Convective patterns in a simulation of solar convection. Shown are (a) the radial velocity (b) the radial vorticity, (c) the horizontal divergence and (d) the temperature perturbation near the outer surface.
High resolution figure
Almost everything we know about the Sun comes from our interpretation of its radiative output. The study of the intensity and polarization of the radiation that we receive from solar regions allows us to infer the thermo-dynamical and magnetic properties of the emitting plasma, if we are able to formulate adequate models of the origin and transport of polarized radiation in the solar atmosphere. In the deeper and denser layers of the visible atmosphere (photosphere), plasma collisions typically ensure that, at each point in the plasma, the ratio of radiation emissivity to absorptivity (source function) is only determined by the local thermal properties of the plasma (local thermodynamic equilibrium, or LTE). Under these special conditions, the mechanisms for the production and transport of polarized radiation are very well understood, and reliable models have been available for at least half a century.
As we move outward in the solar atmosphere (chromosphere and corona), the plasma density rapidly decreases, while at the same time the radiation becomes increasingly anisotropic. Both conditions determine significant departures from LTE, as the atomic equilibrium is now driven mainly by optical pumping by the underlying photospheric radiation. These are also the regions of the solar atmosphere where the topology of the magnetic fields that permeate the heliosphere - finally interacting with the Earth's magnetosphere - takes shape. So the development of adequate models of polarized radiative transfer in these regions, in order to determine the correct magnetic boundary conditions of the heliosphere, is of primary importance for our understanding of solar drivers of Space Weather.
FY07 achievements
1) The inversion of spectro-polarimetric data in one solar filament (i.e., a prominence during its transit over the solar disk) observed in an active region has revealed the presence of magnetic fields in excess of 700 G. These large fields - more than one order of magnitude larger than the observed magnetic strengths in quiet-Sun filaments - pose important questions on the magneto-hydrodynamic stability of these structures.
2) We demonstrated that broadband scattering polarization in hydrogen lines can be produced in a magnetized plasma even in the absence of anisotropic irradiation, due to the presence of micro-turbulent electric fields in plasmas. This result forces a complete revision of the use of hydrogen lines for magnetic diagnostics - in solar structures as well as in laboratory plasmas - and the questioning of results old and new in solar magnetism that have been obtained with these lines (e.g., the measurement of magnetic fields in prominences using the Balmer series of hydrogen).
3) We initiated a study of multi-level effects in the formation of the two well-known Fe I lines at 630 nm, which was motivated by recent spectro-polarimetric observations of these lines in chromospheric emission with the SOT instrument on board Hinode. We were able to prove that these multi-level effects explain the surprising finding that the polarization of these lines by scattering appears to be radially oriented, rather than tangential to the solar limb as one would have expected.
FY08 plans
1) To work on the radiative transfer modeling in a realistic solar atmosphere of the Na I doublet at 589 nm, with the goal of resolving the long standing "enigma" concerning the complicated shape of the linear polarization profile of these lines, produced by scattering near the solar limb.
2) To progress on the theory of the polarized line formation in the presence of coherent scattering (partial redistribution in frequency).
3) To devise plasma diagnostic techniques exploiting the polarization effects of micro-turbulent electric fields on hydrogen lines (e.g., the role of electric-induced dichroism in optically thick plasmas).
4) To refine the atmospheric model adopted for the simulation of the chromospheric Fe I Hinode observations, and to reach a better understanding of the interplay between multi-level effects in Fe I and magnetic fields, in order to assess the diagnostic potential of Fe I limb emission for chromospheric magnetism.
