Mesogranulation and small-scale dynamo action in the quiet Sun

AbstractIn the quiet Sun, convective motions form a characteristic granular pattern, with broad upflows enclosed by a network of narrow downflows. Magnetic fields tend to accumulate in the intergranular lanes, forming localised flux concentrations. One of the most plausible explanations for the appearance of these quiet Sun magnetic features is that they are generated and maintained by dynamo action resulting from the local convective motions at the surface of the Sun. Motivated by this idea, we describe high resolution numerical simulations of nonlinear dynamo action in a (fully) compressible, non-rotating layer of electrically-conducting fluid. The dynamo properties depend crucially upon various aspects of the fluid. For example, the magnetic Reynolds number (Rm) determines the initial growth rate of the magnetic energy, as well as the final saturation level of the dynamo in the nonlinear regime. We focus particularly upon the ways in which the Rm-dependence of the dynamo is influenced by the level of stratification within the domain. Our results can be related, in a qualitative sense, to solar observations.

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Magnetic power spectrum in the undisturbed solar photosphere

Acta Astrophysica Taurica ◽

10.31059/aat.vol1.iss1.pp1-5 ◽

2020 ◽

Vol 1 (1) ◽

pp. 1-5

Author(s):

Valentina Abramenko ◽

Olga Kutsenko

Keyword(s):

Power Spectrum ◽

Power Spectra ◽

Active Regions ◽

Reliable Estimate ◽

Small Scale ◽

Solar Photosphere ◽

Dynamo Action ◽

Quiet Sun ◽

Turbulent Dynamo ◽

Wide Range

Using the magnetic field data obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), an investigation of magnetic power spectra in the undisturbed solar photosphere was performed. The results are as follows. 1) To get a reliable estimate of a magnetic power spectrum from the uniformly distributed quiet-sun magnetic flux, a sample pattern of no less than 300 pixels length should be adopted. With smaller patterns, energy on all observable scales might be overestimated. 2) For patterns of different magnetic intensity (e.g., a coronal hole, a quiet-sun area, an area of supergranulation), the magnetic power spectra in a range of (2.5-10) Mm exhibit very close spectral indices of about -1. The observed spectrum is more shallow than the Kolmogorov-type spectrum (with a slope of -5/3) and it differs from steep spectra of active regions. Such a shallow spectrum cannot be explained by the only direct Kolmogorov’s cascade, but it can imply a small-scale turbulent dynamo action in a wide range of scales: from tens of megameters down to at least 2.5 Mm. On smaller scales, the HMI/SDO data do not allow us to reliably derive the shape of the spectrum. 3) Data make it possible to conclude that a uniform mechanism of the small-scale turbulent dynamo is at work all over the solar surface outside active regions.

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Global MHD phenomena and their importance for stellar surfaces

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311015146 ◽

2010 ◽

Vol 6 (S273) ◽

pp. 141-147

Author(s):

Rainer Arlt

Keyword(s):

Magnetic Fields ◽

Mean Field ◽

Momentum Equation ◽

Small Scale ◽

Dynamo Action ◽

Complex Activity ◽

Mhd Instabilities ◽

Stellar Magnetic Fields ◽

Mean Field Dynamo

AbstractThis review is an attempt to elucidate MHD phenomena relevant for stellar magnetic fields. The full MHD treatment of a star is a problem which is numerically too demanding. Mean-field dynamo models use an approximation of the dynamo action from the small-scale motions and deliver global magnetic modes which can be cyclic, stationary, axisymmetric, and non-axisymmetric. Due to the lack of a momentum equation, MHD instabilities are not visible in this picture. However, magnetic instabilities must set in as a result of growing magnetic fields and/or buoyancy. Instabilities deliver new timescales, saturation limits and topologies to the system probably providing a key to the complex activity features observed on stars.

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Transient brightenings in the quiet Sun detected by ALMA at 3 mm

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037810 ◽

2020 ◽

Vol 638 ◽

pp. A62 ◽

Cited By ~ 4

Author(s):

A. Nindos ◽

C. E. Alissandrakis ◽

S. Patsourakos ◽

T. S. Bastian

Keyword(s):

Solar Atmosphere ◽

Power Law ◽

Small Scale ◽

Solar Chromosphere ◽

Average Intensity ◽

Quiet Sun ◽

Brightness Temperatures ◽

Weak Preference ◽

Thermal Origin ◽

First Time

Aims. We investigate transient brightenings, that is, weak, small-scale episodes of energy release, in the quiet solar chromosphere; these episodes can provide insights into the heating mechanism of the outer layers of the solar atmosphere. Methods. Using Atacama Large Millimeter/submillimeter Array (ALMA) observations, we performed the first systematic survey for quiet Sun transient brightenings at 3 mm. Our dataset included images of six 87″ × 87″ fields of view of the quiet Sun obtained with angular resolution of a few arcsec at a cadence of 2 s. The transient brightenings were detected as weak enhancements above the average intensity after we removed the effect of the p-mode oscillations. A similar analysis, over the same fields of view, was performed for simultaneous 304 and 1600 Å data obtained with the Atmospheric Imaging Assembly. Results. We detected 184 3 mm transient brightening events with brightness temperatures from 70 K to more than 500 K above backgrounds of ∼7200 − 7450 K. All events showed light curves with a gradual rise and fall, strongly suggesting a thermal origin. Their mean duration and maximum area were 51.1 s and 12.3 Mm2, respectively, with a weak preference of appearing at network boundaries rather than in cell interiors. Both parameters exhibited power-law behavior with indices of 2.35 and 2.71, respectively. Only a small fraction of ALMA events had either 304 or 1600 Å counterparts but the properties of these events were not significantly different from those of the general population except that they lacked their low-end energy values. The total thermal energies of the ALMA transient brightenings were between 1.5 × 1024 and 9.9 × 1025 erg and their frequency distribution versus energy was a power law with an index of 1.67 ± 0.05. We found that the power per unit area provided by the ALMA events could account for only 1% of the chromospheric radiative losses (10% of the coronal ones). Conclusions. We were able to detect, for the first time, a significant number of weak 3 mm quiet Sun transient brightenings. However, their energy budget falls short of meeting the requirements for the heating of the upper layers of the solar atmosphere and this conclusion does not change even if we use the least restrictive criteria possible for the detection of transient brightenings.

