Abstract
The characteristics of quasi‐periodic (QP) VLF emissions observed near the equatorial plane by the GEOS satellites are first compared with simultaneous ground data obtained at L ≃ 6. The frequency structure of QP elements is shown to be the same in the magnetosphere and at ground, whereas their amplitude measurement gives evidence of a ≃20‐dB attenuation between GEOS and the ground. Different events are described as a function of their more or less complex relationships with concomitant Pc 3–4 magnetic pulsations. The spectral analysis of the simultaneous QP envelopes and ULF waves permits us to deduce that QP emissions are due to the modulation of a VLF hiss by a compressional component of ULF waves. This component is a harmonic of an ULF standing wave, showing that the classification between type I and type II is not so obvious in space. When QP envelope and ULF periodicities are different, they are shown to correspond to different harmonics of a same fundamental oscillation of the field line. This set of new results is discussed in the light of the quasi‐linear theory of wave‐particle interactions. The modulation rate of nondispersive QP type can be roughly well explained, and some ideas are given about a relaxation mechanism which is able to interpret the frequency modulation of the QP type rising tone.
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