Where did the A, B and C layers go?Pieter-Tjerk de Boer, PA3FWM email@example.com
(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, June 2019.)
Books about radio propagation discuss the various ionised layers in the ionosphere: the D layar at an altitude of about 80 km, the E layer starting at about 90 km, and the F layer (or F1 and F2) above 150 km. These layers arise at different heights because they are related to different gasses that are ionised: the gasses are at different altitudes, and need different ultraviolet wavelengths for ionization.
But why do we use the letters D, E and F, instead of starting at A? Strangely, almost no book explains this. An authorative answer can be found in , where a letter from Sir Edward Appleton (who discovered the various layers in the 1920s) to J.H. Dellinger (who discovered the relationship between solar flares and shortwave blackouts) from 1943 is cited:
In the early work with our broadcasting wavelengths, I obtained reflections from the Kennelly-Heaviside layer, and on my diagrams I used the letter E for the electric vector of the down-coming wave. When therefore in the winter of 1925 I found that I could get reflections from a higher and completely different layer, I used the term F for the electric vector of the waves reflected from it. Then at about the same time I got occasional reflections from a very low height and so naturally used the letter D for the electric vector of the return waves. Then I suddenly realized that I must name these discrete strata and being rather fearful of assuming any finality about my measurements I felt I ought not to call them layers A, B and C since there might be undiscovered layers both below and above them. I therefore felt that the original designation for the electric vector D, E and F might be used for the layers themselves since there was considerable latitude for the naming of any layers that might come to light, as a result of further work.
Some 90 years have passed since then, and the main propagation phenomena are still explained with just the D, E and F layers. Still, Appleton's precaution wasn't quite unneeded, because there are measurements hinting at a "C layer" at an altittude of about 65 km, which e.g. gives a phase shift of VLF signals .
 R. Silberstein: The origin of the current nomenclature for the ionospheric layers. Journal of Atmospheric and Terrestrial Physics, 1959. See also https://www.kb6nu.com/if-the-ionosphere-has-d-e-and-f-layers-what-happened-to-the-a-b-and-c-layers/.
 Fernando Celso Perin Bertoni et al.: Lower ionosphere monitoring by the South America VLF Network (SAVNET): C region occurrence and atmospheric temperature variability. JGR Space Physics, 2013. https://doi.org/10.1002/jgra.50559