STEREO Observations of Heliosheath ENAs in 2008

In 2007, the STE (SupraThermal Electron) sensors of the IMPACT instrument suite on the STEREO A and B spacecraft detected ~4-20 keV ENAs, most likely hydrogen, coming from the nose of heliosphere, with a wide (~60° in longitude), asymmetric double-peak source structure. These ENAs very likely originated from termination-shock-accelerated pickup ions in the heliosheath. Here we report on new STEREO A observations of heliosheath ENAs in 2008, a year later. We find generally the same ENA source structure - an asymmetric double peak straddling the nose of heliosphere, with the two peaks located at virtually same longitudes with comparable fluxes, on average. The observed ENA flux spectra also exhibit a similar double-power-law with a break at ~9-14 keV. Compared to 2007, the average ENA fluxes are almost the same in the direction of Voyage 1 (close to the major peak), while lower by a factor up to 2 in the direction of Voyager 2 (at the edge of ENA source). These observations indicate a rather steady ENA source in the heliosheath over a year, from 2007 to 2008. If the source is pickup ions accelerated at the termination shock, then their acceleration is quite stable. Furthermore, we will present STEREO B observations that are being taken in the next few months.