The Method of Extracting Ion Distributions from TWINS ENA Images

Lessons learned from the extraction of equatorial ion distributions from energetic neutral atom (ENA) images from the satellite IMAGE have been successfully applied to ENA images from TWINS. A full description of the technique will be presented. The method uses a tri-cubic spline expansion to express the equatorial ion distribution as a function of radius, polar angle and pitch angle. Adapting a method based upon Bayesian statistics and derived using the formalism of reproducing kernels by Wahba [Spline Models for Observational Data, SIAM, Philadelphia, 1990], a linear set of equations that minimizes both chi-squared and a penalty function to regularize the solution provides the expansion coefficients and thus the equatorial ion distribution. Using TWINS data, examples will be given at each step of the process. It will also be shown how having two simultaneous views from TWINS enhances the confidence in the deconvolutions and provides the opportunity to obtain ion pitch angle distributions in addition to the spatial distributions.