Pulsar Scintillation Arcs and the Ionized ISM

I review the observations and theory of pulsar scintillation, focusing on the phenomenon of scintillation arcs. It has been known since soon after the discovery of pulsars that dynamic spectra of the sources occasionally show pronounced fringing or criss-cross patterns. It was a surprise, however, when the 2D Fourier analysis of these spectra showed faint, parabolic features in the secondary spectra. I will show evidence that the scintillation arc phenomenon is widespread and that it underpins many other scintillation phenomena. Furthermore, there is often pronounced substructure in the arcs, and it translates along the main arc in a manner that is determined by the proper motion of the pulsar. This substructure may be produced by lens-like features in the ionized ISM that are far out of pressure balance with the warm interstellar medium (WIM). These may, in turn, be related to deterministic structures that cause extreme scattering events. Observations with this technique, which rely on a large flux density and/or a large collecting area, have an angular resolution of about a milliarcsecond. They often show features in the scatter broadened image out to 10 -- 15 times this resolution, however. Thus, single-dish observations can study details in the scattering medium on AU-size scales while covering a relatively large field of view, which scans the sky at the pulsar proper motion speed. We are still learning how to interpret the richly detailed scintillation arc pattern that results, and observational and interpretive surprises continue.