Svetlana Berdyugina: Solar magnetism

Svetlana Berdyugina
Research projects Solar magnetism

(Mathew et al. 2004)
3D sunspot models
Magnetic fields of 2-3 kG in sunspots are already strong enough to cause a noticeable Paschen-Back effect on the fine structure of electronic terms of some molecules. Indeed, observations of sunspot spectra reveal strongly asymmetric Stokes V profiles of many MgH and CN lines. Since molecular lines are formed at different heights of the sunspot atmosphere and are also strongly temperature and pressure sensitive, an analysis of their Stokes profiles provides valuable information on the thermal and magnetic structure of sunspots. Recently, we have updated the SPINOR inversion code created by Frutiger (2000) and carried out first simultaneous inversions of atomic and molecular lines. We obtained very promising results when using simultaneously infrared Fe I and OH lines in the inversions, which allowed us to extend the recovered sunspot model to higher atmospheric layers (Mathew et al. 2003, 2004). We are currently applying our inversion procedure to high-resolution data obtained with the HINODE satellite.

(Zakharov et al., in preparation)

Imaging of sunspots in molecular bands
Based on our theoretical developments, we proposed a novel narrow-band filter centred at the TiO band head at 705.5 nm and successfully used it for observing fine structure of the sunspot umbra and a light-bridge at the Swedish Solar Tower, La Palma (Berger & Berdyugina 2003). TiO formation in sunspots increases the opacity in the coolest umbral regions while the hotter umbral dots remain relatively bright. TiO images thus show the umbral structure with significantly higher contrast than in other pass bands. The TiO filter has been accepted as a standard passband for imaging sunspots at the Advanced Technology Solar Telescope in the future.

(Zakharov et al. 2005)

Magnetic elements in the solar photosphere
Direct imaging of the solar photosphere in molecular bands reveals small-scale magnetic flux concentrations outside sunspots. They are often observed with the G-band filter centered at 430 nm, where strong CH lines significantly absorb the continuum light. Our modeling CH line formation processes in the solar atmosphere combined with realistic ab initio simulations of radiative magnetoconvection successfully explained the brightening of magnetic flux concentrations observed in the G-band (Schssler et al. 2003). This provided a firm basis for "proxy-magnetometry" with G-band images. We predicted also that the contrast of the magnetic flux concentration should be stronger if they are observed with the filter centered at the violet CN band at 388 nm. This was clearly confirmed by the recent observations at the Swedish Solar Tower, La Palma (Zakharov et al. 2005).

(Berdyugina & Fluri 2004)

Turbulent magnetic fields and Hanle effect
Coherent scattering on the Sun produces a wealth of linearly polarised (Stokes Q/I) spectral structures that are poorly correlated with the familiar spectral lines in the ordinary intensity spectrum. One of the big surprises when the richness of the scattering polarization was first uncovered (Stenflo & Keller 1996) was the prominence of molecular contributions, in particular from MgH and C2. While these molecular lines in the intensity spectrum of the quiet Sun are very inconspicuous and barely visible, they dominate the appearance of the polarization spectrum in many spectral regions. Magnetic fields influence the scattering polarisation via the Hanle effect. The main effect when observing near the Sun's limb is Hanle depolarisation, leading to reduced polarisation amplitudes when magnetic fields are present. Due to their different sensitivities to magnetic fields, different spectral lines are affected to different degrees by the Hanle effect. This differential Hanle effect can be used for model-independent diagnostics of spatially unresolved magnetic fields in regimes not accessible with the ordinary Zeeman effect, e.g. weak, turbulent fields of 1-50 G. We developed a theoretical foundation for polarized scattering radiation by molecules and explained qualitatively the enigmatic behaviour of molecular scattering polarization (Berdyugina et al. 2002). We also developed a simple model for scattering polarization in molecular lines and, for the first time, detected the Hanle effect in molecular lines and provided excellent molecular diagnosics of turbulent magnetic fields based on the differential Hanle effect (Berdyugina & Fluri 2004). We continue monitoring of the turbulent magnetic fields in C2 lines.



