No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Quiet sun magnetic fields vs. polar faculae - Local vs. global dynamo?
Abstract
Quiet Sun magnetic fields in the internetwork are almost ubiquitous. Simultaneous observations in infra-red and visible lines and high spatial resolution (<0.5″) data in visible lines show that their field strengths range from below few hundred Gauss to kilo-Gauss. Most of the flux is contained in small-scale, strong-field features located mainly in intergranular lanes. The average unsigned flux density exceeds 20 Gauss. The new detections are confirmed by recent quiet Sun observations in the G band. The generation of the strong fields in the internetwork, which may be due to a local dynamo, poses a challenging problem. – Polar faculae (PFe) are small-scale magnetic features at the polar caps of the Sun. They take part in the solar cycle and are thus likely to be rooted deeply in the solar interior. They are the result of the global dynamo at the solar poles. PFe also possess kilo-Gauss magnetic fields which have the same polarity as the global magnetic field. The rôle of quiet Sun magnetic field structures and of PFe for the dynamics of the corona and for the solar wind are addressed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
... Discussing the origin of the photospheric magnetic field is beyond the scope of this paper. Then, we refer the reader to the literature that deals with the dynamo processes, global and local, which are believed to be at the origin of the emergence, dynamics and cancellation of this magnetic field (e.g., [47][48][49][50][51]). Of interest for this work are the convective transport mechanisms of magnetic elements and the consequent magnetic pattern formation that is intimately linked to the boundaries of the underlying convective structures. ...
The multiscale dynamics associated with turbulent convection present in physical systems governed by very high Rayleigh numbers still remains a vividly disputed topic in the community of astrophysicists, and in general, among physicists dealing with heat transport by convection. The Sun is a very close star for which detailed observations and estimations of physical properties on the surface, connected to the processes of the underlying convection zone, are possible. This makes the Sun a unique natural laboratory in which to investigate turbulent convection in the hard turbulence regime, a regime typical of systems characterized by high values of the Rayleigh number. In particular, it is possible to study the geometry of convection using the photospheric magnetic voids (or simply voids), the quasi-polygonal quiet regions nearly devoid of magnetic elements, which cover the whole solar surface and which form the solar magnetic network. This work presents the most extensive statistics, both in the spatial scales studied (1–80 Mm) and in the temporal duration (SC 23 and SC 24), to investigate the multiscale nature of solar magnetic patterns associated with the turbulent convection of our star. We show that the size distribution of the voids, in the 1–80 Mm range, for the 317,870 voids found in the 692 analyzed magnetograms, is basically described by an exponential function.
Context. The Mn line at 553 nm shows strong spectral features in both intensity and polarization profiles due to the hyperfine structure of the atom. These features, their presence or absence, are known to be dependent on the magnetic regime to which the Mn atom is subject. Aims. Our objective is to disentangle strong kilo-Gauss (kG) fields from relatively weak hecto-Gauss (hG) fields in the quiet sun, and compute relative filling factors on the resolution element. Methods. We observed the 553 nm Mn line in a quiet sun area with the Advanced Stokes Polarimeter, and we introduce an in-line ratio between different spectral features. Filling factors can be retrieved from the measurement of this ratio and the total longitudinal flux. Results. In the photospheric network the kG dominate the magnetic flux, although out of the higher concentration areas the hG fields dominate in surface coverage. For the internetwork (granules and lanes confounded) the hG are dominant both in surface and total flux.
Polar faculae (PF) are bright, small-scale structures measuring a few seconds of arc, populating the polar zones at latitudes
>50°. They possess magnetic fields ranging from 150 to 1,700 Gauss and largely constitute the polar magnetic fields. Where
and how their fields are generated in the solar interior remain open questions. Using measurements of PF rotation rates, we
show that their anchor depths probably lie in subsurface layers at radius r/R
⊙ = 0:94–1.00. If so, the PF fields are possibly generated by a local dynamo in a subsurface shear layer extending to r/R
⊙ > 0:94.
Small-scale solar magnetic flux concentrations are studied in two-dimensional G-band images at very high spatial resolution
and compared with Ca ii H enhancements. Among 970 small-sized G-band intergranular structures (IgS), 45% are co-spatial with isolated locations of
Ca ii H excess and thus considered as magnetic (MIgS); they may be even twice as frequent as the known G-band bright points. The
IgS are recognized in the G-band image by a new algorithm operating in four steps: (1)A set of equidistant detection levels
yields a pattern of primary “cells”; (2)for each cell, the intrinsic intensity profile is normalized to its brightest pixel;
(3)the cell sizes are shrunk by a unitary single-intensity clip; (4)features in contact at an appropriate reference level
are merged by removal of the respective common dividing lines. Optionally, adjoining structures may be excluded from this
merging process (e.g., chains of segmented IgS), referring to the parameterized number and intensity of those pixels where enveloping feature contours
overlap. From the thus recognized IgS pattern, MIgS are then selected by their local Ca ii H contrast and their mean G-band-to-continuum brightness ratio.
