The S1 shell and interstellar magnetic field and gas near the heliosphere.
Abstract (from CDS):
Many studies of the Loop I magnetic superbubble place the Sun at the edges of the bubble. One recent study models the polarized radio continuum of Loop I as two magnetic shells with the Sun embedded in the rim of the "S1" shell. If the Sun is in such a shell, it should be apparent in both the local interstellar magnetic field and the distribution of nearby interstellar material. The properties of these subshells are compared to the interstellar magnetic field and the distribution of interstellar Fe+ and Ca+ within ∼55 pc of the Sun. Although the results are not conclusive, the interstellar magnetic field direction obtained from polarized stars within ∼30 pc is consistent with the interstellar magnetic field direction of the S1 shell. The distribution of nearby interstellar Fe+ with log N(Fe+) < 12.5/cm2 is described equally well by a uniform distribution or an origin in spherical shell-like features. Higher column densities of Fe+ (log N(Fe+)>12.5/cm2) tend to be better described by the path length of the sightline through the S1 and S2 subshells. Column densities of the recombinant ion Ca+ are found to increase with the strength of the interstellar radiation field, rather than with star distance or total pathlength through the two magnetic subshells. The ion Ca+ cannot be used to trace the distribution of local interstellar gas unless the spatial variations in the radiation field are included in the calculation of the ionization balance, in addition to possible abundance variations. The result is that a model of Loop I as composed of two spherical magnetic subshells remains a viable description of the distribution of nearby low density interstellar medium, but is not yet proven.