Temperature fluctuations in HII regions: ionization by Cosmic Rays as a   key mechanism
  We present a detailed model capable of explaining quantitatively the temperature fluctuations observed in luminous, large HII regions. The model is based on two assumptions which we justify on the basis of observations: that the major fraction of the hydrogen in the clouds that form the HII regions is not photoionized and is essentially HI, this HII is lightly ionized by fluxes of low energy cosmic rays (CR) produced by processes originating in the hot stars which illuminate the regions. 
C. Giammanco, J. E. Beckman