Changes in Alpha-to-Proton Ratios During Periodic Solar Wind Density Structures

Abstract:

Trains of mesoscale density structures in the solar wind are often periodic, with length scales between ~100-1000 Mm, and frequencies corresponding to their advected timescales of ~0.1-5 mHz. Previous work has shown that periodic density structure (PDSs) occur at recurrent scale sizes, and that those scale sizes are correlated with the terminator signaling end of the solar cycle. However, only a few event studies have examined the compositional changes associated with PDSs. Compositional changes, such as the alpha-to-proton ratio, are frozen into the plasma low in the corona, and so do not evolve as the solar wind advects and fills the Heliosphere. Therefore, alpha-to-proton ratio changes within PDSs provide critical insights into and constraints on their generation – i.e., whether they are generated in situ during transit to 1 AU, or created as part of solar wind formation. Using 25 years of Wind proton and alpha solar wind data measured at 90-s cadence, we examine the occurrence distributions of periodic density structures. Our analysis utilizes multiple temporal (between 6 and 48 hours) and length-scale (9-72 Gm) spectral analysis windows to fully explore the distribution of alpha-to-proton ratio changes during PDSs placing important constraints on their generation mechanism and the formation of the solar wind.