## 12/25/12

### A real loop of wire will have less induced amperage because part of the induced current will circulate in the Earth, and because the loop current will decay as an ordinary resistive inductor would decay, proportional to exp[-t(R/L)] (so the inductor loop current is constantly decaying as the magnetic field changes, which takes around an hour typically, at the least). For a 24 GW elpipe loop that is 1000 km in diameter:

R = 1.67 ohms (circumferential resistance)
L = 9.9 henries
R/L = 0.17/second (that means the induced current decays to 1/e its original value in 5.93 seconds)

Therefore the current cannot build up as it would in the case of a superconducting loop. The loop current would be substantial during the height of a magnetic storm as severe as 1859 (about 17 kA), but not enough to damage or overheat the conductors per se.

New York Times post based on this discussion (March 19, 2013):
The disturbance at the Earth's surface is far to weak to harm auto electronics, I think. The grid effects come about because of small local changes in the local magnetic field that are amplified because of the great distances involved. The changing magnetic field induces currents to flow in both metallic loops (pipelines as well as the grid) and in the Earth as well. The flowing current in the earth causes the local ground potential to change with position, which causes most of the damage.

In principle, a region can be protected by a superconductive loop around the region (this implies complete exclusion of magnetic effects within the loop, no ground currents: all the changing magnetic field is cancelled by the induced loop current). The 1859 Carrington event is estimated to have involved a 2.0 microtesla geomagnetic field change (out of a total field of ~65 microteslas in the normal gweomagnetic field), over about an hour. What I calculated for the induced current on a 1000 km diameter superconducting loop is ~one trillion amps induced in the loop. This is not practical, but it gives an idea about how much magnetic energy is sloshing around in such an event.