Solar wind acceleration in low density regions

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High speed solar wind is known to originate in polar coronal holes which, however, are made up of two components: bright, high density regions known as plumes, and dark, weakly emitting low density regions known as interplumes. Recent space observations have shown that the width of UV lines is larger in interplume regions [see e.g. 1, 2] while observations of the ratio of the O vi doublet lines at 1032 and 1037 A, at the altitude of 1.7 solar radii, suggest higher outflows in interplume regions than in plumes [3]. These results seem to locate the source of the fast solar wind in the interplume regions. The present work aims at identifying the outflow speed vs. altitude profile of the O vi ions, at heights up to 2 solar radii, both in plumes and interplume regions. To this end, we examined SUMER and UVCS data taken in the North polar coronal hole on June 3, 1996 over the altitude range between 1 and 2 solar radii. A Doppler dimming analysis applied to our data allows us to determine the outflow speed in interplume regions throughout the range covered by the observations. Our results favor interplumes as sources of fast wind. However, models mimicking observations in plume regions will also be discussed.