Fibril Magnetic Fields Research Articles

Effects of Full-sphere Convection on M-dwarf Dynamo Action, Flux Emergence, and Spin-down

We present an analysis of 21 global MHD simulations of the dynamo action taking place in the interiors of fully convective M4 stars. We leverage our past work involving 45 simulations of shell-convecting-type M2 stars in a very similar parameter space to identify properties that their dynamos share and ways in which they differ. We find that the parameter scalings of many fundamental dynamo measures including the ratio of magnetic to kinetic energy and the cycle period are shared not only between the two sets of M-dwarf models but also with the broader literature on K and G dwarfs, suggesting that their convective dynamos are similar in nature. We find that the surface fields of the fully convective stars are stronger and more organized than those in the shell-convecting models and indicate faster spin-down than is observed, for which we suggest possible remediations. We note an abundance of dynamo states that are localized within one hemisphere at a time and find that such states are well described by a quasi-linear superposition of even- and odd-parity eigenmodes with similar amplitudes. We observe that the turbulent stirring of the deep fields of the fully convective models allows them to attain far higher peak amplitudes than those in the tachoclines of the partially convective models. We employ our AI-enabled pipeline, LoopNet, to search for buoyant, fibril magnetic fields in the simulated stars and find nearly twice as many such structures forming in the M4 models, in good agreement with observed differences in their flaring rates.

ABSORPTION OFpMODES BY THIN MAGNETIC FLUX TUBES

We study the interaction between p modes and the many magnetic fibrils that lace the solar convection zone. In particular, we investigate the resulting absorption of p-mode energy by the fibril magnetic field. Through mechanical buffeting, the p modes excite tube waves on the magnetic fibrils—in the form of longitudinal sausage waves and transverse kink waves. The tube waves propagate up and down the magnetic fibrils and out of the p-mode cavity, thereby removing energy from the incident acoustic waves. We compute the absorption coefficient associated with this damping mechanism and model the absorption that would be observed for magnetic plage. We compare our results to the absorption coefficient that is measured using the local-helioseismic technique of ridge-filtered holography. We find that, depending on the mode order and the photospheric boundary conditions, we can achieve absorption coefficients for simulated plage that exceed 50%. The observed increase of the absorption coefficient as a function of frequency is reproduced for all model parameters.

A Viscoelastic Theory of Turbulent Fluid Permeated with Fibril Magnetic Fields

The solar convection zone is a turbulent plasma interacting with a magnetic field. Its magnetic field is often described as fibrillar since it consists of slender flux tubes occupying a small fraction of the total volume. It is well known that plasma flow will exert a force on these magnetic fibrils, but few models have accounted for the back-reaction of the fibrils on the flow. We present a model in which the back-reaction of the fibrils on the flow is manifest as viscoelastic properties. On short timescales the fibrils react elastically with a shear modulus proportional to their overall magnetic energy density. On longer timescales they produce an effective viscosity resulting from collective aerodynamic drag. The viscosity due to flux tubes in the solar convection zone can be comparable to that attributed to turbulence there. These forces might have observable effects on the convection zone flows. Subject headings: convection — magnetic fields — MHD — Sun: interior — Sun: magnetic fields — turbulence

Inferring Mean Electric Currents in Unresolved Fibril Magnetic Fields

It is shown that the curl of the transverse magnetogram of magnetic fields composed of unresolved separate fibrils bears no direct relation to the mean longitudinal electric current density. The mean current density is essentially zero, regardless of the spatial variation and torsion of the mean field.

