Closed strings in uniform magnetic field backgrounds

We consider a class of conformal models describing closed strings in axially symmetric stationary magnetic flux tube backgrounds. These models are closed string analogs of the Landau model of a particle in a magnetic field or the model of an open string in a constant magnetic field. They are interesting examples of solvable unitary conformal string theories with non-trivial 4-dimensional curved space-time interpretation. In particular, their quantum Hamiltonian can be expressed in terms of free fields and the physical spectrum and string partition function can be explicitly determined. In addition to the presence of tachyonic instabilities and existence of critical values of magnetic field the closed string spectrum exhibits also some novel features which were absent in the open string case. (Contribution to the Proceedings of "Strings 95" Conference)