Variations in the Sun’s Meridional Flow over a Solar Cycle

Hathaway, David H.; Rightmire, Lisa

doi:10.1126/science.1181990

Cited by 239 publications

(265 citation statements)

References 23 publications

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“…The meridional flow results often depend on the method used to derive them, e.g., Doppler measurements, magnetic feature tracking and helioseismic inversions of different kinds (global modes, ring diagrams). Surface Doppler (Ulrich, 2010) and helioseismic (Basu and Antia, 2010) meridional flow measurements generally agree with each other, having peak speeds at low latitudes around 25 ∘ on average, whereas magnetic feature tracking meridional flow measurements (Hathaway and Rightmire, 2010) have much lower values peaking at higher latitudes, around 50 ∘ . Magnetic feature tracking methods give a peak at higher latitudes because they do not separate the bulk fluid flow from the effect of supergranular diffusion.…”

Section: Unusual Cycle 23 Minimumsupporting

confidence: 54%

“…The meridional flow speed measurements of Ulrich (2010), Basu and Antia (2010) and Hathaway and Rightmire (2011) do not show evidence of significantly faster flows at active latitudes during cycle 23 than during previous cycles. Moreover, Basu and Antia (2010) concluded that the flow speed variation reported by Hathaway and Rightmire (2010) was associated with a flow pattern that migrated equatorward with the magnetic activity belts. When this pattern was removed from their data, the speed variations vanished.…”

Section: Unusual Cycle 23 Minimummentioning

confidence: 99%

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Solar Magnetism in the Polar Regions

Petrie

2015

Living Rev. Sol. Phys.

100

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This review describes observations of the polar magnetic fields, models for the cyclical formation and decay of these fields, and evidence of their great influence in the solar atmosphere. The polar field distribution dominates the global structure of the corona over most of the solar cycle, supplies the bulk of the interplanetary magnetic field via the polar coronal holes, and is believed to provide the seed for the creation of the activity cycle that follows. A broad observational knowledge and theoretical understanding of the polar fields is therefore an essential step towards a global view of solar and heliospheric magnetic fields. Analyses of both high-resolution and long-term synoptic observations of the polar fields are summarized. Models of global flux transport are reviewed, from the initial phenomenological and kinematic models of Babcock and Leighton to present-day attempts to produce time-dependent maps of the surface magnetic field and to explain polar field variations, including the weakness of the cycle 23 polar fields. The relevance of the polar fields to solar physics extends far beyond the surface layers from which the magnetic field measurements usually derive. As well as discussing the polar fields' role in the interior as seed fields for new solar cycles, the review follows their influence outward to the corona and heliosphere. The global coronal magnetic structure is determined by the surface magnetic flux distribution, and is dominated on large scales by the polar fields. We discuss the observed effects of the polar fields on the coronal hole structure, and the solar wind and ejections that travel through the atmosphere. The review concludes by identifying gaps in our knowledge, and by pointing out possible future sources of improved observational information and theoretical understanding of these fields. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

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Section: Unusual Cycle 23 Minimumsupporting

confidence: 54%

Section: Unusual Cycle 23 Minimummentioning

confidence: 99%

Solar Magnetism in the Polar Regions

Petrie

2015

Living Rev. Sol. Phys.

100

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“…Furthermore, the tilt angles show a very large scatter, possibly due to the convective buffeting of rising flux tubes in the upper layers of the convection zone (Longcope and Choudhuri 2002). c) meridional flow: significant variability of the meridional flow has been detected during cycle 23 (Chou and Dai 2001;Gizon 2004;Hathaway and Rightmire 2010;González Hernández et al 2010). We considered variations of the meridional flow, of the mean sunspot group tilt angle, and of sunspot number during cycle 23 as potential causes of the low polar field during that cycle and used our SFTM to determine which changes of these properties in cycle 23 would be required to reproduce the observations.…”

Section: Possible Cause Of the Weak Polar Field In Cycle 23mentioning

confidence: 99%

Can Surface Flux Transport Account for the Weak Polar Field in Cycle 23?

et al. 2011

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To reproduce the weak magnetic field on the polar caps of the Sun observed during the declining phase of cycle 23 poses a challenge to surface flux transport models since this cycle has not been particularly weak. We use a well-calibrated model to evaluate the parameter changes required to obtain simulated polar fields and open flux that are consistent with the observations. We find that the low polar field of cycle 23 could be reproduced by an increase of the meridional flow by 55% in the last cycle. Alternatively, a decrease of the mean tilt angle of sunspot groups by 28% would also lead to a similarly low polar field, but cause a delay of the polar field reversals by 1.5 years in comparison to the observations.

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“…However, attempts to measuring the MC velocity using small magnetic structures as tracers of the flow (Komm et al 1993, Hathaway & Rightmire 2010) reveal systematic differences from these results. Compared to Doppler measurements (Ulrich 2010) magnetic feature tracking (MFT) speeds can be as much as ≈ 30% smaller.…”

Section: Introductionmentioning

confidence: 81%

Magnetic feature tracking, what determines the speed?

2011

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Abstract. Recent observations revealed that small magnetic elements abundant at the solar surface move poleward with a velocity which seems to be lower than the plasma velocity U . Guerrero et al. (2011) explained this discrepancy as a consequence of diffusive spreading of the magnetic elements due to a positive radial gradient of |U θ |. As the gradient's sign (inferred by local helioseismology) is still unclear, cases with a negative gradient are studied in this paper. Under this condition, the velocity of the magnetic tracers turns out to be larger than the plasma velocity, in disagreement with the observations. Alternative mechanisms for explaining them independently are proposed. For the turbulent magnetic pumping it is shown that it has to be unrealistically strong to reconcile the model with the observations.

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Variations in the Sun’s Meridional Flow over a Solar Cycle

Cited by 239 publications

References 23 publications

Solar Magnetism in the Polar Regions

Solar Magnetism in the Polar Regions

Can Surface Flux Transport Account for the Weak Polar Field in Cycle 23?

Magnetic feature tracking, what determines the speed?

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