Non-stationary Alfvén resonator: New results on Pc1 pearls and IPDP events

• Main Authors: K. Mursula, K. Prikner, F.Z. Feygin, T. Bräysy, J. Kangas, R. Kerttula, P. Pollari, T. Pikkarainen, O.A. Pokhotelov
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2000
• Subjects: EISCAT
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.20.9257; http://spaceweb.oulu.fi/~kalevi/publications/Mursulaetal2000.pdf

We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on December 15, 1984, which subsequently developed to 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 earlier results, we find that the Pc1/IPDP wave frequency observed on ground corresponds to the maximum of the transmission coefficient rather than that of the reection coefficient. 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 modied 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 effect 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 IPDP's on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.