CHAMONIX, FRANCE l 14-18 July 2014
FROM THE MRI TO THE SUN:
a conference to celebrate the 60th birthday of Steven BALBUS
Abstracts
Accretion very
near black holes - slim accretion disks
Marek Abramowicz
I will discuss models of slim accretion disks and compare
them with most recent MHD simulations. I will argue that
the semi-analytic slim disk models provide an excellent
description of black hole
accretion flows, especially very near black holes.
Protostellar disks
Philip Armitage
Protostellar disks are the accreting systems
where the prospects for dramatically improved
observational constraints on turbulence and angular
momentum transport are the best. I will review some of the
physics particular to prostellar disks, where non-ideal
MHD terms play a major role. This led to the idea of a
dead zone of suppressed turbulence, and more recently to
the possibility that the MRI leads to solutions which
transport angular momentum through laminar stresses or
winds. I will discuss some of the challenging open issues:
how does the MRI interact with gravitational instability
in massive disks, whether it is possible to tap the
independent reservoir of thermal energy to drive
turbulence, and how to reconcile the large net magnetic
fluxes that are predicted from star formation with the
much smaller fluxes that are sufficent to drive accretion
via the MRI.
Non-linear evolution of
the MRI in the presence of net vertical magnetic flux
Xuening Bai
Accretion disks are very likely
threaded by external magnetic flux, inherent from disk
formation, yet we are only at the beginning to explore the
physical consequences. I will discuss recent simulations
of the MRI in the presence of weak and strong external
vertical magnetic field, highlighting the results on
angular momentum transport, disk outflow, and magnetic
flux transport. Limitations of the local approximation and
future perspectives will also be discussed.
Nonmodal Growth of the
Magnetorotational Instability and the Dynamo Effect
Amitava
Bhattacharjee
Beyond its role as a prime mechanism for
angular momentum transport in astrophysical disks, the
magnetorotational instability (MRI) has now become a
standard model for the study of MHD stability of
rotating systems with sheared flows and a large-scale
dynamo. In this talk, I will present some recent results
(obtained in collaboration with Jonathan Squire) of the
dynamo in turbulence induced by the MRI in its simplest
possible form --- an unstratified shearing box without a
mean magnetic field. Sustained turbulence --- generating
a strong azimuthal magnetic field --- is possible in
this system, despite the absence of spectral linear
instability. Because of this, nonmodal growth due to the
non-normality of the linear operator plays an important
role in the MRI dynamo, both for axisymmetric and
non-axisymmetric disturbances. With the goal of
understanding the core dynamo process, we have been
studying a quasi-linear version of the shearing box
system, drawing upon interesting new developments in
hydrodynamics. Among the most interesting ideas
resulting from this approach is the existence of a mean
field dynamo instability of homogenous background
turbulence. The instability saturates nonlinearly at
levels consistent with nonlinear simulations and depends
strongly on the magnetic Prandtl number.
Hysteresis in Black
Hole Binary State Transitions
Mitch Begelman
In X-ray binaries, the transition from hard
to soft state typically occurs at a higher luminosity than
the transition from soft to hard. Recent work
has suggested that such hysteresis can result from the
sensitivity of MRI-driven turbulence (and thus the value
of alpha) to such disk properties as magnetic Prandtl
number or net magnetic flux. Phil Armitage and I
have been studying the latter possibility, and I will
discuss our results to date.
Visualizing MRI
turbulence through Compton scattering
Omer Blaes
While MRI turbulence is generally subsonic,
in the radiation pressure dominated flows of high
luminosity accretion onto black holes, the turbulent
velocities can exceed the sound speed in the gas alone,
and even exceed the microscopic thermal speeds of both
ions and electrons. In this regime, Compton scattering
between photons and electrons will be dominated by the
bulk motions of the turbulence, rather than thermal
motions of the electrons.
Bulk Comptonization by the turbulence can therefore
have a direct impact on the radiation spectrum, and I will
present calculations demonstrating this effect.
