Dynamics of long‐period ULF waves in the plasma sheet: Coordinated space and ground observations

Abstract

Spacecraft and ground‐based observations are used to study characteristics of ultralow frequency waves in the plasma sheet from the postmidnight to morning local time sectors in the terrestrial magnetosphere. Field line resonance (FLR) type oscillations with discrete and latitude‐dependent frequencies in the ranges of 1.7–2.0 and 3.0–3.2 mHz are observed in situ by the Time History of Events and Macroscale Interactions during Substorms C (THEMIS C), THEMIS D, THEMIS E, and GOES 12 spacecraft. The ground resonant oscillations in the two mentioned frequency bands are also observed at corresponding spacecraft footprints. Spectral peaks at these frequencies are observed by nearly all ground stations from premidnight to noon, with the larger‐amplitude oscillations occurring in a narrow range of latitudes (3°–6°). The largest wave activity occurred in the magnetic local time of ∼05:00. The ground observations indicate westward propagation for the 1.8 mHz wave activity with an azimuthal wave number of about −2.6. The Poynting vectors from the THEMIS spacecraft show weak net energy flow (antifield aligned) toward the ionosphere of the southern hemisphere. They also show notable net energy flow toward the west. A possible interpretation is that the observed FLRs are driven by cavity and waveguide modes in the nightside outer magnetosphere after a period of long‐lasting northward interplanetary magnetic field.