Periodic Mesoscale Density Structures Comprise a Significant Fraction of the Solar Wind and Are Formed at the Sun

Abstract

AbstractMesoscale density structures in the solar wind are often periodic, with f ∼ 0.1–5 mHz. They are trains of advected density structures with radial length scales of ∼100–10,000 Mm. While studies have shown that these periodic density structures (PDSs) are often formed at the Sun and released into the solar wind, it is unknown what percent of the solar wind at 1 AU is comprised of PDSs from the Sun, as opposed to periodicities formed through dynamics en route. We expand on Kepko et al. (2020, https://doi.org/10.1029/2020ja028037) which analyzed 25 years of in situ solar wind proton data, and include here alphas to examine the compositional characteristics of PDSs. Compositional changes, such as the alpha‐to‐proton ratio (α/p), are frozen into the solar wind plasma low in the corona, and so do not evolve as the solar wind advects and fills the Heliosphere. We find a broad occurrence enhancement in both the proton and α/p distributions between 1 and 3 mHz, centered near ∼2.1 mHz, and demonstrate that this distribution can be modeled assuming ∼30% of the solar wind segments contain a PDS. We find a distinct distribution below 1 mHz, with markedly different α/p characteristics. The α/p indicates that both populations are from the Sun, with likely different generation mechanisms. We conclude by summarizing mechanisms at the Sun that could produce periodic mass release, namely, periodic magnetic reconnection. The lower frequency PDSs likely involve reconnection at S‐web arcs including the heliospheric current sheet, while the higher frequency PDSs may be driven by p‐mode‐related transverse coronal oscillations.