The normal modes of a resonant cavity containing discrete inhomogeneities - The influence of fibril magnetic fields on the solar acoustic oscillations

view Abstract Citations (25) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Normal Modes of a Resonant Cavity Containing Discrete Inhomogeneities: The Influence of Fibril Magnetic Fields on the Solar Acoustic Oscillations Bogdan, Thomas J. ; Cattaneo, Fausto Abstract 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). Publication: The Astrophysical Journal Pub Date: July 1989 DOI: 10.1086/167615 Bibcode: 1989ApJ...342..545B Keywords: Acoustic Frequencies; Cavity Resonators; Solar Magnetic Field; Solar Oscillations; Bessel Functions; Eigenvalues; Hankel Functions; Magnetic Flux; Matrices (Mathematics); Solar Physics; HYDROMAGNETICS; SUN: MAGNETIC FIELDS; SUN: OSCILLATIONS full text sources ADS |

Propagation of Compressive Waves through Fibril Magnetic Fields. III. Waves that Propagate along the Magnetic Field

Propagation of Compressive Waves through Fibril Magnetic Fields. III. Waves that Propagate along the Magnetic Field

Propagation of compressive waves through fibril magnetic fields

view Abstract Citations (49) References (86) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Propagation of Compressive Waves through Fibril Magnetic Fields Bogdan, T. J. ; Zweibel, Ellen G. Abstract The surface effects of interactions between the solar 5-min p-modes and the large-scale fibril magnetic field are discussed using a multiple scattering approach. Attention is given to the propagation of linear disturbances in a two-dimensional, highly conducting magnetized plasma with many parallel flux tubes in pressure equilibrium with a surrounding stationary field-free plasma. Multiple scattering in the fibril half-space is shown to generate acoustic waves that cascade to ever-smaller length scales. The scale reduction, proportional to the depth into the fibril magnetic field, is responsible for the damping of p-mode oscillations observed in plages. Publication: The Astrophysical Journal Pub Date: January 1987 DOI: 10.1086/164889 Bibcode: 1987ApJ...312..444B Keywords: Compression Waves; Solar Magnetic Field; Sunspots; Wave Propagation; Acoustic Scattering; P Waves; Plane Waves; Solar Oscillations; Stellar Models; Solar Physics; HYDROMAGNETICS; SUN: MAGNETIC FIELDS; SUN: OSCILLATIONS; SUN: SUNSPOTS full text sources ADS | Related Materials (2) Part 2: 1987ApJ...318..888B Part 3: 1987ApJ...318..896B

Propagation of compressive waves through fibril magnetic fields. II - Scattering from a slab of magnetic flux tubes

view Abstract Citations (20) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Propagation of Compressive Waves through Fibril Magnetic Fields. II. Scattering from a Slab of Magnetic Flux Tubes Bogdan, T. J. Abstract The reflection/transmission of an acoustic plane wave from/through a slab of parallel, randomly distributed, magnetic flux tubes is worked out in the long wavelength (thin flux tube) limit. These results are then contrasted with the reflection/transmission problem for (i) a uniformly magnetized homogeneous slab and (ii) an infinite half-space of parallel, randomly distributed, flux tubes. A thin slab of magnetic fibrils possesses properties that are qualitatively similar to those of the uniformly magnetized slab. The slab of magnetic fibrils does not, however, exhibit the total reflection or complete transmission of the incident wave that are possible for the uniformly magnetized slab. A thick slab of magnetic fibrils behaves more nearly like the infinite half-space of fibrils: the amplitude of the reflected wave is more-or-less independent of the width of the slab, and the amplitude of the transmitted wave is vanishingly small. The distinction between thin and thick slabs of randomly placed fibrils depends critically upon the properties of the composite magnetic medium the magnetic volume filling factor and the mean flux tube separation and the wavelength of the incident acoustic plane wave. Publication: The Astrophysical Journal Pub Date: July 1987 DOI: 10.1086/165420 Bibcode: 1987ApJ...318..888B Keywords: Acoustic Scattering; Compression Waves; Magnetic Flux; Magnetohydrodynamics; Solar Magnetic Field; Solar Oscillations; Sunspots; Wave Propagation; Geometry; Phase Velocity; Plasma Slabs; Wavelengths; Solar Physics; HYDROMAGNETICS; SUN: MAGNETIC FIELDS; SUN: OSCILLATIONS; SUN: SUNSPOTS full text sources ADS | Related Materials (2) Part 1: 1987ApJ...312..444B Part 3: 1987ApJ...318..896B

