Short description
(English)
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An important task in astronomy is to measure and understand the nature of the chromospheric magnetic field, because it is via the magnetized chromosphere that the photosphere gets coupled to the corona. Little is still known about the chromospheric magnetic fields because they are generally weaker and so notoriously difficult to measure. Direct mapping of the chromospheric magnetic field in selected areas on the solar disk would provide the needed guidance to choose between competing theoretical models. At IRSOL we have developed instrumentation that can detect the chromospheric magnetic field, based on ZIMPOL (Zurich Imaging Polarimeter), the worlwide most precise polarimeter. Together with the Indian Institute of Astrophysics in Bangalore we have theoretically modelled the forward scattering Hanle effect, which allows the direct determination of the small and elusive chromospheric fields. Currently and during the next years we are the only institute that can do such observations, and we apply for a postdoc position to produce for the first time chromospheric magnetic field maps, based on observations to be carried out with ZIMPOL both at IRSOL and at GREGOR on Tenerife. This project would also allow us to better interact with other international research groups in our field and participate in this MP 1104 COST action.
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Abstract
(English)
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The main aim of this project has been to develop a diagnosis method to measure solar chromospheric magnetic fields using the polarization signals created by Hanle and Zeeman effects in forward scattering geometry. Few spectral lines show measurable forward scattering polarization signals on the solar disk; one is the 422.7 nm Calcium spectral line, which is used in our project. Its prominent signals are responding to the magnetic field existing in the lower half of the solar chromosphere, exactly there where vertical velocities and temperature start to grow from their minimum values until reaching their maximums in the solar corona. Both coronal and chromospheric heatings are connected, being expected to respond to the same not yet understood heating processes. Furthermore, there is general consensus about the key role that the magnetic field plays in the heating process. This is why researching the magnetism in the lower chromosphere (where the CaI 422.7 nm line is formed) can lead to key clues for solving the long-standing coronal heating problem. The only way of measuring chromospheric magnetic fields is through spectro-polarimetry. But for doing that properly it is necessary to consider that other physical actors can emulate/modify the polarization signals created by the magnetic field, and this is a key point of our project. The most powerful of those actors is solar dynamics: namely, vertical gradients of velocity and temperature. In the past, when most solar research was focused in the photosphere, this problem was hidden because there the magnetic field effects are dominant over velocity gradients effects, while in the chromosphere it is just the opposite. Such difficulty is then intrinsic to the solar layers we are studying, and it is in fact a common problem that other research groups working with chromospheric polarization will need to tackle sooner or later. Due to the above considerations, the current project has evolved during these two years for aiming at a fundamental and primary objective, nevertheless perfectly aligned with our original goals. Namely, we worked on finding a way of disentangling the effects that solar magnetic field and dynamics introduce in the polarization created in forward scattering. The 4227 CaI line is sensitive to Hanle and Zeeman effects only in the upper atomic level of the transition, which offers us minimal quantum-mechanic effects for facilitating the discrimination between different sources of polarization. We have used state-of-the-art solar chromospheric models developed at Oslo University by Carlsson et al. (2015, in press), which contain realistic motions together with magnetic fields. Finally, we have oriented our calculations to the research of the temporal evolution of the polarization signals. Accounting for those ingredients we are now in a position for understanding and illustrating the physical evolution of the chromosphere through the eyes of the polarization in the CaI 422.7 nm line.
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