They exploded 1.4 mega ton "Starfish" 400km above the Johnston atoll in the Pacific Ocean. It made day out of night and cause auroras which were visible at the equator. It killed telephone communications in Hawai`i and I heard someone say that they remember seeing the flash as a child in New Zealand.

It took earth's magnetic field 10 years to return to normal.

Nicholas Christofilos

a good (and accurate) write-up is here. .. It says: "Starfish Prime, a 1.4 megaton device, was detonated at an altitude of 400 kilometers over Johnston Island."
not 1.4 kilo tons (like you will find on many UFO related pages )

here ... and here is more related interesting information.

or search altavista for more clues