The Program Magreg for Determining the Longitudinal Component of Stellar Magnetic Fields from Spectropolarimetric Observations

Zeeman spectroheliograms of photospheric magnetic fields (longitudinal component) in the CaI 6102.7 Å line are being obtained with the new 61-cm vacuum solar telescope and spectroheliograph, using the Leighton technique. The structure of the magnetic field network appears identical to the bright photospheric network visible in the cores of many Fraunhofer lines and in CN spectroheliograms, with the exception that polarities are distinguished. This supports the evolving concept that solar magnetic fields outside of sunspots exist in small concentrations of essentially vertically oriented field, roughly clumped to form a network imbedded in the otherwise field-free photosphere. A timelapse spectroheliogram movie sequence spanning 6 hr revealed changes in the magnetic fields, including a systematic outward streaming of small magnetic knots of both polarities within annular areas surrounding several sunspots. The photospheric magnetic fields and a series of filtergrams taken at various wavelengths in the Hα profile starting in the far wing are intercompared in an effort to demonstrate that the dark strands of arch filament systems (AFS) and fibrils map magnetic field lines in the chromosphere. An example of an active region in which the magnetic fields assume a distinct spiral structure is presented.

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Infrared Measurements of Stellar Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900124696 ◽

1994 ◽

Vol 154 ◽

pp. 437-447 ◽

Cited By ~ 1

Author(s):

Steven H. Saar

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Pressure Support ◽

Field Research ◽

Future Directions ◽

Rotation Rates ◽

Infrared Measurements ◽

Cool Stars ◽

Rs Cvn Variables ◽

Stellar Magnetic Fields

I review the advantages, techniques, and results of measurement of magnetic fields on cool stars in the infrared (IR). These measurements have generated several important results, including the following: the first data on the magnetic parameters of dMe and RS CVn variables; evidence for field strength confinement by photospheric gas pressure; support for the correlation between magnetic flux and rotation, with possible saturation at high rotation rates; indications of horizontal and/or vertical magnetic field structure; and evidence of spatial variations in B over a stellar surface. I discuss these results in detail, and suggest future directions for IR magnetic field research.

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Observing and Modelling Stellar Magnetic Fields

EAS Publications Series ◽

10.1051/eas/0939001 ◽

2009 ◽

Vol 39 ◽

pp. 1-20 ◽

Cited By ~ 7

Author(s):

J.D. Landstreet

Keyword(s):

Magnetic Fields ◽

Stellar Magnetic Fields

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Global MHD phenomena and their importance for stellar surfaces

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311015146 ◽

2010 ◽

Vol 6 (S273) ◽

pp. 141-147

Author(s):

Rainer Arlt

Keyword(s):

Magnetic Fields ◽

Mean Field ◽

Momentum Equation ◽

Small Scale ◽

Dynamo Action ◽

Complex Activity ◽

Mhd Instabilities ◽

Stellar Magnetic Fields ◽

Mean Field Dynamo

AbstractThis review is an attempt to elucidate MHD phenomena relevant for stellar magnetic fields. The full MHD treatment of a star is a problem which is numerically too demanding. Mean-field dynamo models use an approximation of the dynamo action from the small-scale motions and deliver global magnetic modes which can be cyclic, stationary, axisymmetric, and non-axisymmetric. Due to the lack of a momentum equation, MHD instabilities are not visible in this picture. However, magnetic instabilities must set in as a result of growing magnetic fields and/or buoyancy. Instabilities deliver new timescales, saturation limits and topologies to the system probably providing a key to the complex activity features observed on stars.

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The macrodynamics of α-effect dynamos in rotating fluids

Journal of Fluid Mechanics ◽

10.1017/s0022112075000390 ◽

1975 ◽

Vol 67 (3) ◽

pp. 417-443 ◽

Cited By ~ 140

Author(s):

W. V. R. Maekus ◽

M. R. E. Proctor

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Magnetic Energy ◽

Finite Amplitude ◽

Small Scale ◽

Past Study ◽

Ohmic Loss ◽

General Will ◽

Rotating Systems ◽

Stellar Magnetic Fields

Past study of the large-scale consequences of forced small-scale motions in electrically conducting fluids has led to the ‘α-effect’ dynamos. Various linear kinematic aspects of these dynamos have been explored, suggesting their value in the interpretation of observed planetary and stellar magnetic fields. However, large-scale magnetic fields with global boundary conditions can not be force free and in general will cause large-scale motions as they grow. I n this paper the finite amplitude behaviour of global magnetic fields and the large-scale flows induced by them in rotating systems is investigated. In general, viscous and ohmic dissipative mechanisms both play a role in determining the amplitude and structure of the flows and magnetic fields which evolve. In circumstances where ohmic loss is the principal dissipation, it is found that determination of a geo- strophic flow is an essential part of the solution of the basic stability problem. Nonlinear aspects of the theory include flow amplitudes which are independent of the rotation and a total magnetic energy which is directly proportional to the rotation. Constant a is the simplest example exhibiting the various dynamic balances of this stabilizing mechanism for planetary dynamos. A detailed analysis is made for this case to determine the initial equilibrium of fields and flows in a rotating sphere.

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