Stellar Dynamo Characteristics

AbstractRecent discoveries have shown that magnetic activity is typical of cool stars with deep convective zones and magnetic cycles are found in slowly rotating stars like the sun. The current state of hydromagnetic dynamo theory is reviewed, and simplified models are used in an attempt to isolate the dominant nonlinear processes in stellar dynamos.

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Differential Rotation and Magnetic Activity of the Lower Main Sequence Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100075539 ◽

1980 ◽

Vol 51 ◽

pp. 296-297

Author(s):

G. Belvedere ◽

L. Paterno ◽

M. Stix

Keyword(s):

Spherical Shell ◽

Differential Rotation ◽

Main Sequence ◽

Magnetic Activity ◽

The Sun ◽

Coupling Constant ◽

Main Sequence Stars ◽

Dynamo Theory ◽

Magnetic Cycles ◽

Surface Convection

AbstractWe extend to the lower main sequence stars the analysis of convection interacting with rotation in a compressible spherical shell, already applied to the solar case (Belvedere and Paterno, 1977; Belvedere et al. 1979a). We assume that the coupling constant ε between convection and rotation, does not depend on the spectral type. Therefore we take ε determined from the observed differential rotation of the Sun, and compute differential rotation and magnetic cycles for stars ranging from F5 to MO, namely for those stars which are supposed to possess surface convection zones (Belvedere et al. 1979b, c, d). The results show that the strength of differential rotation decreases from a maximum at F5 down to a minimum at G5 and then increases towards later spectral types. The computations of the magnetic cycles based on the αω-dynamo theory show that dynamo instability decreases from F5 to G5, and then increases towards the later spectral types reaching a maximum at MO. The period of the magnetic cycles increases from a few years at F5 to about 100 years at MO. Also the extension of the surface magnetic activity increases substantially towards the later spectral types. The results are discussed in the framework of Wilson’s (1978) observations.

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Dynamo models of the solar cycle

Living Reviews in Solar Physics ◽

10.1007/s41116-020-00025-6 ◽

2020 ◽

Vol 17 (1) ◽

Cited By ~ 7

Author(s):

Paul Charbonneau

Keyword(s):

Solar Cycle ◽

Large Scale ◽

Chaotic Behavior ◽

Critical Discussion ◽

Solar Convection ◽

Current State ◽

Hydromagnetic Dynamo ◽

Magnetic Cycles ◽

Dynamo Process ◽

Key Questions

AbstractThis paper reviews recent advances and current debates in modeling the solar cycle as a hydromagnetic dynamo process. Emphasis is placed on (relatively) simple dynamo models that are nonetheless detailed enough to be comparable to solar cycle observations. After a brief overview of the dynamo problem and of key observational constraints, I begin by reviewing the various magnetic field regeneration mechanisms that have been proposed in the solar context. I move on to a presentation and critical discussion of extant solar cycle models based on these mechanisms, followed by a discussion of recent magnetohydrodynamical simulations of solar convection generating solar-like large-scale magnetic cycles. I then turn to the origin and consequences of fluctuations in these models and simulations, including amplitude and parity modulation, chaotic behavior, and intermittency. The paper concludes with a discussion of our current state of ignorance regarding various key questions relating to the explanatory framework offered by dynamo models of the solar cycle.

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Dynamo Processes Constrained by Solar and Stellar Observations

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318001424 ◽

2018 ◽

Vol 13 (S340) ◽

pp. 275-280

Author(s):

Maria A. Weber

Keyword(s):

Differential Rotation ◽

Meridional Circulation ◽

Mean Field ◽

Magnetic Activity ◽

Flux Emergence ◽

Dynamo Theory ◽

M Dwarfs ◽

Dynamo Action ◽

Solar Type ◽

Stellar Dynamo

AbstractOur understanding of stellar dynamos has largely been driven by the phenomena we have observed of our own Sun. Yet, as we amass longer-term datasets for an increasing number of stars, it is clear that there is a wide variety of stellar behavior. Here we briefly review observed trends that place key constraints on the fundamental dynamo operation of solar-type stars to fully convective M dwarfs, including: starspot and sunspot patterns, various magnetism-rotation correlations, and mean field flows such as differential rotation and meridional circulation. We also comment on the current insight that simulations of dynamo action and flux emergence lend to our working knowledge of stellar dynamo theory. While the growing landscape of both observations and simulations of stellar magnetic activity work in tandem to decipher dynamo action, there are still many puzzles that we have yet to fully understand.

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Magnetic activity of cool stars in the Hertzsprung-Russell diagram

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312005005 ◽

2011 ◽

Vol 7 (S286) ◽

pp. 296-306

Author(s):

J. H. M. M. Schmitt

Keyword(s):

Magnetic Activity ◽

Maunder Minimum ◽

The Sun ◽

X Ray ◽

Cool Stars ◽

Low Activity

AbstractI review the X-ray emission from cool stars with outer convection zones in comparison to the Sun with a focus on the properties of low-activity stars. I present the recent results of long-term X-ray monitoring which demonstrate the existence of X-ray cycles on stars with known calcium cycles. The evidence of a minimum stellar X-ray flux is presented and arguments are put forward for the view that the Sun in its extended minimum between 2008 - 2009 behaved very much like a Maunder-minimum Sun.

