explanation      blue bibcodes open ADS page with paths to full text
Author name code: cattaneo
ADS astronomy entries on 2022-09-14
author:"Cattaneo, Fausto" 

---------------------------------------------------------
Title: Research Opportunities in Plasma Astrophysics
Authors: Bale, Stuart; Bhattacharjee, Amitava; Cattaneo, Fausto; Drake,
   Jemes; Ji, Hantao; Lee, Marty; Li, Hui; Liang, Edison; Pound, Marc;
   Prager, Stewart; Quataert, Eliot; Remington, Bruce; Rosner, Robert;
   Ryutov, Dmitri; Thomas, Edward, Jr; Zweibel, Ellen
2022arXiv220302406B    Altcode:
  Major scientific questions and research opportunities are described
  on 10 unprioritized plasma astrophysics topics: (1) magnetic
  reconnection, (2) collisionless shocks and particle acceleration, (3)
  waves and turbulence, (4) magnetic dynamos, (5) interface and shear
  instabilities, (6) angular momentum transport, (7) dusty plasmas,
  (8) radiative hydrodynamics, (9) relativistic, pair-dominated and
  strongly magnetized plasmas, (10) jets and outflows. Note that this
  is a conference report from a Workshop on Opportunities in Plasma
  Astrophysics (WOPA, https://w3.pppl.gov/conferences/2010/WOPA/) in
  January 2010, that attracted broad representation from the community
  and was supported by the U.S. Department of Energy, National Aeronautics
  and Space Administration, National Science Foundation, American Physical
  Society's Topical Group for Plasma Astrophysics and Division of Plasma
  Physics, and Center for Magnetic Self-Organization in Laboratory and
  Astrophysical Plasmas. Although there has been much planning and many
  developments in both science and infrastructure since the report was
  written, most of the motivation, priorities, problems and technical
  challenges discussed therein remain unaddressed and are relevant at
  the time of posting.

---------------------------------------------------------
Title: Generation of coherent magnetic fields in periodic (closed)
    and non-periodic (open) domains
Authors: Bhat, Pallavi; Tobias, Steven; Cattaneo, Fausto; Bodo,
   Gianluigi
2020APS..DFDE05005B    Altcode:
  The origin of large-scale magnetic fields in most astrophysical
  systems like the Sun, stars and galaxies remains a challenging open
  problem. Dynamo action in the underlying turbulent fluid is thought
  to be responsible for the emergence of coherent magnetic fields. Due
  to the enormity of magnetic Reynolds numbers in these astrophysical
  systems, current theoretical models of the turbulent dynamo struggle to
  generate large-scale field on fast dynamic timescales. The conservation
  properties of magnetic helicity can constrain the nonlinear evolution
  of the dynamo. We have performed direct numerical simulations of the
  turbulent dynamo to investigate if employing open boundaries relaxes
  the constraint imposed by magnetic helicity conservation. We find
  that in the open systems a net magnetic flux (or system-scale fields)
  of significant strength arises. However, the type of open boundary we
  employ does not alleviate the magnetic Reynolds number (in the range
  explored) dependence in the nonlinear evolution of the large-scale
  fields. Finally, simulations performed across different magnetic Prandtl
  numbers indicate that the behavior of the magnetic helicity evolution
  is affected by flow properties as well. <P />European Research Council.

---------------------------------------------------------
Title: Magnetorotational Turbulence, Dynamo Action and Transport in
    Convective Disks
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Rossi, P.
2019ASSP...55....3B    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Laboratory evidence of dynamo amplification of magnetic fields
    in a turbulent plasma
Authors: Tzeferacos, P.; Rigby, A.; Bott, A. F. A.; Bell, A. R.;
   Bingham, R.; Casner, A.; Cattaneo, F.; Churazov, E. M.; Emig, J.;
   Fiuza, F.; Forest, C. B.; Foster, J.; Graziani, C.; Katz, J.; Koenig,
   M.; Li, C. -K.; Meinecke, J.; Petrasso, R.; Park, H. -S.; Remington,
   B. A.; Ross, J. S.; Ryu, D.; Ryutov, D.; White, T. G.; Reville,
   B.; Miniati, F.; Schekochihin, A. A.; Lamb, D. Q.; Froula, D. H.;
   Gregori, G.
2018NatCo...9..591T    Altcode: 2017arXiv170203016T
  Magnetic fields are ubiquitous in the Universe. The energy density of
  these fields is typically comparable to the energy density of the fluid
  motions of the plasma in which they are embedded, making magnetic fields
  essential players in the dynamics of the luminous matter. The standard
  theoretical model for the origin of these strong magnetic fields is
  through the amplification of tiny seed fields via turbulent dynamo to
  the level consistent with current observations. However, experimental
  demonstration of the turbulent dynamo mechanism has remained elusive,
  since it requires plasma conditions that are extremely hard to re-create
  in terrestrial laboratories. Here we demonstrate, using laser-produced
  colliding plasma flows, that turbulence is indeed capable of rapidly
  amplifying seed fields to near equipartition with the turbulent fluid
  motions. These results support the notion that turbulent dynamo is a
  viable mechanism responsible for the observed present-day magnetization.

---------------------------------------------------------
Title: Magnetic Helicities and Dynamo Action in Magneto-rotational
    Turbulence
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Rossi, P.
2017ApJ...843...86B    Altcode: 2017arXiv170604492B
  We examine the relationship between magnetic flux generation, taken
  as an indicator of large-scale dynamo action, and magnetic helicity,
  computed as an integral over the dynamo volume, in a simple dynamo. We
  consider dynamo action driven by magneto-rotational turbulence (MRT)
  within the shearing-box approximation. We consider magnetically open
  boundary conditions that allow a flux of helicity in or out of the
  computational domain. We circumvent the problem of the lack of gauge
  invariance in open domains by choosing a particular gauge—the winding
  gauge—that provides a natural interpretation in terms of the average
  winding number of pairwise field lines. We use this gauge precisely
  to define and measure the helicity and the helicity flux for several
  realizations of dynamo action. We find in these cases that the system
  as a whole does not break reflectional symmetry and that the total
  helicity remains small even in cases when substantial magnetic flux
  is generated. We find no particular connection between the generation
  of magnetic flux and the helicity or the helicity flux through the
  boundaries. We suggest that this result may be due to the essentially
  nonlinear nature of the dynamo processes in MRT.

---------------------------------------------------------
Title: Numerical modeling of laser-driven experiments aiming to
    demonstrate magnetic field amplification via turbulent dynamo
Authors: Tzeferacos, P.; Rigby, A.; Bott, A.; Bell, A. R.; Bingham,
   R.; Casner, A.; Cattaneo, F.; Churazov, E. M.; Emig, J.; Flocke,
   N.; Fiuza, F.; Forest, C. B.; Foster, J.; Graziani, C.; Katz, J.;
   Koenig, M.; Li, C. -K.; Meinecke, J.; Petrasso, R.; Park, H. -S.;
   Remington, B. A.; Ross, J. S.; Ryu, D.; Ryutov, D.; Weide, K.; White,
   T. G.; Reville, B.; Miniati, F.; Schekochihin, A. A.; Froula, D. H.;
   Gregori, G.; Lamb, D. Q.
2017PhPl...24d1404T    Altcode: 2017arXiv170203015T
  The universe is permeated by magnetic fields, with strengths ranging
  from a femtogauss in the voids between the filaments of galaxy clusters
  to several teragauss in black holes and neutron stars. The standard
  model behind cosmological magnetic fields is the nonlinear amplification
  of seed fields via turbulent dynamo to the values observed. We have
  conceived experiments that aim to demonstrate and study the turbulent
  dynamo mechanism in the laboratory. Here, we describe the design of
  these experiments through simulation campaigns using FLASH, a highly
  capable radiation magnetohydrodynamics code that we have developed,
  and large-scale three-dimensional simulations on the Mira supercomputer
  at the Argonne National Laboratory. The simulation results indicate
  that the experimental platform may be capable of reaching a turbulent
  plasma state and determining the dynamo amplification. We validate and
  compare our numerical results with a small subset of experimental data
  using synthetic diagnostics.

---------------------------------------------------------
Title: What is a large-scale dynamo?
Authors: Nigro, G.; Pongkitiwanichakul, P.; Cattaneo, F.; Tobias, S. M.
2017MNRAS.464L.119N    Altcode:
  We consider kinematic dynamo action in a sheared helical flow at
  moderate to high values of the magnetic Reynolds number (Rm). We find
  exponentially growing solutions which, for large enough shear, take
  the form of a coherent part embedded in incoherent fluctuations. We
  argue that at large Rm large-scale dynamo action should be identified
  by the presence of structures coherent in time, rather than those at
  large spatial scales. We further argue that although the growth rate
  is determined by small-scale processes, the period of the coherent
  structures is set by mean-field considerations.

---------------------------------------------------------
Title: Shear-driven Dynamo Waves in the Fully Nonlinear Regime
Authors: Pongkitiwanichakul, P.; Nigro, G.; Cattaneo, F.; Tobias, S. M.
2016ApJ...825...23P    Altcode:
  Large-scale dynamo action is well understood when the magnetic Reynolds
  number (Rm) is small, but becomes problematic in the astrophysically
  relevant large Rm limit since the fluctuations may control the
  operation of the dynamo, obscuring the large-scale behavior. Recent
  works by Tobias &amp; Cattaneo demonstrated numerically the existence of
  large-scale dynamo action in the form of dynamo waves driven by strongly
  helical turbulence and shear. Their calculations were carried out in
  the kinematic regime in which the back-reaction of the Lorentz force on
  the flow is neglected. Here, we have undertaken a systematic extension
  of their work to the fully nonlinear regime. Helical turbulence and
  large-scale shear are produced self-consistently by prescribing body
  forces that, in the kinematic regime, drive flows that resemble the
  original velocity used by Tobias &amp; Cattaneo. We have found four
  different solution types in the nonlinear regime for various ratios
  of the fluctuating velocity to the shear and Reynolds numbers. Some
  of the solutions are in the form of propagating waves. Some solutions
  show large-scale helical magnetic structure. Both waves and structures
  are permanent only when the kinetic helicity is non-zero on average.

---------------------------------------------------------
Title: Strong-field dynamo action in rapidly rotating convection
    with no inertia
Authors: Hughes, David W.; Cattaneo, Fausto
2016PhRvE..93f1101H    Altcode: 2015arXiv151006220H
  The earth's magnetic field is generated by dynamo action driven by
  convection in the outer core. For numerical reasons, inertial and
  viscous forces play an important role in geodynamo models; however,
  the primary dynamical balance in the earth's core is believed to be
  between buoyancy, Coriolis, and magnetic forces. The hope has been
  that by setting the Ekman number to be as small as computationally
  feasible, an asymptotic regime would be reached in which the correct
  force balance is achieved. However, recent analyses of geodynamo models
  suggest that the desired balance has still not yet been attained. Here
  we adopt a complementary approach consisting of a model of rapidly
  rotating convection in which inertial forces are neglected from the
  outset. Within this framework we are able to construct a branch of
  solutions in which the dynamo generates a strong magnetic field that
  satisfies the expected force balance. The resulting strongly magnetized
  convection is dramatically different from the corresponding solutions
  in which the field is weak.

---------------------------------------------------------
Title: The electromotive force in multi-scale flows at high magnetic
    Reynolds number
Authors: Tobias, Steven M.; Cattaneo, Fausto
2015JPlPh..81f3901T    Altcode:
  &gt; Recent advances in dynamo theory have been made by examining the
  competition between small- and large-scale dynamos at high magnetic
  Reynolds number . Small-scale dynamos rely on the presence of chaotic
  stretching whilst the generation of large-scale fields occurs in
  flows lacking reflectional symmetry via a systematic electromotive
  force (EMF). In this paper we discuss how the statistics of the EMF
  (at high ) depend on the properties of the multi-scale velocity that
  is generating it. In particular, we determine that different scales
  of flow have different contributions to the statistics of the EMF,
  with smaller scales contributing to the mean without increasing
  the variance. Moreover, we determine when scales in such a flow act
  independently in their contribution to the EMF. We further examine the
  role of large-scale shear in modifying the EMF. We conjecture that the
  distribution of the EMF, and not simply the mean, largely determines
  the dominant scale of the magnetic field generated by the flow.

---------------------------------------------------------
Title: A model of plasma heating by large-scale flow
Authors: Pongkitiwanichakul, P.; Cattaneo, F.; Boldyrev, S.; Mason,
   J.; Perez, J. C.
2015MNRAS.454.1503P    Altcode: 2015arXiv150908848P
  In this work, we study the process of energy dissipation triggered
  by a slow large-scale motion of a magnetized conducting fluid. Our
  consideration is motivated by the problem of heating the solar corona,
  which is believed to be governed by fast reconnection events set off by
  the slow motion of magnetic field lines anchored in the photospheric
  plasma. To elucidate the physics governing the disruption of the
  imposed laminar motion and the energy transfer to small scales, we
  propose a simplified model where the large-scale motion of magnetic
  field lines is prescribed not at the footpoints but rather imposed
  volumetrically. As a result, the problem can be treated numerically
  with an efficient, highly accurate spectral method, allowing us to use
  a resolution and statistical ensemble exceeding those of the previous
  work. We find that, even though the large-scale deformations are slow,
  they eventually lead to reconnection events that drive a turbulent
  state at smaller scales. The small-scale turbulence displays many of the
  universal features of field-guided magnetohydrodynamic turbulence like
  a well-developed inertial range spectrum. Based on these observations,
  we construct a phenomenological model that gives the scalings of
  the amplitude of the fluctuations and the energy-dissipation rate as
  functions of the input parameters. We find good agreement between the
  numerical results and the predictions of the model.

