Visualizing the three-dimensional structure of the local interstellar medium and possible physical causes for this structure

Analysis of high-resolution interstellar absorption lines observed in the ultraviolet spectra of nearby stars provides the basis for identifying 15 partially ionized warm "clouds" of interstellar gas located within a few parsecs of the Sun. We show visualizations of these clouds as seen from different directions. The presence of discrete clouds implies one or more physical processes that establish these structures and the presence of gas with very different properties between these clouds. While classical models of the interstellar medium based on the assumptions of pressure and ionization equilibrium in a nonmagnetic medium are not consistent with observations, dynamical simulations without these assumptions are likely more realistic. Possible physical processes that could structure the clouds include ram pressure, magnetic fields, and photoionization by extreme-ultraviolet radiation. We describe the available evidence concerning these three processes.