ENA Time Variation in the IBEX skymaps and at the Poles

Abstract:

After 5 years of IBEX observations and 11 complete skymaps, we can now trace the time evolution of the heliospheric ENA flux at all points in the sky over the course of half a solar cycle. Generally we find that the ENA flux has decreased by 30-40% since the beginning of the mission, with a decline of up to 60% in some parts of the north ecliptic polar region. In the direction of the heliospheric tail, the ENA flux has been steady at all energies, exhibiting no drop in ENA flux, or perhaps even a slight increase. Preliminary analysis of the IBEX ribbon reveals the same fractional drop in flux as observed in adjacent parts of the sky. This has significant implications for the source mechanism for the ribbon.

We also have continuous coverage of the ENA flux directly at the north and south ecliptic poles, and from this we find that from the beginning of the mission until mid-2012, the ENA flux declined at all IBEX-Hi energies and the IBEX-Lo energies above 400 eV. Starting in mid-2012, the ENA flux began to level out and increase, first at 1.7 keV, and then a few months later at 1.1 keV (below this, the statistics are too poor to draw any conclusions). From this we can deduce a distance of about 150 AU to the ENA source region, which is likely within the inner heliosheath. Interestingly, at 2.7 and 4.3 keV, the flux continues to fall, most strongly in the north. This is somewhat unexpected, and might imply that these ENAs originate further out in the heliosheath and/or result from a different physical process.