The first basic assumption of this Paper is to model the solar H incident radiation on the spicule by a negative exponential profile of width Å . Due to the fact that we use a gaussian profile, the average on the shifted velocity distribution of the scatterer can be performed nearly analytically. An important result is that limb darkening enhances the polarization degree together with the fact that isotropic collisions decrease the degree itself. We show that the polarization degree is maximum in a range of anisotropic velocities compatible with the real velocity of matter in spicules.
The order of the polarization degree is for atoms situated at an altitude of km and for an electron density of cm-3 (see Fig. 5). A consequence of the theory is to find a rotation of the polarization direction independent of collisions. Up to now, the radiation scattered by the matter in the spicule is used to determine the velocity of escape projected onto the line of sight. The measurement of both the shift of the profile and the angle gives information on the azimuth of the spicules, which provides a step forward towards the complete determination of the angular geometry of spicules.
The theory we have built is very general and may be improved by the introduction of a weak static magnetic field. This will be done in future work.
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998