Signatures of very slow MHD wave in CIS data inferred from Cluster measurements during sudden magnetosphere compression: a case study

A sudden increase (SI) of the solar wind dynamic pressure detected by the WIND satellite on 07.04.2001 caused a magnetopause displacement near 17:30 UT. The Cluster was located inside the dayside magnetosphere near the magnetopause. Before entering the plasmasheath, the Cluster pair C3 and C4 detected a sharp enhancement of downward moving protons with a time lag of about 25 seconds. We think that proton onset was initiated by the specific MHD-wave excited by the impact of a solar wind irregularity on the dayside magnetopause and propagating tailward. The propagation velocity inferred from the time lag is too small (30 km/s) to connect the wave with compressional or/and Alfven waves which usually are associated with SI. The CIS data allowed also to define that before the SI the Cluster was located in the area with proton pressure gradient outward from the Earth. So, we think that we deal with the waves similar to those discussed earlier by Safargaleev et al. [1986, 2001, 2003] as propagating inside the medium with smooth plasma pressure gradient and being responsible for poleward moving periodic arc-like forms in pulsating and diffuse background auroras in the nightside and periodic ULF-activity of Pc1 range accompanying SI in the dayside.