scholarly journals The second solar spectrum and the hidden magnetism

2008 ◽  
Vol 4 (S259) ◽  
pp. 211-222
Author(s):  
Jan O. Stenflo
Keyword(s):  
Magnetic Flux ◽  
Spatial Resolution ◽  
Zeeman Effect ◽  
Solar Spectrum ◽  
Coherent Scattering ◽  
Small Scale ◽  
Solar Photosphere ◽  
Resolution Limit ◽  
New Paradigm ◽  
Hanle Effect

AbstractApplications of the Hanle effect have revealed the existence of vast amounts of “hidden“ magnetic flux in the solar photosphere, which remains invisible to the Zeeman effect due to cancellations inside each spatial resolution element of the opposite-polarity contributions from this small-scale, tangled field. The Hanle effect is a coherency phenomenon that represents the magnetic modification of the linearly polarized spectrum of the Sun that is formed by coherent scattering processes. This so-called “Second Solar Spectrum” is as richly structured as the ordinary intensity spectrum, but the spectral structures look completely different and have different physical origins. One of the new diagnostic uses of this novel spectrum is to explore the magnetic field in previously inaccessible parameter domains. The earlier view that most of the magnetic flux in the photosphere is in the form of intermittent kG flux tubes with tiny filling factors has thereby been shattered. The whole photospheric volume instead appears to be seething with intermediately strong fields, of order 100G, of significance for the overall energy balance of the solar atmosphere. According to the new paradigm the field behaves like a fractal with a high degree of self-similarity between the different scales. The magnetic structuring is expected to continue down to the 10m scale, 4 orders of magnitude below the current spatial resolution limit.

scholarly journals P-mode induced convective collapse in vertical expanding magnetic flux tubes?

2016 ◽  
Vol 12 (S327) ◽  
pp. 86-93 ◽  
Author(s):  
D. Utz ◽  
T. Van Doorsselaere ◽  
N. Magyar ◽  
M. Bárta ◽  
J. I. Campos Rozo
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Strong Magnetic Field ◽  
Magnetic Flux Tube ◽  
Small Scale ◽  
Solar Photosphere ◽  
Ongoing Work ◽  
Standard Scenario ◽  
Wave Guides ◽  
Collapse Process

AbstractSmall-scale kG strong magnetic field elements in the solar photosphere are often identified as so-called magnetic bright points (MBPs). In principle these MBPs represent the cross-section of a vertical, strong, magnetic flux tube which is expanding with height in the solar atmosphere. As these magnetic elements represent possible MHD wave guides, a significant interest has been already paid to them from the viewpoint of observations and simulations. In this work we would like to shed more light on a possible scenario for the creation of such strong magnetic field concentrations. The accepted standard scenario involves the convective collapse process. In this ongoing work we will show indications that this convective collapse process may become triggered by sufficiently strong pressure disturbances. However, it is highly unlikely that p-mode waves can be of such a strength.

scholarly journals Observations on spatial variations of the Sr I 4607 Å scattering polarization signals at different limb distances with ZIMPOL

2019 ◽  
Vol 630 ◽  
pp. A67 ◽  
Author(s):  
Sajal Kumar Dhara ◽  
Emilia Capozzi ◽  
Daniel Gisler ◽  
Michele Bianda ◽  
Renzo Ramelli ◽  
...  
Keyword(s):  
Spectral Line ◽  
Spatial Scales ◽  
Solar Spectrum ◽  
Spatial Variations ◽  
Stokes Parameters ◽  
Small Scale ◽  
Solar Photosphere ◽  
Local Anisotropy ◽  
Polarization Signal ◽  
The Continuum

Context. The Sr I 4607 Å spectral line shows one of the strongest scattering polarization signals in the visible solar spectrum. The amplitude of this polarization signal is expected to vary at granular spatial scales, due to the combined action of the Hanle effect and the local anisotropy of the radiation field. Observing these variations would be of great interest because it would provide precious information on the small-scale activity of the solar photosphere. At present, few detections of such spatial variations have been reported. This is due to the difficulty of these measurements, which require combining high spatial (∼0.1″), spectral (≤20 mÅ), and temporal resolution (< 1 min) with increased polarimetric sensitivity (∼10−4). Aims. We aim to detect spatial variations at granular scales of the scattering polarization peak of the Sr I 4607 Å line at different limb distances, and to study the correlation with the continuum intensity. Methods. Using the Zurich IMaging POLarimeter (ZIMPOL) system mounted at the GREGOR telescope and spectrograph in Tenerife, Spain, we carried out spectro-polarimetric measurements to obtain the four Stokes parameters in the Sr I line at different limb distances, from μ = 0.2 to μ = 0.8, on the solar disk. Results. Spatial variations of the scattering polarization signal in the Sr I 4607 Å line, with a spatial resolution of about 0.66″, are clearly observed at every μ. The spatial scale of these variations is comparable to the granular size. A statistical analysis reveals that the linear scattering polarization amplitude in this Sr I spectral line is positively correlated with the intensity in the continuum, corresponding to the granules, at every μ.

