How to make the Sun look less like the Sun and more like a star?

AbstractSynoptic maps of the vector magnetic field have routinely been made available from stellar observations and recently have started to be obtained for the solar photospheric field. Although solar magnetic maps show a multitude of details, stellar maps are limited to imaging large-scale fields only. In spite of their lower resolution, magnetic field imaging of solar-type stars allow us to put the Sun in a much more general context. However, direct comparison between stellar and solar magnetic maps are hampered by their dramatic differences in resolution. Here, I present the results of a method to filter out the small-scale component of vector fields, in such a way that comparison between solar and stellar (large-scale) magnetic field vector maps can be directly made. This approach extends the technique widely used to decompose the radial component of the solar magnetic field to the azimuthal and meridional components as well, and is entirely consistent with the description adopted in several stellar studies. This method can also be used to confront synoptic maps synthesised in numerical simulations of dynamo and magnetic flux transport studies to those derived from stellar observations.

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Global magnetic fields: variation of solar minima

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312004723 ◽

2011 ◽

Vol 7 (S286) ◽

pp. 113-122

Author(s):

Andrey G. Tlatov ◽

Vladimir N. Obridko

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Sunspot Cycle ◽

Magnetic Activity ◽

Small Scale ◽

The Sun ◽

Large Scale Magnetic Field ◽

Scale Field ◽

Large Scale Field

AbstractThe topology of the large-scale magnetic field of the Sun and its role in the development of magnetic activity were investigated using Hα charts of the Sun in the period 1887-2011. We have considered the indices characterizing the minimum activity epoch, according to the data of large-scale magnetic fields. Such indices include: dipole-octopole index, area and average latitude of the field with dominant polarity in each hemisphere and others. We studied the correlation between these indices and the amplitude of the following sunspot cycle, and the relation between the duration of the cycle of large-scale magnetic fields and the duration of the sunspot cycle.The comparative analysis of the solar corona during the minimum epochs in activity cycles 12 to 24 shows that the large-scale magnetic field has been slow and steadily changing during the past 130 years. The reasons for the variations in the solar coronal structure and its relation with long-term variations in the geomagnetic indices, solar wind and Gleissberg cycle are discussed.We also discuss the origin of the large-scale magnetic field. Perhaps the large-scale field leads to the generation of small-scale bipolar ephemeral regions, which in turn support the large-scale field. The existence of two dynamos: a dynamo of sunspots and a surface dynamo can explain phenomena such as long periods of sunspot minima, permanent dynamo in stars and the geomagnetic field.

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Reconstruction of Sun-as-Star Magnetic Field Structure and Evolution Using Filament Observations

International Astronomical Union Colloquium ◽

10.1017/s0252921100048156 ◽

1998 ◽

Vol 167 ◽

pp. 493-496

Author(s):

Dmitri I. Ponyavin

Keyword(s):

Magnetic Field ◽

Solar Cycle ◽

Large Scale ◽

Solar Magnetic Field ◽

Close Association ◽

Field Structure ◽

The Sun ◽

Magnetic Field Structure ◽

Large Scale Magnetic Field ◽

Field Organization

AbstractA technique is used to restore the magnetic field of the Sun viewed as star from the filament distribution seen on Hα photographs. For this purpose synoptic charts of the large-scale magnetic field reconstructed by the McIntosh method have been compared with the Sun-asstar solar magnetic field observed at Stanford. We have established a close association between the Sun-as-star magnetic field and the mean magnetic field inferred from synoptic magnetic field maps. A filtering technique was applied to find correlations between the Sun-as-star and large-scale magnetic field distributions during the course of a solar cycle. The correlations found were then used to restore the Sun-as-star magnetic field and its evolution in the late 1950s and 1960s, when such measurements of the field were not being made. A stackplot display of the inferred data reveals large-scale magnetic field organization and evolution. Patterns of the Sun-as-star magnetic field during solar cycle 19 were obtained. The proposed technique can be useful for studying the solar magnetic field structure and evolution during times with no direct observations.

