Time-Variation of ENA Flux Observed by IBEX at the Heliospheric Poles: Has the Recovery Begun?

For nearly five years the Interstellar Boundary Explorer (IBEX) has been making continuous observations of heliospheric energetic neutral atoms (ENAs) arriving from the direction of the ecliptic poles. For the first three years, we have observed a steady decrease in the polar ENA flux in all IBEX-Hi energy passbands. The magnitude and rate of decline was shown to be consistent with the observed decline in solar wind dynamic pressure during the previous solar minimum, once the lag time between solar wind observations at 1 AU and the return time of ENAs from the heliosheath was taken into account (Reisenfeld et al. ApJ, 747, 2012). When it became apparent in 2010 that the solar wind dynamic pressure was beginning to recover, it was expected that within a couple of years, the decline in ENA flux should cease and begin to turn around as well. Numerical models of an asymmetric heliosphere (e.g. Pogorelov et al. ApJ, 668, 2007; Opher et al. SSR, 143, 2009, and others) indicate that the distance to the termination shock (TS) should be significantly shorter toward the south ecliptic pole than toward the north. Thus it was expected that the turnaround in ENA flux should occur first at the south pole, then the north. Another expectation was that the ENA flux should begin recovery at the highest energies first, since such ENAs have a shorter travel time toward IBEX. The latest observations show that for ENAs arriving from the south pole, the data are consistent with a flattening in the ENA flux. At the north pole, there is no indication yet of a turnaround or flattening of the ENA flux, which is consistent with the expected greater distance to the TS. We present a quantitative analysis of the ENA time series for both poles and compare the polar variations to ENA measurements from lower latitudes (where the data are collected with a 6 month cadence), to identify any indication of changes in the overall trend of flux decrease.