  Stand-off magnetometry allows measuring magnetic field at a distance, and can
be employed in geophysical research, hazardous environment monitoring, and
security applications. Stand-off magnetometry based on resonant scattering from
atoms or molecules is often limited by the scarce amounts of detected light.
The situation would be dramatically improved if the light emitted by excited
atoms were to propagate towards the excitation-light-source in a directional
manner. Here, we demonstrate that this is possible by means of mirrorless
lasing. In a tabletop experiment, we detect free-precession signals of
ground-state sodium spins under the influence of a magnetic field by measuring
backward-directed light. This method enables scalar magnetometry in the Earth
field range, that can be extended to long-baseline sensing.