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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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Inverse Energy Cascade in Advanced MHD Turbulence (the RNG Method)

Symposium - International Astronomical Union ◽

10.1017/s0074180900174248 ◽

1993 ◽

Vol 157 ◽

pp. 255-261

Author(s):

N. Kleeorin ◽

I. Rogachevskii

Keyword(s):

Magnetic Field ◽

Large Scale ◽

Magnetic Field Effect ◽

Magnetic Reynolds Number ◽

Energy Cascade ◽

Small Scale ◽

Magnetic Fluctuations ◽

Mhd Turbulence ◽

Inverse Energy Cascade ◽

Large Scale Magnetic Field

The nonlinear (in terms of the large-scale magnetic field) effect of the modification of the magnetic force by an advanced small-scale magnetohydrodynamic (MHD) turbulence is considered. The phenomenon is due to the generation of magnetic fluctuations at the expense of hydrodynamic pulsations. It results in a decrease of the elasticity of the large-scale magnetic field.The renormalization group (RNG) method was employed for the investigation of the MHD turbulence at the large magnetic Reynolds number. It was found that the level of the magnetic fluctuations can exceed that obtained from the equipartition assumption due to the inverse energy cascade in advanced MHD turbulence.This effect can excite an instability of the large-scale magnetic field due to the energy transfer from the small-scale turbulent pulsations. This instability is an example of the inverse energy cascade in advanced MHD turbulence. It may act as a mechanism for the large-scale magnetic ropes formation in the solar convective zone and spiral galaxies.

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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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Solar Dynamo

Oxford Research Encyclopedia of Physics ◽

10.1093/acrefore/9780190871994.013.11 ◽

2020 ◽

Author(s):

Robert Cameron

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Flux ◽

Differential Rotation ◽

Large Scale ◽

Toroidal Field ◽

Solar Dynamo ◽

Small Scale ◽

The Sun ◽

The Magnetic Field

The solar dynamo is the action of flows inside the Sun to maintain its magnetic field against Ohmic decay. On small scales the magnetic field is seen at the solar surface as a ubiquitous “salt-and-pepper” disorganized field that may be generated directly by the turbulent convection. On large scales, the magnetic field is remarkably organized, with an 11-year activity cycle. During each cycle the field emerging in each hemisphere has a specific East–West alignment (known as Hale’s law) that alternates from cycle to cycle, and a statistical tendency for a North-South alignment (Joy’s law). The polar fields reverse sign during the period of maximum activity of each cycle. The relevant flows for the large-scale dynamo are those of convection, the bulk rotation of the Sun, and motions driven by magnetic fields, as well as flows produced by the interaction of these. Particularly important are the Sun’s large-scale differential rotation (for example, the equator rotates faster than the poles), and small-scale helical motions resulting from the Coriolis force acting on convective motions or on the motions associated with buoyantly rising magnetic flux. These two types of motions result in a magnetic cycle. In one phase of the cycle, differential rotation winds up a poloidal magnetic field to produce a toroidal field. Subsequently, helical motions are thought to bend the toroidal field to create new poloidal magnetic flux that reverses and replaces the poloidal field that was present at the start of the cycle. It is now clear that both small- and large-scale dynamo action are in principle possible, and the challenge is to understand which combination of flows and driving mechanisms are responsible for the time-dependent magnetic fields seen on the Sun.

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Observations of solar small-scale magnetic flux-sheet emergence

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834628 ◽

2019 ◽

Vol 622 ◽

pp. L12 ◽

Cited By ~ 2

Author(s):

C. E. Fischer ◽

J. M. Borrero ◽

N. Bello González ◽

A. J. Kaithakkal

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Flux Density ◽

Large Scale ◽

Magnetic Loop ◽

Field Vector ◽

Magnetic Flux Density ◽

Small Scale ◽

Flux Emergence ◽

Vector Information

Aims. Two types of flux emergence were recently discovered in numerical simulations: magnetic loops and magnetic sheet emergence. While magnetic loop emergence has been documented well in recent years using high-resolution full Stokes data from ground-based telescopes as well as satellites, magnetic sheet emergence is still an understudied process. We report here on the first clear observational evidence of a magnetic sheet emergence and characterise its development. Methods. Full Stokes spectra from the Hinode spectropolarimeter were inverted with the Stokes Inversion based on Response functions (SIR) code to obtain solar atmospheric parameters such as temperature, line-of-sight velocities, and full magnetic field vector information. Results. We analyse a magnetic flux emergence event observed in the quiet-Sun internetwork. After a large-scale appearance of linear polarisation, a magnetic sheet with horizontal magnetic flux density of up to 194 Mx cm−2 hovers in the low photosphere spanning a region of 2–3 arcsec. The magnetic field azimuth obtained through Stokes inversions clearly shows an organised structure of transversal magnetic flux density emerging. The granule below the magnetic flux sheet tears the structure apart leaving the emerged flux to form several magnetic loops at the edges of the granule. Conclusions. A large amount of flux with strong horizontal magnetic fields surfaces through the interplay of buried magnetic flux and convective motions. The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic flux at the foot points of the order of ∼1018 Mx. This is one to two orders of magnitude larger than what has been reported for small-scale magnetic loops. The convective flows feed the newly emerged flux into the pre-existing magnetic population on a granular scale.

