Geomagnetic Pulsations in the 1–4 mHz Frequency Range (Pc5/Pi3) in the Magnetotail. Internal and Extramagnetospheric Sources

Abstract

Spectral and spatial parameters of nonstorm Pc5/Pi3 geomagnetic pulsations in the geomagnetic tail are studied based on magnetic measurements on Cluster satellites under different conditions in the interplanetary medium. Both the average values of the magnetic field components in front of the bow shock and fluctuations in the pulsations’ frequency range are considered. Special attention is paid to the conditions of small amplitudes of fluctuations (“zero” perturbation) in order to localize the source of pulsations starting under these conditions. For this purpose, simultaneous data from two satellites located on one or different sides of the magnetopause are used. It is shown that, along with a decrease in the amplitude of fluctuations in front of the bow shock, the amplitude and spatial scale of fluctuations in the magnetosheath change and the coherence between fluctuations on different sides of the magnetopause decreases sharply. This allows us to conclude that the pulsations occurring in the geomagnetic tail under “zero” level fluctuations in front of the bow shock are an intramagnetospheric phenomenon weakly related to the processes beyond the magnetopause.