Quasi-periodic Intensity Disturbances in Polar Plumes

It's known that polar coronal plumes appear to be hazy and ray-like structures. Quasi-periodic disturbances in polar plumes are often observed with Extreme-ultraviolet (EUV) images and are identified as alternating slanted ridges in the distance-time maps with periods of 10-30 minutes. Usually, their propagating speeds range from 60 to 150 km/s. We analyse the intensity variation above polar coronal holes with data from three SDO/AIA bandpass channels (171Å, 193Å, 304Å) by using wavelet analysis and FFT, and produce intensity power distribution images. We find that slender radial structures (which we call fine structures) in these images and their widths are only a few arcsec in the plume and inter-plume regions. We propose that these fine structures could depict the orientation of the magnetic field in the polar coronal hole. Similar to the previous research, intensity disturbances along fine structures have periods of 15-20 min. Besides, the propagating speed of intensity disturbances along the fine structures ranges from more than a dozen kilometers per second at just above the solar limb to 150 km/s around 140'' above the limb. It is easy to identify the 304Å jets in the power images. We find that the intensity variation of jets obtained from the distance-time maps of the 304 line is often inversely correlated with that obtained from the 171 line (which is formed at 0.8MK) at the same position, which suggests that cool jets may be the driving source of intensity disturbances of hotter lines. This study may contribute to our understanding of the fine structures of plumes, the magnetic fields in polar corona hole and the acceleration of the fast solar wind.