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A test particle code is used to study the response of ensembles of particles to a two‐dimensional, time‐dependent model of the geomagnetic tail and to test the proposition (Coroniti, 1981a, b; Büchner and Zelenyi, 1986; Chen and Palmadesso, 1986; Martin, 1986) that the stochasticity of the particle orbits in these fields is an important part of the physical mechanism for magnetospheric substorms. The realistic results obtained for the fluid moments of the particle distribution with this simple model, and their insensitivity to initial conditions, are consistent with this hypothesis.

Far-ir uniaxial-stress experiments reveal the unusual result that the pocket gap modes in KI:Ag+ couple much more to the fully symmetric ( A &I ) strain than to the tetragonal (Eg ) and trigonal ( T2g ) strains; other modes due to monatomic substitutional defects are found to couple with similar strengths to both A &g and Eg strains. Theoretical calculations based on perturbed breathing-shell-model phonons and a simple anharmonicity model suggest the presence of unusual pressure-induced microscopic displacements in the vicinity of the defect s fourth-nearest neighbors. The coupling coefficients of the pocket gap mode at 86.2 cm ' are measured to be temperature dependent, with the Eg coefficient actually changing sign at 12 K; such behavior has not been found for other impurity-induced localized modes. ES IN KI:Ag EOF THE PQCKET QAP MODSTRESS DEPENDEN 11 509 46 Boule no.

Far-IR static-electric-field experiments on KI : Ag+ reveal that the field-induced frequency shifts for the <(pocket,, vibrational gap modes are nearly two orders of magnitude smaller than the corresponding shifts measured for the low-frequency resonant modes. When combined with previously measured stress shifts and a quasi-harmonic perturbed shell-model calculation, these results firmly establish that the silver ion has a significant quadrupolar deformability, which has been proposed to play an important role in the dynamics of the silver ion in other systems.

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