(Mathew et al. 2004)		3D sunspot models Magnetic fields of 2-3 kG in sunspots are already strong enough to cause a noticeable Paschen-Back effect on the fine structure of electronic terms of some molecules. Indeed, observations of sunspot spectra reveal strongly asymmetric Stokes V profiles of many MgH and CN lines. Since molecular lines are formed at different heights of the sunspot atmosphere and are also strongly temperature and pressure sensitive, an analysis of their Stokes profiles provides valuable information on the thermal and magnetic structure of sunspots. Recently, we have updated the SPINOR inversion code created by Frutiger (2000) and carried out first simultaneous inversions of atomic and molecular lines. We obtained very promising results when using simultaneously infrared Fe I and OH lines in the inversions, which allowed us to extend the recovered sunspot model to higher atmospheric layers (Mathew et al. 2003, 2004). We are currently applying our inversion procedure to high-resolution data obtained with the HINODE satellite.
(Zakharov et al., in preparation)		Imaging of sunspots in molecular bands Based on our theoretical developments, we proposed a novel narrow-band filter centred at the TiO band head at 705.5 nm and successfully used it for observing fine structure of the sunspot umbra and a light-bridge at the Swedish Solar Tower, La Palma (Berger & Berdyugina 2003). TiO formation in sunspots increases the opacity in the coolest umbral regions while the hotter umbral dots remain relatively bright. TiO images thus show the umbral structure with significantly higher contrast than in other pass bands. The TiO filter has been accepted as a standard passband for imaging sunspots at the Advanced Technology Solar Telescope in the future.
(Zakharov et al. 2005)		Magnetic elements in the solar photosphere Direct imaging of the solar photosphere in molecular bands reveals small-scale magnetic flux concentrations outside sunspots. They are often observed with the G-band filter centered at 430 nm, where strong CH lines significantly absorb the continuum light. Our modeling CH line formation processes in the solar atmosphere combined with realistic ab initio simulations of radiative magnetoconvection successfully explained the brightening of magnetic flux concentrations observed in the G-band (Schssler et al. 2003). This provided a firm basis for "proxy-magnetometry" with G-band images. We predicted also that the contrast of the magnetic flux concentration should be stronger if they are observed with the filter centered at the violet CN band at 388 nm. This was clearly confirmed by the recent observations at the Swedish Solar Tower, La Palma (Zakharov et al. 2005).
(Berdyugina & Fluri 2004)		Turbulent magnetic fields and Hanle effect Coherent scattering on the Sun produces a wealth of linearly polarised (Stokes Q/I) spectral structures that are poorly correlated with the familiar spectral lines in the ordinary intensity spectrum. One of the big surprises when the richness of the scattering polarization was first uncovered (Stenflo & Keller 1996) was the prominence of molecular contributions, in particular from MgH and C2. While these molecular lines in the intensity spectrum of the quiet Sun are very inconspicuous and barely visible, they dominate the appearance of the polarization spectrum in many spectral regions. Magnetic fields influence the scattering polarisation via the Hanle effect. The main effect when observing near the Sun's limb is Hanle depolarisation, leading to reduced polarisation amplitudes when magnetic fields are present. Due to their different sensitivities to magnetic fields, different spectral lines are affected to different degrees by the Hanle effect. This differential Hanle effect can be used for model-independent diagnostics of spatially unresolved magnetic fields in regimes not accessible with the ordinary Zeeman effect, e.g. weak, turbulent fields of 1-50 G. We developed a theoretical foundation for polarized scattering radiation by molecules and explained qualitatively the enigmatic behaviour of molecular scattering polarization (Berdyugina et al. 2002). We also developed a simple model for scattering polarization in molecular lines and, for the first time, detected the Hanle effect in molecular lines and provided excellent molecular diagnosics of turbulent magnetic fields based on the differential Hanle effect (Berdyugina & Fluri 2004). We continue monitoring of the turbulent magnetic fields in C2 lines.