Solar transition region line broadening: Limb to limb measurements

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 Å 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.