Annals of Geophysics
Incoherent scatter and tilting filter photometer data from Arecibo Observatory were combined to search for a contribution to the 6300 Å predawn enhancement due to impact excitation by photoelectrons from the magnetic conjugate point. Prior to about 105° conjugate solar zenith angle, calculations of the 6300 Å intensities due to dissociative recombination gave good agreement with the observed intensities. Then, with the arrival of conjugate photoelectrons (confirmed by plasma line enhancements), the observed intensities rose above the calculated, the difference increasing until a plateau was achieved at a conjugate solar zenith angle of about 95°. The difference or residual intensity is attributed to direct photoelectron impact excitation. Significant day-to-day variations in the onset angle and intensity have a strong negative correlation with ionospheric electron density.
Wickwar, V.B., The 6300Å Predawn Enhancement: Excitation by Protoelectrons from the Magnetic Conjugate Point, Ann. Geophys., 27, 187–192, 1972.