I assume the inverse square law still applies. Higher altitude = wider coverage = weaker effect — inversely proportional to the square of the distance...?
Yes, with the caveat that there are certain areas within the affected area which receive more or less than you would expect. For example, an area north of detonation will see almost no effect and a horseshoe-shaped area south of detonation will see the greatest effect. It has to do with interaction with the earth’s magnetism.