Flux‐Tube Dynamics and a Model for the Origin of the Local Fluff

Abstract

The Local Fluff is the low-density warm interstellar material found in several small structures in the general vicinity of the Sun, lying within the much larger scale, lower density hot gas of the Local Bubble. A scenario is proposed for the formation of the Local Fluff after the generation and reheating of the Local Bubble by a small series of supernovae over a period of a few million years. In this scenario, a magnetic flux tube, spanning roughly a half-circle along the face of the Bubble surface, separates from the wall except at its anchored ends and, driven by its tension, springs back into the cavity. The tube brings with it material from the inside surface of the Bubble wall, concentrating it during the evolution into a small set of density and velocity structures near the Bubble center, the Local Fluff. A unique hydrocode is described that simultaneously follows the motion of the flux tube through the hot gas and the motion of the material within the tube. Characteristics of the evolution of an initially very symmetric tube geometry are explored over an extensive region of parameter space, and useful analytic results are presented for the characteristics of the single cloud formed. The scenario acquits itself very well in that parameters consistent with those seen in the Local Bubble and Fluff lead to timescales, tube velocities, densities, and fractional ionizations like those observed. Reasons are also given for expecting gas-phase abundances and ionization conditions to vary among the components of the Local Fluff, as suggested by some observations. We present cases in which the initial geometry of the flux tube is somewhat irregular. They lead to the early formation of a number of density peaks within the tube. The individual and interactive evolutions of these regions, as the material as a whole converges toward the Bubble center, lead to an evanescent population of density peaks and velocity features distributed along the central portion of the tube. A single springing flux tube is thus also able to produce an array of structures similar to several components of the Local Fluff. We note that the great success of the model as a whole is tempered by two modest difficulties that may disappear with less idealized modeling of the evolution and with more careful scrutiny of the data. We also describe several predictions. These involve the relative locations of higher and lower stages of ionization within the Local Fluff, the likelihood of velocity gradients within features, the probable near linearity of the overall spatial distribution of material within an individual tube, and the likely direction and magnitude of the magnetic field. The latter should be of interest to modelers of the interaction between the solar wind and the Local Cloud.