A&A 412, 271-280 (2003)
DOI: 10.1051/0004-6361:20031407
N.-E. Raouafi^{} - S. K. Solanki
Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
Received 16 September 2002 / Accepted 3 September 2003
Abstract
The effect of an anisotropic velocity field distribution of scattering ions on the polarization parameters of a spectral line emitted by resonance scattering is considered. The anisotropy of the velocity field distribution can be interpreted in terms of the ion-cyclotron effect that is believed to influence some heavy ions in the solar corona. We present a theoretical study of the Stokes parameters of a spectral line emitted by atoms or ions in the presence of a bi-Maxwellian velocity field distribution. It is found from test calculations that such a distribution measurably changes the polarization properties of the
D_{2} coronal line. Consequently, measurements of the linear polarization of this line may serve as a new diagnostic of a possible bi-Maxwellian velocity distribution. As a preliminary application, the obtained theoretical results are used to interpret the polarization parameters of the
D_{2} coronal line (
)
measured using SUMER/SoHO observations. The obtained results are compatible with SUMER's observations for more reasonable solar wind parameters than for an isotropic velocity field distribution of the scattering ions. Thus, the outflow velocity field vector of the emitting ions is less inclined with respect to the polar axis of the Sun (the minimal value of the polar angle
is
for an anisotropic velocity distribution versus
for an isotropic velocity field distribution). These results are obtained assuming that the re-emitted photons come from a small area in the center of the coronal polar hole, with zero magnetic field. Since SUMER/SoHO observations integrate over the line of sight, the results of the current analysis must be considered preliminary pending computations including an integration along the line of sight.
Key words: polarization - scattering - line: profiles - Sun: corona - Sun: solar wind - Sun: UV radiation
Spectropolarimetry of coronal ultraviolet lines which are sensitive to the effect of the Doppler redistribution due to ion motion and/or to the effect of the coronal magnetic field (Hanle effect, see Mitchell & Zemansky 1934) could yield more accurate information about the physical conditions of the coronal plasma. In fact, spectropolarimetry provides access to the magnetic field vector as well as the velocity field vector (see Raouafi et al. 2002a; Sahal-Bréchot et al. 1998). In contrast, without polarimetric information the spectroscopy of lines only provides partial information on the vectorial quantities, basically through Doppler shifts.
The line (hereafter D_{2} line) is one of the strongest emitted by the solar corona up to high altitudes, as demonstrated by Vial et al. (1980); Kohl et al. (1998); Xing Li et al. (1998); and other papers related to UVCS (Ultraviolet Coronagraph Spectrometer: Kohl et al. 1995 & 1997) aboard of SoHO (the Solar and Heliospheric Observatory: Domingo et al. 1995). The D_{2} line (and also the line, hereafter D_{1}) is formed in the chromosphere-corona transition region. In the corona, the O^{5+} ions are excited by isotropic electronic collisions (which do not create polarization in the Zeeman sub-levels of the excited ions) and by the unpolarized photons with a somewhat anisotropic distribution (due to the center-to-limb brightening) coming from the underlying transition region. The partial anisotropy of the radiation coming from the chromosphere-corona transition region creates the partial linear polarization of the D_{2} coronal line detected for the first time by Raouafi et al. (1999a) in observations performed by the SUMER spectrometer (Solar Ultraviolet Measurements of Emitted Radiation; Wilhelm et al. 1995, 1997; Lemaire et al. 1997) on SoHO. These observations are raster sequences performed at different angles with respect to the polar axis of the Sun by rotating the SoHO spacecraft and moving SUMER's slit laterally. The area common to all the rasters is centered on the polar axis. The observations are described in detail by Raouafi et al. (1999).
