Diagnostics of the Local Interstellar Medium using particles and UV radiation Original Research Article

The chain of processes which govern trajectories and ionization of neutral particles from the circumsolar interstellar gas to the inner heliosphere can now be studied by combining several types of diagnostics: solar backscattered radiation (H and He), direct detection of neutral particles (He), detection of pickup ions of interstellar origin (H, He, C, N and O, etc.). Moreover, continuous observations of the Sun bring the required solar parameters to be included in the modeling and, in parallel, considerable efforts have been made in model developments. A good example of such improvements is the synthetic work done on interstellar helium, detailed by Möbius et al. in this issue. This is useful because helium is a tracer of the initial state of the interstellar gas in the Local Cloud and serves as a zero level for the other species. In particular, the velocity distribution of interstellar neutral hydrogen measured by SOHO/SWAN can be compared with helium characteristics, to quantify the modifications experienced by the H flow at entrance in the heliosphere. This provides a measurement of the interstellar circumsolar plasma pressure. Here we focus on three aspects: the recent EUVE data on the helium 58.4 nm glow, which provide an independent measurement of the flow direction and velocity, and help to understand how helium temperature has been overestimated when analyzing past glow data.