Coherent Structure in Solar Wind C$^{6+}$/C$^{4+}$ Ionic Composition Data During the Quiet-Sun Conditions of 2008

TL;DR

This study identifies characteristic scale sizes and periodicities in solar wind ionic composition data during quiet-sun conditions, revealing distinct transient coherency scales and their prevalence in different solar wind types.

Contribution

It introduces a novel wavelet analysis methodology with significance testing to characterize coherent structures in solar wind charge state data, highlighting scale distributions and their relation to wind types.

Findings

Evidence for a 10-day upper limit on transient coherency.

Identification of 4-5 hour and 35-45 day global periodicities.

Distinct transient coherency scale distributions with two populations.

Abstract

This analysis offers evidence of characteristic scale sizes in solar wind charge state data measured in-situ for thirteen quiet-sun Carrington rotations in 2008. Using a previously established novel methodology, we analyze the wavelet power spectrum of the charge state ratio C$^{6+}$/C$^{4+}$ measured in-situ by ACE/SWICS for 2-hour and 12-minute cadence. We construct a statistical significance level in the wavelet power spectrum to quantify the interference effects arising from filling missing data in the time series (Edmondson et al. 2013), allowing extraction of significant power from the measured data to a resolution of 24 mins. We analyze each wavelet power spectra for transient coherency, and global periodicities resulting from the superposition of repeating coherent structures. From the significant wavelet power spectra, we find evidence for a general upper-limit on individual transient coherency of $\sim$10 days. We find evidence for a set of global periodicities between 4-5 hours and 35-45 days. We find evidence for the distribution of individual transient coherency scales consisting of two distinct populations. Below the $\sim$2 day time scale the distribution is reasonably approximated by an inverse power law, whereas for scales $\gtrsim$2 days, the distribution levels off showing discrete peaks at common coherency scales. In addition, by organizing the transient coherency scale distributions by wind type, we find these larger, common coherency scales are more prevalent and well-defined in coronal hole wind. Finally, we discuss the implications of our results for current theories of solar wind generation and describe future work for determining the relationship between the coherent structures in our ionic composition data and the structure of the coronal magnetic field.