The present paper is the third of a series of papers on the interpretation of the linear polarization parameters of the D_{2} line. The goal is the determination of the velocity field vector in the solar corona, which is dominated by the solar wind at increasing distance from the Sun. Raouafi et al. (2002a) showed that the Doppler redistribution effect (due to the ions' motion with an isotropic velocity distribution and a drift velocity vector) on the linear polarization of the D_{2} coronal line gives results in agreement with SUMER/SoHO observations. However, the obtained solutions correspond to velocity field vectors (solar wind velocity field vectors) with relatively high inclinations with respect to the polar axis. Thus the minimal value of the polar angle of the velocity field vector of the solar wind with respect to the polar axis (Pz) is (see Fig. 1). This is unsatisfactory because although the expansion of the coronal structures in the polar holes is non-radial, these latter are quasi-radial at the altitudes where SUMER/SoHO observations were carried out (1.29 from the center of the Sun) (see Fig. 1 in Wilhelm et al. 1998). Raouafi et al. (2002b) showed also that the combination of the Doppler redistribution effect with the Hanle effect due to the coronal magnetic field gives better results than those obtained for the case of the Doppler redistribution alone. In the present paper, we consider the influence of an anisotropic coronal velocity field on the polarization of the D_{2}. The effect of the magnetic field is not taken into account in the present paper. In fact the expected range of the coronal magnetic strength is at the lower edge of the interval of the D_{2} line's sensitivity to the Hanle effect. For such field strengths the effect of Doppler redistribution is more important than that of the coronal magnetic field. The combined effect of a magnetic field and of anisotropic velocity distribution of the scattering ions will be the subject of a future investigation, together with the effect of the integration along the line of sight.
Observations performed by UVCS/SoHO (Kohl et al. 1995) show that line profiles of some of the minor ions (O^{5+} and Mg^{9+}) of the solar corona indicate the presence of two components. One is a very broad component with a Doppler width corresponding to temperatures of many million Kelvin. This component provides the dominant contribution to the spectral line at high altitudes where evidence of anisotropic velocity field distribution is obtained from the observed line profiles combined with the intensity ratio of the D_{1} and D_{2} lines of the O^{5+} doublet (e.g. Kohl et al. 1998; Cranmer et al. 1999; Cranmer 2001). The outflow speeds of O^{5+} ions are also found to be higher than expected for most of the coronal ions. A promising interpretation of these unexpected observational results proposes the transport of energy by magnetohydrodynamic waves at the cyclotron frequencies of the heavy particles in the local magnetic field (e.g. Kohl et al. 1999; Cranmer 2000; Markovskii 2001; Gary et al. 2001; Isenberg 2001; Axford & McKenzie 2002).
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). and are the angular coordinates of the unitary vector 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 . The angular coordinates of 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 with the polar axis (Pz). | |
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The paper is structured as follows. In Sect. 2, we present theoretical results concerning the Stokes parameters of spectral lines emitted by resonance scattering by atoms or ions in the presence of an anisotropic velocity field distribution. In Sect. 3, we present numerical calculations showing the effect of the anisotropy of the velocity field distribution on the polarization parameters of the re-emitted line. In Sect. 4, as an application of the obtained results, we apply the theory developed in this paper to the interpretation of the polarimetric measurements made by SUMER/SoHO (Wilhelm et al. 1995) of the linear polarization parameters of the D_{2} coronal line and the intensity ratio of the doublet. Finally, in Sect. 5 the results are discussed.
The equations presented by Raouafi (2000, 2002) describing the Stokes parameters of a resonantly scattering spectral line sensitive to the Hanle and Doppler redistribution effects are general. They give the Stokes parameters as a function of the velocity vector of the scattering atoms (or ions) and the local magnetic field vector (the coronal magnetic field vector in the solar case) they are immersed in. However, the relevant equations show that both effects are decoupled. In the limit of zero magnetic field, the results of Sahal-Bréchot et al. (1998) are recovered which were derived considering only the effect of the velocity field. To obtain the magnetic field effect alone it is sufficient to eliminate the dimming term by cancelling the macroscopic velocity field vector of the scattering ions, and we obtain the results of Sahal-Bréchot et al. (1986).
Raouafi (2000 & 2002) derived the general expressions giving the linear polarization of a spectral line created by resonance scattering that is sensitive to the Doppler redistribution due to the motion of the scattering atoms (or ions) and/or to the Hanle effect caused by a local magnetic field. A particular case is that of coronal lines scattered by lithium-like ions (O^{5+}, N^{4+}, C^{3+}, ...). The general form of the Stokes parameters of the re-emitted radiation (here we only take into account the linear polarization) is given by (Eq. (20) in Raouafi 2002)
(1) |
In order to obtain the Stokes profiles as a function of the scattered radiation frequency ,
one needs to average the previous expression over all the atoms or ions having
along the line of sight. This could be achieved by a double integration over the velocity field components
and
in the plane perpendicular to the line of sight (inset of Fig. 1). They are given (in photons s^{-1} cm^{-3} Hz^{-1} str^{-1}) by
(4) |
(5) |
For the radiative term in Eq. (2), the integration over the velocity field components perpendicular to the line of sight is achieved by integrating the velocity field distribution
multiplied by the Gaussian term in the incident line profile
of Eq. (3)
(6) |
(7) |
(9) |
In order to obtain the Stokes parameters integrated over the line profile of the scattered radiation, we need to integrate Eq. (8) over the frequency .
