds. As the
rotation rate is increased, the primary force balance shifts from
nonlinear advection balancing Lorentz forces to a magnetostrophic
balance between Coriolis and Lorentz forces.
Title: Flux-tube geometry and solar wind speed during an activity
cycle
Authors: Pinto, R. F.; Brun, A. S.; Rouillard, A. P.
Bibcode: 2016A&A...592A..65P
Altcode: 2016arXiv160309251P
Context. The solar wind speed at 1 AU shows cyclic variations in
latitude and in time which reflect the evolution of the global
background magnetic field during the activity cycle. It is commonly
accepted that the terminal (asymptotic) wind speed in a given magnetic
flux-tube is generally anti-correlated with its total expansion ratio,
which motivated the definition of widely used semi-empirical scaling
laws relating one to the other. In practice, such scaling laws require
ad hoc corrections (especially for the slow wind in the vicinities
of streamer/coronal hole boundaries) and empirical fits to in situ
spacecraft data. A predictive law based solely on physical principles
is still missing.
Aims: We test whether the flux-tube expansion
is the controlling factor of the wind speed at all phases of the cycle
and at all latitudes (close to and far from streamer boundaries) using
a very large sample of wind-carrying open magnetic flux-tubes. We
furthermore search for additional physical parameters based on
the geometry of the coronal magnetic field which have an influence
on the terminal wind flow speed.
Methods: We use numerical
magneto-hydrodynamical simulations of the corona and wind coupled to
a dynamo model to determine the properties of the coronal magnetic
field and of the wind velocity (as a function of time and latitude)
during a whole 11-yr activity cycle. These simulations provide a large
statistical ensemble of open flux-tubes which we analyse conjointly
in order to identify relations of dependence between the wind speed
and geometrical parameters of the flux-tubes which are valid globally
(for all latitudes and moments of the cycle).
Results: Our
study confirms that the terminal (asymptotic) speed of the solar wind
depends very strongly on the geometry of the open magnetic flux-tubes
through which it flows. The total flux-tube expansion is more clearly
anti-correlated with the wind speed for fast rather than for slow
wind flows, and effectively controls the locations of these flows
during solar minima. Overall, the actual asymptotic wind speeds
attained - especially those of the slow wind - are also strongly
dependent on field-line inclination and magnetic field amplitude at
the foot-points. We suggest ways of including these parameters in
future predictive scaling laws for the solar wind speed.
Title: Global Solar Convective Dynamo with Cycles, Equatorward
Propagation and Grand Minima
Authors: Toomre, Juri; Augustson, Kyle C.; Brun, Allan Sacha; Miesch,
Mark S.
Bibcode: 2016SPD....47.1013T
Altcode:
The 3-D MHD Anelastic Spherical Harmonic (ASH) code, using slope-limited
diffusion, is used to study the interaction of turbulent convection,
rotation and magnetism in a full spherical shell comparable to the solar
convection zone. Here a star of one solar mass, with a solar luminosity,
is considered that is rotating at three times the solar rate. The
dynamo generated magnetic field forms large-scale toroidal wreaths,
whose formation is tied to the low Rossby number of the convection in
this simulation which we have labeled K3S. This case displays prominent
polarity cycles with regular reversals occurring roughly every 6.2
years. These reversals are linked to the weakened differential rotation
and a resistive collapse of the large-scale magnetic field. Distinctive
equatorial migration of the strong magnetic wreaths is seen, arising
from modulation of the differential rotation rather than a dynamo
wave. As the wreaths approach the equator, cross-equatorial magnetic
flux is achieved that permits the low-latitude convection to generate
poloidal magnetic field with opposite polarity. Poleward migration of
such magnetic flux from the equator eventually leads to the reversal of
the polarity of the high-latitude magnetic field. This K3S simulation
also enters an interval with reduced magnetic energy at low latitudes
lasting roughly 16 years (about 2.5 polarity cycles), during which the
polarity cycles are disrupted and after which the dynamo recovers its
regular polarity cycles. An analysis of this striking grand minimum
reveals that it likely arises through the interplay of symmetric and
antisymmetric dynamo families.
Title: Polar cap magnetic field reversals during solar grand minima:
could pores play a role?
Authors: Švanda, Michal; Brun, Allan Sacha; Roudier, Thierry;
Jouve, Laurène
Bibcode: 2016A&A...586A.123S
Altcode: 2015arXiv151106894S
We study the magnetic flux carried by pores located outside active
regions with sunspots and investigate their possible contribution to
the reversal of the global magnetic field of the Sun. We find that they
contain a total flux of comparable amplitude to the total magnetic flux
contained in polar caps. The pores located at distances of 40-100 Mm
from the closest active region systematically have the correct polarity
of the magnetic field to contribute to the polar cap reversal. These
pores can be found predominantly in bipolar magnetic regions. We propose
that during grand minima of solar activity, such a systematic polarity
trend, which is akin to a weak magnetic (Babcock-Leighton-like) source
term, could still be operating but was missed by the contemporary
observers because of the limited resolving power of their telescopes.
Title: Magnetic energy fluxes in close-in star-planet systems
Authors: Strugarek, A.; Brun, A. S.; Matt, S. P.; Réville, V.
Bibcode: 2016IAUS..320..403S
Altcode:
Magnetic interactions between a close-in planet and its host star have
been postulated to be a source of enhanced chromospheric emissions. We
develop three dimensional global models of star-planet systems
under the ideal magnetohydrodynamic (MHD) approximation to explore
the impact of magnetic topology on the energy fluxes induced by the
magnetic interaction. We conduct twin numerical experiments in which
only the magnetic topology of the interaction is altered. We find that
the Poynting flux varies by more than an order of magnitude when varying
the magnetic topology from an aligned case to an anti-aligned case. This
provides a simple and robust physical explanation for on/off enhanced
chromospheric emissions induced by a close-in planet on time-scales
of the order of days to years.
Title: The role of complex magnetic topologies on stellar spin-down
Authors: Réville, Victor; Brun, Allan Sacha; Strugarek, Antoine;
Matt, Sean P.; Bouvier, Jérôme; Folsom, Colin P.; Petit, Pascal
Bibcode: 2016IAUS..320..297R
Altcode:
The rotational braking of magnetic stars through the extraction of
angular momentum by stellar winds has been studied for decades, leading
to several formulations. We recently demonstrated that the dependency
of the braking law on the coronal magnetic field topology can be taken
into account through a simple scalar parameter: the open magnetic
flux. The Zeeman-Doppler Imaging technique has brought the community
a reliable and precise description of the surface magnetic field of
distant stars. The coronal structure can then be reconstructed using
a potential field extrapolation, a technique that relies on a source
surface radius beyond which all field lines are open, thus avoiding a
computationally expensive MHD simulations. We developed a methodology
to choose the best source surface radius in order to estimate open
flux and magnetic torques. We apply this methodology to five K-type
stars from 25 to 584 Myr and the Sun, and compare the resulting torque
to values expected from spin evolution models.
Title: Magnetic Games between a Planet and Its Host Star: The Key
Role of Topology
Authors: Strugarek, A.; Brun, A. S.; Matt, S. P.; Réville, V.
Bibcode: 2015ApJ...815..111S
Altcode: 2015arXiv151102837S
Magnetic interactions between a star and a close-in planet are
postulated to be a source of enhanced emissions and to play a role
in the secular evolution of the orbital system. Close-in planets
generally orbit in the sub-alfvénic region of the stellar wind,
which leads to efficient transfers of energy and angular momentum
between the star and the planet. We model the magnetic interactions
occurring in close-in star-planet systems with three-dimensional,
global, compressible magnetohydrodynamic numerical simulations of a
planet orbiting in a self-consistent stellar wind. We focus on the
cases of magnetized planets and explore three representative magnetic
configurations. The Poynting flux originating from the magnetic
interactions is an energy source for enhanced emissions in star-planet
systems. Our results suggest a simple geometrical explanation for
ubiquitous on/off enhanced emissions associated with close-in planets,
and confirm that the Poynting fluxes can reach powers of the order
of 1019 W. Close-in planets are also shown to migrate due
to magnetic torques for sufficiently strong stellar wind magnetic
fields. The topology of the interaction significantly modifies the
shape of the magnetic obstacle that leads to magnetic torques. As a
consequence, the torques can vary by at least an order of magnitude
as the magnetic topology of the interaction varies.
Title: The Solar-Stellar Connection
Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.;
Pinsonneault, M.
Bibcode: 2015SSRv..196..303B
Altcode: 2014SSRv..tmp...54B; 2015arXiv150306742B
We discuss how recent advances in observations, theory and numerical
simulations have allowed the stellar community to progress in its
understanding of stellar convection, rotation and magnetism and to
assess the degree to which the Sun and other stars share similar
dynamical properties. Ensemble asteroseismology has become a reality
with the advent of large time domain studies, especially from space
missions. This new capability has provided improved constraints
on stellar rotation and activity, over and above that obtained via
traditional techniques such as spectropolarimetry or CaII H&K
observations. New data and surveys covering large mass and age ranges
have provided a wide parameter space to confront theories of stellar
magnetism. These new empirical databases are complemented by theoretical
advances and improved multi-D simulations of stellar dynamos. We trace
these pathways through which a lucid and more detailed picture of
magnetohydrodynamics of solar-like stars is beginning to emerge and
discuss future prospects.
Title: From Solar to Stellar Corona: The Role of Wind, Rotation,
and Magnetism
Authors: Réville, Victor; Brun, Allan Sacha; Strugarek, Antoine;
Matt, Sean P.; Bouvier, Jérôme; Folsom, Colin P.; Petit, Pascal
Bibcode: 2015ApJ...814...99R
Altcode: 2015arXiv150906982R
Observations of surface magnetic fields are now within reach for
many stellar types thanks to the development of Zeeman-Doppler
Imaging. These observations are extremely useful for constraining
rotational evolution models of stars, as well as for characterizing the
generation of the magnetic field. We recently demonstrated that the
impact of coronal magnetic field topology on the rotational braking
of a star can be parameterized with a scalar parameter: the open
magnetic flux. However, without running costly numerical simulations
of the stellar wind, reconstructing the coronal structure of the
large-scale magnetic field is not trivial. An alternative—broadly
used in solar physics—is to extrapolate the surface magnetic field
assuming a potential field in the corona, to describe the opening of
the field lines by the magnetized wind. This technique relies on the
definition of a so-called source surface radius, which is often fixed
to the canonical value of 2.5{R}⊙ . However this value
likely varies from star to star. To resolve this issue, we use our
extended set of 2.5D wind simulations published in 2015 to provide
a criterion for the opening of field lines as well as a simple tool
to assess the source surface radius and the open magnetic flux. This
allows us to derive the magnetic torque applied to the star by the wind
from any spectropolarimetric observation. We conclude by discussing
some estimations of spin-down timescales made using our technique and
compare them to observational requirements.
Title: Recent Advances on Solar Global Magnetism and Variability
Authors: Brun, A. S.; Browning, M. K.; Dikpati, M.; Hotta, H.;
Strugarek, A.
Bibcode: 2015SSRv..196..101B
Altcode: 2013SSRv..tmp..100B
We discuss recent observational, theoretical and numerical progress
made in understanding the solar global magnetism and its short and
long term variability. We discuss the physical process thought to
be at the origin of the solar magnetic field and its 22-yr cycle,
namely dynamo action, and the nonlinear interplay between convection,
rotation, radiation and magnetic field, yielding modulations of the
solar constant or of the large scale flows such as the torsional
oscillations. We also discuss the role of the field parity and dynamo
families in explaining the complex multipolar structure of the solar
global magnetic field. We then present some key MHD processes acting
in the deep radiative interior and discuss the probable topology of
a primordial field there. Finally we summarize how helioseismology
has contributed to these recent advances and how it could contribute
to resolving current unsolved problems in solar global dynamics and
magnetism.
Title: Erratum: Erratum to: The Solar-Stellar Connection
Authors: Brun, A. S.; García, R. A.; Houdek, G.; Nandy, D.;
Pinsonneault, M.
Bibcode: 2015SSRv..196..357B
Altcode: 2015SSRv..tmp...30B
No abstract at ADS
Title: Estimating the Deep Solar Meridional Circulation Using Magnetic
Observations and a Dynamo Model: A Variational Approach
Authors: Hung, Ching Pui; Jouve, Laurène; Brun, Allan Sacha; Fournier,
Alexandre; Talagrand, Olivier
Bibcode: 2015ApJ...814..151H
Altcode: 2017arXiv171002084H
We show how magnetic observations of the Sun can be used in conjunction
with an axisymmetric flux-transport solar dynamo model in order
to estimate the large-scale meridional circulation throughout the
convection zone. Our innovative approach rests on variational data
assimilation, whereby the distance between predictions and observations
(measured by an objective function) is iteratively minimized by means of
an optimization algorithm seeking the meridional flow that best accounts
for the data. The minimization is performed using a quasi-Newton
technique, which requires knowledge of the sensitivity of the objective
function to the meridional flow. That sensitivity is efficiently
computed via the integration of the adjoint flux-transport dynamo
model. Closed-loop (also known as twin) experiments using synthetic
data demonstrate the validity and accuracy of this technique for a
variety of meridional flow configurations, ranging from unicellular and
equatorially symmetric to multicellular and equatorially asymmetric. In
this well-controlled synthetic context, we perform a systematic study of
the behavior of our variational approach under different observational
configurations by varying their spatial density, temporal density,
and noise level, as well as the width of the assimilation window. We
find that the method is remarkably robust, leading in most cases to a
recovery of the true meridional flow to within better than 1%. These
encouraging results are a first step toward using this technique to (i)
better constrain the physical processes occurring inside the Sun and
(ii) better predict solar activity on decadal timescales.
Title: Characterizing the propagation of gravity waves in 3D nonlinear
simulations of solar-like stars
Authors: Alvan, L.; Strugarek, A.; Brun, A. S.; Mathis, S.; Garcia,
R. A.
Bibcode: 2015A&A...581A.112A
Altcode: 2015arXiv150803126A
Context. The revolution of helio- and asteroseismology provides
access to the detailed properties of stellar interiors by studying the
star's oscillation modes. Among them, gravity (g) modes are formed by
constructive interferences between progressive internal gravity waves
(IGWs), propagating in stellar radiative zones. Our new 3D nonlinear
simulations of the interior of a solar-like star allows us to study
the excitation, propagation, and dissipation of these waves.
Aims: The aim of this article is to clarify our understanding of
the behavior of IGWs in a 3D radiative zone and to provide a clear
overview of their properties.
Methods: We use a method of
frequency filtering that reveals the path of individual gravity waves
of different frequencies in the radiative zone.
Results: We are
able to identify the region of propagation of different waves in 2D and
3D, to compare them to the linear raytracing theory and to distinguish
between propagative and standing waves (g-modes). We also show that
the energy carried by waves is distributed in different planes in the
sphere, depending on their azimuthal wave number.
Conclusions:
We are able to isolate individual IGWs from a complex spectrum and to
study their propagation in space and time. In particular, we highlight
in this paper the necessity of studying the propagation of waves in
3D spherical geometry, since the distribution of their energy is not
equipartitioned in the sphere.
Title: Dynamo action and magnetic activity of the giant star Pollux
Authors: Brun, Allan Sacha; Palacios, Ana
Bibcode: 2015IAUGA..2252288B
Altcode:
Recent spectropolarimetric observations of the giant star Pollux have
revealed that it possesses a weak global magnetic field of the order
of a Gauss. Using 3-D nonlinear MHD simulations performed with the
ASH code we study the source of this global magnetic field in this
slowly rotating giant star (Omega*=Omega_sun/20). We find that the
extended convective envelope is able to generate a multi-scales magnetic
field reaching of the order of 10% of the kinetic energy contained in
the envelope. This global field acts such as to suppress the strong
differential rotation present in the purely hydrodynamical progenitor
simulation. When filtering the large scale magnetic field components
(dipole, quadrupole) we find magnetic field of the order of a few
Gauss, hence in qualitative agreeement with observations. Our study
confirms that such slowly rotating convective giants are likely to
possess global magnetic field maintained through contemporaneous dynamo
action and not as the vestige of their past main sequence activity.
Title: The Solar/Stellar Connection
Authors: Brun, Allan Sacha
Bibcode: 2015IAUGA..2244193B
Altcode:
The Sun is the archetype of magnetic star. Its proximity and the
wealth of very high accuracy observations that this has allowed us
to gather over many decades have greatly helped us understanding how
solar-like stars (e.g with a convective envelope) redistribute angular
momentum and generate a cyclic magnetic field. However most models have
been so fine tuned that when they are straightforwardly extended to
other solar-like stars and are compared with the ever growing stellar
magnetism and differential rotation observations the agreement is not
as good as one could hope. In this review I will discuss based on
theoretical considerations and multi-D MHD stellar models what can
be considered as robust properties of solar-like star dynamics and
magnetism and what is still speculative.
Title: Coronal magnetic field and wind of an aging K-type star
Authors: Réville, Victor; Brun, Allan Sacha; Strugarek, Antoine;
Jeffers, Sandra; Folsom, Colin; Marsden, Stephen C.; Petit, Pascal
Bibcode: 2015IAUGA..2249564R
Altcode:
Created at the base of the convective envelope by a nonlinear dynamo
process, the large scale magnetic field of a star evolves with
its rotational history. Beyond the photosphere, magnetic processes
heat the corona above one million Kelvin hence driving a magnetized
wind responsible for the braking of main sequence stars. Hence a
feedback loop tie those processes. Development of Zeeman-Doppler
imaging through spectropolarimetry allows to precisely describe the
surface magnetic field of a large sample of stars. Thus the study of
the coronal structure and magnetic field with age, magnetochoronology,
has developed to extend and complete gyrochronology. We propose a study
of the corona and the wind of a sample of K-type stars of different
age to follow the evolution of its properties from 20 Myr to 8 Gyr
thanks to a set of 3D MHD simulations with the PLUTO code constrained
by spectropolarimetric maps of the surface magnetic field obtained
by the BCool consortium. To perform those simulations we developed a
coherent framework to assess various stellar parameters such as the
equilibrium coronal temperature driving the wind. Those assumptions
have consequences on UV emissions, wind terminal speed and mass loss
that impact planetary systems that could potentially host life.
Title: Grand Minima and Equatorward Propagation in a Cycling Stellar
Convective Dynamo
Authors: Augustson, Kyle; Brun, Allan Sacha; Miesch, Mark; Toomre, Juri
Bibcode: 2015ApJ...809..149A
Altcode: 2014arXiv1410.6547A
The 3D MHD Anelastic Spherical Harmonic code, using slope-limited
diffusion, is employed to capture convective and dynamo processes
achieved in a global-scale stellar convection simulation for a
model solar-mass star rotating at three times the solar rate. The
dynamo-generated magnetic fields possesses many timescales, with
a prominent polarity cycle occurring roughly every 6.2 years. The
magnetic field forms large-scale toroidal wreaths, whose formation is
tied to the low Rossby number of the convection in this simulation. The
polarity reversals are linked to the weakened differential rotation and
a resistive collapse of the large-scale magnetic field. An equatorial
migration of the magnetic field is seen, which is due to the strong
modulation of the differential rotation rather than a dynamo wave. A
poleward migration of magnetic flux from the equator eventually leads to
the reversal of the polarity of the high-latitude magnetic field. This
simulation also enters an interval with reduced magnetic energy at
low latitudes lasting roughly 16 years (about 2.5 polarity cycles),
during which the polarity cycles are disrupted and after which the
dynamo recovers its regular polarity cycles. An analysis of this grand
minimum reveals that it likely arises through the interplay of symmetric
and antisymmetric dynamo families. This intermittent dynamo state
potentially results from the simulation’s relatively low magnetic
Prandtl number. A mean-field-based analysis of this dynamo simulation
demonstrates that it is of the α-Ω type. The timescales that appear
to be relevant to the magnetic polarity reversal are also identified.
Title: Coronal structure of the large scale magnetic field and its
influence on stellar rotation.
Authors: Réville, Victor; Brun, Allan Sacha; Matt, Sean; Strugarek,
Antoine; Bouvier, Jérôme
Bibcode: 2015IAUGA..2249552R
Altcode:
The braking of magnetic stars through the extraction of angular
momentum by stellar winds has been studied for decades, leading
to several formulations as functions of stellar parameters. We
recently demonstrated that the dependency of the braking law on the
coronal magnetic field topology can be taken into account through
a simple scalar parameter : the open magnetic flux. This parameter
can be integrated anywhere beyond the last closed coronal loop in
steady-state. The Zeeman-Doppler Imaging technique has brought the
community a reliable and precise description of the surface magnetic
field of distant stars. However reconstruction of the coronal structure
of the large scale magnetic field without running costly numerical
simulations of the stellar wind is not trivial. An alternative is
to use the classical analytical potential field extrapolation to
describe the opening of the field lines by the magnetized wind but
this technique relies on knowing the so-called radius of the surface
source term which must vary from star to star. To resolve this issue,
we use our extended set of 2.5D wind simulations published in 2015,
to provide a criteria for the field lines opening as well as a simple
tool to assess the surface source term radius and the open magnetic
flux. This allows us to derive the magnetic torque applied to the star
by the wind from any spectropolarimetric observations. We conclude
our talk by discussing the case of 3D wind simulations of the BCool
sample ; whose surface magnetic field has been obtained by ZDI and to
discuss how non-axisymmetry modifies or not our recent findings.
Title: Linking stellar dynamo action to flux emergence and flares
Authors: Brun, Allan Sacha; Pinto, Rui
Bibcode: 2015IAUGA..2244355B
Altcode:
Stars are active magnetic objects. In this talk I will discuss how
the surface activity is linked to its deep internal origin via dynamo
action and flux emergence. Based on 3-D MHD simulations performed
with both ASH and PLUTO codes we will show how turbulence and shear
(either in convection or radiation zones) can help building intense
coherent magnetic structures amidst disorganized magnetic fields that
can subsequently rise and emerge at the stellar surface. Those intense
twisted magnetic features, the amount of magnetic flux they possess and
the shape of the emerged structures are likely the source/ingredients
of the intense magnetic flaring activity seen in most solar-like stars
and in particular of the associated X-ray emission as revealed by our
recent 3-D PLUTO MHD compressible simulations.
Title: Close-in planet migration due to magnetic torques
Authors: Strugarek, Antoine; Brun, Allan Sacha; Matt, Sean; Réville,
Victor
Bibcode: 2015IAUGA..2242256S
Altcode:
The diversity of masses, sizes and orbits of known exoplanets has
prompted recent efforts in the scientific community to explore
the broad range of interactions that can exist between planets
and their host stars. In addition to tidal interactions, planets
orbiting inside the stellar wind Alfv ´en radius can magnetically
interact with their host. These interactions could lead to an angular
momentum transfer between the planet and its host, resulting in a
substantial planetary migration and participating in the dynamical
(in)stability of the system. Among the star-planet interaction (SPI)
models that have been developed, magnetohydrodynamic (MHD) simulations
combine state of the art numerical models of cool star magnetospheres
with simplified models of planets. The advantage of these global,
dynamical models is the ability to assess the effects of SPI in a
self-consistent way, by modelling the full interaction channel from
the planetary magnetosphere down to the lower stellar corona.We will
present our study of global magnetic SPI using the PLUTO code. We first
give an overview of different types of interactions, depending on the
stellar wind and orbital properties. Based on our previous exploratory
2D axisymmetric study, we develop our magnetic interaction model in
3D to tackle the full geometry of the star-wind- planet connection. We
study the formation of Aflv ´en wings and parametrize the key physical
ingredients (magnetic field strength and topology, orbital distance,
stellar wind mass and angular momentum loss rates) controlling the
magnetic torques which lead to planet migration. These torques are shown
to operate on time-scales comparable to tidal torques for sufficiently
compact systems and favorable magnetic topologies.
Title: 3D magnetic interactions of stars with their close-in planets
Authors: Strugarek, Antoine; Brun, Allan Sacha; Matt, Sean; Réville,
Victor
Bibcode: 2015IAUGA..2247838S
Altcode:
Close-in planets generally orbit in a sub-alfv ´enic stellar wind,
where the perturbations they excite in the corona are able to travel
upwind to the stellar surface and potentially induce observable
phenomena. The effective connection between the planet and its host
takes the form of two Aflv ´en wings. Depending on the topology of
the planetary and stellar magnetic fields, the rotation profile of the
corona, and the orbital parameters, it is possible that none, one, or
the two Aflv ´en wings connect together the star and the planet.We
explore the formation and sustainment of Alfv ´en wings in global
three dimensional simulations under the magneto-hydrodynamic formalism
with the PLUTO code. We model the stellar wind of a typical cool
star in which a close-in orbiting planet is introduced as a boundary
condition. By varying the magnetic topologies of the planetary and
stellar magnetic fields, we explore the variety of Alfv ´en wings
that can develop and quantify the Poynting flux flowing through those
wings. We thus provide estimates of the amount of magnetic energy
these magnetic interactions can channel to the lower corona. We also
quantify the phase and latitude offsets that can be expected between
the planetary subpoint on the stellar surface and the actual location
where energy is deposited. We summarize the typical situations (in
terms of magnetic topology, stellar type, and orbital parameters)
where the star-planet magnetic interaction could trigger observable
flares. We conclude by extending our results to the cases of more
complex, non-axisymmetric topologies of the observed magnetic fields
for a few particular stars.
Title: Super-equipartition Convective Dynamo Action in the Cores of
B-Type Stars
Authors: Augustson, Kyle C.; Brown, Benjamin P.; Brun, Allan Sacha;
Toomre, Juri
Bibcode: 2015IAUGA..2258137A
Altcode:
Observations have revealed the presence and topology of magnetic fields
on the surfaces of some main sequence massive stars. These stars
possess a convective core that supports strong dynamo action. This
core is linked to the dynamics of the rest of the star through
overshooting convection and magnetic fields and may influence
the surface magnetism. Such effects are captured through 3-D MHD
simulations of a 10 M⊙ B-type star, using the anelastic
spherical harmonic (ASH) code. These simulations capture the inner 65%
of the star by radius, encompassing the convective core and an extensive
portion of the radiative exterior. Vigorous dynamo action is achieved
in the convective core with self-consistent super-equipartition (SE)
states sustained over a range of rotation rates. Indeed, the ratio
of magnetic to convective kinetic energy shows a distinct scaling
with Elsasser and Coriolis number. The impact of this dynamo action
upon the differential rotation of the core is assessed by contrasting
hydrodynamic and magnetohydrodynamic simulations. The processes that
permit the maintenance of such SE states are examined. We further
study how the magnetic field generated during main-sequence dynamo
action may carry over into later evolutionary stages.
Title: Super-equipartition Convective Dynamo Action in the Cores of
B-Type Stars
Authors: Augustson, Kyle C.; Brown, Benjamin P.; Brun, Allan Sacha;
Toomre, Juri
Bibcode: 2015IAUGA..2257925A
Altcode:
Observations have revealed the presence and topology of magnetic fields
on the surfaces of some main sequence massive stars. These stars
possess a convective core that supports strong dynamo action. This
core is linked to the dynamics of the rest of the star through
overshooting convection and magnetic fields and may influence
the surface magnetism. Such effects are captured through 3-D MHD
simulations of a 10 M⊙ B-type star, using the anelastic
spherical harmonic (ASH) code. These simulations capture the inner 65%
of the star by radius, encompassing the convective core and an extensive
portion of the radiative exterior. Vigorous dynamo action is achieved
in the convective core with self-consistent super-equipartition (SE)
states sustained over a range of rotation rates. Indeed, the ratio
of magnetic to convective kinetic energy shows a distinct scaling
with Elsasser and Coriolis number. The impact of this dynamo action
upon the differential rotation of the core is assessed by contrasting
hydrodynamic and magnetohydrodynamic simulations. The processes that
permit the maintenance of such SE states are examined. We further
study how the magnetic field generated during main-sequence dynamo
action may carry over into later evolutionary stages.
Title: Grand Minima and Equatorward Propagation in a Cycling Stellar
Convective Dynamo
Authors: Augustson, Kyle C.; Brun, Allan Sacha; Miesch, Mark;
Toomre, Juri
Bibcode: 2015IAUGA..2257912A
Altcode:
The 3-D magnetohydrodynamic (MHD) Anelastic Spherical Harmonic (ASH)
code, using slope-limited diffusion, is employed to capture convective
and dynamo processes achieved in a global-scale stellar convection
simulation for a model solar-mass star rotating at three times the solar
rate. The dynamo generated magnetic fields possesses many time scales,
with a prominent polarity cycle occurring roughly every 6.2 years. The
magnetic field forms large-scale toroidal wreaths, whose formation is
tied to the low Rossby number of the convection in this simulation. The
polarity reversals are linked to the weakened differential rotation and
a resistive collapse of the large-scale magnetic field. An equatorial
migration of the magnetic field is seen, which is due to the strong
modulation of the differential rotation rather than a dynamo wave. A
poleward migration of magnetic flux from the equator eventually leads to
the reversal of the polarity of the high-latitude magnetic field. This
simulation also enters an interval with reduced magnetic energy at
low latitudes lasting roughly 16 years (about 2.5 polarity cycles),
during which the polarity cycles are disrupted and after which the
dynamo recovers its regular polarity cycles. An analysis of this
grand minimum reveals that it likely arises through the interplay of
symmetric and antisymmetric dynamo families. This intermittent dynamo
state potentially results from the simulations relatively low magnetic
Prandtl number. A mean-field-based analysis of this dynamo simulation
demonstrates that it is of the α-Ω type. The time scales that appear
to be relevant to the magnetic polarity reversal are also identified.
Title: Gravity waves nonlinear excitation and propagation in
solar-like stars
Authors: Brun, Allan Sacha; Alvan, Lucie; Mathis, Stéphane; Strugarek,
Antoine; Garcia, Rafael
Bibcode: 2015IAUGA..2244249B
Altcode:
Using the ASH code we have made a 3-D model of the full Sun (from r=0
to 0.99 Rsol) coupling nonlinearly its convective envelope to its deep
radiative interior. Solar-like differential rotation is developing
due to the joint action of the Coriolis force on the turbulent
convective motions and the feedback (via thermal wind balance) of a
self-established tachocline at the base of the convective envelope. The
model further self-consistently excite gravity waves and modes due to
the continuous pummeling action of cold convective plumes on the top of
the radiative interior. When compared with the Aarhus oscillation code
we find a very good agreement between the ridges present in the power
spectra and the frequency computed from the 1-D background structure
of the 3-D model. This model allows us to study for the first time
excitation and propagation of gravity waves in 3-D in a star and to
study their visibility through a differentially rotating convective
envelope. We also assess their lifetime, rotational splitting and
radiative damping and found some departures from the linear asymptotic
theory.
Title: Grand Minima and Equatorward Propagation in a Cycling Stellar
Convective Dynamo
Authors: Augustson, Kyle C.; Brun, Allan Sacha; Miesch, Mark;
Toomre, Juri
Bibcode: 2015IAUGA..2258283A
Altcode:
The 3-D magnetohydrodynamic (MHD) Anelastic Spherical Harmonic (ASH)
code, using slope-limited diffusion, is employed to capture convective
and dynamo processes achieved in a global-scale stellar convection
simulation for a model solar-mass star rotating at three times the solar
rate. The dynamo generated magnetic fields possesses many time scales,
with a prominent polarity cycle occurring roughly every 6.2 years. The
magnetic field forms large-scale toroidal wreaths, whose formation is
tied to the low Rossby number of the convection in this simulation. The
polarity reversals are linked to the weakened differential rotation and
a resistive collapse of the large-scale magnetic field. An equatorial
migration of the magnetic field is seen, which is due to the strong
modulation of the differential rotation rather than a dynamo wave. A
poleward migration of magnetic flux from the equator eventually leads to
the reversal of the polarity of the high-latitude magnetic field. This
simulation also enters an interval with reduced magnetic energy at
low latitudes lasting roughly 16 years (about 2.5 polarity cycles),
during which the polarity cycles are disrupted and after which the
dynamo recovers its regular polarity cycles. An analysis of this
grand minimum reveals that it likely arises through the interplay of
symmetric and antisymmetric dynamo families. This intermittent dynamo
state potentially results from the simulations relatively low magnetic
Prandtl number. A mean-field-based analysis of this dynamo simulation
demonstrates that it is of the α-Ω type. The time scales that appear
to be relevant to the magnetic polarity reversal are also identified.
