Emission cores in H and K lines

Calculations are made for the center-limb variations of the K₂ and K₃ components of the solar Ca ii K line using an optically thick model of the chromosphere. The center-limb variations are shown to require an increase of Doppler width with height in the chromosphere and to depend critically upon the location of the point where _D has increased by a factor e. Good agreement with observations is found when, and only when, the increase in _D occurs nearly simultaneously with the increase in chromospheric temperature.