Solar transition region line broadening: Limb to limb measurements

Abstract

Full disk images from SUMER/SoHO taken in He I, C IV and Ne VIII are used to investigate whether there exists a center-to-limb variation in the line width. Both C IV and He I show such a variation but the higher temperature Ne VIII line is relatively constant. For C IV, this corresponds to ~ 3 km s^-1 difference while He I is significantly larger particularly at the limb. This work may suggest that the non-thermal motions are slightly non-isotropic in the transition region and upper chromosphere, with the horizontal unresolved motions exceeding those in the vertical plane however a more probable explanation is that the lines are broadened due to opacity effects. The more slower variation in the C IV 1548 Ä line width could be explained by increasing the opacity from zero at disk center to ~ 1 at the limb. For He I the opacity is significantly grater than unity at the limb. This therefore implying all mass motions in the chromosphere, transition region and corona are isotropic.