Atomic and Ionized Microstructures in the Diffuse Interstellar Medium
  It has been known for half a century that the interstellar medium (ISM) of
our Galaxy is structured on scales as small as a few hundred km, more than 10
orders of magnitude smaller than typical ISM structures and energy input
scales. In this review we focus on neutral and ionized structures on spatial
scales of a few to ~10^4 Astronomical Units (AU) which appear to be highly
overpressured, as these have the most important role in the dynamics and energy
balance of interstellar gas: the Tiny Scale Atomic Structure (TSAS) and Extreme
Scattering Events (ESEs) as the most over-pressured example of the Tiny Scale
Ionized Structures (TSIS). We review observational results and highlight key
physical processes at AU scales. We present evidence for and against
microstructures as part of a universal turbulent cascade and as discrete
structures, and review their association with supernova remnants, the Local
Bubble, and bright stars. We suggest a number of observational and theoretical
programs that could clarify the nature of AU structures. TSAS and TSIS probe
spatial scales in the range of what is expected for turbulent dissipation
scales, therefore are of key importance for constraining exotic and not-well
understood physical processes which have implications for many areas of
astrophysics. The emerging picture is one in which a magnetized, turbulent
cascade, driven hard by a local energy source and acting jointly with phenomena
such as thermal instability, is the source of these microstructures.