---------------------------------------------------------
Title: Global Properties of Fully Convective Accretion Disks from
    Local Simulations
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Ponzo, F.; Rossi, P.
2015ApJ...808..141B    Altcode: 2016arXiv160207334B
  We present an approach to deriving global properties of accretion disks
  from the knowledge of local solutions derived from numerical simulations
  based on the shearing box approximation. The approach consists of a
  two-step procedure. First, a local solution valid for all values of
  the disk height is constructed by piecing together an interior solution
  obtained numerically with an analytical exterior radiative solution. The
  matching is obtained by assuming hydrostatic balance and radiative
  equilibrium. Although in principle the procedure can be carried out
  in general, it simplifies considerably when the interior solution is
  fully convective. In these cases, the construction is analogous to
  the derivation of the Hayashi tracks for protostars. The second step
  consists of piecing together the local solutions at different radii
  to obtain a global solution. Here we use the symmetry of the solutions
  with respect to the defining dimensionless numbers—in a way similar
  to the use of homology relations in stellar structure theory—to obtain
  the scaling properties of the various disk quantities with radius.

---------------------------------------------------------
Title: Energy dynamics and current sheet structure in fluid and
    kinetic simulations of decaying magnetohydrodynamic turbulence
Authors: Makwana, K. D.; Zhdankin, V.; Li, H.; Daughton, W.;
   Cattaneo, F.
2015PhPl...22d2902M    Altcode: 2014arXiv1412.4723M
  Simulations of decaying magnetohydrodynamic (MHD) turbulence
  are performed with a fluid and a kinetic code. The initial
  condition is an ensemble of long-wavelength, counter-propagating,
  shear-Alfvén waves, which interact and rapidly generate strong
  MHD turbulence. The total energy is conserved and the rate of
  turbulent energy decay is very similar in both codes, although
  the fluid code has numerical dissipation, whereas the kinetic code
  has kinetic dissipation. The inertial range power spectrum index
  is similar in both the codes. The fluid code shows a perpendicular
  wavenumber spectral slope of k<SUB>⊥-1.3</SUB> . The kinetic code
  shows a spectral slope of k<SUB>⊥-1.5</SUB> for smaller simulation
  domain, and k<SUB>⊥-1.3</SUB> for larger domain. We estimate that
  collisionless damping mechanisms in the kinetic code can account for
  the dissipation of the observed nonlinear energy cascade. Current
  sheets are geometrically characterized. Their lengths and widths are
  in good agreement between the two codes. The length scales linearly
  with the driving scale of the turbulence. In the fluid code, their
  thickness is determined by the grid resolution as there is no explicit
  diffusivity. In the kinetic code, their thickness is very close to the
  skin-depth, irrespective of the grid resolution. This work shows that
  kinetic codes can reproduce the MHD inertial range dynamics at large
  scales, while at the same time capturing important kinetic physics at
  small scales.

---------------------------------------------------------
Title: Fully Convective Magneto-rotational Turbulence in Large
    Aspect-ratio Shearing Boxes
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Rossi, P.
2015ApJ...799...20B    Altcode:
  We present a numerical study of turbulence and dynamo action in
  stratified shearing boxes with both finite and zero net magnetic
  flux. We assume that the fluid obeys the perfect gas law and has finite
  thermal diffusivity. The latter is chosen to be small enough so that
  vigorous convective states develop. The properties of these convective
  solutions are analyzed as the aspect ratio of the computational domain
  is varied and as the value of the mean field is increased. For the
  cases with zero net flux, we find that a well-defined converged state
  is obtained for large enough aspect ratios. In the converged state,
  the dynamo can be extremely efficient and can generate substantial
  toroidal flux. We identify solutions in which the toroidal field is
  mostly symmetric about the mid-plane and solutions in which it is mostly
  anti-symmetric. The symmetric solutions are found to be more efficient
  at transporting angular momentum and can give rise to a luminosity that
  is up to an order of magnitude larger than the corresponding value
  for the anti-symmetric states. In the cases with a finite net flux,
  the system appears to spend most of the time in the symmetric states.

---------------------------------------------------------
Title: Plasma Heating by Volumetric Large-scale Flows
Authors: Pongkitiwanichakul, P.; Cattaneo, F.; Boldyrev, S.; Mason,
   J.; Perez, J. C.
2014AGUFMSH13C4136P    Altcode:
  We solve an incompressible magnetohydrodynamo system.We simulate a
  series of high-resolution numerical simulations and aim at clarifying
  the mechanisms that lead to heating of the solar corona. The model
  consists of an initially uniform magnetic field that is slowly deformed
  by a volumetric prescribed flow.The response of the system is to
  re-adjust the magnetic configuration by a series of non-ideal events
  that lead to the heating of the plasma. Our simulations show that the
  system develops an MHD turbulent state. The heating is intermittent
  and independent of Lundquist number. The time averaged magnetic and
  kinetic energies also are independent of Lundquist number.Thereafter,
  we propose a phenomenological model to explain our finding. Our model
  provides the scaling laws that fairly describe the heating rate,
  the time averaged magnetic energy, and the time averaged kinetic energy.

---------------------------------------------------------
Title: The statistics of a passive scalar in field-guided
    magnetohydrodynamic turbulence
Authors: Mason, J.; Boldyrev, S.; Cattaneo, F.; Perez, J. C.
2014GApFD.108..686M    Altcode: 2014arXiv1409.7196M
  A variety of studies of magnetised plasma turbulence invoke theories for
  the advection of a passive scalar by turbulent fluctuations. Examples
  include modelling the electron density fluctuations in the interstellar
  medium, understanding the chemical composition of galaxy clusters and
  the intergalactic medium, and testing the prevailing phenomenological
  theories of magnetohydrodynamic turbulence. While passive scalar
  turbulence has been extensively studied in the hydrodynamic case,
  its counterpart in MHD turbulence is significantly less well
  understood. Herein we conduct a series of high-resolution direct
  numerical simulations of incompressible, field-guided, MHD turbulence
  in order to establish the fundamental properties of passive scalar
  evolution. We study the scalar anisotropy, establish the scaling
  relation analogous to Yaglom's law, and measure the intermittency
  of the passive scalar statistics. We also assess to what extent the
  pseudo Alfvén fluctuations in strong MHD turbulence can be modelled as
  a passive scalar. The results suggest that the dynamics of a passive
  scalar in MHD turbulence is considerably more complicated than in the
  hydrodynamic case.

---------------------------------------------------------
Title: Scaling Properties of Small-scale Fluctuations in
    Magnetohydrodynamic Turbulence
Authors: Perez, Jean Carlos; Mason, Joanne; Boldyrev, Stanislav;
   Cattaneo, Fausto
2014ApJ...793L..13P    Altcode: 2014arXiv1409.2728P
  Magnetohydrodynamic (MHD) turbulence in the majority of natural systems,
  including the interstellar medium, the solar corona, and the solar wind,
  has Reynolds numbers far exceeding the Reynolds numbers achievable
  in numerical experiments. Much attention is therefore drawn to the
  universal scaling properties of small-scale fluctuations, which
  can be reliably measured in the simulations and then extrapolated
  to astrophysical scales. However, in contrast with hydrodynamic
  turbulence, where the universal structure of the inertial and
  dissipation intervals is described by the Kolmogorov self-similarity,
  the scaling for MHD turbulence cannot be established based solely on
  dimensional arguments due to the presence of an intrinsic velocity
  scale—the Alfvén velocity. In this Letter, we demonstrate that the
  Kolmogorov first self-similarity hypothesis cannot be formulated for
  MHD turbulence in the same way it is formulated for the hydrodynamic
  case. Besides profound consequences for the analytical consideration,
  this also imposes stringent conditions on numerical studies of MHD
  turbulence. In contrast with the hydrodynamic case, the discretization
  scale in numerical simulations of MHD turbulence should decrease
  faster than the dissipation scale, in order for the simulations to
  remain resolved as the Reynolds number increases.

---------------------------------------------------------
Title: Comment on the numerical measurements of the
    magnetohydrodynamic turbulence spectrum by A. Beresnyak
    (Phys. Rev. Lett. 106 (2011) 075001; MNRAS 422 (2012) 3495; ApJ 784
    (2014) L20)
Authors: Perez, J. C.; Mason, J.; Boldyrev, S.; Cattaneo, F.
2014arXiv1409.8106P    Altcode:
  The inertial-interval energy spectrum of strong magnetohydrodynamic
  (MHD) turbulence with a uniform background magnetic field was observed
  numerically to be close to $k^{-3/2}$ by a number of independent
  groups. A dissenting opinion has been voiced by Beresnyak, A. 2011,
  PRL, 106, 075001-. 2012, MNRAS, 422, 3495-. 2014, ApJ, 784, L20 that
  the spectral scaling is close to $k^{-5/3}$. The conclusions of these
  papers are however incorrect as they are based on numerical simulations
  that are drastically unresolved, so that the discrete numerical scheme
  does not approximate the physical solution at the scales where the
  measurements are performed. These results have been rebutted in our
  more detailed papers Perez, J. C., Mason, J., Boldyrev, S., &amp;
  Cattaneo, F. 2012, PRX, 2, 041005-. 2014, ApJL, 793, L13; here, by
  popular demand, we present a brief and simple explanation of our major
  criticism of Beresnyak's work.

---------------------------------------------------------
Title: On Large-scale Dynamo Action at High Magnetic Reynolds Number
Authors: Cattaneo, F.; Tobias, S. M.
2014ApJ...789...70C    Altcode: 2014arXiv1405.3071C
  We consider the generation of magnetic activity—dynamo waves—in
  the astrophysical limit of very large magnetic Reynolds number. We
  consider kinematic dynamo action for a system consisting of helical
  flow and large-scale shear. We demonstrate that large-scale dynamo
  waves persist at high Rm if the helical flow is characterized by a
  narrow band of spatial scales and the shear is large enough. However,
  for a wide band of scales the dynamo becomes small scale with a further
  increase of Rm, with dynamo waves re-emerging only if the shear is then
  increased. We show that at high Rm, the key effect of the shear is to
  suppress small-scale dynamo action, allowing large-scale dynamo action
  to be observed. We conjecture that this supports a general "suppression
  principle"—large-scale dynamo action can only be observed if there
  is a mechanism that suppresses the small-scale fluctuations.

---------------------------------------------------------
Title: On the Convergence of Magnetorotational Turbulence in
    Stratified Isothermal Shearing Boxes
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Rossi, P.
2014ApJ...787L..13B    Altcode: 2014arXiv1404.6079B
  We consider the problem of convergence in stratified isothermal
  shearing boxes with zero net magnetic flux. We present results with
  the highest resolution to date—up to 200 grid points per pressure
  scale height—that show no clear evidence of convergence. Rather, the
  Maxwell stresses continue to decrease with increasing resolution. We
  propose some possible scenarios to explain the lack of convergence
  based on multi-layer dynamo systems.

---------------------------------------------------------
Title: Fully Convective Magnetorotational Turbulence in Stratified
    Shearing Boxes
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Rossi, P.
2013ApJ...771L..23B    Altcode: 2013arXiv1306.3386B
  We present a numerical study of turbulence and dynamo action in
  stratified shearing boxes with zero magnetic flux. We assume that
  the fluid obeys the perfect gas law and has finite (constant) thermal
  diffusivity. We choose radiative boundary conditions at the vertical
  boundaries in which the heat flux is proportional to the fourth power
  of the temperature. We compare the results with the corresponding cases
  in which fixed temperature boundary conditions are applied. The most
  notable result is that the formation of a fully convective state in
  which the density is nearly constant as a function of height and the
  heat is transported to the upper and lower boundaries by overturning
  motions is robust and persists even in cases with radiative boundary
  conditions. Interestingly, in the convective regime, although the
  diffusive transport is negligible, the mean stratification does not
  relax to an adiabatic state.

---------------------------------------------------------
Title: Computation as a Bridge between the Laboratory and Astrophysics
Authors: Rosner, Robert; Cattaneo, F.
2013AAS...22221201R    Altcode:
  Over the past decade, the many deep connections between terrestrial
  laboratory studies and astrophysics have been powerfully supported
  by modern numerical simulation: These calculations are able to make
  contact with modeling of both physically complex astrophysical phenomena
  and related phenomena observed in far greater detail in terrestrial
  laboratories. We will describe several examples that illustrate the
  power of numerical simulations to bridge laboratory and astrophysical
  studies.

---------------------------------------------------------
Title: Recent results on magnetic plasma turbulence
Authors: Boldyrev, Stanislav; Perez, Jean Carlos; Mason, Joanne;
   Cattaneo, Fausto
2013AIPC.1539..135B    Altcode:
  Magnetic plasma turbulence is observed over a broad range of scales
  in the solar wind. We discuss the results of high-resolution numerical
  simulations of magnetohydrodynamic (MHD) turbulence that models plasma
  motion at large scales and the results of numerical simulations
  of kinetic-Alfvén turbulence that models plasma motion at small,
  sub-proton scales. The simulations, with numerical resolutions up to
  20483 mesh points in the MHD case and 5123 points in kinetic-Alfvén
  case and statistics accumulated over 30 to 150 eddy turnover times,
  constitute, to the best of our knowledge, the largest statistical
  sample of steadily driven three dimensional MHD and kinetic-Alfvén
  turbulence to date.

