2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.( OBSERVATION OF POLAR PLUMES AT HIGH SOLAR ALTITUDES

Using the Large-Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft, we have imaged polar plumes extending 30 from disk center in theR _image plane and D45 in three-dimensional space, a factor of 2È3 farther than previous imaging mea-R _surements and well into the constant-velocity regime of wind Ñow. We Ðnd that the plumes maintain their overall linear morphology and density enhancement to at least this altitude range. Using LASCO photometry and a modeled cylindrical plume geometry, we derive the density excess within the plumes 30 above the Sun (in three dimensions). At this altitude, the plumes are (2È4)] 103 cm~3 above theR _background interplume density, with an estimated plasma b of order 300. The excess electron densities are a factor of 20È30 greater than the average total electron density estimates obtained from extrapo-lation of in situ measurements by Ulysses at 1 AU. The contrast between the high plume excess densities that we observe and the uniformity of the wind seen by Ulysses may best be explained by wind models that include horizontal mixing in the lower heliosphere between 45 and UlyssesÏs altitude of D200