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Session 47 - The Diffuse Interstellar Medium, Progress and Puzzles - II.
Oral session, Tuesday, June 11
Historical Society,
As astronomers, we like to concentrate on what we can directly observe with our spectroscopes and photometers. However, in the interstellar medium magnetic fields and cosmic rays are also important and sometimes dominant. The global structure of galactic magnetic fields should reflect the amplification mechanism (windup of dynamo), but the observations of global structure are in their infancy and results from different techniques are often contradictory. Whatever their origin, magnetic fields in galaxies are strong enough to have significant effects on interstellar gas dynamics, on scales from global, characterizing galactic structure, down to the small scales characteristic of star formation. And, equally importantly, magnetic fields confine cosmic rays--the subrelativistic to ultrarelativistic component of the ISM--to galaxies, and probably play a role in acceleration of cosmic rays as well. Cosmic rays are a primary source of heating, ionization, and dynamical pressure in the ISM. We so-called experts on the interstellar medium admit that there is much we do not know, but a primary cornerstone of our understanding is the ability to categorize the phases of the interstellar gas, with a primary characteristic being thermal pressure--which is only one of many kinds, and is often the least important on a numerical scale. However, we must now face the embarrasing but seemingly secure observational fact that both the neutral and warm ionized media have tiny-scale, high column density structures in which thermal pressures are orders of magnitude larger than what is normally regarded as the total pressure in the interstellar medium. At the other extreme, tiny-scale structure with much smaller densities is revealed by pulsar scintillation, which together with other data imply the existence a Kolmogoroff-like spectrum extending from a.u. to many pc scales.