Context.Faculae at the poles of the Sun, or polar faculae (PFe), take part in the solar magnetic cycle. Their occurrence maximum is shifted by 5–6 years with respect to the sunspot cycle. PFe are stable phenomena, with lifetimes of several hours to days, and harbour magnetic fields of kilo-Gauss strength. Yet their role for the global magnetic field at the solar poles is unknown.Aims.To contribute to the knowledge of the physical properties of PFe and to the understanding of their role in the global magnetism of the Sun.Methods.PFe were observed on 21–24 August 2005 with the Vacuum Tower Telescope at the Observatorio del Teide. The “Göttingen” Fabry-Perot spectrometer was used with a Stokes $V$ polarimeter to scan the Fe i 6173 Å line (Landé factor $g$ = 2.5) and the H$\alpha$ line in two-dimensional fields of view (FOVs). A large observational coverage of the polar caps was obtained. The data were analysed with speckle methods. Magnetic field strengths were determined with the weak field approximation, with the approximation of the strong field regime, and with the centre of gravity (COG) method. Velocities were measured with the COG method and from the zero-crossing of the Stokes $V$ profiles.Results.PFe show a decrease of the continuum and broadband intensity contrast towards the disc centre and no decrease of contrast towards the limb, similar to as faculae in active regions near the equator. Extrapolating from the observed FOVs to the total areas of PF occurrence around the solar poles, we find 4 120 PFe in the northern polar cap and, asymmetrically to this number, 1 250 PFe near the south pole. The total area coverages by PFe are ~7.6$\times$10$^8$ km$^2$ and ~3.4$\times$10$^8$ km$^2$ near the solar north and south poles, respectively. Some of the PFe exhibit magnetic polarities opposite to the global polarity at the time of observation. The resulting total magnetic fluxes in PFe fall short by an order of magnitude from those found in the literature for the fluxes at the polar caps. This also holds if we include magnetic structures which are not related to brightenings. We conclude that with the present spatial resolution of $0 \farcs4{-}0\farcs5$ (FWHM), PFe represent the “large-scale” end of a distribution of unipolar strands near the solar poles. The velocities in PFe show amplitudes of 2 km s$^{-1}$, with systematic up-flows in the Stokes $I$ profile, but no average velocity measured in the $V$ zero-crossings.
This work confirms the existence of the mesogranulation as an intermediate scale of convection in the solar photosphere. The main argument presented is the correlation between horizontal flows obtained with two different methods (Local Correlation Tracking, and gradient of granular intensities), and the pattern observed in magnetograms above a fixed threshold of the magnetic signal. The data used consist of a time series (40 min) of high spatial resolution images of intensity and simultaneous magnetograms. The size of the scales found is 5 arcsec-10 arcsec (3.6-7.2 Mm), but some mesogranules close to supergranular boundaries can be smaller, and appear to be stabilized by the network magnetic fields.
The magnetic field strength within the polar caps of the Sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field. Measurements of the line-of-sight component of the magnetic field generally yield 0.1 to 0.2 mT near times of sunspot minimum. In this paper we report measurements of the polar fields made at the Stanford Solar Observatory using the Fe i line 525.02 nm. We find that the average flux density poleward of 55 latitude is about 0.6 mT peaking to more than 1 mT at the pole and decreasing to 0.2 mT at the polar cap boundary. The total open flux through either polar cap thus becomes about 3 1014 Wb. We also show that observed magnetic field strengths vary as the line-of-sight component of nearly radial fields.
We analyze a time sequence of Inter-Network (IN) magnetograms observed at the solar disk center. Speckle reconstruction techniques provide a good spatial resolution (0$\farcs$5 cutoff frequency) yet maintaining a fair sensitivity (some 20 G). Patches with signal above noise cover 60% of the observed area, most of which corresponds to intergranular lanes. The large surface covered by signal renders a mean unsigned magnetic flux density between 17 G and 21 G (1 G $\equiv$ 1 Mx cm$^{-2}$). The difference depends on the spectral line used to generate the magnetograms ($\ion{Fe}{i}$ $\lambda$6302.5 Å or $\ion{Fe}{i}$ $\lambda$6301.5 Å). Such systematic difference can be understood if the magnetic structures producing the polarization have intrinsic field strengths exceeding 1 kG, and consequently, occupying only a very small fraction of the surface (some 2%). We observe both, magnetic signals changing in time scales smaller than 1 min, and a persistent pattern lasting longer than the duration of the sequence (17 min). The pattern resembles a network with a spatial scale between 5 and 10 arcsec, which we identify as the mesogranulation. The strong dependence of the polarization signals on spatial resolution and sensitivity suggests that much quiet Sun magnetic flux still remains undetected.
