The influence of 10.7‐cm solar‐flux variations on midlatitude daytime ionospheric disturbance conditions

For midlatitude regions, routinely recorded ionograms have been used to investigate whether or not the occurrence levels of (1) daytime first‐hop distortions, (2) daytime second‐hop spread, and (3) nighttime spread F are influenced by 10.7‐cm solar‐flux changes, because of associated upper atmosphere neutral‐particle density (UA‐NPD) changes. Recordings for a 5‐year period (1979–1983) were used. It was found that generally speaking, occurrence levels of these parameters were lower for high solar‐flux values and higher for low solar‐flux values as variations occurred from day to day. These are the results expected if the UA‐NPD changes have an influence on the wave amplitudes of the medium‐scale traveling ionospheric disturbances (MS‐TIDs) thought to be responsible for the recording of these parameters. This association was particularly well defined for the occurrence of the daytime second‐hop spread. It is suggested that non linear effects of the atmospheric gravity waves (AGWs) responsible for the MS‐TIDs may be responsible for generating the daytime small‐scale structures, which experimental results indicate are the ionospheric structures responsible for the second‐hop spread. These structures may possibly be field aligned.