Interpretation of the Second Solar Spectrum of the Sr I 4607 Å Line Observed at THEMIS and Pic-du-Midi (original) (raw)
Related papers
Astronomy and Astrophysics, 2005
This paper presents and interprets some observations of the limb polarization of Sr 4607 Å obtained with the spectropolarimeter of the French-Italian telescope THEMIS in quiet regions close to the solar North Pole on 2002 December 7-9. The linear polarization was measured for a series of limb distances ranging from 4 to 160 arcsec, corresponding to heights of optical depth unity in the line core ranging from about 330 to 220 km, respectively, above the τ 5000 = 1 level. To increase the polarimetric sensitivity, the data were averaged along the spectrograph slit (one arcmin long) set parallel to the solar limb. Since the data show no rotation of the linear polarization direction with respect to the limb direction, the observed depolarization is ascribed to the Hanle effect of a turbulent weak magnetic field, the zero-field polarization being derived from a model. The interpretation is performed by means of an algorithm which describes the process of line formation in terms of the atomic density matrix formalism, the solar atmosphere being described by an empirical, plane-parallel model. The collisional rates entering the model (inelastic collisions with electrons, elastic depolarizing collisions with neutral hydrogen), have been computed by applying fast semi-classical methods having a typical accuracy of the order of 20% or better (see Derouich 2004), leading to 6% inaccuracy on the magnetic field strength determination. We assume a unimodal distribution for the intensity of the turbulent field. The computed intensity profile has been adjusted to the observed one in both depth and width, by varying both microturbulent and macroturbulent velocities. The best adjustment is obtained for respectively 1.87 km s −1 (micro) and 1.78 km s −1 (macro). The evaluation of the magnetic depolarization leads then to the average value of 46 Gauss for the turbulent magnetic field strength, with a gradient of −0.12 Gauss/km. Our results are in very good agreement with the value of 60 Gauss determined at large µ, in the volume-filling field case, by Trujillo Bueno et al. (2004, Nature, 430, 326), using a 3D magneto-convective simulation. This validates our method.
Hanle diagnostics of solar magnetic fields: the SR II 4078 Angstrom line
Astronomy and Astrophysics
The Hanle depolarization and rotation effects in the Sr ii 4078Å line have been explored with the instrumentation at IRSOL (Istituto Ricerche Solari Locarno) by recording the Stokes I, Q, and U line profiles with high spectral resolution and polarimetric accuracy in a large number of regions across the solar disk. From the extracted line parameters we have constructed "Hanle histograms" showing the statistical distributions of the Hanle rotation and depolarization effects. Comparison with theoretical calculations allow these histograms to be understood in terms of magnetic fields with a strength of about 5-10 G, which is similar to the field strengths previously found through analysis of Q/I Hanle depolarization in the Ca i 4227Å line. While small-scale magnetic fields with spatially unresolved angular distributions contribute to the observed Hanle depolarization effects, the observed Hanle rotation effects in Stokes U are due to spatially resolved fields with net large-scale orientations (e.g. global or canopy-type fields). We have also for the first time determined empirical "Hanle efficiency profiles", derived independently for the Hanle rotation and depolarization effects. They show how the Hanle efficiency has its maximum in the Doppler core of the line and then rapidly decreases to become zero in the line wings.
Astronomy & Astrophysics, 2005
This paper presents and interprets some observations of the limb polarization of Sr 4607 Å obtained with the spectropolarimeter of the French-Italian telescope THEMIS in quiet regions close to the solar North Pole on 2002 December 7-9. The linear polarization was measured for a series of limb distances ranging from 4 to 160 arcsec, corresponding to heights of optical depth unity in the line core ranging from about 330 to 220 km, respectively, above the τ 5000 = 1 level. To increase the polarimetric sensitivity, the data were averaged along the spectrograph slit (one arcmin long) set parallel to the solar limb. Since the data show no rotation of the linear polarization direction with respect to the limb direction, the observed depolarization is ascribed to the Hanle effect of a turbulent weak magnetic field, the zero-field polarization being derived from a model. The interpretation is performed by means of an algorithm which describes the process of line formation in terms of the atomic density matrix formalism, the solar atmosphere being described by an empirical, plane-parallel model. The collisional rates entering the model (inelastic collisions with electrons, elastic depolarizing collisions with neutral hydrogen), have been computed by applying fast semi-classical methods having a typical accuracy of the order of 20% or better (see Derouich 2004), leading to 6% inaccuracy on the magnetic field strength determination. We assume a unimodal distribution for the intensity of the turbulent field. The computed intensity profile has been adjusted to the observed one in both depth and width, by varying both microturbulent and macroturbulent velocities. The best adjustment is obtained for respectively 1.87 km s −1 (micro) and 1.78 km s −1 (macro). The evaluation of the magnetic depolarization leads then to the average value of 46 Gauss for the turbulent magnetic field strength, with a gradient of −0.12 Gauss/km. Our results are in very good agreement with the value of 60 Gauss determined at large µ, in the volume-filling field case, by Trujillo Bueno et al. (2004, Nature, 430, 326), using a 3D magneto-convective simulation. This validates our method.
