Global magnetic cycles in rapidly rotating younger suns

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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Magnetic field and chromospheric activity evolution of HD 75332: a rapid magnetic cycle in an F star without a hot Jupiter

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3878 ◽

2020 ◽

Author(s):

E L Brown ◽

S C Marsden ◽

M W Mengel ◽

S V Jeffers ◽

I Millburn ◽

...

Keyword(s):

Magnetic Field ◽

Large Scale ◽

Magnetic Activity ◽

Doppler Imaging ◽

Shallow Convection ◽

Magnetic Cycle ◽

Field Polarity ◽

Chromospheric Activity ◽

Magnetic Cycles ◽

Hot Jupiter

Abstract Studying cool star magnetic activity gives an important insight into the stellar dynamo and its relationship with stellar properties, as well as allowing us to place the Sun’s magnetism in the context of other stars. Only 61 Cyg A (K5V) and τ Boo (F8V) are currently known to have magnetic cycles like the Sun’s, where the large-scale magnetic field polarity reverses in phase with the star’s chromospheric activity cycles. τ Boo has a rapid ∼240 d magnetic cycle, and it is not yet clear whether this is related to the star’s thin convection zone or if the dynamo is accelerated by interactions between τ Boo and its hot Jupiter. To shed light on this, we studied the magnetic activity of HD 75332 (F7V) which has similar physical properties to τ Boo and does not appear to host a hot Jupiter. We characterized its long term chromospheric activity variability over 53 yrs and used Zeeman Doppler Imaging to reconstruct the large-scale surface magnetic field for 12 epochs between 2007 and 2019. Although we observe only one reversal of the large-scale magnetic dipole, our results suggest that HD 75332 has a rapid ∼1.06 yr solar-like magnetic cycle where the magnetic field evolves in phase with its chromospheric activity. If a solar-like cycle is present, reversals of the large-scale radial field polarity are expected to occur at around activity cycle maxima. This would be similar to the rapid magnetic cycle observed for τ Boo, suggesting that rapid magnetic cycles may be intrinsic to late-F stars and related to their shallow convection zones.

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Dynamo generated field emergence through recurrent plasmoid ejections

Proceedings of the International Astronomical Union ◽

10.1017/s174392131101533x ◽

2010 ◽

Vol 6 (S273) ◽

pp. 256-260

Author(s):

Jörn Warnecke ◽

Axel Brandenburg

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Solar Atmosphere ◽

Large Scale ◽

Computational Domain ◽

Dynamo Action ◽

Layer System ◽

Forcing Function ◽

Magnetic Buoyancy ◽

The Magnetic Field

AbstractMagnetic buoyancy is believed to drive the transport of magnetic flux tubes from the convection zone to the surface of the Sun. The magnetic fields form twisted loop-like structures in the solar atmosphere. In this paper we use helical forcing to produce a large-scale dynamo-generated magnetic field, which rises even without magnetic buoyancy. A two layer system is used as computational domain where the upper part represents the solar atmosphere. Here, the evolution of the magnetic field is solved with the stress–and–relax method. Below this region a magnetic field is produced by a helical forcing function in the momentum equation, which leads to dynamo action. We find twisted magnetic fields emerging frequently to the outer layer, forming arch-like structures. In addition, recurrent plasmoid ejections can be found by looking at space–time diagrams of the magnetic field. Recent simulations in spherical coordinates show similar results.

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Turbulent dynamo action at low magnetic Reynolds number

Journal of Fluid Mechanics ◽

10.1017/s002211207000068x ◽

1970 ◽

Vol 41 (2) ◽

pp. 435-452 ◽

Cited By ~ 156

Author(s):

H. K. Moffatt

Keyword(s):

Magnetic Field ◽

Reynolds Number ◽

Large Scale ◽

Isotropic Turbulence ◽

Magnetic Energy ◽

Magnetic Reynolds Number ◽

Dynamo Action ◽

Ohmic Dissipation ◽

Magnetic Energy Density ◽

Magnetic Diffusivity

The effect of turbulence on a magnetic field whose length-scale L is initially large compared with the scale l of the turbulence is considered. There are no external sources for the field, and in the absence of turbulence it decays by ohmic dissipation. It is assumed that the magnetic Reynolds number Rm = u0l/λ (where u0 is the root-mean-square velocity and λ the magnetic diffusivity) is small. It is shown that to lowest order in the small quantities l/L and Rm, isotropic turbulence has no effect on the large-scale field; but that turbulence that lacks reflexional symmetry is capable of amplifying Fourier components of the field on length scales of order Rm−2l and greater. In the case of turbulence whose statistical properties are invariant under rotation of the axes of reference, but not under reflexions in a point, it is shown that the magnetic energy density of a magnetic field which is initially a homogeneous random function of position with a particularly simple spectrum ultimately increases as t−½exp (α2t/2λ3) where α(= O(u02l)) is a certain linear functional of the spectrum tensor of the turbulence. An analogous result is obtained for an initially localized field.

