  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$R_s$).