Deciphering and understanding the small-scale magnetic activity of the quiet solar photosphere should help to solve many of the key problems of solar and stellar physics, such as the magnetic coupling to the outer atmosphere and the coronal heating. At present, we can see only approximately 1 per cent of the complex magnetism of the quiet Sun, which highlights the need to develop a reliable way to investigate the remaining 99 per cent. Here we report three-dimensional radiative transfer modelling of scattering polarization in atomic and molecular lines that indicates the presence of hidden, mixed-polarity fields on subresolution scales. Combining this modelling with recent observational data, we find a ubiquitous tangled magnetic field with an average strength of approximately 130 G, which is much stronger in the intergranular regions of solar surface convection than in the granular regions. So the average magnetic energy density in the quiet solar photosphere is at least two orders of magnitude greater than that derived from simplistic one-dimensional investigations, and sufficient to balance radiative energy losses from the solar chromosphere.
The described investigation is concerned with the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semiempirical models for six components of the quiet solar chromosphere. For a given temperature-height distribution, the solution is obtained of the equations of statistical equilibrium, radiative transfer for lines and continua, and hydrostatic equilibrium to find the ionization and excitation conditions for each atomic constituent. The emergent spectrum is calculated, and a trial and error approach is used to adjust the temperature distribution so that the emergent spectrum is in best agreement with the observed one. The relationship between semiempirical models determined in this way and theoretical models based on radiative equilibrium is discussed by Avrett (1977). Harvard Skylab EUV observations are used to determine models for a number of quiet-sun regions.
Magneto-convection simulations on meso-granule and granule scales near the solar surface are used to study small scale dynamo activity and the emergence and disappearance of magnetic flux tubes.
We present spectro-polarimetric observations of Inter-Network magnetic fields at the solar disk center. A Fabry-Perot spectrometer was used to scan the two Fe I lines at 6301.5 A and 6302.5 A. High spatial resolution (0.5") magnetograms were obtained after speckle reconstruction. The patches with magnetic fields above noise cover approximately 45% of the observed area. Such large coverage renders a mean unsigned magnetic flux density of some 20 G (or 20 Mx/cm^2), which exceeds all previous measurements. Magnetic signals occur predominantly in intergranular spaces. The systematic difference between the flux densities measured in the two iron lines leads to the conclusion that, typically, we detect structures with intrinsic field strengths larger than 1kG occupying only 2% of the surface. Comment: To be published in ApJL. 4 pages and 2 figures
We analyze a time series of high angular resolution magnetograms of quiet Sun Inter-Network (IN) magnetic fields. These magnetograms have a spatial resolution better than 0.5 arcsec, a noise of some 20 G, and they have been obtained at the disk center during the minimum of the solar cycle. The IN regions show a typical unsigned flux density of the order of 15 G. Signals occur, preferentially, in the intergranular lanes, and the strongest signals trace a network with a scale similar to the mesogranulation. All these features are consistent with the IN magnetograms by Dominguez Cernena et al., obtained during the maximum of the solar cycle. Consequently, the unsigned magnetic flux of the structures that give rise to the IN polarization signals does not seem to undergo large variations during the solar cycle. Comment: Accepted for publication in A&A. 7 Pages . 6 Figures
The numbers of faculae at the sun's poles have been estimated from white-light images obtained at the Mount Wilson Observatory during 1970-1990 and have been combined with previous measurements extending back to 1906 when the observations began. The combined measurements now span four complete 22 year cycles and show the following: (1) the numbers of north and south polar faculae were about 50 percent larger around sunspot minimum in 1986 than in 1976, but were still smaller than some of the very large numbers that occurred near sunspot minima in earlier cycles; (2) in 1974, the number of south polar faculae exhibited a short-lived increase which coincided with the arrival of a surge of trailing-polarity flux at the pole, suggesting that similar poleward surges may have been responsible for previously unexplained bursts of faculae such as the one that occurred at the south pole in 1959; and (3) the numbers of polar faculae have been highly correlated with the Wilcox Solar Observatory polar field strengths since these magnetic measurements began in 1976. 10 refs.