This is equivalent to integrating over
and could be also done analytically. For the collisional part of Eq. (8), the integration gives
(11) |
= | |||
(12) |
Figure 2: Top panels give the degree of linear polarization as a function of the azimuthal angle (of the macroscopic velocity field vector ) for different values of the polar angle (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 ( and , respectively) is set to 2 in the present case, which corresponds roughly to km s^{-1}. These curves are obtained for the case of right-angle scattering . | |
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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 present section, we calculate numerically by using the theoretical results obtained in the previous section, the polarization parameters (degree and rotation angle of the direction of linear polarization with respect to the tangent to the solar limb) of the D_{2} line scattered by the coronal ions. For simplicity and in order to compare the present results to those obtained for an isotropic velocity distribution (see Raouafi et al. 2002a), we consider the same system as by Raouafi et al. (2002a). However, we consider a bi-Maxwellian velocity distribution instead of a simple Maxwellian one.
This means that the Doppler width parallel to the magnetic field vector differs from the Doppler width perpendicular to the field. In a first approximation, the Doppler width of the re-emitted line is in this case given by
We consider scattering ions that are located in an infinitesimal volume around the point P on the polar axis at from the solar center. They are illuminated by the partially anisotropic radiation coming from a spherical cap that contains a polar hole with axial symmetry. The polar hole has a radius of . The emitting ions interact also with electrons with an isotropic distribution. The considered electron density^{} is cm^{-3} and the collision coefficient is cm^{-3} s^{-1} (Sahal-Bréchot et al. 1986). We take into account also the limb-brightening function given by Raouafi et al. (2002a). The re-emitted photons are observed at a scattering angle equal to . The Einstein coefficient B_{lu} is determined from the atomic data given by Sahal-Bréchot et al. (1986) and the specific intensity is determined from SUMER/SoHO observations and is equal to erg s^{-1} cm^{-2} str^{-1}.
For weak values of the outflow speed (V<27 km s^{-1}), the Doppler redistribution effect is depolarizing for the re-emitted radiation. The depolarization increases with the outflow speed (compare left and middle top panels of Fig. 2, note the small decrease in the y-axis numbers) and with the polar angle (compare different lines in the top left or middle panels of Fig. 2 which range from for the topmost line to for the lowest). This is until a given limit of around 27 km s^{-1} where the velocity effect becomes polarizing for some ranges of the azimuthal angle (see top-right panel in Fig. 2). Beyond this limit, the depolarization of the scattered radiation due to the Doppler redistribution increases with the outflow speed and decreases with the polar angle (see top panels of Fig. 3). For this range of outflow speeds (beyond the limit of 27 km s^{-1}), the shapes of the curves giving the polarization parameters as a function of the azimuthal angle are similar to those obtained with an isotropic velocity distribution. Curves given by speeds around 27 km s^{-1} are between those obtained for the two ranges of outflow speed (see top-right panel of Fig. 2; for V=27 km s^{-1}).
The curves giving the rotation angle of the direction of linear polarization with respect to the tangent to the solar limb (Px) are quasi-sinusoidal for outflow speeds far away from the limit of V=27 km s^{-1}. The amplitude of these curves has the same behavior as that obtained for the polarization degree (see bottom panels of Figs. 2 and 3). The variation of the amplitude of the curves of the rotation angle of the polarization direction with the outflow speed and the polar angle is clearer than for the polarization degree. The curves giving the polarization parameters as a function of the azimuthal angle for the low speeds are shifted by with respect to those given by values greater than 20 km s^{-1} of the outflow speed (see Figs. 2 and 3).
Note that the amplitudes of the different curves increases with the ratio . The position of the critical limit between weak and strong outflow speeds does not change very much with this ratio (24 km s^{-1} for , 27 km s^{-1} for and 28 km s^{-1} for ).