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Solar magneto-convection

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313002329 ◽

2012 ◽

Vol 8 (S294) ◽

pp. 95-106 ◽

Cited By ~ 4

Author(s):

Manfred Schüssler

Keyword(s):

Active Regions ◽

Small Scale ◽

Solar Photosphere ◽

Surface Layers ◽

Dynamo Action ◽

Near Surface ◽

Wide Range ◽

Recent Developments ◽

Mixed Polarity ◽

The Magnetic Field

AbstractAn overview is given about recent developments and results of comprehensive simulations of magneto-convective processes in the near-surface layers and photosphere of the Sun. Simulations now cover a wide range of phenomena, from whole active regions, over individual sunspots and pores, magnetic flux concentrations and vortices in intergranular lanes, down to the intricate mixed-polarity structure of the magnetic field generated by small-scale dynamo action. The simulations in concert with high-resolution observations have provided breakthroughs in our understanding of the structure and dynamics of the magnetic fields in the solar photosphere.

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High Angular Resolution Stokes V Spectra in Penumbrae

International Astronomical Union Colloquium ◽

10.1017/s0252921100029079 ◽

1993 ◽

Vol 141 ◽

pp. 192-195

Author(s):

J. Sánchez Almeida ◽

V. Martínez Pillet ◽

J. Trujillo Bueno ◽

B.W. Lites

Keyword(s):

Angular Resolution ◽

Small Scale ◽

High Angular Resolution ◽

Quiet Sun ◽

Continuum Intensity

AbstractStokes I and V profiles of Fel 630.1 nm and Fel 630.2 nm observed in a sunspot with an angular resolution ≥0.5” are analyzed. We find asymmetric V-profiles whose shapes are in agreement with previous determinations having lower angular resolution (~5”). We also find a correlation between penumbral small scale variations of continuum intensity and velocity. Although it involves smaller velocities, such correlation resembles that existing in the quiet sun as a consequence of convection.

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Emergence of small-scale magnetic flux in the quiet Sun

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936778 ◽

2020 ◽

Vol 633 ◽

pp. A67 ◽

Cited By ~ 2

Author(s):

I. Kontogiannis ◽

G. Tsiropoula ◽

K. Tziotziou ◽

C. Gontikakis ◽

C. Kuckein ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Morphological Characteristics ◽

Small Scale ◽

X Rays ◽

Coronal Emission ◽

Quiet Sun ◽

Temperature Regimes ◽

Multi Wavelength ◽

Modelling Studies

Context. We study the evolution of a small-scale emerging flux region (EFR) in the quiet Sun, from its emergence in the photosphere to its appearance in the corona and its decay. Aims. We track processes and phenomena that take place across all atmospheric layers; we explore their interrelations and compare our findings with those from recent numerical modelling studies. Methods. We used imaging as well as spectral and spectropolarimetric observations from a suite of space-borne and ground-based instruments. Results. The EFR appears in the quiet Sun next to the chromospheric network and shows all morphological characteristics predicted by numerical simulations. The total magnetic flux of the region exhibits distinct evolutionary phases, namely an initial subtle increase, a fast increase with a Co-temporal fast expansion of the region area, a more gradual increase, and a slow decay. During the initial stages, fine-scale G-band and Ca II H bright points coalesce, forming clusters of positive- and negative-polarity in a largely bipolar configuration. During the fast expansion, flux tubes make their way to the chromosphere, pushing aside the ambient magnetic field and producing pressure-driven absorption fronts that are visible as blueshifted chromospheric features. The connectivity of the quiet-Sun network gradually changes and part of the existing network forms new connections with the newly emerged bipole. A few minutes after the bipole has reached its maximum magnetic flux, the bipole brightens in soft X-rays forming a coronal bright point. The coronal emission exhibits episodic brightenings on top of a long smooth increase. These coronal brightenings are also associated with surge-like chromospheric features visible in Hα, which can be attributed to reconnection with adjacent small-scale magnetic fields and the ambient quiet-Sun magnetic field. Conclusions. The emergence of magnetic flux even at the smallest scales can be the driver of a series of energetic phenomena visible at various atmospheric heights and temperature regimes. Multi-wavelength observations reveal a wealth of mechanisms which produce diverse observable effects during the different evolutionary stages of these small-scale structures.

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Observations of Magnetic Features with the German Solar Telescopes at the Observatorio del Teide/Tenerife

Symposium - International Astronomical Union ◽

10.1017/s0074180900087830 ◽

1990 ◽

Vol 142 ◽

pp. 113-117

Author(s):

F. Kneer ◽

D. Soltau ◽

E. Wiehr

Keyword(s):

Magnetic Field ◽

Fine Structure ◽

Spatial Variation ◽

Velocity Fluctuation ◽

Small Scale ◽

Field Variation ◽

Magnetic Field Variation ◽

Quiet Sun ◽

Magnetic Features ◽

Solar Telescopes

The German solar facilities at the Obsrvatorio del Teide are described first. Then, a few examples of recent results from magnetic features are given: spatial variation and velocity fluctuation of small-scale magnetic fluxtubes in the quiet Sun, Evershed flow and magnetic field in connection with penumbral fine structure, and magnetic field variation in sunspot umbrae.

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