---------------------------------------------------------
Title: Shear-driven dynamo waves at high magnetic Reynolds number
Authors: Tobias, S. M.; Cattaneo, F.
2013Natur.497..463T    Altcode:
  Astrophysical magnetic fields often display remarkable organization,
  despite being generated by dynamo action driven by turbulent flows at
  high conductivity. An example is the eleven-year solar cycle, which
  shows spatial coherence over the entire solar surface. The difficulty
  in understanding the emergence of this large-scale organization is that
  whereas at low conductivity (measured by the magnetic Reynolds number,
  Rm) dynamo fields are well organized, at high Rm their structure is
  dominated by rapidly varying small-scale fluctuations. This arises
  because the smallest scales have the highest rate of strain, and can
  amplify magnetic field most efficiently. Therefore most of the effort
  to find flows whose large-scale dynamo properties persist at high
  Rm has been frustrated. Here we report high-resolution simulations
  of a dynamo that can generate organized fields at high Rm; indeed,
  the generation mechanism, which involves the interaction between
  helical flows and shear, only becomes effective at large Rm. The shear
  does not enhance generation at large scales, as is commonly thought;
  instead it reduces generation at small scales. The solution consists
  of propagating dynamo waves, whose existence was postulated more than
  60 years ago and which have since been used to model the solar cycle.

---------------------------------------------------------
Title: Magnetorotational Turbulence in Stratified Shearing Boxes
    with Perfect Gas Equation of State and Finite Thermal Diffusivity
Authors: Bodo, G.; Cattaneo, F.; Mignone, A.; Rossi, P.
2012ApJ...761..116B    Altcode: 2012arXiv1210.6443B
  We present a numerical study of turbulence and dynamo action in
  stratified shearing boxes with zero mean magnetic flux. We assume
  that the fluid obeys the perfect gas law and has finite (constant)
  thermal diffusivity. The calculations begin from an isothermal state
  spanning three scale heights above and below the mid-plane. After a
  long transient the layers settle to a stationary state in which thermal
  losses out of the boundaries are balanced by dissipative heating. We
  identify two regimes. The first is a conductive regime in which the
  heat is transported mostly by conduction and the density decreases with
  height. In the limit of large thermal diffusivity this regime resembles
  the more familiar isothermal case. The second is the convective regime,
  observed at smaller values of the thermal diffusivity, in which the
  layer becomes unstable to overturning motions, the heat is carried
  mostly by advection, and the density becomes nearly constant throughout
  the layer. In this latter constant-density regime we observe evidence
  for large-scale dynamo action leading to a substantial increase in
  transport efficiency relative to the conductive case.

---------------------------------------------------------
Title: On the Energy Spectrum of Strong Magnetohydrodynamic Turbulence
Authors: Perez, Jean Carlos; Mason, Joanne; Boldyrev, Stanislav;
   Cattaneo, Fausto
2012PhRvX...2d1005P    Altcode: 2012arXiv1209.2011P
  The energy spectrum of magnetohydrodynamic turbulence attracts
  interest due to its fundamental importance and its relevance for
  interpreting astrophysical data. Here we present measurements of
  the energy spectra from a series of high-resolution direct numerical
  simulations of magnetohydrodynamics turbulence with a strong guide
  field and for increasing Reynolds number. The presented simulations,
  with numerical resolutions up to 2048<SUP>3</SUP> mesh points
  and statistics accumulated over 30 to 150 eddy turnover times,
  constitute, to the best of our knowledge, the largest statistical
  sample of steady state magnetohydrodynamics turbulence to date. We
  study both the balanced case, where the energies associated with
  Alfvén modes propagating in opposite directions along the guide field,
  E<SUP>+</SUP>(k<SUB>⊥</SUB>) and E<SUP>-</SUP>(k<SUB>⊥</SUB>), are
  equal, and the imbalanced case where the energies are different. In
  the balanced case, we find that the energy spectrum converges to a
  power law with exponent -3/2 as the Reynolds number is increased,
  which is consistent with phenomenological models that include
  scale-dependent dynamic alignment. For the imbalanced case,
  with E<SUP>+</SUP>&gt;E<SUP>-</SUP>, the simulations show that
  E<SUP>-</SUP>∝k<SUB>⊥</SUB><SUP>-3/2</SUP> for all Reynolds
  numbers considered, while E<SUP>+</SUP> has a slightly steeper
  spectrum at small Re. As the Reynolds number increases, E<SUP>+</SUP>
  flattens. Since E<SUP>±</SUP> are pinned at the dissipation scale and
  anchored at the driving scales, we postulate that at sufficiently high
  Re the spectra will become parallel in the inertial range and scale
  as E<SUP>+</SUP>∝E<SUP>-</SUP>∝k<SUB>⊥</SUB><SUP>-3/2</SUP>.
  Questions regarding the universality of the spectrum and the value of
  the “Kolmogorov constant” are discussed.

---------------------------------------------------------
Title: Numerical simulations of strong incompressible
    magnetohydrodynamic turbulence
Authors: Mason, J.; Perez, J. C.; Boldyrev, S.; Cattaneo, F.
2012PhPl...19e5902M    Altcode: 2012arXiv1202.3474M
  Magnetised plasma turbulence pervades the universe and is
  likely to play an important role in a variety of astrophysical
  settings. Magnetohydrodynamics (MHD) provides the simplest theoretical
  framework in which phenomenological models for the turbulent dynamics
  can be built. Numerical simulations of MHD turbulence are widely used
  to guide and test the theoretical predictions; however, simulating
  MHD turbulence and accurately measuring its scaling properties is far
  from straightforward. Computational power limits the calculations to
  moderate Reynolds numbers and often simplifying assumptions are made in
  order that a wider range of scales can be accessed. After describing
  the theoretical predictions and the numerical approaches that are
  often employed in studying strong incompressible MHD turbulence, we
  present the findings of a series of high-resolution direct numerical
  simulations. We discuss the effects that insufficiencies in the
  computational approach can have on the solution and its physical
  interpretation.

---------------------------------------------------------
Title: Symmetries, Scaling Laws, and Convergence in Shearing-box
    Simulations of Magneto-rotational Instability Driven Turbulence
Authors: Bodo, G.; Cattaneo, F.; Ferrari, A.; Mignone, A.; Rossi, P.
2011ApJ...739...82B    Altcode: 2011arXiv1106.5727B
  We consider the problem of convergence in homogeneous shearing-box
  simulations of magneto-rotationally driven turbulence. When there is
  no mean magnetic flux, if the equations are non-dimensionalized with
  respect to the diffusive scale, the only free parameter in the problem
  is the size of the computational domain. The problem of convergence then
  relates to the asymptotic form of the solutions as the computational
  box size becomes large. By using a numerical code with a high order of
  accuracy we show that the solutions become asymptotically independent
  of domain size. We also show that cases with weak magnetic flux join
  smoothly to the zero-flux cases as the flux vanishes. These results
  are consistent with the operation of a subcritical small-scale dynamo
  driving the turbulence. We conclude that for this type of turbulence
  the angular momentum transport is proportional to the diffusive flux
  and therefore has limited relevance in astrophysical situations.

---------------------------------------------------------
Title: Extended Scaling Laws in Numerical Simulations of
    Magnetohydrodynamic Turbulence
Authors: Mason, Joanne; Perez, Jean Carlos; Cattaneo, Fausto;
   Boldyrev, Stanislav
2011ApJ...735L..26M    Altcode: 2011arXiv1104.1437M
  Magnetized turbulence is ubiquitous in astrophysical systems, where it
  notoriously spans a broad range of spatial scales. Phenomenological
  theories of MHD turbulence describe the self-similar dynamics of
  turbulent fluctuations in the inertial range of scales. Numerical
  simulations serve to guide and test these theories. However,
  the computational power that is currently available restricts the
  simulations to Reynolds numbers that are significantly smaller than
  those in astrophysical settings. In order to increase computational
  efficiency and, therefore, probe a larger range of scales, one often
  takes into account the fundamental anisotropy of field-guided MHD
  turbulence, with gradients being much slower in the field-parallel
  direction. The simulations are then optimized by employing the
  reduced MHD equations and relaxing the field-parallel numerical
  resolution. In this work we explore a different possibility. We propose
  that there exist certain quantities that are remarkably stable with
  respect to the Reynolds number. As an illustration, we study the
  alignment angle between the magnetic and velocity fluctuations in
  MHD turbulence, measured as the ratio of two specially constructed
  structure functions. We find that the scaling of this ratio can be
  extended surprisingly well into the regime of relatively low Reynolds
  number. However, the extended scaling easily becomes spoiled when the
  dissipation range in the simulations is underresolved. Thus, taking the
  numerical optimization methods too far can lead to spurious numerical
  effects and erroneous representation of the physics of MHD turbulence,
  which in turn can affect our ability to identify correctly the physical
  mechanisms that are operating in astrophysical systems.

---------------------------------------------------------
Title: The α-effect in rotating convection: a comparison of numerical
    simulations
Authors: Hughes, D. W.; Proctor, M. R. E.; Cattaneo, F.
2011MNRAS.414L..45H    Altcode: 2011arXiv1103.0754H
  Numerical simulations are an important tool in furthering our
  understanding of turbulent dynamo action, a process that occurs
  in a vast range of astrophysical bodies. It is important in all
  computational work that comparisons are made between different codes
  and, if non-trivial differences arise, that these are explained. In a
  recent paper, Käpylä, Korpi &amp; Brandenburg describe an attempt
  to reproduce some of our results and, by employing a different
  methodology, they arrive at very different conclusions concerning the
  mean electromotive force and the generation of large-scale fields. Here
  we describe why the simulations of Käpylä et al. are simply not
  suitable for a meaningful comparison, since they solve different
  equations, at different parameter values and with different boundary
  conditions. Furthermore, we describe why the 'resetting' method adopted
  by Käpylä et al. to calculate the α-effect is inappropriate, since
  the resulting value of α cannot be related to the evolution of any
  large-scale magnetic field.

---------------------------------------------------------
Title: Magnetic Dynamo Action in Random Flows with Zero and Finite
    Correlation Times
Authors: Mason, Joanne; Malyshkin, Leonid; Boldyrev, Stanislav;
   Cattaneo, Fausto
2011ApJ...730...86M    Altcode: 2011arXiv1101.5181M
  Hydromagnetic dynamo theory provides the prevailing theoretical
  description for the origin of magnetic fields in the universe. Here,
  we consider the problem of kinematic, small-scale dynamo action driven
  by a random, incompressible, non-helical, homogeneous, and isotropic
  flow. In the Kazantsev dynamo model, the statistics of the driving flow
  are assumed to be instantaneously correlated in time. Here, we compare
  the results of the model with the dynamo properties of a simulated
  flow that has similar spatial characteristics as the Kazantsev flow but
  different temporal statistics. In particular, the simulated flow is a
  solution of the forced Navier-Stokes equations and hence has a finite
  correlation time. We find that the Kazantsev model typically predicts
  a larger magnetic growth rate and a magnetic spectrum that peaks at
  smaller scales. However, we show that by filtering the diffusivity
  spectrum at small scales it is possible to bring the growth rates into
  agreement and simultaneously align the magnetic spectra.

---------------------------------------------------------
Title: MHD Dynamos and Turbulence
Authors: Tobias, Steven M.; Cattaneo, Fausto; Boldyrev, Stanislav
2011arXiv1103.3138T    Altcode:
  A review of MHD dynamos and turbulence.

---------------------------------------------------------
Title: On the Generation of Organized Magnetic Fields
Authors: Tobias, S. M.; Cattaneo, F.; Brummell, N. H.
2011ApJ...728..153T    Altcode:
  Motivated by the problem of the origin of astrophysical magnetic fields,
  we introduce two concepts. The first is that of a "system-scale dynamo",
  i.e., a dynamo that can organize magnetic fields on the scale of the
  astrophysical object. The second is that of an "essentially nonlinear
  dynamo". This is a dynamo which relies on a velocity driven by magnetic
  forces and/or magnetic instabilities. We construct a simple framework
  that can be used to study such dynamos and give examples in which
  the evolution is such to generate a system-scale field. We argue that
  this framework provides a valuable complementary approach to the more
  conventional studies based on kinematic mean-field dynamo theory.

---------------------------------------------------------
Title: An Experimental Plasma Dynamo Program for Investigations of
    Fundamental Processes in Heliophysics
Authors: Brown, Benjamin; Forest, Cary; Nornberg, Mark; Zweibel,
   Ellen; Cattaneo, Fausto; Cowley, Steven
2011arXiv1101.0176B    Altcode:
  Plasma experiments in laboratory settings offer unique opportunities
  to address fundamental aspects of the solar dynamo and magnetism
  in the solar atmosphere. We argue here that ground-based laboratory
  experiments have direct connections to NASA based missions and NSF
  programs, and that a small investment in laboratory heliophysics may
  have a high payoff. We advocate for broad involvement in community-scale
  plasma experiments.

---------------------------------------------------------
Title: Dynamo efficiency in compressible convective dynamos with
    and without penetration
Authors: Brummell, Nicholas; Tobias, Steven; Cattaneo, Fausto
2010GApFD.104..565B    Altcode:
  We investigate dynamo action in compressible convection via numerical
  simulations in a Cartesian domain. We directly compare the dynamo
  properties of a fully convective domain with the same domain extended
  to include an underlying stable region. These simulations extend models
  of fully convective domains with open lower boundary conditions to
  a more self-consistent model. We examine whether the extremely slow
  recirculation of the lower region affects the dynamo properties in
  the convection zone. We find that the dynamo properties of the upper
  convective region are essentially unchanged by the addition of the
  lower stable region. After a transient period, dynamo action in the
  convective region not only proceeds as normal, but also extends into
  the region of overshooting flow in the stable region. Downward magnetic
  pumping, long recirculation times and the low percentage of rising
  elements that transit the vertical extent of the domain all fail to
  eliminate the dynamo. Sufficient magnetic field is recirculated or
  remains in the convective region to fuel the local dynamo there. The
  independence of the convective layer from the conditions of the lower
  layer makes the dynamo truly local.

