Determination of Plasma, Pickup Ion, and Suprathermal Particle Spectrum in the Solar Wind Frame of Reference

Abstract

Abstract Particle spectra directly measured by a plasma or low-energy particle experiment on spacecraft often contain instrumental effects due to a limited field of view or angular resolution. It is because the particle distribution function at low energies is highly anisotropic in the spacecraft frame of reference, in which the measurements are made. In this paper, we present a new mathematical method of transforming the particle spectrum to the solar wind frame of reference, where the particle distribution can be assumed to be nearly isotropic. The transformed particle spectrum allows us to investigate the properties of the solar wind, pickup ions, and suprathermal particles without concern for instrumental effects. We apply the method to the measurements made by the Solar Wind Ion Composition Spectrometer on Ulysses. Our results demonstrate that the transformed spectrum can improve the determination of the solar wind density and temperature from the previously published methods. A brief survey of Ulysses data shows that suprathermal ions in the slow solar wind frequently display a velocity distribution very close to the v −5 power law.