Diffusive transport and in situ coalescence of magnetic flux tubes. I - Steady state solutions

view Abstract Citations (6) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffusive transport and in situ coalescence of magnetic flux tubes. I - Steady state solutions Bogdan, T. J. Abstract There is increasing evidence of fibril character in astrophysical magnetic fields. The solar photosphere is an obvious example. The author suggests that the hierarchy of magnetic structures observed in the photosphere may be synthesized through the coalescence of the thin (R ≈ 2 - 3×102km), intense (B ≈ 1-2×103gauss) magnetic fibrils seen to be threading the solar surface. Indeed, the distribution of magnetic flux tube sizes in a large-scale fibril magnetic field is likely to result from the interplay between flux coalescence and fragmentation. To illustrate this point, the author considers the diffusion, and in situ coalescence, of parallel twisted magnetic flux tubes through a slab. The present paper concentrates on steady state solutions for the diffusive transport. Publication: The Astrophysical Journal Pub Date: December 1985 DOI: 10.1086/163718 Bibcode: 1985ApJ...299..510B Keywords: Magnetic Diffusion; Magnetic Field Configurations; Magnetic Flux; Solar Magnetic Field; Convection; Magnetohydrodynamics; Photosphere; Slabs; Steady State; Sunspots; Solar Physics full text sources ADS |

Effect of a fibril magnetic field on solar p-modes

The dispersion relation is obtained for acoustic plane waves that scatter coherently from an ensemble of parallel magnetic flux tubes when the wave vector is perpendicular to the flux-tube axis. When the magnetic flux tubes are distributed uniformly and possess radii that are small compared with the wavelength, the frequency can be calculated exactly. The waves are damped slightly due to a loss of coherence and are shifted downward or upward in frequency relative to a medium devoid of magnetic fibrils, depending primarily on whether the flux tubes are more or less dense than their surroundings. It is suggested that the influence of the fibril magnetic fields observed at the solar surface cannot be ignored in the interpretation of high-1 surface p-mode data.

Dynamical evolution of large-scale, two-dimensional, fibril magnetic fields

view Abstract Citations (25) References (84) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical evolution of large-scale, two-dimensional, fibril magnetic fields Bogdan, T. J. ; Lerche, I. Abstract The dynamical behavior of a large-scale, two-dimensional, fibril magnetic field embedded in an ideal fluid of infinite electrical conductivity is considered. The field is treated statistically as a dilute gas composed of parallel, twisted, magnetic flux tubes in specified velocity and gravitational fields. The salient physical processes are the annihilation/coalescence of flux tubes through binary collisions and flux-tube precipitation, diffusion, and advection caused by their motions. The purpose is to study the size distribution, temporal, and latitudinal variations of emerging solar flux with a fibril magnetic field throughout the solar convection zone. The analysis suggests that the transport and in situ coalescence/annihilation of magnetic flux tubes through the solar convection zone may be responsible for the size spectrum, temporal, and latitudinal variations observed in the emerging magnetic flux during the solar cycle. Such a scenario is consistent with the previous works of Speigel and Weiss (1980), and Schmitt and Rosner (1984), which suggest that solar toroidal fields are produced near the base of the convection zone and are shredded into thin magnetic fibrils through a doubly diffusive magnetic buoyancy instability. Publication: The Astrophysical Journal Pub Date: September 1985 DOI: 10.1086/163489 Bibcode: 1985ApJ...296..719B Keywords: Evolution (Development); Magnetohydrodynamics; Solar Magnetic Field; Sunspots; Advection; Coalescing; Convective Heat Transfer; Integral Equations; Kernel Functions; Magnetic Flux; Solar Physics full text sources ADS |

