Electromagnetic Pulse (EMP) Threat Update

John Kyl's April 16, 2005 op-ed Unready for This Attack from the Washington Post provides a worst-case scenario for a terrorist EMP threat.
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Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack (2004)
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And from GlobalSecurity.org:

"High-altitude EMP (HEMP), is the most significant and, potentially, the most hazardous to our security (of EMP weapons). The explosion of a nuclear burst at an altitude greater than 30 to over 500 kilometers above the earth's surface will produce the above scenario. Due to the very thin to non existent atmosphere at these altitudes, the gamma rays emitted from the explosion will travel radically outward for long distances. Those gamma rays traveling toward the earth's atmosphere are stopped by collisions with atmospheric molecules at altitudes between 20 and40 kilometers. These collisions generate Compton recoil electrons which interact with the earth's magnetic field to produce a downward traveling electromagnetic wave. This high altitude burst will not generate any other nuclear effect at the earth's surface.

However, this type of nuclear explosion also produces a vast ground coverage. Significant HEMP levels occur at the earth's surface out to where the line of sight from the burst contacts the earth's surface. Consequently, a nuclear burst over the central part of the United States at an altitude of 500 kilometers would produce an EMP field that would incapacitate all communications systems in the continental United States. "

Source: Electromagnetic Pulse-From Chaos To A Manageable Solution, Major M. CaJohn, USMC. Global Security.org.