scholarly journals The effect of toroidal magnetic fields in the overshoot layer on the eigenfrequencies of stellar oscillations

1988 ◽  
Vol 123 ◽  
pp. 167-170
Author(s):  
Gaetano Belvedere
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Convection Zone ◽  
High Order ◽  
Stable Region ◽  
Flux Tubes ◽  
Frequency Splitting ◽  
Acoustic Modes ◽  
Stellar Oscillations ◽  
The Magnetic Field

The overshoot layer in stellar convection zones is slightly subadiabatic and can be considered as a stable region for storage of magnetic flux. Belvedere, Pidatella and Stix (1986) estimated the size of the overshoot layer and computed the magnetic field strength, beyond which toroidal flux tubes become unstable to buoyancy, for a number of main sequence spectral types ranging from F5 to K0. Here we estimate the relative frequency perturbation of high order acoustic modes due to the presence of a non-oblique axisymmetric magnetic field in the overshoot layer. We find that increases with the advancing spectral type, the predicted frequency splitting being large enough to be detected by observations, at least for the Sun.We conclude that magnetic field induced frequency splitting of high order acoustic modes may well be due to a toroidal field of relatively moderate strength just beneath the bottom of the convection zone.

V. Heating and Dynamics of Chromosphere and Corona

1988 ◽  
Vol 20 (1) ◽  
pp. 100-102
Author(s):  
G.E. Brueckner
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Acoustic Waves ◽  
Convection Zone ◽  
Flux Tubes ◽  
Magnetic Structures ◽  
Convective Motions ◽  
Field Lines ◽  
The Magnetic Field

The crucial role of magnetic fields in any mechanism to heat the outer solar atmosphere has been generally accepted by all authors. However, there is still no agreement about the detailed function of the magnetic field. Heating mechanisms can be divided up into 4 classes: (I) The magnetic field plays a passive role as a suitable medium for the propagation of Alfvén waves from the convection zone into the corona (Ionson, 1984). (II) In closed magnetic structures the slow random shuffling of field lines by convective motions below the surface induces electric currents in the corona which heat it by Joule dissipation (Heyvaerts and Priest, 1984). (Ill) Emerging flux which is generated in the convection zone reacts with ionized material while magnetic field lines move through the chromosphere, transition zone and corona. Rapid field line annihilation, reconnection and drift currents result in heating and material ejection (Brueckner, 1987; Brueckner et al., 1987; Cook et al., 1987). (IV) Acoustic waves which could heat the corona can be guided by magnetic fields. Temperature distribution, wave motions and shock formation are highly dependent on the geometry of the flux tubes (Ulmschneider and Muchmore, 1986; Ulmschneider, Muchmore and Kalkofen, 1987).

A Dynamo Model for Magnetic Stars with Long Periods

1976 ◽  
Vol 32 ◽  
pp. 39-42
Author(s):  
M. Schüssler
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Convection Zone ◽  
Dynamo Model ◽  
Magnetic Stars ◽  
Order Of Magnitude ◽  
Linear Effects ◽  
The Right ◽  
Right Order ◽  
The Magnetic Field

SummaryA α - effect dynamo model is presented which can be relevant for the group of magnetic stars.with observed periods between 1 y and 72 ys. The model is based on an axisymmetric α2- dynamo including non-linear effects due to the “cut off α- effect”; no differential rotation is taken into account. There are oscilliations of the magnetic field with periods in the right order of magnitude under the assumption of an outer convection zone between R ≥ r ≥.5 R ….7R. In the sense of this model therefore these stars should be young objects passing from their Hayashi track down to the main sequence.

scholarly journals Small Magnetostatic Flux Tubes

1977 ◽  
Vol 4 (2) ◽  
pp. 265-266
Author(s):  
H. C. Spruit
Keyword(s):  
Magnetic Field ◽  
Heat Transport ◽  
Convective Heat ◽  
Convection Zone ◽  
Solar Surface ◽  
Small Scale ◽  
Flux Tubes ◽  
Basic Assumptions ◽  
Magnetic Elements ◽  
The Magnetic Field

In an attempt to interpret the observed properties of small scale magnetic fields at the solar surface, a set of models has been calculated based on the assumption of a magnetostatic equilibrium. The basic assumptions made are: i.The observed magnetic elements are magnetostatic flux tubes.ii.The efficiency of convective heat transport inside the tube is reduced with respect to that in the normal convection zone; the horizontal convective heat transport in the tube is suppressed completely by the magnetic field.iii.Close to the tube, horizontal convective heat transport is reduced due to the proximity of the magnetic field.

Connecting a Star’s Convection Zone with its Corona

1995 ◽  
Vol 12 (2) ◽  
pp. 180-185 ◽  
Author(s):  
D. J. Galloway ◽  
C. A. Jones
Keyword(s):  
Magnetic Field ◽  
Convection Zone ◽  
Flux Rope ◽  
Field System ◽  
Stellar Convection ◽  
Coronal Activity ◽  
Flux Concentration ◽  
Global Understanding ◽  
The Magnetic Field

AbstractThis paper discusses problems which have as their uniting theme the need to understand the coupling between a stellar convection zone and a magnetically dominated corona above it. Interest is concentrated on how the convection drives the atmosphere above, loading it with the currents that give rise to flares and other forms of coronal activity. The role of boundary conditions appears to be crucial, suggesting that a global understanding of the magnetic field system is necessary to explain what is observed in the corona. Calculations are presented which suggest that currents flowing up a flux rope return not in the immediate vicinity of the rope but rather in an alternative flux concentration located some distance away.