Title: Angular momentum transport in stars: From short to long
time scales
Authors: Brun, A. S.; Mathis, S.
Bibcode: 2015exse.book..264B
Altcode:
No abstract at ADS
Title: Soft X-ray emission in kink-unstable coronal loops
Authors: Pinto, R. F.; Vilmer, N.; Brun, A. S.
Bibcode: 2015A&A...576A..37P
Altcode: 2014arXiv1401.0916P
Context. Solar flares are associated with intense soft X-ray
emission generated by the hot flaring plasma in coronal magnetic
loops. Kink-unstable twisted flux-ropes provide a source of magnetic
energy that can be released impulsively and may account for the heating
of the plasma in flares.
Aims: We investigate the temporal,
spectral, and spatial evolution of the properties of the thermal
continuum X-ray emission produced in such kink-unstable magnetic
flux-ropes and discuss the results of the simulations with respect
to solar flare observations.
Methods: We computed the temporal
evolution of the thermal X-ray emission in kink-unstable coronal loops
based on a series of magnetohydrodynamical numerical simulations. The
numerical setup consisted of a highly twisted loop embedded in a region
of uniform and untwisted background coronal magnetic field. We let
the kink instability develop, computed the evolution of the plasma
properties in the loop (density, temperature) without accounting
for mass exchange with the chromosphere. We then deduced the X-ray
emission properties of the plasma during the whole flaring episode.
Results: During the initial (linear) phase of the instability,
plasma heating is mostly adiabatic (as a result of compression). Ohmic
diffusion takes over as the instability saturates, leading to strong
and impulsive heating (up to more than 20 MK), to a quick enhancement of
X-ray emission, and to the hardening of the thermal X-ray spectrum. The
temperature distribution of the plasma becomes broad, with the
emission measure depending strongly on temperature. Significant
emission measures arise for plasma at temperatures higher than 9
MK. The magnetic flux-rope then relaxes progressively towards a
lower energy state as it reconnects with the background flux. The
loop plasma suffers smaller sporadic heating events, but cools down
globally by thermal conduction. The total thermal X-ray emission
slowly fades away during this phase, and the high-temperature
component of the emission measure distribution converges to the
power-law distribution EM ∝ T-4.2. The twist deduced
directly from the X-ray emission patterns is considerably lower than
the highest magnetic twist in the simulated flux-ropes. Movies
associated to Figs. 4 and 5 are available in electronic form at http://www.aanda.org
Title: Simulating Solar Global Magnetism in 3-D
Authors: Brun, A. S.; Strugarek, A.
Bibcode: 2015HiA....16..101B
Altcode:
We briefly present recent progress using the ASH code to model in 3-D
the solar convection, dynamo and its coupling to the deep radiative
interior. We show how the presence of a self-consistent tachocline
influences greatly the organization of the magnetic field and modifies
the thermal structure of the convection zone leading to realistic
profiles of the mean flows as deduced by helioseismology.
Title: Dynamo Modeling of the Kepler F Star KIC 12009504
Authors: Mathur, S.; Augustson, Kyle C.; Brun, A. S.; Garcia, R. A.;
Metcalfe, T. S.
Bibcode: 2015csss...18..365M
Altcode: 2014arXiv1408.5926M
The Kepler mission has collected light curves for almost 4 years. The
excellent quality of these data has allowed us to probe the structure
and the dynamics of the stars using asteroseismology. With the length
of data available, we can start to look for magnetic activity cycles
. The Kepler data obtained for the F star, KIC 12009504 shows a rotation
period of 9.5 days and additional variability that could be due to the
magnetic activity of the star. Here we present recent and preliminary 3D
global-scale dynamo simulations of this star with the ASH and STELEM
codes, capturing a substantial portion of the convection and the
stable radiation zone below it. These simulations reveal a multi-year
activity cycle whose length tentatively depends upon the width of the
tachocline present in the simulation. Furthermore, the presence of a
magnetic field and the dynamo action taking place in the convection
zone appears to help confine the tachocline, but longer simulations
will be required to confirm this.
Title: The Effect of Magnetic Topology on Thermally Driven Wind:
Toward a General Formulation of the Braking Law
Authors: Réville, Victor; Brun, Allan Sacha; Matt, Sean P.; Strugarek,
Antoine; Pinto, Rui F.
Bibcode: 2015ApJ...798..116R
Altcode: 2014arXiv1410.8746R
Stellar wind is thought to be the main process responsible for
the spin down of main-sequence stars. The extraction of angular
momentum by a magnetized wind has been studied for decades, leading
to several formulations for the resulting torque. However, previous
studies generally consider simple dipole or split monopole stellar
magnetic topologies. Here we consider, in addition to a dipolar stellar
magnetic field, both quadrupolar and octupolar configurations, while
also varying the rotation rate and the magnetic field strength. Sixty
simulations made with a 2.5D cylindrical and axisymmetric set-up, and
computed with the PLUTO code, were used to find torque formulations
for each topology. We further succeed to give a unique law that fits
the data for every topology by formulating the torque in terms of
the amount of open magnetic flux in the wind. We also show that our
formulation can be applied to even more realistic magnetic topologies,
with examples of the Sun in its minimum and maximum phases as observed
at the Wilcox Solar Observatory, and of a young K-star (TYC-0486-4943-1)
whose topology has been obtained by Zeeman-Doppler Imaging.
Title: Convective Dynamo Simulation with a Grand Minimum
Authors: Augustson, Kyle C.; Brun, A. S.; Miesch, Mark; Toomre, Juri
Bibcode: 2015csss...18..451A
Altcode: 2015arXiv150304225A
The global-scale dynamo action achieved in a simulation of a Sun-like
star rotating at thrice the solar rate is assessed. The 3-D MHD
Anelastic Spherical Harmonic (ASH) code, augmented with a viscosity
minimization scheme, is employed to capture convection and dynamo
processes in this G-type star. The simulation is carried out in a
spherical shell that encompasses 3.8 density scale heights of the solar
convection zone. It is found that dynamo action with a high degree of
time variation occurs, with many periodic polarity reversals occurring
roughly every 6.2 years. The magnetic energy also rises and falls with
a regular period. The magnetic energy cycles arise from a Lorentz-force
feedback on the differential rotation, whereas the processes leading
to polarity reversals are more complex, appearing to arise from the
interaction of convection with the mean toroidal fields. Moreover,
an equatorial migration of toroidal field is found, which is linked
to the changing differential rotation, and potentially to a nonlinear
dynamo wave. This simulation also enters a grand minimum lasting roughly
20 years, after which the dynamo recovers its regular polarity cycles.
Title: The Mass-dependence of Angular Momentum Evolution in Sun-like
Stars
Authors: Matt, Sean P.; Brun, A. Sacha; Baraffe, Isabelle; Bouvier,
Jérôme; Chabrier, Gilles
Bibcode: 2015ApJ...799L..23M
Altcode: 2014arXiv1412.4786M
To better understand the observed distributions of the rotation rate and
magnetic activity of Sun-like and low-mass stars, we derive a physically
motivated scaling for the dependence of the stellar wind torque on
the Rossby number. The torque also contains an empirically derived
scaling with stellar mass (and radius), which provides new insight
into the mass-dependence of stellar magnetic and wind properties. We
demonstrate that this new formulation explains why the lowest mass
stars are observed to maintain rapid rotation for much longer than
solar-mass stars, and simultaneously why older populations exhibit a
sequence of slowly rotating stars, in which the low-mass stars rotate
more slowly than solar-mass stars. The model also reproduces some
previously unexplained features in the period-mass diagram for the
Kepler field, notably: the particular shape of the "upper envelope"
of the distribution, suggesting that ~95% of Kepler field stars with
measured rotation periods are younger than ~4 Gyr; and the shape of the
"lower envelope," corresponding to the location where stars transition
between magnetically saturated and unsaturated regimes.
Title: Numerical Aspects of 3D Stellar Winds
Authors: Strugarek, A.; Brun, A. S.; Matt, S. P.; Reville, V.
Bibcode: 2015csss...18..589S
Altcode: 2014arXiv1410.3537S
This paper explores and compares the pitfalls of modelling the
three-dimensional wind of a spherical star with a cartesian
grid. Several numerical methods are compared, using either
uniform and stretched grid or adaptative mesh refinement (AMR). An
additional numerical complication is added, when an orbiting planet
is considered. In this case a rotating frame is added to the model
such that the orbiting planet is at rest in the frame of work. The
three-dimensional simulations are systematically compared to an
equivalent two-dimensional, axisymmetric simulation. The comparative
study presented here suggests to limit the rotation rate of the rotating
frame below the rotating frame of the star and provides guidelines for
further three-dimensional modelling of stellar winds in the context
of close-in star-planet interactions.
Title: Upgrading the Solar-Stellar Connection: News about activity
in Cool Stars
Authors: Gunther, H. M.; Poppenhaeger, K.; Testa, P.; Borgniet, S.;
Brun, A. S.; Cegla, H. M.; Garraffo, C.; Kowalski, A.; Shapiro, A.;
Shkolnik, E.; Spada, F.; Vidotto, A. A.
Bibcode: 2015csss...18...25G
Altcode: 2014arXiv1408.3068G
In this splinter session, ten speakers presented results on solar
and stellar activity and how the two fields are connected. This was
followed by a lively discussion and supplemented by short, one-minute
highlight talks. The talks presented new theoretical and observational
results on mass accretion on the Sun, the activity rate of flare stars,
the evolution of the stellar magnetic field on time scales of a single
cycle and over the lifetime of a star, and two different approaches
to model the radial-velocity jitter in cool stars that is due to the
granulation on the surface. Talks and discussion showed how much the
interpretation of stellar activity data relies on the sun and how the
large number of objects available in stellar studies can extend the
parameter range of activity models.
Title: Modelling the Corona of HD 189733 in 3D
Authors: Strugarek, A.; Brun, A. S.; Matt, S. P.; Reville, V.; Donati,
J. F.; Moutou, C.; Fares, R.
Bibcode: 2014sf2a.conf..279S
Altcode: 2014arXiv1411.2494S
The braking of main sequence stars originates mainly from their
stellar wind. The efficiency of this angular momentum extraction
depends on the rotation rate of the star, the acceleration profile of
the wind and the coronal magnetic field. The derivation of scaling
laws parametrizing the stellar wind torque is important for our
understanding of gyro-chronology and the evolution of the rotation
rates of stars. In order to understand the impact of complex magnetic
topologies on the stellar wind torque, we present three-dimensional,
dynamical simulations of the corona of HD 189733. Using the observed
complex topology of the magnetic field, we estimate how the torque
associated with the wind scales with model parameters and compare
those trends to previously published scaling laws.
Title: The influence of the magnetic topology on the wind braking
of sun-like stars.
Authors: Réville, V.; Brun, A. S.; Matt, S. P.; Strugarek, A.;
Pinto, R.
Bibcode: 2014sf2a.conf..509R
Altcode: 2014arXiv1410.8759R
Stellar winds are thought to be the main process responsible for
the spin down of main-sequence stars. The extraction of angular
momentum by a magnetized wind has been studied for decades, leading
to several formulations for the resulting torque. However, previous
studies generally consider simple dipole or split monopole stellar
magnetic topologies. Here we consider in addition to a dipolar stellar
magnetic field, both quadrupolar and octupolar configurations, while
also varying the rotation rate and the magnetic field strength. 60
simulations made with a 2.5D, cylindrical and axisymmetric set-up and
computed with the PLUTO code were used to find torque formulations
for each topology. We further succeed to give a unique law that fits
the data for every topology by formulating the torque in terms of
the amount of open magnetic flux in the wind. We also show that our
formulation can be applied to even more realistic magnetic topologies,
with examples of the Sun in its minimum and maximum phase as observed
at the Wilcox Solar Observatory, and of a young K-star (TYC-0486-4943-1)
whose topology has been obtained by Zeeman-Doppler Imaging (ZDI).
Title: On the Diversity of Magnetic Interactions in Close-in
Star-Planet Systems
Authors: Strugarek, A.; Brun, A. S.; Matt, S. P.; Réville, V.
Bibcode: 2014ApJ...795...86S
Altcode: 2014arXiv1409.5268S
Magnetic interactions between close-in planets and their host star
can play an important role in the secular orbital evolution of the
planets, as well as the rotational evolution of their host. As long as
the planet orbits inside the Alfvén surface of the stellar wind, the
magnetic interaction between the star and the planet can modify the wind
properties and also lead to direct angular momentum transfers between
the two. We model these star-planet interactions using compressible
magnetohydrodynamic (MHD) simulations, and quantify the angular
momentum transfers between the star, the planet, and the stellar
wind. We study the cases of magnetized and non-magnetized planets and
vary the orbital radius inside the Alfvén surface of the stellar
wind. Based on a grid of numerical simulations, we propose general
scaling laws for the modification of the stellar wind torque, for the
torque between the star and the planet, and for the planet migration
associated with the star-planet magnetic interactions. We show that
when the coronal magnetic field is large enough and the star is rotating
sufficiently slowly, the effect of the magnetic star-planet interaction
is comparable to tidal effects and can lead to a rapid orbital decay.
Title: On dynamo action in the giant star Pollux: first results
Authors: Palacios, Ana; Brun, Allan Sacha
Bibcode: 2014IAUS..302..363P
Altcode: 2013arXiv1312.3132P
We present preliminary results of a 3D MHD simulation of the convective
envelope of the giant star Pollux for which the rotation period and
the magnetic field intensity have been measured from spectroscopic
and spectropolarimetric observations. This giant is one of the first
single giants with a detected magnetic field, and the one with the
weakest field so far. Our aim is to understand the development and
the action of the dynamo in its extended convective envelope.
Title: Rotation and magnetism of solar-like stars: from scaling laws
to spot-dynamos
Authors: Brun, Allan Sacha
Bibcode: 2014IAUS..302..114B
Altcode:
The Sun is the archetype of magnetic star and its proximity coupled
with very high accuracy observations has helped us understanding how
solar-like stars (e.g with a convective envelope) redistribute angular
momentum and generate a cyclic magnetic field. However most solar models
have been so fine tuned that when they are applied to other solar-like
stars the agreement with observations is not good enough. I will thus
discuss, based on theoretical considerations and multi-D MHD stellar
models, what can be considered as robust properties of solar-like
star dynamics and magnetism and what is still speculative. I will
derive scaling laws for differential rotation and magnetic energy as
a function of stellar parameters, discuss recent results of stellar
dynamo models and define the new concept of spot-dynamo, e.g. global
dynamo that develops self-consistent magnetic buoyant structures that
emerge at the surface.
Title: Theoretical seismology in 3D: nonlinear simulations of internal
gravity waves in solar-like stars
Authors: Alvan, L.; Brun, A. S.; Mathis, S.
Bibcode: 2014A&A...565A..42A
Altcode: 2014arXiv1403.4052A
Context. Internal gravity waves (IGWs) are studied for their impact
on the angular momentum transport in stellar radiation zones and
the information they provide about the structure and dynamics of
deep stellar interiors. We present the first 3D nonlinear numerical
simulations of IGWs excitation and propagation in a solar-like star.
Aims: The aim is to study the behavior of waves in a realistic 3D
nonlinear time-dependent model of the Sun and to characterize their
properties.
Methods: We compare our results with theoretical
and 1D predictions. It allows us to point out the complementarity
between theory and simulation and to highlight the convenience, but
also the limits, of the asymptotic and linear theories.
Results:
We show that a rich spectrum of IGWs is excited by the convection,
representing about 0.4% of the total solar luminosity. We study the
spatial and temporal properties of this spectrum, the effect of thermal
damping, and nonlinear interactions between waves. We give quantitative
results for the modes' frequencies, evolution with time and rotational
splitting, and we discuss the amplitude of IGWs considering different
regimes of parameters.
Conclusions: This work points out the
importance of high-performance simulation for its complementarity
with observation and theory. It opens a large field of investigation
concerning IGWs propagating nonlinearly in 3D spherical structures. The
extension of this work to other types of stars, with different masses,
structures, and rotation rates will lead to a deeper and more accurate
comprehension of IGWs in stars.
Title: Detailed analysis of internal waves in stars
Authors: Brun, Allan Sacha; Alvan, Lucie
Bibcode: 2014emfi.confE...4B
Altcode:
No abstract at ADS
Title: 3D simulations of internal gravity waves in solar-like stars
Authors: Alvan, Lucie; Brun, Allan Sacha; Mathis, Stéphane
Bibcode: 2014IAUS..301..375A
Altcode:
We perform numerical simulations of the whole Sun using the 3D anelastic
spherical harmonic (ASH) code. In such models, the radiative and
convective zones are non-linearly coupled and in the radiative interior
a wave-like pattern is observed. For the first time, we are thus able
to model in 3D the excitation and propagation of internal gravity waves
(IGWs) in a solar-like star's radiative zone. We compare the properties
of our waves to theoretical predictions and results of oscillation
calculations. The obtained good agreement allows us to validate the
consistency of our approach and to study the characteristics of IGWs. We
find that a wave's spectrum is excited up to radial order n=58. This
spectrum evolves with depth and time; we show that the lifetime of
the highest-frequency modes must be greater than 550 days. We also
test the sensitivity of waves to rotation and are able to retrieve the
rotation rate to within 5% error by measuring the frequency splitting.
Title: Modeling magnetized star-planet interactions: boundary
conditions effects
Authors: Strugarek, Antoine; Brun, Allan Sacha; Matt, Sean P.;
Reville, Victor
Bibcode: 2014IAUS..300..330S
Altcode: 2013arXiv1311.3902S
We model the magnetized interaction between a star and a close-in planet
(SPMIs), using global, magnetohydrodynamic numerical simulations. In
this proceedings, we study the effects of the numerical boundary
conditions at the stellar surface, where the stellar wind is driven,
and in the planetary interior. We show that is it possible to design
boundary conditions that are adequate to obtain physically realistic,
steady-state solutions for cases with both magnetized and unmagnetized
planets. This encourages further development of numerical studies,
in order to better constrain and undersand SPMIs, as well as their
effects on the star-planet rotational evolution.
Title: Flux emergence in a magnetized convection zone
Authors: Pinto, Rui; Brun, Allan Sacha
Bibcode: 2014cosp...40E2550P
Altcode:
We study the influence of a dynamo magnetic field on the buoyant rise
and emergence of twisted magnetic flux-ropes, and their influence
on the global external magnetic field. We ran three-dimensional MHD
numerical simulations using the ASH code and analysed the dynamical
evolution of such buoyant flux-ropes from the bottom of the convection
zone until the post-emergence phases. The actual flux-emergence episode
is preceded by a localised increase of radial velocity, density and
current density at the top of the convection zone. During the buoyant
rise, the flux-rope's magnetic field strength and density scale as
B~rho(alpha) , with alpha≤sssim 1. The properties of initial phases
of the buoyant rise are determined essentially by the flux-rope's
properties and the convective flows and are, in consequence, in good
agreement with previous studies. However, the effects of the interaction
of the background dynamo field become increasingly stronger as the
flux-ropes evolve. The threshold for the initial magnetic field
amplitude is slightly increased by the presence of the background
dynamo field, even if it is on average much weaker than the flux-rope's
field. The geometry and relative orientation of the magnetic field in
the flux-ropes with respect to that in the background magnetic field
influences the resulting rise speeds, zonal flows amplitudes (which
develop within the flux-ropes) and surface signatures of magnetic
flux emergence. This strongly influences the morphology, duration
and amplitude of the surface shearing and Poynting flux associated
with magnetic flux-rope emergence, which are key ingredients to the
current coronal eruption triggering scenarios. The actual magnetic
flux emergence is consistently preceded by strong and localised
radial velocity enhancements at the place where the flux rope will
emerge. The emerged magnetic flux is in most of the cases studied
enough to influence the global surface magnetic field. In some cases,
the emergence reinforces the system's global polarity reversal while
in some others it inhibits the background dynamo from doing so. The
fraction of magnetic flux which remains attached to the flux-rope
is slowly spread out in latitude, diffused and assimilated by the
background dynamo field.
Title: Soft X-ray emission in kink-unstable coronal loops
Authors: Pinto, Rui; Vilmer, Nicole; Brun, Allan Sacha
Bibcode: 2014cosp...40E2552P
Altcode:
Solar flares are associated with intense soft X-ray emission generated
by the hot flaring plasma in coronal magnetic loops. We investigate
the temporal, spectral and spatial evolution of the properties of the
thermal X-ray emission produced in simulated kink-unstable magnetic
flux-ropes. The numerical setup used consists of a highly twisted
loop embedded in a region of uniform and untwisted background coronal
magnetic field. The magnetic flux-rope reconnects with the background
flux after the triggering of the kink instability and is then allowed
to relax to a lower energy state. Strong ohmic heating leads to strong
and quick heating (up to more than 15 MK), to a strong peak of X-ray
emission and to the hardening of the thermal X-ray spectrum. The
emission pattern is often filamentary and the amount of twist deduced
from the X-ray emission alone is considerably lower than the maximum
twist in the simulated flux-ropes. The flux-rope plasma becomes strongly
multi-thermal during the flaring episode. The emission measure evolves
into a bi-modal distribution as a function of temperature during the
saturation phase, and later converges to the power-law distribution
mathrm{EM}~ T(-4.2) (during the relaxation/cooling) phase. These soft
X-ray emission properties are maintained for a large range of coronal
magnetic field strength, plasma density and flux-rope twist values.
Title: Solar wind and coronal rotation during an activity cycle
Authors: Pinto, Rui; Brun, Allan Sacha
Bibcode: 2014cosp...40E2551P
Altcode:
The properties of the solar wind flow are strongly affected by the
time-varying strength and geometry of the global background magnetic
field. The wind velocity and mass flux depend directly on the size and
position of the wind sources at the surface, and on the geometry of
the magnetic flux-tubes along which the wind flows. We address these
problems by performing numerical simulations coupling a kinematic
dynamo code (STELEM) evolve in a 2.5D axisymmetric coronal MHD code
(DIP) covering an 11 yr activity cycle. The latitudinal distribution
of the calculated wind velocities agrees with in-situ (ULYSSES,
HELIO) and radio measurements (IPS). The transition from fast to slow
wind flows can be explained in terms of the high overall flux-tube
superradial expansion factors in the vicinities of coronal streamer
boundaries. We found that the Alfvén radii and the global Sun's
mass loss rate vary considerably throughout the cycle (by a factor
4.5 and 1.6, respectively), leading to strong temporal modulations
of the global angular momentum flux and magnetic braking torque. The
slowly varying magnetic topology introduces strong non-uniformities
in the coronal rotation rate in the first few solar radii. Finally,
we point out directions to assess the effects of surface transient
phenomena on the global properties of the solar wind.
Title: The spectrometer telescope for imaging X-rays (STIX) on board
Solar Orbiter
Authors: Vilmer, Nicole; Krucker, Samuel; Karol Seweryn, D. .;
Orleanski, Piotr; Limousin, Olivier; Meuris, Aline; Brun, Allan Sacha;
Grimm, Oliver; Groebelbauer, HansPeter; Rendtel, J.
Bibcode: 2014cosp...40E3527V
Altcode:
The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10
instruments on board Solar Orbiter, a confirmed M-class mission of the
European Space Agency (ESA) within the Cosmic Vision program scheduled
to be launched in 2017. STIX applies a Fourier-imaging technique using
a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of
32 pixelized CdTe detectors to provide imaging spectroscopy of solar
thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The
paper presents the status of the instrument for the Critical Design
Review to be held with ESA in June 2014. Particular emphasis is given
to the CdTe hybrid detector called Caliste-SO for high resolution
hard X-ray spectroscopy from 4 to 150 keV: Characterizations of the
first production batch are reported. Caliste-SO spectrometer units
could also fulfill the needs for the SORENTO instrument of the Russian
Interhelioprobe mission currently in assessment study.
Title: 3D simulations of internal gravity waves in solar-like stars
Authors: Alvan, L.; Brun, A. -S.; Mathis, S.
Bibcode: 2013sf2a.conf...77A
Altcode:
We perform numerical simulations of the whole Sun using the 3D
anelastic ASH code. In such models, the radiative and convective zones
are non-linearly coupled and in the radiative interior a wave-like
pattern is observed. For the first time, we are thus able to modelize
in 3D the excitation and propagation of IGWs in a solar-like star's
radiative zone. We compare the properties of our waves to theoretical
predictions and results of oscillation calculations. The good agreement
obtained allow us to validate the consistency of our approach and to
study the characteristics of IGWs. In the 3D domain, we focus on the
excitation of IGWs and on the form of their spectrum where we suspect
that both g-modes and propagative waves are present.
Title: World-leading science with SPIRou - The nIR spectropolarimeter
/ high-precision velocimeter for CFHT
Authors: Delfosse, X.; Donati, J. -F.; Kouach, D.; Hébrard, G.; Doyon,
R.; Artigau, E.; Bouchy, F.; Boisse, I.; Brun, A. S.; Hennebelle, P.;
Widemann, T.; Bouvier, J.; Bonfils, X.; Morin, J.; Moutou, C.; Pepe,
F.; Udry, S.; do Nascimento, J. -D.; Alencar, S. H. P.; Castilho,
B. V.; Martioli, E.; Wang, S. Y.; Figueira, P.; Santos, N. C.
Bibcode: 2013sf2a.conf..497D
Altcode: 2013arXiv1310.2991D
SPIRou is a near-infrared (nIR) spectropolarimeter / velocimeter
proposed as a new-generation instrument for CFHT. SPIRou aims in
particular at becoming world-leader on two forefront science topics,
(i) the quest for habitable Earth-like planets around very- low-mass
stars, and (ii) the study of low-mass star and planet formation in
the presence of magnetic fields. In addition to these two main goals,
SPIRou will be able to tackle many key programs, from weather patterns
on brown dwarf to solar-system planet atmospheres, to dynamo processes
in fully-convective bodies and planet habitability. The science
programs that SPIRou proposes to tackle are forefront (identified
as first priorities by most research agencies worldwide), ambitious
(competitive and complementary with science programs carried out on
much larger facilities, such as ALMA and JWST) and timely (ideally
phased with complementary space missions like TESS and CHEOPS). SPIRou
is designed to carry out its science mission with maximum efficiency
and optimum precision. More specifically, SPIRou will be able to
cover a very wide single-shot nIR spectral domain (0.98-2.35 μm) at a
resolving power of 73.5K, providing unpolarized and polarized spectra
of low-mass stars with a ∼15% average throughput and a radial velocity
(RV) precision of 1 m/s.
Title: Dynamo Action and Magnetic Cycles in F-type Stars
Authors: Augustson, Kyle C.; Brun, Allan Sacha; Toomre, Juri
Bibcode: 2013ApJ...777..153A
Altcode:
Magnetic activity and differential rotation are commonly observed
features on main-sequence F-type stars. We seek to make contact with
such observations and to provide a self-consistent picture of how
differential rotation and magnetic fields arise in the interiors
of these stars. The three-dimensional magnetohydrodynamic anelastic
spherical harmonic code is employed to simulate global-scale convection
and dynamo processes in a 1.2 M ⊙ F-type star at two
rotation rates. The simulations are carried out in spherical shells
that encompass most of the convection zone and a portion of the stably
stratified radiative zone below it, allowing us to explore the effects
a stable zone has upon the morphology of the global-scale magnetic
fields. We find that dynamo action with a high degree of time variation
occurs in the star rotating more rapidly at 20 Ω⊙, with
the polarity of the mean field reversing on a timescale of about 1600
days. Between reversals, the magnetic energy rises and falls with a
fairly regular period, with three magnetic energy cycles required to
complete a reversal. The magnetic energy cycles and polarity reversals
arise due to a linking of the polar-slip instability in the stable
region and dynamo action present in the convection zone. For the more
slowly rotating case (10 Ω⊙), persistent wreaths of
magnetism are established and maintained by dynamo action. Compared
to their hydrodynamic progenitors, the dynamo states here involve a
marked reduction in the exhibited latitudinal differential rotation,
which also vary during the course of a cycle.
Title: Cycling Dynamo in a Young Sun: Grand Minima and Equatorward
Propagation
Authors: Augustson, Kyle; Brun, Allan Sacha; Miesch, Mark Steven;
Toomre, Juri
Bibcode: 2013arXiv1310.8417A
Altcode:
We assess the global-scale dynamo action achieved in a simulation of
a sun-like star rotating at three times the solar rate. The 3-D MHD
Anelastic Spherical Harmonic code, using slope-limited diffusion,
is employed to capture convection and dynamo processes in such a
young sun. The simulation is carried out in a spherical shell that
encompasses 3.8 density scale heights of the solar convection zone. We
find that dynamo action with a high degree of time variation occurs,
with many periodic polarity reversals every 6.2 years. The magnetic
energy also rises and falls with a regular period, with two magnetic
energy cycles required to complete a polarity cycle. These magnetic
energy cycles arise from a Lorentz-force feedback on the differential
rotation, whereas the polarity reversals are present due to the
spatial separation of the equatorial and polar dynamos. Moreover,
an equatorial migration of toroidal field is found, which is linked
to the changing differential rotation and to a near-surface shear
layer. This simulation also enters a grand minimum lasting roughly 20
years, after which the dynamo recovers its regular polarity cycles.
Title: On the role of asymmetries in the reversal of the solar
magnetic field
Authors: Brun, A. S.; Derosa, M. L.; Hoeksema, J. T.
Bibcode: 2013IAUS..294...75B
Altcode:
We study how the solar magnetic field evolves from antisymmetric
(dipolar) to symmetric (quadrupolar) state during the course of
its 11-yr cycle. We show that based on equatorial symmetries of the
induction equation, flux transport solar mean field dynamo models excite
mostly the antisymmetric (dipolar) family whereas a decomposition of the
solar magnetic field data reveals that both families should be excited
to similar amplitude levels. We propose an alternative solar dynamo
solution based on North-South asymmetry of the meridional circulation
to better reconcile models and observations.
Title: Flux Emergence in a Magnetized Convection Zone
Authors: Pinto, R. F.; Brun, A. S.
Bibcode: 2013ApJ...772...55P
Altcode: 2013arXiv1305.2159P
We study the influence of a dynamo magnetic field on the buoyant rise
and emergence of twisted magnetic flux ropes and their influence on the
global external magnetic field. We ran three-dimensional MHD numerical
simulations using the ASH code (anelastic spherical harmonics) and
analyzed the dynamical evolution of such buoyant flux ropes from the
bottom of the convection zone until the post-emergence phases. The
global nature of this model can only very crudely and inaccurately
represent the local dynamics of the buoyant rise of the implanted
magnetic structure, but nonetheless allows us to study the influence
of global effects, such as self-consistently generated differential
rotation and meridional circulation, and of Coriolis forces. Although
motivated by the solar context, this model cannot be thought of as a
realistic model of the rise of magnetic structures and their emergence
in the Sun, where the local dynamics are completely different. The
properties of initial phases of the buoyant rise are determined
essentially by the flux-rope's properties and the convective flows
and consequently are in good agreement with previous studies. However,
the effects of the interaction of the background dynamo field become
increasingly strong as the flux ropes evolve. During the buoyant rise
across the convection zone, the flux-rope's magnetic field strength
scales as Bvpropρα, with α <~ 1. An increase of
radial velocity, density, and current density is observed to precede
flux emergence at all longitudes. The geometry, latitude, and relative
orientation of the flux ropes with respect to the background magnetic
field influences the resulting rise speeds, zonal flow amplitudes
(which develop within the flux ropes), and the corresponding surface
signatures. This influences the morphology, duration and amplitude of
the surface shearing, and the Poynting flux associated with magnetic
flux-rope emergence. The emerged magnetic flux influences the system's
global polarity, leading in some cases to a polarity reversal while
inhibiting the background dynamo from doing so in others. The emerged
magnetic flux is slowly advected poleward while being diffused and
assimilated by the background dynamo field.
