Weak electrostatic waves near the upper hybrid frequency: A comparison between theory and experiment

Two types of generating mechanisms have been proposed to explain the observations of weak electrostatic waves near the local upper hybrid frequency fuh. In this paper data from the wave and plasma diagnostic experiments on GEOS 1 are presented with simultaneously observed particle data in an attempt to identify the generating mechanism. The electron data contain two sources of free energy: a highly anisotropic "pancake" distribution in the warm (20 eV to ∼1 keV) plasma and a loss cone type distribution in the hot (>few keV) plasma. This leads us to consider a coherent generating mechanism. A model of the plasma distribution, with cold, warm and hot components is constructed and linear instability calculations are presented. It is shown that the relatively large resonant perpendicular velocities in the loss cone distribution generate instabilities at small perpendicular wave numbers preferentially in the upper hybrid band since the wave frequency can lie midway between the gyroharmonics where cyclotron damping is reduced. The calculations are able to reproduce the spectral characteristics observed by previous spacecraft over a range of cold plasma densities. The temporal growth rates are comparable to those obtained previously for strong electrostatic emissions near fuh, but it is shown that the unstable wave number region is an order of magnitude smaller. Thus it is argued that the waves remain weak since they are quickly refracted out of resonance. It is demonstrated that when there is a significant amount of warm plasma present the strongest natural emission can be used to derive the total plasma density, cold plus warm plus hot, and not the cold plasma density, and that the warm plasma introduces new regions of small group Velocities which result in fine structure in the wave spectrum. The effects of the warm plasma and particularly the pancake distribution are discussed in connection With the ray propagation of these and other classes of much stronger emissions.

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