Outflow velocity of interplume regions at the base of Polar Coronal Holes

We report on SUMER/SOHO observations at 1.05 Ro of a well identified interplume region in a South Pole Coronal Hole. Combination of Doppler shifts and Doppler dimming measurements allowed to determine, for the first time, the total wind outflow velocity (approximate to 67 km s(-1)) at this height. Our calculations of the outflow velocity benefit from co-spatial and almost co-temporal observations. This large outflow velocity is a strong argument in favour of the interplumes being the main soul-ce of the fast solar wind. We find that the mass flux density through the observed interplume is 4.8 10(-10) g cm(-2)s(-1) which yields 10(-15) g cm(-2)s(-1) at 1 AU with an expansion factor of 11.