The Interface between the Outer Heliosphere and the
The Interface between the Outer Heliosphere and the Inner LISM (see arxiv: 1910: 12043) Jeffrey Linsky JILA and APS/University of Colorado. Interstellar Probe Workshop New York City October 16 -18, 2019
The heliosphere is surrounded by interstellar clouds containing neutral hydrogen • Plasma models of the outer heliosphere assume that the heliosphere is surrounded by interstellar gas containing neutral hydrogen. • The models predict a hydrogen wall where inflowing neutral hydrogen charge exchanges with solar wind protons producing a region of slowed down, heated and denser neutral hydrogen. • Red-shifted absorption in the Lyman-α line by hydrogen in the hydrogen wall is detected in stellar spectra for many lines of sight. • This supports the plasma models of the outer heliosphere, but the reality is far more complex and relevant for ISP science
Temperature and densities in a model of the interaction between the solar wind and the ISM Richardson & Stone (2009)
The heliosphere is NOT surrounded by the Local Interstellar Cloud (LIC) • Radial velocity measurements for neutral and ions in the ISM toward nearby stars resolve the LISM into 15 velocity vectors (clouds). • Nearest 4 clouds are LIC, G seen toward α Cen (1. 3 pc), Blue seen toward Sirius (2. 6 pc), and Aql seen toward 61 Cyg (3. 5 pc).
Morphology and kinematics of the LIC • 62 lines of sight through the LIC allow us to compute a 3 D model of the LIC. • The Sun is at or just beyond the edge of the LIC (covers 45% of the sky). • EUVE, IBEX, and Ulysses have measured the flow of neutral He from the ISM with inflow speed 2 km/s faster and direction 2 -3 degrees off from the LIC vector. • Evidence that the heliosphere is not inside of the LIC but maybe at an edge with a different flow vector.
What shapes and ionizes the surrounding clouds? • From the 3 D model of the LIC, we compute the view from the center looking out through the hydrogen absorption. • Very uneven structure with a large “hydrogen hole” in the direction of ϵ CMa (E), β CMa (B), and Sirius (S). • ϵ CMa is the brightest EUV (photoionizing source). It shapes the LIC and produces Stromgren shells at the outer edges of clouds facing the star.
Strömgren sphere and Strömgren shell • Strömgren (1939) described the ionized (H II) region around a hot star that emits EUV photons. • Strömgren sphere radius is distance out to which photoionizations exceed recombinations. Hydrogen is fully ionized (H II gas). • Strömgren shell thickness δ is length of mean free path for neutral hydrogen atoms. • For n(HI) = 0. 2 cm^{-3}, δ≃0. 2 pc.
Which cloud is in the direction of the hydrogen hole? • Blue cloud (blue outline) overlaps most of the hydrogen hole (dashed line outline). • The blue cloud is moving at 6 km/s in the direction of the heliosphere likely pushed by the gas in the H II region ionized by ϵ CMa. • ϵ CMa and Sirius both show absorption by the thin Stromgren shells at the edges of the LIC and Blue clouds.
What will ISP encounter as it leaves the heliosphere? • It depends on the direction it is sent and the cloud it enters. • Hydrogen density in the hydrogen wall may peak near 300 AU and the pristine ISM is beyond 600 AU (Zank Ap. J 763, 20 (2013)) • I suggest targeting ϵ CMa because we should see increasing ionization from the incident EUV radiation. Galactic coordinates: l=240, b=-11. Ecliptic coordinates: �� =111, �� =-51.
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