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In 1964 several rockets were fired in Hammaguir, Algeria, in order to study the sporadic E region.

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We report here the first direct measurements of the effect of high‐altitude nuclear explosions on ionization density above the F2 layer. The Birmingham RF electron density probe carried in satellite Ariel (perigee 400 km, apogee 1200 km, inclination 54°, period 101.4 minutes) has monitored the ambient electron density along the satellite orbit from the date of launch on April 27, 1962 [Sayers et al., 1962]. After the high‐altitude explosion near Johnston Island (latitude 16°N, longitude 169°W) at 0900 UT on July 9, 1962, abnormally high electron densities were recorded for a few minutes on each of five successive satellite orbits. The times and places at which these high densities were observed are listed in Table 1, together with values of ‘enhanced’ and ‘normal’ electron densities, the latter estimated from similar orbits on the day before the explosion. The explosion occurred on the nighttime side of the earth, and the ‘enhanced’ electron densities recorded by the satellite are similar to normal daytime densities at the same places. The location of the enhanced ionization along the satellite orbit in terms of latitude and longitude is shown in Figure 1, in which Johnston Island, its conjugate point, and the magnetic equator are also indicated.