Non-stationary Alfven resonator: new results on Pc1 pearls and IPDP events

We analyse a Pc1 pearl event observed by the Finnish search-coil magnetomet er network on 15 December 1984, which subsequently developed into a structu red IPDP after a substorm onset. The EISCAT radar was simultaneously monito ring the mid- to high-latitude ionosphere, We have calculated the ionospher ic resonator properties during the different phases of the event using EISC AT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the g round corresponds to the maximum of the transmission coefficient rather tha n that of the reflection coefficient. This casts strong doubts on the bounc ing wave packet model of Pc1 pearls. Instead, we present evidence for an al ternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, w hereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator pro perties can be modified during the event so that the resonator eigenfrequen cy increases according to the observed frequency increase during the IPDP p hase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultane ous increase of the resonator eigenfrequency. The need for such a complicat ed matching of the two factors explains the rarity of IPDPs on the ground d espite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle. (C) 2000 Elsevier Science Ltd. All rights reser ved.