The Heliosphere and the Local Interstellar Medium

Abstract

In a first section I briefly review recent advances in the knowledge of the interstellar helium flow in the heliosphere. Because neutral helium traces physical conditions in the Local interstellar Cloud (LIC), the precise determination of its parameters is mandatory if one wants to accurately quantify the modifications that other species suffer across the heliospheric interface. The substantial progress resulted from a coordinated analysis held at the International Space Science Institute (Bern). In short, all parameters are now better constrained and there are no more discrepancies among the results obtained from different types of detections. In a second section, I describe recent advances in the description of the local interstellar medium, which have a potential impact on the heliosphere. The pressure of the hot gas surrounding the LIC and filling the Local cavity is found to be significantly smaller, if one takes into account the newly discovered heliospheric diffuse X‐ray emission, which mimics thermal hot gas emission. Because the gas pressure difference between the LIC and the hot medium is thought to be compensated for by the LIC magnetic pressure, this has the effect of constraining the LIC magnetic field to a smaller value than previously estimated. New HST and FUSE limits on the columns of intermediate ions forming at the conductive interfaces between the local clouds and the hot gas of the local cavity also suggest a lower hot gas pressure. However, the 35–40% ionization degree of helium in the local clouds, supposedly due to the EUV emission from those conductive interfaces, is now very difficult to explain if they are as “weak” as suggested by the recent results. Helium ionization still remains to be explained. On the other hand, the new maps of the Local Cavity boundaries reveal a number of “tunnels” linking it to surrounding hot “bubbles”, and suggest a lower pressure in “our” bubble as compared with those neighboring “bubbles”. Hot gas may have been (or is still) flowing into the local cavity from these hotter regions, and helium ionization may be linked to this motion.