---------------------------------------------------------
Title: Dynamic Alignment and Exact Scaling Laws in Magnetohydrodynamic
    Turbulence
Authors: Boldyrev, Stanislav; Mason, Joanne; Cattaneo, Fausto
2009ApJ...699L..39B    Altcode:
  Magnetohydrodynamic (MHD) turbulence is pervasive in astrophysical
  systems. Recent high-resolution numerical simulations suggest that
  the energy spectrum of strong incompressible MHD turbulence is E(k
  <SUB>bottom</SUB>) vprop k <SUP>-3/2</SUP> <SUB>bottom</SUB>. So
  far, there has been no phenomenological theory that simultaneously
  explains this spectrum and satisfies the exact analytic relations
  for MHD turbulence due to Politano &amp; Pouquet. Indeed, the
  Politano-Pouquet relations are often invoked to suggest that
  the spectrum of MHD turbulence instead has the Kolmogorov scaling
  -5/3. Using geometrical arguments and numerical tests, here we analyze
  this seeming contradiction and demonstrate that the -3/2 scaling and
  the Politano-Pouquet relations are reconciled by the phenomenon of
  scale-dependent dynamic alignment that was recently discovered in
  MHD turbulence.

---------------------------------------------------------
Title: Problems with kinematic mean field electrodynamics at high
    magnetic Reynolds numbers
Authors: Cattaneo, F.; Hughes, D. W.
2009MNRAS.395L..48C    Altcode: 2009MNRAS.tmpL.213C; 2008arXiv0805.2138C
  We discuss the applicability of the kinematic α-effect formalism at
  high magnetic Reynolds numbers. In this regime, the underlying flow is
  likely to be a small-scale dynamo, leading to the exponential growth
  of fluctuations. Difficulties arise with both the actual calculation of
  the α coefficients and their interpretation. We argue that although the
  former may be circumvented - and we outline several procedures by which
  the α coefficients can be computed in principle - the interpretation
  of these quantities in terms of the evolution of the large-scale field
  may be fundamentally flawed.

---------------------------------------------------------
Title: Aspect Ratio Dependence in Magnetorotational Instability
    Shearing Box Simulations
Authors: Mignone, Andrea; Ferrari, Attilio; Bodo, Gianluigi; Rossi,
   Paola; Cattaneo, Fausto
2009ASSP...13...77M    Altcode: 2009pjc..book...77M
  Three-dimensional numerical simulations of the magnetorotational
  instability in the shearing box approximation with a nonzero net flux
  are presented. By changing the size of the computational domain in
  the radial direction relative to the vertical box height, we find, in
  agreement with previous studies, that transport of angular momentum
  (associated with the so-called "channel solution") is strongly
  intermittent and maximized for boxes of unit aspect ratio. On the
  other hand, in boxes with larger aspect ratio the intermittent behavior
  disappears and angular momentum transport is inhibited.

---------------------------------------------------------
Title: The influence of horizontal boundaries on Ekman circulation
    and angular momentum transport in a cylindrical annulus
Authors: Obabko, Aleksandr V.; Cattaneo, Fausto; F Fischer, Paul
2008PhST..132a4029O    Altcode: 2008arXiv0806.4630O
  We present numerical simulations of circular Couette flow in
  axisymmetric and fully three-dimensional geometry of a cylindrical
  annulus inspired by Princeton magnetorotational instability (MRI) liquid
  gallium experiment. The incompressible Navier-Stokes equations are
  solved with the spectral element code Nek5000 incorporating realistic
  horizontal boundary conditions of differentially rotating rings. We
  investigate the effect of changing rotation rates (Reynolds number)
  and of the horizontal boundary conditions on flow structure, Ekman
  circulation and associated transport of angular momentum through the
  onset of unsteadiness and three-dimensionality. A mechanism for the
  explanation of the dependence of the Ekman flows and circulation on
  horizontal boundary conditions is proposed. <P />First International
  Conference 'Turbulent Mixing and Beyond' held on 18-26 August 2007
  at the Abdus Salam International Centre for Theoretical Physics,
  Trieste, Italy.

---------------------------------------------------------
Title: Convective Dynamos with Penetration, Rotation, and Shear
Authors: Tobias, Steven M.; Cattaneo, Fausto; Brummell, Nicholas H.
2008ApJ...685..596T    Altcode:
  We investigate the dynamo properties of Boussinesq, penetrative
  convection in the presence of rotation and large-scale velocity shear
  flows. Several numerical experiments are conducted in a local Cartesian
  computational domain in which the relative stability of the lower stable
  layer, the rotation rate, the supercriticality of the convection, and
  the strength of the imposed shear flow are varied. Once a statistically
  steady hydrodynamic state is achieved for any set of parameters, a
  weak seed magnetic field is added and the subsequent dynamo evolution
  is followed. In all cases studied, the weak seed field is initially
  amplified exponentially and then eventually saturates in a stationary
  magnetohydrodynamic (MHD) state. Even in the presence of penetration,
  rotation, and shear, the field is predominantly small-scale. We
  analyze the reasons for this and make suggestions as to possible
  further mechanisms that may lead to large-scale field generation.

---------------------------------------------------------
Title: Limited Role of Spectra in Dynamo Theory: Coherent versus
    Random Dynamos
Authors: Tobias, Steven M.; Cattaneo, Fausto
2008PhRvL.101l5003T    Altcode:
  We discuss the importance of phase information and coherence times
  in determining the dynamo properties of turbulent flows. We compare
  the kinematic dynamo properties of three flows with the same energy
  spectrum. The first flow is dominated by coherent structures with
  nontrivial phase information and long eddy coherence times, the second
  has random phases and long-coherence time, the third has nontrivial
  phase information, but short coherence time. We demonstrate that
  the first flow is the most efficient kinematic dynamo, owing to the
  presence of sustained stretching and constructive folding. We argue
  that these results place limitations on the possible inferences of
  the dynamo properties of flows from the use of spectra alone, and that
  the role of coherent structures must always be accounted for.

---------------------------------------------------------
Title: Aspect ratio dependence in magnetorotational instability
    shearing box simulations
Authors: Bodo, G.; Mignone, A.; Cattaneo, F.; Rossi, P.; Ferrari, A.
2008A&A...487....1B    Altcode: 2008arXiv0805.1172B
  Aims: We study the changes in the properties of turbulence driven
  by the magnetorotational instability in a shearing box, as the
  computational domain size in the radial direction is varied relative
  to the height. <BR />Methods: We perform 3D simulations in the
  shearing box approximation, with a net magnetic flux, and we consider
  computational domains with different aspect ratios. <BR />Results:
  We find that in boxes of aspect ratio unity the transport of angular
  momentum is strongly intermittent and dominated by channel solutions in
  agreement with previous work. In contrast, in boxes with larger aspect
  ratios, the channel solutions and the associated intermittent behavior
  disappear. <BR />Conclusions: There is strong evidence that, as the
  aspect ratio becomes larger, the characteristics of the solution become
  aspect ratio independent. We conclude that shearing box calculations
  with an aspect ratio of unity or near unity may introduce spurious
  effects.

---------------------------------------------------------
Title: Numerical measurements of the spectrum in magnetohydrodynamic
    turbulence
Authors: Mason, Joanne; Cattaneo, Fausto; Boldyrev, Stanislav
2008PhRvE..77c6403M    Altcode: 2007arXiv0706.2003M
  We report the results of an extensive set of direct numerical
  simulations of forced, incompressible, magnetohydrodynamic
  (MHD) turbulence with a strong guide field. The aim is to
  resolve the controversy regarding the power-law exponent
  ( α , say) of the field-perpendicular energy spectrum
  E(k<SUB>⊥</SUB>)∝k<SUB>⊥</SUB><SUP>α</SUP> . The two main
  theoretical predictions α=-3/2 and α=-5/3 have both received
  some support from differently designed numerical simulations. Our
  calculations have a resolution of 512<SUP>3</SUP> mesh points, a strong
  guide field, and an anisotropic simulation domain and implement a broad
  range of large-scale forcing routines, including those previously
  reported in the literature. Our findings indicate that the spectrum
  of well-developed, strong incompressible MHD turbulence with a strong
  guide field is E(k<SUB>⊥</SUB>)∝k<SUB>⊥</SUB><SUP>-3/2</SUP> .

---------------------------------------------------------
Title: Numerical Simulations of Strong MHD Turbulence
Authors: Mason, J.; Cattaneo, F.; Boldyrev, S.
2007APS..DPPGP8028M    Altcode:
  Magnetohydrodynamic turbulence plays an important role in many
  astrophysical phenomena, including the solar wind, angular momentum
  transport in accretion disks and interstellar scintillation. Despite
  more than 40 years of investigations much within the subject remains
  controversial. Recently a new theory has been developed [1, 2]. It
  predicts a scale-dependent dynamic alignment between the velocity and
  magnetic fluctuations and leads to the field-perpendicular energy
  spectrum E(k)k<SUP>-3</SUP>/2. Here we discuss this new theory and
  present the results of a series of numerical tests. Quantities measured
  include the alignment angle, the spectrum and the third order structure
  functions for which the exact relations due to Politano &amp; Pouquet
  [3] hold. <BR />[1] Boldyrev, S. (2005) Astrophys. J. 626, L37. <BR
  />[2] Boldyrev, S. (2006) Phys. Rev. Lett. 96, 115002. <BR />[3]
  Politano, H. &amp; Pouquet, A. (1998) Geophys. Res. Lett. 25, 273.

---------------------------------------------------------
Title: New directions in the theory of hydromagnetic dynamos
Authors: Cattaneo, Fausto
2007APS..DPPUT2001C    Altcode:
  In dynamo theory a distinction is made between small- and large-scale
  dynamo action. The former refers to the generation of magnetic fields
  on scales smaller than or comparable with the characteristic scale of
  the velocity. It is now widely believed that in a turbulent fluid,
  small-scale dynamo action is always possible provided the magnetic
  Reynolds number is sufficiently high. Large-scale dynamo action, on
  the other hand, refers to the generation of large-scale fields, i.e
  the generation of magnetic flux which is of considerable importance in
  many astrophysical situations. The traditional view is that large-scale
  generation occurs via an inverse cascade of magnetic helicity driven
  by turbulence lacking reflectional symmetry. This view, however, is
  becoming increasingly at odds with numerical simulations that show that
  the cascade is either absent or ineffective. The question then arises
  of what are the alternative mechanisms that can leads to the generation
  of large-scale fields. I will discuss two possible resolutions: one
  based on the role of boundary conditions in releasing the constraints
  of helicity conservation, the other based on the existence of special
  classes of velocity fields that are generated by magnetic instabilities
  and that are particularly suited to dynamo action. In both cases I
  will discuss important analogies between the astrophysical and the
  laboratory situations.

---------------------------------------------------------
Title: Spectra and structure of MHD turbulence
Authors: Boldyrev, S.; Perez, J. C.; Mason, J.; Cattaneo, F.
2007APS..DPPPO7001B    Altcode:
  We present recent results on magnetohydrodynamic (MHD) turbulent
  cascades. We concentrate on the physical processes that determine
  the structure of MHD turbulence in the regimes of weak and strong
  turbulence, and discuss the corresponding turbulent spectra. The results
  are compared with numerical simulations and geophysical (solar wind)
  and astrophysical (interstellar scintillation) observations.

---------------------------------------------------------
Title: The Spectrum and Small-Scale Structures of Magnetohydrodynamic
    Turbulence
Authors: Mason, J.; Boldyrev, S.; Cattaneo, F.
2007ASPC..365..315M    Altcode:
  A theory of incompressible MHD turbulence in the presence of a strong
  guiding magnetic field is described. Numerical results that show
  good agreement with the theory are also presented. The theory was
  developed in tet{boldyrev,boldyrev2} and predicts that in the plane
  perpendicular to the guiding field, the velocity and magnetic field
  fluctuations align within a scale dependent angle θ<SUB>λ</SUB>∝
  λ<SUP>1/4</SUP>. The phenomena is known as dynamic alignment and leads
  to the field-perpendicular energy spectrum E(k<SUB>⊥</SUB>)∝
  k<SUB>⊥</SUB><SUP>-3/2</SUP>. The results are of interest in
  understanding turbulence in the interstellar medium and may provide
  a natural explanation for the recent observations of interstellar
  scintillations of PSR 0329+54 and PSR J0437-4715 tep{shishov,smirnova}.

---------------------------------------------------------
Title: Challenges to the theory of solar convection
Authors: Cattaneo, F.
2007IAUS..239...35C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamic Alignment in Driven Magnetohydrodynamic Turbulence
Authors: Mason, Joanne; Cattaneo, Fausto; Boldyrev, Stanislav
2006PhRvL..97y5002M    Altcode: 2006astro.ph..2382M
  Motivated by recent analytic predictions, we report numerical
  evidence showing that in driven incompressible magnetohydrodynamic
  turbulence the magnetic- and velocity-field fluctuations locally
  tend to align the directions of their polarizations. This dynamic
  alignment is stronger at smaller scales with the angular mismatch
  between the polarizations decreasing with the scale λ approximately
  as θ<SUB>λ</SUB>∝λ<SUP>1/4</SUP>. This can naturally lead to a
  weakening of the nonlinear interactions and provide an explanation
  for the energy spectrum E(k)∝k<SUP>-3/2</SUP> that is observed in
  numerical experiments of strongly magnetized turbulence.

