Latitudinal and energy dependence of energetic neutral atom spectral indices measured by the Interstellar Boundary Explorer

We investigate the latitudinal and energy dependence of the globally distributed 0.5–6 keV energetic neutral atom (ENA) spectra measured by the Interstellar Boundary Explorer (IBEX) during the first 3 yrs of the mission. Our results are: (1) the ENA spectral indices at the two lowest energies (0.89 and 1.47 keV) exhibit no clear trend with ecliptic latitude θ, while those at ∼2.29 and ∼3.41 keV exhibit a clear latitudinal pattern; flatter spectra occur above 60° latitude and steeper spectra occur ±30° of the equator. (2) The latitudinal dependence of the spectral indices at different energies can be represented by the cosine function $\gamma =a_0+a_{1}cos\left(a_2\theta \right)$ with unique offsets, amplitudes, and phase angles; the higher energy ENA indices transition to successively larger amplitudes within ±45° of the equator. Our results confirm the previously reported latitudinal organization of the ENA spectra and their remarkable similarity to that of the solar wind (SW) speed observed by Ulysses in the inner heliosphere. While earlier studies showed that the ∼0.5–6 keV globally distributed ENA spectral indices could be represented as single power laws over much of the sky, our new results indicate that this is an over-simplification because the spectral indices have an energy and latitude dependence. This dependence is an important factor that must be taken into consideration by models and simulations that seek to map the IBEX ENA observations back to the latitudinal profile of the SW speed structure observed in the inner heliosphere.