Analyses of convective flows and spatial gradients in energetic ion observations

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Abstract
We present two techniques for analyzing bulk flows, ϵ × B convection and pressure gradients in magnetospheric plasmas using the measured anisotropy of three‐dimensional energetic ion (>30 keV) distributions from the Medium Energy Particle Instrument experiment on the ISEE 1 spacecraft. The first technique uses a Lorentz (Compton ‐ Getting) transform into the appropriate rest frame (plasma or ϵ × B) in an iterative fashion, converging to the unique velocity that removes the first‐order anisotropy from the flux angular distribution. Gradients are distinguished from ϵ × B convection on the basis of the dependence on energy of the anisotropy. The gradients are described by assuming an exponential spatial dependence and obtaining a characteristic gradient scale length. The second technique involves a first moment of the distribution function, in which the shift in velocity space of the distribution function is obtained at two phase space density values. Based upon the energy (velocity) dependence of the shift, it can be determined whether convection or a gradient is responsible for the shift. By assuming an MHD generator condition (which may apply particularly in the magnetospheric boundary layer), the convection contribution to the velocity shift can be assumed to be perpendicular to the pressure gradient contribution, thus rendering the two shifts separable. Using this assumption, we obtain both a gradient scale and a lower limit to the convection electric field in one (noniterative) calculation.