scholarly journals Theoretical Models of Magnetic Flux Tubes: Structure and Dynamics

1994 ◽  
Vol 154 ◽  
pp. 407-421
Author(s):  
O Steiner
Keyword(s):  
Magnetic Flux ◽  
Line Emission ◽  
Theoretical Models ◽  
Small Scale ◽  
Solar Photosphere ◽  
Model Calculations ◽  
Flux Tubes ◽  
Wide Range ◽  
Magnetic Flux Tubes ◽  
Lower Chromosphere

Two types of model calculations for small scale magnetic flux tubes in the solar atmosphere are reviewed. In the first kind, one follows the temporal evolution governed by the complete set of the MHD and radiative transfer equations to a (quasi) stationary solution. From such a solution the continuum contrasts of a photospheric flux tube in the visible and in the infrared continuum at 1.6 μm have been computed and are briefly discussed. The second, more empirical type of method assumes the flux tubes to be in magnetohydrostatic equilibrium. It is computationally faster and more flexible and allows us to explore a wide range of parameters. Models and insights obtained from such parameter studies are discussed in some detail. These include an explanation for the peculiar variation of the area asymmetry of Stokes V profiles across the solar disk in terms of mass motions in the surroundings of magnetic flux tubes.Furthermore, a two-dimensional model of the lower chromosphere that has been developed is presented. Emphasis is laid on the effect of thermal bifurcation of the lower chromosphere on the structure of the chromospheric magnetic field. If the cool carbon monoxide clouds, observed in the infrared, occupy the non-magnetic regions, the flux tubes expand very strongly and form a magnetic canopy with an almost horizontal base. This has consequences for the spatial distribution of the Ca II K spectral line emission.Finally, some consideration is given to the formation and destruction of intense magnetic flux tubes in the solar photosphere. The formation is described as a consequence of the flux expulsion process that leads to a convective instability. A possible observational signature of this mechanism is proposed.

12 Solar radiation & structure

1997 ◽  
Vol 23 (1) ◽  
pp. 149-163
Keyword(s):  
High Resolution ◽  
Recent Work ◽  
Spatial Resolution ◽  
Mean Free Path ◽  
Solar Photosphere ◽  
Image Correction ◽  
Resolution Limit ◽  
Earth’S Atmosphere ◽  
Earth's Atmosphere ◽  
Solar Telescopes

Spatial structures in the solar photosphere are likely to be seen down to scales of the order of the photon mean free path, which is about 70 km in the lower photosphere. This scale corresponds to an angle of O.”1 at disk center. Structures associated with magnetic fields may be expected on even smaller scales. Existing solar telescopes typically have diameters of slightly less than one meter. Hence, even in the visible part of the spectrum, the scales of solar structures extend out to the diffraction limit of current solar telescopes. Therefore, the achievable spatial resolution is limited by turbulence in the Earth’s atmosphere (seeing). This has led to the development of various techniques to overcome this resolution limit and achieve diffraction-limited resolution. This report covers selected highlights and recent work done in the context of high-resolution techniques published in the period from July 1, 1993 to June 30, 1996. Due to the lack of space the report remains necessarily incomplete, and I apologize to all the authors of important contributions that are not cited here. This review does not cover space and balloon-borne instruments that try to achieve high spatial resolution by observing from above the Earth’s atmosphere. Recent work on ground-based high-resolution techniques has been collected in the proceedings of the 13thSacramento Peak Summer Workshop on Real Time and Post Facto Solar Image Correction (Radick 1993).

scholarly journals [no title]

