L'interaction entre le flot d'helium neutre d'origine interstellaire et l'heliosphere

Elastic collisions between neutral helium atoms from interstellar origin and protons from both the interstellar plasma in the heliospheric interface and the solar wind inside the heliosphere modify parameters of the helium gas. Upstream the heliopause, the helium atoms release momentum to the plasma, which results in a decrease of their bulk velocity, and remove energy from the plasma, which results in an increase of their temperature. Inside the heliosphere, the major part of the energy communicated by solar wind protons to helium atoms is transferred through scarce frontal collisions, which results in the creation of a hot tenuous He component in the downstream (with respect to the sun) region. The modification of the spatial He distribution function near the sun due to slightly deflected atoms is negligible in first approximation. Lastly, we emphasize that the collisional effect is probably insufficient to explain the high temperature of helium derived from photometric measurements of the interplanetary medium at 58.4 nm (16 000 K). By taking into account two other effects which are the light transfer in the He resonance line inside the focusing cone and a possible latitudinal anisotropy of the solar ionizing EUV flux, leading both to a lowering of the contrast of the cone, we show that the temperature discrepancy between H (8 000 K : spectral measurements : line width, Doppler shift) and He (16 000 K : photometric measurements) can be reasonably explained by the combined actions of these three effects : elastic collisions, light transfer at 58.4 nm inside the focusing cone, solar EUV flux latitudinal anisotropy.