PA3FWM is a well-known http://websdr.ewi.utwente.nl:8901/ The creator of this, whom I recently saw on his personal website (which has a lot of useful information), has kindly allowed me to repost it for learning purposes!
https://www.pa3fwm.nl/technotes/tn19c.html
Where did layers A, B, and C go?
Pieter-Tjerk de Boer, PA3FWM pa3fwm@amsat.org
(This is an adapted version of an article I wrote for the Dutch amateur radio magazine Electron, published in June 2019.)
Books on radio propagation discuss the various layers of the ionosphere: the D layer, approximately 80 km high, and the E layer, which begins at around 90 km, as well as the F layer (or F1 and F2) above 150 km. These layers exist at different altitudes because they are associated with different ionized gases: these gases are found at different altitudes and require different ultraviolet wavelengths to ionize.
But why do we use the letters D, E, and F, instead of starting with A? Strangely, very few books explain this. The authoritative answer can be found in [3], which quotes a letter written by Sir Edward Appleton (who discovered the different layers) to JH Dellinger (who discovered the relationship between solar flares and shortwave radio blackouts) in 1943:
In my early work with radio waves, I observed reflections from the Kennelly-Heaviside layer. In my diagrams, I used the letter E to represent the electric vector of the downward wave. Therefore, in the winter of 1925, when I discovered that reflections could be obtained from a higher and completely different layer, I used the term F to represent the electric vector of the wave reflected from that layer. Around the same time, I occasionally received reflections from very low heights, so it was natural for me to use the letter D to represent the electric vector of the returning wave. Then, I suddenly realized that I needed to name these discrete layers, and I was very concerned about making any assumptions about my measurements having a definitive outcome. I felt that I should not call them A, B, and C layers because there might be undiscovered layers below and above them. Therefore, I thought that the original names for the electric vectors D, E, and F could be used for the layers themselves, as any possible naming of the layers that might be revealed would have a considerable degree of freedom.
Approximately 90 years have passed, and the main observed phenomena can still only be explained using layers D, E, and F. However, Appleton's preventative measures are not entirely unnecessary, as measurements suggest that a "C layer" exists at an altitude of approximately 65 kilometers, for example, this layer causes a phase shift in VLF signals [4].
Reference:
[3] R. Silberstein: The Origin of the Current Ionospheric Nomenclature. Journal of Atmospheric and Oceanic Physics, 1959. Also see https://www.kb6nu.com/if-the-ionosphere-has-de-and-f-layers-what-happened-to-the-ab-and-c -layers/.
[4] Fernando Celso Perin Bertoni et al.: South American VLF Network (SAVNET) Low-altitude ionospheric monitoring: Occurrence and changes in atmospheric temperature in the C region. JGR - Geophysics, 2013.https://doi.org/10.1002/jgra.50559
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