Solar-System-Scale Microstructure in the Diffuse Interstellar Medium
Source: Astrophysics and Space Science, Volume 288, Number 3, 2003 , pp. 369-375(7)
Abstract:The most remarkable discovery of recent years in the study of the interstellar medium has been the identification of spatial structures of sizes down to those of a few times the mean Earth-Sun distance (an Astronomical Unit, AU). Solar-System Scale microstructures are detected in cool clouds (temperatures 100 K in which the hydrogen number density is of order 102 cm-3, and which are about a parsec in extent (1 parsec = 2 × 105 AU). Some microstructures are believed to be much denser (by factors of about 100) than the ambient gas, and therefore significantly over-pressured. They are necessarily short-lived, and dissipate on timescales 10 y. They do not originate in conventional nonmagnetic hydrodynamic processes, and their existence indicates that our knowledge of interstellar dynamics has been incomplete. Here, we summarize the evidence for the existence of microstructures, and propose that any that are significantly overpressured are formed in postshock regions of high magnetic pressure due to the excitation of slow-mode magnetosonic waves by the nonlinear steepening of disturbances generated by the Kelvin-Helmholtz and other instabilities. Less overpressured microstructures can be formed by the same nonlinear processes operating in a region in which the magnetic contribution to the pressure is less dominant.
Document Type: Research article
Affiliations: 1: Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK 2: Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT, UK 3: Department of Physics and Astronomy, University College London, London WCIE 6BT, UK
Publication date: 2003-01-01