The contribution of plumes to the solar wind has been subject to hot debate in the past decades .
The EUV Imaging Spectrometer ( EIS ) on board Hinode provides a unique means to deduce outflow velocities at coronal heights via direct Doppler shift measurements of coronal emission lines .
Such direct Doppler shift measurements were not possible with previous spectrometers .
We measure the outflow velocity at coronal heights in several on-disk long-duration plumes , which are located in coronal holes and show significant blue shifts throughout the entire observational period .
In one case , a plume is measured 4 hours apart .
The deduced outflow velocities are consistent , suggesting that the flows are quasi-steady .
Furthermore , we provide an outflow velocity profile along the plumes , finding that the velocity corrected for the line-of-sight effect can reach 10 km s ^ { -1 } at 1.02 R _ { \odot } , 15 km s ^ { -1 } at 1.03 R _ { \odot } , and 25 km s ^ { -1 } at 1.05 R _ { \odot } .
This clear signature of steady acceleration , combined with the fact that there is no significant blue shift at the base of plumes , provides an important constraint on plume models .
At the height of 1.03 R _ { \odot } , EIS also deduced a density of 1.3 \times 10 ^ { 8 } cm ^ { -3 } , resulting in a proton flux of about 4.2 \times 10 ^ { 9 } cm ^ { -2 } s ^ { -1 } scaled to 1AU , which is an order of magnitude higher than the proton input to a typical solar wind if a radial expansion is assumed .
This suggests that , coronal hole plumes may be an important source of the solar wind .