Turbulent magnetic fields in the quiet Sun: A search for cyclic variations
Turbulent magnetic fields fill most of the volume of the solar atmosphere. However, their spatial and temporal variations are still unknown. Since 2007, during the current solar minimum, we are periodically monitoring several wavelength regions in the solar spectrum to search for variations of the turbulent magnetic field in the quiet Sun. These fields, which are below the resolution limit, can be detected via the Hanle effect which influences the scattering polarization signatures (Q/I) in the presence of magnetic fields. We present a description of our program and first results showing that such a synoptic program is complementary to the daily SOHO magnetograms for monitoring small-scale magnetic fields.
Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields
Astronomy & Astrophysics, 2008
Context. The physics of the solar chromosphere depends in a crucial way on its magnetic structure. However there are presently very few direct magnetic field diagnostics available for this region. Aims. Here we investigate the diagnostic potential of the Hanle effect on the Ba ii D2 line resonance polarization for the determination of weak chromospheric turbulent magnetic fields. Methods. The line formation is described with a non-LTE polarized radiative transfer model taking into account partial frequency redistribution with an equivalent two-level atom approximation, in the presence of depolarizing collisions and the Hanle effect. We investigate the line sensitivity to temperature variations in the low chromosphere and to elastic collision with hydrogen atoms. We compare center-to-limb variations of the intensity and linear polarization profiles observed at THEMIS in 2007 to our numerical results. Results. We show that the line resonance polarization is very strongly affected by partial frequency redistribution effects both in the line central peak and in the wings. Complete frequency redistribution cannot reproduce the polarization observed in the line wings. The line is weakly sensitive to temperature differences between warm and cold components of the chromosphere. The effects of elastic collisions with hydrogen atoms and of alignment transfer due to multi-level coupling with the metastable 2 D 5/2 levels have been studied in a recent paper showing that they depolarize the 2 P 3/2 level of the line. In the conditions where the line is formed we estimate the amount of depolarization due to this mechanism as a factor of 0.7 to 0.65. If we first neglect this effect and determine the turbulent magnetic field strength required to account for the observed line polarization, we obtain values between 20 G and 30 G. We show that this overestimates the magnetic strength by a factor between 1.7 and 2. Applying these correction factors to our previous estimates, we find that the turbulent magnetic field strength is between 10 G and 18 G. Conclusions. Because of its low sensitivity to temperature variations, the solar Ba ii D2 line appears as a very good candidate for the diagnosis of weak magnetic fields in the low chromosphere (z ≥ 900 km) by means of the Hanle effect.
2016
We present some theoretical predictions concerning the amplitude and magnetic sensitivity of the linear polarization signals produced by scattering processes in the hydrogen Lyα line of the solar transition region. To this end, we have calculated the atomic level polarization (population imbalances and quantum coherences) induced by anisotropic radiation pumping in semi-empirical and hydrodynamical models of the solar atmosphere, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. The line-center amplitudes of the emergent linear polarization signals are found to vary typically between 0.1% and 1%, depending on the scattering geometry and the strength and orientation of the magnetic field. The results shown here encourage the development of UV polarimeters for sounding rockets and space telescopes with the aim of opening up a diagnostic window for magnetic field measurements in the upper chromosphere and transition region of the Sun.
