Exploring the time dispersion of the IBEX-Hi ENA spectra at the ecliptic poles

Since launch the Interstellar Boundary Explorer (IBEX) has produced more than five maps of hydrogen Energetic Neutral Atom (ENA) emission from the edge of the solar system. The maps were constructed for the energies from 0.71 to 4.3 keV. At these energies and for a travel distance of the order of 100 AU, the travel time difference between the slowest and the fastest ENA is more than a year. Therefore, when constructing spectra from a single map, the slower ENAs left the source at an earlier time than the faster ones. If the source yields a steady rate of ENAs and the extinction does not vary too much, the spectral shape is time independent. However, while the extinction of ENAs has been fairly constant during the first two and a half years, the source has changed locally, and thus the spectra derived from a single map may not represent the conditions at the source. IBEX's orbit configuration allows continuous sampling of the Ecliptic poles where one can monitor those changes over time. For a given source distance and a source thickness we construct spectra assuming that the measured ENAs have left the source at roughly the same time from the source. To accomplish this, we apply time lag corrections to the signal at different ENA energies that take into account the travel time difference. We show that the spectral shape at the poles exhibits a statistically significant change with time, though the changes are small.