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 R☉ from disk center in the image plane and ~45 R☉ in three-dimensional space, a factor of 2-3 farther than previous imaging measurements and well into the constant-velocity regime of wind flow. We find 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 R☉ above the Sun (in three dimensions). At this altitude, the plumes are (2-4) × 103 cm-3 above the background interplume density, with an estimated plasma β of order 300. The excess electron densities are a factor of 20-30 greater than the average total electron density estimates obtained from extrapolation 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 R☉ and Ulysses's altitude of ~200 R☉.