Discussion following the Reports by Weaver and Field

Extract

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:

1. (i) The hydrogen surface density or column density N_H = ʃ n_H dl (N_H is sometimes called the hydrogen measure HM).

2. (ii) The dispersion measure DM = ʃ n_e dl (DM is often called the electron surface density N_e).

3. (iii) The rotation measure RM = c₁ ʃ n_eB_‖ dl.

4. (iv) The emission measure EM = ʃ n_e² dl.

   In these definitions, n_H represents the hydrogen density, n_e 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 n_H and n_e 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:

5. (v) The free-free absorption coefficient k(v) = c₂(v)n_e²T^−3/2 (v is the frequency).

6. (vi) The free-free emissivity ɛ(v)=c₃(v)n_e²T^−1/2 (only at radio wavelengths). c₂ and c₃ depend also on T, but only rather weakly.