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Magnetic Fields in Galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311017546 ◽

2010 ◽

Vol 6 (S271) ◽

pp. 135-144

Author(s):

Ellen G. Zweibel

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Cosmic Time ◽

Origin And Evolution ◽

Cosmological Problem ◽

Background Turbulence ◽

Multicomponent Gas ◽

Magnetic Buoyancy ◽

Galactic Dynamos

AbstractThe origin and evolution of magnetic fields in the Universe is a cosmological problem. Although exotic mechanisms for magneotgenesis cannot be ruled out, galactic magnetic fields could have been seeded by magnetic fields from stars and accretion disks, and must be continuously regenerated due to the ongoing replacement of the interstellar medium. Unlike stellar dynamos, galactic dynamos operate in a multicomponent gas at low collisionality and high magnetic Prandtl number. Their background turbulence is highly compressible, the plasma β ~ 1, and there has been time for only a few large exponentiation times at large scale over cosmic time. Points of similarity include the importance of magnetic buoyancy, the large range of turbulent scales and tiny microscopic scales, and the coupling between the magnetic field and certain properties of the flow. Understanding the origin and maintenance of the large scale galactic magnetic field is the most challenging aspect of the problem.

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Variability in the Power Spectrum of Solar Five-Minute Oscillations

International Astronomical Union Colloquium ◽

10.1017/s0252921100095749 ◽

1983 ◽

Vol 66 ◽

pp. 411-425

Author(s):

Frank Hill ◽

Juri Toomre ◽

Laurence J. November

Keyword(s):

Large Scale ◽

Solar Rotation ◽

Temporal Frequency ◽

Power Spectra ◽

Giant Cells ◽

Spectral Lines ◽

Data Sets ◽

Data Set ◽

Temporal Sampling ◽

Velocity Changes

AbstractTwo-dimensional power spectra of solar five-minute oscillations display prominent ridge structures in (k, ω) space, where k is the horizontal wavenumber and ω is the temporal frequency. The positions of these ridges in k and ω can be used to probe temperature and velocity structures in the subphotosphere. We have been carrying out a continuing program of observations of five-minute oscillations with the diode array instrument on the vacuum tower telescope at Sacramento Peak Observatory (SPO). We have sought to establish whether power spectra taken on separate days show shifts in ridge locations; these may arise from different velocity and temperature patterns having been brought into our sampling region by solar rotation. Power spectra have been obtained for six days of observations of Doppler velocities using the Mg I λ5173 and Fe I λ5434 spectral lines. Each data set covers 8 to 11 hr in time and samples a region 256″ × 1024″ in spatial extent, with a spatial resolution of 2″ and temporal sampling of 65 s. We have detected shifts in ridge locations between certain data sets which are statistically significant. The character of these displacements when analyzed in terms of eastward and westward propagating waves implies that changes have occurred in both temperature and horizontal velocity fields underlying our observing window. We estimate the magnitude of the velocity changes to be on the order of 100 m s -1; we may be detecting the effects of large-scale convection akin to giant cells.

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Large-scale dynamo action of magnetized Taylor–Couette flows

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa293 ◽

2020 ◽

Vol 493 (1) ◽

pp. 1249-1260

Author(s):

G Rüdiger ◽

M Schultz

Keyword(s):

Magnetic Field ◽

Large Scale ◽

Mean Field ◽

Single Mode ◽

Eddy Diffusivity ◽

Magnetic Reynolds Number ◽

Dynamo Model ◽

Turbulence Energy ◽

Dynamo Action ◽

Taylor Couette

ABSTRACT A conducting Taylor–Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler–Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining α effect and eddy diffusivity. If both degenerated modes with m = ±1 are excited with the same power then the global α effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite α effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an α2 dynamo. Moreover, an αΩ dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm ≃ 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, ηT ≃ η. By basically lower or larger eddy diffusivities the dynamo instability is suppressed.