Three
modes of mass-loss from the
microquasar SS433
Katherine Blundell
Instability,
magnetism, and differential rotation in fully convective
stars
Matthew Browning
In stars like the Sun, the interface between
the convective envelope and the radiative core has been
widely thought to play a crucial role in generating
organized, cyclical magnetic fields. But sufficiently
low-mass stars (as well as pre-main sequence and
sub-stellar objects) are convective throughout their
interiors, and so presumably do not possess such an
interface; a generic theoretical expectation has therefore
been that such stars should harbor magnetic dynamos very
different from those in solar-like stars. I will discuss
how this expectation has been partly borne out, but partly
confounded, by recent observations and theoretical
modeling of dynamos in fully convective stars. In
particular, I will review 3-D MHD simulations of
convection and magnetism in such stars, and highlight how
the dynamo process in these models depends on rotation
rate and stellar mass.
Finally, I will also describe some of the impact
the dynamo-generated magnetism has on flows and heat
transport.
The Solar tachocline: a
shallow-water perspective
James Cho
I shall review turbulence in the
traditional shallow-water model and its MHD extension. focus will be
on conservation laws and changes caused by the addition
of the magnetic field
-- in particular, on how inverse cascade-induced
jet formation is modified.
Possible application of the simple model to
the solar tachocline is discussed.
Heating and cooling
processes in the (diffuse and neutral) ISM
Bruce T. Draine
At constant density, the temperature
of the gas in the diffuse neutral ISM is the result of the
competition between heating processes that add heat to the
gas, and cooling processes that remove it. The heating is
generally thought to be dominated by elastic scattering of
energetic electrons ejected from H and He by cosmic rays
or X-rays, or electrons ejected from nanoparticles (dust
grains) by ultraviolet photons. Some additional heating comes from
energetic H2 molecules produced by catalytic formation of
H2 on dust, photoionization of atoms other than H or He,
photoexcitation and photodissociation of molecules, and
gradual damping of MHD turbulence and waves, but these are
normally estimated to be of secondary importance.
Cooling of the gas - removal of
thermal energy - occurs by excitation of atoms, ions, and
molecules by inelastic scattering of thermal electrons and
atoms, followed by radiative decay of the excited states. Additional
cooling due to inelastic collisions of gas particles with
colder dust grains is generally unimportant, except in
very dense regions.
Balancing heating and cooling allows
us to understand the range of temperatures in the diffuse
neutral ISM, including the coexistence of a cool phase and
a warm phase at the same pressure.
There is, however, evidence (which
will be reviewed) that diffuse molecular clouds often
contain regions where the gas temperature is much higher
than expected based on balancing the above-cited heating
and cooling processes.
The clear implication is that additional heating
processes are present, capable of transforming either
mechanical or magnetic energy into heat.
Intermittency
of
interstellar turbulence : a new playground for
theorists, observers and numericists
Edith Falgarone
Turbulence in
galaxies stands at the crossroad of a wide variety of
cosmic processes, including star formation and stellar
feedback. As such, it is key in the self-regulation of
the open cycle of matter and energy, mainly powered by
stars, that keeps the interstellar medium (ISM) far out
of thermodynamic equilibrium. A fundamental property of
turbulence was recognized in the early 1960's: its
intermittency in space and time. Since then,
intermittency has been observed and characterized in
laboratory flows and the terrestrial atmosphere.
Magnetic fields and compressibility of the ISM make
turbulent intermittency more elusive. Yet, statistical
studies start to be significant and slowly disclose its
properties. Some of the first steps of chemistry in the
diffuse ISM that have been known for decades to require supra-thermal
energy, can now be understood in the framework of
turbulent intermittency. Unexpectedly, chemistry is
therefore providing
unique diagnostics and tracers of the energy
trail in ISM turbulence.
Thermodynamics
of
the dead zone inner edge in protoplanetary disks
Julien Faure
In
protoplanetary disks, the inner boundary between an MRI
active and inactive region has recently been suggested to
be a promising site for planet formation, thanks to the
trapping of solid at the boundary itself or in vortices
generated by the Rossby Wave instability.
However,
numerical models of the boundary have so far considered
only the case of an isothermal equation of state while the
disk thermodynamics and the turbulent dynamics are
entwined because of the thermal ionization.
Using
the Godunov code RAMSES, we have performed a 3D global
numerical simulation of a protoplanetary disk, including
thermodynamical effects and a simple model for the
resistivity dependence with temperature. The comparison
with a 2D viscous simulation has been extensively used to
identify the physical processes at play.
We find
that, surprisingly, a vortex forming at the interface
migrates inward, penetrates inside the active zone until
beeing destroyed by turbulent motions. A new vortex forms
few tens of orbits later at the interface and migrates
too.
