What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): What will ISP detect?
Abstract
The Interstellar Probe (ISP) will provide the first directmeasurements of interstellar gas and dust when it travels far beyond theheliopause where the solar wind no longer influences the ambient medium.We summarize in this presentation what we have been learning about the VLISMfrom 20 years of remote observations with the high-resolution spectrographson the Hubble Space Telescope. Radial velocity measurements of interstellarabsorption lines seen in the lines of sight to nearby stars allow us tomeasure the kinematics of gas flows in the VLISM. We find that the heliosphereis passing through a cluster of warm partially ionized interstellar clouds.The heliosphere is now at the edge of the Local Interstellar Cloud (LIC) andheading in the direction of the slighly cooler G cloud. Two other warm clouds(Blue and Aql) are very close to the heliosphere. We find that there is alarge region of the sky with very low neutral hydrogen column density, whichwe call the hydrogen hole. In the direction of the hydrogen hole is thebrightest photoionizing source, the star Epsilon Canis Majoris (CMa). Extremeultraviolet photons from this star produce a Stromgren sphere region ofionized gas as large as the Local Cavity (extending to 100-200 parsecs)and produce Stromgren shells at the outer regions of the local warm cloudsincluding the LIC.When the ISP passes beyond the hydrogen wall at a distance of about 500 AU,it will likely enter the outer edge of the LIC where photoionization fromEpsilon CMa plays an important role. Analysis of Hubble observations ofinterstellar absorption proves estimates of the densities, temperature,pressure, and flow properties of the main portion of the LIC, but we havelittle informtion on these properties at the LIC's edge. Comparison with theinflow vector of neutral helium measured by IBEX and Ulysses indicates aslightly different flow speed and direction than the mean flow of the LIC gas.ISP will provide direct measurements of the flow and gas properties of thispoorly understood region. In particular, ISP will provide information onhow photoionization from Epsilon CMa influences warm clouds through ionization,heating, and perhaps pressure balance. This information may resolve questionsconcerning the magnetic field surrounding the heliosphere.
- Publication:
-
European Planetary Science Congress
- Pub Date:
- September 2020
- DOI:
- 10.5194/epsc2020-68
- Bibcode:
- 2020EPSC...14...68L