Title: On gravity waves in the Sun
Authors: Brun, Allan Sacha; Alvan, Lucie; Strugarek, Antoine; Mathis,
Stéphane; García, Rafael A.
Bibcode: 2013JPhCS.440a2043B
Altcode:
We briefly present our recent progress to model in 3-D the excitation
and propagation of internal waves in the deep solar radiative
interior. By modeling a rotating spherical convection zone on top of
a radiative interior with a realistic seismically calibrated stable
stratification (i.e solar-like Brunt-Väisälä frequency), we are
able to generate a large spectrum of internal waves and modes thanks
to the continuous pummeling of convective plumes. When comparing with
an adiabatic oscillation code we find a good overall agreement and
confirm that those waves are gravity waves.
Title: Magnetic Energy Cascade in Spherical Geometry. I. The Stellar
Convective Dynamo Case
Authors: Strugarek, A.; Brun, A. S.; Mathis, S.; Sarazin, Y.
Bibcode: 2013ApJ...764..189S
Altcode: 2013arXiv1301.1606S
We present a method to characterize the spectral transfers of
magnetic energy between scales in simulations of stellar convective
dynamos. The full triadic transfer functions are computed thanks to
analytical coupling relations of spherical harmonics based on the
Clebsch-Gordan coefficients. The method is applied to mean field αΩ
dynamo models as benchmark tests. From a physical standpoint, the
decomposition of the dynamo field into primary and secondary dynamo
families proves very instructive in the αΩ case. The same method is
then applied to a fully turbulent dynamo in a solar convection zone,
modeled with the three-dimensional MHD Anelastic Spherical Harmonics
code. The initial growth of the magnetic energy spectrum is shown
to be non-local. It mainly reproduces the kinetic energy spectrum
of convection at intermediate scales. During the saturation phase,
two kinds of direct magnetic energy cascades are observed in regions
encompassing the smallest scales involved in the simulation. The first
cascade is obtained through the shearing of the magnetic field by the
large-scale differential rotation that effectively cascades magnetic
energy. The second is a generalized cascade that involves a range
of local magnetic and velocity scales. Non-local transfers appear to
be significant, such that the net transfers cannot be reduced to the
dynamics of a small set of modes. The saturation of the large-scale
axisymmetric dipole and quadrupole is detailed. In particular, the
dipole is saturated by a non-local interaction involving the most
energetic scale of the magnetic energy spectrum, which points to the
importance of the magnetic Prandtl number for large-scale dynamos.
Title: Global dynamics of subsurface solar active regions
Authors: Jouve, L.; Brun, A. S.; Aulanier, G.
Bibcode: 2013ApJ...762....4J
Altcode: 2012arXiv1211.7251J
We present three-dimensional numerical simulations of a magnetic
loop evolving in either a convectively stable or unstable rotating
shell. The magnetic loop is introduced into the shell in such a
way that it is buoyant only in a certain portion in longitude, thus
creating an Ω-loop. Due to the action of magnetic buoyancy, the loop
rises and develops asymmetries between its leading and following legs,
creating emerging bipolar regions whose characteristics are similar
to those of observed spots at the solar surface. In particular, we
self-consistently reproduce the creation of tongues around the spot
polarities, which can be strongly affected by convection. We further
emphasize the presence of ring-shaped magnetic structures around our
simulated emerging regions, which we call "magnetic necklace" and
which were seen in a number of observations without being reported
as of today. We show that those necklaces are markers of vorticity
generation at the periphery and below the rising magnetic loop. We also
find that the asymmetry between the two legs of the loop is crucially
dependent on the initial magnetic field strength. The tilt angle of the
emerging regions is also studied in the stable and unstable cases and
seems to be affected both by the convective motions and the presence
of a differential rotation in the convective cases.
Title: Magnetic Wreaths and Cycles in Convective Dynamos
Authors: Nelson, Nicholas J.; Brown, Benjamin P.; Brun, Allan Sacha;
Miesch, Mark S.; Toomre, Juri
Bibcode: 2013ApJ...762...73N
Altcode: 2012arXiv1211.3129N
Solar-type stars exhibit a rich variety of magnetic activity. Seeking
to explore the convective origins of this activity, we have carried out
a series of global three-dimensional magnetohydrodynamic simulations
with the anelastic spherical harmonic code. Here we report on the
dynamo mechanisms achieved as the effects of artificial diffusion are
systematically decreased. The simulations are carried out at a nominal
rotation rate of three times the solar value (3 Ω⊙), but
similar dynamics may also apply to the Sun. Our previous simulations
demonstrated that convective dynamos can build persistent toroidal flux
structures (magnetic wreaths) in the midst of a turbulent convection
zone and that high rotation rates promote the cyclic reversal of
these wreaths. Here we demonstrate that magnetic cycles can also be
achieved by reducing the diffusion, thus increasing the Reynolds and
magnetic Reynolds numbers. In these more turbulent models, diffusive
processes no longer play a significant role in the key dynamical
balances that establish and maintain the differential rotation and
magnetic wreaths. Magnetic reversals are attributed to an imbalance
in the poloidal magnetic induction by convective motions that is
stabilized at higher diffusion levels. Additionally, the enhanced
levels of turbulence lead to greater intermittency in the toroidal
magnetic wreaths, promoting the generation of buoyant magnetic loops
that rise from the deep interior to the upper regions of our simulated
domain. The implications of such turbulence-induced magnetic buoyancy
for solar and stellar flux emergence are also discussed.
Title: On close-in magnetized star-planet interactions
Authors: Strugarek, A.; Brun, A. S.; Matt, S.
Bibcode: 2012sf2a.conf..419S
Altcode: 2013arXiv1301.5239S
We present 2D magnetohydrodynamic simulations performed with the PLUTO
code to model magnetized star-planet interactions. We study two simple
scenarios of magnetized star-planet interactions: the unipolar and
dipolar} interactions.Despite the simplified hypotheses we consider in
the model, the qualitative behavior of the interactions is very well
recovered. These encouraging results promote further developments
of the model to obtain predictions on the effect and the physical
manifestation of magnetized star--close-in planet interactions.
Title: 3D simulations of internal gravity waves in stellar interiors
Authors: Alvan, L.; Brun, A. S.; Mathis, S.
Bibcode: 2012sf2a.conf..289A
Altcode:
We investigate the excitation and propagation of internal gravity waves
by penetrative convective plumes using the 3D anelastic simulation
code ASH. The study of the waves' properties is of high importance
for helio- and asteroseismology and to understand how waves transport
angular momentum and may establish the observed rotation profile of the
solar radiative zone. After illustrating basic properties of g-modes
in terms of simple ray-theory, we show that the rich field of gravity
waves obtained with our 3D model is in good agreement with theoretical
predictions concerning the period spacing of g-modes.
Title: New Era in 3-D Modeling of Convection and Magnetic Dynamos
in Stellar Envelopes and Cores
Authors: Toomre, J.; Augustson, K. C.; Brown, B. P.; Browning, M. K.;
Brun, A. S.; Featherstone, N. A.; Miesch, M. S.
Bibcode: 2012ASPC..462..331T
Altcode:
The recent advances in asteroseismology and spectropolarimetry are
beginning to provide estimates of differential rotation and magnetic
structures for a range of F and G-type stars possessing convective
envelopes, and in A-type stars with convective cores. It is essential
to complement such observational work with theoretical studies based
on 3-D simulations of highly turbulent convection coupled to rotation,
shear and magnetic fields in full spherical geometries. We have so
employed the anelastic spherical harmonic (ASH) code, which deals
with compressible magnetohydrodynamics (MHD) in spherical shells, to
examine the manner in which the global-scale convection can establish
differential rotation and meridional circulations under current
solar rotation rates, and these make good contact with helioseismic
findings. For younger G stars rotating 3 to 5 times faster than
the current Sun, the convection establishes ever stronger angular
velocity contrasts between their fast equators and slow poles, and
these are accompanied by prominent latitudinal temperature contrasts as
well. Turning to MHD simulation of magnetic dynamo action within these
younger G stars, the resulting magnetism involves wreaths of strong
toroidal magnetic fields (up to 50 to 100 kG strengths) in the bulk
of the convection zone, typically of opposite polarity in the northern
and southern hemispheres. These fields can persist for long intervals
despite being pummeled by the fast convective downflows, but they can
also exhibit field reversals and cycles. Turning to shallower convective
envelopes in the more luminous F-type stars that range in mass from 1.2
to 1.4 solar masses and for various rotation rates, we find that the
convection can again establish solar-like differential rotation profiles
with a fast equator and slow poles, but the opposite is achieved at
the slower rotation rates. The F stars are also capable of building
strong magnetic fields, often as wreaths, through dynamo action. We
also consider dynamo action within the cores of rotating A-type stars,
finding that striking super-equipartition magnetic fields can be built
there. These families of 3-D simulations are showing that a new era of
detailed stellar modeling is becoming feasible through rapid advances
in supercomputing, and these have the potential to help interpret and
possibly even guide some of the observational efforts now under way.
Title: Convection and Differential Rotation in F-type Stars
Authors: Augustson, Kyle C.; Brown, Benjamin P.; Brun, Allan Sacha;
Miesch, Mark S.; Toomre, Juri
Bibcode: 2012ApJ...756..169A
Altcode:
Differential rotation is a common feature of main-sequence spectral
F-type stars. In seeking to make contact with observations and to
provide a self-consistent picture of how differential rotation is
achieved in the interiors of these stars, we use the three-dimensional
anelastic spherical harmonic (ASH) code to simulate global-scale
turbulent flows in 1.2 and 1.3 M ⊙ F-type stars
at varying rotation rates. The simulations are carried out in
spherical shells that encompass most of the convection zone and a
portion of the stably stratified radiative zone below it, allowing
us to explore the effects of overshooting convection. We examine
the scaling of the mean flows and thermal state with rotation rate
and mass and link these scalings to fundamental parameters of the
simulations. Indeed, we find that the differential rotation becomes
much stronger with more rapid rotation and larger mass, scaling as
ΔΩvpropM 3.9Ω0.6 0. Accompanying the
growing differential rotation is a significant latitudinal temperature
contrast, with amplitudes of 1000 K or higher in the most rapidly
rotating cases. This contrast in turn scales with mass and rotation
rate as ΔTvpropM 6.4Ω1.6 0. On
the other hand, the meridional circulations become much weaker with
more rapid rotation and with higher mass, with their kinetic energy
decreasing as KEMCvpropM -1.2Ω-0.8
0. Additionally, three of our simulations exhibit
a global-scale shear instability within their stable regions that
persists for the duration of the simulations. The flow structures
associated with the instabilities have a direct coupling to and impact
on the flows within the convection zone.
Title: Fast Rotating Solar-like Stars Using Asteroseismic Datasets
Authors: García, R. A.; Ceillier, T.; Campante, T. L.; Davies, G. R.;
Mathur, S.; Suárez, J. C.; Ballot, J.; Benomar, O.; Bonanno, A.;
Brun, A. S.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Deheuvels,
S.; Elsworth, Y.; Handberg, R.; Hekker, S.; Jiménez, A.; Karoff, C.;
Kjeldsen, H.; Mathis, S.; Mosser, B.; Pallé, P. L.; Pinsonneault, M.;
Régulo, C.; Salabert, D.; Silva Aguirre, V.; Stello, D.; Thompson,
M. J.; Verner, G.; PE11 Team of Kepler WG#1
Bibcode: 2012ASPC..462..133G
Altcode: 2011arXiv1109.6488G
The NASA Kepler mission is providing an unprecedented set of
asteroseismic data. In particular, short-cadence light-curves (∼ 60
s samplings), allow us to study solar-like stars covering a wide range
of masses, spectral types and evolutionary stages. Oscillations have
been observed in around 600 out of 2000 stars observed for one month
during the survey phase of the Kepler mission. The measured light
curves can present features related to the surface magnetic activity
(starspots) and, thus we are able to obtain a good estimate of the
surface (differential) rotation. In this work we establish the basis
of such research and we show a potential method to find stars with
fast surface rotation.
Title: Solar Magnetic Field Reversals and the Role of Dynamo Families
Authors: DeRosa, M. L.; Brun, A. S.; Hoeksema, J. T.
Bibcode: 2012ApJ...757...96D
Altcode: 2012arXiv1208.1768D
The variable magnetic field of the solar photosphere exhibits periodic
reversals as a result of dynamo activity occurring within the solar
interior. We decompose the surface field as observed by both the Wilcox
Solar Observatory and the Michelson Doppler Imager into its harmonic
constituents, and present the time evolution of the mode coefficients
for the past three sunspot cycles. The interplay between the various
modes is then interpreted from the perspective of general dynamo
theory, where the coupling between the primary and secondary families
of modes is found to correlate with large-scale polarity reversals
for many examples of cyclic dynamos. Mean-field dynamos based on the
solar parameter regime are then used to explore how such couplings may
result in the various long-term trends in the surface magnetic field
observed to occur in the solar case.
Title: Understanding the Solar Inner Magnetism and Dynamics
Authors: Brun, A. S.; Strugarek, A.
Bibcode: 2012ASPC..454....3B
Altcode:
The observations of solar magnetic activity by the satellite Hinode
confirm the large range of spatial and temporal scales present on the
Sun's surface and the complexity of the flows and fields. How such
magnetic field is generated, amplified, maintained and emerge over the
course of the solar cycle is key to determine in order to progress
in our understanding of the Sun. It is believed that dynamo action
in and at the base of the convective envelope is the main source of
solar magnetism. Further the radiative interior of the Sun may also
posses a primordial field whose influences on the dynamo generated
field needs to be studied. We propose in this paper to make a brief
review of our effort to model in 3-D the Sun's inner magnetism and
the coupling between its convective and radiative zones and how such
magnetism may emerge at the solar surface.
Title: Convection and differential rotation properties of G and K
stars computed with the ASH code
Authors: Matt, S. P.; Do Cao, O.; Brown, B. P.; Brun, A. S.
Bibcode: 2011AN....332..897M
Altcode: 2011arXiv1111.5585M
The stellar luminosity and depth of the convective envelope vary rapidly
with mass for G- and K-type main sequence stars. In order to understand
how these properties influence the convective turbulence, differential
rotation, and meridional circulation, we have carried out 3D dynamical
simulations of the interiors of rotating main sequence stars, using the
anelastic spherical harmonic (ASH) code. The stars in our simulations
have masses of 0.5, 0.7, 0.9, and 1.1 M_⊙, corresponding to spectral
types K7 through G0, and rotate at the same angular speed as the Sun. We
identify several trends of convection zone properties with stellar mass,
exhibited by the simulations. The convective velocities, temperature
contrast between up- and downflows, and meridional circulation
velocities all increase with stellar luminosity. As a consequence
of the trend in convective velocity, the Rossby number (at a fixed
rotation rate) increases and the convective turnover timescales decrease
significantly with increasing stellar mass. The three lowest mass cases
exhibit solar-like differential rotation, in a sense that they show
a maximum rotation at the equator and minimum at higher latitudes,
but the 1.1 M_⊙ case exhibits anti-solar rotation. At low mass, the
meridional circulation is multi-cellular and aligned with the rotation
axis; as the mass increases, the circulation pattern tends toward a
unicellular structure covering each hemisphere in the convection zone.
Title: Modeling the Dynamical Coupling of Solar Convection with the
Radiative Interior
Authors: Brun, Allan Sacha; Miesch, Mark S.; Toomre, Juri
Bibcode: 2011ApJ...742...79B
Altcode:
The global dynamics of a rotating star like the Sun involves the
coupling of a highly turbulent convective envelope overlying a
seemingly benign radiative interior. We use the anelastic spherical
harmonic code to develop a new class of three-dimensional models
that nonlinearly couple the convective envelope to a deep stable
radiative interior. The numerical simulation assumes a realistic solar
stratification from r = 0.07 up to 0.97R (with R the solar radius),
thus encompassing part of the nuclear core up through most of the
convection zone. We find that a tachocline naturally establishes itself
between the differentially rotating convective envelope and the solid
body rotation of the interior, with a slow spreading that is here
diffusively controlled. The rapid angular momentum redistribution in
the convective envelope leads to a fast equator and slow poles, with a
conical differential rotation achieved at mid-latitudes, much as has
been deduced by helioseismology. The convective motions are able to
overshoot downward about 0.04R into the radiative interior. However,
the convective meridional circulation there is confined to a smaller
penetration depth and is directed mostly equatorward at the base
of the convection zone. Thermal wind balance is established in the
lower convection zone and tachocline but departures are evident in
the upper convection zone. Internal gravity waves are excited by the
convective overshooting, yielding a complex wave field throughout the
radiative interior.
Title: Towards a 3D dynamo model of the PMS star BP Tau
Authors: Bessolaz, N.; Brun, A. S.
Bibcode: 2011AN....332.1045B
Altcode:
Studying how convective and magnetic properties of pre-main sequence
stars change during their evolution towards the zero-age main
sequence is a growing area of research triggered by the development
of efficient spectropolarimeters. 3D simulations can help to identify
the key parameters to understand the diversity (strength, topology) of
magnetic fields observed. We present results of a dynamo computation
done with the ASH code for a 0.7 M_⊙ pre-main sequence star with
a 7.6 day rotation period which is nearly fully convective, using a
realistic stratification contrast to resolve 90 % of the convective
zone. This star corresponds to the target star BP Tau already observed
with spectropolarimetry (Donati et al. 2008). We particularly compare
the magnetic field properties found in our simulation with the
observational constraints.
Title: Global-scale Magnetism (and Cycles) in Dynamo Simulations of
Stellar Convection Zones
Authors: Brown, B. P.; Browning, M. K.; Brun, A. S.; Miesch, M. S.;
Toomre, J.
Bibcode: 2011ASPC..448..277B
Altcode: 2011arXiv1101.0171B; 2011csss...16..277B
Young solar-type stars rotate rapidly and are very magnetically
active. The magnetic fields at their surfaces likely originate in their
convective envelopes where convection and rotation can drive strong
dynamo action. Here we explore simulations of global-scale stellar
convection in rapidly rotating suns using the 3-D MHD anelastic
spherical harmonic (ASH) code. The magnetic fields built in these
dynamos are organized on global-scales into wreath-like structures
that span the convection zone. We explore one case rotates five times
faster than the Sun in detail. This dynamo simulation, called case
D5, has repeated quasi-cyclic reversals of global-scale polarity. We
compare this case D5 to the broader family of simulations we have been
able to explore and discuss how future simulations and observations
can advance our understanding of stellar dynamos and magnetism.
Title: The 3D Nature of Convective Dynamos
Authors: Miesch, M.; Brown, B.; Nelson, N.; Browning, M.; Brun, A. S.;
Toomre, J.
Bibcode: 2011AGUFMSH23D..01M
Altcode:
Solar observations throughout the extended minimum between cyles 23 and
24 have highlighted the intrinsically three-dimensional (3D) nature of
the solar magnetic field. These include prominent multipolar components
and low-latitude coronal holes observed with STEREO, asymmetric
surface flux distributions in photospheric magnetograms, ond global,
multi-scale magnetic linkages revealed by SDO. Axisymmetric mean-field
dynamo models cannot capture this complexity, which ultimately arises
from turbulent convection. The solar dynamo is a convective dynamo;
convection is clearly responsible for the diversity of solar magnetic
activity we observe, generating and organizing magnetic fields both
directly by turbulent induction and indirectly via mean flows and MHD
instabilities. Simulations of convective dynamos reveal the 3D nature
of how large-scale magnetic fields are generated and provide insight
into the intricate topology of the solar magnetic field, apparent
even during solar minimum. I will describe recent work on the role of
helicity and shear in magnetic self-organization and promising first
steps toward linking convective dynamos with flux emergence.
Title: Magnetic confinement of the solar tachocline: The oblique
dipole
Authors: Strugarek , A.; Brun, A. S.; Zahn, J. -P.
Bibcode: 2011AN....332..891S
Altcode: 2011arXiv1112.1319A
3D MHD global solar simulations coupling the turbulent convective
zone and the radiative zone have been carried out. Essential features
of the Sun such as differential rotation, meridional circulation and
internal waves excitation are recovered. These realistic models are
used to test the possibility of having the solar tachocline confined by
a primordial inner magnetic field. We find that the initially confined
magnetic fields we consider open into the convective envelope. Angular
momentum is transported across the two zones by magnetic torques and
stresses, establishing the so-called Ferarro's law of isorotation. In
the parameter space studied, the confinement of the magnetic field by
meridional circulation penetration fails, also implying the failure of
the tachocline confinement by the magnetic field. Three-dimensional
convective motions are proven responsible for the lack of magnetic
field confinement. Those results are robust for the different magnetic
field topologies considered, i.e. aligned or oblique dipole.
Title: Effects of turbulent pumping on stellar activity cycles
Authors: Do Cao, O.; Brun, A. S.
Bibcode: 2011AN....332..907D
Altcode: 2011arXiv1112.1321D
Stellar magnetic activity of solar like stars is thought to be due
to an internal dynamo. While the Sun has been the subject of intense
research for refining dynamo models, observations of magnetic cyclic
activity in solar type stars have become more and more available,
opening a new path to understand the underlying physics behind stellar
cycles. For instance, it is key to understand how stellar rotation rate
influences magnetic cycle period P_cyc. Recent numerical simulations
of advection-dominated Babcock Leighton models have demonstrated that
it is difficult to explain this observed trend given a) the strong
influence of the cycle period to the meridional circulation amplitude
and b) the fact that 3D models indicate that meridional flows become
weaker as the rotation rate increases. In this paper, we introduce
the turbulent pumping mechanism as another advective process capable
also of transporting the magnetic fields. We found that this model
is now able to reproduce the observations under the assumption that
this effect increases as \Omega2. The turbulent pumping
becomes indeed another major player able to circumvent the meridional
circulation. However, for high rotation rates (\Omega ≃ 5 \Omega_⊙),
its effects dominate those of the meridional circulation, entering a
new class of regime dominated by the advection of turbulent pumping
and thus leading to a cyclic activity qualitatively different from
that of the Sun.
Title: Anelastic convection-driven dynamo benchmarks
Authors: Jones, C. A.; Boronski, P.; Brun, A. S.; Glatzmaier, G. A.;
Gastine, T.; Miesch, M. S.; Wicht, J.
Bibcode: 2011Icar..216..120J
Altcode:
Benchmark solutions for fully nonlinear anelastic compressible
convection and dynamo action in a rotating spherical shell are
proposed. Three benchmarks are specified. The first is a purely
hydrodynamic case, which is steady in a uniformly drifting frame. The
second is a self-excited saturated dynamo solution, also steady in
a drifting frame. The third is again a self-excited dynamo but is
unsteady in time, and it has a higher Rayleigh number than the steady
dynamo benchmark. Four independent codes have been tested against
these benchmarks, and very satisfactory agreement has been found. This
provides an accurate reference standard against which new anelastic
codes can be tested.
Title: Exploring the Deep Convection and Magnetism of A-type stars
Authors: Featherstone, Nicholas; Browning, Matthew; Brun, Allan Sacha;
Toomre, Juri
Bibcode: 2011APS..DPPN10003F
Altcode:
A-type stars have both a near-surface layer of fast convection that
can excite acoustic modes and a deep zone of core convection whose
properties may be probed with asteroseismology. Many A-type stars also
exhibit large magnetic spots that are often attributed to surviving
primordial fields of global scale in the intervening radiative zone. We
have explored the potential for core convection in rotating A-type
stars to build strong magnetic fields through dynamo action. Using the
ASH code, we model the inner 30% by radius of a two solar mass A-type
star, rotating at four times the solar rate and capturing the convective
core and a portion of the overlying radiative envelope. Convection in
these stars drives a strong retrograde differential rotation and yields
a core that is prolate in shape. When dynamo action is admitted, the
convection generates strong magnetic fields largely in equipartition
with the dynamics. Remarkably, introducing a modest but large-scale
external field threading the radiative envelope (which may be of
primordial origin) can substantially alter the turbulent dynamics
of the convective interior. The resulting convection establishes a
complex assembly of helical rolls that link distant portions of the
core and yield magnetic fields of super-equipartition strength.
Title: Buoyant Magnetic Loops in a Global Dynamo Simulation of a
Young Sun
Authors: Nelson, Nicholas J.; Brown, Benjamin P.; Brun, Allan Sacha;
Miesch, Mark S.; Toomre, Juri
Bibcode: 2011ApJ...739L..38N
Altcode: 2011arXiv1108.4697N
The current dynamo paradigm for the Sun and Sun-like stars places the
generation site for strong toroidal magnetic structures deep in the
solar interior. Sunspots and starspots on Sun-like stars are believed
to arise when sections of these magnetic structures become buoyantly
unstable and rise from the deep interior to the photosphere. Here, we
present the first three-dimensional global magnetohydrodynamic (MHD)
simulation in which turbulent convection, stratification, and rotation
combine to yield a dynamo that self-consistently generates buoyant
magnetic loops. We simulate stellar convection and dynamo action in
a spherical shell with solar stratification, but rotating three times
faster than the current solar rate. Strong wreaths of toroidal magnetic
field are realized by dynamo action in the convection zone. By turning
to a dynamic Smagorinsky model for subgrid-scale turbulence, we here
attain considerably reduced diffusion in our simulation. This permits
the regions of strongest magnetic field in these wreaths to rise toward
the top of the convection zone via a combination of magnetic buoyancy
instabilities and advection by convective giant cells. Such a global
simulation yielding buoyant loops represents a significant step forward
in combining numerical models of dynamo action and flux emergence.
Title: Astrophysical Dynamics: From Stars to Galaxies
Authors: Brummell, Nicholas H.; Brun, A. Sacha; Miesch, Mark S.;
Ponty, Yannick
Bibcode: 2011IAUS..271.....B
Altcode:
Preface; 1. The Sun and stars: observational constraints, theories and
models; 2. Galaxies: observational constraints, theories and models;
3. Nonlinear astrophysics; 4. Cosmic magnetism; 5. Astrophysical
turbulence; 6. Posters; Author index; Subject index.
Title: Convection and dynamo action in B stars
Authors: Augustson, Kyle C.; Brun, Allan S.; Toomre, Juri
Bibcode: 2011IAUS..271..361A
Altcode: 2010arXiv1011.1016A
Main-sequence massive stars possess convective cores that likely
harbor strong dynamo action. To assess the role of core convection
in building magnetic fields within these stars, we employ the 3-D
anelastic spherical harmonic (ASH) code to model turbulent dynamics
within a 10 Msolar main-sequence (MS) B-type star rotating
at 4 Ωsolar. We find that strong (900 kG) magnetic fields
arise within the turbulence of the core and penetrate into the stably
stratified radiative zone. These fields exhibit complex, time-dependent
behavior including reversals in magnetic polarity and shifts between
which hemisphere dominates the total magnetic energy.
Title: Magnetic confinement of the solar tachocline: influence of
turbulent convective motions
Authors: Strugarek, Antoine; Brun, Allan Sacha; Zahn, Jean-Paul
Bibcode: 2011IAUS..271..399S
Altcode:
We present the results of 3D simulations, performed with the ASH
code, of the nonlinear, magnetic coupling between the convective and
radiative zones in the Sun, through the tachocline. Contrary to the
predictions of Gough & McIntyre (1998), a fossil magnetic field,
deeply buried initially in the solar interior, will penetrate into
the convection zone. According to Ferraro's law of iso-rotation, the
differential rotation of the convective zone will thus expand into
the radiation zone, along the field lines of the poloidal field.
Title: Dipolar and Quadrupolar Magnetic Field Evolution over Solar
Cycles 21, 22, and 23
Authors: DeRosa, M. L.; Brun, A. S.; Hoeksema, J. T.
Bibcode: 2011IAUS..271...94D
Altcode:
Time series of photospheric magnetic field maps from two observatories,
along with data from an evolving surface-flux transport model,
are decomposed into their constituent spherical harmonic modes. The
evolution of these spherical harmonic spectra reflect the modulation
of bipole emergence rates through the solar activity cycle, and the
subsequent dispersal, shear, and advection of magnetic flux patterns
across the solar photosphere. In this article, we discuss the evolution
of the dipolar and quadrupolar modes throughout the past three solar
cycles (Cycles 21-23), as well as their relation to the reversal of
the polar dipole during each solar maximum, and by extension to aspects
of the operation of the global solar dynamo.
Title: Global-scale wreath-building dynamos in stellar convection
zones
Authors: Brown, Benjamin P.; Browning, Matthew K.; Brun, Allan Sacha;
Miesch, Mark S.; Toomre, Juri
Bibcode: 2011IAUS..271...78B
Altcode: 2010arXiv1011.0445B
When stars like our Sun are young they rotate rapidly and are very
magnetically active. We explore dynamo action in rapidly rotating suns
with the 3-D MHD anelastic spherical harmonic (ASH) code. The magnetic
fields built in these dynamos are organized on global-scales into
wreath-like structures that span the convection zone. Wreath-building
dynamos can undergo quasi-cyclic reversals of polarity and such behavior
is common in the parameter space we have been able to explore. These
dynamos do not appear to require tachoclines to achieve their spatial
or temporal organization. Wreath-building dynamos are present to some
degree at all rotation rates, but are most evident in the more rapidly
rotating simulations.
Title: Global magnetic cycles in rapidly rotating younger suns
Authors: Nelson, Nicholas J.; Brown, Benjamin P.; Browning, Matthew
K.; Brun, Allan Sacha; Miesch, Mark S.; Toomre, Juri
Bibcode: 2011IAUS..273..272N
Altcode: 2010arXiv1010.6073N
Observations of sun-like stars rotating faster than our current
sun tend to exhibit increased magnetic activity as well as magnetic
cycles spanning multiple years. Using global simulations in spherical
shells to study the coupling of large-scale convection, rotation,
and magnetism in a younger sun, we have probed effects of rotation
on stellar dynamos and the nature of magnetic cycles. Major 3-D MHD
simulations carried out at three times the current solar rotation
rate reveal hydromagnetic dynamo action that yields wreaths of strong
toroidal magnetic field at low latitudes, often with opposite polarity
in the two hemispheres. Our recent simulations have explored behavior in
systems with considerably lower diffusivities, achieved with sub-grid
scale models including a dynamic Smagorinsky treatment of unresolved
turbulence. The lower diffusion promotes the generation of magnetic
wreaths that undergo prominent temporal variations in field strength,
exhibiting global magnetic cycles that involve polarity reversals. In
our least diffusive simulation, we find that magnetic buoyancy coupled
with advection by convective giant cells can lead to the rise of
coherent loops of magnetic field toward the top of the simulated domain.
