“…In more recent times this same approach has been used in the investigation of a number of questions involving radio propagation in the ionosphere [e.g., Appleton, 1928;Kelso, 1960Kelso, , 1961Brandstatter, 1963;Ross, 1965;Kirnura and Nishina, 1967;Bennett, 1968Bennett, , 1969Bennett, , 1971aBennett, , 1971b. General investigations of the associated variations have been made by Kelso [1960Kelso [ , 1961 and Bennett [1969].…”