Heliospheric interstellar H temperature from SOHO/SWAN H cell data

We show a first comparison between selected SOHO/SWAN H cell data recorded in 1996-1997 and a simple classical "hot model" of the interstellar (IS) H flow in the inner heliosphere. Our,goal is to obtain some constraints on th e interplanetary background Ly-a profiles, for the first time without any a ssumption on the H cell characteristics. For this purpose the H cell optica l thickness and its temperature are free parameters of the study, but we as sume that the direction of the flow and the allowed range for the upwind li ne-of-sight apparent Doppler shift are known from previous studies. We derive apparent temperatures (or line-of-sight (LOS) temperatures) betwe en 11,000 and 20,000 K according to the direction. This implies a significa nt broadening with respect to the linewidths expected for a flow at the sam e temperature as the interstellar helium flow (6,000 +/- 1000 K) in the opt ically thin approximation. Radiative transfer is probably responsible for a fraction of this effect, and heating at the heliospheric interface for the remaining. The best solutions are found for an upwind velocity of 26 km s( -1), in excellent agreement with an independent study by Quemerais et al. ( 1999), and for very similar H cell absorption width and temporal decrease. The deceleration of interstellar H at heliopause crossing is found to be be tween 2.5 and 4.5 km s(-1) We also use one particular H cell absorption map to derive directly from th e data how the LOS temperature (or linewidth) varies with the angle with th e wind direction. Interestingly, we measure a temperature minimum between t he upwind and crosswind directions, while classical models predict a monoto nic increase of the LOS temperature from upwind to downwind. We believe tha t this behavior is the first evidence for the existence of two distinct pop ulations at different velocities (primary and secondary IS atoms), as predi cted by heliosphere-interstellar gas interface models. If confirmed, this s hould be an extremely good diagnostic of the interface.