Diurnal variation of ionospheric electron content obtained from doppler slopes of simultaneous satellite signals

All Doppler methods for measuring electron content using simultaneously emitted rocket or satellite cw signals of different frequencies are based on the assumption that those signals travel towards the observer along nearly identical paths. For satellites this assumption has not been discussed earlier. It is best met at high elevation and therefore the data used in this paper is based on the measurement of Doppler slopes of high elevation orbits at the points of inflection. In addition, the comparison of the observed time moments at which Doppler inflection of different frequencies occurs provides a mean to select only those data which are actually simultaneous. Data selected in accordance with these principles can be evaluated on the basis of straight line propagation. The actual data describe the diurnal variation of the integrated electron density during November - December 1961. This variation resembles the known trend of the diurnal variation of f0F2during December of a quiet year, as well as of the diurnal variation of the hmaxF2. Attention is focussed on an apparent enhancement of the content about midnight. This enhancement is considered to be indicative of vertical downward electron transport.