Abstract
The main features of the ultraviolet, visible and infrared spectra of aurora are described. Excitation processes by auroral primary particles are summarized. A simple approach is followed to provide an understanding of the main factors influencing the dependence of the relative intensity of features of the auroral spectrum on incident particle energy and atmospheric parameters. The status of the use of various optical measurements to infer average particle energies and total energy fluxes is reviewed including comments on precautions to be observed and limitations to be considered in the application and interpretation of optical measurements.
