 Johnston Island is in the northern hemisphere, as were all of the planned "Operation Fishbowl" nuclear detonation locations.It was known from previous high altitude tests, as well as from theoretical work done in the late 1950s, that high-altitude nuclear tests produce a number of unique geophysical phenomena at the opposite end of the magnetic field line of the Earth's magnetic field.According to the standard reference book on nuclear weapon effects by the United States Department of Defense, "For the high-altitude tests conducted in 1958 and 1962 in the vicinity of Johnston Island, the charged particles entered the atmosphere in the northern hemisphere between Johnston Island and the main Hawaiian Islands, whereas the conjugate region was in the vicinity of the Samoan, Fiji, and Tonga Islands.