Title: Magnetic Cycles and Meridional Circulation in Global Models
of Solar Convection
Authors: Miesch, Mark S.; Brown, Benjamin P.; Browning, Matthew K.;
Brun, Allan Sacha; Toomre, Juri
Bibcode: 2011IAUS..271..261M
Altcode: 2010arXiv1009.6184M
We review recent insights into the dynamics of the solar convection
zone obtained from global numerical simulations, focusing on two recent
developments in particular. The first is quasi-cyclic magnetic activity
in a long-duration dynamo simulation. Although mean fields comprise
only a few percent of the total magnetic energy they exhibit remarkable
order, with multiple polarity reversals and systematic variability
on time scales of 6-15 years. The second development concerns the
maintenance of the meridional circulation. Recent high-resolution
simulations have captured the subtle nonlinear dynamical balances with
more fidelity than previous, more laminar models, yielding more coherent
circulation patterns. These patterns are dominated by a single cell in
each hemisphere, with poleward and equatorward flow in the upper and
lower convection zone respectively. We briefly address the implications
of and future of these modeling efforts.
Title: Exploring the deep convection and magnetism of A-type stars
Authors: Featherstone, Nicholas A.; Browning, Matthew K.; Brun,
Allan Sacha; Toomre, Juri
Bibcode: 2011IAUS..273..111F
Altcode:
A-type stars have both a near-surface layer of fast convection that
can excite acoustic modes and a deep zone of core convection whose
properties may be probed with asteroseismology. Many A-type stars also
exhibit large magnetic spots that are often attributed to surviving
primordial fields of global scale in the intervening radiative zone. We
have explored the potential for core convection in rotating A-type
stars to build strong magnetic fields through dynamo action. These
3-D simulations using the ASH code provide guidance on the nature
of differential rotation and magnetic fields that may be present in
the deep interiors of these stars, thus informing the asteroseismic
deductions now becoming feasible. Our models encompass the inner 30%
by radius of a two solar mass A-type star, rotating at four times
the solar rate and capturing the convective core and a portion of the
overlying radiative envelope. Convection in these stars drives a strong
retrograde differential rotation and yields a core that is prolate in
shape. When dynamo action is admitted, the convection generates strong
magnetic fields largely in equipartition with the dynamics. Remarkably,
introducing a modest but large-scale external field threading the
radiative envelope (which may be of primordial origin) can substantially
alter the turbulent dynamics of the convective interior. The resulting
convection involves a complex assembly of helical rolls that link
distant portions of the core and stretch and advect magnetic field,
ultimately yielding magnetic fields of super-equipartition strength.
Title: Hunting down giant cells in deep stellar convective zones
Authors: Bessolaz, Nicolas; Brun, Allan Sacha
Bibcode: 2011IAUS..271..365B
Altcode:
3D high resolution simulations for the convective zone of a 4Myr
old 0.7 Msolar pre-main sequence star in gravitational
contraction are carried out with different radial density contrast
using the pseudo spectral ASH code (Brun et al. 2004). We extract
giant cells signal from the complex surface convective patterns by
using a wavelet analysis. We then characterize them by estimating
their lifetime and rotation rate according to the density contrast.
Title: Magnetic confinement of the solar tachocline: II. Coupling
to a convection zone
Authors: Strugarek, A.; Brun, A. S.; Zahn, J. -P.
Bibcode: 2011A&A...532A..34S
Altcode: 2011arXiv1107.3665S
Context. The reason for the observed thinness of the solar tachocline
is still not well understood. One of the explanations that have been
proposed is that a primordial magnetic field renders the rotation
uniform in the radiation zone.
Aims: We test here the validity
of this magnetic scenario through 3D numerical MHD simulations that
encompass both the radiation zone and the convection zone.
Methods: The numerical simulations are performed with the anelastic
spherical harmonics (ASH) code. The computational domain extends from
0.07R⊙ to 0.97R⊙.
Results: In the
parameter regime we explored, a dipolar fossil field aligned with
the rotation axis cannot remain confined in the radiation zone. When
the field lines are allowed to interact with turbulent unstationary
convective motions at the base of the convection zone, 3D effects
prevent the field confinement.
Conclusions: In agreement with
previous work, we find that a dipolar fossil field, even when it is
initially buried deep inside the radiation zone, will spread into the
convective zone. According to Ferraro's law of iso-rotation, it then
imprints on the radiation zone the latitudinal differential rotation
of the convection zone, which is not observed.
Title: Coupling the Solar Dynamo and the Corona: Wind Properties,
Mass, and Momentum Losses during an Activity Cycle
Authors: Pinto, Rui F.; Brun, Allan Sacha; Jouve, Laurène; Grappin,
Roland
Bibcode: 2011ApJ...737...72P
Altcode: 2011arXiv1106.0882P
We study the connections between the Sun's convection zone and the
evolution of the solar wind and corona. We let the magnetic fields
generated by a 2.5-dimensional (2.5D) axisymmetric kinematic dynamo
code (STELEM) evolve in a 2.5D axisymmetric coronal isothermal
magnetohydrodynamic code (DIP). The computations cover an 11 year
activity cycle. The solar wind's asymptotic velocity varies in latitude
and in time in good agreement with the available observations. The
magnetic polarity reversal happens at different paces at different
coronal heights. Overall the Sun's mass-loss rate, momentum flux, and
magnetic braking torque vary considerably throughout the cycle. This
cyclic modulation is determined by the latitudinal distribution of the
sources of open flux and solar wind and the geometry of the Alfvén
surface. Wind sources and braking torque application zones also vary
accordingly.
Title: Assimilating Data into an αΩ Dynamo Model of the Sun:
A Variational Approach
Authors: Jouve, Laurène; Brun, Allan Sacha; Talagrand, Olivier
Bibcode: 2011ApJ...735...31J
Altcode: 2011arXiv1105.0626J
We have developed a variational data assimilation technique for the Sun
using a toy αΩ dynamo model. The purpose of this work is to apply
modern data assimilation techniques to solar data using a physically
based model. This work represents the first step toward a complete
variational model of solar magnetism. We derive the adjoint αΩ dynamo
code and use a minimization procedure to invert the spatial dependence
of key physical ingredients of the model. We find that the variational
technique is very powerful and leads to encouraging results that will
be applied to a more realistic model of the solar dynamo.
Title: Magnetic Cycles in a Convective Dynamo Simulation of a Young
Solar-type Star
Authors: Brown, Benjamin P.; Miesch, Mark S.; Browning, Matthew K.;
Brun, Allan Sacha; Toomre, Juri
Bibcode: 2011ApJ...731...69B
Altcode: 2011arXiv1102.1993B
Young solar-type stars rotate rapidly and many are magnetically
active. Some appear to undergo magnetic cycles similar to the 22 yr
solar activity cycle. We conduct simulations of dynamo action in rapidly
rotating suns with the three-dimensional magnetohydrodynamic anelastic
spherical harmonic (ASH) code to explore dynamo action achieved in
the convective envelope of a solar-type star rotating at five times
the current solar rotation rate. We find that dynamo action builds
substantial organized global-scale magnetic fields in the midst of the
convection zone. Striking magnetic wreaths span the convection zone
and coexist with the turbulent convection. A surprising feature of this
wreath-building dynamo is its rich time dependence. The dynamo exhibits
cyclic activity and undergoes quasi-periodic polarity reversals where
both the global-scale poloidal and toroidal fields change in sense on
a roughly 1500 day timescale. These magnetic activity patterns emerge
spontaneously from the turbulent flow and are more organized temporally
and spatially than those realized in our previous simulations of the
solar dynamo. We assess in detail the competing processes of magnetic
field creation and destruction within our simulations that contribute to
the global-scale reversals. We find that the mean toroidal fields are
built primarily through an Ω-effect, while the mean poloidal fields
are built by turbulent correlations which are not well represented by
a simple α-effect. During a reversal the magnetic wreaths propagate
toward the polar regions, and this appears to arise from a poleward
propagating dynamo wave. As the magnetic fields wax and wane in
strength and flip in polarity, the primary response in the convective
flows involves the axisymmetric differential rotation which varies on
similar timescales. Bands of relatively fast and slow fluid propagate
toward the poles on timescales of roughly 500 days and are associated
with the magnetic structures that propagate in the same fashion. In
the Sun, similar patterns are observed in the poleward branch of the
torsional oscillations, and these may represent poleward propagating
magnetic fields deep below the solar surface.
Title: Hunting for Giant Cells in Deep Stellar Convective Zones
Using Wavelet Analysis
Authors: Bessolaz, Nicolas; Brun, Allan Sacha
Bibcode: 2011ApJ...728..115B
Altcode: 2011arXiv1101.1943B
We study the influence of stratification on stellar turbulent
convection near the stellar surface and at various depths by carrying
out three-dimensional, high-resolution hydrodynamic simulations with
the Anelastic Spherical Harmonic code. Four simulations with different
radial-density contrasts corresponding to different aspect ratios for
the same underlying 4 Myr, 0.7 M sun pre-main-sequence star
model are performed. We highlight the existence of giant cells that are
embedded in the complex surface convective patterns using a wavelet
and time-correlation analysis. Next, we study their properties, such
as lifetime, aspect ratio, and spatial extension, in the different
models according to the density contrast. We find that these giant
cells have a lifetime larger than the stellar period, with a typical
longitudinal width of 490 Mm and a latitudinal extension increasing with
the radial-density contrast, surpassing 50° in the thickest convective
zone. Their rotation rate is much larger than the local differential
rotation rate, also increasing with radial-density contrast. However,
their spatial coherence as a function of depth decreases with density
contrast due to the stronger shear present in these more stratified
cases.
Title: Magnetic Cycles in a Wreath-Building Dynamo Simulation of a
Young Solar-type Star
Authors: Brown, Benjamin; Miesch, M. S.; Browning, M. K.; Brun, A. S.;
Nelson, N. J.; Toomre, J.
Bibcode: 2011AAS...21724222B
Altcode: 2011BAAS...4324222B
Stars like the Sun build global-scale magnetic fields though dynamo
processes in their convection zones. There, global-scale plasma motions
couple with rotation and likely drive cycles of magnetic activity,
though the exact processes at work in solar and stellar dynamos remain
elusive. Observations of younger suns indicate that they rotate quite
rapidly, have strong magnetic fields at their surfaces, and show
signs of cyclic activity. Here we explore recent 3-D MHD simulations
of younger, more rapidly rotating solar-type stars conducted with
the anelastic spherical harmonic (ASH) code. These simulations of
global-scale convection and dynamo action produce strikingly organized
magnetic structures in the bulk of their convection zones. Wreaths of
magnetic field fill the convection zone and can undergo regular cycles
of polarity reversal. Indeed, we find that cyclic behavior is a common
feature throughout the parameter space we have explored. Though these
magnetic wreaths can coexist with tachoclines of penetration and shear,
they do not rely on that internal boundary layer for their formation or
persistence. Tachoclines may play a less critical role in the stellar
dynamos of younger Suns than has been supposed in solar dynamo theory.
Title: Assessing the Deep Interior Dynamics and Magnetism of A-type
Stars
Authors: Featherstone, Nicholas A.; Browning, Matthew K.; Brun,
Allan Sacha; Toomre, Juri
Bibcode: 2011JPhCS.271a2068F
Altcode:
A-type stars have both a shallow near-surface zone of fast convection
that can excite acoustic modes and a deep zone of core convection
whose properties may be studied through asteroseismology. Many A stars
also exhibit large magnetic spots as they rotate. We have explored the
properties of core convection in rotating A-type stars and their ability
to build strong magnetic fields. These 3-D simulations using the ASH
code may serve to inform asteroseismic deductions of interior rotation
and magnetism that are now becoming feasible. Our models encompass the
inner 30% by radius of a 2 solar mass A-type star, capturing both the
convective core and some of the overlying radiative envelope. Convection
can drive a column of strong retrograde differential rotation and
yield a core prolate in shape. When dynamo action is admitted, the
convection is able to generate strong magnetic fields largely in
equipartition with the dynamics. Introducing a modest external field
(which may be of primordial origin) into the radiative envelope can
substantially alter the turbulent dynamics of the convective core,
yielding magnetic fields of remarkable super-equipartition strength. The
turbulent convection involves a complex assembly of helical rolls that
link distant portions of the core and stretch and advect magnetic field
into broad swathes of strong toroidal field. These simulations reveal
that supercomputing is providing a perspective of the deep dynamics
that may become testable with asteroseismology for these stars.
Title: Visualization with SDvision of ASH Stellar MHD Simulations
Authors: Pomaréde, D.; Brun, A.
Bibcode: 2010ASPC..434..378P
Altcode: 2010adass..19..378P
Numerical simulation s are playing a leading role in the study
of astrophysical objects. The ASH program is used to perform
high-resolution three-dimensional simulations of the MHD processes
occurring in the convection zone of the Sun and other stars. The size
and complexity of the data produced in these simulations require to use
special software tools at the post-treatment, visualization and analysis
stages. The SDvision graphical interface is developed to provide an
interactive and immersive visualization of such data. In this paper, we
describe the different rendering capabilities provided by this program.
Title: Stochastic excitation of gravity modes in massive main-sequence
stars
Authors: Samadi, R.; Belkacem, K.; Goupil, M. J.; Dupret, M. -A.;
Brun, A. S.; Noels, A.
Bibcode: 2010Ap&SS.328..253S
Altcode: 2009Ap&SS.tmp..240S
We investigate the possibility that gravity modes can be stochastically
excited by turbulent convection in massive main-sequence (MS) stars. We
build stellar models of MS stars with masses M=10 M ⊙,15
M ⊙, and 20 M ⊙. For each model, we then
compute the power supplied to the modes by turbulent eddies in the
convective core (CC) and the outer convective zones (OCZ). We found
that, for asymptotic gravity modes, the major part of the driving
occurs within the outer iron convective zone, while the excitation
of low n order modes mainly occurs within the CC. We compute the mode
lifetimes and deduce the expected mode amplitudes. We finally discuss
the possibility of detecting such stochastically-excited gravity modes
with the CoRoT space-based mission.
Title: A Spherical Harmonic Analysis of the Evolution of the
Photospheric Magnetic Field, and Consequences for the Solar Dynamo
Authors: DeRosa, Marc L.; Hoeksema, J. T.; Brun, A. S.
Bibcode: 2010AAS...21631701D
Altcode: 2010BAAS...41..898D
Time series of synoptic maps from several observatories, along with data
from an evolving surface-flux transport model, are analyzed in terms
of their spherical harmonic decomposition. The characteristics of these
spherical harmonic spectra, such as the relative amplitudes of various
harmonic modes, at different phases of the solar cycle are shown. We
illustrate how the rise and decline of the flux emergence rates, and
the associated reversal of the polar dipole, throughout a sunspot
cycle are reflected in the evolution of the various harmonic mode
coefficients. We further discuss the interplay between the low-degree
modes, in particular the dipole and quadrupole, and how such dynamics
may trigger the reversal of the polar dipole during solar maximum.
Title: Core Convection and Dynamos in Spectral Type O and B Stars
Authors: Augustson, Kyle; Brun, A. S.; Toomre, J.
Bibcode: 2010AAS...21642301A
Altcode: 2010BAAS...41..835A
Recent observations have revealed that about one-third of O and B type
stars have strong magnetic fields at their surfaces. It is currently
unclear where these fields originate. In order to address this question,
we examine the effects of core convection and magnetic dynamo processes
within massive O and B stars with simulations in rotating spherical
shells using the 3-D Spherical Harmonic (ASH) magnetohydrodynamic code.
Title: Persistent Magnetic Wreaths in a Rapidly Rotating Sun
Authors: Brown, Benjamin P.; Browning, Matthew K.; Brun, Allan Sacha;
Miesch, Mark S.; Toomre, Juri
Bibcode: 2010ApJ...711..424B
Altcode: 2010arXiv1011.2831B
When our Sun was young it rotated much more rapidly than
now. Observations of young, rapidly rotating stars indicate that many
possess substantial magnetic activity and strong axisymmetric magnetic
fields. We conduct simulations of dynamo action in rapidly rotating
suns with the three-dimensional magnetohydrodynamic anelastic spherical
harmonic (ASH) code to explore the complex coupling between rotation,
convection, and magnetism. Here, we study dynamo action realized in the
bulk of the convection zone for a system rotating at 3 times the current
solar rotation rate. We find that substantial organized global-scale
magnetic fields are achieved by dynamo action in this system. Striking
wreaths of magnetism are built in the midst of the convection zone,
coexisting with the turbulent convection. This is a surprise, for
it has been widely believed that such magnetic structures should be
disrupted by magnetic buoyancy or turbulent pumping. Thus, many solar
dynamo theories have suggested that a tachocline of penetration and
shear at the base of the convection zone is a crucial ingredient for
organized dynamo action, whereas these simulations do not include
such tachoclines. We examine how these persistent magnetic wreaths
are maintained by dynamo processes and explore whether a classical
mean-field α-effect explains the regeneration of poloidal field. We
find that the global-scale toroidal magnetic fields are maintained by an
Ω-effect arising from the differential rotation, while the global-scale
poloidal fields arise from turbulent correlations between the convective
flows and magnetic fields. These correlations are not well represented
by an α-effect that is based on the kinetic and magnetic helicities.
Title: Is the solar convection zone in strict thermal wind balance?
Authors: Brun, A. S.; Antia, H. M.; Chitre, S. M.
Bibcode: 2010A&A...510A..33B
Altcode: 2009arXiv0910.4954B
Context. The solar rotation profile is conical rather than cylindrical
as it could be expected from classical rotating fluid dynamics
(e.g. Taylor-Proudman theorem). Thermal coupling to the tachocline,
baroclinic effects and latitudinal transport of heat have been suggested
to explain this peculiar state of rotation.
Aims: To test the
validity of thermal wind balance in the solar convection zone using
helioseismic inversions for both the angular velocity and fluctuations
in entropy and temperature.
Methods: Entropy and temperature
fluctuations obtained from 3D hydrodynamical numerical simulations of
the solar convection zone are compared with solar profiles obtained from
helioseismic inversions.
Results: The temperature and entropy
fluctuations in 3D numerical simulations have smaller amplitude in
the bulk of the solar convection zone than those derived from seismic
inversions. Seismic inversion provides variations of temperature from
about 1 K at the surface to up to 100 K at the base of the convection
zone while in 3D simulations they are of an order of 10 K throughout
the convection zone up to 0.96 R⊙. In 3D simulations,
baroclinic effects are found to be important to tilt the isocontours
of Ω away from a cylindrical profile in most of the convection zone,
helped by Reynolds and viscous stresses at some locations. By contrast
the baroclinic effect inverted by helioseismology is much larger than
what is required to yield the observed angular velocity profile.
Conclusions: The solar convection does not appear to be in strict
thermal wind balance, Reynolds stresses must play a dominant role in
setting not only the equatorial acceleration but also the observed
conical angular velocity profile.
Title: Towards understanding the global magnetism of the Sun and
solar-like stars
Authors: Brun, Allan Sacha
Bibcode: 2010IAUS..264..161B
Altcode:
The Sun and solar-like stars possess intense and cyclic magnetic
activity. In order to understand how this comes about we have developed
series of 2-D and 3-D models in order to simulate their global dynamics
and magnetism. We here report on our latest findings.
Title: Interior and Exterior Clues of Solar Activity
Authors: Turck-Chièze, S.; Brun, A. S.; Duez, V.; García, R. A.;
Mathis, S.; Piau, L.; Salabert, D.; Pallé, P. L.; Jiménez-Reyes,
S. J.; Mathur, S.; Simoniello, R.; Robillot, J. M.
Bibcode: 2010ASSP...19..368T
Altcode: 2010mcia.conf..368T
Two research paths are described to obtain better understanding
of the origin of global solar activity. First, observations with
a multichannel resonant spectrometer may reveal the dynamics of the
solar core, the tachocline, and the temporal evolution of activity
between the photosphere and chromosphere. Such new observations will
deliver constraints for 3D simulations of solar activity. Second, we
examine the ab-initio introduction of a non-force-free field expressed
in spherical harmonics into the solar structure equations and estimate
its impact on the inner and subsurface layers, its time evolution,
and its role in angular momentum transport.
Title: Status of 3D MHD Models of Solar Global Internal Dynamics
Authors: Brun, A. S.
Bibcode: 2010ASSP...19...96B
Altcode: 2010mcia.conf...96B
This is a brief report on the decade-long effort by our group to model
the Sun's internal magnetohydrodynamics in 3D with the ASH code.
Title: Exploring the P cyc vs. P rot relation
with flux transport dynamo models of solar-like stars
Authors: Jouve, L.; Brown, B. P.; Brun, A. S.
Bibcode: 2010A&A...509A..32J
Altcode: 2009arXiv0911.1947J
Aims: Understand stellar magnetism and test the validity of
the Babcock-Leighton flux transport mean field dynamo models with
stellar activity observations
Methods: 2-D mean field dynamo
models at various rotation rates are computed with the STELEM code
to study the sensitivity of the activity cycle period and butterfly
diagram to parameter changes and are compared to observational data. The
novelty is that these 2-D mean field dynamo models incorporate scaling
laws deduced from 3-D hydrodynamical simulations for the influence
of rotation rate on the amplitude and profile of the meridional
circulation. These models make also use of observational scaling laws
for the variation of differential rotation with rotation rate.
Results: We find that Babcock-Leighton flux transport dynamo models
are able to reproduce the change in topology of the magnetic field
(i.e. toward being more toroidal with increasing rotation rate) but
seem to have difficulty reproducing the cycle period vs activity period
correlation observed in solar-like stars if a monolithic single cell
meridional flow is assumed. It may however be possible to recover
the P_cyc vs. P_rot relation with more complex meridional flows,
if the profile changes in a particular assumed manner with rotation
rate.
Conclusions: The Babcock-Leighton flux transport dynamo
model based on single cell meridional circulation does not reproduce the
P cyc vs. P rot relation unless the amplitude of
the meridional circulation is assumed to increase with rotation rate
which seems to be in contradiction with recent results obtained with
3-D global simulations.
Title: Modelling the Sun and Stars in 3-D
Authors: Brun, A. S.
Bibcode: 2010EAS....44...81B
Altcode: 2011EAS....44...81B
Stars can be seen as modern physics laboratory from which fundamental
processes as diverse as atomic physics or turbulence can be studied
and understood. Being able to model accurately their structure, dynamic
and evolution is thus of fundamental importance and is the subject of
intense research. In this short review we will present some of the
numerical simulations in three dimensions performed in recent years
to model such complex and nonlinear objects, focussing mostly our
discussion on results obtained with the anelastic spherical harmonic
(ASH) code. Using the Sun as a reference star, we wish to gain insight
and to constrain magnetohydrodynamical processes (such as Reynolds
and Maxwell stresses, meridional circulations, differential rotation
(i.e. ω-effect), thermal wind, α-effect) at the origin of the solar
small and large scale dynamics and magnetism. We will then extend our
study to other stars, such as young Suns, massive stars or evolved
RGB stars in order to identify which processes are at the origin of
their significantly different dynamics.
Title: Solar Convective Dynamo Action With A Tachocline
Authors: Featherstone, Nicholas; Brun, A. S.; Miesch, M. S.; Brown,
B. P.; Toomre, J.
Bibcode: 2010AAS...21532202F
Altcode: 2010BAAS...42..323F
We present continuing simulations of solar-like convection penetrating
into the tachocline at the base of the convection zone and examine
the resulting dynamo action. Prior simulations using the 3-D anelastic
spherical harmonic (ASH) code of convection in a full spherical shell
admitting penetration into a tachocline have yielded differential
rotation profiles whose latitudinal contrast is considerably smaller
than in simulations without penetration. We believe that the relatively
soft stabilizing entropy gradients in the overshooting regions may
have resulted in unusually strong circulations that worked against the
Reynolds stresses, thus diminishing the differential rotation. Here we
turn to ASH simulations with more realistic stiffer entropy gradients
and reduced diffusivities in the radiative zone. We report on the
hydrodynamic balances achieved within the region of penetration that
allows the convection zone to return to differential rotation profiles
in closer accord with helioseismic deductions, including possessing
a tachocline of shear. We then examine the possibilities for dynamo
action in this system and find that weak wreathes of toroidal field,
similar to those found in simulations of faster rotating suns, are
realized in the convection zone. Convective pumping of these fields
into the tachocline leads to the generation of strong axisymmetric
toroidal fields there, with oppositely signed polarities about the
equator. We examine the temporal variation of these magnetic fields
as well as their effects on the angular momentum transport within the
bulk of the convection zone.
Title: Wreath-Building Dynamos in Rapidly Rotating Suns
Authors: Brown, Benjamin; Browning, M. K.; Brun, A. S.; Miesch, M. S.;
Toomre, J.
Bibcode: 2010AAS...21542415B
Altcode: 2010BAAS...42..332B
When stars like our Sun are young, they rotate quite
rapidly. Observations of these young suns indicate that they generally
possess strong magnetic activity. Here we explore 3-D MHD simulations
of dynamo action in rapidly rotating suns. Our simulations with
the anelastic spherical harmonic (ASH) code extend from 0.72 to
0.97 solar radii and thus span the bulk of the stellar convection
zone. We find that these stars achieve strong dynamo action, and
naturally build remarkable global-scale magnetic structures in their
convection zones. These wreaths of magnetism fill the convection zone
and retain coherence over long epochs despite being embedded in the
turbulent convection. This is in striking contrast to many theories
of the global solar dynamo, which is thought to require a tachocline
of shear and penetration at the base of the convection zone to achieve
such structures. Wreath-building dynamos can undergo repeated cycles of
magnetic polarity reversal, with the global-scale magnetic structures
changing their sense on thousand day timescales.
Title: Three-Dimensional Simulations of Solar and Stellar Dynamos:
The Influence of a Tachocline
Authors: Miesch, M. S.; Browning, M. K.; Brun, A. S.; Toomre, J.;
Brown, B. P.
Bibcode: 2009ASPC..416..443M
Altcode: 2008arXiv0811.3032M
We review recent advances in modeling global-scale convection and
dynamo processes with the Anelastic Spherical Harmonic (ASH) code. In
particular, we have recently achieved the first global-scale solar
convection simulations that exhibit turbulent pumping of magnetic
flux into a simulated tachocline and the subsequent organization and
amplification of toroidal field structures by rotational shear. The
presence of a tachocline not only promotes the generation of mean
toroidal flux, but it also enhances and stabilizes the mean poloidal
field throughout the convection zone, promoting dipolar structure with
less frequent polarity reversals. The magnetic field generated by a
convective dynamo with a tachocline and overshoot region is also more
helical overall, with a sign reversal in the northern and southern
hemispheres. Toroidal tachocline fields exhibit little indication of
magnetic-buoyancy instabilities, but may be undergoing magneto-shear
instabilities.
Title: Dynamo Action and Wreaths of Magnetism in a Younger Sun
Authors: Brown, B. P.; Browning, M. K.; Brun, A. S.; Miesch, M. S.;
Toomre, J.
Bibcode: 2009ASPC..416..369B
Altcode:
When our Sun was younger it rotated much more rapidly. Observations of
many young stars indicate that magnetic activity and perhaps dynamo
action are stronger in the rapidly rotating suns. Here we use the
anelastic spherical harmonic (ASH) code to explore 3-D MHD simulations
of the dynamo action that might occur in such younger suns. As a great
surprise, we find that coherent global-scale structures of toroidal
magnetic field are formed in the bulk of the convection zone. These
wreaths of magnetism persist for long periods of time amidst the still
turbulent convection. In contrast to previous solar dynamo simulations,
the wreaths of magnetism formed in these more rapidly rotating suns
do not require a tachocline of penetration and shear at the base of
the convection zone for their creation or survival.
Title: Effects of Fossil Magnetic Fields on Convective Core Dynamos
in A-type Stars
Authors: Featherstone, Nicholas A.; Browning, Matthew K.; Brun,
Allan Sacha; Toomre, Juri
Bibcode: 2009ApJ...705.1000F
Altcode:
The vigorous magnetic dynamo action achieved within the convective cores
of A-type stars may be influenced by fossil magnetic fields within their
radiative envelopes. We study such effects through three-dimensional
simulations that model the inner 30% by radius of a 2 M sun
A-type star, capturing the convective core and a portion of the
overlying radiative envelope within our computational domain. We
employ the three-dimensional anelastic spherical harmonic code to
model turbulent dynamics within a deep rotating spherical shell. The
interaction between a fossil field and the core dynamo is examined by
introducing a large-scale magnetic field into the radiative envelope
of a mature A star dynamo simulation. We find that the inclusion of
a twisted toroidal fossil field can lead to a remarkable transition
in the core dynamo behavior. Namely, a super-equipartition state can
be realized in which the magnetic energy built by dynamo action is
10-fold greater than the kinetic energy of the convection itself. Such
strong-field states may suggest that the resulting Lorentz forces should
seek to quench the flows, yet we have achieved super-equipartition
dynamo action that persists for multiple diffusion times. This is
achieved by the relative co-alignment of the flows and magnetic fields
in much of the domain, along with some lateral displacements of the
fastest flows from the strongest fields. Convection in the presence of
such strong magnetic fields typically manifests as 4-6 cylindrical rolls
aligned with the rotation axis, each possessing central axial flows that
imbue the rolls with a helical nature. The roll system also possesses
core-crossing flows that couple distant regions of the core. We find
that the magnetic fields exhibit a comparable global topology with
broad, continuous swathes of magnetic field linking opposite sides of
the convective core. We have explored several poloidal and toroidal
fossil field geometries, finding that a poloidal component is essential
for a transition to super-equipartition to occur.
Title: Numerical Simulations of a Rotating Red Giant
Star. I. Three-dimensional Models of Turbulent Convection and
Associated Mean Flows
Authors: Brun, A. S.; Palacios, A.
Bibcode: 2009ApJ...702.1078B
Altcode:
With the development of one-dimensional stellar evolution codes
including rotation and the increasing number of observational data for
stars of various evolutionary stages, it becomes more and more possible
to follow the evolution of the rotation profile and angular momentum
distribution in stars. In this context, understanding the interplay
between rotation and convection in the very extended envelopes of giant
stars is very important considering that all low- and intermediate-mass
stars become red giants after the central hydrogen burning phase. In
this paper, we analyze the interplay between rotation and convection
in the envelope of red giant stars using three-dimensional numerical
experiments. We make use of the Anelastic Spherical Harmonics code to
simulate the inner 50% of the envelope of a low-mass star on the red
giant branch. We discuss the organization and dynamics of convection,
and put a special emphasis on the distribution of angular momentum in
such a rotating extended envelope. To do so, we explore two directions
of the parameter space, namely, the bulk rotation rate and the Reynolds
number with a series of four simulations. We find that turbulent
convection in red giant stars is dynamically rich, and that it is
particularly sensitive to the rotation rate of the star. Reynolds
stresses and meridional circulation establish various differential
rotation profiles (either cylindrical or shellular) depending on the
convective Rossby number of the simulations, but they all agree that the
radial shear is large. Temperature fluctuations are found to be large
and in the slowly rotating cases, a dominant ell = 1 temperature dipole
influences the convective motions. Both baroclinic effects and turbulent
advection are strong in all cases and mostly oppose one another.
