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Non-adiabatic entropy increase in the solar wind governed by velocity spikes
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DOI：10.1038/s41550-026-02923-5.png)
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Abstract
En 中文
The solar wind cools more slowly than adiabatic expansion predicts, implying the presence of an additional heat source. A key unknown is the physical carriers that transport energy into the solar wind. Here we track the same fast solar wind stream from its source to Earth’s orbit. Using a rare quasi-radial alignment of Solar Orbiter near 0.38 au and wind near 1 au, together with remote sensing from Chinese Hα Solar Explorer and Solar Dynamics Observatory, we show that intermittent velocity spikes are the dominant energy carriers that drive the solar wind’s non-adiabatic evolution. These spikes dissipate during propagation, reducing the cooling rate and increasing plasma entropy. The resulting temperature and entropy at 1 au are in good agreement with predictions from Alfvénic turbulence theory. The solar wind source region shows abundant magnetic reconnection activity, probably providing the initial fluctuations that seed the formation of velocity spikes. Our results provide observational constraints on how intermittent velocity spikes and Alfvénic turbulence shape the thermodynamic evolution of collisionless plasmas in the heliosphere. By tracking the same fast solar wind stream from Solar Orbiter to Earth, this study shows that intermittent velocity spikes transport energy and contribute to entropy increase, helping explain why the solar wind cools more slowly than adiabatic expansion predicts.
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