The role of turbulence in the warm ionized medium

We discuss the role of turbulence in establishing the observed emission measures and ionization of the warm ionized medium. A Monte Carlo radiative transfer code applied to a snapshot of a simulation of a supernova-driven, multi-temperature, stratified ISM reproduces the essential observed features of the WIM. In recent years, it has become clear that turbulence plays a central role in establishing the observed conditions in the warm ionized medium (WIM) of the Galaxy. Although the WIM was first detected four decades ago, the 1996 completion of the Wisconsin Hα Mapper (WHAM; Haffner et al. 2003) made possible large-scale studies of the physical conditions in the medium, particularly through the combination of Hα and collisionally excited metal lines. These and other studies have established that the WIM is nearly fully ionized (Reynolds et al. 1998), primarily by the O star Lyman continuum flux (Reynolds 1990). However, the mechanism by which ionizing photons escape the immediate vicinity of the O stars near the Galactic plane to reach the observed 1.0− 1.5 kpc scale height of the WIM (Savage & Wakker 2009) has been the subject of numerous investigations (Miller & Cox 1993; Dove & Shull 1994; Ciardi et al. 2002; Wood & Mathis 2004; Haffner et al. 2009). These studies have generally found that the propagation of ionizing photons to |z| ∼ 1 kpc in an ISM with the observed densities requires the presence of low-density paths through the ISM, but none produce the required low-density paths in a physically motivated way. Meanwhile, the WIM has most often been modeled as consisting of either photoionized shells surrounding neutral clouds (McKee & Ostriker 1977) or as discrete clouds occupying some fraction of the total volume of the Galaxy (Reynolds 1991). However, this picture is clearly incomplete. The WIM has a high Reynolds number and therefore is turbulent (Benjamin 1999), as seen observationally (Armstrong et al. 1995). The recent advent of 3D simulations of supernova-driven turbulence in a multiphase ISM which cover dynamically significant time and size scales (Avillez & Breitschwerdt 2004; Joung & Mac Low 2006) has made possible detailed studies of the interplay between the physical conditions revealed by optical emission lines and turbulence, helping to address the long-standing puzzle of the ionization of the medium. 1. Distribution of emission measure First, we consider the global distribution of Hα emission measures observed in the WHAM Northern Sky Survey (Haffner et al. 2003). We derived the emission measure, EM ≡ ∫ ne ds, from the Hα surface brightness assuming a temperature of 8000 K. We