---------------------------------------------------------
Title: What is a flux tube? On the magnetic field topology of buoyant
    flux structures
Authors: Cattaneo, Fausto; Brummell, Nicholas H.; Cline, Kelly S.
2006MNRAS.365..727C    Altcode: 2005MNRAS.tmp.1117C
  We study the topology of field lines threading buoyant magnetic flux
  structures. The magnetic structures, visually resembling idealized
  magnetic flux tubes, are generated self-consistently by numerical
  simulation of the interaction of magnetic buoyancy and a localized
  velocity shear in a stably stratified atmosphere. Depending on the
  parameters, the system exhibits varying degrees of symmetry. By
  integrating along magnetic field lines and constructing return maps,
  we show that, depending on the type of underlying behaviour, the
  stages of the evolution, and therefore the degree of symmetry, the
  resulting magnetic structures can have field lines with one of three
  distinct topologies. When the x-translational and y-reflectional
  symmetries remain intact, magnetic field lines lie on surfaces but
  individual lines do not cover the surface. When the y symmetry is
  broken, magnetic field lines lie on surfaces and individual lines do
  cover the surface. When both x and y symmetries are broken, magnetic
  field lines wander chaotically over a large volume of the magnetically
  active region. We discuss how these results impact our simple ideas
  of a magnetic flux tube as an object with an inside and an outside,
  and introduce the concept of `leaky' tubes.

---------------------------------------------------------
Title: Magnetic-Field Generation in Helical Turbulence
Authors: Boldyrev, Stanislav; Cattaneo, Fausto; Rosner, Robert
2005PhRvL..95y5001B    Altcode: 2005astro.ph..4588B
  We investigate analytically the amplification of a weak magnetic
  field in a homogeneous and isotropic turbulent flow lacking
  reflectional symmetry (helical turbulence). We propose that the
  spectral distributions of magnetic energy and magnetic helicity can
  be found as eigenmodes of a self-adjoint, Schrödinger-type system of
  evolution equations. We argue that large-scale and small-scale magnetic
  fluctuations cannot be effectively separated, and that the conventional
  α model is, in general, not an adequate description of the large-scale
  dynamo mechanism. As a consequence, the correct numerical modeling of
  such processes should resolve magnetic fluctuations down to the very
  small, resistive scales.

---------------------------------------------------------
Title: Simulations of magneto-convection in the solar photosphere.
    Equations, methods, and results of the MURaM code
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
   Emonet, T.; Linde, T.
2005A&A...429..335V    Altcode:
  We have developed a 3D magnetohydrodynamics simulation code for
  applications in the solar convection zone and photosphere. The code
  includes a non-local and non-grey radiative transfer module and takes
  into account the effects of partial ionization. Its parallel design
  is based on domain decomposition, which makes it suited for use on
  parallel computers with distributed memory architecture. We give a
  description of the equations and numerical methods and present the
  results of the simulation of a solar plage region. Starting with
  a uniform vertical field of 200 G, the processes of flux expulsion
  and convective field amplification lead to a dichotomy of strong,
  mainly vertical fields embedded in the granular downflow network and
  weak, randomly oriented fields filling the hot granular upflows. The
  strong fields form a magnetic network with thin, sheet-like structures
  extending along downflow lanes and micropores with diameters of up to
  1000 km which form occasionally at vertices where several downflow
  lanes merge. At the visible surface around optical depth unity,
  the strong field concentrations are in pressure balance with their
  weakly magnetized surroundings and reach field strengths of up to 2
  kG, strongly exceeding the values corresponding to equipartition with
  the kinetic energy density of the convective motions. As a result of
  the channelling of radiation, small flux concentrations stand out as
  bright features, while the larger micropores appear dark in brightness
  maps owing to the suppression of the convective energy transport. The
  overall shape of the magnetic network changes slowly on a timescale
  much larger than the convective turnover time, while the magnetic flux
  is constantly redistributed within the network leading to continuous
  formation and dissolution of flux concentrations. <P />Appendices A-D
  are only available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: Magnetic field generation in Kolmogorov turbulence
Authors: Boldyrev, Stanislav; Cattaneo, Fausto
2004AIPC..733..137B    Altcode:
  We analyze the initial, kinematic stage of magnetic field evolution
  in an isotropic and homogeneous turbulent conducting fluid with a
  “rough” velocity field, v(l) ~ l<SUP>α</SUP>, α &lt; 1. This
  regime is relevant to the problem of magnetic field generation in
  turbulent fluids with small magnetic Prandtl number, i.e. with ohmic
  resistivity much larger than viscosity. Our interest in motivated by
  the following question: suppose that smooth fluctuations of velocity
  field are able to amplify a weak magnetic field, would the magnetic
  field be still amplified if the fluid motion becomes strongly turbulent,
  i.e. non-smooth? Quite paradoxically, turbulence can be dangerous for
  magnetic field generation. We propose that the smaller the magnetic
  Prandtl number, the larger the magnetic Reynolds number that is
  needed to excite magnetic fluctuations. This implies that numerical
  or experimental investigations of magnetohydrodynamical turbulence
  with small Prandtl numbers need to achieve extremely high resolution
  in order to describe magnetic phenomena adequately.

---------------------------------------------------------
Title: Magnetic-Field Generation in Kolmogorov Turbulence
Authors: Boldyrev, Stanislav; Cattaneo, Fausto
2004PhRvL..92n4501B    Altcode: 2003astro.ph.10780B
  We analyze the initial, kinematic stage of magnetic field evolution
  in an isotropic and homogeneous turbulent conducting fluid with a
  rough velocity field, v(l)∼l<SUP>α</SUP>, α&lt;1. This regime is
  relevant to the problem of magnetic field generation in fluids with
  small magnetic Prandtl number, i.e., with Ohmic resistivity much larger
  than viscosity. We propose that the smaller the roughness exponent
  α, the larger the magnetic Reynolds number that is needed to excite
  magnetic fluctuations. This implies that numerical or experimental
  investigations of magnetohydrodynamic turbulence with small Prandtl
  numbers need to achieve extremely high resolution in order to describe
  magnetic phenomena adequately.

---------------------------------------------------------
Title: The Generation of Surface Magnetic Fields
Authors: Cattaneo, F.; Emonet, T.
2004cosp...35.4443C    Altcode: 2004cosp.meet.4443C
  The most readily observable manifestation of convection at the solar
  surface is the granulation. Granules with a characteristic size of
  1,000 km and a lifetime of 5 min are too small and too short-lived
  to be significantly affected by the solar rotation. Thus, the upper
  layers of the convective zone are in a state of strongly turbulent,
  non-helical convection. Because of the high electrical conductivity
  of the solar plasma, the magnetic Reynmolds number of the granulation
  is large (&gt; 10^5). These considerations, suggest that the granular
  flows can act as local small-scale dynamos, generating disordered
  small-scale magnetic fields with lifetimes comparable to that of the
  granulation. Numerical simulations support this conclusion showing
  that intense highly intermittent fields can readily be generated
  provided the magnetic Reynolds number is large enough. One interesting
  aspect of the solar small-scale dynamo problem is related to the
  extremely small value of the plasma viscosity; much smaller than the
  magnetic diffusivity. In this regime (small magnetic Prandtl number)
  the velocity has strong fluctuations at the magnetic diffusion scale
  with profound consequences for the operation of the dynamo. In this
  talk I will address how our ideas of dynamo action, mostly based on
  smooth velocity fields, must be modified to account for this fact

---------------------------------------------------------
Title: Dynamo Action Driven by Shear and Magnetic Buoyancy
Authors: Cline, Kelly S.; Brummell, Nicholas H.; Cattaneo, Fausto
2003ApJ...599.1449C    Altcode:
  We present direct numerical simulations based on the full MHD
  equations of dynamo action in a nonrotating, convectively stable layer
  containing a forced, localized velocity shear. The dynamo operates
  by the interaction of two MHD processes: the production of toroidal
  magnetic field from poloidal field by the shear, and the regeneration
  of poloidal loops from toroidal field due to the combined action
  of magnetic buoyancy and Kelvin-Helmholtz instabilities. The nature
  of the dynamo process is such that it can occur only if the initial
  magnetic fields exceed a critical value that typically depends on the
  magnetic Reynolds number. As such, this dynamo does not operate in the
  kinematic limit. Several different behaviors are observed, including
  steady dynamo production and cyclic as well as chaotic activity. In
  the cyclic regimes, the dynamo process exhibits polarity reversals
  and periods of reduced activity.

---------------------------------------------------------
Title: Dynamos in small magnetic Prandtl number fluids
Authors: Cattaneo, Fausto
2003APS..DPPKM1003C    Altcode:
  In liquid metals and dense stellar plasmas, the magnetic Prandtl
  number Pm--the ratio between viscosity and magnetic diffusivity--is
  a very small number (10<SUP>-5</SUP>-10<SUP>-9</SUP>). Consequently
  the scales of dissipation of velocity and magnetic field are widely
  separated, and dynamo processes must operate in the inertial range
  of the turbulence. Two questions naturally arise. First, can the
  dynamo operate in such a strongly turbulent environment? Second, if
  it operates, what is the typical strength of the resulting magnetic
  field? I will argue that dynamo action is possible even at moderate
  magnetic Reynolds numbers, provided that either the turbulence
  spectrum is not too flat, or that favorable coherent structures are
  present. Concerning the strength of the generated dynamo field, I will
  present numerical results suggesting that a magnetic Prandtl number
  independent regime is reached even for values of Pm only slightly
  smaller than unity. If this result were valid in general it would
  imply that numerical models with Pm near unity could be adequate to
  describe many aspects of the dynamics of fluids with Pm very small.

---------------------------------------------------------
Title: Future Perspectives of Laboratory Plasma Astrophysics II:
    Liquid Metal and Basic Plasma Experiments
Authors: Ji, H.; Cattaneo, F.; Colgate, S.; Cowley, S.; Forest, C.;
   Lathrop, D.; Gekelman, W.
2003APS..DPPKM1007J    Altcode:
  This is for a panel discussion slot for mini- conference on laboratory
  plasma astrophysics II: Liquid Metal and Basic Plasma Experiments

---------------------------------------------------------
Title: On the Formation of Magnetic Structures by the Combined Action
    of Velocity Shear and Magnetic Buoyancy
Authors: Cline, Kelly S.; Brummell, Nicholas H.; Cattaneo, Fausto
2003ApJ...588..630C    Altcode:
  Using numerical simulations of a compressible, stably stratified,
  magnetohydrodynamical (MHD) flow, we investigate a mechanism for
  producing a series of rising tubelike magnetic structures. In this
  process, a steadily forced shear flow stretches a weak poloidal
  background magnetic field to create a strong toroidal field that is
  magnetically buoyant. The subsequent evolution of this system depends on
  the parameters: At moderate magnetic Reynolds numbers (R<SUB>m</SUB>),
  the system reaches a stable nonstatic equilibrium. At larger values
  of R<SUB>m</SUB>, this equilibrium becomes unstable to a shear-buoyant
  instability, involving a modification of the background velocity shear
  by the magnetically induced buoyant poloidal flow. The system then
  produces a series of buoyant magnetic structures at regular intervals
  that are expelled from the region of strong velocity shear. Even
  higher R<SUB>m</SUB> causes the magnetic intensity of the structures
  to strengthen and the intervals between expulsion events to become
  irregular. For large enough kinetic Reynolds numbers (R<SUB>e</SUB>),
  the magnetic modification of the background shear can trigger a
  secondary three-dimensional Kelvin-Helmholtz instability that can
  twist the magnetic structures into a helical shape.

---------------------------------------------------------
Title: On the Interaction between Convection and Magnetic Fields
Authors: Cattaneo, Fausto; Emonet, Thierry; Weiss, Nigel
2003ApJ...588.1183C    Altcode:
  Turbulent convection in the solar photosphere can act as a small-scale
  dynamo, maintaining a disordered magnetic field that is locally
  intense. On the other hand, convection is inhibited in the presence
  of a strong, externally imposed magnetic field, as for instance,
  in a sunspot. Large-scale, three-dimensional, numerical experiments
  on highly nonlinear magnetoconvection in a Boussinesq fluid show
  that there is a continuous transition from a dynamo regime through
  a convective regime to an oscillatory regime as the strength of the
  imposed magnetic field is progressively increased. The patterns found
  in these different regimes are described and analyzed.

---------------------------------------------------------
Title: Magnetic Dynamos: Numerical Simulations and Experimental Statu
Authors: Cattaneo, Fausto
2003APS..APR.U4004C    Altcode:
  Dynamo action--the process of magnetic field generation by the motions
  of an electrically conducting fluids--is often invoked to explain the
  origin of magnetic fields in stars and planets. In many situations
  of astrophysical interest, the plasma motions that drive the dynamos
  are characterised by a large magnetic Reynolds number--the ratio of
  diffusive to convective timescales-- and a small magnetic Prandtl
  number--the ratio of viscosity to magnetic diffusivity, implying that
  that the dynamos operate in highly turbulent environments, where
  the velocity fluctuates on scales much smalled than the lentgth at
  which the magnetic diffusion becomes important. These constraint pose
  significant challenges to the numerical and experimental attempts to
  model astrophysical dynamos. In this talk I will review the present
  state of numerical and laboratory dynamo experiments as well as the
  efforts to extend it towards more realistic astrophysical regimes.

