This sub-subseries consists of 25 files, containing correspondence between Edward Appleton and colleagues, amateur observers, institutions, etc. It includes comments on research and publications, including. U.R.S.I. [Union Radio-Scientifique International] reports), data, observations and photographs. Many of the exchanges deal with the special activity observed in 1942 and 1946, but there is also earlier material relating to observations in the later 1930s and some correspondence on later research projects. Some of the letters refer to the 'hissing phenomenon', as the earlier observations were called. The correspondence is presented alphabetically, with dates and a brief note of content and is followed by reports and papers on solar noise.