Pc 4 Cavity mode wave frequency variation associated with inward motion of the magnetopause during interplanetary shock compression

A cavity mode wave, referring to a trapped or radially standing fast mode wave between different magnetospheric boundaries, has been developed in theory and reported in observation studies. In this study, we present an interplanetary shock (IPS)-induced cavity mode wave event observed outside the plasmasphere on 31 August 2017 with multispacecraft measurements. The phase delay of 90 degrees between the azimuthal electric field and compressional magnetic field indicates that the fast-mode wave triggered by the IPS is a standing wave, presumably radially trapped in the cavity between the magnetopause and plasmapause. Taking advantage of the location of VAP-B spacecraft right outside the plasmapause and the AARI ground-based high-latitude array mapped in the noon sector, it is suggested that the observed compressional wave associates to cavity mode with its inner boundary at the plasmapause and its outer boundary at the magnetopause. The peak frequency of the wavelet spectrum of the compressional magnetic field increases from 10.5 to 12.5 mHz, which is consistent with the theoretically calculated cavity eigenfrequencies before and after the IPS. We also provide the first evidence that the peak frequency of the cavity mode increases due to the inward motion of the magnetopause during IPS compression.