Multi-Point Cluster Observations of VLF Risers, Fallers and Hooks at and Near 
the Plasmapause

J. S. Pickett, O. Santolik, S. W. Kahler, A. Masson, M. L. Adrian, 
D. A. Gurnett, T. F. Bell, H. Laakso, M. Parrot, P. Decreau, A. Fazakerley, 
N. Cornilleau-Wehrlin, A. Balogh, and M. Andre


The four Cluster Wideband (WBD) plasma wave receivers occasionally observe
electromagnetic triggered wave emissions at and near the plasmapause. We present
the remarkable cases of such observations. These triggered emissions consist of
very fine structured VLF risers, fallers and hooks in the frequency range of 1.5
to 3.5 kHz with frequency drifts for the risers on the order of 1 kHz/s. They
appear to be triggered out of the background whistler mode waves (hiss) that are
usually observed in this region, as well as from narrowband, constant frequency
emissions. Occasionally, identical, but weaker, emissions are seen to follow the
initial triggered emissions. When all the Cluster spacecraft are relatively
close (< 800 km, with interspacecraft separations of around 100Ð200 km), the
triggered emissions are correlated across all the spacecraft. The triggered
emissions reported here are observed near the perigee of the Cluster spacecraft
(around 4Ð5 RE) within about 20 degrees, north or south, of the magnetic equator
at varying magnetic local times and generally at times of low to moderate Kp. In
at least one case they have been observed to be propagating toward the magnetic
equator at group velocities on the order of 5-9 X 10E7 m/s. The triggered
emissions are observed in the region of steep density gradient either leading up
to or away from the plasmasphere where small-scale density cavities are often
encountered. Through analysis of images from the EUV instrument onboard the
IMAGE spacecraft, we provide evidence that Cluster may sometimes be immersed in
a low density channel or other complex Examples of the various types of
triggered emissions are provided which show the correlations across spacecraft.
Supporting density data are included in order to determine the location of the
plasmapause. A nonlinear gyroresonance wave-particle interaction mechanism is
discussed as one possible generation mechanism.

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in Multiscale Processes in Earth's Magnetosphere: From Interball to Cluster, 
J.-A. Sauvaud and Z. Nemecek (eds.), Kluwer, pp. 307-328, 2004.