Journal article
Evidence for Solar Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind
Astrophysical journal. Letters, Vol.1008(1), L1
09/01/2026
DOI: 10.3847/2041-8213/ae984e
Abstract
The influence of collisional age ( A c ) on the alpha-to-proton temperature ratio ( T α / T p ) has been explored in the past. However, the modulation of this ratio with respect to the solar cycle has remained unexplored so far. We show solar cycle modulation of T α / T p and A c using nearly three decades of in situ observations from the Wind spacecraft across distinct solar wind speed regimes and solar activity phases. Our results reveal that in the slow solar wind with velocity <400 km s −1 , where A c happens to be typically >1, the ratio T α / T p stays close to unity. This suggests frequent Coulomb collisions efficiently iron out temperature differences. In contrast, in the fast wind with a velocity >500 km s −1 , where A c happens to be typically <1, mass-proportional heating is most pronounced, with T α / T p often exceeding 4. The intermediate speed regime (400–500 km s −1 ) represents a gradual transition between the slow and fast wind populations in terms of their solar cycle dependence. This behavior reflects the changing dominance of high-speed streams from polar coronal holes during minima to denser slow wind during maxima. These results suggest that mass-proportional ion heating at 1 au is not solely governed by local collisional physics but is significantly modulated by the solar cycle-dependent variations in the solar wind sources.
Details
- Title: Subtitle
- Evidence for Solar Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind
- Creators
- Aakash Gupta - Physical Research LaboratoryDibyendu ChakrabartyLeon Ofman - Goddard Space Flight CenterGregory G. Howes - University of Iowa
- Resource Type
- Journal article
- Publication Details
- Astrophysical journal. Letters, Vol.1008(1), L1
- DOI
- 10.3847/2041-8213/ae984e
- ISSN
- 2041-8205
- eISSN
- 2041-8213
- Publisher
- IOP
- Grant note
- Physical Research Laboratory (PRL): N.A.
We gratefully acknowledge the principal investigators of the Solar Wind Experiment (SWE) on board NASA's Wind mission for generating the data and making them publicly available. L.O. and Y. acknowledge support from NASA grant 80NSSC24K0724, NSF grant AGS-2300961, and NASA's GSFC through Cooperative Agreement 80NSSC21M0180 to the Catholic University of America, Partnership for Heliophysics and Space Environment Research (PHaSER). Y. acknowledges the support of the College of Liberal Arts and Sciences at the University of Iowa. G.G.H. acknowledges support from NASA grant 80NSSC24K124. We also thank World Data Center-SILSO, Royal Observatory of Belgium, Brussels, for the monthly mean sunspot number data used in this Letter. We express our sincere gratitude to the Department of Space, Government of India, for supporting this work.
- Language
- English
- Date published
- 09/01/2026
- Academic Unit
- Physics and Astronomy
- Record Identifier
- 9985221334702771
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