This report to the statutory Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack by Dr. Mark Schneider, National Institute for Public Policy, (2007) is available at http://www.nipp.org/wp-content/uploads/2014/12/EMP-Paper-Final-November07.pdf . It has a special chapter on the North Korean threat, which given the direct threats made by the North Koreans in recent weeks should be taken very seriously. Whilst we have no way of knowing whether NK’s “thermonuclear bomb” has been optimised for enhanced EMP (as opposed to explosive output) there are clear indications that it could have been designed that way from the beginning, given that much of China’s nuclear arsenal has gone that way.
Above all, as deterrent devices, nukes are not strictly intended to be used, but they get their defensive value from the catastrophic nature of their results. Since improving the ICBMs accuracy is unimportant for EMP threat, that relieves NK of the necessity of doing all that complex work.
Citing an August 2005 article that appeared in a South Korean Defense Ministry journal, the report says (Page 11):
What North Korea’s Kim Jong Il would do is to first explode nuclear weapons at a
high altitude . . . while destroying electronic devices and computers and paralyzing
the functions of military strongpoints, logistics plants, and cities . . . [If] it is exploded
at a high altitude of 100km or so . . . all kinds of electrical machinery and, in
particular, electronic devices are damaged. More seriously, many of the artificial
satellites orbiting from 400 to 800km above the earth get demolished. Then, neither
satellite telephone nor GPS could be used, so while the US military, which depends
on satellites, immediately falls into a panic and becomes combat incapable, other
nations around the world that used these satellites would also be greatly affected.