How Good Is the Bipolar Approximation of Active Regions for Surface Flux Transport?

Yeates, Anthony R.

doi:10.1007/s11207-020-01688-y

Cited by 30 publications

(31 citation statements)

References 44 publications

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“…The database is clearly not ideal for this purpose. The BMR databases compiled more recently by Yeates et al (2007) and by Yeates (2020), although of excellent quality, are not suitable either as they tabulate the physical properties of each BMR at the time it crosses the central meridian, not when it newly emerges.…”

Section: Comparison Tomentioning

confidence: 99%

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The relationship between bipolar magnetic regions and their sunspots

Yeo

Solanki

Krivova

et al. 2021

A&A

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Context. The relationship between bipolar magnetic regions (BMRs) and their sunspots is an important property of the solar magnetic field, but it is not well constrained. One consequence is that it is a challenge for surface flux transport models (SFTMs) based on sunspot observations to determine the details of BMR emergence, which they require as input, from such data. Aims. We aimed to establish the relationship between the amount of magnetic flux in newly emerged BMRs and the area of the enclosed sunspots, and examine the results of its application to an established SFTM. Methods. Earlier attempts to constrain BMR magnetic flux were hindered by the fact that there is no extensive and reliable record of the magnetic and physical properties of newly emerged BMRs currently available. We made use of the empirical model of the relationship between the disc-integrated facular and network magnetic flux and the total surface coverage by sunspots reported in a recent study. The structure of the model is such that it enabled us to establish, from these disc-integrated quantities, an empirical relationship between the magnetic flux and sunspot area of individual newly emerged BMRs, circumventing the lack of any proper BMR database. Results. Applying the constraint on BMR magnetic flux derived here to an established SFTM retained its key features, in particular its ability to replicate various independent datasets and the correlation between the model output polar field at the end of each cycle and the observed strength of the following cycle. The SFTM output indicates that facular and network magnetic flux rises with increasing sunspot magnetic flux at a slowing rate such that it appears to gradually saturate. This is analogous to what earlier studies comparing disc-integrated quantities sensitive to the amount of faculae and network present to sunspot indices had reported. The activity dependence of the ratio of facular and network flux to sunspot flux is consistent with the findings of recent studies: although the Sun is faculae-dominated (such that its brightness is mostly positively correlated with activity), it is only marginally so as facular and network brightening and sunspot darkening appear to be closely balanced.

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Section: Comparison Tomentioning

confidence: 99%

“…Otherwise, SFTMs usually make use of solar magnetograms (e.g. Whitbread et al 2018;Virtanen et al 2018;Yeates 2020) or Ca II K spectroheliograms (e.g. Virtanen et al 2019) instead.…”

Section: Introductionmentioning

confidence: 99%

The relationship between bipolar magnetic regions and their sunspots

Yeo

Solanki

Krivova

et al. 2021

A&A

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“…We adopt a symmetric solar bipole model (Yeates 2020) that can be scaled to represent larger starspots. The centroids of the positive and negative polarities are placed at (𝑠 + , 𝜙 + ) and (𝑠 − , 𝜙 − ), respectively.…”

Section: Bipole Modelmentioning

confidence: 99%

Torus-Stable Zone Above Starspots

Sun,

Török,

DeRosa

2021

Preprint

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Whilst intense solar flares are almost always accompanied by a coronal mass ejection (CME), reports on stellar CMEs are rare, despite the frequent detection of stellar 'super flares'. The torus instability of magnetic flux ropes is believed to be one of the main driving mechanisms of solar CMEs. Suppression of the torus instability, due to a confining background coronal magnetic field that decreases sufficiently slowly with height, may contribute to the lack of stellar CME detection. Here we use the solar magnetic field as a template to estimate the vertical extent of this 'torus-stable zone' (TSZ) above a stellar active region. For an idealised potential field model comprising the fields of a local bipole (mimicking a pair of starspots) and a global dipole, we show that the upper bound of the TSZ increases with the bipole size, the dipole strength, and the source surface radius where the coronal field becomes radial. The boundaries of the TSZ depend on the interplay between the spots' and the dipole's magnetic fields, which provide the local-and global-scale confinement, respectively. They range from about half the bipole size to a significant fraction of the stellar radius. For smaller spots and an intermediate dipole field, a secondary TSZ arises at a higher altitude, which may increase the likelihood of 'failed eruptions'. Our results suggest that the low apparent CME occurrence rate on cool stars is, at least partially, due to the presence of extended TSZs.

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“…The random convective motions can be treated as a diffusion process (Leighton 1964), which can be implemented in surface flux transport models (SFTMs) as a random walk. Estimates of the diffusion rate D from observations typically indicate D = 250 km 2 s −1 (Jafarzadeh et al 2014), but higher values up to D = 500 km 2 s −1 have also been reported (Wang et al 2002;Yeates 2020).…”

Section: Introductionmentioning

confidence: 99%

“…Inclusion of the inflows into global SFTMs improve the match to the global dipole for the solar cycles 13 to 21 (Cameron & Schüssler 2012), and can account for the excess strength of the polar field at activity minimum in simulations, by effectively reducing the tilt angle of active regions (Cameron et al 2010;Yeates 2014). However, recently Yeates (2020) argued that this excess strength can be a result of the bipolar approximation of the active regions. On the other hand, Yeates (2014) found that the incorporation of the inflows (in form of a perturbation of the meridional flow) delays the dipole reversal times for solar cycle 23 with respect to the observed cycle.…”

Section: Introductionmentioning

confidence: 99%

Testing solar surface flux transport models in the first days after active region emergence

Gottschling,

Schunker,

Birch

et al. 2021

Preprint

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Context. Active regions (ARs) play an important role in the magnetic dynamics of the Sun. Solar surface flux transport models (SFTMs) are used to describe the evolution of the radial magnetic field at the solar surface. The models are kinematic in the sense that the radial component of the magnetic field behaves like passively advected corks. There is however uncertainty about using these models in the early stage of active region evolution, where dynamic effects might be important. Aims. We aim to test the applicability of SFTMs in the first days after the emergence of active regions by comparing them with observations. The models we employ range from passive evolution to models where the inflows around active regions are included. Methods. We simulate the evolution of the surface magnetic field of 17 emerging active regions using a local surface flux transport simulation. The regions are selected such that they do not form fully-fledged sunspots that exhibit moat flows. The simulation includes diffusion and advection by a velocity field, for which we test different models. For the flow fields, we use observed flows from local correlation tracking of solar granulation, as well as parametrizations of the inflows around active regions based on the gradient of the magnetic field. To evaluate our simulations, we measure the cross correlation between the observed and the simulated magnetic field, as well as the total unsigned flux of the ARs, over time. We also test the validity of our simulations by varying the starting time relative to the emergence of flux. Results. We find that the simulations using observed surface flows can reproduce the evolution of the observed magnetic flux. The effect of buffeting of the field by supergranulation can be described as a diffusion process. The SFTM is applicable after 90 % of the peak total unsigned flux of the active region has emerged. Diffusivities in the range between D = 250 to 720 km 2 s −1 are consistent with the evolution of the AR flux in the first five days after this time. We find that the converging flows around emerging active regions are not important for the evolution of the total flux of the AR in these first five days; their effect of increasing flux cancellation is balanced by the decrease of flux transport away from the AR.

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How Good Is the Bipolar Approximation of Active Regions for Surface Flux Transport?

Cited by 30 publications

References 44 publications

The relationship between bipolar magnetic regions and their sunspots

The relationship between bipolar magnetic regions and their sunspots

Torus-Stable Zone Above Starspots

Testing solar surface flux transport models in the first days after active region emergence

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