In December 1924 the first observations reflected by the ionosphere at steep incidence were made by Appleton, Barnett.Breit, and Tuve. Their research has often been marked as the beginning of modern physics of the upper atmosphere [see Vasilyev, 1967; Villmann, 1967; Bronsten and Grisin, 1970].
It is perhaps fair to place the beginning of aeronomic research at some special studies made at the Berlin Observatory some 40 years earlier. The Krakatoa eruption stirred up a worldwide interest in the intense twilight phenomena. Although earlier in the nineteenth century W. Bezold had studied twilights for some years, 1883 marked the beginning of extensive twilight research. The astronomer Otto Jesse (1838–1901) of the Berlin Observatory continued the twilight observations ]Jesse, 1884]. Within the framework of these observations, in 1885 noctilucent clouds appeared in the twilight. They were first noted by Backhause at Bad Kissingen and a few days later by Jesse in Berlin. In Russia they were observed by Ceraskij (1849–1925).