Ground-based magnetometer observations of Pc 5 micropulsation periods suggest these events are resonant oscillations of geomagnetic field lines. Although most individual events are essentially monochromatic, the average periods increase with latitude and hence field line length. Walker et al. (1979) have use STARE (Scandinavian twin auroral radar experiment) radar observations of ionospheric electric fields in support of the monochromatic resonance theory. Transient Pc 5 events whose period increases with latitude have also been observed by STARE. These events have the characteristics of toroidal eigenmode oscillations of individual shells of the geomagnetic field. The period variation with latitude is also consistent with that derived from averaged periods and time of flight calculations.