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In recent years the STARE system has been used to analyze Pc 5 pulsations of several different types. In this paper a detailed description of a new type of pulsation is given. The event described occurred during local magnetic afternoon; it had small amplitude (IEI •< 5 mV m-• in the ionosphere), a period which varied from 220 to 385 s during the event, and a large azimuthal wave number (m -35) which varied such as to keep the azimuthal phase velocity approximately constant for a given geomagnetic latitude; propagation was geomagnetic westward. When mapped into the equatorial plane, the properties of the wave strongly suggest that the wave phase velocity was determined by hot proton drift motions near the equatorial plane. The observational results are compared with theoretical properties of the drift mirror instability. While the results do not contradict the predicted properties, it is felt that further theoretical development is required. 1. (typically --•20 mV m-J); (3) amplitude profile sharply peaked in latitude (half-power width -1 ø of latitude); (4) phase decrease with increasing latitude of about 180 ø across the amplitude peak; (5) small change of phase with longitude, •<5 ø per degree of longitude, corresponding to m •< 5 if the wave is modeled with azimuthal variation • exp (im &), where & is the azimuthal angle; and (6) possibility of sudden changes in phase with time. This type of wave is usually considered to be a field line resonance driven by an instability at the magnetopause, such as the Kelvin-Helmholtz instability [e.g., Southwood, 1974; Chen and Hasegawa, 1974; Walker, 1980]. The second type is the transient Pc 5 [Poulter and Nielsen, 1982], which has the following characteristics: (1) a marked period increase with increasing latitude, consistent with the period variation of independently resonating field shells; (2) a dominant north-south electric field component; (3) an amplitude profile with no strong localization in latitude but with magnitude comparable to the driven Pc 5 type; the amplitude decays rapidly with time, in contrast to many of the driven resonance events; and (4) no evidence of significant phase variation with longitude. Poulter and Nielsen [1981] interpret these events as decaying resonances of

Recently, emphasis in modelling ULF pulsations has begun to shift away from steady‐state driving mechanisms towards impulsive and non‐steady sources. We propose a model allowing numerical solution of the coupled hydromagnetic wave equations with arbitrary azimuthal asymmetry in a cylindrical magnetospheric geometry. General time‐dependent stimuli can be applied at the outer (magnetopause) boundary, an arbitrary Alfvén speed distribution can be defined within the boundary, and ionospheric Joule dissipation is included. Initial results are presented which show that impulsive stimuli at the magnetopause can set up compressional cavity resonances which drive transverse field‐line resonances within the magnetosphere, in general agreement with recent predictions. For a realistic ionospheric dissipation, and azimuthal wavenumber ∼3, it is found that transient transverse mode solutions are of comparable importance to monochromatic solutions near the resonant field line.

The Earth's magnetosphere is highly structured, in terms of both magnetic field and plasma characteristics. This structure has a profound influence on the propagation of plasma waves, especially ultra-low-frequency (ULF) waves With mHz frequencies, which have wavelengths comparable with typical magnetospheric dimensions. In thk review we illustrate bow the basic theory of ULF hydromagnetic wave propagation in an infinite, homogeneous, uniformly magnetized plasma has been extensively modified to cope with the requirements of applying it to the magnetosphere, a natural laboratory for the physics of uLF waves.We consider the field-line-guided Alfvdn wave modes and the isotropic fast hydromagnetic wave modes, and show how the existence of magnetospheric boundaries can affect the structure of both modes. The finite length of geomagnetic field lines can quantize the frequencies of Alfvdn waves by forcing the establishment of standing waves on field lines. The natural variation of Alfvh speed and field-line length in the magnetosphere then allows the possibility of a continuum of eigenfrequencies for each harmonic of a field-line standing wave. The magnetosphere can also be considered as a cavity in which fast cavity modes may exist, with frequencies quantized by the three-dimensional cavity shape.Azimuthal variation in a system With radial Alfvh speed structure leads to the idea of 'field-line resonance', in which a quasi-monochromatic compressional wave (for example a surface wave or a cavity wave) can match frequency (resonate) with a particular magnetic field line, and hence can transfer energy to a narrow region of field lines around the resonance line.The concept of resonance extends also into the regime of energetic magnetospheric plasma. Hot particles in a spatially structured magnetic field have characteristic drift frequencies perpendicular to the magnetic field and bounce frequencies parallel to the magnetic field. Resonance between waves and particles takes place when wave frequencies match either the drift or bounce frequency of a suitable subset of particles, or a combination of harmonics of both. M v e s either grow or damp as a result of this resonance, depending on the detailed structure of particle distributions and energy availability. We discuss in particular the evolution of ideas concerning the so-called 'drift-mirror' instability in the magnetosphere.