Type Pi Magnetic field pulsations at very high latitudes and their relation to plasma convection in the magnetosphere

Abstract

Type Pi (broadband) magnetic field pulsation activity near the dayside cusp and in the polar cap has been investigated using data recorded at Sachs Harbour (76.2°N, IL (invariant latitude)) and at Thule, Greenland (86.3°N, IL). Most of the pulsation activity observed at these sites must be classsified as type Pi, and this activity occurs mainly in midday hours. In summer, the midday Pi intensities are on the same order as at auroral oval sites in the night sector. Pi amplitudes are smaller in winter at Thule than in summer in the ratio 1∶3. Pi activity is always present at these very high latitudes in day hours in summer, with a minimum observed amplitude of 14 nT at Sachs and 5 nT at Thule in June–July, 1977. A study of isolated substorms having sharp onsets revealed a clear tendency for Pi activity to appear in the polar cap preceding the substorm, with a median lag between onset of Pi at Thule and onset of substorm near 1½ hours. We believe that this class of Pi activity indicates the presence of plasma convecting antisunwards over the polar cap. Polar cap and cusp Pi activity also tends to be seen during the substorms. The Pi pulsations appear to be caused by three‐dimensional current loops that are driven by the magnetospheric electric field at times of enhanced plasma convection. The response of the current loop is closely related to the conductivity of the E region portion of the loop, and this characteristic explains the tendencies for larger Pi amplitudes to be observed in the dayside and summer time polar cap and at locations where auroral precipitation increases the E region conductivity.