Spatially averaged ( > 50 ^ { \prime \prime } ) EUV spectral lines in the transition region of solar quiet regions are known to be redshifted .
Because the mechanism underlying this phenomenon is unclear , we require additional physical information on the lower corona for limiting the theoretical models .
To acquire this information , we measured the Doppler shifts over a wide coronal temperature range ( \log T [ \mathrm { K } ] = 5.7 – 6.3 ) using the spectroscopic data taken by the Hinode EUV Imaging Spectrometer .
By analyzing the data over the center-to-limb variations covering the meridian from the south to the north pole , we successfully measured the velocity to an accuracy of 3 \mathrm { km } \mathrm { s } ^ { -1 } .
Below \log T [ \mathrm { K } ] = 6.0 , the Doppler shifts of the emission lines were almost zero with an error of 1 – 3 \mathrm { km } \mathrm { s } ^ { -1 } ; above this temperature , they were blueshifted with a gradually increasing magnitude , reaching -6.3 \pm 2.1 \mathrm { km } \mathrm { s } ^ { -1 } at \log T [ \mathrm { K } ] = 6.25 .