| Mesoscale structure in the solar wind (~tens to several thousands of Mm) are created in two possible ways: those that come directly from the Sun, and those formed through processing en route as the solar wind advects outward. Complexity arises from a competition between the imposed and injected structures that survive from the Sun and turbulence and other dynamical evolution. Understanding the source and evolution of solar wind structures is important because it contains information on how the Sun forms the solar wind and it constrains the physics of turbulent processes. Mesoscale structures also comprise the ground state of space weather, continually buffeting planetary magnetospheres. Periodic trains of mesoscale structures in solar wind density have been observed throughout the inner heliosphere with in situ data from the Helios, ACE, and Wind spacecraft, as well as remotely in STEREO/COR2 and STEREO/HI1 white light imaging data. In this talk, we describe recent research of the first observations of periodic trains of mesoscale structures in solar wind density observed by the Wide-field Imager for Parker Solar PRobe (WISPR). While some periodic density structures may be a consequence of the development of dynamics en route, many are remnants of the formation and release of the solar wind, and thus provide important constraints on solar wind models. The observation of periodic density structures so near to the Sun allows us to begin disentangling how much structure is created during solar wind formation, versus how much is due to evolution as the solar wind advects outward. |