scholarly journals The Solar-Stellar Connection and Disconnection

2004 ◽  
Vol 219 ◽  
pp. 11-28 ◽  
Author(s):  
Klaus G. Strassmeier
Keyword(s):  
Magnetic Field ◽  
Meridional Circulation ◽  
Magnetic Activity ◽  
Driving Mechanism ◽  
The Sun ◽  
Dynamo Action ◽  
Convective Envelope ◽  
Surface Rotation ◽  
Single Process ◽  
Solar Type

The study of stellar activity is now an almost classical astronomical topic. The first Ca ii-H&K observations were made a hundred years ago by Eberhard & Schwarzschild1 and many thousand papers were published after its rediscovery some three decades ago by O. C. Wilson. The complexity of the atmospheric and interior magnetic activity as observed on the Sun is hard, if not impossible, to extrapolate to solar-type stars. So far there is no solar twin found, despite that it appears that just a single process acts as the driving mechanism for activity in all atmospheric layers and partially even in the convective envelope: the dynamodriven magnetic field. In this paper, I will try to give examples where the solar analogy holds and where it is clearly not appropriate, putting some emphases on differential surface rotation and meridional circulation. I stress the importance of mapping stellar surfaces as fingerprints of the underlying dynamo action and directly measure surface magnetic fields.

Solar Rotation, Irradiance Changes and Climate

1994 ◽  
Vol 143 ◽  
pp. 244-251
Author(s):  
Elizabeth Nesme-Ribes ◽  
Dmitry Sokoloff ◽  
Robert Sadourny
Keyword(s):  
Magnetic Fields ◽  
Internal Rotation ◽  
Solar Rotation ◽  
Total Solar Irradiance ◽  
Magnetic Activity ◽  
Convective Envelope ◽  
Surface Rotation ◽  
Solar Type ◽  
Activity Cycles ◽  
The Impact

Magnetic activity cycles for solar-type stars are believed to originate from non-uniform internal rotation. To determine this depthwise angular velocity distribution, helioseismology is a valuable source of information. Surface rotation, as traced by sunspot motion, is a well-observed parameter with data going back to the beginning of the telescopic era. This long sunspot series can be used in understanding the behaviour of the Sun’s surface rotation, the connection with its internal rotation, and thereby its magnetic activity. Apparent solar diameter is another important parameter. This is related to the structure of the convective envelope and how it reacts to the presence of magnetic fields. Both these parameters are related to the solar output, and can provide a surrogate for total solar irradiance, by way of a theoretical modeling of the response of the convective zone to the emergence of periodic magnetic fields. The impact of solar variability on the terrestrial climate is also addressed.

scholarly journals Dynamo Processes Constrained by Solar and Stellar Observations

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.

scholarly journals Rotational Transport Processes

2004 ◽  
Vol 215 ◽  
pp. 336-345 ◽  
Author(s):  
Suzanne Talon
Keyword(s):  
Magnetic Field ◽  
Gravity Waves ◽  
Meridional Circulation ◽  
Chemical Transport ◽  
Transport Processes ◽  
Momentum Transport ◽  
The Sun ◽  
Physical Mechanisms ◽  
Self Consistent ◽  
Solar Type

In this review, I discuss physical mechanisms leading to momentum and chemical transport in stars. Various instabilities leading to turbulence are discussed. I then present a self-consistent description of rotational mixing under the action of turbulence and meridional circulation in 1D models. Limitations of the model are discussed, both in terms of an extra mechanism for momentum transport in the Sun and solar-type stars (magnetic field and/or gravity waves) and in terms of our understanding of turbulent properties.

scholarly journals A Quantitative Study of Magnetic Flux Transport on the Sun

1983 ◽  
Vol 102 ◽  
pp. 273-278 ◽  
Author(s):  
N.R. Sheeley ◽  
J.P. Boris ◽  
T.R. Young ◽  
C.R. DeVore ◽  
K.L. Harvey
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Quantitative Study ◽  
Differential Rotation ◽  
Meridional Circulation ◽  
Diffusion Constant ◽  
The Sun ◽  
Flux Transport ◽  
Field Reversal ◽  
Best Fit

A computational model, based on diffusion, differential rotation, and meridional circulation, has been developed to simulate the transport of magnetic flux on the Sun. Using Kitt Peak magnetograms as input, we have determined a best-fit diffusion constant by comparing the computed and observed fields at later times. Our value of 730 ± 250 km2/s is consistent with Leighton's (1964) estimate of 770–1540 km2/s and is significantly larger than Mosher's (1977) estimate of 200–400 km2/s. This suggests that diffusion may be fast enough to account for the observed polar magnetic field reversal without requiring a significant assist from meridional currents.