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C I 5380 Å: A SPECTRAL LINE WITH LOW INFLUENCE FROM STARSPOTS DYNAMICS AND MAGNETIC CYCLES

International Journal of Modern Physics Conference Series ◽

10.1142/s2010194512008410 ◽

2012 ◽

Vol 18 ◽

pp. 178-181

Author(s):

D. SOUTO ◽

J. D. DO NASCIMENTO

Keyword(s):

Radial Velocity ◽

Activity Cycle ◽

Magnetic Activity ◽

Spectral Lines ◽

The Sun ◽

Total Irradiance ◽

Modulation Signal ◽

Solar Magnetic Activity ◽

Activity Cycles ◽

Magnetic Cycles

In the Sun-as-a-star Project, the sun was observed spectroscopically and photometrically for more than 25 years in order to determine variability and luminosity changes. This project detected systematic longterm decrease in the total irradiance as a consequence of the solar magnetic activity cycle (scale of years) and variability on solar activity from a time scale of days-months. The solar magnetic activity cycles could mimic the radial velocity modulation signal of a long-period companion in several spectral lines. This effect is an important limitation for the exoplanet searches programs using the radial velocity technique. The Lomb-Scargle periodogram analysis of the Sun-as-a-star spectroscopic data shows that the photospheric line C I 5380 Å and other 11 lines seems to not show significant influence from the rotational or cromospheric magnetic activity modulation. Thus, our analysis suggest that C I 5380 Å line could be used in programs that require extremely line stability.

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FK Comae and the Evolution of Close Binaries

Symposium - International Astronomical Union ◽

10.1017/s0074180900122582 ◽

1992 ◽

Vol 151 ◽

pp. 403-406

Author(s):

David P. Huenemoerder ◽

Lawrence W. Ramsey ◽

Derek L. Buzasi ◽

Harold L. Nations

Keyword(s):

Binary Star ◽

Magnetic Activity ◽

Close Binary ◽

Close Binaries ◽

Evolutionary Status ◽

Dynamo Theory ◽

Cool Stars ◽

Depth Study ◽

Important Object ◽

Close Binary Star

FK Com is an enigma among active cool stars. A rotationally driven magnetic dynamo is an explanation for activity. Youth and binarity are two causes of rapid rotation, but FK Com is old, single, and rotating near breakup. Much studied optical and ultraviolet data have not revealed the cause of its unusual activity. It is an important object in a more complete understanding of close binary star evolution and of the dynamo theory of magnetic activity. In 1989, we executed a coordinated ultraviolet and intensive optical spectroscopic and photometric campaign to better elucidate its characteristics. Data relevant to its evolutionary status, namely its radial velocity variations, will be discussed here, while an in depth study of the chromospheric activity will be deferred to a more detailed publication.

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Rossby Waves in Astrophysics

Space Science Reviews ◽

10.1007/s11214-021-00790-2 ◽

2021 ◽

Vol 217 (1) ◽

Author(s):

T. V. Zaqarashvili ◽

M. Albekioni ◽

J. L. Ballester ◽

Y. Bekki ◽

L. Biancofiore ◽

...

Keyword(s):

Large Scale ◽

Rossby Waves ◽

Magnetic Activity ◽

Future Research ◽

Rotating Stars ◽

Spatial And Temporal Scales ◽

Physical Role ◽

Astrophysical Objects ◽

Exoplanetary Atmospheres ◽

Rapidly Rotating Stars

AbstractRossby waves are a pervasive feature of the large-scale motions of the Earth’s atmosphere and oceans. These waves (also known as planetary waves and r-modes) also play an important role in the large-scale dynamics of different astrophysical objects such as the solar atmosphere and interior, astrophysical discs, rapidly rotating stars, planetary and exoplanetary atmospheres. This paper provides a review of theoretical and observational aspects of Rossby waves on different spatial and temporal scales in various astrophysical settings. The physical role played by Rossby-type waves and associated instabilities is discussed in the context of solar and stellar magnetic activity, angular momentum transport in astrophysical discs, planet formation, and other astrophysical processes. Possible directions of future research in theoretical and observational aspects of astrophysical Rossby waves are outlined.

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Learning about Stellar Dynamos from Long–term Photometry of Starspots

International Astronomical Union Colloquium ◽

10.1017/s0252921100079914 ◽

1991 ◽

Vol 130 ◽

pp. 353-369 ◽

Cited By ~ 2

Author(s):

Douglas S. Hall

Keyword(s):

Differential Rotation ◽

Rotation Period ◽

Turnover Time ◽

Rotating Stars ◽

Photometric Variability ◽

Solar Type ◽

Magnetic Cycles ◽

The Many ◽

Rapidly Rotating Stars

AbstractSpottedness, as evidenced by photometric variability in 277 late-type binary and single stars, is found to occur when the Rossby number is less than about 2/3. This holds true when the convective turnover time versus B–V relation of Gilliland is used for dwarfs and also for subgiants and giants if their turnover times are twice and four times longer, respectively, than for dwarfs. Differential rotation is found correlated with rotation period (rapidly rotating stars approaching solid-body rotation) and also with lobe-filling factor (the differential rotation coefficient k is 2.5 times larger for F = 0 than F = 1). Also reviewed are latitude extent of spottedness, latitude drift during a solar-type cycle, sector structure and preferential longitudes, starspot lifetimes, and the many observational manifestations of magnetic cycles.

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