Title: Three-Dimensional Nonlinear Evolution of a Magnetic Flux
Tube in a Spherical Shell: Influence of Turbulent Convection and
Associated Mean Flows
Authors: Jouve, Laurène; Brun, Allan Sacha
Bibcode: 2009ApJ...701.1300J
Altcode: 2009arXiv0907.2131J
We present the first three-dimensional magnetohydrodynamics study in
spherical geometry of the nonlinear dynamical evolution of magnetic
flux tubes in a turbulent rotating convection zone (CZ). These
numerical simulations use the anelastic spherical harmonic code. We
seek to understand the mechanism of emergence of strong toroidal fields
through a turbulent layer from the base of the solar CZ to the surface
as active regions. To do so, we study numerically the rise of magnetic
toroidal flux ropes from the base of a modeled CZ up to the top of our
computational domain where bipolar patches are formed. We compare the
dynamical behavior of flux tubes in a fully convective shell possessing
self-consistently generated mean flows such as meridional circulation
(MC) and differential rotation, with reference calculations done in
a quiet isentropic zone. We find that two parameters influence the
tubes during their rise through the CZ: the initial field strength
and amount of twist, thus confirming previous findings in Cartesian
geometry. Further, when the tube is sufficiently strong with respect
to the equipartition field, it rises almost radially independently
of the initial latitude (either low or high). By contrast, weaker
field cases indicate that downflows and upflows control the rising
velocity of particular regions of the rope and could in principle
favor the emergence of flux through Ω-loop structures. For these
latter cases, we focus on the orientation of bipolar patches and find
that sufficiently arched structures are able to create bipolar regions
with a predominantly east-west orientation. Meridional flow seems to
determine the trajectory of the magnetic rope when the field strength
has been significantly reduced near the top of the domain. Appearance
of local magnetic field also feeds back on the horizontal flows thus
perturbing the MC via Maxwell stresses. Finally differential rotation
makes it more difficult for tubes introduced at low latitudes to reach
the top of the domain.
Title: Wreathes of Magnetism in Rapidly Rotating Suns
Authors: Brown, Benjamin P.; Browning, Matthew K.; Miesch, Mark S.;
Brun, Allan Sacha; Toomre, Juri
Bibcode: 2009arXiv0906.2407B
Altcode:
When our Sun was young it rotated much more rapidly than
now. Observations of young, rapidly rotating stars indicate that
many possess substantial magnetic activity and strong axisymmetric
magnetic fields. We conduct simulations of dynamo action in rapidly
rotating suns with the 3-D MHD anelastic spherical harmonic (ASH)
code to explore the complex coupling between rotation, convection
and magnetism. Here we study dynamo action realized in the bulk of
the convection zone for two systems, rotating at three and five times
the current solar rate. We find that substantial organized global-scale
magnetic fields are achieved by dynamo action in these systems. Striking
wreathes of magnetism are built in the midst of the convection zone,
coexisting with the turbulent convection. This is a great surprise,
for many solar dynamo theories have suggested that a tachocline of
penetration and shear at the base of the convection zone is a crucial
ingredient for organized dynamo action, whereas these simulations do
not include such tachoclines. Some dynamos achieved in these rapidly
rotating states build persistent global-scale fields which maintain
amplitude and polarity for thousands of days. In the case at five
times the solar rate, the dynamo can undergo cycles of activity,
with fields varying in strength and even changing polarity. As the
magnetic fields wax and wane in strength, the primary response in
the convective flows involves the axisymmetric differential rotation,
which begins to vary on similar time scales. Bands of relatively fast
and slow fluid propagate toward the poles on time scales of roughly 500
days. In the Sun, similar patterns are observed in the poleward branch
of the torsional oscillations, and these may represent a response to
poleward propagating magnetic field deep below the solar surface.
Title: Mean-Field Generation in Turbulent Convective Dynamos: The
Role of a Tachocline
Authors: Miesch, Mark S.; Browning, M. K.; Brun, A. S.; Brown, B. P.;
Toomre, J.
Bibcode: 2009SPD....40.0406M
Altcode:
Turbulent dynamos tend to generate turbulent magnetic fields. The
Sun exhibits such disordered fields but it also exhibits large-scale
magnetic activity patterns of striking order, including cyclically
varying sunspot distributions and a reversing dipole moment. The
challenge of global solar dynamo theory is to account for such
order. Rotational shear almost certainly plays an essential role,
placing the solar tachocline at center stage. Here we present global
simulations of convective dynamos with and without a tachocline,
focusing on how the presence of a tachocline alters mean field
generation. The presence of a tachocline not only promotes the
generation of mean toroidal flux, but it also enhances and stabilizes
the mean poloidal field throughout the convection zone, promoting
dipolar structure with less frequent polarity reversals. Magnetic fields
generated in the presence of a tachocline are more helical overall,
with opposite senses among hemispheres and among mean and fluctuating
components. Toroidal tachocline fields exhibit little indication of
magnetic buoyancy instabilities but may be undergoing magneto-shear
instabilities.
Title: Marching Toward More Realistic Penetration of Convection into
a Tachocline
Authors: Featherstone, Nicholas; Brun, A. S.; Miesch, M. S.; Toomre, J.
Bibcode: 2009SPD....40.0803F
Altcode:
The solar convection zone has provided many challenges for the
theoretical modeling of dynamics within our nearest star. The
tachocline, a region of strong shear near the base of the convection
zone, has received much attention due to its likely role in the
generation of the global-scale magnetic fields. The establishment and
maintenance of the solar tachocline has been variously attributed to
angular momentum transport via gravity waves, magnetic torques and
anisotropic mixing processes. Self consistently capturing the turbulent
dynamics of the convection zone and underlying radiative zone through
3-D numerical modeling is difficult due to the wide range of scales
involved. Prior simulations using the 3-D anelastic spherical harmonic
(ASH) code of convection in a full spherical shell admitting penetration
into a stable region below have yielded differential rotation profiles
whose latitudinal contrast is considerably smaller than in simulations
without penetration. We believe that the relatively soft stabilizing
entropy gradients in the overshooting regions may have resulted
in unusually strong circulations that worked against the Reynolds
stresses, thus diminishing the differential rotation. Here we turn
to ASH simulations with more realistic stiffer entropy gradients and
reduced diffusivities in the radiative zone. We report on the balances
achieved within the region of penetration that allows the convection
zone to return to differential rotation profiles in closer accord with
helioseismic deductions, including possessing a tachocline of shear.
Title: On MHD rotational transport, instabilities and dynamo action
in stellar radiation zones
Authors: Mathis, Stéphane; Brun, A. -S.; Zahn, J. -P.
Bibcode: 2009IAUS..259..421M
Altcode:
Magnetic field and their related dynamical effects are thought
to be important in stellar radiation zones. For instance, it has
been suggested that a dynamo, sustained by a m = 1 MHD instability
of toroidal magnetic fields (discovered by Tayler in 1973), could
lead to a strong transport of angular momentum and of chemicals in
such stable regions. We wish here to recall the different magnetic
transport processes present in radiative zone and show how the dynamo
can operate by recalling the conditions required to close the dynamo
loop (BPol → BTor → BPol). Helped
by high-resolution 3D MHD simulations using the ASH code in the solar
case, we confirm the existence of the m = 1 instability, study its
non-linear saturation, but we do not detect, up to a magnetic Reylnods
number of 105, any dynamo action.
Title: Impact of large-scale magnetic fields on stellar structure
and evolution
Authors: Duez, Vincent; Mathis, S.; Brun, A. S.; Turck-Chièze, S.
Bibcode: 2009IAUS..259..177D
Altcode:
We study the impact on the stellar structure of a large-scale magnetic
field in stellar radiation zones. The field is in magneto-hydrostatic
(MHS) equilibrium and has a non force-free character, which allows
us to study its influence both on the mechanical and and on the
energetic balances. This approach is illustrated in the case
of an Ap star where the magnetic field matches at the
surface with an external potential one. Perturbations of the stellar
structure are semi-analytically computed. The relative importance of
the magnetic physical quantities is discussed and a hierarchy, aiming
at distinguishing various refinement degrees in the implementation
of a large-scale magnetic field in a stellar evolution code, is
established. This treatment also allows us to deduce the gravitational
multipolar moments and the change in effective temperature associated
with the presence of a magnetic field.
Title: Large Scale Flows in the Solar Convection Zone
Authors: Brun, Allan Sacha; Rempel, Matthias
Bibcode: 2009SSRv..144..151B
Altcode: 2008SSRv..tmp..173B
We discuss the current theoretical understanding of the large scale
flows observed in the solar convection zone, namely the differential
rotation and meridional circulation. Based on multi-D numerical
simulations we describe which physical processes are at the origin of
these large scale flows, how they are maintained and what sets their
unique profiles. We also discuss how dynamo generated magnetic field
may influence such a delicate dynamical balance and lead to a temporal
modulation of the amplitude and profiles of the solar large scale flows.
Title: Impact of a Large-Scale Magnetic Field on Stellar Structure
Authors: Duez, V.; Mathis, S.; Brun, A. S.; Turck-Chièze, S.
Bibcode: 2009AIPC.1121...55D
Altcode:
We present the derivation of non force-free magneto-hydrostatic (MHS)
equilibria in spherical geometry, supposing any prescription for the
toroidal current. This allows us to study the influence on the stellar
structure of a large-scale magnetic field, both on the mechanical
and on the energetical balances. Two cases illustrate this approach:
(i) the field is buried below a given radius, in order to model
deep fossil magnetic fields in solar-like stars; (ii) the internal
field matches at the surface with an external potential magnetic
field that corresponds to fossil fields in more massive stars. The
stellar structure perturbations are semi-analytically computed in both
cases. This allows us to establish a hierarchy between the orders of
magnitude of the different terms. Finally, the limit of validity of
the linear perturbation is discussed.
Title: Stellar Convection and Magnetism across the H-R diagram:
Theory and Models
Authors: Brun, A. S.
Bibcode: 2009EAS....39..153B
Altcode:
Stars constitute undoublty one of the elementary blocks of the Universe
and play as such a central role in determining for instance its chemical
evolution. They can be seen as modern physics laboratory from which
fundamental processes as diverse as atomic physics or turbulence can be
studied and understood. Being able to model accurately their structure,
dynamic and evolution is thus of fundamental importance and is the
subject of intense research. In this short lecture we will first discuss
the basic equations and processes, such as convection, turbulence,
rotation, instabilities and dynamo action that are at the origin of
the magnetic field observed in stars. We will then present some of the
numerical simulations in three dimensions performed in recent years
to model such complex objects and their nonlinear behavior, focussing
mainly on results obtained with the anelastic spherical harmonic (ASH)
code. Using the Sun as a reference star, we wish to gain insight the
various magnetohydrodynamical processes that shape its large scale
dynamics and magnetism, such as the Reynolds and Maxwell stresses, and
the ω and α-effects. We will then extend our study to other stars,
such as young Suns, massive stars or evolved RGB stars in order to
identify which processes are at the origin of their significantly
different dynamics.
Title: Stochastic excitation of nonradial modes. II. Are solar
asymptotic gravity modes detectable?
Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Dupret, M. A.;
Brun, A. S.; Baudin, F.
Bibcode: 2009A&A...494..191B
Altcode: 2008arXiv0810.0602B
Context: Detection of solar gravity modes remains a major challenge to
our understanding of the inner parts of the Sun. Their frequencies
would enable the derivation of constraints on the core physical
properties, while their amplitudes can put severe constraints on the
properties of the inner convective region.
Aims: Our purpose
is to determine accurate theoretical amplitudes of solar g modes and
estimate the SOHO observation duration for an unambiguous detection
of individual modes. We also explain differences in theoretical
amplitudes derived from previous works.
Methods: We investigate
the stochastic excitation of modes by turbulent convection, as
well as their damping. Input from a 3D global simulation of the
solar convective zone is used for the kinetic turbulent energy
spectrum. Damping is computed using a parametric description of the
nonlocal, time-dependent, convection-pulsation interaction. We then
provide a theoretical estimation of the intrinsic, as well as apparent,
surface velocity.
Results: Asymptotic g-mode velocity amplitudes
are found to be orders of magnitude higher than previous works. Using
a 3D numerical simulation from the ASH code, we attribute this to
the temporal-correlation between the modes and the turbulent eddies,
which is found to follow a Lorentzian law rather than a Gaussian one, as
previously used. We also find that damping rates of asymptotic gravity
modes are dominated by radiative losses, with a typical life time of 3
× 105 years for the ell=1 mode at ν=60 μHz. The maximum
velocity in the considered frequency range (10-100 μHz) is obtained
for the ell=1 mode at ν=60 μHz and for the ell=2 at ν=100 μHz. Due
to uncertainties in the modeling, amplitudes at maximum i.e. for ell=1
at 60 μHz can range from 3 to 6 mm s-1. The upper limit
is too high, as g modes would have been easily detected with SOHO,
the GOLF instrument, and this sets an upper constraint mainly on the
convective velocity in the Sun.
Title: Solar Dynamo and Magnetic Self-Organization
Authors: Kosovichev, A. G.; Arlt, R.; Bonanno, A.; Brandenburg,
A.; Brun, A. S.; Busse, F.; Dikpati, M.; Hill, F.; Gilman, P. A.;
Nordlund, A.; Ruediger, G.; Stein, R. F.; Sekii, T.; Stenflo, J. O.;
Ulrich, R. K.; Zhao, J.
Bibcode: 2009astro2010S.160K
Altcode:
No abstract at ADS
Title: Large Scale Flows in the Solar Convection Zone
Authors: Brun, Allan Sacha; Rempel, Matthias
Bibcode: 2009odsm.book..151B
Altcode:
We discuss the current theoretical understanding of the large scale
flows observed in the solar convection zone, namely the differential
rotation and meridional circulation. Based on multi-D numerical
simulations we describe which physical processes are at the origin of
these large scale flows, how they are maintained and what sets their
unique profiles. We also discuss how dynamo generated magnetic field
may influence such a delicate dynamical balance and lead to a temporal
modulation of the amplitude and profiles of the solar large scale flows.
Title: Rapidly Rotating Suns and Active Nests of Convection
Authors: Brown, Benjamin P.; Browning, Matthew K.; Brun, Allan Sacha;
Miesch, Mark S.; Toomre, Juri
Bibcode: 2008ApJ...689.1354B
Altcode: 2008arXiv0808.1716B
In the solar convection zone, rotation couples with intensely turbulent
convection to drive a strong differential rotation and achieve complex
magnetic dynamo action. Our Sun must have rotated more rapidly in
its past, as is suggested by observations of many rapidly rotating
young solar-type stars. Here we explore the effects of more rapid
rotation on the global-scale patterns of convection in such stars and
the flows of differential rotation and meridional circulation, which
are self-consistently established. The convection in these systems is
richly time-dependent, and in our most rapidly rotating suns a striking
pattern of localized convection emerges. Convection near the equator in
these systems is dominated by one or two nests in longitude of locally
enhanced convection, with quiescent streaming flow in between them at
the highest rotation rates. These active nests of convection maintain a
strong differential rotation despite their small size. The structure of
differential rotation is similar in all of our more rapidly rotating
suns, with fast equators and slower poles. We find that the total
shear in differential rotation Δ Ω grows with more rapid rotation,
while the relative shear Δ Ω/Ω0 decreases. In contrast,
at more rapid rotation, the meridional circulations decrease in energy
and peak velocities and break into multiple cells of circulation in
both radius and latitude.
Title: Impact of Large-Scale Magnetic Fields on Stellar Structure
and Prospectives on Stellar Evolution
Authors: Duez, V.; Mathis, S.; Brun, A. -S.; Turck-Chièze, S.
Bibcode: 2008sf2a.conf..459D
Altcode:
The influence of large-scale magnetic fields on stellar structure
and stellar evolution is semi-analytically considered. The magnetic
field is derived for a given axisymmetric azimuthal current, and is non
force-free, acting thus directly on the stellar structure by modifying
the hydrostatic balance. We discuss the relative importance of the
various terms associated with the magnetic field in the mechanical
and thermal balances before implementing its effects in a 1D stellar
evolution code in a way that preserves its geometrical properties. Our
purpose is illustrated by the case of an internal magnetic field
matching at the surface of an Ap star with an external potential and
multipolar magnetic field.
Title: Dynamical aspects of stellar physics
Authors: Zahn, J. -P.; Brun, A. -S.; Mathis, S.
Bibcode: 2008sf2a.conf..341Z
Altcode:
Several manifestations of the dynamics of stellar interiors are
briefly presented, with emphasis on the most recent developments in
their numerical simulation.
Title: Impact of Large-Scale Magnetic Fields on Solar Structure
Authors: Duez, V.; Mathis, S.; Brun, A. -S.; Turck-Chièze, S.;
Le Poncin-Lafitte, C.
Bibcode: 2008sf2a.conf..463D
Altcode:
We here focus on the impact of large-scale magnetic fields on the solar
structure from its core up to its surface by treating semi-analytically
the Magneto-HydroStatic (MHS) equilibria of a self-gravitating spherical
shell. Then, the modifications of the internal structure of the Sun
introduced by such a field are deduced, and the resulting multipolar
gravitational moments are obtained.
Title: Hydrodynamical Simulations of Turbulent Convection in a
Rotating Red Giant Star
Authors: Palacios, A.; Brun, A. S.
Bibcode: 2008IAUS..252..175P
Altcode:
We present 3-D hydrodynamical simulations of the extended turbulent
convective envelope of a low-mass red giant star. These simulations,
computed with the ASH code, aim at understanding the redistribution
of angular momentum and heat in extended turbulent convection zones of
these giant stars. We focus our study on the effects of turbulence and
of the rotation rate on the convective patterns and on the distribution
of angular momentum within the inner 50% of the convective envelope
of such stars.
Title: On MHD rotational transport, instabilities and dynamo action
in stellar radiation zones
Authors: Mathis, S.; Zahn, J. -P.; Brun, A. -S.
Bibcode: 2008IAUS..252..255M
Altcode:
Magnetic field is an essential dynamical process in stellar radiation
zones. Moreover, it has been suggested that a dynamo action, sustained
by a MHD instability which affects the toroidal axisymmetric magnetic
field, could lead to a strong transport of angular momentum and of
chemicals in such regions. Here, we recall the different magnetic
transport and mixing processes in radiative regions. Next, we show
that the dynamo cannot operate as described by Spruit (2002) and
recall the condition required to close the dynamo loop. We perform
high-resolution 3D simulations with the ASH code, where we observe
indeed the MHD instability, but where we do not detect any dynamo
action, contrary to J. Braithwaite (2006). We conclude on the picture
we get for magnetic transport mechanisms in radiation zones and the
associated consequences for stellar evolution.
Title: A solar mean field dynamo benchmark
Authors: Jouve, L.; Brun, A. S.; Arlt, R.; Brandenburg, A.; Dikpati,
M.; Bonanno, A.; Käpylä, P. J.; Moss, D.; Rempel, M.; Gilman, P.;
Korpi, M. J.; Kosovichev, A. G.
Bibcode: 2008A&A...483..949J
Altcode:
Context: The solar magnetic activity and cycle are linked to an
internal dynamo. Numerical simulations are an efficient and accurate
tool to investigate such intricate dynamical processes.
Aims:
We present the results of an international numerical benchmark
study based on two-dimensional axisymmetric mean field solar dynamo
models in spherical geometry. The purpose of this work is to provide
reference cases that can be analyzed in detail and that can help in
further development and validation of numerical codes that solve such
kinematic problems.
Methods: The results of eight numerical
codes solving the induction equation in the framework of mean field
theory are compared for three increasingly computationally intensive
models of the solar dynamo: an αΩ dynamo with constant magnetic
diffusivity, an αΩ dynamo with magnetic diffusivity sharply varying
with depth and an example of a flux-transport Babcock-Leighton dynamo
which includes a non-local source term and one large single cell of
meridional circulation per hemisphere. All cases include a realistic
profile of differential rotation and thus a sharp tachocline.
Results: The most important finding of this study is that all codes
agree quantitatively to within less than a percent for the αΩ dynamo
cases and within a few percent for the flux-transport case. Both
the critical dynamo numbers for the onset of dynamo action and the
corresponding cycle periods are reasonably well recovered by all
codes. Detailed comparisons of butterfly diagrams and specific cuts of
both toroidal and poloidal fields at given latitude and radius confirm
the good quantitative agreement.
Conclusions: We believe that
such a benchmark study will be a very useful tool since it provides
detailed standard cases for comparison and reference.
Title: Global models of the magnetic Sun
Authors: Brun, Allan Sacha; Jouve, Laurène
Bibcode: 2008IAUS..247...33B
Altcode: 2007IAUS..247...33B
We briefly present recent simulations of the internal magnetism of the
Sun with the 3-D ASH code and with the 2-D STELEM code. The intense
magnetism of the Sun is linked to local and global dynamo action within
our star. We focus our study on how magnetohydrodynamical processes in
stable (radiative) or unstable (convective) zones, nonlinearly interact
to establish the solar differential rotation, meridional circulation,
confine the tachocline, amplify and organise magnetic fields and how
magnetic flux emerge to the surface. We also test the robustness of
flux transport dynamo models to various profiles of circulation.
Title: Interactive Visualization of Astrophysical Plasma Simulations
with SDvision
Authors: Pomarède, D.; Fidaali, Y.; Audit, E.; Brun, A. S.; Masset,
F.; Teyssier, R.
Bibcode: 2008ASPC..385..327P
Altcode:
SDvision is a graphical interface developed in the framework of
IDL Object Graphics and designed for the interactive and immersive
visualization of astrophysical plasma simulations. Three-dimensional
scalar and vector fields distributed over regular mesh grids or more
complex structures such as adaptive mesh refinement data or multiple
embedded grids, as well as N-body systems, can be visualized in a
number of different, complementary ways. Various implementations of
the visualization of the data are simultaneously proposed, such as
3D isosurfaces, volume projections, hedgehog and streamline displays,
surface and image of 2D subsets, profile plots, particle clouds. The
SDvision widget allows to visualize complex, composite scenes both
from outside and from within the simulated volume. This tool is used
to visualize data from RAMSES, a hybrid N-body and hydrodynamical
code which solves the interplay of dark matter and the baryon gas
in the study of cosmological structures formation, from HERACLES, a
radiation hydrodynamics code used in particular to study turbulence in
interstellar molecular clouds, from the ASH code dedicated to the study
of stellar magnetohydrodynamics, and from the JUPITER multi-resolution
code used in the study of protoplanetary disks formation.
Title: Influence of a global magnetic field on stellar structure
Authors: Duez, V.; Brun, A. S.; Mathis, S.; Nghiem, P. A. P.;
Turck-Chièze, S.
Bibcode: 2008MmSAI..79..716D
Altcode:
The theoretical framework we have developed to take into account
the influence of a global axisymmetric magnetic field on stellar
structure and evolution is described. The prescribed field, possibly
time-dependent, is expanded in the vectorial spherical harmonics
basis. Hydrostatic equilibrium and energetic balance are consequently
modified. Convection's efficiency and onset are also revised. Finally,
our numerical strategy and the results one can expect from the
implementation of those theoretical results are discussed.
Title: Structure and Evolution of Giant Cells in Global Models of
Solar Convection
Authors: Miesch, Mark S.; Brun, Allan Sacha; DeRosa, Marc L.;
Toomre, Juri
Bibcode: 2008ApJ...673..557M
Altcode: 2007arXiv0707.1460M
The global scales of solar convection are studied through
three-dimensional simulations of compressible convection carried out
in spherical shells of rotating fluid that extend from the base of
the convection zone to within 15 Mm of the photosphere. Such modeling
at the highest spatial resolution to date allows study of distinctly
turbulent convection, revealing that coherent downflow structures
associated with giant cells continue to play a significant role in
maintaining the differential rotation that is achieved. These giant
cells at lower latitudes exhibit prograde propagation relative to
the mean zonal flow, or differential rotation, that they establish,
and retrograde propagation of more isotropic structures with vortical
character at mid and high latitudes. The interstices of the downflow
networks often possess strong and compact cyclonic flows. The
evolving giant-cell downflow systems can be partly masked by the
intense smaller scales of convection driven closer to the surface,
yet they are likely to be detectable with the helioseismic probing that
is now becoming available. Indeed, the meandering streams and varying
cellular subsurface flows revealed by helioseismology must be sampling
contributions from the giant cells, yet it is difficult to separate
out these signals from those attributed to the faster horizontal flows
of supergranulation. To aid in such detection, we use our simulations
to describe how the properties of giant cells may be expected to vary
with depth and how their patterns evolve in time.
Title: Stellar convection simulations
Authors: Brun, A.; Miesch, Mark
Bibcode: 2008SchpJ...3.4278B
Altcode:
No abstract at ADS
Title: Nonlinear simulations of magnetic instabilities in stellar
radiation zones: The role of rotation and shear
Authors: Brun, A. S.
Bibcode: 2007AN....328.1137B
Altcode:
Using the 3-dimensional ASH code, we have studied numerically the
instabilities that occur in stellar radiation zones in presence
of large-scale magnetic fields, rotation and large-scale shear. We
confirm that some configurations are linearly unstable, as predicted
by Tayler and collaborators, and we determine the saturation level of
the instability. We find that rotation modifies the peak of the most
unstable wave number of the poloidal instability but not its growth
rate as much as in the case of the m=1 toroidal instability for which
it is changed to σ = σ_A2/Ω. Further in the case with
rotation and shear, we found no sign of the dynamo mechanism suggested
recently by Spruit even though we possess the essential ingredients
(Tayler's m=1 instability and a large scale shear) supposedly at work.
Title: 3-D non-linear evolution of a magnetic flux tube in a spherical
shell: The isentropic case
Authors: Jouve, L.; Brun, A. S.
Bibcode: 2007AN....328.1104J
Altcode: 2007arXiv0712.3408J
We present recent 3-D MHD numerical simulations of the non-linear
dynamical evolution of magnetic flux tubes in an adiabatically
stratified convection zone in spherical geometry, using the anelastic
spherical harmonic (ASH) code. We seek to understand the mechanism
of emergence of strong toroidal fields from the base of the solar
convection zone to the solar surface as active regions. We confirm
the results obtained in cartesian geometry that flux tubes that are
not twisted split into two counter vortices before reaching the top of
the convection zone. Moreover, we find that twisted tubes undergo the
poleward-slip instability due to an unbalanced magnetic curvature force
which gives the tube a poleward motion both in the non-rotating and in
the rotating case. This poleward drift is found to be more pronounced on
tubes originally located at high latitudes. Finally, rotation is found
to decrease the rise velocity of the flux tubes through the convection
zone, especially when the tube is introduced at low latitudes.
Title: Simulation of turbulent convection in a slowly rotating red
giant star
Authors: Palacios, A.; Brun, A. S.
Bibcode: 2007AN....328.1114P
Altcode:
The first 3-D non-linear hydrodynamical simulation of the inner
convective envelope of a rotating low mass red giant star is
presented. This simulation, computed with the ASH code, aims at
understanding the redistribution of angular momentum and heat in
extended convection zones. The convection patterns achieved in the
simulation consist of few broad and warm upflows surrounded by a network
of cool downflows. This asymmetry between up and downflows leads to
a strong downward kinetic energy flux, that must be compensated by
an overluminous enthalpy flux in order to carry outward the total
luminosity of the star. The influence of rotation on turbulent
convection results in the establishment of large-scale mean flows:
a strong radial differential rotation and a single cell poleward
meridional circulation per hemisphere. A detailed analysis of angular
momentum redistribution reveals that the meridional circulation
transports angular momentum outward in the radial direction and
poleward in the latitudinal direction, with the Reynolds stresses
acting in the opposite direction. This simulation indicates that the
classical hypothesis of mixing length theory and solid-body rotation
in the envelope of red giants assumed in 1-D stellar evolution models
are unlikely to be realized and thus should be reconsidered.
Title: Rapid rotation, active nests of convection and global-scale
flows in solar-like stars
Authors: Brown, B. P.; Browning, M. K.; Brun, A. S.; Miesch, M. S.;
Toomre, J.
Bibcode: 2007AN....328.1002B
Altcode: 2008arXiv0801.1672B
In the solar convection zone, rotation couples with intensely turbulent
convection to build global-scale flows of differential rotation and
meridional circulation. Our sun must have rotated more rapidly in its
past, as is suggested by observations of many rapidly rotating young
solar-type stars. Here we explore the effects of more rapid rotation
on the patterns of convection in such stars and the global-scale
flows which are self-consistently established. The convection in
these systems is richly time dependent and in our most rapidly
rotating suns a striking pattern of spatially localized convection
emerges. Convection near the equator in these systems is dominated by
one or two patches of locally enhanced convection, with nearly quiescent
streaming flow in between at the highest rotation rates. These active
nests of convection maintain a strong differential rotation despite
their small size. The structure of differential rotation is similar
in all of our more rapidly rotating suns, with fast equators and
slower poles. We find that the total shear in differential rotation,
as measured by latitudinal angular velocity contrast, \Delta \Omega,
increases with more rapid rotation while the relative shear, \Delta
\Omega/ \Omega, decreases. In contrast, at more rapid rotation the
meridional circulations decrease in both energy and peak velocities and
break into multiple cells of circulation in both radius and latitude.
Title: Dynamo action in simulations of penetrative solar convection
with an imposed tachocline
Authors: Browning, M. K.; Brun, A. S.; Miesch, M. S.; Toomre, J.
Bibcode: 2007AN....328.1100B
Altcode:
We summarize new and continuing three-dimensional spherical shell
simulations of dynamo action by convection allowed to penetrate
downward into a tachocline of rotational shear. The inclusion
of an imposed tachocline allows us to examine several processes
believed to be essential in the operation of the global solar dynamo,
including differential rotation, magnetic pumping, and the stretching
and organization of fields within the tachocline. In the stably
stratified core, our simulations reveal that strong axisymmetric
magnetic fields (of ∼ 3000 G strength) can be built, and that those
fields generally exhibit a striking antisymmetric parity, with fields
in the northern hemisphere largely of opposite polarity to those in
the southern hemisphere. In the convection zone above, fluctuating
fields dominate over weaker mean fields. New calculations indicate
that the tendency toward toroidal fields of antisymmetric parity
is relatively insensitive to initial magnetic field configurations;
they also reveal that on decade-long timescales, the magnetic fields
can briefly enter (and subsequently emerge from) states of symmetric
parity. We have not yet observed any overall reversals of the field
polarity, nor systematic latitudinal propagation.
Title: Dynamo action in the presence of an imposed magnetic field
Authors: Featherstone, N. A.; Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2007AN....328.1126F
Altcode:
Dynamo action within the cores of Ap stars may offer intriguing
possibilities for understanding the persistent magnetic fields observed
on the surfaces of these stars. Deep within the cores of Ap stars,
the coupling of convection with rotation likely yields magnetic dynamo
action, generating strong magnetic fields. However, the surface fields
of the magnetic Ap stars are generally thought to be of primordial
origin. Recent numerical models suggest that a primordial field in
the radiative envelope may possess a highly twisted toroidal shape. We
have used detailed 3-D simulations to study the interaction of such a
twisted magnetic field in the radiative envelope with the core-dynamo
operating in the interior of a 2 solar mass A-type star. The resulting
dynamo action is much more vigorous than in the absence of such a
fossil field, yielding magnetic field strengths (of order 100 kG)
much higher than their equipartition values relative to the convective
velocities. We examine the generation of these fields, as well as the
growth of large-scale magnetic structure that results from imposing
a fossil magnetic field.
Title: Strong Dynamo Action in Rapidly Rotating Suns
Authors: Brown, Benjamin P.; Browning, Matthew K.; Brun, Allan Sacha;
Miesch, Mark S.; Nelson, Nicholas J.; Toomre, Juri
Bibcode: 2007AIPC..948..271B
Altcode: 2008arXiv0801.1684B
Stellar dynamos are driven by complex couplings between rotation and
turbulent convection, which drive global-scale flows and build and
rebuild stellar magnetic fields. When stars like our sun are young,
they rotate much more rapidly than the current solar rate. Observations
generally indicate that more rapid rotation is correlated with stronger
magnetic activity and perhaps more effective dynamo action. Here
we examine the effects of more rapid rotation on dynamo action in a
star like our sun. We find that vigorous dynamo action is realized,
with magnetic field generated throughout the bulk of the convection
zone. These simulations do not possess a penetrative tachocline of shear
where global-scale fields are thought to be organized in our sun, but
despite this we find strikingly ordered fields, much like sea-snakes
of toroidal field, which are organized on global scales. We believe
this to be a novel finding.
