Nonthermal Motions in a Polar Coronal Hole Measured with Hinode/EIS during an on-Orbit Partial Solar Eclipse on 2017 August 21
We have performed a spectroscopic observation over the south polar coronal hole (PCH) with the Hinode Extreme-ultraviolet (EUV) Imaging Spectrometer (EIS) during an on-orbit partial solar eclipse. In this partial eclipse, the Moon passed through the EIS observing area that was set in the south PCH at the height of 0.9─1.4 solar radii. Using the lunar occultation, we have corrected for the scattered light contamination from bright regions of the Sun that is present in the dark PCH emission line profiles. The nonthermal width of the corrected emission line profile in the PCH increases from the limb toward the high-altitude corona. It has also been confirmed that the nonthermal width tends to decrease beyond ∼1.2 solar radii. These results are consistent with the model in which outward-propagating Alfvén waves start being dissipated at ∼1.2 solar radii, as previously reported. The reduced energy within ∼1.4 solar radii contributes to atmospheric heating and the initial acceleration for the solar wind in the low corona. The remaining energy flux at 1.4 solar radii may be dissipated in the distant corona and is sufficient to provide the additional acceleration required to drive the fast solar wind.