Observations of Sudden Ionospheric Disturbances

Observations were made of the effect of sudden ionospheric disturbances on radio waves reflected from, and transmitted through, the ionosphere. The results of simultaneous measurements of the phase changes on waves of frequency 2-6 Mc/s are shown to indicate that they are due to an increase in the electron density which extends to above the maximum of the E layer, and possibly up to 150 km. Simultaneous measurements of the increases in absorption of waves in the frequency range 2 to 24 Mc/s (which include measurements of cosmic radio noise) show that there is usually an important increase in absorption in the lowest regions of the ionosphere where the electronic collision frequency is 10 7 or even 10 8 sec -1 . These conclusions are consistent with the observations of other workers. The above mentioned phenomena were also compared with phase changes observed on very long waves, but since there was found to be a large variation in the individual aspects of different sudden ionospheric disturbances it was not possible to draw any detailed conclusions.