Doppler redistribution of anisotropic radiation and resonance polarizationin moving scattering media - I. Theory revisited in the density matrix formalism

Under the light of recent developments of the theory of matter-radiation in teraction in the presence of magnetic field applied to non-LTE spectropolar imetry in astrophysics, we have revisited the theory of anisotropic resonan ce line scattering in moving media by means of the density-matrix formulati on. 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 theo ry into operation. It has been observed by the ultraviolet spectrograph SUM ER of SOHO in different regions of the solar wind acceleration region; it i s partially formed by resonance scattering of the incident underlying trans ition region radiation which competes (and can predominate) with electron c ollisional excitation at the low densities which prevail at these high alti tudes. The theory which is developed hereafter not only shows that this line is sh ifted and its intensity dimmed by the Doppler effect, due to the matter vel ocity field of the solar wind, but also predicts that it is linearly polari zed, owing to the anisotropy of the incident radiation field; its two linea r polarization parameters, degree and direction of polarization, are sensit ive to the matter velocity field vector. Our results show that the interpretation of polarimetric data, associated t o the shift and the Doppler-dimming effect, may offer a method of diagnosti c of the complete velocity field vector, provided that the partial anisotro py of the incident radiation field be taken into account. In fact such a di agnostic is currently missing. Yet its interest is crucial to understand va rious problems in astrophysics, such as stellar winds, and especially the a cceleration mechanisms of the solar wind. It is also essential for a dynami cal modelling of solar structures.