---------------------------------------------------------
Title: Polarization of Photospheric Lines from Turbulent Dynamo
    Simulations
Authors: Sánchez Almeida, J.; Emonet, T.; Cattaneo, F.
2003ApJ...585..536S    Altcode: 2002astro.ph.11175S; 2002astro.ph.11175A
  We employ the magnetic and velocity fields from turbulent dynamo
  simulations to synthesize the polarization of a typical photospheric
  line. The synthetic Stokes profiles have properties in common with
  those observed in the quiet Sun. The simulated magnetograms present a
  level of signal similar to that of the Inter-Network regions. Asymmetric
  Stokes V profiles with two, three, and more lobes appear in a natural
  way. The intensity profiles are broadened by the magnetic fields
  in fair agreement with observational limits. Furthermore, the Hanle
  depolarization signals of the Sr I λ4607 Å line turn out to be within
  the solar values. Differences between synthetic and observed polarized
  spectra can also be found. There is a shortage of Stokes V asymmetries,
  which we attribute to a deficit of structuring in the magnetic and
  velocity fields from the simulations as compared to the Sun. This
  deficit may reflect the fact that the Reynolds numbers of the numerical
  data are still far from solar values. We consider the possibility that
  intense and tangled magnetic fields, like those in the simulations,
  exist in the Sun. This scenario has several important consequences. For
  example, less than 10% of the existing unsigned magnetic flux would
  be detected in present magnetograms. The existing flux would exceed
  by far that carried by active regions during the maximum of the solar
  cycle. Detecting these magnetic fields would involve improving the
  angular resolution, the techniques to interpret the polarization
  signals, and to a lesser extent, the polarimetric sensitivity.

---------------------------------------------------------
Title: The Polarized Spectrum Emerging from Fast Dynamo Simulations
Authors: Sánchez Almeida, J.; Emonet, T.; Cattaneo, F.
2003ASPC..307..293S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Simulation of Solar Magnetoconvection
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
   Emonet, T.; Linde, T.
2003IAUS..210..157V    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Formation of buoyant magnetic structures by a localized
    velocity shear
Authors: Brummell, N.; Cline, K.; Cattaneo, F.
2002MNRAS.329L..73B    Altcode:
  Motivated by considerations of the solar tachocline, we study the
  generation of strong buoyant magnetic structures by a sheared velocity
  field localized in a convectively stable background, using non-linear
  three-dimensional (3D) magnetohydrodynamic (MHD) simulations. The shear
  flow can spontaneously create strong tube-like toroidal (streamwise)
  magnetic structures from an imposed weak uniform poloidal (cross-stream)
  magnetic field. The structures are magnetically buoyant and therefore
  rise, and may evolve further to a rich variety of geometries, including
  kinked or arched shapes. The emergence process can repeat indefinitely
  with a characteristic period. These mechanisms may be relevant to the
  MHD processes in the solar tachocline and the creation and emergence
  of solar active regions.

---------------------------------------------------------
Title: On the nonlinear nature of the turbulent α-effect
Authors: Cattaneo, Fausto; Hughes, David W.; Thelen, Jean-Claude
2002HiA....12..733C    Altcode:
  Galactic magnetic fields are, typically, modelled by mean-field dynamos
  involving the α-effect. Here we consider, very briefly, some of the
  issues involving the nonlinear dependence of α on the mean field.

---------------------------------------------------------
Title: On the Origin of the Solar Mesogranulation
Authors: Cattaneo, Fausto; Lenz, Dawn; Weiss, Nigel
2001ApJ...563L..91C    Altcode:
  The observed properties of mesogranules are related to structures found
  in idealized numerical experiments on turbulent convection. We describe
  results obtained for three-dimensional Boussinesq convection in a layer
  with a very large aspect ratio. There are two distinct cellular patterns
  at the surface. Energy-transporting convection cells (corresponding
  to granules in the solar photosphere) have diameters comparable to
  the layer depth, while macrocells (corresponding to mesogranules)
  are several times larger. The motion acts as a small-scale turbulent
  dynamo, generating a disordered magnetic field that is concentrated
  at macrocellular corners and, to a lesser extent, in the lanes that
  join them. These results imply that mesogranules owe their origin to
  collective interactions between the granules.

---------------------------------------------------------
Title: Small-Scale Photospheric Fields: Observational Evidence and
    Numerical Simulations
Authors: Emonet, Thierry; Cattaneo, Fausto
2001ApJ...560L.197E    Altcode:
  Observations suggest that magnetic fields at the solar photosphere
  may be structured below the limit of the present resolution. We
  argue that numerical simulations could be used in a complementary
  way to observations in order to study the small-scale structure of
  photospheric fields. We present a number of illustrative examples.

---------------------------------------------------------
Title: Solar dynamo theory : Solar dynamo theory: a new look at the
    origin of small-scale magnetic fields
Authors: Cattaneo, Fausto; Hughes, David W.
2001A&G....42c..18C    Altcode:
  Fausto Cattaneo and David W Hughes delve beneath the surface of the
  Sun with numerical models of turbulent convection. <P />Although
  magnetic dynamo action is traditionally associated with rotation,
  fast dynamo theory shows that chaotic flows, even without rotation,
  can act as efficient small-scale dynamos. Indeed, numerical simulations
  suggest that granular and supergranular convection may generate locally
  a substantial part of the field in the quiet photosphere.

---------------------------------------------------------
Title: Numerical simulations of self-excited dynamos
Authors: Cattaneo, F.; Brummell, N.; Cline, K.
2001sps..proc..122C    Altcode:
  We present results of numerical simulations of self-consistent dynamo
  action in electrically conducting fluids. Two cases are discussed in
  details. In the first, dynamo action is driven by turbulent convection;
  in the second, it is driven by a combination of velocity shear and
  magnetic buoyancy instabilities. In both cases the magnetic field is
  generated by fluid motions with no net helicity and in the absence of
  rotational effects.

---------------------------------------------------------
Title: Effects of Limited Resolution on the Inferred Structure of
    Photospheric Magnetic Fields
Authors: Emonet, T.; Cattaneo, F.
2001ASPC..236..355E    Altcode: 2001aspt.conf..355E
  No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamics: Magnetoconvection
Authors: Cattaneo, F.
2000eaa..bookE2223C    Altcode:
  Convective motions occur naturally in layers of fluid heated from
  below. In the Sun turbulent convection carries most of the luminosity
  over the outer 30% by radius of the star. Because of the high electrical
  conductivity of the solar plasma convective motions interact with
  magnetic fields. The modified form of convection that occurs in a
  conducting fluid when externally imposed magnetic fields...

---------------------------------------------------------
Title: Dynamo action driven by convection: the influence of magnetic
    boundary conditions
Authors: Thelen, J. -C.; Cattaneo, F.
2000MNRAS.315L..13T    Altcode:
  We study the influence of different magnetic boundary conditions
  on the generation of magnetic fields by turbulent convection. It is
  found that the structure and strength of the generated field in the
  vicinity of the boundary is strongly dependent on the choice of boundary
  conditions. In the convective interior, however, the solutions remain
  largely insensitive to the boundary conditions. In all cases the overall
  efficiency of the dynamo process remains high with a steady state
  magnetic energy density between 12 and 25per cent of the turbulent
  kinetic energy, and peak field values exceeding the equipartition
  level. These results support the idea that the solar granulation may
  constitute a dynamo source for magnetic fields in the quiet photosphere.

---------------------------------------------------------
Title: The Solar Dynamos
Authors: Cattaneo, F.
2000SPD....31.0402C    Altcode: 2000BAAS...32Q.835C
  Magnetic activity on the Sun presents us with an interesting
  dichotomy. On large spatial and temporal scales the solar magnetic field
  displays a remarkable degree of organization. The 11 years cadence of
  the solar cycle, Hales' polarity law, and the systematic drift of the
  regions of emergence of active regions towards the equator throughout
  the solar cycle are all indicative of a powerful organizing process. On
  small spatial and temporal scales, the Solar magnetic field appears
  random and chaotic. It is interesting that recent advances in dynamo
  theory provide us with a unified approach to solar magnetic activity
  whereby both large and small scales emerge naturally as dynamo processes
  associated with rotationally constrained and unconstrained scales of
  motions in the convection zone (or directly below it). According to
  this view all coherent scales of motions produce magnetic structures of
  comparable coherence length. Those that are further endowed with lack of
  reflectional symmetry by virtue of being rotationally constrained are
  further associated with inverse cascades that can generate magnetic
  structures on larger scales still. The picture that emerges is one
  in which dynamo action proceeds on different time scales all over the
  convection zone. But only in very special regions, like for instance the
  solar tachocline, is the magnetic field organized on large scales. This
  idea provides a natural explanation for the origin of active regions,
  ephemeral regions, and intra--network fields.

---------------------------------------------------------
Title: Surface Dynamics of the Solar Granular Flow
Authors: Lenz, D. D.; Cattaneo, F.
1999AAS...194.2106L    Altcode: 1999BAAS...31..858L
  Flow patterns on the solar surface can provide insight into the dynamics
  of the solar convection zone and photosphere. Since basic properties of
  the solar fluid can be very difficult to observe directly, simulations
  of solar convection are necessary to explore the interplay between the
  solar fluid and surface features. We investigate flow dynamics on the
  surface of a convective fluid and discuss the relationship of the flow
  patterns to observable characteristics. Passive floaters advected by
  the surface velocity field rapidly collect in the downflow lanes. At
  large times, the floaters collect in quasi-stagnant regions that
  are long-lived with respect to the granular pattern. We identify
  this larger, longer-lived pattern with mesogranular flows. The
  relationship between the mesogranules and the surface distribution
  of magnetic features is discussed. This work is supported by NASA
  SR&amp;T NAG5-4953 and by a TRACE subgrant from Lockheed Martin to
  the University of Chicago.

---------------------------------------------------------
Title: A Dynamo Driven by Turbulent Thermal Convection.
Authors: Thelen, J. -C.; Cattaneo, F.
1999AAS...194.2105T    Altcode: 1999BAAS...31..858T
  We consider the possibility that a significant part of the magnetic
  field in the quiet photosphere is generated locally by dynamo action
  associated with the granular flow. This argument is based on recent
  advances in fast dynamo theory, which show that almost any complicated,
  chaotic flow generates some magentic field. Numerical simulations
  of convectively driven, non-rotating, dynamos, in domains with large
  aspect ratios, have been performed in order to verify this idea. Our
  results show that thermally driven turbulent convection can indeed be
  an efficient dynamo even in the absence of rotation in term of both
  growth rate and final amplitude of the generated field. Some aspects of
  the resulting magnetic field strength and the degree of intermittency,
  as well as the influence of the boundary conditions on the behaviour
  of the dynamo will be discussed.

---------------------------------------------------------
Title: On the Origin of Magnetic Fields in the Quiet Photosphere
Authors: Cattaneo, Fausto
1999ApJ...515L..39C    Altcode:
  We consider the possibility that a substantial fraction of the magnetic
  field in the quiet photosphere is generated locally by dynamo action
  associated with the granular and supergranular flows. The argument is
  based on recent advances in the theory of fast dynamos and is supported
  by large-scale numerical simulations that show that thermally driven
  turbulent convection can indeed be an efficient source of small-scale,
  highly intermittent magnetic fields. Some aspects of the resulting
  magnetic field, such as its strength and degree of intermittency,
  are discussed.

---------------------------------------------------------
Title: Nonlinear Dynamo Action in a Time-Dependent ABC Flow
Authors: Brummell, N. H.; Cattaneo, F.; Tobias, S. M.
1999ASPC..178...23B    Altcode: 1999sdnc.conf...23B
  No abstract at ADS

---------------------------------------------------------
Title: Dynamo Theory and the Origin of Small Scale Magnetic Fields
Authors: Cattaneo, F.
1999ASSL..239..119C    Altcode: 1999msa..proc..119C
  Dynamo action describes the sustained conversion of kinetic energy
  into magnetic energy throughout the bulk of an electrically conducting
  fluid. Dynamo processes are commonly invoked to explain the origin
  of the solar cycle and of the large scale component of the solar
  magnetic field. The origin of small scale magnetic fields can also be
  understood in terms of dynamo processes. Recent advances in the theory
  of dynamo operating in fluids with high electrical conductivity --
  fast dynamos, indicate that most sufficiently complicated chaotic flows
  should act as dynamos. The resulting magnetic fields have a complex
  structure characterised by amplitude and polarity fluctuations on small
  scales. Some of the geometrical properties of these fluctuations can
  be described in terms of scaling behaviour and exponents. We review
  some of these ideas and illustrate them by simple examples. Possible
  extensions to the (nonlinear) dynamical regime are also discussed.

---------------------------------------------------------
Title: The Solar Dynamo Problem
Authors: Cattaneo, F.
1997ASSL..225..201C    Altcode: 1997scor.proc..201C
  The solar dynamo problem is reviewed in the light of recent developments
  in dynamo theory. We distinguish between the generation of magnetic
  fields on scales smaller than the velocity correlation length-small
  scale dynamo, and larger than the velocity correlation length-large
  scale dynamo. We argue that small scale dynamo action is likely to
  occur everywhere in the convection zone. The field thus generated
  however is disordered both in space and time. Large scale dynamo
  action on the other hand is responsible for the activity cycle and
  the large scale organization of the solar field. The existence of a
  large scale dynamo is related to the breaking of symmetries in the
  underlying field of turbulence.

