When plasma in the polar cap F region becomes highly structured, patches, irregularities, and scintillations of HF signals may be observed. The topic of this paper is not the mechanism for structuring or distributing the plasma but rather the source of the plasma. By understanding the plasma source we gain insight into the specification and forecasting of ionospheric structures and irregularities as required for space weather applications. The two major sources of polar cap F region plasma are the solar EUV radiation and the auroral precipitation. The region over which solar EUV production occurs is readily modeled. In contrast, the auroral precipitation is not subject to diurnal or seasonal dependences in the same predictable manner; the auroral precipitation can almost be viewed as stochastic within certain geomagnetic coordinate constraints. In this study we use a physical model to separate the effects of solar EUV and auroral precipitation. We find that the auroral contribution does provide a far-from-negligible baseline level of polar cap F region plasma, upon which is superimposed the UT and seasonally dependent TOI. This baseline level of ionization is very difficult to predict or forecast since it is determined by plasma flux tube histories through extended regions of the auroral oval over several hours. This result raises the need for more advanced auroral precipitation modeling in order to obtain improved space weather specification. The inclusion of soft auroral precipitation is especially important since it can be a significant source of F region plasma.