Solar coronal magnetic field diagnostics through polarimetric forward modelling of the Hanle effect
Astronomy & Astrophysics, 2011
Context. Progress in the solution to some of the most outstanding open problems of solar physics, such as coronal heating, solar wind acceleration, the generation and triggering of explosive events like flares and CMEs, hinges on the provision of a more stringent estimate of the solar magnetic field coordinates. Aims. We seek a way to infer the magnetic field of the solar atmosphere. A very promising way of doing this is by using the Hanle effect in resonance scattering in the Lα line of the solar atmosphere. Methods. By forward modelling the known scattering effects in the presence of magnetic fields, i.e. rotation of the plane of polarisation and depolarisation of the linear polarisation parameters, and by comparing them to observations, one could potentially uncover the magnetic morphology and restrict its intensity range. We simulate the effects of simple dipole configurations along the coordinate axes and analyse the outcome through two kinds of graphs (i.e. the difference in angle of the plane of linear polarisation with respect to the field-free case, and the relative depolarisation). Results. The graphs are either symmetric, anti-symmetric or asymmetric with respect to the (y, z) plane. This is explained by invoking two symmetry operations and taking into account that the magnetic field is a pseudovector. We also show the polarimetric effects of active regions and use them pairwise with the magnetic field due to dipoles to analyse the polarimetric signatures of magnetic field line loops. Inspired by the famous TRACE image, we finally show what one could expect from polarimetry performed on the region of the solar atmosphere displayed in the image. Conclusions. By combining the two complementary remote sensing techniques, i.e. the Zeeman and the Hanle effect, in all thinkable ways with tracers such as the images revealed by TRACE, SOHO, STEREO, etc., we hope one day to be able to infer the solar magnetic field coordinates. Much theoretical and instrumental work still lies ahead, however.
Solar turbulent magnetic fields: surprisingly homogeneous distribution during the solar minimum
Astronomy & Astrophysics, 2010
Context. Small-scale, weak magnetic fields are ubiquitous in the quiet solar atmosphere. Yet their properties and temporal and spatial variations are not well known. Aims. We have initiated a synoptic program, carried out at the Istituto Ricerche Solari Locarno (IRSOL), to investigate both turbulent, mixed-polarity magnetic fields and nearly horizontal, directed fields and their variation with the solar cycle. Methods. Through spectropolarimetric observations we monitor linear and circular polarization at the solar limb (5 on the disk) at five positional angles (N, NW, S, SW, W) with the sensitivity of ∼10 −5 . In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141 Å region including two C 2 triplets and three Fe i lines. Linear polarization in these lines arises from scattering and can be modified via the Hanle effect in the presence of turbulent magnetic fields. Through the application of the differential Hanle effect to the C 2 R-triplet line ratios and the use of a simplified line formation model, we are able to infer a strength of turbulent magnetic fields while using the P-triplet to further restrict it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux densities of horizontally directed fields. Results. We conclude that weak fields were evenly distributed over the Sun during this solar minimum. The turbulent field strength was at least 4.7 ± 0.2 G, and it did not vary during the last two years. This result was complemented with earlier, mainly unpublished measurements in the same region, which extend our set to nearly one decade. A statistical analysis of these all data suggests that there could be a very small variation of the turbulent field strength (3σ-limit) since the solar maximum in 2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal flux densities of 3-8 G.
THE HANLE EFFECT OF THE HYDROGEN Lyα LINE FOR PROBING THE MAGNETISM OF THE SOLAR TRANSITION REGION
The Astrophysical Journal, 2011
We present some theoretical predictions concerning the amplitude and magnetic sensitivity of the linear polarization signals produced by scattering processes in the hydrogen Lyα line of the solar transition region. To this end, we have calculated the atomic level polarization (population imbalances and quantum coherences) induced by anisotropic radiation pumping in semi-empirical and hydrodynamical models of the solar atmosphere, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. The line-center amplitudes of the emergent linear polarization signals are found to vary typically between 0.1% and 1%, depending on the scattering geometry and the strength and orientation of the magnetic field. The results shown here encourage the development of UV polarimeters for sounding rockets and space telescopes with the aim of opening up a diagnostic window for magnetic field measurements in the upper chromosphere and transition region of the Sun.
Measurement of the evolution of the magnetic field of the quiet photosphere during a solar cycle
arXiv: Solar and Stellar Astrophysics, 2017
The solar photosphere is filled by a magnetic field which is tangled on scales much smaller than the resolution capability of solar telescopes. This hidden magnetic field can be investigated via the Hanle effect. In 2007 we started a synoptic program to explore if the magnetic flux of the quiet photosphere varies with the solar cycle. For this purpose we applied a differential Hanle effect technique based on observations of scattering polarization in C$_2$ molecular lines around 514.0 nm, taken generally every month. Our results now span almost one complete solar cycle.