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Sun-like Stars: magnetic fields, cycles and exoplanets

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316003288 ◽

2015 ◽

Vol 11 (A29A) ◽

pp. 360-364

Author(s):

Rim Fares

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Observational Constraints ◽

The Sun ◽

Field Generation ◽

Large Scale Magnetic Field ◽

Planetary Orbits ◽

Activity Cycles ◽

Magnetic Cycles

AbstractIn Sun-like stars, magnetic fields are generated in the outer convective layers. They shape the stellar environment, from the photosphere to planetary orbits. Studying the large-scale magnetic field of those stars enlightens our understanding of the field properties and gives us observational constraints for field generation dynamo models. It also sheds light on how “normal” the Sun is among Sun-like stars. In this contribution, I will review the field properties of Sun-like stars, focusing on solar twins and planet hosting stars. I will discuss the observed large-scale magnetic cycles, compare them to stellar activity cycles, and link that to what we know about the Sun. I will also discuss the effect of large-scale stellar fields on exoplanets, exoplanetary emissions (e.g. radio), and habitability.

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Large Scale Solar Magnetic Fields: Temporal Variations

Symposium - International Astronomical Union ◽

10.1017/s0074180900182580 ◽

2004 ◽

Vol 219 ◽

pp. 552-556 ◽

Cited By ~ 4

Author(s):

R. Knaack ◽

J. O. Stenflo

Keyword(s):

Magnetic Field ◽

Time Series ◽

Large Scale ◽

Solar Magnetic Field ◽

Temporal Variations ◽

Solar Photosphere ◽

Solar Cycles ◽

Radial Magnetic Field ◽

Quasi Biennial Oscillation ◽

Harmonic Decomposition

We have investigated the temporal evolution of the solar magnetic field during solar cycles 20, 21 and 22 by means of spherical harmonic decomposition and subsequent time series analysis. A 33 yr and a 25 yr time series of daily magnetic maps of the solar photosphere, recorded at the Mt. Wilson and NSO/Kitt Peak observatories respectively, were used to calculate the spherical coefficients of the radial magnetic field. Fourier and wavelet analysis were then applied to deduce the temporal variations. We compare the results of the two datasets and present examples of zonal modes which show significant variations, e. g. with a period of approx. 2.0—2.5 years. We provide evidence that this quasi-biennial oscillation originates mainly from the southern hemisphere. Furthermore, we show that low degree modes with odd l — m exhibit periods of 29.2 and 28.1 days while modes with even l — m show a dominant period of 26.9 days. A resonant modal structure of the solar magnetic field (apart from the 22 yr cycle) has not been found.

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Investigating Magnetic Activity in the Galactic Centre by Global MHD Simulation

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316012461 ◽

2016 ◽

Vol 11 (S322) ◽

pp. 137-140

Author(s):

Takeru K. Suzuki ◽

Yasuo Fukui ◽

Kazufumi Torii ◽

Mami Machida ◽

Ryoji Matsumoto ◽

...

Keyword(s):

Magnetic Field ◽

Gas Flow ◽

Rotation Frequency ◽

Magnetic Activity ◽

Time Dependent ◽

Galactic Centre ◽

Dependent Manner ◽

Mhd Simulation ◽

Magnetic Buoyancy ◽

Bulge Region

AbstractBy performing a global magnetohydrodynamical (MHD) simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic centre (GC) region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches ≳ 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. In addition, Parker instability (magnetic buoyancy) creates vertical magnetic structure, which would correspond to observed molecular loops, and frequently excited vertical flows. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which would contribute to the outflow from the bulge.

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The Toroidal Magnetic Field inside the Sun

International Astronomical Union Colloquium ◽

10.1017/s0252921100079616 ◽

1991 ◽

Vol 130 ◽

pp. 187-189

Author(s):

V.N. Krivodubskij ◽

A.E. Dudorov ◽

A.A. Ruzmaikin ◽

T.V. Ruzmaikina

Keyword(s):

Magnetic Field ◽

Large Scale ◽

Convection Zone ◽

The Sun ◽

Poloidal Magnetic Field ◽

Radial Gradient ◽

The Core ◽

Large Scale Magnetic Field ◽

Magnetic Buoyancy ◽

The Magnetic Field

Analysis of the fine structure of the solar oscillations has enabled us to determine the internal rotation of the Sun and to estimate the magnitude of the large-scale magnetic field inside the Sun. According to the data of Duvall et al. (1984), the core of the Sun rotates about twice as fast as the solar surface. Recently Dziembowski et al. (1989) have showed that there is a sharp radial gradient in the Sun’s rotation at the base of the convection zone, near the boundary with the radiative interior. It seems to us that the sharp radial gradients of the angular velocity near the core of the Sun and at the base of the convection zone, acting on the relict poloidal magnetic field Br, must excite an intense toroidal field Bф, that can compensate for the loss of the magnetic field due to magnetic buoyancy.

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