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 highlatitude station (Sodankylä; L=5.1) and a mid-latitude station (Nurmijärvi; L=3.3) during 18 storms occurring in low solar activity years. The storms were divided into two groups according to the their intensity as measured by the minimum value of the Dst index. Pc1 activity was studied from the day of storm sudden commencement onwards during six consecutive days. Having recently published the average results for all 18 storms [Kerttula et al., 1999], we concentrate here on the effect of magnetic storm intensity on wave properties. The source of structured Pc1s was found to be at lower latitudes during intense storms, in agreement with the lower latitude of the ring current during intense storms. Also, the source of unstructured Pc1s, the plasmasheet ions, was found to shift to lower latitudes during intense storms but this change was only observed in the early recovery phase. The great depletion of structured Pc1s on ground during storm main and early recovery phase, which is in apparent disagreement with space observations and model calculations, is even more dramatic for intense storms. This further emphasizes the significance of the ionospheric conditions for wave observations on ground, and suggests that the depletion is due to deterioration of the ionospheric Alfvén resonator during storm main phase. Moreover, the results support the idea [Kerttula et al., 1999] that Pc1 occurrence maximum during late recovery phase is related to the improved ionospheric amplification and propagation conditions, rather than the outward expansion of the plasmapause.