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 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.

scholarly journals Surface Abundances of Light Elements as Diagnostics of Transport Processes in the Sun and Solar-type stars

1990 ◽  
Vol 121 ◽  
pp. 437-448
Author(s):  
A. Baglin ◽  
Y. Lebreton
Keyword(s):  
Solar Surface ◽  
Rotation Velocity ◽  
Transport Processes ◽  
Physical Parameters ◽  
The Sun ◽  
Light Elements ◽  
Cosmic Abundance ◽  
Solar Type ◽  
The Moment ◽  
Slow Transport

AbstractObservations of the surface abundances of lithium, beryllium and helium-3 in the Sun and in solar-type stars of different ages should be interpreted in a coherent way. The abundance of lithium at the surface of a star decreases slowly with age; for stars of the same age it decreases with mass and a dependence on the rotation velocity is suggested. The solar surface lithium is depleted by a factor of 100 relative to the cosmic abundance while an He-3 enrichment of 15% at the solar surface during evolution is suggested.Observations favour the hypothesis of a slow transport process at work between the outer convective zone and the radiative interior of these stars. Orders of magnitude of the transport coefficient as well as its dependence upon the physical parameters can be inferred from surface abundances of light elements, but at the moment we are far from producing a completely consistent modelization.

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 Study of the effects of magnetic braking on the lithium abundances of the Sun and solar-type stars

2020 ◽  
Vol 496 (2) ◽  
pp. 1343-1354
Author(s):  
R Caballero Navarro ◽  
A García Hernández ◽  
A Ayala ◽  
J C Suárez
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Globular Clusters ◽  
Main Sequence ◽  
The Sun ◽  
Depletion Rate ◽  
Solar Type ◽  
Magnetic Braking ◽  
Physical Phenomena ◽  
The Impact

ABSTRACT The study of lithium (Li) surface abundance in the Sun and young stellar globular clusters which are seemingly anomalous in present-day scenarios, as well as the influence of rotation and magnetic braking (MB) on its depletion during pre-main sequence (PMS) and main sequence (MS). In this work, the effects of rotational mixing and of the rotational hydrostatic effects on Li abundances are studied by simulating several grids of PMS and MS rotating and non-rotating models. Those effects are combined with the additional impact of the MB (with magnetic field intensities ranging between 3.0 and 5.0 G). The data obtained from simulations are confronted by comparing different stellar parameters. The results show that the surface Li abundance for the Sun-like models at the end of the PMS and throughout the MS decreases when rotational effects are included, that is the Li depletion rate for rotating models is higher than for non-rotating ones. This effect is attenuated when the MB produced by a magnetic field is present. This physical phenomena impacts also the star effective temperature (Teff) and its location in the HR diagram. The impact of MB in Li depletion is sensitive to the magnetic field intensity: the higher it is, the lower the Li destruction. A direct link between the magnetic fields and the convective zone (CZ) size is observed: stronger magnetic fields produce shallower CZ’s. This result suggests that MB effect must be taken into consideration during PMS if we aim to reproduce Li abundances in young clusters.

Current understanding of mesospheric transport processes

1987 ◽  
Vol 323 (1575) ◽  
pp. 655-666 ◽  
Keyword(s):  
Gravity Waves ◽  
Chemical Constituents ◽  
Transport Processes ◽  
Small Scale ◽  
Dynamical Structure ◽  
Strongly Coupled ◽  
Physical Mechanisms ◽  
Waves And Tides ◽  
The Subject ◽  
And Diffusion

The chemistry and transport processes taking place in the mesosphere (approximately 50—85 km altitude) are strongly coupled. In recent years, the physical mechanisms influencing the transport processes of the mesosphere have become better understood, through both theoretical and observational studies. Perhaps most importantly, a theoretical framework for describing the momentum deposition and diffusion due to breaking small-scale gravity waves has been developed and shown to result in thermal, chemical and dynamical structure that closely resembles observations. The transport due to planetary waves and tides has also been the subject of study. In this paper, the transport processes influencing mesospheric distributions of chemical constituents are briefly reviewed.