Contents: Magnetic fields: instrumentation, observational techniques and models. Dynamics and evolution of magnetic fields. Magnetic field and coupling through atmospheric structuring. Magnetic fields and coupling of the physical processes. Prospects and opportunities. Posters.
The radial component of the magnetic field in the southern hemisphere has been measured at Ulysses as it traveled from the equator to -80.2° latitude and returned. The radial component multiplied by the square of the radial distance, i.e., BRr2, averaged over 77 day intervals (three solar rotations) is approximately constant at -3.5 nT and shows no evidence of a dependence on heliographic latitude. To discriminate against possible time variations, the measurements have been compared with simultaneous observations in the ecliptic by IMP-8. The two sets of observations agree very well confirming the absence of a significant latitude gradient. Since the sun's dipolar magnetic field component is strong at this phase of the sunspot cycle, it is inferred that magnetic flux from the polar cap is transported to lower latitudes in the solar wind source region to produce a uniform radial field. Such a configuration would be expected if magnetic stresses are influencing the solar wind flow near the sun and are contributing to a non-radial deflection. Estimates of the flux of open field lines (3×1014 webers), of the non-radial solar wind expansion (~3) and the polar cap magnetic field strength (~5 Gauss) are derived.
We derive the poleward migration trajectory diagram of the filament bands for the years 1915–1982 from the H-alpha synoptic charts. We find that the global solar activity commences soon after the polar field reversal in the form of two components in each hemisphere. The first component we identify with the polar faculae that appear at latitudes 40–70 and migrate polewards. The second and the more powerful component representing the sunspots shows up at 40 latitudes 5–6 years later and drifts equatorward giving rise to the butterfly diagram. Thus the global solar activity is described by the faculae and the sunspots that occur at different latitude belts and displaced in time by 5–6 years. This gives rise to the prolonged duration for the global solar activity lasting for 16–18 years as against the 11 years which has come about based only on the spots. The two components match with the pattern of the coronal emission in 5303 line. Finally, we show that the two components of activity also match with the pattern of excess shear associated with the torsional oscillations on the Sun and this provides a link between the torsional oscillations and the magnetic activity.
We have defined the duration of polar magnetic activity as the time interval between two successive polar reversals. The epochs of the polarity reversals of the magnetic field at the poles of the Sun have been determined (1)by the time of the final disappearance of the polar crown filaments and (2)by the time between the two neighbouring reversals of the magnetic dipole configuration (l=1) from the H synoptic charts covering the period 1870–2001. It is shown that the reversals for the magnetic dipole configuration (l=1) occur on an average 3.30.5 years after the sunspot minimum according to the H synoptic charts (TableI) and the Stanford magnetograms (TableIII). If we set the time of the final disappearance of the polar crown filaments (determined from the latitude migration of filaments) as the criterion for deciding the epoch of the polarity reversal of the polar fields, then the reversal occurs on an average 5.80.6 years from sunspot minimum (last column of TableI). We consider this as the most reliable diagnostic for fixing the epoch of reversals, as the final disappearance of the polar crown filaments can be observed without ambiguity. We show that shorter the duration of the polar activity cycle (i.e., the shorter the duration between two neighbouring reversals), the more intense is the next sunspot cycle. We also notice that the duration of polar activity is always more in even solar cycles than in odd cycles whereas the maximum Wolf numbers W
\max is always higher for odd solar cycles than for even cycles. Furthermore, we assume there is a secular change in the duration of the polar cycle. It has decreased by 1.2 times during the last 120 years.
Faculae on the polar caps of the Sun, in short polar faculae (PFe), are investigated. They take part in the magnetic solar cycle. Here, we study the fine structures of PFe, their magnetic fields and their dynamics on short time scales. The observations stem from several periods in 2001 and 2002. They consist of spectropolarimetric data (Stokes $I$ and $V$) taken in the $\ion{Fe}{i}$ 6301.5 and 6302.5 Å and $\ion{Fe}{ii}$ 6149.3 Å lines with the Gregory-Coudé Telescope (GCT) and the Vacuum Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. At the VTT, the “Göttingen” two-dimensional Fabry-Perot spectrometer was used. It allows image reconstruction with speckle methods resulting in spatial resolution of approximately 0$\farcs$25 for broadband images and 0$\farcs$5 for magnetograms. The application of singular value decomposition yielded a polarimetric detection limit of $\vert V\vert\approx2$ $\times$ $10^{-3}I_{\rm c}$. We find that PFe, of size of 1´´ or larger, possess substantial fine structure of both brightness and magnetic fields. The brightness and the location of polar facular points change noticeably within 50 s. The facular points have strong, kilo-Gauss magnetic fields, they are unipolar with the same polarity as the global, poloidal magnetic field. The ambient areas, however, exhibit weak flux features of both polarities, as in the quiet Sun near disk center. Strong upflows of 1 km s$^{-1}$ are detected in the intensity profiles of PFe. The zero-crossings of the Stokes $V$ profiles yield an average velocity of the magnetized plasma of $v_{{\rm FT}}\approx-0.4$ km s$^{-1}$ (towards the observer).