scholarly journals Stellar activity and rotation of the planet host Kepler-17 from long-term space-borne photometry

2019 ◽  
Vol 626 ◽  
pp. A38 ◽  
Author(s):  
A. F. Lanza ◽  
Y. Netto ◽  
A. S. Bonomo ◽  
H. Parviainen ◽  
A. Valio ◽  
...  
Keyword(s):  
Maximum Entropy ◽  
Differential Rotation ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Model Parameters ◽  
The Sun ◽  
Level Of Activity ◽  
Spot Model ◽  
Rotational Evolution ◽  
Solar Type

Context. The study of young Sun-like stars is fundamental to understanding the magnetic activity and rotational evolution of the Sun. Space-borne photometry by the Kepler telescope provides unprecedented datasets to investigate these phenomena in Sun-like stars. Aims. We present a new analysis of the entire Kepler photometric time series of the moderately young Sun-like star Kepler-17 accompanied by a transiting hot Jupiter. Methods. We applied a maximum-entropy spot model to the long-cadence out-of-transit photometry of the target to derive maps of the starspot filling factor versus the longitude and the time. These maps are compared to the spots occulted during transits to validate our reconstruction and derive information on the latitudes of the starspots. Results. We find two main active longitudes on the photosphere of Kepler-17, one of which has a lifetime of at least ∼1400 days although with a varying level of activity. The latitudinal differential rotation is of solar type, that is, with the equator rotating faster than the poles. We estimate a minimum relative amplitude ΔΩ/Ω between ∼0.08 ± 0.05 and 0.14 ± 0.05, our determination being affected by the finite lifetime of individual starspots and depending on the adopted spot model parameters. We find marginal evidence of a short-term intermittent activity cycle of ∼48 days and an indication of a longer cycle of 400−600 days characterized by an equatorward migration of the mean latitude of the spots as in the Sun. The rotation of Kepler-17 is likely to be significantly affected by the tides raised by its massive close-by planet. Conclusion. We confirm the reliability of maximum-entropy spot models to map starspots in young active stars and characterize the activity and differential rotation of this young Sun-like planetary host.

scholarly journals Solar and Stellar Dynamo Waves Under Asymptotic Investigation

1998 ◽  
Vol 167 ◽  
pp. 415-418
Author(s):  
Kirill M. Kuzanyan
Keyword(s):  
Magnetic Field ◽  
Temporal Distribution ◽  
Active Regions ◽  
Magnetic Activity ◽  
Dynamo Model ◽  
The Sun ◽  
Asymptotic Investigation ◽  
Spatio Temporal ◽  
Dynamo Wave ◽  
The Magnetic Field

AbstractThe main magnetic activity of the Sun can be visualised by Maunder butterfly diagrams which represent the spatio-temporal distribution of sunspots. Besides sunspots there are other tracers of magnetic activity, like filaments and active regions, which are observable over a wider latitudinal range of the Sun. Both these phenomena allow one to consider a complete picture of solar magnetic activity, which should be explained in the framework of one relatively simple model.A kinematic αѡ-dynamo model of the magnetic field’s generation in a thin convection shell with nonuniform helicity for large dynamo numbers is considered in the framework of Parker’s migratory dynamo. The obtained asymptotic solution of equations governing the magnetic field has a form of a modulated travelling dynamo wave. This wave propagates over the most latitudes of the solar hemisphere equatorwards, and the amplitude of the magnetic field first increases and then decreases with the propagation. Over the subpolar latitudes the dynamo wave reverses, there the dynamo wave propagates polewards and decays with latitude. Butterfly diagrams are plotted and analyzed.There is an attractive opportunity to develop a more quantitatively precise model taking into account helioseismological data on differential rotation and fitting the solar observational data on the magnetic field and turbulence, analyzing the helicity and the phase shift between toroidal and poloidal components of the field.

scholarly journals Mean field dynamo action in shear flows. I: fixed kinetic helicity

2020 ◽  
Vol 495 (4) ◽  
pp. 4557-4569 ◽  
Author(s):  
Naveen Jingade ◽  
Nishant K Singh
Keyword(s):  
Magnetic Field ◽  
Mean Field ◽  
Magnetic Activity ◽  
Turnover Time ◽  
Cycle Period ◽  
Dynamo Action ◽  
Navier Stokes Equation ◽  
Axisymmetric Mode ◽  
Dynamo Wave ◽  
Mean Field Dynamo

