We study the quantum mechanics of a charged particle on a constant curvature
noncommutative Riemann surface in the presence of a constant magnetic field. We
formulate the problem by considering quantum mechanics on the noncommutative
AdS_2 covering space and gauging a discrete symmetry group which defines a
genus-g surface. Although there is no magnetic field quantization on the
covering space, a quantization condition is required in order to have
single-valued states on the Riemann surface. For noncommutative AdS_2 and
subcritical values of the magnetic field the spectrum has a discrete Landau
level part as well as a continuum, while for overcritical values we obtain a
purely noncommutative phase consisting entirely of Landau levels.Comment: 18 pages, 1 figure, final version to appear in Nucl.Phys.