1977 ◽  
Vol 4 (2) ◽  
pp. 223-239 ◽  
Author(s):  
J. Harvey
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Spatial Resolution ◽  
Small Scale ◽  
Solar Photosphere ◽  
The Sun ◽  
Solar Magnetic Fields ◽  
Structure And Dynamics ◽  
Physical Problems ◽  
Observational Techniques

If the Sun is observed like a star, without spatial resolution, its magnetic field seldom exceeds 1 Gauss. But with high spatial resolution the field is seen to be largely concentrated into kG structures. Observations of the structure and dynamics of solar magnetic fields can therefore provide a guide to the nature of magnetic fields of other stars which cannot be resolved. Solar activity and the structure of the chromosphere and inner corona are intimately linked with magnetism and a complete understanding of these features often depends on magnetic field details. There are unsolved physical problems involving solar magnetic fields which have challenged many physicists. For example, confinement of small-scale fields in kG structures is a problem of current interest (Parker, 1976; Piddington, 1976; Spruit, 1976). Solar observers are no less challenged since the Sun presents us with a complicated magnetic field having a range of scales from global to less than the scale of our best observations as illustrated in Figures 1, 2, and 3. This paper is a survey of observational techniques and results at the small-scale end of the spectrum of sizes in the solar photosphere. This topic has been frequently reviewed (e.g. Athay, 1976; Beckers, 1976; Deubner, 1975; Howard, 1972; Mullan, 1974; Severny, 1972; Stenflo, 1975) so that recent work is emphasized here.

scholarly journals Radiative transfer modeling of the Hanle effect in convective atmospheres

2006 ◽  
Vol 2 (S239) ◽  
pp. 44-51
Author(s):  
Javier Trujillo Bueno
Keyword(s):  
Radiative Transfer ◽  
Magnetic Flux ◽  
Magnetic Energy ◽  
Radiative Energy ◽  
Energy Losses ◽  
Solar Photosphere ◽  
Hanle Effect ◽  
Molecular Lines ◽  
Outer Atmosphere ◽  
Radiative Transfer Modeling

AbstractThis paper summarizes the results of a recent investigation on the Hanle effect in atomic and molecular lines, which indicates that there is a vast amount of “hidden” magnetic energy and (unsigned) magnetic flux in the internetwork regions of the quiet solar photosphere. This hidden magnetic energy, localized in the (intergranular) downflowing plasma of the solar photosphere, is carried mainly by tangled fields at sub-resolution scales with strengths between the equipartition field values and ∼1 kG, and is more than sufficient to compensate the radiative energy losses of the solar outer atmosphere.

scholarly journals Theoretical Aspects of Small-Scale Photospheric Magnetic Fields

1990 ◽  
Vol 138 ◽  
pp. 161-179
Author(s):  
M. Schüssler
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Thermal Structure ◽  
Theoretical Description ◽  
Small Scale ◽  
Solar Photosphere ◽  
Quasi Equilibrium ◽  
Magnetic Structures ◽  
Magnetic Elements ◽  
Work Done

The state of theoretical description of small-scale concentrated magnetic fields in the solar photosphere (excluding oscillations and wave propagation) is reviewed with emphasis on work done since 1982. The processes which probably lead to the formation of strong fields (flux expulsion, convective collapse) are discussed in some detail and the present understanding of the subsequent (quasi-)equilibrium state is summarized. We consider in particular the magnetic and thermal structure of the basic magnetic flux concentrations (magnetic elements) and stress the importance of radiative transfer effects, e.g. the horizontal heat exchange with the surroundings and the effect of radiation from the hot bottom and walls on the upper layers. Velocity fields within and around magnetic flux concentrations are discussed with emphasis on shift and asymmetry of the observed Stokes V-profiles which have recently been understood in terms of a downflow in the immediate vicinity outside magnetic structures. Reconnection and instabilities are considered as possible destruction processes for magnetic elements.

scholarly journals The diagnostic potential of the weak field approximation for investigating the quiet Sun magnetism: the Si I 10 827 Å line

2019 ◽  
Vol 628 ◽  
pp. A47 ◽  
Author(s):  
N. G. Shchukina ◽  
J. Trujillo Bueno
Keyword(s):  
Magnetic Field ◽  
Spatial Resolution ◽  
3D Model ◽  
Field Approximation ◽  
Weak Field ◽  
Small Scale ◽  
Solar Photosphere ◽  
Diagnostic Potential ◽  
Weak Field Approximation ◽  
The Magnetic Field

