Concluding remarks, session 2: Structure, ionization sources

There are three major topics which arose in Session 2 on which I would like to comment, namely, maintenance of the night ionosphere, particle ionization, and modelling. 1. THE NIGHT IONOSPHERE The maximum densities of the E and intermediate layers decay at night to values of the order of 3×10 3 cm −3 . Taking a low value of α ≈ 10 −8 cm 3 sec −1 , then N e = 4.96×10 4 cm −3 at sunset and the electrons would completely disappear within an hour after production ceases. Not only does the E ‐layer ionization persist but evidence (Geller et al .) indicates that after midnight, the ion production in the upper E region increases markedly. Incidentally, ionospheric total content data show that the content of the F region can also increase around midnight. Hence, a source of plasma is required.