Observational constraints on the radial evolution of O⁶+ temperature and differential flow in the inner heliosphere
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
Over decades of solar wind observations, heavy ions have been observed to have a higher temperature and flow faster than protons in the solar corona and heliosphere. Remote observations have largely been limited to the low corona (<4 R⊙), while in situ observations for heavy ions (Z > 2) have only been sampled at 1 au and beyond. As a result, theories that address heavy ion heating and acceleration remain largely unconstrained. With the launch of Solar Orbiter, heavy ion kinetics can be probed closer to the Sun, as close as the orbit of Mercury (65 R⊙), to examine their radial behavior. Through a statistical analysis of O6+, this work provides a comprehensive analysis of the velocity and temperature of O6+ from 0.3 to 1 au. The study finds that the O6+ relative drift, normalized to the local Alfvén speed, and its temperature compared to protons both decrease with distance from the Sun and show some speed dependence. The O6+ temperature is well fit by a single temperature adiabatic profile across all wind speeds, suggesting that there is no significant heating at these heliocentric distances, which is in contrast to what is observed for protons and He2+. Alfvénic fluctuations, some with full 180∘ field rotation, create momentary negative differential streaming where the speed of O6+ trails the protons. The amount of negative differential streaming gradually increases at larger distances. These results provide critical constraints on the proposed mechanisms seeking to describe ion heating and acceleration in the solar wind.
Date Issued
2025-09-10
Date Acceptance
2025-08-10
Citation
Astrophysical Journal Letters, 2025, 990 (2)
ISSN
2041-8205
Publisher
IOP Publishing
Journal / Book Title
Astrophysical Journal Letters
Volume
990
Issue
2
Copyright Statement
© 2025. TheAuthor(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
License URL
Subjects
ALFV-AND-EACUTE,N WAVES
ALPHA-PARTICLES
Astronomy & Astrophysics
DRIVEN INSTABILITIES
HEATING RATES
HEAVY-IONS
ION-SCALE WAVES
KINETIC-PROPERTIES
MINOR IONS
PARKER SOLAR PROBE
Physical Sciences
Science & Technology
WIND OBSERVATIONS
Publication Status
Published
Article Number
L60
Date Publish Online
2025-09-08