... The Si viii line width measurements and N e estimates based on SUMER observations are combined with LASCO and UVCS output to provide an overview of its variations with height above a polar coronal hole. ... Our data suggests that the MHD waves responsible for the excess line broadening tends to become non­linear as it reaches 1.2 R fi . ... All solar wind modelling requires knowledge of both the electron density and non­thermal velocity at the base of the coronal hole as boundary conditions. ... Fig...