ABSTRACT We study mean field dynamo action in a background linear shear flow by employing pulsed renewing flows with fixed kinetic helicity and non-zero correlation time (τ). We use plane shearing waves in terms of time-dependent exact solutions to the Navier–Stokes equation as derived by Singh & Sridhar (2017). This allows us to self-consistently include the anisotropic effects of shear on the stochastic flow. We determine the average response tensor governing the evolution of mean magnetic field, and study the properties of its eigenvalues that yield the growth rate (γ) and the cycle period (Pcyc) of the mean magnetic field. Both, γ and the wavenumber corresponding to the fastest growing axisymmetric mode vary non-monotonically with shear rate S when τ is comparable to the eddy turnover time T, in which case, we also find quenching of dynamo when shear becomes too strong. When $\tau /T\sim {\cal O}(1)$, the cycle period (Pcyc) of growing dynamo wave scales with shear as Pcyc ∝ |S|−1 at small shear, and it becomes nearly independent of shear as shear becomes too strong. This asymptotic behaviour at weak and strong shear has implications for magnetic activity cycles of stars in recent observations. Our study thus essentially generalizes the standard αΩ (or α2Ω) dynamo as also the α effect is affected by shear and the modelled random flow has a finite memory.

scholarly journals Dynamo Action in Evolved Stars

1991 ◽  
Vol 130 ◽  
pp. 336-341
Author(s):  
David F. Gray
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Rotation Rate ◽  
Magnetic Activity ◽  
Dynamo Action ◽  
Rotation Rates ◽  
Evolved Stars ◽  
Solar Type ◽  
Limiting Value

AbstractEvolved stars tell us a great deal about dynamos. The granulation boundary shows us where solar-type convection begins. Since activity indicators also start at this boundary, it is a good bet that solar-type convection is an integral part of dynamo activity for all stars. The rotation boundary tells us where the magnetic fields of dynamos become effective in dissipating angular momentum, and rotation beyond the boundary tells us the limiting value needed for a dynamo to function. The observed uniqueness of rotation rates after the rotation boundary is crossed can be understood through the rotostat hypothesis. Quite apart from the reason for the unique rotation rate, its existence can be used to show that magnetic activity of giants is concentrated to the equatorial latitudes, as it is in the solar case. The coronal boundary in the H-R diagram is probably nothing more than a map of where rotation becomes too low to sustain dynamo activity.

scholarly journals On dynamo action in the giant star Pollux: first results

2013 ◽  
Vol 9 (S302) ◽  
pp. 363-364 ◽  
Author(s):  
Ana Palacios ◽  
Allan Sacha Brun
Keyword(s):  
Magnetic Field ◽  
Magnetic Field Intensity ◽  
Rotation Period ◽  
Dynamo Action ◽  
Convective Envelope ◽  
Giant Star ◽  
First Results ◽  
The One ◽  
3D Mhd Simulation ◽  
The Magnetic Field

AbstractWe present preliminary results of a 3D MHD simulation of the convective envelope of the giant star Pollux for which the rotation period and the magnetic field intensity have been measured from spectroscopic and spectropolarimetric observations. This giant is one of the first single giants with a detected magnetic field, and the one with the weakest field so far. Our aim is to understand the development and the action of the dynamo in its extended convective envelope.

scholarly journals Global magnetic cycles in rapidly rotating younger suns

2010 ◽  
Vol 6 (S273) ◽  
pp. 272-275 ◽  
Author(s):  
Nicholas J. Nelson ◽  
Benjamin P. Brown ◽  
Matthew K. Browning ◽  
Allan Sacha Brun ◽  
Mark S. Miesch ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Solar Rotation ◽  
Giant Cells ◽  
Temporal Variations ◽  
Opposite Polarity ◽  
Magnetic Activity ◽  
Dynamo Action ◽  
Magnetic Buoyancy ◽  
Magnetic Cycles

AbstractObservations of sun-like stars rotating faster than our current sun tend to exhibit increased magnetic activity as well as magnetic cycles spanning multiple years. Using global simulations in spherical shells to study the coupling of large-scale convection, rotation, and magnetism in a younger sun, we have probed effects of rotation on stellar dynamos and the nature of magnetic cycles. Major 3-D MHD simulations carried out at three times the current solar rotation rate reveal hydromagnetic dynamo action that yields wreaths of strong toroidal magnetic field at low latitudes, often with opposite polarity in the two hemispheres. Our recent simulations have explored behavior in systems with considerably lower diffusivities, achieved with sub-grid scale models including a dynamic Smagorinsky treatment of unresolved turbulence. The lower diffusion promotes the generation of magnetic wreaths that undergo prominent temporal variations in field strength, exhibiting global magnetic cycles that involve polarity reversals. In our least diffusive simulation, we find that magnetic buoyancy coupled with advection by convective giant cells can lead to the rise of coherent loops of magnetic field toward the top of the simulated domain.

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