Compared to the effect of an isotropic velocity field distribution (a Maxwellian with a drift velocity field vector, for more details see Raouafi et al. 2002a), an anisotropic velocity field distribution has an important effect on the direction of linear polarization. The effect on the degree of linear polarization is less important (see Figs. 2 and 3 and compare with Figs. 8-10 of Raouafi et al. 2002a). In fact, with an anisotropic velocity field distribution we obtain more rotation of the direction of linear polarization for all the polar angle values. Significant values of the rotation angle of the linear polarization are obtained for high outflow speeds beyond the second limit.
The SUMER spectrometer operating on board the SoHO spacecraft is sensitive to the linear polarization of the observed light due to oblique reflections in the instrument, which have different reflection coefficients for orthogonal linear polarizations (37% at 1000 Å) (Wilhelm et al. 1995; Hassler et al. 1997). It has been used to analyze the linear polarization of the D_{2} coronal line. The net linear polarization is obtained by subtracting two intensity measurements from each other, one obtained before, the other after rotating the whole SoHO spacecraft by around its optical axis. The observations recorded by SUMER/SoHO on March 19 1996 are described by Raouafi et al. (1999 & 2002a). The obtained degree of linear polarization integrated over the line profile is found to be equal to . The rotation angle of the direction of linear polarization with respect to the tangent to the solar limb is found to be equal to . The mean intensity ratio of the doublet is found to be (for more details, see Raouafi et al. 2002a).
Raouafi et al. (2002a) found that the Doppler redistribution effect due to the scattering ions' motion, together with an isotropic velocity field distribution (Maxwellian with a drift velocity field that could be equated to the outflow velocity field vector of the solar wind) reproduces very well the measured values. It provides strong constraints on the outflow speed of the solar wind in the polar holes. However, the velocity field vectors corresponding to the obtained solutions have a relatively high inclination angle with respect to the polar axis of the Sun (minimal value of the inclination angle is with respect to the polar axis (Pz) in Fig. 1). In the polar hole of the solar corona, one expects that velocity field vectors are more or less radial directly above the pole (neglecting contributions from fore- or background material). Raouafi et al. (2002b) showed also that the combination of the Doppler redistribution effect with the Hanle effect due to the coronal magnetic field gives better results. It sets constraints on the solar wind velocity field and on the coronal magnetic field vectors in the polar holes at a heliocentric distance of . Nonetheless the inclination angles with respect to the polar axis of the vectors corresponding to the numerical solutions remain relatively high.
In the present section, we apply the theoretical results obtained above to the case of the D_{2} coronal line. We consider an anisotropic bi-Maxwellian velocity field distribution. The outflow speed of the solar wind is given by the drift velocity field vector parallel to the coronal magnetic field. Both the strength and direction of the velocity vector are allowed to vary.
As in Raouafi et al. (2002a & 2002b), we calculate the degree and rotation angle of the direction of linear polarization with respect to the tangent to the solar limb ((Px) in Fig. 1). We calculate also the intensity ratio of the doublet D_{1} and D_{2}. After comparison of the obtained results with the measured values, we keep only velocity field vectors which give linear polarization degree in the observed range , rotation angle of the direction of linear polarization in the range and an intensity ratio of the doublet within the range . We first carry out the calculations for the case which is consistent with the UVCS/SoHO measurements.
In the present case, we consider ions to be contained in an infinitesimal volume on the polar axis at a heliocentric distance of . The influence of an extended distribution of the gas along the line-of-sight will be studied in a forthcoming paper. They re-emit by resonant scattering the partially anisotropic and unpolarized radiation coming from the underlying chromosphere-corona transition region. They are also excited by isotropic electron collisions that do not create polarization in the Zeeman levels of the excited ions. The electron density is taken to be equal to cm^{-3}. The re-emitted photons are observed under a scattering angle of . These are the same conditions considered by Raouafi et al. (2002a,b) to study the effect of the Doppler redistribution on the polarization of the D_{2} line and the intensity ratio of the D_{2} and D_{1} lines of the ion.