Title: Convective Core Dynamos of A-type Stars in the Presence of
Fossil Magnetic Fields
Authors: Featherstone, N. A.; Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2007AIPC..948..279F
Altcode:
The persistent magnetic fields of Ap stars are generally thought to
be of primordial origin, but dynamo generation of magnetic fields may
offer alternative possibilities. Deep within the interiors of such
stars, vigorous core convection likely couples with rotation to yield
magnetic dynamo action, generating strong magnetic fields. Recent
numerical models suggest that a primordial field remaining from the
star's formation may possess a highly twisted toroidal shape in the
radiative interior. We have used detailed 3-D simulations to study the
interaction of such a magnetic field with a dynamo generated within the
core of a 2 solar mass A-type star. Dynamo action realized under these
circumstances is much more vigorous than in the absence of a fossil
field in the radiative envelope, yielding magnetic field strengths (of
order 100 kG) much higher than their equipartition values relative to
the convective velocities. We examine the generation of these fields,
as well as their effect on the complex dynamics of the convective core.
Title: Global Models of Solar Convection
Authors: Miesch, Mark S.; Browning, Matthew K.; Brun, Allan Sacha;
Toomre, Juri
Bibcode: 2007AIPC..948..149M
Altcode:
Convection is fundamental and enigmatic enough to rank high on any
pundit's list of unsolved problems in stellar physics. It is responsible
in large part for why stars shine since most stellar interiors are
at least partially convective. Furthermore, convection plays an
essential role in how stars build magnetic fields. Magnetism in turn
accounts for most short-term solar and stellar variability. Despite
its ubiquity, stellar convection is challenging to model theoretically
or numerically. In this paper we provide an overview of some recent
insights into solar and stellar convection obtained from high-resolution
numerical simulations. Thanks to continuing advances in high performance
computing technology, such simulations continue to achieve unprecedented
parameter regimes revealing turbulent dynamics inaccessible to previous
models. Here we focus in particular on the subtle and profound influence
of the complex boundary layers which exist near the top and bottom of
the solar convection zone.
Title: Simulations of Turbulent Convection in Rotating Young Solarlike
Stars: Differential Rotation and Meridional Circulation
Authors: Ballot, Jérôme; Brun, Allan Sacha; Turck-Chièze, Sylvaine
Bibcode: 2007ApJ...669.1190B
Altcode: 2007arXiv0707.3943B
We present the results of three-dimensional simulations of the deep
convective envelope of a young (10 Myr) 1 Msolar star,
obtained with the anelastic spherical harmonic code. Since young stars
are known to be faster rotators than their main-sequence counterparts,
we have systematically studied the impact of the stellar rotation
speed, by considering stars spinning up to 5 times as fast as the
Sun. The aim of these nonlinear models is to understand the complex
interactions between convection and rotation. We discuss the influence
of the turbulence level and of the rotation rate on the intensity
and the topology of the mean flows. For all of the computed models,
we find a solar-type superficial differential rotation, with an
equatorial acceleration, and meridional circulation that exhibits a
multicellular structure. Even if the differential rotation contrast
ΔΩ decreases only marginally for high rotation rates, the meridional
circulation intensity clearly weakens according to our simulations. We
have also shown that, for Taylor numbers above a certain threshold
(Ta>~109), the convection can develop a vacillating
behavior. Since simulations with high turbulence levels and rotation
rates exhibit strongly cylindrical internal rotation profiles, we
have considered the influence of baroclinic effects at the base of the
convective envelope of these young Suns to see whether such effects can
modify the otherwise near-cylindrical profiles to produce more conical,
solarlike profiles.
Title: On the role of meridional flows in flux transport dynamo models
Authors: Jouve, L.; Brun, A. S.
Bibcode: 2007A&A...474..239J
Altcode: 2007arXiv0712.3200J
Context: The Sun is a magnetic star whose magnetism and cyclic activity
is linked to the existence of an internal dynamo.
Aims: We aim
to understand the establishment of the solar magnetic 22-yr cycle,
its associated butterfly diagram and field parity selection through
numerical simulations of the solar global dynamo. Inspired by recent
observations and 3D simulations that both exhibit multicellular flows
in the solar convection zone, we seek to characterise the influence of
various profiles of circulation on the behaviour of solar mean-field
dynamo models. We focus our study on a number of specific points: the
role played by these flows in setting the cycle period and the shape of
the butterfly diagram and their influence on the magnetic field parity
selection, namely the field parity switching from an antisymmetric,
dipolar field configuration to a symmetric, mostly quadrupolar one,
that has been discussed by several authors in the recent literature.
Methods: We are using 2D mean field flux transport Babcock-Leighton
numerical models in which we test several types of meridional flows:
1 large single cell, 2 cells in radius and 4 cells per hemisphere.
Results: We confirm that adding cells in latitude tends to speed
up the dynamo cycle whereas adding cells in radius more than triples
the period. We find that the cycle period in the four cells model is
less sensitive to the flow speed than in the other simpler meridional
circulation profiles studied. Moreover, our studies show that adding
cells in radius or in latitude seems to favour the parity switching to
a quadrupolar solution.
Conclusions: According to our numerical
models, the observed 22-yr cycle and dipolar parity is easily reproduced
by models including multicellular meridional flows. On the contrary, the
resulting butterfly diagram and phase relationship between the toroidal
and poloidal fields are affected to a point where it is unlikely that
such multicellular meridional flows persist for a long period of time
inside the Sun, without having to reconsider the model itself.
Title: On magnetic instabilities and dynamo action in stellar
radiation zones
Authors: Zahn, J. -P.; Brun, A. S.; Mathis, S.
Bibcode: 2007A&A...474..145Z
Altcode: 2007arXiv0707.3287Z
Context: We examine the MHD instabilities arising in the radiation
zone of a differentially rotating star, in which a poloidal field of
fossil origin is sheared into a toroidal field.
Aims: We focus
on the non-axisymmetric instability that affects the toroidal magnetic
field in a rotating star, which was first studied by Pitts and Tayler
in the non-dissipative limit. If such an instability were able to mix
the stellar material, it could have an impact on the evolution of the
star. According to Spruit, it could also drive a dynamo.
Methods:
We compare the numerical solutions built with the 3-dimensional ASH code
with the predictions drawn from an analytical study of the Pitts &
Tayler instability.
Results: The Pitts & Tayler instability
is manifestly present in our simulations, with its conspicuous m=1
dependence in azimuth. But its analytic treatment used so far is too
simplified to be applied to the real stellar situation. Although the
instability generated field reaches an energy comparable to that of the
mean poloidal field, that field seems unaffected by the instability:
it undergoes Ohmic decline, and is neither eroded nor regenerated
by the instability. The toroidal field is produced by shearing
the poloidal field and it draws its energy from the differential
rotation. The small scale motions behave as Alfvén waves; they cause
negligible eddy-diffusivity and contribute little to the net transport
of angular momentum.
Conclusions: In our simulations we observe
no sign of dynamo action, of either mean field or fluctuation type,
up to a magnetic Reynolds number of 10^5. However the Pitts &
Tayler instability is sustained as long as the differential rotation
acting on the poloidal field is able to generate a toroidal field of
sufficient strength. But in the Sun such a poloidal field of fossil
origin is ruled out by the nearly uniform rotation of the deep interior.
Title: Joint Discussion 17 Highlights of recent progress in the
seismology of the Sun and Sun-like stars
Authors: Bedding, Timothy R.; Brun, Allan S.; Christensen-Dalsgaard,
Jørgen; Crouch, Ashley; De Cat, Peter; García, Raphael A.; Gizon,
Laurent; Hill, Frank; Kjeldsen, Hans; Leibacher, John W.; Maillard,
Jean-Pierre; Mathis, S.; Rabello-Soares, M. Cristina; Rozelot,
Jean-Pierre; Rempel, Matthias; Roxburgh, Ian W.; Samadi, Réza; Talon,
Suzanne; Thompson, Michael J.
Bibcode: 2007HiA....14..491B
Altcode:
The seismology and physics of localized structures beneath the surface
of the Sun takes on a special significance with the completion in
2006 of a solar cycle of observations by the ground-based Global
Oscillation Network Group (GONG) and by the instruments on board the
Solar and Heliospheric Observatory (SOHO). Of course, the spatially
unresolved Birmingham Solar Oscillation Network (BiSON) has been
observing for even longer. At the same time, the testing of models of
stellar structure moves into high gear with the extension of deep probes
from the Sun to other solar-like stars and other multi-mode pulsators,
with ever-improving observations made from the ground, the success of
the MOST satellite, and the recently launched CoRoT satellite. Here
we report the current state of the two closely related and rapidly
developing fields of helio- and asteroseimology.
Title: Can a dynamo operate in stellar radiation zones?
Authors: Zahn, J. -P.; Brun, A. S.; Mathis, S.
Bibcode: 2007sf2a.conf..566Z
Altcode:
We examine the MHD instabilities arising in the radiation zone of a
differentially rotating star, in which a poloidal field of fossil origin
is sheared into a toroidal field. The numerical solutions built with the
3-dimensional ASH code are compared with the predictions drawn from an
analytical study of the Pitts & Tayler instability. This instability
is manifestly present in our simulations, with its conspicuous m=1
dependence in azimuth. However, although the instability generated
field reaches an energy comparable to that of the mean poloidal field,
that field seems unaffected by the instability: it undergoes Ohmic
decline, and is neither eroded nor regenerated by the instability. The
instability is sustained as long as the differential rotation acting on
the poloidal field is able to generate a toroidal field of sufficient
strength but, up to a magnetic Reynolds number of 10^5, we observe
no sign of dynamo action, of either mean field or fluctuation type,
contrary to what was suggested by Spruit.
Title: Challenges of magnetism in the turbulent Sun
Authors: Brun, Allan Sacha; Miesch, Mark S.; Toomre, Juri
Bibcode: 2007IAUS..239..488B
Altcode:
No abstract at ADS
Title: Simulations of solar magnetic dynamo action in the convection
zone and tachocline
Authors: Browning, Matthew K.; Miesch, Mark S.; Brun, Allan Sacha;
Toomre, Juri
Bibcode: 2007IAUS..239..510B
Altcode:
No abstract at ADS
Title: On the interactions of turbulent convection and rotation in
RGB stars
Authors: Palacios, Ana; Brun, Allan S.
Bibcode: 2007IAUS..239..431P
Altcode: 2006astro.ph.10040P
We have performed the first three-dimensional non-linear simulation
of the turbulent convective envelope of a rotating 0.8 Msun RGB star
using the ASH code. Adopting a global typical rotation rate of a tenth
of the solar rate, we have analyzed the dynamical properties of the
convection and the transport of angular momentum within the inner 50%
in radius of the convective envelope. The convective patterns consist
of a small number of large cell, associated with fast flows (about
3000 m/s) and large temperature fluctuations (about 300 K) in order to
carry outward the large luminosity (L* = 400 Lsun) of the star. The
interactions between convection and rotation give rise to a large
radial differential rotation and a meridional circulation possessing
one cell per hemisphere, the flow being poleward in both hemisphere. By
analysing the redistribution of angular momentum, we find that the
meridional circulation transports the angular momentum outward in the
radial direction, and poleward in the latitudinal direction, and that
the transport by Reynolds stresses acts in the opposite direction. From
this 3-D simulation, we have derived an average radial rotation profile,
that we will ultimately introduce back into 1-D stellar evolution code.
Title: Magnetic Dynamo Action In The Convective Cores Of A-type
Stars In The Presence Of Fossil Fields
Authors: Featherstone, Nicholas; Browning, M. K.; Brun, A. S.;
Toomre, J.
Bibcode: 2007AAS...210.1702F
Altcode: 2007BAAS...39Q.117F
The intense surface magnetism of Ap stars has attracted much
scrutiny. The observed persistent fields are generally thought to be
of primordial origin, but dynamo generation of magnetic fields may
offer alternative possibilities. Deep within the interiors of such
stars, vigorous core convection likely couples with rotation to yield
magnetic dynamo action, generating strong magnetic fields. Recent
numerical models suggest that a primordial field remaining from the
star’s formation may possess a highly twisted toroidal shape in the
radiative interior. We have used detailed 3-D simulations to study the
interaction of such a magnetic field with a dynamo generated within the
core of a 2 solar mass A-type star. Dynamo action realized under these
circumstances is much more vigorous than in the absence of a fossil
field in the radiative envelope, yielding magnetic field strengths (of
order 100 kG) much higher than their equipartition values relative to
the convective velocities. We examine the generation of these fields,
as well as their effect on the complex dynamics of the convective core.
Title: Structure and Evolution of Giant Cells in Global Models of
Solar Convection
Authors: Miesch, Mark S.; Brun, A. S.; De Rosa, M. L.; Toomre, J.
Bibcode: 2007AAS...210.2217M
Altcode: 2007BAAS...39..127M
We present the highest-resolution simulations of global-scale solar
convection so far achieved, dealing with turbulent compressible
flows interacting with rotation in a full spherical shell. The
three-dimensional simulation domain extends from 0.71R-0.98R, close
enough to the photosphere to overlap with solar subsurface weather
(SSW) maps inferred from local helioseismology. The convective patterns
achieved are complex and continually evolving on a time scale of several
days. However, embedded within the intricate downflow network near
the surface are coherent downflow lanes associated with giant cells
which persist for weeks to months and which extend through much of the
convection zone. These coherent downflow lanes are generally confined
to low latitudes and are oriented in a north-south direction. The low
dissipation in these simulations permits a more realistic balance of
forces which yields differential rotation and meridional circulation
profiles in good agreement with those inferred from helioseismology.
Title: Rapid Rotation And Nests Of Convection In Solar-like Stars
Authors: Brown, Benjamin; Browning, M. K.; Brun, A. S.; Miesch, M. S.;
Toomre, J.
Bibcode: 2007AAS...210.1703B
Altcode: 2007BAAS...39..117B
Earlier in its life our Sun must have rotated considerably more
rapidly, given that its magnetized wind slowly carries away angular
momentum. Indeed many G-type stars are found to rotate rapidly, and
their deep convective envelopes and the dynamos operating there must
sense the effects of rotation. Here we use 3-D simulations to study
the differential rotation and patterns of convection established in
these more rapidly rotating stars. Our simulations with the anelastic
spherical harmonic (ASH) code capture the deep solar convection
zone with a solar-like radial stratification and within a spherical
geometry, which admits global-scale flows. We explore a range of
rotation rates from 1 to 10 times the solar rotation rate. Convection
in the equatorial regions of these rapidly rotating stars shows strong
longitudinal modulation. At the fastest rotation rates, convection is
restricted to active nests spanning compact regions in longitude, with
quiescent streaming flow filling the regions in between. These nests
of convection persist for long periods and drive a strong differential
rotation. Convection at high latitudes is more isotropic but couples
to the equatorial regions through the meridional circulations present
throughout the shell.
Title: Dynamo Action, Magnetic Activity, And Rotation In F Stars
Authors: Augustson, Kyle; Brown, B. P.; Brun, A. S.; Toomre, J.
Bibcode: 2007AAS...210.1701A
Altcode: 2007BAAS...39..117A
The origin of stellar magnetic fields must rest with dynamo
processes occurring deep within a star. Observations of F-type stars
suggest unusual relations between their rotation rates and magnetic
activity. Generally in cooler stars, magnetic activity increases with
more rapid rotation, but, in F-type stars, there is observational
evidence for a sharp transition from this behavior around spectral
type F5. Stars hotter than F5 show an anti-correlation between
magnetic activity and rotation: more rapidly rotating stars seem
to possess weaker magnetic fields, possibly because they have less
efficient dynamos. We have conducted 3-D simulations of compressible
MHD convection with the anelastic spherical harmonic (ASH) code,
in order to study F-type star convection zone dynamics in rotating
spherical shells. Our initial radial stratification is based on stellar
models of stars in the narrow mass range between 1.2 and 1.4 solar
masses. We exhibit the resulting differential rotation profiles and
rich convective behavior realized as the rotation rates of the stars
are increased. We also discuss our preliminary foray into studying
the magnetic dynamo achieved within several models, considering the
effects of rotation rates.
Title: Strong Global Dynamo Action in a Younger Sun
Authors: Brown, Benjamin; Brun, A. S.; Miesch, M. S.; Toomre, J.
Bibcode: 2007AAS...210.2414B
Altcode: 2007BAAS...39..130B
Stellar dynamos are powered by the coupling of rotation, convection and
the global scale flows which are established in these systems. Our Sun
has lost angular momentum through its magnetized wind and once rotated
more rapidly than it currently does. We explore the nature of dynamo
action in a younger sun rotating five times its current rate. Our
explorations employ 3-D simulations of compressible MHD convection
within a spherical shell extending from 0.72 to 0.97 solar radii using
the anelastic spherical harmonic (ASH) code on massively parallel
supercomputers. The dynamo which naturally arises in this convective
system is vigorous and builds organized magnetic structures which
fill the bulk of the convection zone. This is in striking contrast
to the global dynamo thought to operate in the current sun, which
appears to require the pumping of magnetic field into a tachocline of
shear at the base of the convection zone to generate similar magnetic
structures. Particularly in the equatorial regions, we find strong
toroidal fields ( 30 kG) coexisting with the turbulent convection. This
dynamo system exhibits cyclic behavior, with the large-scale toroidal
and poloidal fields switching their polarity.
Title: Towards using modern data assimilation and weather forecasting
methods in solar physics
Authors: Brun, A. S.
Bibcode: 2007AN....328..329B
Altcode:
We discuss how data assimilation and forecasting methods developed
in Earth's weather prediction models could be used to improve our
capability to anticipate solar dynamical phenomena and assimilate the
huge amount of data that new solar satellites, such as SDO or Hinode,
will provide in the coming years. We illustrate with some simple
examples such as the solar magnetic activity cycle, the eruption
of CMEs, the real potential of such methods for solar physics. We
believe that we now need to jointly develop solar forecasting models,
whose purpose are to assimilate observational data in order to improve
our predictability power, with ``first principle'' solar models, whose
purpose is to understand the underpinning physical processes behind the
solar dynamics. These two complementary approaches should lead to the
development of a solar equivalent of Earth's general circulation model.
Title: Magnetic confinement of the solar tachocline
Authors: Brun, A. S.; Zahn, J. -P.
Bibcode: 2006A&A...457..665B
Altcode: 2006astro.ph.10069B
Context: .We study the physics of the solar tachocline (i.e. the thin
transition layer between differential rotation in the convection
zone and quasi uniform rotation in the radiative interior), and
related MHD instabilities.
Aims: .We have performed 3D MHD
simulations of the solar radiative interior to check whether a fossil
magnetic field is able to prevent the spread of the tachocline.
Methods: .Starting with a purely poloidal magnetic field and
a latitudinal shear meant to be imposed by the convection zone at
the top of the radiation zone, we have investigated the interactions
between magnetic fields, rotation and shear, using the spectral code
ASH on massively parallel supercomputers.
Results: .In all
cases we have explored, the fossil field diffuses outward and ends
up connecting with the convection zone, whose differential rotation
is then imprinted at latitudes above ≈40° throughout the radiative
interior, according to Ferraro's law of isorotation. Rotation remains
uniform in the lower latitude region which is contained within closed
field lines. We find that the meridional flow cannot stop the inward
progression of the differential rotation. Further, we observe the
development of non-axisymmetric magnetohydrodynamic instabilities,
first due to the initial poloidal configuration of the fossil field,
and later to the toroidal field produced by shearing the poloidal field
through the differential rotation. We do not find dynamo action as
such in the radiative interior, since the mean poloidal field is not
regenerated. But the instability persists during the whole evolution,
while slowly decaying with the mean poloidal field; it is probably
sustained by small departures from isorotation.
Conclusions:
.According to our numerical simulations, a fossil magnetic field cannot
prevent the radiative spread of the tachocline, and thus it is unable
to enforce uniform rotation in the radiation zone. Neither can the
observed thinness of that layer be invoked as a proof for such an
internal fossil magnetic field.
Title: The DynaMICS perspective
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.624E..24T
Altcode: 2006soho...18E..24T
No abstract at ADS
Title: On the possible existence of localised vacillating convection
state in rapidly rotating young solar-like stars
Authors: Ballot, J.; Brun, A. S.; Turck-Chièze, S.
Bibcode: 2006ESASP.624E.108B
Altcode: 2006soho...18E.108B
No abstract at ADS
Title: Dynamo Action in the Solar Convection Zone and Tachocline:
Pumping and Organization of Toroidal Fields
Authors: Browning, Matthew K.; Miesch, Mark S.; Brun, Allan Sacha;
Toomre, Juri
Bibcode: 2006ApJ...648L.157B
Altcode: 2006astro.ph..9153B
We present the first results from three-dimensional spherical shell
simulations of magnetic dynamo action realized by turbulent convection
penetrating downward into a tachocline of rotational shear. This permits
us to assess several dynamical elements believed to be crucial to the
operation of the solar global dynamo, variously involving differential
rotation resulting from convection, magnetic pumping, and amplification
of fields by stretching within the tachocline. The simulations reveal
that strong axisymmetric toroidal magnetic fields (about 3000 G in
strength) are realized within the lower stable layer, unlike in the
convection zone where fluctuating fields are predominant. The toroidal
fields in the stable layer possess a striking persistent antisymmetric
parity, with fields in the northern hemisphere largely of opposite
polarity to those in the southern hemisphere. The associated mean
poloidal magnetic fields there have a clear dipolar geometry, but
we have not yet observed any distinctive reversals or latitudinal
propagation. The presence of these deep magnetic fields appears to
stabilize the sense of mean fields produced by vigorous dynamo action
in the bulk of the convection zone.
Title: The Role of Multi cellular Meridional Flows in Setting the
Cycle Period and Field Parity in Solar Dynamo Models
Authors: Jouve, L.; Brun, A. S.
Bibcode: 2006IAUJD...8E..12J
Altcode:
Inspired by recent observations and 3-D simulations that both
exhibit multicellular flows in the solar convective zone, we seek
to characterize the influence of such flows on the behaviour of
solar dynamo models. We focused on two particular points: the role
played by these flows in setting the cycle period and the so-called
magnetic field parity issue, namely the field parity switching from
an antisymmetric, dipolar field configuration to a symmetric, mostly
quadrupolar one, that has already been discussed by several authors in
the recent literature. Using a 2-D mean field Babcock-Leighton (B-L)
model of the solar dynamo, we confirm that adding cells in latitude
tends to speed up the dynamo cycle whereas we find that adding cells
in radius increases the cycle period by more than 60%. Moreover, our
studies show that adding cells both in radius and in latitude imposes
symmetry conservation: the presence of more complex mean meridional
flows in the model suppresses the switching of the field parity from a
dipolar configuration to a quadrupolar one, thus resolving the parity
issue seen in classical B-L solar dynamo models.
Title: What can 3-D global simulations teach us about the solar
turbulent convection zone, differential rotation and meridional
circulation?
Authors: Brun, A. S.
Bibcode: 2006IAUJD..17E...5B
Altcode:
Understanding the complex solar surface convection zone and associated
mean flows is a great challenge of modern astrophysics. Thanks to
helioseismology and powerful parallel supercomputers, we are starting
to make significant progress in developing a coherent picture of the
solar internal dynamics. We here report on the recent advances made in
modelling in three dimensions in a spherical shell, with the ASH code,
the solar turbulent convection zone and its nonlinear and thermal
coupling with the boundary layer called the tachocline. We find that
baroclinic balance (i.e. thermal wind) and Reynolds stresses are key
players in establishing the observed solar differential rotation. The
associated meridional circulation is found to possess a multi-cell
structure both in radius and latitude. Such a profile, if confirmed by
deep helioseismic inversions, could significantly impact our current
understanding of the solar global dynamo.
Title: Scientific Objectives of the Novel Formation Flying Mission
Aspiics
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.617E.164L
Altcode: 2006soho...17E.164L
No abstract at ADS
Title: The Influence on the 22-Year Solar Cycle of Multicellular
Meridional Flows
Authors: Jouve, L.; Brun, A. S.
Bibcode: 2006ESASP.617E..40J
Altcode: 2006soho...17E..40J
No abstract at ADS
Title: The EUV Variability Experiment (EVE) on the Solar Dynamics
Observatory (SDO): Science Plan and Instrument Overview
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.617E.165W
Altcode: 2006soho...17E.165W
No abstract at ADS
Title: The Dynamics Project
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.617E.162T
Altcode: 2006soho...17E.162T
No abstract at ADS
Title: The Solar Internal Magnetism: Putting together More Pieces
of the Puzzle
Authors: Brun, A. S.; Jouve, L.
Bibcode: 2006ESASP.617E..54B
Altcode: 2006soho...17E..54B
No abstract at ADS
Title: Rising flux tubes in a spherical convective shell
Authors: Jouve, L.; Brun, A. S.
Bibcode: 2006sf2a.conf..473J
Altcode:
We discuss recent 3D MHD numerical simulations computed with the ASH
code of the non-linear dynamical evolution of magnetic flux tubes in
a three-dimensional spherical convective zone, that maintains self
consistently a solar like differential rotation and a large scale
meridional circulation. We seek to understand the mechanism of emergence
of strong toroidal fields from the base of the solar convection zone to
the surface and their interactions with convection and its associated
mean flows. We find that mainly two parameters influence the tubes
during their rise through the convection zone: the degree of initial
twist of the field lines and the initial strength of the magnetic
field inside the tube.
Title: Magnetic instabilities in stellar radiation zones
Authors: Brun, A. S.; Zahn, J. -P.
Bibcode: 2006sf2a.conf..451B
Altcode:
Using the 3-dimensional ASH code, we have studied numerically the
instabilities that occur in stellar radiation zones in presence of
large-scale magnetic fields and differential rotation. We confirm
that some configurations are linearly unstable, as predicted by
Tayler and collaborators, and we determine the saturation level of
the instability. However we found no sign of the dynamo mechanism
suggested recently by Spruit.
Title: Localized Nests of Convection in Rapidly Rotating Suns
Authors: Brown, Benjamin; Browning, M.; Brun, A.; Toomre, J.
Bibcode: 2006SPD....37.3205B
Altcode: 2006BAAS...38..258B
Many solar-like stars rotate more rapidly than the sun. Through
their magnetized winds, these stars gradually lose angular momentum
and spin down. By similar processes, our Sun must have rotated more
rapidly in the past than it currently does. We explore the effects
of more rapid rotation upon turbulent stellar convection, studying
full spherical shells that admit global scale flows. We conduct 3-D
simulations of compressible turbulent convection with the anelastic
spherical harmonic (ASH) code on massively parallel supercomputers. For
simplicity, we adopt the radial stratification of the present-day sun
and examine global scale convection in a zone extending from 0.72 to
0.97 solar radii, and consider a range of rotation rates from 1 to 5
times the solar rotation rate. With increasing rotation we observe
that convection at low latitudes becomes spatially modulated in
strength, yielding localized nests of strong convection. These nests
are persistent over very long periods and propagate in longitude at
slower rates than individual convective structures within them. It
is striking that a strong differential rotation is achieved by these
modulated states. The convection at high latitudes is more isotropic
but influenced by the meridional circulations present throughout the
shell. Weak modulation can be recognized even at the solar rotation
rate, with some implications for active magnetic longitudes in the Sun.
Title: Solar Differential Rotation Influenced by Latitudinal Entropy
Variations in the Tachocline
Authors: Miesch, Mark S.; Brun, Allan Sacha; Toomre, Juri
Bibcode: 2006ApJ...641..618M
Altcode:
Three-dimensional simulations of solar convection in spherical shells
are used to evaluate the differential rotation that results as thermal
boundary conditions are varied. In some simulations a latitudinal
entropy variation is imposed at the lower boundary in order to take
into account the coupling between the convective envelope and the
radiative interior through thermal wind balance in the tachocline. The
issue is whether the baroclinic forcing arising from tachocline-induced
entropy variations can break the tendency for numerical simulations of
convection to yield cylindrical rotation profiles, unlike the conical
profiles deduced from helioseismology. As the amplitude of the imposed
variation is increased, cylindrical rotation profiles do give way to
more conical profiles that exhibit nearly radial angular velocity
contours at midlatitudes. Conical rotation profiles are maintained
primarily by the resolved convective heat flux, which transmits entropy
variations from the lower boundary into the convective envelope, giving
rise to baroclinic forcing. The relative amplitude of the imposed
entropy variations is of order 10-5, corresponding to a
latitudinal temperature variation of about 10 K. The role of thermal
wind balance and tachocline-induced entropy variations in maintaining
the solar differential rotation is discussed.
Title: The origin of the solar cyclic activities: the DynaMICS project
Authors: Turck-Chieze, S.; Brun, A. S.; Garcia, R. A.; Jiménez-Reyes,
S. J.; Palle, P.; Dynamics Team
Bibcode: 2006cosp...36.2001T
Altcode: 2006cosp.meet.2001T
In order to better estimate the earth climatic variations at scales
corresponding to decennia or centuries it appears more and more
important to understand the internal origin of the solar magnetic
cyclic activities together with the evolution of the internal solar
rotation profile It is the only way to be able to predict how they
will evolve in the future The seismic techniques are totally adapted
to this knowledge and an enriched information will allow to interpret
the solar global variations as irradiance luminosity at different
wavelengths and will measure temporal global mode characteristics which
must be linked to the total magnetic fluxes ldots Our main objectives
are to predict the characteristics of the coming solar cycles and to
determine if there is different origins for the longer solar cycles or
if it is only a temporal evolution of the eleven cycle 22 years which
produces grand minima or maxima SDO is well adapted to progress on the
convective zone with increased resolution in comparison with the SoHO
mission it will allow to improve the 11 year solar cycle predictions In
complementarity we consider very important to get a general description
of the dynamics of the solar radiative zone which contains the main
part of the solar mass and to understand the interconnection between
magnetic fields of the radiative zone and of the convective zones Such
information stays today poorly known even SoHO results on the solar
radiative zone through acoustic and gravity modes are very promising
to pursue this investigation In this
Title: Spectral magnetohydrodynamic simulations of the sun and stars
Authors: Brun, A. S.
Bibcode: 2006EAS....21..181B
Altcode:
The purpose of this lecture is two fold: first, to describe a powerful
numerical technic, namely the spectral method, to solve the compressible
(anelastic) magnetohydrodynamic (MHD) equations in spherical geometry
and then to discuss some recent numerical applications to study stellar
dynamics and magnetism. We thus start by describing the semi-implicit,
anelastic spherical harmonic (ASH) code. In this code, the main field
variables are projected into spherical harmonics for their horizontal
dimensions and into Chebyshev polynomials for their radial direction. We
then present, high resolution 3 D MHD simulations of the convective
region of A- and G-type stars in spherical shells. We have chosen to
model A and G-type stars because they represent good proxies to study
and understand stellar dynamics and magnetism given their strikingly
different internal “up-side-down” structure and magnetic activity
level. In particular, we discuss the nonlinear interactions between
turbulent convection, rotation and magnetic fields and the possibility
for such flows and fields to lead to dynamo action. We find that both
core and envelope turbulent convective zones are efficient at inducing
strong mostly non-axisymmetric fields near equipartition but at the
expense of damping the differential rotation present in the purely
hydrodynamic progenitor solutions.
Title: a Challenging Turbulent Magnetic Sun
Authors: Brun, A. S.