scholarly journals Corotation resonances for gravity waves and their impact on angular momentum transport in stellar interiors

2013 ◽  
Vol 9 (S301) ◽  
pp. 377-378
Author(s):  
Lucie Alvan ◽  
Stéphane Mathis ◽  
Thibaut Decressin
Keyword(s):  
Angular Momentum ◽  
Gravity Waves ◽  
Stellar Evolution ◽  
Transport Processes ◽  
Momentum Transport ◽  
Critical Layer ◽  
Secondary Source ◽  
Angular Momentum Transport ◽  
Critical Layers ◽  
Stellar Interiors

AbstractGravity waves, which propagate in radiation zones, can extract or deposit angular momentum by radiative and viscous damping. Another process, poorly explored in stellar physics, concerns their direct interaction with the differential rotation and the related turbulence. In this work, we thus study their corotation resonances, also called critical layers, that occur where the Doppler-shifted frequency of the wave approaches zero. First, we study the adiabatic and non-adiabatic propagation of gravity waves near critical layers. Next, we derive the induced transport of angular momentum. Finally, we use the dynamical stellar evolution code STAREVOL to apply the results to the case of a solar-like star. The results depend on the value of the Richardson number at the critical layer. In the first stable case, the wave is damped. In the other unstable and turbulent case, the wave can be reflected and transmitted by the critical layer with a coefficient larger than one: the critical layer acts as a secondary source of excitation for gravity waves. These new results can have a strong impact on our understanding of angular momentum transport processes in stellar interiors along stellar evolution where strong gradients of angular velocity can develop.

scholarly journals The relation between stellar magnetic field geometry and chromospheric activity cycles – I. The highly variable field of ε Eridani at activity minimum

2017 ◽  
Vol 471 (1) ◽  
pp. L96-L100 ◽  
Author(s):  
S. V. Jeffers ◽  
S. Boro Saikia ◽  
J. R. Barnes ◽  
P. Petit ◽  
S. C. Marsden ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Activity Cycle ◽  
Doppler Imaging ◽  
The Sun ◽  
Chromospheric Activity ◽  
Large Scale Magnetic Field ◽  
Large Scale Field ◽  
Solar Type ◽  
Field Geometry

Abstract The young and magnetically active K dwarf ε Eridani exhibits a chromospheric activity cycle of about 3 yr. Previous reconstructions of its large-scale magnetic field show strong variations at yearly epochs. To understand how ε Eridani’s large-scale magnetic field geometry evolves over its activity cycle, we focus on high-cadence observations spanning 5 months at its activity minimum. Over this time-span, we reconstruct three maps of ε Eridani’s large-scale magnetic field using the tomographic technique of Zeeman–Doppler imaging. The results show that at the minimum of its cycle, ε Eridani’s large-scale field is more complex than the simple dipolar structure of the Sun and 61 Cyg A at minimum. Additionally, we observe a surprisingly rapid regeneration of a strong axisymmetric toroidal field as ε Eridani emerges from its S-index activity minimum. Our results show that all stars do not exhibit the same field geometry as the Sun, and this will be an important constraint for the dynamo models of active solar-type stars.

A Search for Dyson Spheres around Late-Type Stars in the Solar Neighborhood II

1997 ◽  
Vol 161 ◽  
pp. 707-709 ◽  
Author(s):  
Jun Jugaku ◽  
Shiro Nishimura
Keyword(s):  
Solar Neighborhood ◽  
The Sun ◽  
Late Type ◽  
Solar Type

AbstractWe continued our search for partial (incomplete) Dyson spheres associated with 50 solar-type stars (spectral classes F, G, and K) within 25 pc of the Sun. No candidate objects were found.

Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

2000 ◽  
Vol 179 ◽  
pp. 263-264
Author(s):  
K. Sundara Raman ◽  
K. B. Ramesh ◽  
R. Selvendran ◽  
P. S. M. Aleem ◽  
K. M. Hiremath
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Ultra Violet ◽  
Active Regions ◽  
Morphological Properties ◽  
Energy Storage And Conversion ◽  
The Sun ◽  
X Rays ◽  
The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

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