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Magnetic Flux Transport in Protostellar Accretion Disks

International Astronomical Union Colloquium ◽

10.1017/s0252921100043451 ◽

1997 ◽

Vol 163 ◽

pp. 692-692

Author(s):

John Contopoulos ◽

Arieh Königl

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Accretion Disks ◽

Large Scale ◽

Turbulent Viscosity ◽

Flux Transport ◽

Disk Structure ◽

Large Scale Magnetic Field ◽

Field Lines ◽

Open Magnetic Field

AbstractCentrifugally driven winds from the surfaces of magnetized accretion disks are a leading candidate for the origin of bipolar outflows and have also been recognized as an attractive mechanism for removing the angular momentum of the accreted matter. The origin of the open magnetic field lines that thread the disk in this scenario is, however, still uncertain. One possibility is that the field lines are transported through the disk, but previous studies have shown that this process is inefficient in disks with turbulent viscosity and diffusivity. Here we examine whether the efficiency can be increased if angular momentum is transported from the disk surfaces by large-scale magnetic fields instead of radially by viscous stresses. In this picture, the removal of angular momentum is associated with the establishment of a global poloidal electric current driven by the radial EMF in the disc, and it does not necessarily need to involve super-Alfvénic outflows. We address this problem in the context of protostellar systems and present representative solutions of the time evolution of a resistive disk that is initially threaded by a uniform vertical field anchored at a large distance from its surfaces. We assume that the angular momentum transport in the disk is controlled by the large-scale magnetic field and take into account the influence of the field on the disk structure.

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The Large-Scale Magnetic Field and Sunspot Cycles

International Astronomical Union Colloquium ◽

10.1017/s025292110006440x ◽

2000 ◽

Vol 179 ◽

pp. 161-162

Author(s):

V. I. Makarov ◽

A. G. Tlatov

Keyword(s):

Magnetic Field ◽

Spherical Harmonics ◽

Legendre Polynomials ◽

Large Scale ◽

Magnetic Moments ◽

Radial Component ◽

Wolf Number ◽

Spherical Functions ◽

Large Scale Magnetic Field ◽

The Magnetic Field

Extended abstractWe report on the correlation between the large scale magnetic field and sunspot cycles during the last 80 years that was found by Makarovet al. (1999) and Makarov & Tlatov (2000) in H-αspherical harmonics of the large scale magnetic field for 1915–1999. The sum of intensities of the low modes 1 = 1 and 3, A(t), was used for comparison with the Wolf number, W(t). It was shown that the large scale magnetic field cycles, A(t), precede the sunspot cycles, W(t), by 5.5 years.Let us consider the behaviour in time of the harmonics with low numbers 1 = 1 and 1 = 3. The radial component B(r) of the magnetic field may be expanded in terms of the spherical harmonicswhereθandϕare the latitude and longitude,are Legendre polynomials andandare coefficients of expansion on the spherical functions.The magnetic moments of a dipole (1 = 1) and an octopole (1 = 3) are determined by the following equations:Let us enter the parameter describing their intensity,

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Advection/diffusion of large scale magnetic field in accretion disks

Nonlinear Processes in Geophysics ◽

10.5194/npg-16-77-2009 ◽

2009 ◽

Vol 16 (1) ◽

pp. 77-81 ◽

Cited By ~ 5

Author(s):

R. V. E. Lovelace ◽

G. S. Bisnovatyi-Kogan ◽

D. M. Rothstein

Keyword(s):

Magnetic Field ◽

Large Scale ◽

Turbulent Viscosity ◽

Pressure Ratio ◽

Rotational Instability ◽

Small Scale ◽

Magnetic Diffusivity ◽

Large Scale Magnetic Field ◽

Scale Field ◽

Large Scale Field

Abstract. Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr(z) which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P=viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work.

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