Abstract
The polar cap absorption (PCA) especially in twilight condition is considered taking into account of negative ion O- which is possibly formed through the charge transfer process, O-2+O→O2+O-+1.2eV(6) and it follows the negative ion in the upper mesosphere is not O-2 but O- in general, [O-]/[O-2]_??_α6/d(O-)[O] where α6 be presumably 10-11 cm3 sec-1 or so. Then the screening effect of O3 Chappuis bands absorption becomes important.