scholarly journals Simulating Solar Global Magnetism in 3-D

2012 ◽  
Vol 10 (H16) ◽  
pp. 101-103
Author(s):  
A. S. Brun ◽  
A. Strugarek
Keyword(s):  
Magnetic Field ◽  
Recent Progress ◽  
Convection Zone ◽  
Thermal Structure ◽  
Solar Convection ◽  
Self Consistent ◽  
The Mean ◽  
The Magnetic Field

AbstractWe briefly present recent progress using the ASH code to model in 3-D the solar convection, dynamo and its coupling to the deep radiative interior. We show how the presence of a self-consistent tachocline influences greatly the organization of the magnetic field and modifies the thermal structure of the convection zone leading to realistic profiles of the mean flows as deduced by helioseismology.

scholarly journals On the stability of a general magnetic field topology in stellar radiative zones

2019 ◽  
Vol 82 ◽  
pp. 365-371
Author(s):  
K. Augustson ◽  
S. Mathis ◽  
A. Strugarek
Keyword(s):  
Magnetic Field ◽  
Global Change ◽  
Magnetic Fields ◽  
Potential Energy ◽  
Massive Stars ◽  
Spherical Geometry ◽  
Stable Region ◽  
Magnetic Field Topology ◽  
The Stability ◽  
The Magnetic Field

This paper provides a brief overview of the formation of stellar fossil magnetic fields and what potential instabilities may occur given certain configurations of the magnetic field. One such instability is the purely magnetic Tayler instability, which can occur for poloidal, toroidal, and mixed poloidal-toroidal axisymmetric magnetic field configurations. However, most of the magnetic field configurations observed at the surface of massive stars are non-axisymmetric. Thus, extending earlier studies in spherical geometry, we introduce a formulation for the global change in the potential energy contained in a convectively-stable region for both axisymmetric and non-axisymmetric magnetic fields.

scholarly journals Blazhko Effect and Magnetic Field in RR Lyrae

2000 ◽  
Vol 176 ◽  
pp. 318-319
Author(s):  
M. Chadid ◽  
D. Gillet ◽  
K. Kolenberg ◽  
C. Aerts
Keyword(s):  
Magnetic Field ◽  
Pulsation Frequency ◽  
Frequency Splitting ◽  
Rr Lyrae ◽  
Blazhko Effect ◽  
Magnetic Model ◽  
Observational Techniques ◽  
The Magnetic Field

AbstractThe recent detection of a frequency splitting around the pulsation frequency and its harmonics points towards the magnetic model to explain the Blazhko effect. Here we show that it is urgent to confirm with modern observational techniques the existence of the magnetic field in RR Lyrae.

scholarly journals What are the Radio Filaments Near the Galactic Center?

1996 ◽  
Vol 169 ◽  
pp. 247-261 ◽  
Author(s):  
Mark Morris
Keyword(s):  
Magnetic Field ◽  
Galactic Center ◽  
Galactic Plane ◽  
Local Source ◽  
Flux Tubes ◽  
Flux Lines ◽  
Magnetic Flux Tubes ◽  
The Magnetic Field ◽  
Relativistic Particles ◽  
Projected Distance

A population of nonthermally-emitting radio filaments tens of parsecs in length has been observed within a projected distance of ∼130 pc of the Galactic center. More or less perpendicular to the Galactic plane, they appear to define the flux lines of a milligauss magnetic field. The characteristics of the known filaments are summarized. Three fundamental questions raised by these structures are discussed: 1) Do they represent magnetic flux tubes embedded within an ubiquitous, dipole magnetic field permeating the inner Galaxy, but which have been illuminated by some local source of relativistic particles, or are they instead isolated, self-sustaining current paths with an approximately force-free magnetic configuration in pressure equilibrium with the interstellar medium? 2) What is the source of either the magnetic field or the current? and 3) What is the source of the relativistic particles which provide the illuminating synchrotron radiation? We are nearer an answer to the the last of these questions than to the others, although several interesting models have been proposed.

scholarly journals A new weak-field magnetic DA white dwarf in the local 20 pc volume

2019 ◽  
Vol 628 ◽  
pp. A1 ◽  
Author(s):  
J. D. Landstreet ◽  
S. Bagnulo
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
Field Component ◽  
Main Sequence ◽  
Mean Field ◽  
Weak Field ◽  
Line Of Sight ◽  
Limited Sample ◽  
Field Decay ◽  
The Magnetic Field

We report the discovery of a new magnetic DA white dwarf (WD), WD 0011 − 721, which is located within the very important 20 pc volume-limited sample of the closest WDs to the Sun. This star has a mean field modulus ⟨|B|⟩ of 343 kG, and from the polarisation signal we deduce a line-of-sight field component of 75 kG. The magnetic field is sufficiently weak to have escaped detection in classification spectra. We then present a preliminary exploration of the data concerning the frequency of such fields among WDs with hydrogen-rich atmospheres (DA stars). We find that 20 ± 5% of the DA WDs in this volume have magnetic fields, mostly weaker than 1 MG. Unlike the slow field decay found among the magnetic Bp stars of the upper main sequence, the WDs in this sample show no evidence of magnetic field or flux changes over several Gyr.

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