Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events
✍ Scribed by K. Mursula; K. Prikner; F.Z. Feygin; T. Bräysy; J. Kangas; R. Kerttula; P. Pollari; T. Pikkarainen; O.A. Pokhotelov
- Book ID
- 104407977
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 622 KB
- Volume
- 62
- Category
- Article
- ISSN
- 1364-6826
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✦ Synopsis
We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid-to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the dierent phases of the event using EISCAT observations. Contrary to the earlier results, we ®nd that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coecient rather than that of the re¯ection coecient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modi®ed during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined eect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.