Numerical Simulations of Quiet Sun Magnetism: On the Contribution From a Small-Scale Dynamo

Rempel, Matthias

doi:10.1088/0004-637x/789/2/132

Cited by 264 publications

(479 citation statements)

References 43 publications

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“…Karak & Brandenburg 2015)? Answering these questions will not only improve our understanding of the solar cycle but also that of the underlying dynamo theories, as there is still a debate about the possibility of an acting surface dynamo being responsible for the formation of small-scale solar magnetic fields (see Petrovay & Szakaly 1993;Lites 2012;Rempel 2014).…”

Section: Introductionmentioning

confidence: 99%

Long-term trends of magnetic bright points

Utz

Müller

Thonhofer

et al. 2015

A&A

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Context. The Sun shows an activity cycle that is caused by its varying global magnetic field. During a solar cycle, sunspots, i.e. extended regions of strong magnetic fields, occur in activity belts that are slowly migrating from middle to lower latitudes, finally arriving close to the equator during the cycle maximum phase. While this and other facts about the strong extended magnetic fields have been well known for centuries, much less is known about the solar cycle evolution of small-scale magnetic fields. Thus the question arises if similar principles exist for small-scale magnetic fields. Aims. To address this question, we study magnetic bright points (MBPs) as proxies for such small-scale, kG solar magnetic fields. This study is based on a homogeneous data set that covers a period of eight years. The number of detected MBPs versus time is analysed to find out if there is an activity cycle for these magnetic features too and, if so, how it is related to the sunspot cycle.Methods. An automated MBP identification algorithm was applied to the synoptic Hinode/SOT G-band data over the period November 2006 to August 2014, i.e. covering the decreasing phase of Cycle 23 and the rise, maximum, and early decrease of Cycle 24. This data set includes, at the moment of investigation, a total of 4162 images, with about 2.9 million single MBP detections. Results. After a careful preselection and monthly median filtering of the data, the investigation revealed that the number of MBPs close to the equator is coupled to the global solar cycle but shifted in time by about 2.5 yr. Furthermore, the instantaneous number of detected MBPs depends on the hemisphere, with one hemisphere being more prominent, i.e. showing a higher number of MBPs. After the end of Cycle 23 and at the starting point of Cycle 24, the more active hemisphere changed from south to north. Clear peaks in the detected number of MBPs are found at latitudes of about ±7 • , in congruence with the positions of the sunspot belts at the end of the solar cycle. Conclusions. These findings suggest that there is indeed a coupling between the activity of MBPs close to the equator with the global magnetic field. The results also indicate that a significant fraction of the magnetic flux that is visible as MBPs close to the equator originates from the sunspot activity belts. However, even during the minimum of MBP activity, a percentage as large as 60% of the maximum number of detected MBPs has been observed, which may be related to solar surface dynamo action.

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Section: Introductionmentioning

confidence: 99%

Long-term trends of magnetic bright points

Utz

Müller

Thonhofer

et al. 2015

A&A

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“…Although they found in some cases small-scale dynamo action, these simulations are highly idealized and far from the real Sun. Subsequently, Vögler et al (2005; see also Vögler & Schüssler 2007;Pietarila Graham et al 2010;Rempel 2014) have gone one step further to make these models more realistic by including radiation and ionization. However, these simulations are local in the sense that they cannot study the effects of the global dynamo-generated large-scale field on the small-scale field.…”

Section: Introductionmentioning

confidence: 99%

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

Karak

Brandenburg

2015

ApJ

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The small-scale magnetic field is ubiquitous at the solar surface-even at high latitudes. From observations we know that this field is uncorrelated (or perhaps even weakly anticorrelated) with the global sunspot cycle. Our aim is to explore the origin, and particularly the cycle dependence, of such a phenomenon using three-dimensional dynamo simulations. We adopt a simple model of a turbulent dynamo in a shearing box driven by helically forced turbulence. Depending on the dynamo parameters, large-scale (global) and small-scale (local) dynamos can be excited independently in this model. Based on simulations in different parameter regimes, we find that, when only the large-scale dynamo is operating in the system, the small-scale magnetic field generated through shredding and tangling of the large-scale magnetic field is positively correlated with the global magnetic cycle. However, when both dynamos are operating, the small-scale field is produced from both the small-scale dynamo and the tangling of the large-scale field. In this situation, when the large-scale field is weaker than the equipartition value of the turbulence, the small-scale field is almost uncorrelated with the large-scale magnetic cycle. On the other hand, when the large-scale field is stronger than the equipartition value, we observe an anticorrelation between the smallscale field and the large-scale magnetic cycle. This anticorrelation can be interpreted as a suppression of the smallscale dynamo. Based on our studies we conclude that the observed small-scale magnetic field in the Sun is generated by the combined mechanisms of a small-scale dynamo and tangling of the large-scale field.

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“…Another representation of the thermal and magnetic structure of the quiet solar photosphere is provided by magnetoconvection simulations with zero net magnetic flux, such as those of Vögler & Schüssler (2007) and Rempel (2014). These simulations show a complex small-scale magnetic field that results from dynamo amplification of a weak seed field.…”

Section: Introductionmentioning

confidence: 99%

The impact of surface dynamo magnetic fields on the solar iron abundance

Shchukina

Bueno

2015

A&A

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Most chemical abundance determinations ignore that the solar photosphere is significantly magnetized by the ubiquitous presence of a small-scale magnetic field. A recent investigation has suggested that there should be a significant impact on the derived iron abundance, owing to the magnetically induced changes on the photospheric temperature and density structure (indirect effect). The three-dimensional (3D) photospheric models used in that investigation have non-zero net magnetic flux values and stem from magnetoconvection simulations without small-scale dynamo action. Here we address the same problem by instead using 3D models of the quiet solar photosphere that result from a state-of-the-art magneto-convection simulation with small-scale dynamo action, where the net magnetic flux is zero. One of these 3D models has negligible magnetization, while the other is characterized by a mean field strength of 160 Gauss in the low photosphere. With such 3D models we carried out spectral synthesis for a large set of Fe i lines to derive abundance corrections, taking the above-mentioned indirect effect and the Zeeman broadening of the intensity profiles (direct effect) into account. We conclude that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo, then its impact on the determination of the solar iron abundance is negligible.

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Numerical Simulations of Quiet Sun Magnetism: On the Contribution From a Small-Scale Dynamo

Cited by 264 publications

References 43 publications

Long-term trends of magnetic bright points

Long-term trends of magnetic bright points

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

The impact of surface dynamo magnetic fields on the solar iron abundance

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