We consider a bi-Maxwellian velocity distribution with widths in the direction parallel to the coronal magnetic field and in the direction perpendicular to the coronal magnetic field vector that correspond to different values of the ratio and satisfy Eq. (13). These parameters are qualitatively consistent with the results obtained from UVCS/SoHO, although the latter refer to larger radial distances from the Sun's surface. For the numerical calculations, the outflow speed varies from 0 to 100 km s^{-1}, the polar angle varies from to and the azimuth angle varies from to . For the interpretation of the numerical results, we adopt the polar angle ranges considered by Raouafi et al. (2002a,b):
Figure 4: 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 D_{2} line and of the intensity ratio of the doublet. The top panel corresponds to and the bottom one corresponds to . 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 36-42 km s^{-1}. The lowest value of the polar angle of the velocity field vector giving good results is equal to 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 was found equal to . 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 Fig. 5 we summarize the minimum angle obtained for different values of the ratio . Clearly, drops rapidly from its value at ( ) before reaching an asymptotic value of close to for . Due to the unrealistically narrow profiles resulting from the requirement of simultaneously also satisfying Eq. (13) we consider only to be realistic.
The observations are thus reproduced for more reasonable parameters in the case of than for . Are the Stokes measurements of the lines thus providing support for an anisotropic temperature and eventually for ion-cyclotron waves? To test the uniqueness of the result obtained in this section we consider, for comparison, the opposite case, namely in the following section.
For values of the ratio smaller than 1, we also obtain results compatible with the observations. The obtained results are similar to those obtained for the case of an isotropic velocity distribution. The minimal value of the polar angle is and corresponds to . We are aware that in the coronal case, one expects that the ion temperature in the direction perpendicular to the magnetic field be higher than that in the parallel direction. These calculations are just for purposes of comparison. They demonstrate that the sign of the difference cannot be determined with sufficient accuracy from current SUMER/SoHO observations.
Table 1: Limits of the outflow speeds providing results in agreement with the observations and corresponding to different values of the ratio and to different ranges of the polar angle (weak inclinations, weak + medium inclinations and weak + medium+ strong inclinations, respectively). The outflow speed V is in km s^{-1}.
Figure 5: Minimal values of the polar angle of the outflow velocity vector plotted as a function of the ratio . | |
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We have considered the effect of an anisotropic velocity distribution on the intensity and polarization properties of optically thin spectral lines. We have then applied the general theory to the specific case of the D_{1} and D_{2} lines of O VI. Evidence for an anisotropic velocity distribution in the solar corona has been provided by observations made by the UVCS instrument on SoHO (Kohl et al. 1998, 1999). The anisotropy of the velocity field distribution can be interpreted in terms of ion cyclotron waves which, according to theoretical models, could exist everywhere in the solar corona (Marsch 1999; Marsch & Tu 2001; Tu & Marsch 2001; Vocks & Marsch 2001, 2002).
We find that the velocity field anisotropy has a significant effect on the direction of linear polarization. It also influences the degree of linear polarization and the intensity ratio, but these effects are less important for the velocities we consider here. In fact, the polar angle of velocity field vectors giving results compatible with the observations goes down to ; while for an isotropic velocity field distribution the lowest value of the polar angle is . Since the SUMER/SoHO data we compare with were recorded directly above the pole in a coronal hole, the results obtained by taking into account a bi-Maxwellian velocity field distribution are more apropriate than those obtained with a simple Maxwellian velocity field distribution.
In both cases and (independently of the physical significance of these two cases, particularly the second one), the obtained results fit very well the measured values. Note that up until this date at very low altitudes in the corona (< from the center of the Sun) there is no observational proof of an anisotropic temperature distribution of the scattering ions. The UVCS/SoHO data refer to , while SUMER/SoHO data, which we compare to our calculations, were obtained at . The relevant UVCS/SoHO observations at high altitudes (> from solar center) are interpreted such that the ion temperature in the direction perpendicular to the coronal magnetic field is much higher than that in the direction parallel to the magnetic field vector.
In summary, we have shown that the linear polarization measured in coronal holes above the solar limb can serve as a diagnostic of an anisotropic velocity distribution. A simple analysis provides results that favor the presence of an anisotropic velocity distribution. However, there remains a need to find more concrete evidence of an anisotropic temperature at the heights considered here. To enhance the reliability of the current results we need to study the simultaneous influence of a magnetic field and an anisotropic velocity distribution, as well as the effect of the 3D geometry of the coronal magnetic field on the observations (e.g., influence of the presence of inhomogeneities on the radiation integrated along the line of sight).
Acknowledgements
The authors gratefully acknowledge an anonymous referee for constractive critics and E. Marsch for helpful discussions.
The quantities ,
,
and
first appearing in Eq. (7) are given by
= | |||
= | |||
= | |||
(A.1) |
(A.2) |
(A.3) |