Under laboratory conditions, a magnetic field acting on an atom removes the degeneracy with respect to the directions of the angular momentum (Zeeman and Paschen-Back effects) and causes a slight diamagnetic shift of the high-lying energy levels; however, the inner structure of the atom (electron density distribution) and its energy spectrum (neglecting the splitting of the levels in a magnetic field, which is small compared to the atomic electron binding energy) remain virtually unchanged. The effect of a magnetic field becomes significant when the field strength is sufficiently large, such that the cyclotron energy of a free electron, hω_c = heH/cm, when m is the electron mass, becomes comparable to the electron binding energy in the atom, and the magnetic length a_H = (ch/eH)~(1/2) is of the order of the atomic radius. In the case of the hydrogen atom, for example, this implies the field strength H ～ 10~9 Oe, which is beyond the reach of modern experiments. We therefore depend on astrophysical observations or experiments on model systems when wish to draw information on the properties of atoms in strong magnetic fields.
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