Continuing along my oft-contineud series of questions relating to terraforming, I return to an idea of giving Mars a magnetic field which I had hitherto dismissed. The idea is that a huge superconducting ring or rings, (basically a magnetised hoop) is built around a planet’s equator, where, once magnetised, it produces a magnetic field which the planet is presumably lacking. Presumably the ring is buried at least a kilometre below the surface, in order to prevent it having to cross oceans/forests/have bridges built over it.
However, finding no particular specifics regarding this process, I would like to ask: would the hoop require an external source of power? Or would the magnetic field it produce be permanent? And am I right in assuming this ring would be buried below ground?