Spin Index Determination of the Hawkeye Spacecraft

Abstract

We have developed a method to despin data from the Hawkeye spacecraft (1974-1978) that involves analyzing the variation of solar array voltage with sun phase angle. This new method is needed because the optical aspect system on the Hawkeye spacecraft failed four months into its mission, after which the spacecraft spin index could be determined only for limited periods using two other methods. Adding our new solar array technique nearly doubles the available data. The Hawkeye mission was designed to study high-latitude, high-altitude magnetosphere (exterior cusp, surrounding magnetopause and boundary layers, the polar cap, and Auroral field lines). The scientific payload was designed to measure the in situ plasma, magnetic, and electric fields. Besides Hawkeye, only the Highly Eccentric Orbiting Satellite-2 (HEOS-2) systematically explored this region of the Earth's geospace. Determining the spin index for the rest of the data, and subsequently despinning these data and representing them in a geophysically meaningful coordinate system, is still important due to the uniqueness of Hawkeye's orbit. The solar array method presented is based on searching parameter space for the solution that results in the best self-organization in spin index of the solar array voltage measurements. The spin index computed using this method is compared against the spin index determined from other methods. The resulting despun magnetic field data are compared with magnetic field models and with in situ magnetic field measurements in the solar wind made by the IMP-8 spacecraft.