Variation of Hydrogen Energetic Neutral Atom Flux in the Subsolar Magnetosheath as a Function of Solar Cycle

We analyzed observations of energetic neutral atoms (ENAs) of hydrogen collected from the subsolar magnetosheath by the Interstellar Boundary Explorer (IBEX) in Solar Cycle 24. We looked for long-term solar activity-related variation and correlation between the H ENA flux and the solar wind parameters. The H ENA flux in the energy range from 0.5 to 6 keV increased during the maximum and declining phases of the solar cycle. This H ENA flux increase happened during a period of enhanced solar wind dynamic pressure observed at 1 au. We study the relation between the ENA flux and solar wind pressure in the subsolar magnetosheath and conclude that the increase in the ENA flux is a consequence of enhanced ENA production. It is because of the increased solar wind pressure during the declining phase of the solar activity that moved the subsolar magnetosheath earthward into a region of denser hydrogen geocorona, thus increasing the H ENA production. Furthermore, we report a positive correlation between the H ENA flux in the subsolar magnetosheath and the solar wind pressure for the first four energy steps observed by IBEX-Hi and a decrease for the highest energy acquired, which indicates the presence of a different parent ion population for those ENAs from that of the core solar wind. Our study also shows that the ENA flux correlates differently with the solar wind density and speed as a function of increasing energy of ENAs.