Aims. We aim to investigate the validity of the weak field approximation (WFA) for determining magnetic fields in quiet regions of the solar photosphere using the polarization caused by the Zeeman effect in the Si I10 827 Å line.Methods. We solved the NLTE line formation problem by means of multilevel radiative transfer calculations in a three-dimensional (3D) snapshot model taken from a state-of-the-art magneto-convection simulation of the small-scale magnetic activity in the quiet solar photosphere. The 3D model used is characterized by a surface mean magnetic field strength of about 170 G. The calculated Stokes profiles were degraded because of the atmospheric turbulence of Earth and light diffraction by the telescope aperture. We apply the WFA to the StokesI,Q,U,Vprofiles calculated for different seeing conditions and for the apertures of the VTT, GREGOR, EST and DKIST telescopes. We compare the inferred longitudinal and transverse components of the magnetic field with the original vertical and horizontal fields of the 3D model.Results. We find that with a spatial resolution significantly better than 0.5″ the surface maps of the magnetic field inferred from the Stokes profiles of the Si I10 827 Å line applying the WFA are close to the magnetic field of the model on the corrugated surface, corresponding to line optical depth unity at Δλ ≈ 0.1 Å for a disk-center line of sight. The correlation between them is relatively high, except that the inferred longitudinal and transverse components of the magnetic field turn out to be lower than in the 3D model.Conclusions. The use of the WFA for interpreting high-spatial-resolution spectropolarimetric observations of the Si I10 827 Å line obtained with telescopes like GREGOR, EST, and DKIST allows the longitudinal and transverse components of the magnetic field to be retrieved with reasonable precision over the whole quiet solar photosphere, the result being worse for telescopes of lower aperture.

scholarly journals Photospheric Flow Fields and Properties of Embedded Small-scale Magnetic Flux Concentrations

2001 ◽  
Vol 203 ◽  
pp. 205-207
Author(s):  
S. P. Rajaguru ◽  
R. Srikanth ◽  
S. S. Hasan
Keyword(s):  
High Resolution ◽  
Magnetic Flux ◽  
Flow Field ◽  
Convective Flow ◽  
Flow Fields ◽  
Small Scale ◽  
Solar Photosphere ◽  
Local Correlation ◽  
Flow Cells ◽  
Correlation Tracking

The association between the different scales of convection on the solar photosphere and the field strengths/flux content of discrete magnetic flux concentrations is analyzed using simultaneously recorded SOHO/MDI high resolution filtergrams and magnetograms. The convective flow field is derived using the Local Correlation Tracking (LCT) technique. The locations and strengths of the flux elements with respect to the flow cells are analyzed to obtain information about different scales of convection.

scholarly journals Spectropolarimetric diagnostics of unresolved magnetic fields in the quiet solar photosphere

2012 ◽  
Vol 8 (S294) ◽  
pp. 107-118 ◽  
Author(s):  
Nataliya G. Shchukina ◽  
Javier Trujillo Bueno
Keyword(s):  
Magnetic Field ◽  
Magnetic Energy ◽  
Zeeman Effect ◽  
Three Dimensional ◽  
Solar Photosphere ◽  
Magnetic Energy Density ◽  
Hanle Effect ◽  
Molecular Lines ◽  
Surface Convection ◽  
Ubiquitous Presence

AbstractA few years before the Hinode space telescope was launched, an investigation based on the Hanle effect in atomic and molecular lines indicated that the bulk of the quiet solar photosphere is significantly magnetized, due to the ubiquitous presence of an unresolved magnetic field with an average strength 〈B〉, ≈ 130 G. It was pointed out also that this “hidden” field must be much stronger in the intergranular regions of solar surface convection than in the granular regions, and it was suggested that this unresolved magnetic field could perhaps provide the clue for understanding how the outer solar atmosphere is energized. In fact, the ensuing magnetic energy density is so significant that the energy flux estimated using the typical value of 1 km/s for the convective velocity (thinking in rising magnetic loops) or the Alfvén speed (thinking in Alfvén waves generated by magnetic reconnection) turns out to be substantially larger than that required to balance the chromospheric energy losses. Here we present a brief review of the research that led to such conclusions, with emphasis on a new three-dimensional radiative transfer investigation aimed at determining the magnetization of the quiet Sun photosphere from the Hanle effect in the Sr I 4607 Å line and the Zeeman effect in Fe I lines.

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