---------------------------------------------------------
Title: Nonlinear saturation of the turbulent α effect
Authors: Cattaneo, Fausto; Hughes, David W.
1996PhRvE..54.4532C    Altcode:
  We study the saturation of the turbulent α effect in the nonlinear
  regime. A numerical experiment is constructed based on the full
  nonlinear magnetohydrodynamics equations that allows the α effect
  to be measured for different values of the mean magnetic field. The
  object is to distinguish between two possible theories of nonlinear
  saturation. It is found that the results are in close agreement with
  the theories that predict strong suppression and are incompatible with
  those that predict that the turbulent α effect persists up to mean
  fields of order of the equipartition energy.

---------------------------------------------------------
Title: Suppression of chaos in a simplified nonlinear dynamo model
Authors: Cattaneo, Fausto; Hughes, David W.; Kim, Eun-Jin
1996PhRvL..76.2057C    Altcode:
  A simplified nonlinear dynamo model is constructed that allows the
  transition from the kinematic to the dynamic regime to be studied in
  detail. We apply this construction to a chaotic flow recently studied in
  the context of fast dynamo action. It is found that the structure of the
  magnetic field in the two regimes is markedly different. Furthermore,
  the saturation of the exponential growth of the magnetic field is
  achieved by a drastic suppression of the chaotic properties of the flow.

---------------------------------------------------------
Title: Turbulent Dynamics in the Solar Convection Zone
Authors: Brummell, Nicholas; Cattaneo, Fausto; Toomre, Juri
1995Sci...269.1370B    Altcode:
  Observations of the sun reveal highly complex flows and magnetic
  structures that must result from turbulent convection in the solar
  envelope. A remarkable degree of large-scale coherence emerges
  from the small-scale turbulent dynamics, as seen in the cycles of
  magnetic activity and in the differential rotation profile of this
  star. High-performance computing now permits numerical simulations
  of compressible turbulence and magnetohydrodynamics with sufficient
  resolution to show that compact structures of vorticity and magnetic
  fields can coexist with larger scales. Such structured turbulence
  is yielding transport properties for heat and angular momentum at
  considerable variance with earlier models. These simulations are
  elucidating the coupling of turbulent fluid motions with rotation
  and magnetic fields, which must control the interlinked differential
  rotation and magnetic dynamo action.

---------------------------------------------------------
Title: Fluctuations in Quasi-Two-Dimensional Fast Dynamos
Authors: Cattaneo, Fausto; Kim, Eun-Jin; Proctor, Michael; Tao, Louis
1995PhRvL..75.1522C    Altcode:
  The ratio R<SUB>1</SUB> between the average magnetic energy and the
  square-averaged flux plays an important role in the study of nonlinear
  dynamos, as a measure of the efficiency of a dynamo at generating
  flux. For large values of R<SUB>m</SUB>, R<SUB>1</SUB> displays a
  scaling behavior of the type R<SUB>1</SUB>~R<SUP> n</SUP><SUB>m</SUB>,
  where R<SUB>m</SUB> is the magnetic Reynolds number. We show by direct
  numerical evaluation that n depends sensitively on the flow complexity
  for small-scale dynamos. Furthermore, by relating n to the cancellation
  exponent and the correlation dimension of the magnetic field, we argue
  that n is not likely to be close to zero in general.

---------------------------------------------------------
Title: On the Spatial Distribution of Magnetic Fields on the Solar
    Surface
Authors: Tao, L.; Du, Y.; Rosner, R.; Cattaneo, F.
1995ApJ...443..434T    Altcode:
  Recent measurements of solar surface magnetic fields suggest that the
  spatial distribution of these fields is fractal. In order to understand
  the physical basis for such geometric complexity, we study here the
  advection of magnetic flux tubes relatively simple random motions on
  the surface of a fluid and investigate the spatial statistics of the
  resulting surface field. While this study does not directly address
  the question of why solar surface fields have the observed spatial
  structure, it is designed to build our intuition about how surface
  flows lead to complex spatial structuring of magnetic fields. As part
  of our study, we discuss the various methods by which one can describe
  the spatial distribution of the surface magnetic flux and relate them
  mathematically; this turns out to be a crucial point of our work since,
  as we show, a number of previous analyses have misinterpreted the
  analysis procedures for determining fractal dimensions. Our principal
  result is the explicit demonstration that simple random flows lead
  to magnetic flux spatial distributions with a multifractal dimension
  spectrum. Furthermore, we demonstrate that this magnetic spatial
  structure is generic, i.e., is characteristic of a very large class
  of random flows.

---------------------------------------------------------
Title: On the Origin of “Dividing Lines” for Late-Type Giants
    and Supergiants
Authors: Rosner, R.; Musielak, Z. E.; Cattaneo, F.; Moore, R. L.;
   Suess, S. T.
1995ApJ...442L..25R    Altcode:
  We show how a change in the nature of the stellar dyanmo can lead to
  a transition in the topological character of stellar magnetic fields
  of evolved stars, from being mainly closed on the blueward side of the
  giant tracks in the Hertzsprung-Russell (H-R) diagram to being mainly
  open on their redward side. If such a topological transition occurs,
  then these stars naturally segregate into two classes: those having hot
  coronae on the blueward side, and those having massive cool winds on the
  redward side, thus leading naturally to the so-called dividing lines.

---------------------------------------------------------
Title: On the Effects of a Weak Magnetic Field on Turbulent Transport
Authors: Cattaneo, F.
1994ApJ...434..200C    Altcode:
  We discuss the effects of a weak large-scale magnetic field on turbulent
  transport. We show by means of a series of two-dimensional numerical
  experiments that turbulent diffusion can be effectively suppressed
  by a (large scale) magnetic field whose energy is small compared to
  equipartition. The suppression mechanism is associated with a subtle
  modification of the Lagrangian energy spectrum, and it does not require
  any substantial reduction of the turbulent amplitude. We exploit the
  relation between diffusion and random walking to emphasize that the
  effect of a large-scale magnetic field is to induce a long-term memory
  in the field of turbulence. The implications of the general case of
  three-dimensional transport are briefly discussed.

---------------------------------------------------------
Title: Convection and Magnetic Fields
Authors: Cattaneo, Fausto
1994ASPC...68..108C    Altcode: 1994sare.conf..108C
  No abstract at ADS

---------------------------------------------------------
Title: Magnetoconvection (Invited Review)
Authors: Cattaneo, F.
1994smf..conf..261C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the Generation of Sound by Turbulent Convection. I. A
    Numerical Experiment
Authors: Bogdan, Thomas J.; Cattaneo, Fausto; Malagoli, Andrea
1993ApJ...407..316B    Altcode:
  Motivated by the problem of the origin of the solar p-modes, we study
  the generation of acoustic waves by turbulent convection. Our approach
  uses the results of high-resolution 3D simulations as the experimental
  basis for our investigation. The numerical experiment describes the
  evolution of a horizontally periodic layer of vigorously convecting
  fluid. The sound is measured by a procedure, based on a suitable
  linearization of the equations of compressible convection that allows
  the amplitude of the acoustic field to be determined. Through this
  procedure we identify unambiguously some 400 acoustic modes. The
  total energy of the acoustic field is found to be a fraction of a
  percent of the kinetic energy of the convection. The amplitudes of the
  observed modes depend weakly on (horizontal) wavenumber but strongly on
  frequency. The line widths of the observed modes typically exceed the
  natural linewidths of the modes as inferred from linear theory. This
  broadening appears to be related to the (stochastic) interaction
  between the modes and the underlying turbulence which causes abrupt,
  episodic events during which the phase coherence of the modes is lost.

---------------------------------------------------------
Title: Evidence for the suppression of the alpha-effect by weak
    magnetic fields.
Authors: Tao, L.; Cattaneo, F.; Vainshtein, S. I.
1993spd..conf..303T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Turbulent magnetic transport effects and their relation to
    magnetic field intermittency.
Authors: Vainshtein, S. I.; Tao, L.; Cattaneo, F.; Rosner, R.
1993spd..conf..311V    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Evidence for Transonic Flows in the Solar Granulation
Authors: Nesis, A.; Bogdan, T. J.; Cattaneo, F.; Hanslmeier, A.;
   Knoelker, M.; Malagoli, A.
1992ApJ...399L..99N    Altcode:
  High-resolution observations of the solar granulation are interpreted
  in the light of recent numerical simulations of compressible
  convection. The observations show a negative correlation between
  the width of suitably chosen, nonmagnetic lines and the continuum
  intensity. This result is consistent with a model of granular convection
  where regions of supersonic horizontal flow form intermittently in
  the vicinity of the downflow lanes. We conjecture that the observed
  line broadening in the regions of low intensity is caused by enhanced
  turbulent fluctuations generated by the passage of shock fronts bounding
  the regions of supersonic motion.

---------------------------------------------------------
Title: Nonlinear Restrictions on Dynamo Action
Authors: Vainshtein, Samuel I.; Cattaneo, Fausto
1992ApJ...393..165V    Altcode:
  Astrophysical dynamos operate in the limit of small magnetic
  diffusivity. In order for magnetic reconnection to occur, very small
  magnetic structures must form so that diffusion becomes effective. The
  formation of small-scale fields is accompanied by the stretching of
  the field lines and therefore by an amplification of the magnetic
  field strength. The back reaction of the magnetic field on the motions
  leads to the eventual saturation of the dynamo process, thus posing
  a constraint on the amount of magnetic flux that can be generated by
  dynamo action, It is argued that in the limit of small diffusivity
  only a small amount of flux, many orders of magnitude less than the
  observed fluxes, can be created by dynamo processes.

---------------------------------------------------------
Title: Numerical Models of Stellar Convection (Invited Review)
Authors: Cattaneo, F.; Malagoli, A.
1992ASPC...26..139C    Altcode: 1992csss....7..139C
  No abstract at ADS

---------------------------------------------------------
Title: What is a stellar dynamo?
Authors: Cattaneo, F.; Hughes, D. W.; Weiss, N. O.
1991MNRAS.253..479C    Altcode:
  Numerical simulations of turbulent stellar dynamos are now feasible. The
  characteristic time-scale for kinematic behavior is related to the
  turnover time of the turbulent eddies. Results from idealized 2D
  models show that the Lorentz force alters the velocity field, allowing
  transient magnetic activity to persist for intervals much longer than
  the expected turbulent decay time. For 3D flows a characteristic time
  Te is defined for turbulent diffusion to act, based on the rate at which
  magnetic energy is dissipated, and it is asserted that there is a dynamo
  only if the field survives for times much longer than Te. This criterion
  is then applied to cyclic magnetic activity in late-type stars.

---------------------------------------------------------
Title: Development of hard-turbulent convection in two dimensions:
    Numerical evidence
Authors: Werne, J.; Deluca, E. E.; Rosner, R.; Cattaneo, F.
1991PhRvL..67.3519W    Altcode:
  New numerical evidence for a transition to hard turbulence in 2D
  Boussinesq convection is presented. These 2D simulations agree with
  some, but not all, experimental results for the scaling properties of
  3D hard turbulence. The transition to 2D hard turbulence, as measured
  by a change in the Nusselt-Rayleigh scaling law, coincides with a
  gradual change in the velocity probability distribution from Gaussian
  to exponential form and with the development of a “well-mixed”
  central region.

---------------------------------------------------------
Title: Suppression of Turbulent Transport by a Weak Magnetic Field
Authors: Cattaneo, Fausto; Vainshtein, Samuel I.
1991ApJ...376L..21C    Altcode:
  Two-dimensional numerical simulations with high spatial resolution
  are used to study the effects of a large-scale magnetic field on
  its turbulent transport. It is commonly believed that the nonlinear
  back-reaction of the magnetic field on the turbulence becomes important
  when the field strength is close to equipartition. However, turbulent
  diffusion is effectively reduced even when the large-scale field is
  much weaker than equipartition.

---------------------------------------------------------
Title: On Magnetic Diffusion in a Turbulent Fluid
Authors: Vainshtein, S.; Cattaneo, F.; Rosner, R.
1991BAAS...23.1049V    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Turbulent Compressible Convection
Authors: Cattaneo, Fausto; Brummell, Nicholas H.; Toomre, Juri;
   Malagoli, Andrea; Hurlburt, Neal E.
1991ApJ...370..282C    Altcode:
  Numerical simulations with high spatial resolution (up to 96-cubed
  gridpoints) are used to study three-dimensional, compressible
  convection. A sequence of four models with decreasing viscous
  dissipation is considered in studying the changes in the flow structure
  and transport properties as the convection becomes turbulent.

---------------------------------------------------------
Title: The Organization of Turbulent Convection
Authors: Brummell, Nicholas; Cattaneo, Fausto; Malagoli, Andrea;
   Toomre, Juri; Hurlburt, Neal E.
1991LNP...388..187B    Altcode: 1991ctsm.conf..187B
  Highly resolved numerical simulations are used to study
  three-dimensional, compressible convection. The viscous dissipation is
  sufficiently low that the flow divides itself in depth into two distinct
  regions: (i) an upper thermal boundary layer containing a smooth flow
  with a granular appearance, and (ii) a turbulent interior pierced
  by the strongest downflows from the surface layer. Such downflows
  span the whole depth of the unstable layer, are temporally coherent,
  and are thermodynamically well correlated. A remarkable property of
  such convection, once it becomes turbulent, is that the enthalpy and
  kinetic fluxes carried by the strong downflows nearly cancel, for they
  are of opposite sense and nearly equal in amplitude. Thus, although the
  downflows serve to organize the convection and are the striking feature
  that emerges from effects of compressibility, it is the small-scale,
  disorganized turbulent motions (between the coherent downflow structures
  that serve as the principal carriers of net convected flux.

