As the velocity vectors of the two clouds were refined with new observations, it was noted that the interstellar flow into the solar system was intermediate between the LIC and G cloud as defined by Redfield & Linsky ( 2008). The ensemble of nearby interstellar material has been denoted the complex of local interstellar clouds (CLIC Slavin & Frisch 2002). Nevertheless, the equivalence between the LIC and the VLISM near the Sun has remained a widely accepted paradigm.Įxtended observations of absorption lines toward multiple stars from the Hubble Space Telescope (HST) led to the definition of 15 discrete warm clouds within 15 pc of the Sun (Redfield & Linsky 2008). Further observations confirmed that the absorption lines toward the stars located close to the Galactic center do not show absorption from the LIC, implying that the Sun must be within a small fraction of a parsec from the LIC edge (Wood et al. Further observations indicated apparent consistency of the AG cloud flow with the very local interstellar medium (VLISM) observed in the heliosphere, and consequently, the cloud was renamed the Local Interstellar Cloud (LIC Bertin et al. The absorption lines toward nearby stars initially revealed two clouds in the direct neighborhood of the Sun called Galactic (G) and Anti-Galactic (AG) clouds based on their position in the sky relative to the Galactic center (Lallement & Bertin 1992). The velocity structure of these clouds suggested the presence of nearby interstellar shocks (Grzedzielski & Lallement 1996), possibly aligned with the interstellar magnetic field shaping the heliosphere (Frisch et al. Individual clouds show velocity deviations from a rigid-body flow and indicate that the flow is decelerating as it moves outward in the superbubble (Frisch et al. Local clouds appear to originate from an evolved superbubble associated with Loop I (Frisch et al. The local interstellar medium consists of a low-density fully ionized cavity called the Local Bubble extending over 100 pc from the Sun (Frisch & York 1983 Breitschwerdt 1998) and smaller warm (3000–13,000 K) partially ionized interstellar clouds inside this cavity (Redfield & Linsky 2008). The structure and equilibrium in this region require further studies using in situ and telescopic observations. The observed asymmetry of the interstellar helium distribution function also supports this interaction. Interactions between these clouds explain the substantially higher density of the interstellar hydrogen near the Sun and toward stars located within the interaction region of these two clouds. This finding shows that the Sun travels through a mixed-cloud interstellar medium composed of material from both these clouds. We find that, contrary to the widespread viewpoint that the Sun resides inside the LIC, the locally observed velocity of the interstellar neutral helium is consistent with a linear combination of the velocities of the LIC and G cloud, but not with either of these two velocities. We combine recent results from Ulysses, IBEX, and STEREO observations to find a more accurate estimate of the velocity and temperature of the very local interstellar medium. Their bulk heliocentric velocities can be compared with the interstellar neutral helium flow velocity obtained from space-based experiments. The two nearest clouds-the Local Interstellar Cloud (LIC) and Galactic (G) cloud-move toward each other. High-resolution spectroscopic measurements of interstellar absorption lines in spectra of nearby stars show absorption components from more than a dozen warm partially ionized clouds within 15 pc of the Sun. On its journey through the Galaxy, the Sun passes through diverse regions of the interstellar medium.
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