POLAR PLUME ANATOMY - RESULTS OF A COORDINATED OBSERVATION

On 7 and 8 March 1996, the SOHO spacecraft and several other space- an d ground-based observatories cooperated in the most comprehensive obse rvation to date of solar polar plumes. Based on simultaneous data from five instruments, we describe the morphology of the plumes observed o ver the south pole of the Sun during the SOHO observing campaign. Indi vidual plumes have been characterized from the photosphere to approxim ately 15 R. yielding a coherent portrait of the features for more quan titative future studies. The observed plumes arise from small (similar to 2-5 arc sec diameter) quiescent, unipolar magnetic flux concentrat ions, on chromospheric network cell boundaries. They are denser and co oler than the surrounding coronal hole through which they extend, and are seen clearly in both Fe IX and Fe XII emission lines, indicating a n ionization temperature between 1.0-1.5 x 10(6) K. The plumes initial ly expand rapidly with altitude, to a diameter of 20-30 Mm about 30 Mm off the surface. Above 1.2 R., plumes are observed in white light (as 'coronal rays') and extend to above 12 R.. They grow superradially th roughout their observed height, increasing their subtended solid angle (relative to disk center) by a factor of similar to 10 between 1.05 R . and 4-5 R., and by a total factor of 20-40 between 1.05 R. and 12 R. . On spatial scales larger than 10 arc sec, plume structure in the low er corona (R < 1.3 R.) is observed to be steady-state for periods of a t least 24 hours; however, on spatial scales smaller than 10 arc sec, plume XUV intensities vary by 10-20% (after background subtraction) on a time scale of a few minutes.