The superimposed photospheric and stellar wind variability of the O-type supergiant alpha Cam

(Abridged) We provide empirical constraints on the different physical components that can act to yield temporal variability in predominantly or partially wind-formed optical lines of luminous OB stars, and thus potentially affect the reliable determination of fundamental parameters, including mass-loss rates via clumped winds. Using time-series spectroscopy from epochs spread over $\sim$ 4 years, a case study of the O9.5 supergiant $\alpha$ Cam is presented. We demonstrate that the He I 5876 (2$^3$P$^0$--3$^3$D) line is an important diagnostic for photospheric and wind variability in this star. The optical line profiles of $\alpha$ Cam are affected by (i) deep-seated fluctuations close to, or at, the photosphere, (ii) atmospheric velocity gradients, and (iii) large-scale stellar wind structure. This study provides new empirical perspectives on accurate line-synthesis modelling of stellar wind signatures in massive luminous stars. Using a pure Halpha line-synthesis code we interpret maximum changes in the red-ward and peak emission of $\alpha$ Cam in terms of mass-loss rate differences in the range $\sim$ 5.1 $\times$ 10$^{-6}$ to 6.5 $\times$ 10$^{-6}$ M$_\odot$ yr$^{-1}$. However, the models generally fail to reproduce the morphology of blueward (possibly absorptive) regions of the profiles.