Structure of the heliosheath from HSTOF energetic neutral atoms measurements

Abstract

Context. From the year 1996 until now, High energy Suprathermal Time Of Flight sensor (HSTOF) on board Solar and Heliospheric Observatory (SOHO) has been measuring the heliospheric energetic neutral atoms (ENA) flux between +/- 17 degrees from the ecliptic plane. At present it is the only ENA instrument with the energy range within that of Voyager LECP energetic ion measurements. The energetic ion density and thickness of the inner heliosheath along the Voyager 1 trajectory are now known, and the ENA flux in the HSTOF energy range coming from the Voyager 1 direction may be estimated. Aims. We use HSTOF ENA data and Voyager 1 energetic ion spectrum to compare the regions of the heliosheath observed by HSTOF and Voyager 1. Methods. We compared the HSTOF ENA flux data from the forward and flank sectors of the heliosphere observed in various time periods between the years 1996 and 2010 and calculated the predicted ENA flux from the Voyager 1 direction using the Voyager 1 LECP energetic ion spectrum and including the contributions of charge exchange with both neutral H and He atoms. Results. The ratio between the HSTOF ENA flux from the ecliptic longitude sector 210-300 degrees (the LISM apex sector) for the period 1996-1997 to the estimated ENA flux from the Voyager 1 direction is similar to 1.3, but decreases to similar to 0.6 for the period 1996 2005 and similar to 0.3 for 1998-2006. For the flank longitude sectors (120-210 degrees and 300-30 degrees), the ratio also tends to decrease with time from similar to 0.6 for 1996-2005 to similar to 0.2 for 2008 2010. We discuss implications of these results for the energetic ion distribution in the heliosheath and the structure of the heliosphere.