A mechanism of anticorrelation in the occurrence of ULF electromagnetic noise resonance structure and Pc 1 magnetic pulsations through the solar activity cycle

The observed anticorrelation between the Ii-year solar activity cycle and t he appearance of Pc 1 geomagnetic pulsations as well as the resonance struc ture of the electromagnetic noise spectrum (SRS) in the frequency range 0.1 -10 Hz is explained on the basis of the role of the ionospheric Alfven reso nator (IAR) in the formation of both phenomena. The reflection coefficient R of Alfven waves from the IAR determines the threshold of the cyclotron in stability (CI) which is the cause of Pc 1 pulsations. At the same time, the quality of the IAR, which varies synchronously with R, controls the SRS mo dulation depth. It is shown that the decrease of the maximum value of \R\ d uring the maximum phase of the solar activity cycle remarkably increases th e CI threshold and sharply decreases the modulation depth of SRS, thus decr easing the occurrence probability of both phenomena. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.