Quasi-periodic compressive waves in polar plumes

The observation of polar plumes in the south polar coronal hole, carried out on 7 March 1996 by the Solar and Heliospheric Observatory (SOHO), are analyzed. These polar plumes are cool density structures that arise from morphologically unipolar magnetic footpoints. Data from the extreme ultraviolet imaging telescope show quasi-periodic perturbations in the brightness of the Fe IX and X line emissions at 171 A from polar plumes. The perturbations have periods of 10 to 15 min, and repeat for several cycles suggesting that they are compressive waves propagating through the plume at or near the Alfven speed. Possible explanations for the observed phenomenon are proposed.