Extremal Black Holes in Strong Magnetic Fields: Near-Horizon Geometries   and Meissner Effect
  For extremal black holes, one can construct simpler, limiting spacetimes that describe the geometry near degenerate horizons. Since these spacetimes are known to have enhanced symmetry, the limiting objects coincide for different solutions. We show that this occurs for strongly magnetised Kerr-Newman solution, and how this is related to physical Meissner effect of expulsion of magnetic fields from extremal black holes. 
Filip Hejda, Ji\v{r}\'i Bi\v{c}\'ak
gr-qc