Heliospheric energetic neutral atoms as a means to determine the anomalous cosmic ray spectrum at the termination shock

We demonstrate the possibility of deriving the anomalous cosmic ray (ACR) spectrum at the solar wind termination shock and the strength of the shock using energetic neutral H and He atoms (ENA) measurements at 1 AU. Assuming that the energetic neutral atoms detected by CELIAS/HSTOF on board SOHO are of ACR origin, we consider $\chi^2$ fits to the ACR termination shock spectrum based on approximations to the ACR distribution downstream of the shock. We show that the method becomes much more effective when the low-energy ENA data are combined with the available high-energy ACR data from Voyager spacecraft. The results imply a weak termination shock (compression ratio $\sim2.9$).