Storm‐time Pc1 activity at high and middle latitudes

Abstract

We study structured and unstructured Pc1 pulsations observed at a high‐latitude station (Sodankylä; L = 5.1) and a midlatitude station (Nurmijärvi; L = 3.3) during 18 storms occurring in low solar activity years (1976–1978 and 1984–1988). Pc1 activity was studied from the day of storm sudden commencement (denoted by day 0) onward during six consecutive days. While unstructured pulsations are only weakly affected, structured pulsations are greatly dependent on storm evolution. During the storm main phase they nearly vanish on the ground, despite strong wave activity in space. Structured Pc1 activity increases from day 0 to day 4 by a factor of about 4–5, reaching maximum occurrence on day 4 at both stations. Also, the average daily frequency of structured Pc1s increases from day 0 to a maximum on day 3 at Nurmijärvi or day 4 at Sodankylä. The diurnal distribution of structured Pc1s suffers a dramatic change during the storm. On days 1 and 2, structured pulsations are strongly concentrated in the evening sector, but during the later recovery (days 3–5) the activity shifts to the morning sector. The latitudinal similarity of structured Pc1 occurrence and the daily evolution of wave frequency argue against the model according to which the outward expansion of plasmapause causes the maximum wave occurrence on the ground on day 4. We also note that the strong maximum of structured Pels during the late storm recovery phase is not supported by the model calculations of the magnetospheric wave source or by direct observations of waves in space. Instead, we argue that the ionospheric resonator and propagation conditions which strongly affect wave observations on the ground are deteriorated during the storm main and early recovery phases, impeding wave propagation to the ground. The subsequent recovery of the ionospheric conditions leads to the maximum occurrence of structured Pc1s on the ground during the late storm recovery phase.