Danny
Pity the effect is so transient, as otherwise we would seem to have a good defence against nuclear attack ! (but then, I suppose, a way would soon be found to shield these vital parts from the radiation belt).
The US followed-up the ARGUS tests by a couple more tests (ORANGE and TEAK) at a lower altitude and later in 1962 a much more ambitious test "STARFISH PRIME" was detonated. The design yield was 1.4 megatons, but STARFISH was bungled and its electromagnetic pulse (EMP) is reputed to have knocked out several satellites in low Earth orbit and caused damage to several others including the UK's first satellite, Ariel 1. Damage was also caused to about 300 street lights in Hawaii - 900 miles away!
The after effects of the detonation of STARFISH were being observed two years after the test.
More about STARFISH PRIME here:
https://en.wikipedia.org/wiki/Starfish_Prime
Exciting days if one had a background in radio!
FWIW
A few years ago during a visit to the National Archives in Kew I asked to see any files regarding the communications outage at Gan in 1958. Seems file/s are available but not in the NA. NA referred me to the RAF Historical Branch, but enquiries there drew a partial blank, files existed, but the official in charge of and holding of these files was on extended sick leave (well over three months) and no one else was available to field a query, so I never did see what the official UK reaction / response to this event was, which is a pity.