In this
paper, we characterize this vortex life cycle and discuss
its implications for planet formation at the dead/active
interface.
Radiation
MHD In Global Simulations Of Protoplanetary Disks.
Mario Flock
In this talk we present our
newest results related to the thermal and dynamical
evolution of gas and dust in turbulent protoplanetary
disks.
The magneto–rotational instability quickly causes
magneto–hydrodynamic turbulence and heating in the disk.
The disk midplane temperature raises to a new equilibrium.
A roughly flat vertical temperature profile establishes in
the disk's optically thick region. The present work
demonstrates for the first time that global radiation
magneto–hydrodynamic simulations of turbulent
protoplanetary disks are feasible with current
computational facilities. This opens up the windows to a
wide range of studies of the dynamics of protoplanetary
disks, especially their inner parts for which there are
significant observational constraints.
Recent
progress in numerical modeling of accretion flows
Charles F. Gammie
The discovery of the magnetorotational instability in 1991
initiated a flurry of numerical investigations into its
nonlinear evolution that continue to this day. I
will review recent work on the saturation of disk
turbulence, particularly disk turbulence around black
holes, describe some upcoming relevant observations, and
point out some outstanding problems for future
investigation.
Meteorite
evidence
for sequential star formation in a hierarchichal ISM.
Matthieu Gounelle
Short-lived radionuclides
are radioactive elements with half-life significantly
shorter than the age of the Solar System. Their decay
products can be found in meteorites indicating that some
of them such as 26Al (T1/2 = 0.74 Myr) or 60Fe (T1/2 =
2.6 Myr) were alive in the nascent Solar System. I will
review the proposed origin for short-lived
radionuclides, and show that the past presence of 26Al
and 60Fe in the nascent Solar System can be attributed
to massive stars and trace sequential star formation in
a hierarchical ISM. I will specifically focus on the
last episode of star formation that lead to our Sun
formation in a dense shell accumulated around a massive
star.
John F. Hawley
Disk accretion is one of the
most fundamental processes in the universe. Extensive
observations have revealed a wide range of astrophysical
phenomena in which accretion plays a significant, or even
fundamental, role. The
magnetorotational instability (MRI) provides the root
mechanism by which accretion occurs. This talk will
review the basic physics of the MRI, as well as present
some (now) historical recollections of the research that
Steve Balbus and I carried out to discover and elucidate
the properties of this instability.
Chondrules:
Our
eyes in protoplanetary disks
Emmanuel Jacquet
Primitive meteorites provide irreplaceable constraints on
the conditions of our past protoplanetary disks. They
consist in agglomerations of various millimeter-sized
solids native to the solar nebula, in particular the
ubiquitous chondrules, which are silicate spherules
presumably resulting of transient high-temperature
episodes in the disk. Yet despite their ubiquity, and the
wealth of data gathered on them -- e.g. age range,
physico-chemical conditions of formation, individual
paleomagnetic records etc. --, the nature of their
formation mechanism, obviously an important process in the
disk, remains elusive. I will discuss some recent advances
in the field, and the (even more numerous!) questions they
raise.
Angular
Momentum Transport in the Lab
Hantao Ji
This
talk is to summarize rigorous efforts in the lab to
demonstrate and study the mechanisms of rapid angular
momentum transport relevant to accretion disks, including
the MRI. Isolating and minimizing effects
due to artificial boundaries, which are inherent to
terrestrial experiments, has been a particular challenge.
Nonetheless, significant
insights relevant to accretion disks have been already
obtained, with many surprises, perhaps even to Steve.
The recent achievements and future prospects of these
efforts will be discussed in this talk in three
categories: hydrodynamic, magnetohydrodynamic,
and gas/plasma experiments.
Hydrodynamic stability of disks
Hubert
Klahr
Keplerian
disks have proven to be extremely stable to perturbations,
when magnetic fields are not in operation. But disks
around young stars are complicated entities, very similar
to planetary atmospheres.
There is a radial temperature gradient driven by
stellar irradiation, which leads to a thermal wind, e.g.
vertical shear. The temperature gradient leads also to a
height dependent radial stratification that can be
radially buoyant. Without thermal relaxation these
disks are stable, but with the right amount of cooling and
heating for instance by the radiative transport of heat,
one can drive a Goldreich-Schubert-Fricke Instability
(see for instance Nelson et al 2013) or a
Convective Overstability (Klahr and Hubbard 2014;
Lyra 2014). In this talk I discuss some recent
results from linear stability analysis and numerical
experiments.
