Geotail observations of low‐frequency waves from 0.001 to 16 Hz during the November 24, 1996, Geospace Environment Modeling substorm challenge event

Abstract

We present observations of low‐frequency waves from 0.001 Hz up to the lower hybrid frequency during the time periods corresponding to the growth, expansion, and recovery phases of a substorm that occurred from ∼ 2100 to 2310 UT on November 24, 1996, while Geotail was located in the magnetotail at XGSE ∼ 25 RE and 0100 LT. Large‐amplitude waves near the lower hybrid frequency were observed when large‐scale density and magnetic field gradients were present during thinning of the plasma sheet at substorm onset and the beginning of the recovery phase. Later in the recovery phase, waves near the lower hybrid frequency were observed close to the neutral sheet in localized areas with strong small‐scale density and magnetic field gradients. Fluctuations of the electric and magnetic fields near the ion gyrofrequency were also observed during this event; however, the amplitudes were smaller than those often observed in the near tail in association with substorms. Compressional fluctuations of the magnetic field in the Pi2 frequency range were observed close to substorm onset. Although Geotail was not in the region considered in recent studies of Pi2 pulsations, it may still be possible that the magnetic field fluctuations observed by Geotail during the November 24, 1996, substorm are related to this phenomenon since little is known about propagation of the compressional pulses and generation of the waves.