Electron densities less than 100 electron cm-3in the topside ionosphere

Data obtained from Alouette II topside-sounder ionograms have been used to study the regions of occurrence of low electron number density, N ≤ 100 electron cm^-3, from December 1965 to April 1968. The study investigates the diurnal, seasonal, and spatial variations in the height and frequency of occurrence of the low electron density regions below 3000 km, within the limits imposed by the Alouette II orbit. In the northern-hemisphere winter near sunspot minimum, low electron densities were observed at all local times. A few examples were found in the summer (northem hemisphere) but only for local times around midnight and in the region of the main (midlatitude) trough. Most of the low electron densities were observed at geomagnetic latitudes greater than 65° near noon and greater than 55° near midnight, although they were observed at geomagnetic latitudes as low as 45° in the southern hemisphere near midnight. The low height cutoff of occurrence of N ≤ 100 electron cm^-3showed a diurnal variation, with heights of 1400 km at night and about 2400 km near noon. This diurnal effect shifted upward in height during years of increased solar activity. The observations are shown to be consistent with the removal of light ions over regions in which the magnetic field lines are swept back into the tail of the magnetosphere.