Bibcode: 2005ESASP.600E...3B
Altcode: 2005ESPM...11....3B; 2005dysu.confE...3B
No abstract at ADS
Title: The influence of multicellular meridional flows in setting
the cycle period in solar dynamo models
Authors: Jouve, L.; Brun, A. S.
Bibcode: 2005sf2a.conf..763J
Altcode:
Inspired by recent observations and 3D simulations that both exhibit
multicellular meridional circulation flows in the solar convection zone,
we seek to characterize the influence of such multiple flows in setting
the magnetic solar cycle period. For most existing mean field dynamo
models of flux transport type, the flow is assumed to be constituted of
a large single cell per meridional quadrant extending from the surface
(where it is poleward) to slightly below the core-envelope interface
(where it is equatorward). Here we study the influence of adding
meridional cells both in latitude and in radius. We confirm that 2
cells in latitude speeds up the cycle but we find on the contrary that
2 cells in radius significantly increases the cycle period.
Title: Simulations of Core Convection in Rotating A-Type Stars:
Magnetic Dynamo Action
Authors: Brun, Allan Sacha; Browning, Matthew K.; Toomre, Juri
Bibcode: 2005ApJ...629..461B
Altcode: 2006astro.ph.10072B
Core convection and dynamo activity deep within rotating A-type
stars of 2 Msolar are studied with three-dimensional
nonlinear simulations. Our modeling considers the inner 30% by
radius of such stars, thus capturing within a spherical domain the
convective core and a modest portion of the surrounding radiative
envelope. The magnetohydrodynamic (MHD) equations are solved using the
anelastic spherical harmonic (ASH) code to examine turbulent flows and
magnetic fields, both of which exhibit intricate time dependence. By
introducing small seed magnetic fields into our progenitor hydrodynamic
models rotating at 1 and 4 times the solar rate, we assess here how
the vigorous convection can amplify those fields and sustain them
against ohmic decay. Dynamo action is indeed realized, ultimately
yielding magnetic fields that possess energy densities comparable to
that of the flows. Such magnetism reduces the differential rotation
obtained in the progenitors, partly by Maxwell stresses that transport
angular momentum poleward and oppose the Reynolds stresses in the
latitudinal balance. In contrast, in the radial direction we find
that the Maxwell and Reynolds stresses may act together to transport
angular momentum. The central columns of slow rotation established in
the progenitors are weakened, with the differential rotation waxing and
waning in strength as the simulations evolve. We assess the morphology
of the flows and magnetic fields, their complex temporal variations,
and the manner in which dynamo action is sustained. Differential
rotation and helical convection are both found to play roles in
giving rise to the magnetic fields. The magnetism is dominated by
strong fluctuating fields throughout the core, with the axisymmetric
(mean) fields there relatively weak. The fluctuating magnetic fields
decrease rapidly with radius in the region of overshooting, and the
mean toroidal fields less so due to stretching by rotational shear.
Title: On the Coupled Influence of Rotation and Magnetism in
Convective Core of A-type Stars
Authors: Brun, A. S.
Bibcode: 2005EAS....17..203B
Altcode:
We briefly report on an ongoing numerical project that aims at
simulating, with the ASH code, the intricate magnetohydrodynamic
processes present in the inner region of A-type stars. We mainly focus
our attention, on the dynamics of the convective core, its associated
differential rotation and meridional ciculation and the dynamo action
than can occur in such tubulent MHD system. We indeed find that magnetic
fields with amplitude greater than 10 kG are likely to be present in
the core of A-type stars.
Title: Magnetohydrodynamic 3-D Models of the Solar Convection Zone
Authors: Brun, Allan Sacha
Bibcode: 2005HiA....13...94B
Altcode:
We discuss recent progresses made in modelling the complex
magnetohydrodynamics of the Sun using our anelastic spherical harmonics
(ASH) code on massively parallel computers. We have conducted 3--D
MHD simulations of compressible convection in spherical shells to
study the coupling between convection rotation and magnetic field
in seeking to understand how the solar differential rotation is
established and maintained. The resulting convection within domains
that capture a good fraction of the bulk of the solar convection zone
is highly time dependent and intricate and is dominated by intermittent
upflows and networks of strong downflows (i.e. plumes). These plumes
play a significant role in yielding Reynolds stresses that serve to
redistribute angular momentum leading to angular velocity profiles
that make good contact with helioseismic deductions. Such complex
convective flows are efficient in amplifying the magnetic energy near
equipartition. The resulting magnetic fields are found to concentrate
around the downflowing networks and to have significant north-south
asymmetry and helicity. But these strong fields yield Maxwell stresses
that seek to speed up the poles and destroy the agreement with
helioseismic observations. So for a given angular velocity profile
the level of magnetism that the Sun can sustain is likely to be limited.
Title: Simulations of core convection and resulting dynamo action
in rotating A-type stars
Authors: Browning, Matthew K.; Brun, Allan S.; Toomre, Juri
Bibcode: 2004IAUS..224..149B
Altcode: 2004astro.ph..9703B
We present the results of 3-D nonlinear simulations of magnetic dynamo
action by core convection within A-type stars of 2 M⊙ with
a range of rotation rates. We consider the inner 30% by radius of such
stars, with the spherical domain thereby encompassing the convective
core and a portion of the surrounding radiative envelope. The
compressible Navier-Stokes equations, subject to the anelastic
approximation, are solved to examine highly nonlinear flows that span
multiple scale heights, exhibit intricate time dependence, and admit
magnetic dynamo action. Small initial seed magnetic fields are found
to be amplified greatly by the convective and zonal flows. The central
columns of strikingly slow rotation found in some of our progenitor
hydrodynamic simulations continue to be realized in some simulations
to a lesser degree, with such differential rotation arising from the
redistribution of angular momentum by the nonlinear convection and
magnetic fields. We assess the properties of the magnetic fields thus
generated, the extent of the convective penetration, the magnitude of
the differential rotation, and the excitation of gravity waves within
the radiative envelope.
Title: Core Convection and Dynamo Action in Rotating A-type Stars
Authors: Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2004AAS...205.3403B
Altcode: 2004BAAS...36.1402B
We have carried out 3-D simulations of core convection and dynamo
activity within A-type stars of two solar masses at a range of rotation
rates. Our models consider the inner 30% by radius of such stars, thus
capturing the entire convective core and a portion of the surrounding
radiative envelope within the spherical computational domain. Using
the anelastic spherical harmonic (ASH) code on massively parallel
supercomputers, we solve the compressible MHD equations to examine
highly nonlinear and evolving flows and magnetic fields. Vigorous
dynamo action is realized, with initial seed magnetic fields amplified
by many orders of magnitude and sustained against ohmic decay. The
resulting complex magnetism possesses energy densities comparable to
that in the flows, is structured on many scales, and serves to modify
the convective and zonal flows that gave rise to it. The differential
rotation established in progenitor hydrodynamic simulations is weakened,
and waxes and wanes in strength as the simulations evolve. We discuss
the morphology and evolution of the flows and magnetic fields,
the penetrative properties of the convection, and the nature of the
dynamo process.
Title: Turbulent Convection in Young Solar-like Stars: Influence
of rotation
Authors: Ballot, J.; Brun, A. S.; Turck-Chièze, S.
Bibcode: 2004sf2a.conf..197B
Altcode: 2004sf2a.confE.266B
The study of the relationship between X-ray emission and rotation in
young stars (Feigelson et al. 2003) and observations of magnetic-field
topology of such stars with Zeeman-Doppler Imaging (Donati et al. 2003)
indicate that the dynamo processes differ from those operating in main
sequence stars. In this context, 3-D numerical simulations have been
started. The first step is to study the purely hydrodynamic case. We
have simulated the convective shell of a young sun (10 Myr) with the
Anelastic Spherical Harmonic (ASH) code. We have studied the angular
momentum transfer, the meridional circulation and the differential
rotation in this shell. We have also studied the effects of different
rotation rates (1, 2 and 5 solar rate).
Title: Turbulent Convection and Dynamo Action in A- and G-type stars
Authors: Brun, A. S.
Bibcode: 2004sf2a.conf..207B
Altcode:
No abstract at ADS
Title: Simulations of Core Convection and Dynamo Activity in Rotating
A-Type Stars
Authors: Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2004ESASP.559..349B
Altcode: 2004soho...14..349B
No abstract at ADS
Title: Differential Rotation when the Sun Spun Faster
Authors: Brown, B. P.; Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2004ESASP.559..341B
Altcode: 2004soho...14..341B
No abstract at ADS
Title: D MHD Simulations of the Solar Convection Zone and Tachocline
Authors: Brun, A. S.
Bibcode: 2004ESASP.559..271B
Altcode: 2004soho...14..271B
No abstract at ADS
Title: Global-Scale Turbulent Convection and Magnetic Dynamo Action
in the Solar Envelope
Authors: Brun, Allan Sacha; Miesch, Mark S.; Toomre, Juri
Bibcode: 2004ApJ...614.1073B
Altcode: 2006astro.ph.10073B
The operation of the solar global dynamo appears to involve many
dynamical elements, including the generation of fields by the intense
turbulence of the deep convection zone, the transport of these fields
into the tachocline region near the base of the convection zone,
the storage and amplification of toroidal fields in the tachocline by
differential rotation, and the destabilization and emergence of such
fields due to magnetic buoyancy. Self-consistent magnetohydrodynamic
(MHD) simulations that realistically incorporate all of these processes
are not yet computationally feasible, although some elements can now
be studied with reasonable fidelity. Here we consider the manner in
which turbulent compressible convection within the bulk of the solar
convection zone can generate large-scale magnetic fields through dynamo
action. We accomplish this through a series of three-dimensional
numerical simulations of MHD convection within rotating spherical
shells using our anelastic spherical harmonic (ASH) code on massively
parallel supercomputers. Since differential rotation is a key ingredient
in all dynamo models, we also examine here the nature of the rotation
profiles that can be sustained within the deep convection zone as strong
magnetic fields are built and maintained. We find that the convection
is able to maintain a solar-like angular velocity profile despite the
influence of Maxwell stresses, which tend to oppose Reynolds stresses
and thus reduce the latitudinal angular velocity contrast throughout
the convection zone. The dynamo-generated magnetic fields exhibit a
complex structure and evolution, with radial fields concentrated in
downflow lanes and toroidal fields organized into twisted ribbons
that are extended in longitude and achieve field strengths of up to
5000 G. The flows and fields exhibit substantial kinetic and magnetic
helicity although systematic hemispherical patterns are only apparent in
the former. Fluctuating fields dominate the magnetic energy and account
for most of the back-reaction on the flow via Lorentz forces. Mean
fields are relatively weak and do not exhibit systematic latitudinal
propagation or periodic polarity reversals as in the Sun. This may
be attributed to the absence of a tachocline, i.e., a penetrative
boundary layer between the convection zone and the deeper radiative
interior possessing strong rotational shear. The influence of such a
layer will await subsequent studies.
Title: Solar Differential Revealed by Helioseismology and Simulations
of Deep Shells of Turbulent Convection
Authors: Toomre, J.; Brun, A. S.
Bibcode: 2004IAUS..215..326T
Altcode:
No abstract at ADS
Title: Erratum: ``Looking for Gravity-Mode Multiplets with the GOLF
Experiment aboard SOHO'' (ApJ,
604, 455 [2004])
Authors: Turck-Chièze, S.; García, R. A.; Couvidat, S.; Ulrich,
R. K.; Bertello, L.; Varadi, F.; Kosovichev, A. G.; Gabriel, A. H.;
Berthomieu, G.; Brun, A. S.; Lopes, I.; Pallé, P.; Provost, J.;
Robillot, J. M.; Roca Cortés, T.
Bibcode: 2004ApJ...608..610T
Altcode:
As a result of an error at the Press, the second panel of Figure 9
was repeated twice in the top row of the printed, black-and-white
version of this figure, and the first panel was omitted. This error
appears in the print edition and the PDF and postscript (PS) versions
available with the electronic edition of the journal, although the
panels of the color figure displayed in the electronic article itself
are correct. Please see below for the corrected print version of Figure
9. The Press sincerely regrets the error.
Title: Looking Deep Within an A-type Star: Core Convection Under
the Influence of Rotation
Authors: Brun, A. S.; Browning, M.; Toomre, J.
Bibcode: 2004IAUS..215..388B
Altcode: 2003astro.ph..2598B
The advent of massively parallel supercomputing has begun to permit
explicit 3--D simulations of turbulent convection occurring within the
cores of early-type main sequence stars. Such studies should complement
the stellar structure and evolution efforts that have so far largely
employed 1--D nonlocal mixing length descriptions for the transport,
mixing and overshooting achieved by core convection. We have turned
to A-type stars as representative of many of the dynamical challenges
raised by core convection within rotating stars. The differential
rotation and meridional circulations achieved deep within the star by
the convection, the likelihood of sustained magnetic dynamo action
there, and the bringing of fresh fuel into the core by overshooting
motions, thereby influencing main sequence lifetimes, all constitute
interesting dynamical questions that require detailed modelling
of global-scale convection. Using our anelastic spherical harmonic
(ASH) code tested on the solar differential rotation problem, we have
conducted a series of 3--D spherical domain simulations that deal with
a simplified description of the central regions of rotating A-type
stars, i.e a convectively unstable core is surrounded by a stable
radiative envelope. A sequence of 3--D simulations are used to assess
the properties of the convection (its global patterns, differential
rotation, meridional circulations, extent and latitudinal variation of
the overshooting) as transitions are made between laminar and turbulent
states by changing the effective diffusivities, rotation rates, and
subadiabaticity of the radiative exterior. We report on the properties
deduced from these models for both the extent of penetration and the
profile of rotation sustained by the convection.
Title: Simulations of Core Convection Dynamos in Rotating A-type Stars
Authors: Browning, M.; Brun, A. S.; Toomre, J.
Bibcode: 2004IAUS..215..376B
Altcode:
No abstract at ADS
Title: Simulations of Core Convection and Dynamo Activity in A-type
Stars at a Range of Rotation Rates
Authors: Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2004AAS...204.0707B
Altcode: 2004BAAS...36..786B
We present the results of nonlinear 3--D simulations of magnetic dynamo
action by core convection within A-type stars of 2 solar masses, at a
range of rotation rates. We consider the inner 30% by radius of such
stars, with the spherical domain thereby encompassing the convective
core and a portion of the surrounding radiative envelope. We solve the
compressible Navier-Stokes equations in the anelastic approximation to
examine highly nonlinear flows that span multiple scale heights, exhibit
intricate time dependence, and admit magnetic dynamo action. Small
initial seed magnetic fields are found to be amplified greatly by the
convective and zonal flows. The central columns of strikingly slow
rotation found in some of our progenitor hydrodynamic simulations
continue to be realized in some simulations to a lesser degree,
with such differential rotation arising from the redistribution of
angular momentum by the nonlinear convection and magnetic fields. We
assess the properties of the magnetic fields thus generated and the
magnitude of the differential rotation sustained as the rotation rate
in our simulations is varied.
Title: On the interaction between differential rotation and magnetic
fields in the Sun
Authors: Brun, Allan Sacha
Bibcode: 2004SoPh..220..333B
Altcode:
We have performed 3-D numerical simulations of compressible convection
under the influence of rotation and magnetic fields in spherical
shells. They aim at understanding the subtle coupling between
convection, rotation and magnetic fields in the solar convection
zone. We show that as the magnetic Reynolds number is increased in the
simulations, the magnetic energy saturates via nonlinear dynamo action,
to a value smaller but comparable to the kinetic energy contained in
the shell, leading to increasingly strong Maxwell stresses that tend
to weaken the differential rotation driven by the convection. These
simulations also indicate that the mean toroidal and poloidal magnetic
fields are small compared to their fluctuating counterparts, most of
the magnetic energy being contained in the non-axisymmetric fields. The
intermittent nature of the magnetic fields generated by such a turbulent
convective dynamo confirms that in the Sun the large-scale ordered
dynamo responsible for the 22-year cycle of activity can hardly be
located in the solar convective envelope.
Title: Looking for Gravity-Mode Multiplets with the GOLF Experiment
aboard SOHO
Authors: Turck-Chièze, S.; García, R. A.; Couvidat, S.; Ulrich,
R. K.; Bertello, L.; Varadi, F.; Kosovichev, A. G.; Gabriel, A. H.;
Berthomieu, G.; Brun, A. S.; Lopes, I.; Pallé, P.; Provost, J.;
Robillot, J. M.; Roca Cortés, T.
Bibcode: 2004ApJ...604..455T
Altcode:
This paper is focused on the search for low-amplitude solar gravity
modes between 150 and 400 μHz, corresponding to low-degree, low-order
modes. It presents results based on an original strategy that looks
for multiplets instead of single peaks, taking into consideration
our knowledge of the solar interior from acoustic modes. Five years
of quasi-continuous measurements collected with the helioseismic GOLF
experiment aboard the SOHO spacecraft are analyzed. We use different
power spectrum estimators and calculate confidence levels for the
most significant peaks. This approach allows us to look for signals
with velocities down to 2 mm s-1, not far from the limit
of existing instruments aboard SOHO, amplitudes that have never been
investigated up to now. We apply the method to series of 1290 days,
beginning in 1996 April, near the solar cycle minimum. An automatic
detection algorithm lists those peaks and multiplets that have a
probability of more than 90% of not being pure noise. The detected
patterns are then followed in time, considering also series of 1768 and
2034 days, partly covering the solar cycle maximum. In the analyzed
frequency range, the probability of detection of the multiplets
does not increase with time as for very long lifetime modes. This is
partly due to the observational conditions after 1998 October and the
degradation of these observational conditions near the solar maximum,
since these modes have a ``mixed'' character and probably behave as
acoustic modes. Several structures retain our attention because of
the presence of persistent peaks along the whole time span. These
features may support the idea of an increase of the rotation in the
inner core. There are good arguments for thinking that complementary
observations up to the solar activity minimum in 2007 will be decisive
for drawing conclusions on the presence or absence of gravity modes
detected aboard the SOHO satellite.
Title: Simulations of Core Convection in Rotating A-Type Stars:
Differential Rotation and Overshooting
Authors: Browning, Matthew K.; Brun, Allan Sacha; Toomre, Juri
Bibcode: 2004ApJ...601..512B
Altcode: 2003astro.ph.10003B
We present the results of three-dimensional simulations of core
convection within A-type stars of 2 Msolar, at a range of
rotation rates. We consider the inner 30% by radius of such stars,
thereby encompassing the convective core and some of the surrounding
radiative envelope. We utilize our anelastic spherical harmonic
code, which solves the compressible Navier-Stokes equations in the
anelastic approximation, to examine highly nonlinear flows that can
span multiple scale heights. The cores of these stars are found to
rotate differentially, with central cylindrical regions of strikingly
slow rotation achieved in our simulations of stars whose convective
Rossby number (Roc) is less than unity. Such differential
rotation results from the redistribution of angular momentum by the
nonlinear convection that strongly senses the overall rotation of
the star. Penetrative convective motions extend into the overlying
radiative zone, yielding a prolate shape (aligned with the rotation
axis) to the central region in which nearly adiabatic stratification
is achieved. This is further surrounded by a region of overshooting
motions, the extent of which is greater at the equator than at the
poles, yielding an overall spherical shape to the domain experiencing
at least some convective mixing. We assess the overshooting achieved
as the stability of the radiative exterior is varied and the weak
circulations that result in that exterior. The convective plumes
serve to excite gravity waves in the radiative envelope, ranging from
localized ripples of many scales to some remarkable global resonances.
Title: Simulations of core convection in rotating A-type stars:
Magnetic dynamo action
Authors: Browning, M. K.; Brun, A. S.; Toomre, J.
Bibcode: 2003AAS...203.8502B
Altcode: 2003BAAS...35.1342B
We present the results of 3--D simulations of core convection dynamos
within A-type stars of 2 solar masses, at a range of rotation rates. The
inner 30% by radius of such stars are considered in our calculations,
with the spherical domain thereby encompassing the convective core and
some of the surrounding radiative envelope. We utilize our anelastic
spherical harmonic (ASH) code to examine highly nonlinear flows that can
admit magnetic dynamo action. Small initial seed magnetic fields are
found to be amplified greatly by the convective and zonal flows. The
resulting global fields possess structure on many scales, are strong
enough to influence the convective flows themselves, and persist for
as long as we have continued our calculations. The central columns of
strikingly slow rotation found in some of our progenitor hydrodynamic
simulations continue to be realized here to a lesser degree, with
such differential rotation arising from the redistribution of angular
momentum by the nonlinear convection and magnetic fields. We assess the
properties of the magnetic fields, the extent of convective penetration,
and the excitation of gravity waves within the radiative envelope.
Title: Solar Differential Rotation and Magnetism: a 3--D MHD View
Authors: Brun, Allan Sacha; Toomre, Juri
Bibcode: 2003IAUJD..12E...7B
Altcode:
We discuss recent progresses made in modelling the complex
magnetohydrodynamics of the Sun using our anelastic spherical harmonics
(ASH) code on massively parallel computers. We have conducted 3--D
MHD simulations of compressible convection in spherical shells to
study the coupling between convection rotation and magnetic field
in seeking to understand how the solar differential rotation is
established and maintained. The resulting convection within domains
that capture a good fraction of the bulk of the solar convection zone
is highly time dependent and intricate and is dominated by intermittent
upflows and networks of strong downflows (i.e. plumes). These plumes
play a significant role in yielding Reynolds stresses that serve to
redistribute angular momentum leading to angular velocity profiles
that make good contact with helioseismic deductions. Such complex
convective flows are efficient in amplifying the magnetic energy near
equipartition. The resulting magnetic fields are found to concentrate
around the downflowing networks and to have significant north-south
asymmetry and helicity. But these strong fields yield Maxwell stresses
that seek to speed up the poles and destroy the agreement with
helioseismic observations. So for a given angular velocity profile
the level of magnetism that the Sun can sustain is likely to be limited
Title: On Stellar Dynamo Processes and Differential Rotation
Authors: Brun, A. S.
Bibcode: 2003EAS.....9..179B
Altcode: 2003astro.ph..2600B
Many stars exhibit strong magnetic fields, some of which are thought
to be of primordial origin and others a sign of magnetic dynamo
processes. We briefly review the results of observational studies of
solar-type stars seeking to evaluate the linkage between rotation
rate and possible magnetic cycles of activity. Clearly turbulent
convection and rotation within spherical shell geometries provide
ingredients essential for dynamo action. However, intensive efforts
over several decades in solar research have demonstrated that it is no
easy matter to achieve cyclic magnetic activity that is in accord with
observations. Helioseismology has revealed that an essential element
for the global solar dynamo is the presence of a tachocline of shear at
the base of the solar convection zone, leading to the likely operation
of an interface dynamo. We review the crucial elements for achieving
a cyclic magnetic activity. We then discuss some of our current 3 D
MHD simulations of solar turbulent convection in spherical shells that
yield differential rotation profiles which make good contact with some
of the helioseismic findings. We show that such turbulent motions can
amplify and sustain magnetic field in the bulk of the convective zone
whose strength are sufficient to feed back both upon the convection
and its global circulations.
Title: Solar Differential Rotation and Magnetism: a 3-D MHD View
Authors: Brun, Allan Sacha
Bibcode: 2003IAUJD...3E..22B
Altcode:
We discuss recent progresses made in modelling the complex
magnetohydrodynamics of the Sun using our anelastic spherical harmonics
(ASH) code on massively parallel computers. We have conducted 3--D
MHD simulations of compressible convection in spherical shells to
study the coupling between convection rotation and magnetic field
in seeking to understand how the solar differential rotation is
established and maintained. The resulting convection within domains
that capture a good fraction of the bulk of the solar convection zone
is highly time dependent and intricate and is dominated by intermittent
upflows and networks of strong downflows (i.e. plumes). These plumes
play a significant role in yielding Reynolds stresses that serve to
redistribute angular momentum leading to angular velocity profiles
that make good contact with helioseismic deductions. Such complex
convective flows are efficient in amplifying the magnetic energy near
equipartition. The resulting magnetic fields are found to concentrate
around the downflowing networks and to have significant north-south
asymmetry and helicity. But these strong fields yield Maxwell stresses
that seek to speed up the poles and destroy the agreement with
helioseismic observations. So for a given angular velocity profile
the level of magnetism that the Sun can sustain is likely to be limited.
Title: Solar Turbulence and Magnetism Studied Within a Rotating
Convective Spherical Shell
Authors: Brun, A. S.; Toomre, J.
Bibcode: 2003ASPC..293..134B
Altcode: 2003astro.ph..2593B; 2003tdse.conf..134B
We discuss recent advances made in modelling the complex
magnetohydrodynamics of the Sun using our anelastic spherical harmonics
(ASH) code. We have conducted extensive 3--D simulations of compressible
convection in rotating spherical shells with and without magnetic
fields, to study the coupling between global-scale convection and
rotation in seeking to understand how the solar differential rotation
is established and maintained. Such simulations capable of studying
fairly turbulent convection have been enabled by massively parallel
supercomputers. The resulting convection within domains that capture
a good fraction of the bulk of the convection zone is highly time
dependent and intricate, and is dominated by intermittent upflows and
networks of strong downflows. A high degree of coherent structures
involving downflowing plumes can be embedded in otherwise chaotic flow
fields. These vortical structures play a significant role in yielding
Reynolds stresses that serve to redistribute angular momentum, leading
to differential rotation profiles with pole-to-equator contrasts of
about 30% in angular velocity, Omega, and some constancy along radial
lines at mid latitudes, thereby making good contact with deductions
from helioseismology. When a magnetic field is introduced, a dynamo
regime can be found that does not destroy the strong differential
rotation achieved in pure hydrodynamics cases. The magnetic fields
are found to concentrate around the downflowing networks and to have
significant north-south asymmetry and helicity.
Title: Seismic tests for solar models with tachocline mixing
Authors: Brun, A. S.; Antia, H. M.; Chitre, S. M.; Zahn, J. -P.
Bibcode: 2002A&A...391..725B
Altcode: 2002astro.ph..6180B
We have computed accurate 1-D solar models including both a macroscopic
mixing process in the solar tachocline as well as up-to-date
microscopic physical ingredients. Using sound speed and density
profiles inferred through primary inversion of the solar oscillation
frequencies coupled with the equation of thermal equilibrium, we
have extracted the temperature and hydrogen abundance profiles. These
inferred quantities place strong constraints on our theoretical models
in terms of the extent and strength of our macroscopic mixing, on the
photospheric heavy elements abundance, on the nuclear reaction rates
such as S11 and S34 and on the efficiency of
the microscopic diffusion. We find a good overall agreement between
the seismic Sun and our models if we introduce a macroscopic mixing
in the tachocline and allow for variation within their uncertainties
of the main physical ingredients. From our study we deduce that the
solar hydrogen abundance at the solar age is Xinv=0.732+/-
0.001 and that based on the 9Be photospheric depletion,
the maximum extent of mixing in the tachocline is 5% of the solar
radius. The nuclear reaction rate for the fundamental pp reaction is
found to be S11(0)=4.06+/- 0.07 10-25 MeV barns,
i.e., 1.5% higher than the present theoretical determination. The
predicted solar neutrino fluxes are discussed in the light of the new
SNO/SuperKamiokande results.
Title: Turbulent Convection under the Influence of Rotation:
Sustaining a Strong Differential Rotation
Authors: Brun, Allan Sacha; Toomre, Juri
Bibcode: 2002ApJ...570..865B
Altcode: 2002astro.ph..6196B
The intense turbulence present in the solar convection zone is a major
challenge to both theory and simulation as one tries to understand the
origins of the striking differential rotation profile with radius and
latitude that has been revealed by helioseismology. The differential
rotation must be an essential element in the operation of the solar
magnetic dynamo and its cycles of activity, yet there are many
aspects of the interplay between convection, rotation, and magnetic
fields that are still unclear. We have here carried out a series
of three-dimensional numerical simulations of turbulent convection
within deep spherical shells using our anelastic spherical harmonic
(ASH) code on massively parallel supercomputers. These studies of the
global dynamics of the solar convection zone concentrate on how the
differential rotation and meridional circulation are established. We
have addressed two issues raised by previous simulations with
ASH. First, can solutions be obtained that possess the apparent
solar property that the angular velocity Ω continues to decrease
significantly with latitude as the pole is approached? Prior
simulations had most of their rotational slowing with latitude
confined to the interval from the equator to about 45°. Second, can a
strong latitudinal angular velocity contrast ΔΩ be sustained as the
convection becomes increasingly more complex and turbulent? There was
a tendency for ΔΩ to diminish in some of the turbulent solutions
that also required the emerging energy flux to be invariant with
latitude. In responding to these questions, five cases of increasingly
turbulent convection coupled with rotation have been studied along
two paths in parameter space. We have achieved in one case the slow
pole behavior comparable to that deduced from helioseismology and
have retained in our more turbulent simulations a consistently strong
ΔΩ. We have analyzed the transport of angular momentum in establishing
such differential rotation and clarified the roles played by Reynolds
stresses and the meridional circulation in this process. We have found
that the Reynolds stresses are crucial in transporting angular momentum
toward the equator. The effects of baroclinicity (thermal wind) have
been found to have a modest role in the resulting mean zonal flows. The
simulations have produced differential rotation profiles within the bulk
of the convection zone that make reasonable contact with ones inferred
from helioseismic inversions, namely, possessing a fast equator, an
angular velocity difference of about 30% from equator to pole, and
some constancy along radial lines at midlatitudes. Future studies must
address the implications of the tachocline at the base of the convection
zone, and the near-surface shear layer, on that differential rotation.
Title: Mixing in the solar tachocline
Authors: Brun, Allan Sacha
Bibcode: 2002HiA....12..282B
Altcode:
We conduct numerical simulations of updated solar models including
a physical treatment of the tachocline (Spiegel & Zahn 1992),
the rotational transition layer localized at the base of the solar
convection zone. We first describe what is the current understanding
of this thin shear layer. We then show that we improve substantially
the agreement between the models and the observed Sun by taking into
account the macroscopic mixing occurring within this region.
Title: Solar Neutrino Emission Deduced from a Seismic Model
Authors: Turck-Chièze, S.; Couvidat, S.; Kosovichev, A. G.; Gabriel,
A. H.; Berthomieu, G.; Brun, A. S.; Christensen-Dalsgaard, J.; García,
R. A.; Gough, D. O.; Provost, J.; Roca-Cortes, T.; Roxburgh, I. W.;
Ulrich, R. K.
Bibcode: 2001ApJ...555L..69T
Altcode:
Three helioseismic instruments on the Solar and Heliospheric Observatory
have observed the Sun almost continuously since early 1996. This
has led to detailed study of the biases induced by the instruments
that measure intensity or Doppler velocity variation. Photospheric
turbulence hardly influences the tiny signature of conditions in the
energy-generating core in the low-order modes, which are therefore very
informative. We use sound-speed and density profiles inferred from GOLF
and MDI data including these modes, together with recent improvements
to stellar model computations, to build a spherically symmetric
seismically adjusted model in agreement with the observations. The
model is in hydrostatic and thermal balance and produces the present
observed luminosity. In constructing the model, we adopt the best
physics available, although we adjust some fundamental ingredients,
well within the commonly estimated errors, such as the p-p reaction
rate (+1%) and the heavy-element abundance (+3.5%); we also examine the
sensitivity of the density profile to the nuclear reaction rates. Then,
we deduce the corresponding emitted neutrino fluxes and consequently
demonstrate that it is unlikely that the deficit of the neutrino fluxes
measured on Earth can be explained by a spherically symmetric classical
model without neutrino flavor transitions. Finally, we discuss the
limitations of our results and future developments.
