This paper shows that, contrary to previous explanations, the apparent undulating motion of the equatorial F region peak at sunrise is produced mainly by photochemistry rather than dynamics. Our study is based on an investigation of the behavior of the early morning ionosphere observed by a Digital Ionosonde at Trivandrum, India, and an In-coherent backscatter radar measurements at Jicamarca, Peru.
Electrodynamics is not responsible for the sunrise undulation, but plays an indirect role in the detection of the sunrise effect by simultaneously lowering during the night the peak height and decreasing the density.