Satellite observations of the spatial extent and structure of Pc 345 pulsations near the magnetospheric equator

Simultaneous observations of Pc 345 pulsations by five satellites in the pre‐noon local time sector at and near synchronous orbit are examined. The periods of these simultaneous pulsations are not the same at the different observation points. We attribute this difference to site dependent resonant conditions. The spatial properties of this temporal phenomenon are demonstrated with observations by ISEE‐1 and ‐2 as they pass through oscillations in a spatially limited region. Fundamental and second harmonic standing Alfvén waves are observed simultaneously on the same field line. The periods are consistent with model predictions when the measured plasma compositionwhich by mass consists mainly of singly ionized oxygenis taken into account.