## Doppler redistribution of anisotropic radiation and resonance polarization in moving scattering media## I. Theory revisited in the density matrix formalism
Under the light of recent developments of the theory of matter-radiation interaction in the presence of magnetic field applied to non-LTE spectropolarimetry in astrophysics, we have revisited the theory of anisotropic resonance line scattering in moving media by means of the density-matrix formulation. This has led us to present a theoretical method of determination of the matter velocity field vector in the solar wind acceleration region. The example of the O vi 103.2 nm line has been chosen for putting this theory into operation. It has been observed by the ultraviolet spectrograph Sumer of Soho in different regions of the solar wind acceleration region; it is partially formed by resonance scattering of the incident underlying transition region radiation which competes (and can predominate) with electron collisional excitation at the low densities which prevail at these high altitudes. The theory which is developed hereafter not only shows that this line is shifted and its intensity dimmed by the Doppler effect, due to the matter velocity field of the solar wind, but also predicts that it is linearly polarized, owing to the anisotropy of the incident radiation field; its two linear polarization parameters, degree and direction of polarization, are sensitive to the matter velocity field vector. Our results show that the interpretation of polarimetric data, associated to the shift and the Doppler-dimming effect, may offer a method of diagnostic of the complete velocity field vector, provided that the partial anisotropy of the incident radiation field be taken into account. In fact such a diagnostic is currently missing. Yet its interest is crucial to understand various problems in astrophysics, such as stellar winds, and especially the acceleration mechanisms of the solar wind. It is also essential for a dynamical modelling of solar structures.
This article contains no SIMBAD objects. ## Contents- 1. Introduction
- 2. Basic theory
- 3. Application to the O VI line 103.2 nm of the solar corona
- 3.1. Asymptotic limit: perfectly directive case
- 3.2. Symmetries of the problem
- 3.2.1. Symmetry due to the change of the direction of propagation of the incident light into the opposite sense
- 3.2.2. Symmetry with respect to the preferred direction of the incident radiation
- 3.2.3. Symmetry with respect to the center of scattering
*A* - 3.2.4. Symmetry with respect to the scattering plane
- 4. Conclusion
- Acknowledgements
- Appendix A: stokes parameters of the scattered radiation
- References
© European Southern Observatory (ESO) 1998 Online publication: November 9, 1998 |