Optical testing of the High-Resolution Plasmasphere Observatory prototype

We present the result of optical testing of a prototype of the High-Resolution Plasmasphere Observatory (HRPO), an imaging camera designed to measure the macroscale dynamics of fine-scale plasmaspheric density structure, i.e., features with a scale size of ≤0.1 RE. We report laboratory measurements of the HRPO prototype to validate the model-predicted average spot diameter of 0.05 deg ± 0.01 deg [root-mean-squared (RMS)] or 0.08 deg ± 0.03 deg (geometric). From testing at nine different field angles, we find the average measured spot diameter is 0.05 deg ± 0.01 deg (RMS) or 0.09 deg ± 0.02 deg (full-width 1/10 maximum), demonstrating that HRPO’s lab-measured angular resolution is consistent with the model prediction and between four and seven times smaller than the 0.36 deg needed for Nyquist (two-point) sampling of 0.1 RE fine-scale plasmasphere structure from a notional 8 RE geocentric radius orbit. We compute simulated HRPO images to demonstrate the imager’s expected performance for realistic plasmaspheric brightness distributions. HRPO’s improved angular resolution is capable of capturing smaller density structures, but only with sufficient count rates either from the brighter regions of images or with adaptive space or time binning in the dimmer regions.