From MRI
turbulence to photons
Julian Krolik
MHD turbulence driven by the MRI
accounts for the internal stress driving accretion; it
also implies dissipation capable of supplying the energy
for the photons we observe. In recent
years, it has become possible to combine numerical
simulations of accretion dynamics with radiation transfer
techniques to predict the photon output of accretion in a
manner nearly free of phenomenological models.
This talk will review a number of results that have
emerged, focusing on the case of accretion onto black
holes. MHD processes turn out to have
interesting, and sometimes surprising, consequences for
the thermal, coronal, and fluorescence components in terms
of their luminosity, spectrum, variability, and
polarization.
How to tap
free-energy gradients with anisotropic diffusion
Matthew Kunz
Much of Steve's published work
throughout the 1990s and early 2000s focused on the
curious ability of magnetic fields to replace
conserved-quantity gradients with free-energy gradients as
the discriminating quantities of stability. That they do
so not just dynamically -- the most prominent example
being the MRI -- but also passively -- by placing
stringent constraints on the nature of viscous, resistive,
and conductive flows -- lends further credence to what has
become one of Steve's classic mantras, that "even the
tiniest of magnetic fields can have dramatic consequences
for the macroscopic stability of astrophysical plasmas."
In this talk, I will review a veritable alphabet soup of
free-energy-gradient instabilities driven by anisotropic
diffusion, focusing on both the mathematical similarity of
their dispersion relations and the impact they might have
on astrophysical systems. These include shear
instabilities in poorly ionized gas driven by ambipolar
diffusion and the Hall effect, as well as magneto-viscous
forms of convection and rotational instability in weakly
collisional plasmas.
Revisiting
the
linear MRI in quasi-global geometries
Henrik Latter
The mathematical physics of the linear ideal MRI is
particularly well trodden territory. Indeed the lucid
early treatment of Balbus & Hawley (1991, 1998, etc)
in the local and slow limit has become canonical: the
introduction to almost any research talk on accretion
disks would seem incomplete without a reference to fluid
blobs, magnetic tethers, and mechanical springs. In my
talk, however, the linear ideal MRI will not be limited to
the introduction. I
will be focusing on the linear theory throughout,
revisiting attractive, and overlooked, results in global
geometries: vertically stratified boxes and cylindrical
disk models. The
MRI in both cases exhibits (a) the local incompressible
dispersion relation, despite the global nature of the
modes, (b) convenient analytic approximations to the
global eigenfunctions, and (c) channel flows that remain
approximate nonlinear solutions until the plasma beta
approaches one.
Thanatology
in
protoplanetary discs
Geoffroy Lesur
The existence of magnetically
driven turbulence in protoplanetary discs has been a
central question since the discovery of the
magnetorotational instability (MRI). Early models
considered Ohmic diffusion only and led to a scenario of
layered accretion, in which a magnetically ``dead'' zone
in the disc midplane
is embedded within magnetically ``active'' surface layers
at distances of about 1--10 au from the central
protostellar object. Recent work has suggested that a
combination of Ohmic dissipation and ambipolar
diffusion can render both the midplane and surface layers
of the disc inactive and that torques due to magnetically
driven outflows are required to explain the observed
accretion rates.
In this talk, I will present recent results revisiting
this problem including all three non-ideal MHD effects:
Ohmic diffusion, Ambipolar diffusion and the Hall effect.
I will show in particular that the Hall effect can
``revive'' dead zones by providing a large scale magnetic
torque in the disc midplane, potentially leading to
significant accretion rates. Implications for the global
evolution of protoplanetary discs will be discussed.
The
role of magnetic fields in star formation
Christopher McKee
Magnetic forces in the
diffuse interstellar medium are much greater than the
forces due to self-gravitation, thereby precluding star
formation there. Historically, it has been conjectured
that ambipolar diffusion, in which neutral molecules
contract relative to the magnetized ions, is essential for
allowing gravitational forces to exceed magnetic forces in
star-forming clouds. However, observations of magnetic
field strengths in molecular cloud cores have failed to
find evidence of cores that are magnetically dominated.
