Proton specific entropy as a proxy for the $O^{7+}/O^{6+}$ charge state ratio over heliocentric distance

TL;DR

The paper investigates whether proton specific entropy $S_p$ can serve as a high-cadence proxy for the oxygen charge-state ratio $O^{7+}/O^{6+}$ across the inner heliosphere ($0.28$–$1$ AU) using Solar Orbiter measurements. It demonstrates a robust anti-correlation between $\log(S_p)$ and $\log(O^{7+}/O^{6+})$ that persists across wind types and radial distances, and identifies an effective polytropic index $\gamma_{eff}\approx 1.29$ that conserves $S_p$ with distance. By classifying wind into Fast Wind (FSW), Slow Alfvénic Wind (SASW), and Slow Non-Alfvénic Wind (SSW), the study derives an equivalent $S_p$ threshold of $3.33\pm1.26$ corresponding to the CH/non-CH boundary and shows distinct $S_p$–$O^{7+}/O^{6+}$ signatures per wind type. These results suggest that $S_p$ can be used to infer solar wind source regions in the inner heliosphere when direct charge-state measurements are unavailable, motivating future near-Sun conjunctions and model-driven tests to refine the proxy across varying distances.

Abstract

While the fast solar wind has well-established origins in coronal holes, the source of the slow solar wind remains uncertain. Compositional metrics, such as heavy ion charge state ratios are set in the lower corona, providing insights into solar wind source regions. However, prior to the launch of Solar Orbiter, in situ measurements of heavy ion charge state were limited to distances of 1 AU and beyond. We investigate proton specific entropy as a proxy for the oxygen charge state ratio ($O^{7+}/O^{6+}$),which generally becomes frozen-in below ~1.8 Rsun, leveraging observations from Solar Orbiter's Heavy Ion Sensor and Proton and Alphas Sensor covering 0.28 to 1 AU. Our analysis confirms a strong anti-correlation between specific entropy and the oxygen charge state ratio that persists over a broad range of distances in the inner heliosphere. We categorize observed solar wind into fast solar wind, slow Alfvenic solar wind, and slow solar wind, identifying clear distinctions in specific entropy values and charge state ratios across these types. The work demonstrates the potential to use proton specific entropy as a classifier of solar wind source regions throughout the heliosphere. By establishing the $S_p$-$O^{7+}/O^{6+}$ relationship and quantifying its radial dependence, the specific entropy can be used as a quantity to identify the solar wind source region in the absence of in-situ charge state measurements. This motivates future studies as to the applicability of this proxy to near-Sun observations (such as Parker Solar Probe) and throughout the inner heliosphere.

Figures (8)

