A geometrical model for polar plumes observed by STEREO SECCH
  Solar polar plumes are bright radial rays rooted at the sun's polar areas. They are widely believed to have the structure of expanding tube. A four degree polynomial function was assumed to represent the change of cross section diameter as a function of height and four unknown parameters were calculated with measured average widths of total 31 plumes at 4 heights (1.04$R_s$,1.10$R_s$,1.16$R_s$,1.20$R_s$). 
Beidi Peng
astro-ph.SR