SIMULTANEOUS OBSERVATION OF PC-5 PULSATIONS IN THE DAWN AND DUSK LOW-LATITUDE BOUNDARY-LAYER

A Pc 5-type pulsation (T similar or equal to 6min) event, which persis ted for similar to 19h on 15-16 February 1978, was observed by the IMP -8 spacecraft inside the dusk low-latitude boundary layer (LLBL), whil e at the same time the ISEE-1 spacecraft was in the dawn LLBL or the n earby plasma sheet, and the IMP-7 spacecraft was upstream of the Earth 's bow shock. The waves are observed as mixed mode compressional and t ransverse Pc 5 pulsations in the magnetic field, and are also often ob served in the plasma density, plasma velocity and energetic ion intens ities. At the beginning of the event an interplanetary discontinuity o ccurs, where the solar wind density falls from similar to 15 cm(-3) to similar to 3 cm(-3) and the velocity from similar to 680 km s(-1) to similar to 560km s(-1). The dynamic pressure on the magnetosphere is r educed abruptly and the pulsations occur simultaneously with the new m agnetospheric configuration. The geomagnetic activity remains extremel y low during the event, following a preceding disturbed period. Under such circumstances our major findings are the following: (a) the frequ ency of the dawn LLBL waves is twice (f similar or equal to 5 mHz) the frequency of the pulse trains at the duskside LLBL; (b) the magnetic field amplitude peaks in the LLBL coincide with plasma density maxima and plasma velocity minima; (c) the magnitude of the magnetic field an d the energetic ion fluxes are anticorrelated; (d) the thickness of th e LLBL on the plane Z(GSM) = 0 and at X similar or equal to - 19 R(E) is estimated to be similar to 6.5 R(E) throughout the event; (e) the d usk LLBL and the dawn sector plasma sheet reveals Pc 5-type magnetic f ield pulsations at the same time, and furthermore, inside the plasma s heet the amplitude of the Pc 5 waves modulates the coexisting Pc 3 mod e signals in the form of wave packets. The second harmonic of the magn etic held magnitude at ISEE-1 is interpreted as a spatial feature limi ted to the close vicinity of the plane Z(GSM) = 0. AS candidate excita tion mechanisms of the Pc 5 pulsations the following are proposed: (a) during the initial stage (similar to 1 h) of the event, the exo-magne tospheric solar wind discontinuity; fb) during the main stage( similar to 18 h) of the event, the Kelvin-Helmholtz instability at the inner edge of the LLBL and, simultaneously, a single wave mode generated via wave-particle interactions in the ring current. A global cavity mode in the Pc 5 frequency band is also likely to exist.