Analysis of Zeeman observations of magnetic fields in
molecular cloud cores sheds light on the intrinsic
distribution of field strengths. It is also possible to
use ideal magnetohydrodynamic (MHD) simulations to
"observe" a turbulent
molecular cloud on a computer. Such simulations provide
the full 3D field and show how tangling of the field lines
reduces the field measured by the Zeeman effect. Magnetic
fields are effective at extracting angular momentum from
the gas accreting onto a protostar, and many simulations
have found that protostellar (and therefore
protoplanetary) disks cannot form in the presence of
observed interstellar magnetic fields. When turbulence is
included, however, rotating protostellar disks can indeed
form in the presence of magnetic fields, suggesting that
ideal MHD in the presence of turbulence is not ideal.
As the Sun turns:
Differential rotation and meridional circulation in
stellar convection zones
Mark Miesch
The solar differential rotation (DR) is among
the most fundamental and enduring problems in
astrophysical fluid dynamics. Its discovery dates back to the mid
19th century when Sir Richard Carrington was
able to map the latitudinal variation of the solar surface
rotation by tracking sunspots, revealing the fluid nature
of the solar interior.
Modern measurements using various observing
techniques closely match Carrington’s result, indicating
that the solar DR has not changed by more than a few
percent in over 150 years.
Though the story of the solar DR is not complete,
the plot has thickened in the last three decades. The
helioseismology revolution has revealed the internal
rotation profile of the Sun as increasingly sophisticated
supercomputer models unravel the nonlinear dynamics of
solar convection, mean flows, and magnetism. These advances
are supplemented by continuing stellar observations and
theoretical insights (including those by our man of honor)
that contribute perspective and context. One of the
realizations from this work is that the DR is intimately
linked dynamically with the meridional circulation (MC)
and that both are intimately linked to solar magnetism. In particular,
it is the MC that may set the pace of the 11-year solar
cycle. I
will review our current understanding of the solar DR and
MC and how this understanding extends to other stars. Highlights
include the solar near-surface shear layer, potential
pitfalls of mean-field modeling, and the “convection
conundrum”: why recent observational and modeling results
are challenging our understanding of convective heat and
angular momentum transport.
Planetesimal
and
planet migration and growth in turbulent disks
Richard P. Nelson
Turbulence in
protoplanetary discs can have a profound influence on the
formation and evolution of planets. Turbulent density
fluctuations can act as a source of stochastic forcing
that excites the eccentricities and inclinations of
planetesimals and planets, influencing their collisional
outcomes. The stochastic forcing also introduces a random
walk component to the orbital evolution, influencing
planetary migration and the radial mixing of
planetesimals. Furthermore, the angular momentum transport
associated with disc turbulence may provide the effective
viscous stress needed to prevent the saturation of the
corotation torques experienced by low mass planets, and
drives the gas accretion onto forming giant planets, as
well as their type II migration. In this talk I will
review recent
work that has examined these issues, and discuss how
recent developments in our understanding of protoplanetary
disc dynamics may influence the theory of planet
formation.
Dynamics and
instability of eccentric discs
Gordon Ogilvie, Adrian Barker
Eccentric accretion discs composed of variably elliptical
Keplerian orbits are found in many binary stars and can
also be formed when stars or planets are tidally
disrupted. Their dynamics is potentially an
important aspect of planet disc interaction. We
formulate a local model of an eccentric disc that
generalizes the shearing sheet to include the elliptical
geometry of the reference orbit and the oscillatory
compression associated with an eccentricity
gradient. Stresses computed in the local model feed
into the equations determining the large-scale evolution
of the shape and mass distribution of the disc.
Eccentric discs lack vertical hydrostatic equilibrium and
undergo nonlinear vertical oscillations that can become
extreme for eccentricities above about 0.5. The
associated stresses significantly modify the behaviour of
eccentric discs from two-dimensional models. We
compute these solutions and their linear stability to
locally axisymmetric disturbances. Inertial waves
are parametrically destabilized with growth rates that are
significantly larger than in two-dimensional models.
The nonlinear outcome of this instability may generate
hydrodynamic activity in astrophysical discs and limit the
eccentricities that can be achieved.
