Multisatellite observations of resonant hydromagnetic waves

Most measurements of long period ULF pulsations have come from ground based and single satellite observations. The observations have given strong support to the idea that these waves are resonant standing hydromagnetic waves on geomagnetic field lines. Simultaneous ground-satellite observations provide further details of the pulsation structure and are useful for examining the effect of the ionosphere on the transmission of the waves to the ground. Recently, multisatellite observations have been used to provide further insight into the nature of pulsations and we review the results obtained using this technique. Among the results presented are those from the ISEE 1 and 2 spacecraft which are closely spaced in identical orbits, making it possible to distinguish temporal from spatial structure in waves. The ISEE spacecraft have made measurements of resonant region widths and resonance harmonics. In addition, examples are shown of recent multisatellite observations of the global nature of some pulsations and the localization of Pi2 pulsations in space.