Title: Low-Degree Low-Order Solar p Modes As Seen By GOLF On
board SOHO
Authors: García, R. A.; Régulo, C.; Turck-Chièze, S.; Bertello,
L.; Kosovichev, A. G.; Brun, A. S.; Couvidat, S.; Henney, C. J.;
Lazrek, M.; Ulrich, R. K.; Varadi, F.
Bibcode: 2001SoPh..200..361G
Altcode:
Data recovered from the GOLF experiment on board the ESA/NASA SOHO
spacecraft have been used to analyze the low-order low-degree
solar velocity acoustic-mode spectrum below ν=1.5 mHz (i.e.,
1≤n≤9,l≤2). Various techniques (periodogram, RLAvCS,
homomorphic-deconvolution and RLSCSA) have been used and compared to
avoid possible biases due to a given analysis method. In this work,
the acoustic resonance modes sensitive to the solar central region
are studied. Comparing results from the different analysis techniques,
10 modes below 1.5 mHz have been identified.
Title: Helioseismic Tests of Solar Models
Authors: BRUN, A. S.
Bibcode: 2001AGUSM..SP21C01B
Altcode:
We first discuss what we could learn from an updated 1--D standard
solar model including a treatment of the shear layer present at the
base of the convective zone, the so called tachocline. This thin
layer is related to the transition from differential rotation in the
convection zone to almost uniform rotation in the radiative interior
and is now clearly established by helioseismic inversions. We find that
a time dependent treatment of the tachocline improves significantly
the agreement between computed and observed surface chemical species,
such as the 7Li as well as reduces the discrepancies between
the model's internal structure and the Sun (Brun, Turck-Chièze &
Zahn 1999). We then turn to 3--D spherical anelastic simulations of the
solar convection performed on massively parallel computers with our ASH
code. We focus our attention on the establishment of the global scale
flows such as the differential rotation and the meridional circulation,
by looking closely at the angular momentum transport balance and the
influence of the thermal wind. By doing so, we will also make use of the
accurate helioseismic data and show how the angular rotation profile
in our simulations is beginning to approach the differential rotation
character inferred from the observations (Brun & Toomre 2001).
Title: Mean flows in rotating turbulent convective shells
Authors: Brun, Allan Sacha; Toomre, Juri
Bibcode: 2001ESASP.464..619B
Altcode: 2001soho...10..619B
We conduct numerical simulations of turbulent compressible convection
within rotating spherical shells to model solar differential rotation
and meridional circulation. These 3-D simulations are carried out on
massively parallel computers using the Anelastic Spherical Harmonic
(ASH) code. The evolution of such convection is studied in four cases
which sample several paths in achieving highly turbulent flows that are
able to drive a strong differential rotation from equator to pole. The
resulting angular velocity Ω profiles make reasonable contact with
many aspects of the solar rotation profiles inferred from helioseismic
inversions of both MDI and GONG data. The substantial contrast in Ω
of order 30% achieved in our simulations of turbulent convection is
considerably greater than realized in previous studies.
Title: Turbulent Convection and Subtleties of Differential Rotation
Within the Sun
Authors: Toomre, J.; Brun, A. Sacha; De Rosa, M.; Elliott, J. R.;
Miesch, M. S.
Bibcode: 2001IAUS..203..131T
Altcode:
Differential rotation and cycles of magnetic activity are
intimately linked dynamical processes within the deep shell of
highly turbulent convection occupying the outer 200 Mm below the
solar surface. Helioseismology has shown that the angular velocity
Ω within the solar convection zone involves strong shear layers
both near the surface and especially at its base near the interface
with the radiative interior. The tachocline of radial shear there
that varies with latitude is thought to be the site of the global
magnetic dynamo. Most recent continuous helioseismic probing with
MDI on SOHO and from GONG have revealed systematic temporal changes
in Ω with the advancing solar cycle. These include propagating bands
of zonal flow speedup extending from the surface to a depth of about
70 Mm, distinctive out-of-phase vacillations in Ω above and below the
tachocline with a period of about 1.3 years near the equator, a changing
pattern of meridional circulation cells with broken symmetries in the
two hemispheres, and complex speedups and slowdowns in the bulk of
the convection zone. We review these helioseismic findings and their
implications. We also describe current 3-D numerical simulations of
anelastic rotating convection in full spherical shells used to study
the differential rotation that can be established by such turbulence
exhibiting coherent structures. These simulations enabled by massively
parallel computers are making promising contact with aspects of the
Ω profiles deduced from helioseismology, but challenges remain.
Title: The solar tachocline: Where do we stand?
Authors: Brun, Allan Sacha
Bibcode: 2001ESASP.464..273B
Altcode: 2001soho...10..273B
This paper reviews some of the basic features of the tachocline of
shear present at the base of the solar convective zone. We discuss some
aspects of its dynamics and evaluate processes capable of stopping the
spread of the shear deeper into the radiative interior. By taking into
account the macroscopic mixing occuring within this thin layer, we can
improve substantially the agreement between recent 1-D solar models
and quantities inferred from observing the Sun, such as the radial
sound speed profile or the photospheric abundances of light elements.
Title: Structure of the Solar Core: Effect of Asymmetry of Peak
Profiles
Authors: Basu, S.; Turck-Chièze, S.; Berthomieu, G.; Brun, A. S.;
Corbard, T.; Gonczi, G.; Christensen-Dalsgaard, J.; Provost, J.;
Thiery, S.; Gabriel, A. H.; Boumier, P.
Bibcode: 2000ApJ...535.1078B
Altcode: 2000astro.ph..1208B
Recent studies have established that peaks in solar oscillation
power spectra are not Lorentzian in shape but have a distinct
asymmetry. Fitting a symmetric Lorentzian profile to the peaks,
therefore, produces a shift in frequency of the modes. Accurate
determination of low-frequency modes is essential to infer the structure
of the solar core by inversion of the mode frequencies. In this paper
we investigate how the changes in frequencies of low-degree modes
obtained by fitting symmetric and asymmetric peak profiles change the
inferred properties of the solar core. We use data obtained by the
Global Oscillations at Low Frequencies (GOLF) project on board the
SOHO spacecraft. Two different solar models and inversion procedures
are used to invert the data in order to determine the sound speed in
the solar core. We find that for a given set of modes no significant
difference in the inferred sound speed results from taking asymmetry
into account when fitting the low-degree modes.
Title: Solar modelling: Theory and Verification
Authors: Turck-Chièze, S.; Brun, A. S.; Garcia, R. A.
Bibcode: 2000NuPhS..87..162T
Altcode:
After 30 years of investigation, the solar neutrino problem is still
puzzling but the perspectives are extremely encouraging, due to the
large improvements obtained on the experimental side including nuclear
reaction rates, high statistics in neutrino detections, precise acoustic
mode properties and hope to detect gravity modes. The present status,
including the differences between neutrino predictions and neutrino
flux detections, confirms the general features of solar modelling
proposed in the sixties but reveals a rich field of Astrophysics and
Particle Physics. The helioseismic investigation of the solar interior
with the satellite SOHO begins to offer a complete verification of the
solar structure, the introduction of dynamical effects will modify the
neutrino emissions and the solar properties on rotation and magnetic
field could appear important for the solution of the puzzle if the
properties of the neutrinos (mass and magnetic moment) are revealed
more complex than thought at the beginning
Title: Erratum: Standard Solar Models in the Light of New Helioseismic
Constraints. II. Mixing below the Convective Zone
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. P.
Bibcode: 2000ApJ...536.1005B
Altcode:
In the paper ``Standard Solar Models in the Light of New
Helioseismic Constraints. II. Mixing below the Convective Zone''
by A. S. Brun, S. Turck-Chièze, and J. P. Zahn (525, 1032 [1999]),
several corrections are required: 1. The words ``greater than''
just after equation (11) for the definition of rbcz
should be removed. 2. The beginning of first sentence of the next
paragraph should read: ``With the latitudinal dependence of the angular
velocity at the base of the convection zone borrowed from Thompson et
al. (1996), Ωbcz/2π=456-72x2-42x4
nHz,'' instead of ``Ωbcz>/2π=456-72x2-
42x4.'' 3. In the footnote to Table 1, ``Rbzc''
should be ``Rbcz,'' as it is appears for ``Tbcz''
in the same footnote. 4. In Table 2, in the ``Parameters'' column,
``i0'' should be ``Z0,'' as in Table 1. 5. In
Table 3, ``Observaton'' should be ``Observation.'' The Press sincerely
regrets these errors.
Title: Influence of the Tachocline on Solar Evolution.
Authors: Brun, A. S.; Zahn, J. -P.
Bibcode: 2000NYASA.898..113B
Altcode: 2000astro.ph..1510B
Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition from
differential rotation in the convection zone to almost uniform rotation
in the radiative interior, the tachocline. At present, this layer
extends only over a few percent of the solar radius and no definitive
explanations have been given for this thiness. Following Spiegel and
Zahn (1992, Astron. Astrophys.), who invoke anisotropic turbulence to
stop the spread of the tachocline deeper in the radiative zone as the
Sun evolves, we give some justifications for their hypothesis by taking
into account recent results on rotating shear instability (Richard and
Zahn 1999, Astron. Astrophys.). We study the impact of the macroscopic
motions present in this layer on the Sun's structure and evolution by
introducing a macroscopic diffusivity $D_T$ in updated solar models. We
find that a time dependent treatment of the tachocline significantly
improves the agreement between computed and observed surface chemical
species, such as the $^7$Li and modify the internal structure of the
Sun (Brun, Turck-Chièze and Zahn, 1999, in Astrophys. J.).
Title: Mixing in the Solar Tachocline
Authors: Brun, A. S.
Bibcode: 2000IAUJD...5E..15B
Altcode:
Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition
from differential rotation in the convection zone to almost uniform
rotation in the radiative interior, the tachocline. This layer extents
only over a few percent of the solar radius at the present day and no
definitive explanations have been given for this thinness. Following
Spiegel & Zahn (1992), who invoke anisotropic turbulence to stop
the spread of the tachocline deeper in the radiative zone as the Sun
evolves, we give some justifications for their hypothesis by taking
into account recent results on rotating shear instability (Richard &
Zahn 1999). Then we study the impact of the macroscopic motions present
in this layer on the Sun's structure and evolution by introducing a
macroscopic diffusivity DT in updated solar models. We find
that a time dependent treatment of the tachocline improves significantly
the agreement between computed and observed surface chemical species,
such as the 7Li and modify the internal structure of the Sun
(Brun, Turck-Chièze & Zahn 1999).
Title: The tachocline and lithium history in solar-like stars
Authors: Piau, L.; Turck-Chièze, S.; Brun, A. S.
Bibcode: 2000ASPC..198..303P
Altcode: 2000scac.conf..303P
No abstract at ADS
Title: Standard Solar Models in the Light of New Helioseismic
Constraints. II. Mixing below the Convective Zone
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. P.
Bibcode: 1999ApJ...525.1032B
Altcode: 1999astro.ph..6382B
In previous work, we have shown that recent updated standard solar
models cannot reproduce the radial profile of the sound speed at
the base of the convective zone and fail to predict the photospheric
lithium abundance. In parallel, helioseismology has shown that the
transition from differential rotation in the convective zone to almost
uniform rotation in the radiative solar interior occurs in a shallow
layer called the tachocline. This layer is presumably the seat of a
large-scale circulation and of turbulent motions. Here we introduce a
macroscopic transport term in the structure equations that is based on
a hydrodynamical description of the tachocline proposed by Spiegel &
Zahn, and we calculate the mixing induced within this layer. We discuss
the influence of different parameters that represent the tachocline
thickness, the Brunt-Väisälä frequency at the base of the convective
zone, and the time dependence of this mixing process along the Sun's
evolution. We show that the introduction of such a process inhibits
the microscopic diffusion by about 25%. Starting from models including
a pre-main-sequence evolution, we obtain (1) a good agreement with
observed photospheric chemical abundance of light elements such as
3He, 4He, 7Li, and 9Be;
(2) a smooth composition gradient at the base of the convective zone;
and (3) a significant improvement of the sound-speed square difference
between the seismic Sun and the models in this transition region when
we allow the photospheric heavy-element abundance to adjust, within
the observational incertitude, as a result of the action of this mixing
process. The impact on neutrino predictions is also discussed.
Title: The Helioseismic Constraints on 7Li and
9Be from SOHO
Authors: Brun, A. S.; Turck-Chièze, S.
Bibcode: 1999ASPC..171...64B
Altcode: 1999lcrr.conf...64B
No abstract at ADS
Title: Mixing Below the Solar Convective Zone
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. -P.
Bibcode: 1999ASPC..173..293B
Altcode: 1999sstt.conf..293B
No abstract at ADS
Title: Standard Solar Models in the Light of New Helioseismic
Constraints. I. The Solar Core
Authors: Brun, A. S.; Turck-Chièze, S.; Morel, P.
Bibcode: 1998ApJ...506..913B
Altcode: 1998astro.ph..6272B
In this paper, we examine a new, updated solar model that takes
advantage of the recent reexamination of nuclear reaction rates and
the microscopic diffusion of helium and heavy elements. Our best model
fits the helioseismic data reasonably well, giving the base of the
convective zone at Rbcz = 0.715, the photospheric helium in
mass fraction as 0.243, and the sound-speed square difference between
the Sun and the model as δc2/c2 < 1%. This
model leads to a reestimate of neutrino fluxes, giving 7.18 SNU for
the chlorine experiment, 127.2 SNU for the gallium detector, and 4.82
106 cm-2 s-1 for the 8B
neutrino flux. Acoustic-mode predictions are also estimated. We then
consider the radiative zone and discuss what we learn from such a
model when confronted with the present helioseismic constraints from
space experiments aboard SOHO. We present three models that respect
these constraints and better fit the seismic observations by taking
advantage of the known physical uncertainties--nuclear reaction rates,
CNO abundances, and microscopic diffusion. We also study some current
questions, such as the possibility of mixing in the nuclear core, the
revision of the solar radius, and the influence of the solar age. We
conclude that the standard model, inside its inherent uncertainties, is
robust in light of the present acoustic-mode detection and that mixing
in the core is not really favored, even though a proper understanding of
the angular momentum evolution with time has not yet been reached. The
initial solar helium abundance seems more and more constrained;
this study supports an initial abundance between 0.273 and 0.277 in
mass fraction. This analysis allows us to define minimal values for
neutrino predictions, compatible with present seismic results. We
note that a reduction of about 30% in chlorine and water detectors,
which is more than half the discrepancy with the experimental results,
is still supported by the present study. This work also emphasizes the
fact that acoustic-mode determination does not put strong constraints
on the nuclear plasma characteristics. Finally, we estimate g-mode
frequencies in a range that may be accessible to the satellite SOHO;
these results emphasize the substantially improved sensitivity of these
modes to details of the nuclear solar core, and show the frequency
dependence of these modes for the different models previously discussed.
Title: Sensitivity of the Sound Speed to the Physical Processes
Included in the Standard Solar Model
Authors: Turck-Chièze, S.; Basu, S.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Gabriel, M.; Morel, P.;
Provost, J.; Turcotte, S.; GOLF Team
Bibcode: 1998ESASP.418..555T
Altcode: 1998soho....6..555T
The accuracy of the present seismic data allows us to check the
solar internal sound speed down to the core. This is a great support
to check the hypothesis of the classical stellar evolution and to
predict the neutrino fluxes. The interpretation of these measurements
supposes an accurate determination of the structure of the standard
solar model as a first step. It is why a continuing effort has been
devoted to the knowledge of the physical quantities included in this
framework. In this poster we present 6 different solar models calculated
by different groups of the GOLF consortium. These models include the
most recent progress in atomic physics and nuclear physics. Then, we
discuss the sensitivity of the sound speed difference, between GOLF+MDI
observations and models, to different ingredients, in peculiar to the
opacity coefficients and the determination of the solar age.
Title: Macroscopic Processes in the Solar Interior
Authors: Brun, A. S.; Turck-Chièze, S.; Zahn, J. P.
Bibcode: 1998ESASP.418..439B
Altcode: 1998astro.ph..7090B; 1998soho....6..439B
With the recent results of heliseismology aboard SOHO, the solar models
are more and more constrained (Brun, Turck-Chièze et Morel 1998)
. New physical processes, mainly connected to macroscopic motions, must
be introduced to understand these news observations. In this poster,
we present solar models with such macroscopic motions, as turbulent
pressure in the outer layers, mixing due to the tachocline (Spiegel and
Zahn 1992), and some mixing in the core (Morel and Schatzman 1996). From
our results, we could say that: (1) Mixing in the core is unlikely (δ
c2/c2 > 2%) (2) Turbulent pressure improves
the absolute value of the acoustic modes frequencies (~5 μ Hz at 4 mHz)
(3) And mixing in a tachocline of thickness of 0.05 plus or minus 0.03
Rodot (Corbard et al. 1997) looks promising.
Title: Predictions of the Solar Neutrino Fluxes and the Solar Gravity
Mode Frequencies from the Solar Sound Speed Profile
Authors: Turck-Cheèze, S.; Brun, A. S.; Garcia, R. A.
Bibcode: 1998ESASP.418..549T
Altcode: 1998soho....6..549T
Recently, a lot of theoretical and experimental efforts have been
performed in order to improve the knowledge of the nuclear reaction
rates, screening, opacity calculations which are useful for a good
theoretical representation of the Sun. We shall present these new works:
recompilation of all the cross sections useful for the solar fusion
(Aldelberger et al 1998), measurements of the (3He,3He) and (7Be, p)
cross sections, new calculations on screening enhancement, introduction
of more heavy elements in the opacity coefficient calculations
(Rogers 1998). The main progress will be discussed through their
effects on solar models, neutrino and acoustic predictions (Brun,
Turck-Chièze and Morel 1998). A peculiar attention will be devoted
to the confrontation with recent neutrino measurements. One may
notice that these improvements play a signifant role at the level of
accuracy we are able to reach with present seismology and that they are
extremely important for a reasonable interpretation of what we learn
from helioseismology on the radiative region and more precisely on the
solar core. Considering the recent progress done by the ground networks
and the SOHO satellite in helioseismology, the authors suggest new
laboratory experiments on large lasers in order to disentangle different
physical processes. Perspectives of what we prepare for the near future
to better disentangle the neutrino puzzle will be illustrated.
Title: First View of the Solar Core from GOLF Acoustic Modes
Authors: Turck-Chièze, S.; Basu, S.; Brun, A. S.;
Christensen-Dalsgaard, J.; Eff-Darwich, A.; Lopes, I.; Pérez
Hernández, F.; Berthomieu, G.; Provost, J.; Ulrich, R. K.; Baudin,
F.; Boumier, P.; Charra, J.; Gabriel, A. H.; Garcia, R. A.; Grec,
G.; Renaud, C.; Robillot, J. M.; Roca Cortés, T.
Bibcode: 1997SoPh..175..247T
Altcode:
After 8 months of nearly continuous measurements the GOLF instrument,
aboard SOHO, has detected acoustic mode frequencies of more than 100
modes, extending from 1.4 mHz to 4.9 mHz. In this paper, we compare
these results with the best available predictions coming from solar
models. To verify the quality of the data, we examine the asymptotic
seismic parameters; this confirms the improvements achieved in solar
models during the last decade.
Title: Book-Review - Atlas of Selected Areas
Authors: Brun, A.; Vehrenberg, H.
Bibcode: 1984AExpr...1T..81B
Altcode:
No abstract at ADS
Title: Atlas photométrique des constellations.
Authors: Brun, A.; Brun, M.
Bibcode: 1979apc..book.....B
Altcode:
No abstract at ADS
Title: B. V. Kukarkin, 1909 October 30 - 1977 September 15.
Authors: Brun, A.
Bibcode: 1978AFOEV..12....3B
Altcode:
No abstract at ADS
Title: A propos du télescope de Schmidt
Authors: Brun, A.
Bibcode: 1974LAstr..88..107B
Altcode:
No abstract at ADS
Title: EE Cephei, une algolide à très longue période.
Authors: Brun, A.
Bibcode: 1974AFOEV...8...34B
Altcode:
No abstract at ADS
Title: Un grand astronome : Harlow Shapley (1885-1972)
Authors: Brun, A.
Bibcode: 1973LAstr..87..209B
Altcode:
No abstract at ADS
Title: A propos d'étoiles variables
Authors: Brun, A.
Bibcode: 1972LAstr..86..361B
Altcode:
No abstract at ADS
Title: Chronique des observateurs d'étoiles variables
Authors: Brun, A.
Bibcode: 1971LAstr..85..412B
Altcode:
No abstract at ADS
Title: Note aux variabilistes
Authors: Brun, A.
Bibcode: 1970LAstr..84..517B
Altcode:
No abstract at ADS
Title: Bd +28 838
Authors: Brun, A.
Bibcode: 1970IBVS..443....4B
Altcode:
No abstract at ADS
Title: Notice nécrologique : Roger Weber (1903-1969)
Authors: Brun, A.
Bibcode: 1970LAstr..84...79B
Altcode:
No abstract at ADS
Title: Gamma Sagittae étoile variable ?
Authors: Brun, A.
Bibcode: 1970LAstr..84...82B
Altcode:
No abstract at ADS
Title: Etoiles Variables Nouvelles au Nord de Beta Tauri
Authors: Brun, A.
Bibcode: 1969IBVS..409....1B
Altcode:
No abstract at ADS
Title: Conseils aux observateurs d'étoiles variables
Authors: Brun, A.
Bibcode: 1966LAstr..80..283B
Altcode:
No abstract at ADS
Title: V. Sagittæ, post-nova singulière
Authors: Brun, A.
Bibcode: 1965LAstr..79..136B
Altcode:
No abstract at ADS
Title: 37 étoiles variables nouvelles dans Lacerta
Authors: Brun, A.
Bibcode: 1964JO.....47...45B
Altcode:
No abstract at ADS
Title: Une remarquable algolide RW Tauri
Authors: Brun, A.
Bibcode: 1963LAstr..77..457B
Altcode:
No abstract at ADS
Title: Mouvement propre rapide d'une étoile faible se projetant
sur la nébuleuse du tourbillon M 51
Authors: Brun, A.
Bibcode: 1963LAstr..77..228B
Altcode:
No abstract at ADS
Title: Une étoile variable extraordinaire
Authors: Brun, A.
Bibcode: 1963LAstr..77..166B
Altcode:
No abstract at ADS
Title: 26 étoiles variables nouvelles aux environs de la "Selected
Area n° 21"
Authors: Brun, A.
Bibcode: 1963JO.....46..126B
Altcode:
No abstract at ADS
Title: Ce que peut faire un amateur dans le domaine des étoiles
variables
Authors: Brun, A.
Bibcode: 1962LAstr..76...92B
Altcode:
No abstract at ADS
Title: Révision des 139 Selected Areas
Authors: Brun, A.
Bibcode: 1962JO.....45..329B
Altcode:
No abstract at ADS
Title: Y a-t-il de la matière obscure dans l'espace intergalactique?
Authors: Brun, A.
Bibcode: 1960LAstr..74..219B
Altcode:
No abstract at ADS
Title: Un type nouveau d'étoile variable
Authors: Brun, A.
Bibcode: 1960LAstr..74..184B
Altcode:
No abstract at ADS
Title: A catalogue of 9867 stars in the Southern Hemisphere with
proper motions exceeding 0".2 annually
Authors: Brun, A.
Bibcode: 1957Brun..C......0B
Altcode:
No abstract at ADS
Title: Atlas des étoiles variables du type U Geminorum
Authors: Brun, A.; Petit, M.
Bibcode: 1957PZ.....12...18B
Altcode:
No abstract at ADS
Title: Étoile variable nouvelle, Nova probable dans M 31
Authors: Brun, A.; Texereau, J.
Bibcode: 1956LAstr..70..416B
Altcode:
No abstract at ADS
Title: Le déplacement du pôole céleste de 1900 à 2100
Authors: Brun, A.
Bibcode: 1956LAstr..70..345B
Altcode:
No abstract at ADS
Title: UV Persei, variable à long cycle du type U geminorum
Authors: Brun, A.
Bibcode: 1956JO.....39Q..37B
Altcode:
No abstract at ADS
Title: Observations de la variable 30.1934 Dra.
Authors: Brun, A.
Bibcode: 1956JO.....39...46B
Altcode:
No abstract at ADS
Title: Observations de la variable 30. 1934
Authors: Brun, A.
Bibcode: 1956JO.....39R..37B
Altcode:
No abstract at ADS
Title: RX UMa.
Authors: Brun, A.
Bibcode: 1956JO.....39...48B
Altcode:
No abstract at ADS
Title: Une Algolide extraordinaire : Nova Herculis 1934
Authors: Brun, A.
Bibcode: 1955LAstr..69..120B
Altcode:
No abstract at ADS
Title: L'idée géniale de B. Schmidt et ses conséquences pour les
progrès de l'optique et de l'astronomie
Authors: Brun, A.
Bibcode: 1953LAstr..67..420B
Altcode:
No abstract at ADS
Title: On demande des observateurs
Authors: Brun, A.
Bibcode: 1953LAstr..67..203B
Altcode:
No abstract at ADS
Title: Le Telescope de Schmidt.
Authors: Brun, A.
Bibcode: 1940LAstr..54..193B
Altcode:
No abstract at ADS
Title: Sur un Télescope de Newton a Monture Particulière
Authors: Brun, A.
Bibcode: 1939LAstr..53..185B
Altcode:
No abstract at ADS
Title: Une Nouvelles Etoile Variable du Type U Geminorum
Authors: Brun, A.
Bibcode: 1938LAstr..52..321B
Altcode:
No abstract at ADS
Title: Nouvelles de la Science, Varietes, Informations.
Authors: D'Evreinoff, Victor; Courteville, M.; Brun, A.; Girod, Paul;
Bachelard, Raymond; Lumiere, Louis; Hamon, A.
Bibcode: 1937LAstr..51..431D
Altcode:
No abstract at ADS
Title: Une Etoile Binaire a Eclipse Supergeante VV Cephei.
Authors: Brun, A.
Bibcode: 1937LAstr..51..298B
Altcode:
No abstract at ADS
Title: La nebuleuse d'Orion et ses etoiles variables.
Authors: Brun, A.
Bibcode: 1935POLyo...1...12B
Altcode:
No abstract at ADS
Title: La Pluie d'Etoiles Filantes du 9 Octobre 1933.
Authors: Flammarion, G. C.; Esclangon, Ernest; Fichot, M. E.;
Danjon, A.; Baillaud, Rene; Quenisset, F.; Isaac Roberts-Klumpke,
Dorothea; Touchet, Em.; Hamon, A.; de Kerolver, M.; Fournier, G.;
Bidault de L'Isle, G.; Thibault, Ed.; Belin, Abel; Le Coultre, F.;
Schlumberger, Rene; Brun, A.; Joulia, Abbe E.; Memery, Henri; Roguet,
Daniel; Agostinho, J.; Blain-Dejardin; Douillet, E.; Moye, Marcel;
Bernson, Reysa; Luizard, Marcel
Bibcode: 1933LAstr..47..489F
Altcode:
No abstract at ADS
Title: La nébuleuse d'Orion et ses étoiles variables
Authors: Brun, A.
Bibcode: 1932POLyo...1K...1B
Altcode:
Fig I. Répartition de 108 étoiles variables. En pointillé,
la région cartographiée, Fig 2. Carte d'ensemble, Fig 3. Partie
centrale de M42 et M43, Fig 4. Nébulosités autour de c42 F1, Fig
5. Le trapèze. Carré de 90'' de cõté I. Catalogue des étoiles de
la nebuleuse d'Orion, II. Etoiles Variables de la Nébuleuse d'Orion,
III. étoiles Soupçonnees de variabilite, IV. Etoiles du Catalogue
de Bond non Retrouvées sur les Photos, V. Étoiles du Catalogue de
la Carte du Ciel non Retrouvées sur les Photos
Title: Bulletin de l'Observatoire de Lyon: Février 1931
Authors: Bloch, M.; Brun, A.
Bibcode: 1931BuLyo..13A..19B
Altcode:
No abstract at ADS
Title: Etoile Filante télescopique double
Authors: Brun, A.
Bibcode: 1927BuLyo...9A..89B
Altcode:
No abstract at ADS
Title: 201276 - V26 = SZ Cephel
Authors: Brun, A.
Bibcode: 1926BuLyo...8...60B
Altcode:
No abstract at ADS
Title: Nouvelles de la Science, Varietes, Bibliographie.
Authors: de Paolis, Armand; Grouiller, H.; Brun, A.; Jarry-Desloges,
R.; Muraour, Henri; Perrier, G.; Garbes, Mauirce; Cantenot, Louis
Bibcode: 1926LAstr..40..181D
Altcode:
No abstract at ADS
Title: Observation de la Trainee d'un Bolide.
Authors: Brun, A.
Bibcode: 1926LAstr..40...38B
Altcode:
No abstract at ADS
Title: Courbe de lumière et éléments provisoires de l'étoiles
variable 194080 Cephei
Authors: Brun, A.
Bibcode: 1923BuLyo...6...79B
Altcode:
No abstract at ADS
Title: Observations de L'Etoile Variable
Authors: Brun, A.
Bibcode: 1922BuLyo...5Q..10B
Altcode:
No abstract at ADS
Title: La Surface Solaire pendant le Mois de Fé
Authors: Brun, A.
Bibcode: 1922BuLyo...5...53B
Altcode:
No abstract at ADS
Title: Sur l'inexistence dans le ciel de quelques étoiles du grand
Catalogue de Bonn (Bonner Durchmusterung)
Authors: Brun, A.
Bibcode: 1922BuLyo...5..126B
Altcode:
No abstract at ADS
Title: Montures d'Instruments pour l'Observation des Etoiles variables
Authors: Brun, A.
Bibcode: 1920BuLyo...4C...1B
Altcode: 1920BuLyo...4Q...1B
No abstract at ADS
Title: La Position dans le Ciel des Points Equinoxiaux et les Tres
Anciennes Observations.
Authors: Brun, A.
Bibcode: 1920LAstr..34..419B
Altcode:
No abstract at ADS
Title: Les Etoiles Variables a Longue Periode.
Authors: Brun, A.
Bibcode: 1919LAstr..33..397B
Altcode:
No abstract at ADS
Title: L'Etoile Variable Cassiopee.
Authors: Brun, A.
Bibcode: 1919LAstr..33..125B
Altcode:
No abstract at ADS
Title: Nouvelles de la Science, Varietes.
Authors: Fayet; Vinter-Hansen, Julie-Marie; Brun, A.
Bibcode: 1919LAstr..33...41F
Altcode:
No abstract at ADS
Title: Découverte d'Une Étoile Variable
Authors: Brun, A.
Bibcode: 1917LAstr..31..220B
Altcode:
No abstract at ADS
Title: Nouvelles de la Science, Varietes. La variable SZ Cephee.
Authors: Brun, A.
Bibcode: 1916LAstr..30..353B
Altcode:
No abstract at ADS
Title: Observations de R Grande Ourse en 1913.
Authors: Brun, A.
Bibcode: 1915LAstr..29..214B
Altcode:
No abstract at ADS
Title: Sur l'absence dans le ciel d'une étoile du Catalogue
astrographique
Authors: Brun, A.
Bibcode: 1914AN....197..165B
Altcode:
No abstract at ADS
Title: Observations d'Etoiles Variables.
Authors: Brun, A.
Bibcode: 1914LAstr..28..363B
Altcode:
No abstract at ADS
Title: Une nouvelle variable 29.1913 Cephei
Authors: Brun, A.
Bibcode: 1913AN....196..385B
Altcode: 1914AN....196..385B
No abstract at ADS
Title: La Coloration des Etoiles
Authors: Brun, A.
Bibcode: 1913LAstr..27..314B
Altcode:
No abstract at ADS