Proton Specific Entropy as a Proxy for the O7+/O6+ Charge State Ratio over Heliocentric Distance
4 affiliations
- 1University of California
- 2University of Michigan
- 3Center for Astrophysics ∣ Harvard & Smithsonian
- 4NASA Goddard Space Flight Center
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. 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 R ⊙ , leveraging observations from Solar Orbiter’s Heavy Ion Sensor and Proton and Alpha Sensor covering 0.28–1 au. Our analysis confirms a strong anticorrelation between specific entropy and the oxygen charge state ratio that persists over a broad range of distances in the inner heliosphere. We categorize observations into fast solar wind, slow Alfvénic solar wind, and slow solar wind, identifying thresholds in specific entropy values and charge state ratios across these types that can be used to distinguish solar wind source regions. This 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 from Parker Solar Probe, and throughout the inner heliosphere.
Related
- Multispacecraft Constrained Modeling of Solar Energetic Particle Transport across Different Heliocentric Radii and Heliolongitudes
- The role of specific entropy in the onset of the luminosity bump
- Proton acceleration at interplanetary shocks: A numerical study for the HENON mission
- Solar-wind proton heating and its effect on the temperature anisotropy evolution between 0.05 and 1 au
- Neutrino Emission from Proton Photon Jet Interactions in Microquasars
BibTeX
@article{collard2026proton,
title = {{Proton Specific Entropy as a Proxy for the O7+/O6+ Charge State Ratio over Heliocentric Distance}},
author = {Jack D. Collard and Tamar Ervin and Ryan M. Dewey and Yeimy J. Rivera and Aidan J. Nakhleh and Jean-Baptiste Dakeyo and Samuel T. Badman and Trevor A. Bowen and John W. Bonnell and Nicholeen M. Viall and Susan T. Lepri and Jim M. Raines and Stuart D. Bale},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aeacc8},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeacc8},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Proton Specific Entropy as a Proxy for the O7+/O6+ Charge State Ratio over Heliocentric Distance
AU - Jack D. Collard
AU - Tamar Ervin
AU - Ryan M. Dewey
AU - Yeimy J. Rivera
AU - Aidan J. Nakhleh
AU - Jean-Baptiste Dakeyo
AU - Samuel T. Badman
AU - Trevor A. Bowen
AU - John W. Bonnell
AU - Nicholeen M. Viall
AU - Susan T. Lepri
AU - Jim M. Raines
AU - Stuart D. Bale
PY - 2026
DA - 2026/10/28
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aeacc8
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aeacc8
AB - 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. 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 R ⊙ , leveraging observations from Solar Orbiter’s Heavy Ion Sensor and Proton and Alpha Sensor covering 0.28–1 au. Our analysis confirms a strong anticorrelation between specific entropy and the oxygen charge state ratio that persists over a broad range of distances in the inner heliosphere. We categorize observations into fast solar wind, slow Alfvénic solar wind, and slow solar wind, identifying thresholds in specific entropy values and charge state ratios across these types that can be used to distinguish solar wind source regions. This 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 from Parker Solar Probe, and throughout the inner heliosphere.
SN - 1538-4357
SN - 0004-637X
ER -