Abstract
We surveyed the temporal evolution of the latitude and energy dependence of the ∼0.5–6 keV energetic neutral atom (ENA) spectral indices in the globally distributed flux (GDF) observed by the Interstellar Boundary Explorer from 2009 to 2017. We show that (1) the ENA fluxes in the GDF diminish substantially at all latitudes from 2009 to 2016, with the largest decreases occurring above ∼2.73 keV in the northern hemisphere; and (2) the latitudinal ordering of the >1.36 keV ENA spectral indices during 2009–2013 is disrupted during 2014–2017, with the indices exhibiting a north–south asymmetry. This north–south asymmetry is due to the asymmetric decrease in the GDF and the resulting occurrence of significantly steeper spectra in the northern hemisphere. The latitudinal organization of the ENA spectral indices during 2009–2013 and its disruption during 2013–2017 are remarkably similar to that of the solar wind (SW) speed profile observed in the inner heliosphere, thus confirming that the latitudinal profile of the ENA spectral indices in the GDF is a direct manifestation of the SW latitudinal speed profile following a ∼2–3 yr delay due to the SW-ENA recycling time. Based on these results and the SW speed profiles during 2014–2017, we speculate that the ENA spectral indices during 2018–2020 will not exhibit the systematic latitudinal ordering observed during 2009–2014, but instead will show relatively constant and similar values at all energies and all latitudes. There may also be a small north–south asymmetry wherein the northern hemisphere ENA spectra will be slightly flatter compared to those in the southern hemisphere.
