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This paper presents a direct method of obtaining a riometer quiet day curve of the correct shape. The method is simple, and the only input information required is the early morning riometer readings. The corrected curve is shown to be necessary in studies of average ionospheric absorption levels and their diurnal, seasonal, and solar cycle variations.

This paper discusses the general characteristics of synchrotron radiation from high‐energy electrons trapped in the earth's magnetic field and develops methods for calculating the radio noise that will be received at the ground. The synchrotron radiation from trapped electrons created by the nuclear explosion of July 9, 1962, is calculated. These calculations, based on electron flux density measurements in satellites, agree well with observations made at several radio frequencies in the 20–100 Mc/s frequency range at stations near the magnetic equator. It is concluded that the observed radio noise is generated primarily at heights above the magnetic equator between 1.2 and 1.6 earth radii and that it is a very insensitive measure of electrons moving at heights above about 1.6 earth radii because of the weak magnetic field at these heights.

Recent studies at a geomagnetic latitude of 54°N have shown a pronounced peak in the occurrence of HF radar echoes from field‐aligned irregularities near the time of local sunset. It is shown that these results are consistent with previous high‐latitude observations of spread F if the effects of ionospheric refraction are considered. It is also noted that the echoes occurring near sunset differ from the usual nighttime echoes in range, in the amount of range spread, and in their relation to magnetic activity.