Observations of magnetospheric ULF waves in connection with the Kelvin‐Helmholtz instability at Mercury

Abstract

AbstractThe magnetic field data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is investigated to establish the presence of magnetospheric ultralow frequency (ULF) waves in connection with 131 previously observed nonlinear Kelvin‐Helmholtz (KH) waves at Mercury. Distinct ULF wave signatures are detected in 44 out of the 131 magnetospheric traversals prior to or after observing the KH waves. Of these ULF events, 39 out of 44 are highly coherent at the frequency of maximum power spectral density and occur more often on the dayside magnetosphere than away from it. The waves observed at the dayside magnetosphere, which appear mainly at the duskside and naturally following the KH wave occurrence asymmetry, are significantly different from the eveningside or morningside events and have the following distinct wave characteristics: a polarization mainly in the perpendicular (azimuthal) direction to the mean magnetic field, a wave normal angle closer to the parallel than the perpendicular direction, an absolute ellipticity increasing away from noon, almost exclusively a right‐hand polarization, and frequencies in the narrow range of 0.02–0.04 Hz (well below the local Na+ gyrofrequency and in the same range as the KH waves). The results strongly suggest that the large majority of the ULF waves at the dayside observed in this study are driven by KH waves at the magnetopause and that they occur in the vicinity of a field line resonance, which in turn manifests the importance of the Kelvin‐Helmholtz instability in terms of energy and momentum transport throughout Mercury's magnetosphere.