What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): What will ISP detect?

Abstract

<p>The Interstellar Probe (ISP) will provide the first direct<br />measurements of interstellar gas and dust when it travels far beyond the<br />heliopause where the solar wind no longer influences the ambient medium.<br />We summarize in this presentation what we have been learning about the VLISM<br />from 20 years of remote observations with the high-resolution spectrographs<br />on the Hubble Space Telescope. Radial velocity measurements of interstellar<br />absorption lines seen in the lines of sight to nearby stars allow us to<br />measure the kinematics of gas flows in the VLISM. We find that the heliosphere<br />is passing through a cluster of warm partially ionized interstellar clouds.<br />The heliosphere is now at the edge of the Local Interstellar Cloud (LIC) and<br />heading in the direction of the slighly cooler G cloud. Two other warm clouds<br />(Blue and Aql) are very close to the heliosphere. We find that there is a<br />large region of the sky with very low neutral hydrogen column density, which<br />we call the hydrogen hole. In the direction of the hydrogen hole is the<br />brightest photoionizing source, the star Epsilon Canis Majoris (CMa). Extreme<br />ultraviolet photons from this star produce a Stromgren sphere region of<br />ionized gas as large as the Local Cavity (extending to 100-200 parsecs)<br />and produce Stromgren shells at the outer regions of the local warm clouds<br />including the LIC.</p> <p>When the ISP passes beyond the hydrogen wall at a distance of about 500 AU,<br />it will likely enter the outer edge of the LIC where photoionization from<br />Epsilon CMa plays an important role. Analysis of Hubble observations of<br />interstellar absorption proves estimates of the densities, temperature,<br />pressure, and flow properties of the main portion of the LIC, but we have<br />little informtion on these properties at the LIC's edge. Comparison with the<br />inflow vector of neutral helium measured by IBEX and Ulysses indicates a<br />slightly different flow speed and direction than the mean flow of the LIC gas.<br />ISP will provide direct measurements of the flow and gas properties of this<br />poorly understood region. In particular, ISP will provide information on<br />how photoionization from Epsilon CMa influences warm clouds through ionization,<br />heating, and perhaps pressure balance. This information may resolve questions<br />concerning the magnetic field surrounding the heliosphere.</p>