Early theoretical studies of geomagnetic pulsations dealt with the oscillation of individual magnetic lines of force in the toroidal mode. In recent years it has become increasingly clear that the pulsations are organized in the azimuthal direction so that treatment of an oscillating surface of field lines may need to be considered rather than oscillations of individual field lines. In this paper evidence for the azimuthal dependence of Pc 4, 5 pulsation characteristics will be presented. In addition, the latitudinal characteristics of the ground perturbation pattern and the local time variation of these pulsations will be discussed in the context of some of the source mechanisms proposed for long period micropulsation activity.

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Observations of Pc waves in the magnetosphere are reviewed with emphasis on the discussion of satellite-ground correlations. Gross features of wave occurrence in the Pc 3-5 frequency range are shown to be well summarized by taking into account the polarization characteristics of the waves with respect to the ambient magnetic field. In the Pc 4-5 frequency range, azimuthally polarized-transverse waves, which occur predominantly on the morning side of the magnetosphere, show a good correlation with ground Pc events. Radially polarized waves dominate in the afternoon and dusk sectors and are less correlated with ground pulsations than azimuthal waves. Compressional waves, such as stormtime Pc 5 observed at synchronous altitude in the afternoon and compressional Pc 5 observed with HEOS 1 in the dusk, have not yet been identified on the ground. Only compressional waves in space, which belong to the radial class and show a strong ground correlation, are giant pulsations as observed in the morning sector on the ground. Although the problem remains to be studied further, the difference in degree of ground-satellite correlations of Pc waves seems to reflect the spatial extent of respective wave phenomena. As for most of ground-correlated Pc waves, observations appear to yield evidence that a rotation of the wave ellipse orientation occurs between the magnetosphere and the ground as predicted by theory.

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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)R_E 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.

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Magnetometers carried by spacecraft in synchronous orbit show that the magnetic field is almost continuously agitated by the presence of hydromagnetic waves. A variety of waves have been identified, several of which are always associated with certain phases of magnetospheric substorms. These types include Pi 2 bursts and Pi pulsations near midnight and IPDP and mixed mode Pc4-5 waves near dusk. In this report we describe the characteristics of these waves and discuss possible directions for further work.

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We review some ideas developed over the past decade about the generation of low frequency waves in the magnetosphere by wave particle resonance. We emphasise the importance of wave symmetry about the equator, the particular type of particle distributions present in the magnetosphere and differences between drift and bounce resonance. Many arguments are independent of the precise wave mode. Later in the paper we concentrate on the problem of Alfvén wave generation by ring current protons. Recent analysis of measurements at synchronous orbit have yielded supporting experimental evidence.

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Numerous studies have established that various properties of geomagnetic pulsations are linked to various properties of the solar wind. The linkage in most cases is rather loose, suggesting that combinations of factors must be involved in generation and control of pulsation activity. We review briefly the most significant observational results and we describe and discuss critically the two most prominent models for external generation of magnetospheric waves. We present arguments favoring joint application of the models, wherein perturbations in the magnetosheath resulting from favorable interplanetary field orientation are delivered to the magnetopause, transferred directly into the subsolar magnetosphere, and amplified into surface waves on the flank of the magnetosphere by Kelvin-Helmholtz instability at high solar wind speed. This combination of circumstances can account for experimental correlations of pulsation occurrence with interplanetary field orientation, periods with interplanetary field strength, and amplitudes with solar wind velocity.

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It has recently become apparent that auroral radars are powerful tools for the investigation of ULF pulsations in the Pc 5 range. Auroral radar observations of pulsation effects in the ionosphere are reviewed. The STARE radar system is described and its measurements of the pulsation electric field in the ionosphere outlined. It has been established that this electric field is entirely consistent with the field of a hydromagnetic field-line resonance. The rotation of the pulsation magnetic field through 90° by the ionosphere is confirmed. Statistical studies of occurrence show some consistency with the behaviour expected for solar-wind driven instabilities on the magnetopause. The use of STARE data to deduce equatorial plasma density, and height integrated ionospheric conductivities is also discussed. These measurements have been used to estimate energy input into the magnetosphere.

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The theory that long-period geomagnetic pulsations are due to hydromagnetic waves resonating in a magnetosphere bounded by an anisotropically conducting ionosphere, insulating atmosphere, and conducting earth, is reviewed. The history of the subject is outlined, covering the effects of the ionospheric boundary on: pulsations seen at ground level and in the ionosphere (ionospheric screening, 90° rotation of polarisation, attenuation of short wavelengths, mode coupling via Hall conductance); and wave systems above the ionosphere (wave-boundary impedance matching, damping, phase variation along the geomagnetic field, and allowed resonances). Coupling of non-axisymmetric modes within the magnetosphere is also mentioned, and a summary of the present picture given, with suggestions for future work.

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We show that the Doppler shift on whistler mode signals from VLF transmitters may be used to infer the magnetic field of an hydromagnetic wave above the ionosphere in the magnetosphere. This inferred field can then be compared with that measured on the ground. We examine one event in detail and show the data imply that a π/2 rotation of the orientation of the wave ellipse has occurred between the magnetosphere and the ground.

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