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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS3088_Fig1.eps} \end{figure}$ Figure 1: Definition of the different axes and angles used for the calculation of the effect of an anisotropy of the velocity field distribution on the polarization parameters of a spectral line. The scattering atoms or ions are located in a small volume around P. They interact with electrons with an isotropic velocity distribution and with the radiation coming from the transition region located in the spherical cap (shaded area on the solar disk). $\alpha$ and $\beta$ are the angular coordinates of the unitary vector ${\textbf {\textit {n}}}$ that is directed along a radiation beam coming from a small area centered on M. The velocity field distribution of the atoms and ions is considered to be a bi-Maxwellian with a drift velocity field vector ${\textbf {\textit {V}}}$ . The angular coordinates $(\eta,~\psi)$ of ${\textbf {\textit {V}}}$ in the frame (Pxyz) are given in the panel in the top-right of the present figure. The line of sight (PZ) is in the scattering plane (yPz) and makes an angle $\theta$ with the polar axis (Pz).
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In the text

$\begin{figure} \par\includegraphics[width=18cm,clip]{MS3088_Fig2.eps} \end{figure}$ Figure 2: Top panels give the degree of linear polarization as a function of the azimuthal angle $\psi$ (of the macroscopic velocity field vector ${\textbf {\textit {V}}}$ ) for different values of the polar angle $\eta$ (see lower right panel for a legend) and for outflow speeds of 10, 20 and 27 km s^-1 (left, middle and right panels, respectively). The bottom panels exhibit the rotation angle of the direction of linear polarization with respect to the tangent to the solar limb. The ratio of the widths of the velocity field distribution in the directions parallel and perpendicular to the magnetic field ( $\alpha_{\!_{/\!/}}$ and $\alpha_{\!_\perp}$ , respectively) is set to 2 in the present case, which corresponds roughly to $\alpha_{\!_\perp}\sim54$ km s^-1. These curves are obtained for the case of right-angle scattering $(\theta =-\pi /2)$ .
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In the text

$\begin{figure} \par\includegraphics[width=17.4cm,clip]{MS3088_Fig3.eps} \end{figure}$ Figure 3: The same as Fig. 2, but for outflow speeds of 35, 40 and 50 km s^-1 (left, middle and right panels, respectively).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS3088_Fig4.eps} \end{figure}$ Figure 4: $(\eta,~\psi)$ diagrams giving the angular coordinates of velocity field vectors with strengths of 41 km s^-1 that correspond to numerical solutions in agreement with SUMER/SoHO measurements of the polarization and width of the D₂ line and of the intensity ratio of the $O {\sc vi}$ doublet. The top panel corresponds to $\displaystyle({\alpha _{\!_\perp}}/{\alpha_{\!_{/\!/}}}=1.5)$ and the bottom one corresponds to $\displaystyle({\alpha _{\!_\perp}}/{\alpha_{\!_{/\!/}}}=3)$ . The velocity field distribution underlying the shaded areas is considered to be a bi-Maxwellian with a drift velocity field parallel to the magnetic field vector. Velocity field vectors with weak inclinations that reproduce the data lie within the range $\sim$ 36-42 km s^-1. The lowest value of the polar angle of the velocity field vector giving good results is equal to $8\hbox {$^\circ $ }$ and corresponds to a speed of 41 km s^-1. It is lower than for an isotropic velocity field distribution where the lowest value of $\eta$ was found equal to $17\hbox {$^\circ $ }$ . For comparison, the valid solutions obtained for speeds of 41 km s^-1 for an isotropic velocity field distribution lie inside the contours overplotted on each panel (see Raouafi et al. 2002a).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS3088_Fig5.eps} \end{figure}$ Figure 5: Minimal values of the polar angle $\eta$ of the outflow velocity vector plotted as a function of the ratio $(\alpha_{\!_\perp}/\alpha_{\!_{/\!/})}$ .
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In the text

$\begin{figure} \par\includegraphics[width=17.4cm,clip]{MS3088_Fig3.eps} \end{figure}$	Figure 3: The same as Fig. 2, but for outflow speeds of 35, 40 and 50 km s^-1 (left, middle and right panels, respectively).
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS3088_Fig5.eps} \end{figure}$	Figure 5: Minimal values of the polar angle $\eta$ of the outflow velocity vector plotted as a function of the ratio $(\alpha_{\!_\perp}/\alpha_{\!_{/\!/})}$ .
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