---------------------------------------------------------
Title: A new twist to the solar cycle
Authors: Cattaneo, Fausto; Chiueh, Tzihong; Hughes, David W.
1990MNRAS.247P...6C    Altcode:
  Recent numerical simulations of magnetic buoyancy instabilities suggest
  a new mechanism for the variation with the solar cycle in the scale
  and structure of surface magnetic flux. The nonlinear evolution of
  a predominantly toroidal field is found to depend crucially on the
  distribution of the weaker poloidal ingredient. For certain field
  configurations large, helical magnetic fragments are produced; for
  others the escaping field is small-scale and untwisted. We propose
  that the observed structural variations in flux may be accounted for by
  small changes in the twist of a deep-seated field. The large fragments
  will appear at the surface as active regions, which dominate at solar
  maximum, while the small-scale field will emerge as ephemeral regions
  which constitute practically all of the flux at solar minimum.

---------------------------------------------------------
Title: Turbulent Supersonic Convection in Three Dimensions
Authors: Malagoli, Andrea; Cattaneo, Fausto; Brummell, Nicholas H.
1990ApJ...361L..33M    Altcode:
  Previous numerical calculations of two-dimensional, compressible
  convection are extended to three dimensions, using a higher order
  Godunov scheme. The results show that the flow readily becomes
  supersonic in the upper boundary layer, where shock structures form
  intermittently in the vicinity of the strong downflow lanes. The
  convection as a whole is strongly time-dependent and evolves on a
  time scale comparable to the sound crossing time. The motions in
  the upper layers are characterized by the rapid expansion of the
  upward-moving fluid elements. In the interior, most of the heat is
  carried by a small fraction of the fluid residing in strong, highly
  coherent downflows. The remaining fluid is dominated by small-scale,
  disorganized turbulent motions.

---------------------------------------------------------
Title: Three-dimensional compressible convection at low Prandtl
    numbers.
Authors: Toomre, Juri; Brummell, Nicholas; Cattaneo, Fausto; Hurlburt,
   Neal E.
1990CoPhC..59..105T    Altcode:
  Numerical simulations are used to study fully compressible thermal
  convection at large Rayleigh numbers. The authors present results from a
  sequence of three-dimensional simulations that reveal a transition from
  gradually-evolving laminar convection to nearly turbulent convection
  as the Prandtl number is reduced from a value of unity to one-tenth.

---------------------------------------------------------
Title: Numerical simulations of soft and hard turbulence: Preliminary
    results for two-dimensional convection
Authors: Deluca, E. E.; Werne, J.; Rosner, R.; Cattaneo, F.
1990PhRvL..64.2370D    Altcode:
  We report results on the transition from soft to hard turbulence in
  simulations of 2D Boussinesq convection. The computed probability
  densities for temperature fluctuations are exponential in form in both
  soft and hard turbulence, unlike what is observed in experiments; in
  contrast, we obtain a change in the Nusselt number scaling on Rayleigh
  number in good agreement with the 3D experiments.

---------------------------------------------------------
Title: Supersonic Convection
Authors: Cattaneo, Fausto; Hurlburt, Neal E.; Toomre, Juri
1990ApJ...349L..63C    Altcode:
  Numerical simulations with high spatial resolution are used to study
  that the combined effects of stratification, pressure gradients,
  and nonadiabatic processes can lead to the formation of regions of
  supersonic motions near the upper thermal boundary layer. Within
  these regions, the dynamics is dominated by nonstationary shock
  structures. These form near the downflow sites and propagate upstream
  along the boundary layer to the upflow regions where they weaken and
  eventually disappear. The shock cycle, consisting of the formation,
  propagation, and disappearance of shock structures, has a time scale
  comparable to the sound crossing time over a portion of the convective
  cell, giving rise to vigorous time dependence in the convection.

---------------------------------------------------------
Title: Multiple states for quasi-geostrophic channel flows
Authors: Cattaneo, Fausto; Hart, John E.
1990GApFD..54....1C    Altcode:
  We consider nonlinear baroclinic instabilities of two-layer
  quasi-geostrophic flow in a rectilinear channel. The full potential
  vorticity equations are shown to possess a countable infinity of
  invariant wavenumber sets. Each set is composed of a particular
  pattern in wavenumber space in which many Fourier modes have zero
  energy. Solutions with initial conditions confined to a particular
  wavenumber pattern will remain forever in that pattern. There is also a
  general asymmetric state with non-zero energy in all wavenumbers. The
  final state of a long-time evolution calculation depends on initial
  conditions and internal stability.

---------------------------------------------------------
Title: The Normal Modes of a Resonant Cavity Containing Discrete
Inhomogeneities: The Influence of Fibril Magnetic Fields on the
    Solar Acoustic Oscillations
Authors: Bogdan, Thomas J.; Cattaneo, Fausto
1989ApJ...342..545B    Altcode:
  Motivated by considerations of the interaction between fibril magnetic
  fields and solar p-modes, the acoustic spectrum of a cylindrical
  cavity filled with ideal gas in which a number of magnetic flux
  tubes are embedded is studied. A formalism, based on the T-matrix
  approach to acoustic scattering, is developed which can be used to
  determine the eigenfrequencies and eigenfunctions for any arbitrary
  distribution of flux tubes. For weak scatterers, the frequency shifts
  and velocity eigenfunctions are calculated using perturbation theory
  for the cases of a single flux tube and a random distribution of up to
  100 flux tubes. The results of this 'exact' approach are used to give
  a critical appraisal of the predictions of theories based on some form
  of averaging, such as the one discussed recently by Bogdan and Zweibel
  (1987).

---------------------------------------------------------
Title: Magnetic buoyancy instabilities of a sheared magnetic layer.
Authors: Cattaneo, F.; Tzihong, Chiueh; Hughes, D. W.
1989BAPS...34.1294C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Two and Three-Dimensional Simulations of Compressible
    Convection
Authors: Cattaneo, F.; Hurlburt, N. E.; Toomre, J.
1989ASIC..263..415C    Altcode: 1989ssg..conf..415C
  No abstract at ADS

---------------------------------------------------------
Title: The Nonlinear Breakup of the Sun's Toroidal Field
Authors: Hughes, D. W.; Cattaneo, F.
1989ASSL..156...31H    Altcode: 1989admf.proc...31H
  There are good reasons for believing that the sun has a strong toroidal
  magnetic field in the stably stratified region of convective overshoot
  sandwiched between the radiative zone and convective zone proper. The
  magnetic field in this region is modeled by studying the behavior of
  a layer of uniform field embedded in a subadiabatic atmosphere. Since
  the field can support extra mass, such a configuration is top-heavy,
  and instabilities of the Rayleigh-Taylor type can occur. Numerical
  integration of the two-dimensional compressible MHD equations makes it
  possible to follow the evolution of this instability into the nonlinear
  regime. The initial buoyancy-driven instability of the magnetic field
  gives rise to strong shearing motions, thereby exciting secondary
  Kelvin-Helmholtz instabilities which wrap the gas into regions of
  intense vorticity. The somewhat surprising subsequent motions are
  determined primarily by the strong interactions between vortices.

---------------------------------------------------------
Title: The nonlinear breakup of a magnetic layer - Instability to
    interchange modes
Authors: Cattaneo, F.; Hughes, D. W.
1988JFM...196..323C    Altcode:
  Motivated by considerations of the solar toroidal magnetic field,
  the behavior of a layer of uniform magnetic field embedded in a
  convectively stable atmosphere is studied. Since the field can support
  extra mass, such a configuration is top-heavy and thus instabilities
  of the Rayleigh-Taylor type can occur. For both static and rotating
  basic states, the evolution of the interchange modes (no bending of
  the field lines) is followed by integrating numerically the nonlinear
  compressible MHD equations. The initial Rayleigh-Taylor instability
  of the magnetic field gives rise to strong shearing motions, thereby
  exciting secondary Kelvin-Helmholtz instabilities which wrap the
  gas into regions of intense vorticity. The subsequent motions are
  determined primarily by the strong interactions between vortices which
  are responsible for the rapid disruption of the magnetic layer.

---------------------------------------------------------
Title: Topology of Plumes in Nonlinear Compressible Convection
Authors: Toomre, J.; Cattaneo, F.; Hurlburt, N. E.
1988BAAS...20..678T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Mean advection effects in turbulence
Authors: Cattaneo, F.; Hughes, D. W.; Proctor, M. R. E.
1988GApFD..41..335C    Altcode:
  The connection between the effective convection velocities for scalar
  and vector fields due to the action of turbulence is discussed. An
  explicit relation between the two is calculated for the special case of
  two-dimensional flows and fields and it is shown that both velocities
  are zero for homogeneous turbulence. The calculation leads to new
  insights on the rde of symmetry in determining the advection velocity
  for the case of a vector field.

---------------------------------------------------------
Title: Magnetic fields in the overshoot zone: the great escape.
Authors: Cattaneo, F.; Hughes, D. W.
1987NASCP2483..101C    Altcode: 1987tphr.conf..101C
  In order that magnetic flux be confined within the solar interior for
  times comparable to the solar cycle period it has been suggested that
  the bulk of the solar toroidal field is stored in the convectively
  stable overshoot region situated beneath the convection zone
  proper. Such a magnetic field, though, is still buoyant and is therefore
  subject to Rayleigh-Taylor type instabilities. The model problem of an
  isolated region of magnetic field embedded in a convectively stable
  atmosphere is considered. The fully nonlinear evolution of the two
  dimensional interchange of modes is studied, thereby shedding some
  light on one of the processes responsible for the escape of flux from
  the solar interior.

---------------------------------------------------------
Title: A new look at the instability of a stratified horizontal
    magnetic field
Authors: Hughes, D. W.; Cattaneo, F.
1987GApFD..39...65H    Altcode:
  Although the undular instabilities of a stratified horizontal magnetic
  field have been studied in a number of contexts we believe that the
  physical mechanism responsible for the instability has not been fully
  explained. In this paper we present a new explanation of why these
  instabilities occur, considering in detail the differing cases of
  two-dimensional and three-dimensional motions.

---------------------------------------------------------
Title: Nonlinear dynamos: A complex generalization of the Lorenz
    equations
Authors: Jones, C. A.; Weiss, N. O.; Cattaneo, F.
1985PhyD...14..161J    Altcode:
  Plane nonlinear dynamo waves can be described by a sixth order system
  of nonlinear ordinary differential equations which is a complex
  generalization of the Lorenz system. In the regime of interest
  for modelling magnetic activity in stars there is a sequence of
  bifurcations, ending in chaos, as a stability parameter D (the
  dynamo number) is increased. We show that solutions undergo three
  successive Hopf bifurcations, followed by a transition to chaos. The
  system possesses a symmetry and can therefore be reduced to a fifth
  order system, with trajectories that lie on a 2-torus after the
  third bifurcation. As D is then increased, frequency locking occurs,
  followed by a sequence of period-doubling bifurcations that leads to
  chaos. This behaviour is probably caused by the Shil'nikov mechanism,
  with a (conjectured) homoclinic orbit when D is infinite.

---------------------------------------------------------
Title: Oscillatory convection in sunspots.
Authors: Cattaneo, F.
1984ESASP.220...47C    Altcode: 1984ESPM....4...47C
  The structure of oscillatory convection in the layers below the
  umbral photosphere is discussed. A linear stability analysis of a
  simple model problem describing a polytropic layer with a vertical
  magnetic field shows that when the Alfvén speed and sound speed are
  comparable oscillatory convection has a mixed character exhibiting
  properties of both fast and slow magnetoacoustic waves. The analysis
  further reveals that in this regime overstability is possible even in
  convectively stable layers. The nature of the destabilizing mechanism
  is briefly discussed.

---------------------------------------------------------
Title: Periodic and aperiodic dynamo waves
Authors: Weiss, N. O.; Cattaneo, F.; Jones, C. A.
1984GApFD..30..305W    Altcode:
  In order to show that aperiodic magnetic cycles, with Maunder
  minima, can occur naturally in nonlinear hydromagnetic dynamos, we
  have investigated a simple nonlinear model of an oscillatory stellar
  dynamo. The parametrized mean field equations in plane geometry have a
  Hopf bifurcation when the dynamo number D=1, leading to Parker's dynamo
  waves. Including the nonlinear interaction between the magnetic field
  and the velocity shear results in a system of seven coupled nonlinear
  differential equations. For D&gt;1 there is an exact nonlinear
  solution, corresponding to periodic dynamo waves. In the regime
  described by a fifth order system of equations this solution remains
  stable for all D and the velocity shear is progressively reduced by
  the Lorentz force. In a regime described by a sixth order system, the
  solution becomes unstable and successive transitions lead to chaotic
  behaviour. Oscillations are aperiodic and modulated to give episodes
  of reduced activity.

---------------------------------------------------------
Title: Compressible magnetoconvection
Authors: Cattaneo, Fausto
1984PhDT.......174C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Periodic and aperiodic behaviour in stellar dynamos
Authors: Cattaneo, F.; Weiss, N. O.; Jones, C. A.
1983IAUS..102..307C    Altcode:
  A simple parameterized mean field dynamo model has been constructed
  that includes the dynamical interaction between the magnetic field and
  differential rotation. This system of seven coupled nonlinear ordinary
  differential equations has finite amplitude oscillatory solutions
  (corresponding to Parker's dynamo waves) when the dynamo number (D)
  is greater than one. Two regimes were studied. In the first, the
  velocity shear is reduced by the Lorentz force and there are stable
  periodic solutions for all dynamo numbers greater than one. In the
  second there is a transition from strictly periodic oscillations to
  aperiodic (chaotic) behavior as D is increased. This simple example
  shows that nonlinear hydromagnetic dynamos can produce aperiodic cycles,
  with Maunder minima, as observed in the sun and other late-type stars.