Improved Calculation of Interstellar Pickup Proton Properties from Ulysses/SWICS Data

Integrating the particle spectra of ions in the solar wind allows for the determination of the bulk plasma properties such as the density and temperature of interstellar pickup ions. These results are important for validating simulations of the interaction between the solar wind and the local interstellar medium. Including the effects of interstellar pickup ions in these simulations can significantly modify the outer heliospheric boundaries. We build on the particle spectra from [1], which are generated from measurements made by the Solar Wind Ion Composition Spectrometer (SWICS) instrument on the Ulysses spacecraft with a new method of converting the particle distribution functions from the spacecraft frame to the solar wind frame. This approach offers an improvement upon the standard narrow-beam approximation. We calculate the moments of these particle spectra to determine the bulk plasma properties of the interstellar pickup ions. We focus on pickup protons (H⁺) for this study. We compare our density and temperature results to those in [2], where the particle spectra are converted to the solar wind frame using the narrow-beam approximation. The results are given for twelve-hour intervals over two months during the "Halloween 2003" solar storms. In general, our pickup ion density is higher than that calculated in [2]. Our results are promising for the purpose of studying interstellar pickup ions during quiet times as well as during solar events. The results of this analysis will be used to validate simulations of the interaction between the solar wind and the local interstellar medium. This will make it possible to reveal the contribution of pickup ions in more detail.

[1] Zhang, M. et al. 2019, Determination of Plasma, Pickup Ion, and Suprathermal Particle Spectrum in the Solar Wind Frame of Reference. The Astrophysical Journal, 871:60 (9pp). https://doi.org/10.3847/1538-4357/aaf509

[2] Intriligator, D. S. et al. 2012, Pickup Protons: Comparisons using the three-dimensional MHD HHMS PI model and Ulysses SWICS measurements. Journal of Geophysical Research, 117, A06104. https://doi.org/10.1029/2011JA017424