Inherent Length Scales of Periodic Mesoscale Density Structures in the Solar Wind Over Two Solar Cycles

It is now well established through multiple event and statistical studies that the solar wind at 1 AU contains contains periodic, mesoscale (L similar to 100-1,000 Mm) structures in the proton density. Composition variations observed within these structures and remote sensing observations of similar structures in the young solar wind indicate that at least some of these periodic structures originate in the solar atmosphere as a part of solar wind formation. Viall et al. (2008, ) analyzed 11 years of data from the Wind spacecraft near L1 and demonstrated a recurrence to the observed length scales of periodic structures in the solar wind proton density. In the time since that study, Wind has collected 14 additional years of solar wind data, new moment analysis of the Wind SWE data is available, and new methods for spectral background approximation have been developed. In this study, we analyze 25 years of Wind data collected near L1 and produce occurrence distributions of statistically significant periodic length scales in proton density. The results significantly expand upon the Viall et al. (2008, ) study and further show a possible relation of the length scales to solar "termination" events. Plain Language Summary The plasma and magnetic field in the solar atmosphere flows away from the Sun, filling interplanetary space. This plasma is called the solar wind, and it constantly bombards all of the planets in the solar system. The solar wind is composed of mesoscale structures-larger than scales where particle dynamics are important, but smaller than global scales-of increased density, and therefore pressure. A subgroup of mesoscale density structures is of order the size of Earth's magnetosphere, and often quasi-periodic. These periodic density structures are an important driver of dynamics in Earth's space environment. In this study, we examine the statistics of the size scales of these structures using 25 years, or approximately two solar cycles, of solar wind data measured by the Wind spacecraft. We confirm earlier work showing a persistence of particular length scales of the periodicities and find a possible relation of the length scales to the end of a Hale magnetic cycle. In addition to their driving of magnetospheric dynamics, periodic density structures are a tracer of solar wind formation. Their lengths scales and evolution are an important constraint of solar wind formation.