LATITUDE, ENERGY, AND TIME VARIATIONS IN THE ENERGETIC NEUTRAL ATOM SPECTRAL INDICES MEASURED BY THE INTERSTELLAR BOUNDARY EXPLORER (IBEX)

Abstract

We investigate the latitude, energy, and temporal variations of the ∼0.5–6 keV energetic neutral atom (ENA) fluxes and spectral indices measured by the Interstellar Boundary Explorer ( IBEX ) outside of the Ribbon from 2009 to 2013. By combining all-sky maps for years 1–3 and years 4–5, we find that the >1.2 keV globally distributed (GD) ENA fluxes at all latitudes decrease by ∼25% from years 1–3 to years 4–5, but there is no change (<5%) in the corresponding spectral indices. We also show that the latitudinal profile and energy-dependence of the ≥1 keV ENA spectral indices outside the Ribbon exhibit no significant time differences between years 1–3 and 4–5, lending strong support for the notion that the highly organized, persistent energy dependence and latitudinal pattern of the ENA spectral indices during 2009–2014 are determined by that of the solar wind (SW) speed observed in the inner heliosphere during the deep solar minimum conditions of 2006–2010. In contrast, the ∼25% decrease in the >1.2 keV ENA fluxes at all latitudes from years 1–3 to years 4–5 occurs because the magnitude of the driving SW parameters (SW density or dynamic pressure) diminished significantly from 2006 to 2010. Based on the reconstructed latitudinal and temporal profiles of SW parameters from 2011 to 2014, i.e., during the rising phase of solar cycle 24, we suggest that the GD ENA fluxes in years 2014–2017, i.e., in maps 6–9, will either stabilize or increase and the latitudinal pattern and energy dependence of the corresponding spectral indices will be disrupted.