Wave-Particle Interactions.

Plasma waves play a significant role in many of the macroscopic geospace processes discussed above, particularly in the acceleration, transport, and loss of energetic particle populations through pitch angle scattering into the loss cone (Inan, 1987). A better understanding of the mechanisms by which waves are generated, propagate from the source region, and dissipate their energy is required. For some type of waves, observations are extensive, and theories sophisticated, but a full understanding of all the processes involved has yet to be achieved (Bering, 1989). Progress in this area will be aided by combining satellite-borne experiments at or near the source regions with ground-based observations. The latter, such as from SESAME, have an important contribution to make, mainly because the region of interest is accessible much longer from the ground than for an orbiting spacecraft, especially if a network of observatories is employed. Complementary modelling studies, especially those which incorporate composition, and hot and cold plasma populations (e.g. HOTRAY, Horne, 1989) will be particularly valuable.

The intensity of magnetospheric wave activity at Halley (Jenkins, 1988), which is a function of the weighted average of the wave energy incident downwards on the ionosphere within approximately 1,000 km of the receiver, is related to the injected flux of particles at resonant energies. In particular, the signatures of the waves and resonant particles in space, observed by GGS satellites, in comparison with the ground data, should enable the role which these wave-particle interactions play in the overall temporal and spatial distribution of energetic electron populations to be established.

Wave generation resulting from the cyclotron resonance plasma instability is known to be accompanied by energetic particle precipitation caused by pitch angle scattering into the loss cone (Imhof et al., 1989). Thus the wave intensity may be related to precipitation fluxes, and the simultaneous observations of other expected manifestations of this precipitation, such as changes in ionospheric conductivity, optical emission intensity and Trimpi activity (Smith and Cotton, 1990), will test our understanding of these processes.

With the network of spaced receivers (Halley and the AGOs) all measuring intensity, polarisation, and arrival bearing of the received waves, it will be possible to map the complex spatial structure of VLF wave activity in some detail. It is expected to be a function both of the spatial distribution of energetic particle fluxes giving rise to the emissions, and the field-aligned density irregularities (whistler ducts) which are thought to guide the wave energy into the ionosphere. Simultaneous satellite and ground data should enable these contributions to be distinguished.

The polar ionosphere contains a rich `zoo' of irregularities of many scale sizes resulting from a variety of processes, including plasma instabilities, wave-particle interactions, and structured particle precipitation. Irregularities place important constraints on many practical space-based communications/navigation systems, and they also play a critical role in the guiding, focusing and scattering of some classes of natural plasma waves. The combination of space-based imaging, particle and electric field measurements, with ground-based SESAME data, should lead to a greater understanding of the nature and formation processes of irregularities. One example would be to understand the factors affecting the spectral width measurements made by the PACE radars (Baker et al., 1991).

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