“to momentarily energize the Earth magnetic field”... hm.
A nuclear explosion has two effects that one could connect with the subject of magnetic fields:
- The EMP. This is very quick. Like any EM signal, it just follows the wave solution of the Maxwell equations (which are in air very linear, so the amplitude isn't really important), meaning it spreads out at the speed of light, well ahead of the pressure and ionisation effects. The lower-frequency parts will partly be reflected at the ionosphere, you may get some whistler-mode dispesion. This isn't really special to the nuclear bomb (it also happens with lightning transients), and it's completely independent of the Earth's static magnetic field. Yes, the EMP itself also has a magnetic component, but this is short-lived and actually not that strong; only because it's a dynamic field with a strong electrical component does the pulse destroy electronics. (Magnetically, it's much weaker that the disturbances solar flares can cause on Earth.)
- The ionisation. A nuclear bomb turns a significant volume of air into plasma, both through the heating and through radiation. In plasma, you don't have separate dynamics for the gas and EM fields anymore, but both are linked together in magnetohydrodynamics. However, not much air is heated so much† that, at the high densities you have a surface level, you'd actually have electrical conductivity for a long time – recombination removes most of the ions quickly. Only a high voltage would be able to sustain a current (and thus effect a magnetic field), which again is just what happens in a lightning, but that too is short-lived.
So, no, I don't think there's much interesting to be said here.
It might also be worth noting that the Tsar Bomba actually did explode at quite high latitude, 74°N. At that time the magnetic pole itself was only at 75°N (albeit on the western hemisphere, so the explosion wasn't actually at very high magnetic latitude).
†The hard radiation which does most of the initial heating in the fireball does so precisely because it interacts strongly with air, but that also means it doesn't reach very far. The components that reach through air can immediately burn surfaces miles away, but they don't much heat the air they pass through. Only the mechanical (i.e. acoustic) shock wave causes a heating again, but this is transient.