Effect of magnetic storm intensity on Pc1 activity at high and mid-latitudes

We study the properties of structured and unstructured Pc1 pulsations at a high-latitude station (Sodankyla; L = 5.1) and a mid-latitude station (Nurm ijarvi; L = 3.3) during 18 storms occurring in low solar activity years. Th e storms were divided into two groups according to their intensity as measu red by the minimum value of the D-st index. Pc1 activity was studied from t he day of the storm sudden commencement onwards during six consecutive days . Having recently published the average results for all 18 storms [Kerttula et al., J. Geophys. Res. (2000) in press], we concentrate here on the effe ct of magnetic storm intensity on wave properties. The source of structured Pc1s was found to be at lower latitudes during intense storms, in agreemen t with the lower latitude of the ring current during intense storms. Also, the source of unstructured Pc1s, the plasmasheet ions, was found to shift t o lower latitudes during intense storms but this change was only observed i n the early recovery phase. The great depletion of structured Pc Is on grou nd during the storm main and early recovery phase, which is in apparent dis agreement with space observations and model calculations, is even more dram atic for intense storms. This further emphasizes the significance of the io nospheric conditions for wave observations on ground, and suggests that the depletion is due to deterioration of the ionospheric Alfven resonator duri ng the storm main phase. Moreover, the results support the idea [Kerttula e t al., J. Geophys. Res. 62 (2000) 299-309] that Pc1 occurrence maximum duri ng late recovery phase is related to the improved ionospheric amplification and propagation conditions, rather than the outward expansion of the plasm apause. (C) 2001 Elsevier Science Ltd. All rights reserved.