New forms of
convection & turbulence: The MTI & HBI in galaxy
clusters
Ian Parrish
I will review the discovery of the
magnetothermal instability (MTI) by Steve Balbus and its
subsequent study from early numerical work through current
simulations of the MTI in galaxy clusters. This
fascinating instability taps into the temperature gradient
as a source of free energy (as opposed to the entropy
gradient in Schwarzschild convection). I will explain
the linear and nonlinear physics as well as the saturation
process of the instability.
In the outer part of the intracluster medium, the
MTI driven by the outwardly decreasing temperature
gradient can drive vigorous convection. The resultant
non-thermal pressure support from this vigorous convection
has direct implications for using SZ measurements of
clusters in cosmological surveys. Finally, I will
also mention the closely-related heat-flux-driven buoyancy
instability (HBI) that also operates in clusters and
discuss its possible effect in driving bimodality in
cluster cores.
Topology and
magnetic field strength in spherical an elastic dynamo
simulations
Ludovic Petitdemange
Numerical modelling of convection driven
dynamos in the Boussinesq approximation revealed
fundamental characteristics of the dynamo-generated
magnetic fields and the fluid flow. Because these results
were obtained for an incompressible fluid of constant
density, their validity for gas planets and stars remains
to be assessed. A common approach is to take some density
stratification into account with the so-called anelastic
approximation.
The validity of previous results obtained in
the Boussinesq approximation is tested for anelastic
models. We point out and explain specific differences
between both types of models, in particular, with respect
to the field geometry and the field strength.
Our investigations are based on a systematic
parameter study of spherical dynamo models in the
anelastic and Boussinesq approximations.
The dichotomy of dipolar and multipolar
(oscillatory) dynamos identified in Boussinesq simulations
is also present in our sample of anelastic models. Dipolar
models require that the typical length scale of convection
is an order of magnitude larger than the Rossby radius.
However, the distinction between both classes of models is
somewhat less explicit than in previous studies. This is
mainly due to two reasons: we found a number of models
with a considerable equatorial dipole contribution and an
intermediate overall dipole field strength. Furthermore, a
large density stratification may hamper the generation of
dipole dominated magnetic fields. Previously proposed
scaling laws, such as those for the field strength, are
similarly applicable to anelastic models. It is not clear,
however, if this consistency necessarily implies similar
dynamo processes in both settings. We discuss how these
findings relate to previous models and to stellar and
planetary observations.
An accretion disc
instability induced by a temperature sensitive alpha
parameter
Will Potter
The thin disc alpha parameter is usually
assumed to be a constant in analyses, however, from both
theoretical considerations and simulations we expect alpha
to be variable. I will show that if alpha is not a
constant but depends on the magnetic Prandtl number (as
suggested by simulations) it can produce an instability in
the disc. This instability generates cyclic flaring in the
inner disc which could help to explain the complicated
flaring behaviour of observed X-ray binary systems.
Convection in galaxy
cluster plasmas
Eliot Quataert
The outer parts of galaxy clusters are
unstable to the magnetothermal instability if the
temperature decreases at large radii. I
summarize the physics that sets the temperature gradients
in galaxy clusters, the saturation of the magnetothermal
instability, and its implications for galaxy cluster
plasmas.
Accretion in (initially) unmagnetized
collisionless plasmas
Eliot Quataert
We study the stability and angular momentum
transport in initially unmagnetized collisionless plasmas. Conceptually,
this bridges the gap between hydrodynamic,
magnetohydrodynamic, and magnetized kinetic models of disk
dynamics and transport.
We show that initially unmagnetized rotating
collisionless plasmas are unstable to electromagnetic
instabilities (specifically, the Weibel instability). This
leads to the generation of a magnetic field and outward
transport of angular momentum. The stress is dominated by the
anisotropic pressure contribution (i.e., "collisionless
viscosity"). The
amplification of the magnetic field saturates with a
cyclotron frequency somewhat larger than the disk rotation
frequency, leaving the plasma in a state that is likely
unstable to the MRI.
This mechanism may be important for the
astrophysical origin of magnetic fields at high redshift.
