Discussion following the Reports by Weaver and Field

Abstract

This long, very intensive, and partially very confused discussion, has been rearranged in six sections: (1) Direct and Indirect Evidence of X-rays and Low-Energy Cosmic Rays; (2) The Boundary Layer between the Stable Gas Phases; (3) Theoretical Aspects of Interstellar Gas Dynamics and the Formation of Clouds; (4) Observational Aspects of Interstellar Gas Dynamics and the Formation of Clouds; (5) Observations of the Rarefied, Neutral Intercloud Medium and of the Interstellar Electron Density; (6) The Dynamical Theory of Hii Regions. Section 2 has been transferred from the Discussion on Monday, September 8 (Chapter 2). To Section 6 have been added remarks made during various discussions. A couple of remarks have been transferred to other Discussions. Part of the Discussion (in Section 5) was very confused; an attempt has been made to condense and to make as much sense as possible out of what was said. For the convenience of the reader I recapitulate a few concepts, the (mis)-use of which lead partially to the confusion: (i)The hydrogen surface density or column density NH = ʃ nH dl (NH is sometimes called the hydrogen measure HM).(ii)The dispersion measure DM = ʃ ne dl (DM is often called the electron surface density Ne).(iii)The rotation measure RM = c1 ʃ neB‖ dl.(iv)The emission measure EM = ʃ ne2 dl.In these definitions, nH represents the hydrogen density, ne the electron density, B‖ the component of the magnetic field strength along the line of sight and l the distance along the line of sight. Conventionally nH and ne are expressed in cm−3, B in μG and l in pc. In addition there are two combinations of these quantities (and of the electron temperature T) involved in the Discussion:(v)The free-free absorption coefficient k(v) = c2(v)ne2T−3/2 (v is the frequency).(vi)The free-free emissivity ɛ(v)=c3(v)ne2T−1/2 (only at radio wavelengths). c2 and c3 depend also on T, but only rather weakly.