Ground-based and Polar spacecraft observations of a giant (Pg) pulsation and its associated source mechanism

Multi-instrument observations of a Pg pulsation, which occurred on the morning of May 16, 1998, are reported. The wave signature was observed simultaneously on the ground, by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and in the ionosphere by the Doppler Pulsation Experiment (DOPE) high resolution HF Doppler sounder. The wave occurred in the morning sector and possessed an azimuthal wave number, m, of 30±5 with a westward phase propagation. Shortly before the Pg commenced, energetic particle instruments on board the Polar spacecraft detected protons with a non-Maxwellian energy distribution drifting westward toward the location of IMAGE and DOPE. An investigation has been undertaken to determine whether these particles were involved in the wave-particle interaction considered responsible for generating the Pg pulsation. Proton energies of around 7 keV, which occur at the low-energy edge of the unstable distribution (where ∂ƒ/∂W>0), satisfy the drift-bounce resonance relation, ω − mω[subscript d] = Nω[subscript b], for N=1. This result indicates that this particular wave is likely to be the result of a drift-bounce resonance mechanism and that it has an even mode standing wave structure in the magnetosphere. This result is discussed in terms of previous observations of Pgs.