Quasiperiodic modulation of the Pc1 geomagnetic pulsations: An unsettled problem

Recently, it has been suggested that the standard model for interpretation of the Pc1 (pearl) pulsations, which is based on the concept of wave packet oscillating along the geomagnetic field lines by reflection from the ionosphere in the opposite hemispheres, is to be revised. The necessity to modify the standard model is clearly demonstrated by observations. However, it seems premature to reject this model completely, since the excitation of Pc1 pulsations is impossible without reflection of the waves from the ionosphere or from some turning points in the magnetosphere. We are inclined to believe that such turning points exist in the magnetosphere. Our hypothesis is that the Pel quasiperiodic wave packets observed over the Earth's surface are the result of radiation from the magnetosphere-borne open resonator in which the oscillating ion cyclotron wave packets are generated owing to instability of the multicomponent magnetospheric plasma. We emphasize that much remains to be learned about the modulation of the Pc1 waves, and it should be realized that a number of unresolved questions refers equally to alternative approaches to this problem.