The Physics of the
Intracluster Medium and AGN feedback in galaxy clusters
Christopher Reynolds
In the central regions of cooling-core galaxy
clusters, the activity of a central active galactic
nucleus (AGN) is generally believed to heat the
intracluster medium (ICM), thereby preventing a cooling
catastrophe and the unchecked growth of the central galaxy
— these systems provide the cleanest environments to
observe AGN feedback at work. However, the
actual physical processes involved in this AGN-cluster
feedback are complex and subtle. The actual
physical process by which the AGN-injected energy is
thermalized remains unknown and is almost certainly
associated with plasma-scale processes. Equally
mysterious is the mechanism by which the AGN is fueled (on
parsec scales) at a rate that is fine-tuned to balance
cooling in the ICM core (on 100kpc scales). Finally,
even the background ICM atmosphere is dynamically complex,
with conduction-driven MHD instabilities driving
turbulence from the free-energy in the background
temperature gradient. In this talk, I shall
summarize the current observational constraints, as well
as today’s theoretical challenges and progress, on
AGN-cluster feedback models.
The dark side of the
accretion disk dynamo
François Rincon, A. Riols, C. Cossu, G.
Lesur, G. I. Ogilvie, P.-Y. Longaretti
Even though the magnetorotational dynamo
("zero net-flux" MRI) has long been considered one of the
possible bootstrapping processes of turbulent angular
momentum transport in accretion disks, whether it can
actually be excited in Keplerian shear flow in the
astrophysically relevant regime of magnetic Prandtl number
Pm<<1 very much remains an open question. I will present
an overview of recent collaborative research aiming at
deciphering the challenging dynamical complexity of this
subcritical dynamo transition. I will first show that the
emergence of three-dimensional chaos and transient MHD
turbulence in this problem is primarily associated with
global homoclinic and heteroclinic bifurcations involving
the stable and unstable manifolds of three-dimensional
nonlinear MRI dynamo cycles born out of saddle node
bifurcations. I will then explain how such solutions may
be harnessed to learn more about the physics of the
transition as a whole, and will present new results
indicating that turbulent magnetic diffusion makes the
excitation and sustainment of the dynamo at moderate
magnetic Reynolds number Rm increasingly difficult for
decreasing Pm, resulting in an increase of the critical Rm
of the dynamo for increasing kinematic Reynolds number Re,
in agreement with earlier numerical results. The potential
implications of these results for the accretion disk
dynamo will finally be discussed.
Stratified turbulence in
galaxy cluster cores
Alex Schekochihin, Irina Zhuravleva, Eugene
Churazov, Federico Mogavero, Scott Melville, Matt
Kunz, Francois Rincon, Steve Cowley
I will discuss some basic properties of
stratified turbulence in galaxy cluster cores and the
implications for what's observed. In particular, what's
observed quite well is density fluctuations across a
decent scale range, whence one can make inferences about
the turbulence and, therefore, about energy
dissipation rates in cluster cores. I will review some
recent work on this subject [1,2,3]. I will then
discuss various schemes for the evolution of
the magnetic field in a weakly
collisionless plasma [4]: what we have learned recently
[5,6,7] and what that might imply for the
role of magnetisation in the dynamics and
thermodynamics [8] of cluster cores.
References:
[1] I. Zhuravleva et al., ApJL, in press
(2014) arXiv:1404.5306
[2] E. Churazov et al., MNRAS 421, 1123
(2012)
[3] I. Zhuravleva et al., "Turbulent heating
in the brightest galaxy clusters" (to appear)
[4] F. Mogavero and A. A. Schekochihin, MNRAS
440, 3226 (2014)
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Observational signatures of
MRI-driven turbulence in protoplanetary disks:
Connecting numerical simulations with ALMA
Jacob
Simon
Protoplanetary
disks
play a key role in star and planet formation
processes. Turbulence in these disks, which arises
from the magnetorotational instability (MRI), not only
causes accretion of mass onto the central star, but also
sets the conditions for processes such as dust settling,
planetesimal formation, and planet migration.
However, the exact nature of this turbulence is still not
very well constrained in these systems.
In this
talk, I will describe new work, utilizing both
state-of-the-art numerical simulations and high resolution
radio observations, to directly link numerical predictions
for the turbulent velocity structure of protoplanetary
disks to observations by the Atacama Large Millimeter
Array (ALMA). ALMA’s unprecedented resolution will allow
us to generate a three-dimensional view of disk turbulence
by measuring the turbulent broadening component of
molecular lines at different disk heights and radii. A
direct comparison between the observed turbulence values
and those obtained from simulations will strongly
constrain our theoretical understanding of these disks and
the conditions under which planetary systems develop.