Stellar Fibril Magnetic Systems - Part Three - Convective Counterflow

view Abstract Citations (12) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar Fibril Magnetic Systems - Part Three - Convective Counterflow Parker, E. N. Abstract The linearized equations of motion of a fibril magnetic field and the mean fluid motion through that field are used to illustrate the convective counterflow effect. The calculations show that starting from static equilibrium, a vertical impulse applied to a cluster of fibrils (represented by a cluster of parallel circular cylinders) may result in the fluid moving either upward or downward across the cluster, depending on the eccentricity of the cross section of the cluster and the strength of the aerodynamic drag coupling the fluid and fibrils. Specifically, a flattened cluster of fibrils, with a width in excess of twice the thickness, may be carried in the opposite direction from the initial applied impulse by the associated counter convective motion. It appears that this counterflow effect may operate in the upper few thousand km of the convective zone of the Sun, where it would play a role in suppressing the buoyant rise of magnetic flux tubes to the surface. Hence the convective counterflow may take an active part in controlling the emergence of flux tubes through the surface of a star like the Sun, perhaps regulating to some degree the form and general level of the activity of the star. Publication: The Astrophysical Journal Pub Date: July 1985 DOI: 10.1086/163274 Bibcode: 1985ApJ...294...57P full text sources ADS |

The Dynamics of Fibril Magnetic Fields - Part Four - Trapping in Closed Convective Rolls

The Dynamics of Fibril Magnetic Fields - Part Four - Trapping in Closed Convective Rolls

The dynamics of fibril magnetic fields. III - Fibril configurations in steady flows. IV - Trapping in closed convective rolls

The present investigation has the objective to provide an illustration of the equilibrium form of a hypothetical fibril field beneath the surface of the sun. Equilibrium fibril paths in static and moving atmospheres are considered. The provided examples are related to horizontal flows in an atmosphere in which the two anchor points of the arched fibril lie along the flow direction relative to each other. A brief introduction to the subject of the motion of horizontal fibrils in convective rolls is also presented, taking into account some implications for the sun. The equations of motion for a slender, buoyant flux tube extending horizontally along a closed convective roll are solved to illustrate the motion of the flux tube relative to the fluid, and attention is given to theoretical possibilities for the escape of flux tubes from a large horizontal convective roll in the sun.

The dynamics of fibril magnetic fields. I - Effect of flux tubes on convection. II - The mean field equations

view Abstract Citations (28) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The dynamics of fibril magnetic fields. I - Effect of flux tubes on convection. Parker, E. N. Abstract Observations have established the general fibril state of the magnetic field at the visible surface of the sun. This extraordinary state of the field, in separate intense flux tubes, implies that the convection has a relatively permanent, closed topology, in spite of the large Reynolds number. This paper explores some of the effects of the separate flux tubes on the convective motions, generally pushing the convective cells toward aligning their downdrafts with the flux tubes so as to minimize the dissipation. Publication: The Astrophysical Journal Pub Date: May 1982 DOI: 10.1086/159907 Bibcode: 1982ApJ...256..292P Keywords: Magnetic Flux; Solar Atmosphere; Solar Magnetic Field; Convective Flow; Energy Dissipation; Reynolds Number; Topology; Solar Physics full text sources ADS |

The Dynamics of Fibril Magnetic Fields - Part Two - the Mean Field Equations

A variety of scenarios for the origin and activity of magnetic fields in the convective zone of the Sun have been put forth in recent years. In particular, a number of authors have urged that the magnetic field is in a generally fibril state, much as observed at the surface, with a variety of suggestions as to the consequences. The present paper works out the mean field equations for intense thin fibril of fixed cross section under the assumption of some significant local ordering. The aim is to establish to what degree new effects may arise as a result of the fibril state. It is shown that the fibril state of a mean field B/sub i/ enhances the tension in the field and the buoyancy of the field by the compression factor m of the field in the individual fibrils. New qualitative effects appear in the slip velocity u/sub i/ of the fibrils through the ambient fluid and in the neutral point reconnection of neighboring fibrils. Some of the novel features are illustrated in later papers in this series. One of the most important consequences is the simplification of the theory of turbulent diffusion by eliminating the looping and tanglingmore » that arises when a continuum field is carried in a chaotic turbulent flow. We have been unable to find any effects that would admit of a new concept for the origin of the solar magnetic fields.« less