Average motion of emerging solar active region polarities

Schunker, Hannah; Baumgartner, Christian; Birch, A. C.; Cameron, R. H.; Braun, D. C.; Gizon, Laurent

doi:10.1051/0004-6361/201937322

Cited by 22 publications

(28 citation statements)

References 38 publications

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“…These observation results are in general agreement with those of Kosovichev and Stenflo (2008). However, Schunker et al (2020) found it is still surprising that the active regions emerge with an east-west alignment since the thin flux tube model predicts a systematic tilt angle has developed at the apex of the emerging loop. With a simple model calculation (Apendix C Schunker et al 2020), they found that the observed average north-south separation motion of the polarities during the emergence is not consistent with an emerging loop with an initial constant tilt at the apex.…”

Section: Active Region Tiltssupporting

confidence: 87%

“…Using the line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamic Observatory (SDO), Schunker et al (2020) have measured the evolution of the tilt angles of 153 emerging active regions. It is found that for the beginning phase (Phase 1) of the flux emergence (during which the polarity separation speed is increasing, lasting about 0.5 days after the time of emergence), the active region polarities are on average east-west aligned with a zero mean tilt angle.…”

Section: Active Region Tiltsmentioning

confidence: 99%

“…• The results of the thin flux tube simulations of rising flux tubes through the solar convective envelope (Section 5.1) have been updated with the works by Weber et al that incorporate the influence of the giant-cell convection and the mean flows on the motion of the rising flux tube. Results from the recent observational study of the evolution of active region tilt angle during flux emergence by Schunker et al (2020) is added in Section 5.1.2. Section 5.1.5 has been deleted.…”

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confidence: 99%

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Magnetic fields in the solar convection zone

Fan

2021

Living Rev Sol Phys

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It has been a prevailing picture that active regions on the solar surface originate from a strong toroidal magnetic field stored in the overshoot region at the base of the solar convection zone, generated by a deep seated solar dynamo mechanism. This article reviews the studies in regard to how the toroidal magnetic field can destabilize and rise through the convection zone to form the observed solar active regions at the surface. Furthermore, new results from the global simulations of the convective dynamos, and from the near-surface layer simulations of active region formation, together with helioseismic investigations of the pre-emergence active regions, are calling into question the picture of active regions as buoyantly rising flux tubes originating from the bottom of the convection zone. This article also gives a review on these new developments.

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Section: Active Region Tiltssupporting

confidence: 87%

Section: Active Region Tiltsmentioning

confidence: 99%

mentioning

confidence: 99%

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Magnetic fields in the solar convection zone

Fan

2021

Living Rev Sol Phys

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“…The converging flow toward the center of the active region as well as the prograde flow at the position of the leading polarity around the time of emergence are both present in all four subsamples. Joy's law is clearly visible after emergence, consistent with Schunker et al (2020), who showed that active regions on average emerge east-west aligned, and that Joy's law becomes evident two days after emergence. The tilt angle increases toward the higher-latitude subsamples and in time.…”

Section: Flows As a Function Of Time And Latitudesupporting

confidence: 86%

Evolution of solar surface inflows around emerging active regions

Gottschling

Schunker

Birch

et al. 2021

A&A

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Context. Solar active regions are associated with Evershed outflows in sunspot penumbrae, moat outflows surrounding sunspots, and extended inflows surrounding active regions. Extended inflows have been identified around established active regions with various methods. The evolution of these inflows and their dependence on active region properties as well as their effect on the global magnetic field are not yet understood. Aims. We aim to understand the evolution of the average inflows around emerging active regions and to derive an empirical model for these inflows. We expect that this can be used to better understand how the inflows act on the diffusion of the magnetic field in active regions. Methods. We analyzed horizontal flows at the surface of the Sun using local correlation tracking of solar granules observed in continuum images of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We measured average flows of a sample of 182 isolated active regions up to seven days before and after their emergence onto the solar surface with a cadence of 12 h. About half of the active regions in the sample developed sunspots with moat flows in addition to the surrounding inflows. We investigated the average inflow properties with respect to active region characteristics of total flux and latitude. We fit a model to these observed inflows for a quantitative analysis. Results. We find that converging flows of about 20–30 m s−1 are first visible one day prior to emergence, in agreement with recent results. These converging flows are present regardless of the active region properties of latitude or flux. We confirm a recently found prograde flow of about 40 m s−1 at the leading polarity during emergence. We find that the time after emergence when the latitudinal inflows increase in amplitude depends on the flux of the active region, ranging from one to four days after emergence and increasing with flux. The largest extent of the inflows is up to about 7 ± 1° away from the center of the active region within the first six days after emergence. The inflow velocities have amplitudes of about 50 m s−1.

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“…During the emergence of active regions, their magnetic polarities move apart, and develop a tilt angle, with the leading polarity closer to the equator than the trailing polarity (e.g. Schunker et al 2020, for a review see van Driel-Gesztelyi & Green 2015). This is consistent with the footpoints of the flux being connected to the subsurface field, and separating due to the action of magnetic tension and drag force (Chen et al 2017;Schunker et al 2019).…”

Section: Introductionmentioning

confidence: 63%

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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Average motion of emerging solar active region polarities

Cited by 22 publications

References 38 publications

Magnetic fields in the solar convection zone

Magnetic fields in the solar convection zone

Evolution of solar surface inflows around emerging active regions

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

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