Charting Interstellar Medium near the Heliosphere

Observations of absorption lines in the spectra of nearby stars show that the local interstellar medium within 15 pc from the Sun includes more than a dozen warm partially ionized interstellar clouds. The two closest clouds near the heliosphere are the local interstellar cloud (LIC) and galactic cloud (GC). Since a certain column density of absorption material from a cloud is required for identification in the spectra, the closest structure cannot be unambiguously resolved using telescopic observations alone. Interstellar neutral (ISN) helium atoms penetrating the heliosphere allow for derivation of the flow velocity and temperature in front of the heliosphere. The high precision combined parameters obtained from the IBEX, Ulysses, and STEREO results show that the Sun is moving with a speed corresponding to a linear combination of the LIC and GC velocities. Consequently, the Sun is likely not in a singular interstellar cloud, but a mixed interstellar cloud medium (MICM) formed in a collision of the LIC and GC. The existence of the MICM is further supported by the higher local density of ISN hydrogen observed in the heliosphere and toward some nearby stars. Furthermore, the asymmetry of the ISN helium distribution function is aligned with the relative motion of these two clouds. Hence, various populations in the VLISM cannot be in equilibrium, and thus their velocities and temperatures may differ. Consequently, the interstellar boundary conditions used for heliosphere modeling require revision.