Large-scale low-frequency ULF signals in the terrestrial magnetosphere are expected to have the structure of standing waves along the ambient magnetic field and to propagate across the field. By making simple assumptions regarding the wave form, we show that wave polarization in a plane perpendicular to the magnetic field B may be interpreted in terms of the signal structure across B. Two cases are distinguished, corresponding to a pure Alfv¿n mode (transverse) wave and a purely compressional wave. Examining published ULF data, we find that many compressional signals appear to have transverse polarization more appropriate to the Alfv¿n mode. We conclude that the compressional signals are commonly coupled to Alfv¿nic disturbances, but there is evidence of purely Alfv¿nic signals at times.