Solar wind H+ fluxes at 1 AU for solar cycles 23 and 24

Abstract

The solar wind consists of electrons and ion species having kinetic energy ≲10 keVs. Ion stream is mainly composed of protons. Their flux magnitude is especially important for radiation hardness assurance scientists who qualify materials for future space missions. Materials irreversibly degrade as soon as are exposed to solar radiation. Hence, proton differential and integral flux spectra play a crucial role in properly estimating radiation loads deposited within satellite functional components. Up to now, the wind fluxes were read out, e.g., from figures made for specific periods of a solar cycle. This method is not effective and is a source of a flux, and further on, a fluence uncertainty. The objective of this work was to calculate and tabulate the solar wind differential and integral proton fluxes, which can be used further on to evaluatespace mission proton fluence spectra. Solar wind proton bulk velocity and number density from four space solar observatories (SOHO, ACE, WIND, and DSCOVR) were numerically processed to achieve that goal. Fluxes were tabulated and represented in calendar year periods.Now, the radiation hardness assurance community can quickly access, compute and compare SW proton fluxes from different energy ranges, calendar years, and satellite data sources. Proton fluxes presented here indicate magnitude variations along with the solar cycle period. A correlation between the number of sunspots and the proton average flux was found, i.e., the larger the number of spots, the larger the flux magnitude. More importantly, it was discovered that proton fluxes indicate huge variations in magnitude for a selected year based on different satellite data sources. For some years, the difference reaches 240%. This value is reported for the first time and shows that to have a complete picture of the SW proton stream population, one must compare data from all four satellites. Also, it is reported that satellites, for a given calendar year, record different ranges of proton velocity and number density values. This finding, even stronger, suggests a necessity of comparing SW data from as many solar observatories as possible. The spread in the proton flux magnitude (240%) has a direct implication for planning and executing satellite material on-ground radiation test campaigns. Not carefully choosing a proton flux may result in a false impression of how examined material would degrade under an interplanetary space radiation environment. In this article, recommendations for selecting proper proton spectra are given.