SuperDARN observations of the driver wave associated with FLRs

Abstract

Ultra‐low frequency (ULF) field line resonances (FLRs) cause oscillations in F‐region plasma flows and can be detected in SuperDARN measured line‐of‐sight (l‐o‐s) velocities. In this paper, we characterize a ULF wave event with coordinated use of SuperDARN HF radars, optical instrumentation, ground based and space based magnetometers. On December 26, 2000 from 00:00–04:00 UT, the SuperDARN Pykkvibaer radar observes first and second harmonic FLR signatures at 0.8 mHz, while the Kodiak and Hankasalmi radars simultaneously observe the driver wave on open field lines at exactly the same 0.8 mHz frequency. These observations show that SuperDARN can provide a diagnostic of MHD wave propagation on open field lines and potentially be used to monitor MHD wave transmission across the magnetopause, and through the outer regions of the magnetosphere. MHD waves in the outer magnetosphere, which couple to FLRs, are seldom observed, and as far as we know this is the first report of a ground based observation of the driver wave. The observation of first and second harmonic FLRs in SuperDARN data is a unique and useful observation in the sense that it supports the theoretical body of work on expected behavior of FLRs, and their potential use in estimating magnetospheric properties such as density and magnetic topology. During the time interval of interest, Geotail is in the solar wind just outside the dawn flank region, and observes clear oscillations in the IMF Bz component at 0.8 mHz. High coherence is shown between the Geotail Bz oscillations and the radar Doppler velocities at 0.8 mHz, confirming that the 0.8 mHz FLR harmonic and the driver wave on open field lines is directly driven by the 0.8 mHz oscillation in the solar wind. Discrete ULF oscillations in the solar wind as direct drivers of ULF waves in the magnetosphere is a controversial topic, and the results reported here add to a growing body of evidence in support of direct solar wind drivers of ULF waves.