- M Faurobert-Scholl
- N Feautrier
- F Machefert
- K Petrovay
- A Spielfiedel
Faurobert-Scholl, M., Feautrier, N., Machefert, F., Petrovay, K.,
Spielfiedel, A.: 1995, A&A 298, 289
- B W Lites
- H Socas-Navarro
Lites, B.W., Socas-Navarro, H.: 2004, ApJ 613, 600
- K Petrovay
- G Szakaly
Petrovay, K., Szakaly, G.: 1993, A&A 274, 543
- Trujillo Bueno
- J Shchukina
- N G Asensio Ramos
Trujillo Bueno, J., Shchukina, N.G., Asensio Ramos, A.: 2004, Nature 430, 326
- M Waldmeier
Waldmeier, M.: 1962, ZA 54, 260
- J Sánchez Almeida
- I Márquez
- J A Bonet
- I Domínguez Cerdeña
- R Muller
- C J Schrijver
- A M Title
Sánchez Almeida, J., Márquez, I., Bonet, J.A., Domínguez Cerdeña,
I., Muller, R.: 2004, ApJ 609, L91
Schrijver, C.J., Title, A.M.: 2003, ApJ 597, L165
Sheeley jr., N.R.: 1964, ApJ 140, 731
- J Sánchez Almeida
- B W Lites
Sánchez Almeida, J., Lites, B.W.: 2000, ApJ 532, 1215
- Domínguez Cerdeña
- I Sánchez Almeida
- J Kneer
Domínguez Cerdeña, I., Sánchez Almeida, J., Kneer, F.: 2003b,
A&A 407, 741
- H Socas-Navarro
- Martínez Pillet
- V Lites
Socas-Navarro, H., Martínez Pillet, V., Lites, B.W.: 2004, ApJ 611,
1139
- F Cattaneo
Cattaneo, F.: 1999, ApJ 515, L39
- I Domínguez Cerdeña
Domínguez Cerdeña, I.: 2003, A&A 412, L65
- I Domínguez Cerdeña
- J Sánchez Almeida
- F Kneer
Domínguez Cerdeña, I., Sánchez Almeida, J., Kneer, F.: 2003b,
A&A 407, 741
- J Sánchez Almeida
Sánchez Almeida, J.: 2003, A&A 411, 615
- J Sánchez Almeida
- I Domínguez Cerdeña
- F Kneer
Sánchez Almeida, J., Domínguez Cerdeña, I., Kneer, F.: 2003a, ApJ
597, L180
- J Sánchez Almeida
- I Márquez
- J A Bonet
- I Domínguez Cerdeña
- R Muller
Sánchez Almeida, J., Márquez, I., Bonet, J.A., Domínguez Cerdeña,
I., Muller, R.: 2004, ApJ 609, L91
- C J Schrijver
- A M Title
- N R Sheeley Jr
Schrijver, C.J., Title, A.M.: 2003, ApJ 597, L165
Sheeley jr., N.R.: 1964, ApJ 140, 731
- N R Sheeley Jr
Sheeley jr., N.R.: 1991, ApJ 374, 386
- H Socas-Navarro
- V Martínez Pillet
- B W Lites
Socas-Navarro, H., Martínez Pillet, V., Lites, B.W.: 2004, ApJ 611,
1139
The Sun -An Introduction
- R F Stein
- Å Nordlund
- L Svalgaard
- T L Duvall Jr
- P H Scherrer
Stein, R.F., Nordlund,Å.: 2002, ESA SP-505, 83 (IAU Coll. 188)
Stix, M.: 2002, The Sun -An Introduction, 2nd ed., Springer, Berlin
Svalgaard, L., Duvall jr., T.L., Scherrer, P.H.: 1978, SoPh 58, 225
- J E Vernazza
- E H Avrett
- R Loeser
Vernazza, J.E., Avrett, E.H., Loeser, R.: 1981, ApJS 45, 635
- M Waldmeier
Waldmeier, M.: 1955, ZA 38, 37