Understanding our global heliosphere with UV observations: Unique opportunities on Interstellar Probe

<p class="p1">Solar Lyman-a emission re-radiated from H atoms incoming to the heliosphere from interstellar medium is a powerful tool to probe globally plasma properties both at the heliosphere boundary and near the Sun. H Lyman-α line profiles reflect velocity distributions of low energy H atoms in the heliosphere which hold information about the plasma near the heliopause. H Lyman α intensities as observed at 1 AU serve as diagnostic of global properties of the solar wind. In this talk we will review what we have learned about the global heliospheric interaction from H Lyman-a observations from inside the heliosphere on SWAN/SOHO, Voyages/UVS and New Horizons/Alice missions. Outward trajectory of Interstellar Probe going through the outer heliosphere to the interstellar medium (ISM) up to 1000 AU enable unique science opportunities to explore global interaction between the solar wind and local ISM by observing for the first time Lyman-a emission from outside of the heliosphere. We will report a progress of UV working group in outlining primary science questions on the nature of the global heliosphere and Local Interstellar Cloud, planning observation strategy, measurement requirements and synergies with planetary UV observations for potential KBO fly-by.</p>

What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): morphology of the Local Interstellar Cloud, its hydrogen hole, Stromgren Shells, and 60Fe accretion

<p> We describe the very local interstellar medium (VLISM)<br>immediately outside of the outer heliosphere. The VLISM consists <br>of four partially ionized clouds - the Local Interstellar Cloud (LIC), <br>G cloud, Blue cloud, and Aql cloud that are in contact with the outer <br>heliosphere, and ionized gas produced by extreme-UV radiation <br>primarily from the star Epsilon CMa. We construct the <br>three-dimensional shape of the LIC based on interstellar line <br>absorption along 62 sightlines and show that in the direction of <br>Epsilon CMa, Beta CMa, and Sirius B the neutral hydrogen column <br>density from the center of the LIC looking outward is a minimum. <br>We call this region the ``hydrogen hole''. In this direction, the <br>presence of Blue cloud absorption and the absence of LIC absorption <br>can be simply explained by the Blue cloud lying just outside of the <br>heliosphere. We propose that the outer edge of the Blue cloud is a <br>Str\"omgren shell driven toward the heliosphere by high pressures in <br>the H II region. The outer edges of other clouds facing Epsilon CMa <br>are likely also Stromgren shells. Unlike previous models, the LIC<br>surrounds less than half of the heliosphere.</p><p>We find that the vectors of neutral and ionized helium flowing<br>through the heliosphere are inconsistent with the mean LIC flow <br>vector and describe several possible explanations. The ionization<br>of nearby intercloud gas is consistent with photo-ionization by <br>Epsilon CMa and hot white dwarfs without requiring additional <br>sources of ionization or million degree plasma. In the upwind <br>direction, the heliosphere is passing through an environment of <br>several LISM clouds, which may explain the recent influx of <br>interstellar grains containing 60Fe from supernova ejecta measured <br>in Antarctica snow. The Sun will leave the outer partof the LIC <br>in less than 1900 years, perhaps this year, to either enter the <br>partially ionized G cloud or a highly ionized intercloud layer. <br>The heliosphere will change in either scenario. An instrumented <br>deep space probe sending back in situ plasma and magnetic field <br>measurements from 500-1,000 AU is needed to understand the <br>heliosphere environment rather than integrated data along the <br>sightlines to stars.  </p>