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 3×103 cm−3. Taking a low value of α ≈ 10−8 cm3 sec−1, then Ne = 4.96×104 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.