抄録
The technique of using simultaneous data from more than one spacecraft to study hydromagnetic waves in space is proving to be a very powerful one which will undoubtedly become more widespread in the future. We review the results obtained using this technique thus far. All of the work has involved longer period (>20s) pulsations and most of the data has come from near geostationary orbit. Attempts to measure the thickness of field line resonance regions have not so far been conclusive, but they suggest that values of about (1/2)RE are typical. Signals in the Pc 3 and 4 bands in the morning are coherent over at least 20° of longitude while in the afternoon they are much more localized. One afternoon source is resonance with hot ions. Pi 2's on the nightside can also be confined to small regions of longitude.
