Effects of fast and slow solar wind on the energetic neutral atom (ENA) spectra measured by the interstellar boundary explorer (IBEX) at the heliospheric poles

We study the energy dependence of ∼0.5-6keV energetic neutral atom (ENA) spectra in the southern heliospheric polar region obtained during five six-month sky maps measured by IBEX-Hi. We calculate the spectral slopes in the south pole in four different energy bands, namely, ∼0.7-1.1keV, 1.1-1.7keV, ∼1.7-2.7keV, and ∼2.7-4.3keV. We show (1) a persistent flattening of the ENA spectrum between ∼1 and 2keV, (2) significantly different modes (2.31, 1.58, 0.97, and 1.44) for the distributions of the slopes in the four different energy bands, and (3) a general decrease with increasing energy in the widths (FWHM) and mode fluctuations (their spread) of the slope distributions. We also compare the averaged ENA spectra measured at the south pole and at mid-latitudes. We conclude that the flattening between ∼1 and 2keV in the polar spectrum (spectral break) is produced by an enhancement of ENAs created by charge exchange between interstellar neutrals and pick-up ions in the fast solar wind.