An analysis has been made of bottomside spread‐F data obtained at sunspot minimum at stations separated in latitude but grouped about 80° west longitude. At this part of the solar cycle, the temporal variations of the occurrence of the frequency‐spreading component of spread F change with geomagnetic latitude in a manner comparable to that at sunspot maximum, the main differences being the following. The low‐latitude region of high occurrence expands to higher latitudes as the sunspot number decreases. The high‐latitude region of high occurrence is not so strong in sunspot minimum as in sunspot maximum. A third region of high occurrence about 10° latitude wide centered on 50° geomagnetic latitude, which is not so obvious at sunspot maximum, appears at sunspot minimum. This midlatitude region of occurrence on the bottomside is identified with the topside spread F attributed to the ducting process. A comparison of the high‐ and low‐latitude regions of high bottomside frequency‐spreading occurrence with their topside counterparts results in the conclusion that, at these latitudes, the occurrence properties of topside ducting are not substantially different from those of topside scattering.

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An analysis has been made of reliable spread-F data obtained from IGY f plots for ionosonde stations grouped about longitudes 75øW and 120øE. The temporal variations of occurrence of the frequency-spreading component of spread F are found to change with latitude, these changes having a certain symmetry about the geomagnetic equator rather than about the geographic or dip equators. Four regions with different occurrence characteristics appear' two are regions of high activity, the auroral and equatorial regions; and the other two are regions of lower activity, the middle latitude and polar regions. The season of minimum occurrence in the equatorial region changes from the northern summer solstice in the American zone (75øW) to the southern summer solstice in the Far Eastern zone (120øE). This longitude effect, which led Reber to postulate the existence of a spread-F equator, is shown to be due to the interaction of some form of direct or indirect solar control with the magnetic control of the incidence of the phenomenon.

SummaryVirtual range versus frequency (P'f) records of the ionosphere made at Brisbane (lat. 27·5 oS., long. 152·9 °E.) during 1952 and 1953 have been examined. It is found that occasionally neither the o nor the x mode of propagation penetrates the Fa layer at a unique frequency, the upward sweeping traces either being blurred out over a range of penetration frequencies (diffuseness) or possessing a fine structure (penetrationfrequency multiplicity). Temporal analysis of the occurrence of these. effects reveals that they occur only at night; penetration-frequency multiplicity appearing more frequently in the hours before dawn, while the diurnal distribution of diffuseness has a maximum between 01 00 and 0500 hr in summer and represents a more even distribution between 2100 and 0500 hr in winter. The seasonal distribution has a pronounced peak in the winter months and minima in the equinoctial months. These data are compared with the world-wide picture of these variations as it has emerged from the recent literature.The observations are interpreted in terms of scattering from the clouds of enhanced ionization near the Fa-layer maximum which are believed to be responsible for the scintillation