Numerical
models of the MRI: Past, present, and future.
James M. Stone
I will review some
of what we have learned about the nonlinear regime of the
MRI from numerical simulations. I will start at the beginning, with
the first two-dimensional results published by Hawley
& Balbus in a companion paper to the linear analysis
presented in the MRI discovery paper. I will then
highlight some of the important discoveries made through
simulations since then, including results from the first
three-dimensional simulations, the first vertically
stratified models, and the first fully global models. I
will summarize what we have learned as additional physics
has been added, including finite dissipation, non-ideal
MHD, and radiation transport. Many of these topics continue to be
the focus of current research, and I will give a brief
overview of some of the important problems and puzzles
being addressed in current work. Finally I will highlight some
challenges for the future.
Magnetic
Wreaths and Dynamo Cycles in Sun-like Stars
Juri Toomre
The building and
cycling of global-scale magnetic fields in turbulent
convection envelopes of stars like the Sun involve many
dynamical elements, with rotation and shear figuring
prominently. Using 3-D MHD global simulations with our
Anelastic Spherical Harmonic (ASH) code, our studies have
revealed that remarkable wreaths of strong magnetic field
can be built in the bulk of the convection zone by dynamo
action. These wreaths possess toroidal fields often with
opposite polarity in the two hemispheres, with some of the
field pumped downward into the tachocline to be further
amplified by shear.
The sense of the magnetic fields can reverse over
decade-long time scales.
Some models that had gone through many reversing
cycles even went into brief quiescent intervals like a
Maunder Minimum, and then resumed their cycling. Further,
we have obtained loops of magnetic field that break off
spontaneously from the wreaths and rise toward the
surface, providing a path for flux emergence. All of these
processes involve sensitive balances between magnetic
field amplification and dissipation that occur over a
range of scales, with multi-scale interactions being
critical to the field evolution.
Transition to turbulence in
Couette and Poiseuille flows
Laurette Tuckerman
Shear flows, i.e.
Couette and Poiseuille flows, become turbulent for
Reynolds
numbers at which they are linearly stable. Although this
transition is not
completely understood, a number of advances have been made
in recent years. For transitional Reynolds numbers, the
flow takes the form of coexisting laminar and turbulent
regions whose geometric, statistical and dynamical
properties are now well characterized. Critical Reynolds
numbers have been defined probabilistically and determined
numerically. The force balance between the turbulent and
laminar regions has elucidated the maintenance of the
coexistence regimes.
A
connection between the MRI and Elastodynamics
Geoffrey Vasil
I will present a new
simple model for the MRI in a weakly nonlinear regime. Conducting a
systematic reduction of the full problem with reasonable
simplifying assumptions leads to a particularly simple
nonlinear dispersive wave equation governing the dynamics
of the instability near onset. Surprisingly, the reduced dynamics
corresponds to that of an elastic buckling beam under an
applied load. This
very simple reduced model helps explain the saturation of
the MRI via flux and momentum conservation principles with
direct analogies to transport of compression and tension
stress in a bent elastic rod. I will briefly discuss possible
applications of this model to the dynamics of the solar
near-surface shear layers, and magnetic dynamo.
Magnetic
drift in molecular cloud cores, protoplanetary discs,
and the solar chromosphere
Mark Wardle
Flux-freezing breaks
down under the low levels of ionisation in molecular cloud
cores and protoplanetary discs. The dominant processes are ambipolar
diffusion and hall drift, which enable slippage of
magnetic flux through the predominantly neutral gas. The nature of
the field line drift through the bulk neutral component of
the gas is as important as its magnitude. Under ambipolar
diffusion, magnetic field lines in the direction of the
local magnetic stress; the drift is accompanied by
dissipation associated with collisions between charged and
neutral species. The
Hall effect introduces a drift perpendicular to the local
magnetic stresses that is unaccompanied by dissipation. Hall drift
dominates ambipolar diffusion over a wide range of radii
in protoplanetary disks and likely plays a significant
role during gravitational collapse of cloud cores. I shall outline
the physics underlying magnetic drift in a partially
ionised medium and then discuss applications to
gravitational collapse, magnetorotational instability and
jet acceleration in protoplanetary discs. Similar
considerations also apply on small scales in the
partially-ionised solar chromosphere.