• Figure 1: Charge state ratio temporal evolution and correlation with proton-specific entropy as measured by Solar Orbiter from January 2022 through April 2023. Panel (a): Rolling Pearson correlation coefficient (r) between log($\mathrm{S_p}$) and log($\mathrm{O^{7+}/O^{6+}}$), computed using a 7-day time-based window. Panel (b) Timeseries of $\mathrm{O^{7+}/O^{6+}}$ (blue, left axis) and $\mathrm{S_p}$ (orange, right axis) The $\mathrm{S_p}$ axes has been flipped. Panel (c) Log-log distribution of $\mathrm{O^{7+}/O^{6+}}$ versus $\mathrm{S_p}$ separated into five cross-helicity intervals. Each colored contour encloses the densest 45% of data within its bin, and corresponding log-log correlation coefficients are listed in the legend. The dashed horizontal line at $\mathrm{O^{7+}/O^{6+}}$ = 0.145 denotes the upper threshold for solar wind emerging from CHs Zhao-2009. Panel (d): Histogram of the rolling Pearson correlation values from panel (a). The vertical line marks the mean ($\mu \approx -.54$).
• Figure 2: Time series and statistical comparison of the correlation between $\mathrm{O^{7+}/O^{6+}}$ and $\mathrm{S_p}$ for different polytropic indices. Panel (a) shows a time series of radial solar wind velocity colored by the absolute cross helicity, an indicator of Alfvénic fluctuations. Panel (b) displays the oxygen charge state ratio as measured by SWA/HIS, including measurement uncertainties (shown as red error bars). Panel (c) shows the specific proton entropy determined from SWA/PAS measurements for three different polytropic indices ($\gamma = \frac{5}{3}$ (blue), 1.5 (orange), and 1.35 (green)), illustrating how different assumptions about solar wind thermodynamics influence the relationship between these parameters. Panel (d-f) presents histograms of the rolling correlation between log($\mathrm{S_p}$) and log($\mathrm{O^{7+}/O^{6+}}$) computed over 7-day windows for each from January 2022 through April 2023. The vertical black line in each histogram marks the mean correlation coefficient (also listed in legend).
• Figure 3: Panel (a) shows the evolution of $\mathrm{O^{7+}/O^{6+}}$ and (b) shows the evolution of $\mathrm{S_p}$ as a function of distance. A weighted least squares regression is applied to calculate the best fit line (black dashed) and the 95% confidence interval is shaded in gray. Panel (c) is equivalent but for our $\gamma_{eff}$ value of 1.29. Panel (d) shows the absolute value of the correlation coefficient as a function of distance (plotted against the central distance) for different radial bin sizes. The size of the marker corresponds to the number of samples within each bin (between $\sim$1000 and $\sim$11000) depending on the bin sizes. The dashed vertical lines show the radial location corresponding to the maximum correlation between the parameters. The 10 $\mathrm{R_{\odot}}$ bin size also peaks at 135 $\mathrm{R_{\odot}}$ (like the 30 $\mathrm{R_{\odot}}$ bin size).
• Figure 4: Comparison between in-situ solar wind classification parameters ($\mathrm{\sigma_C}$ and $v_R$) with in-situ metrics of solar wind source regions ($\mathrm{S_p}$ and $\mathrm{O^{7+}/O^{6+}}$). Panel (a) shows a 1d distribution of solar wind speeds associated with the time range of interest (Jan 2022 - April 2023). Panel (b) and (c) show normalized 2d distributions of $v_R$ and $\mathrm{S_p}$ and $\mathrm{O^{7+}/O^{6+}}$, where $\mathrm{S_p}$ is computed with $\gamma_{eff}$. The dashed vertical line shows the 423 $\mathrm{km\ s^{-1}}$ threshold from Alterman-2025. Panel (d) shows the 2d distribution of $\mathrm{\sigma_C}$ and $\mathrm{O^{7+}/O^{6+}}$. Joint distributions are normalized to the maximum count in each distribution.
• Figure 5: Normalized 2d histograms (to the maximum value in the joint distribution) showing the $\mathrm{O^{7+}/O^{6+}}$ measurement from Solar Orbiter/HIS compared with the $\mathrm{S_p}$ computed from the MAG and PAS measurements ($\gamma_{eff} =1.29$). The vertical dashed line at $\mathrm{O^{7+}/O^{6+}}$ = 0.145 is the upper threshold for CH wind Zhao-2009Wang-2016. Panel (a) shows all the wind observed by Solar Orbiter (January 2022 to April 2023). Panels (b), (c), and (d) show the corresponding plot for the fast solar wind, slow Alfvénic wind, and classically slow wind. Wind streams are categorized as in Table \ref{['tab:categorization']}. The dashed black lines shows the best fit to the distribution while the gray shading shows the 95% confidence interval of the least squares fit. The dashed pink lines (pink shaded region) show the intersection of the mean best fit line with the Zhao-2009 0.145 threshold for the $\mathrm{O^{7+}/O^{6+}}$ ratio. The shaded pink region corresponds to the overlap with the 95% (gray shaded) confidence interval of the fit where the vertical dashed pink line is at $\mathrm{S_p} = 3.33$.