21340 km of superconducting tape could take rather some time to fabricate. Then, of course you need to get it there, and lay it out, and construct 20k refrigeration plants. Then, extract enough nitrogen from the atmosphere (1.89% concentration) for refrigerant; you'll need about 4M tons of it. You also need 21340 km of cryo-safe piping bonded to the tape, which probably weighs another 4M tons. Plus heat insulation.
Leakage from 21340 km of tubing, even if just migration of single molecules through the walls of the tube, would require continuous injection of fresh nitrogen.
And, I guess, you had better protect it from meteorite strikes. A planet-sized magnetic field suddenly imploding would seem to release enough energy to vaporize your whole tape. Maybe you should have two of them, parallel, 100km apart so they are unlikely to both be taken out by the same meteorite.
