Accelerating and Supersonic Density Disturbances in Solar Polar Plumes

Propagating intensity disturbances in solar coronal holes are often considered as wave propagations or mass flows. By applying the differential emission measure technique for the extreme ultraviolet images taken by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we analyze the propagation speed of density disturbances of plume structure in an off-limb coronal hole for a given temperature. We construct the map of cross-correlation between density profile for a given height and the profile at the height of 50 Mm. The evolution of density disturbances is well fitted by the second-order polynomial. The acceleration is calculated to be 36 m s^-2. The initial speed is 134 km s^-1 which is comparable with the sound speed given by the DEM-weighted temperature. Hence, density disturbances are accelerating and supersonic at around the base of the solar corona. The excess speed relative to the sound speed is ∼ 30 km s^-1 at the height of 1.23 solar radii, which is consistent with the Doppler speeds and Doppler dimming speeds observed by different instruments. The extrapolated sonic distance of the excess speed is ∼ 2.16 solar radii which is consistent with those of solar winds. The lower limit of the mass flux corresponds to 7% of the global solar wind. Hence, we interpret that the observed density disturbances are slow magnetoacoustic waves propagating in subsonic and accelerating solar winds.