  The oscillating magnetic field produced by unbalanced currents in
radio-frequency ion traps induces transition frequency shifts and sideband
transitions that can be harmful to precision spectroscopy experiments. Here, we
describe a methodology, based on two-photon spectroscopy, for determining both
the strength and direction of rf-induced magnetic fields without modifying any
DC magnetic bias field or changing any trap RF power. The technique is readily
applicable to any trapped-ion experiment featuring narrow linewidth
transitions.
