Solar transition region line broadening: Limb to limb measurements
L. Teriaca and
Received 14 December 1999 / Accepted 4 February 2000
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 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.
Key words: Sun: activity Sun: corona Sun: general Sun: transition region Sun: UV radiation
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© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000