Coherence time scales of long-period ULF variations of magnetic field in vicinity of the night-side magnetopause

We examine magnetic field variations within the frequency range of several millihertz (Pc5-6/Pi3 geomagnetic pulsations) in the near-Earth magnetotail and adjacent flank magnetosheath regions, using data from Cluster satellites for 2016. Dependence of spectral coherence on interval length is analyzed for a satellite pair Cluster-1 and Cluster-4 at different satellite positions relative to the magnetopause. It is shown that absolute coherence and the rate of its decline with increasing time interval length differ for the longitudinal and transverse magnetic field components, as well as for different satellite positions. We also present a case study of a coherent pulsation recorded in the magnetosheath at low solar wind velocity and weak fluctuations in front of the bow shock.