Detection of ultralow-frequency cavity modes using spacecraft data

[1] The cold, magnetized plasma in the Earth’s magnetosphere supports two ultralowfrequency plasma wave modes. Both these modes may exhibit resonant oscillations in the magnetosphere cavity. Theoretical and numerical studies have predicted the existence of cavity/waveguide resonance modes, yet experimental evidence is sparse. In this paper we detail the expected structure of these modes using both one dimensional (1-D) and three-dimensional (3-D) magnetohydrodynamic (MHD) numerical models. The cavity/ waveguide mode structures are examined in order to develop experimental detection methods suitable for spacecraft electric and magnetic field perturbation data. Cavity mode resonances in the 1-D model suggest a detection method based on wave polarization using the radial (bx) and field-aligned (bz) magnetic perturbations. However, when implemented, this method failed to identify cavity/waveguide modes in the magnetic field data recorded by Active Magnetospheric Particle Tracer Explorers/CCE for events that showed pronounced field line resonances in the azimuthal (by) channel. An examination of data from a 3-D MHD numerical simulation showed that the cavity/waveguide resonant signature was identified best in bz component data. Consequently, a wave mode detection method using the bz data from two spatially separated satellites is discussed. Magnetometer data examples from the ISEE 1 and 2 spacecraft show that field line resonances appear in the by data even when the coherence length of the bz data is less than 0.4 RE.