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Helium Abundance

Helium makes up, on average, between 0 and 4.19% of the solar wind. In fast wind, the helium abundance is fixed at the 4.19% value. In slow wind, the helium abundance is highly variable. The differences in this behavior are due to differences in how slow and fast wind are born at the Sun. The sources of the fast wind have magnetic fields that are continuously open to interplanetary space. The sources of the slow wind are only intermittently open to interplanetary space. The processes governing how these sources open and the frequency with which they open lead to the highly variable slow wind helium abundance.

Bilinear fits to the helium abundance as a function of solar wind speed.
The solar wind helium abundance is constant in fast wind. In slow wind, it is highly variable. The blue dot indciates the speed and abundance where the slope changes. This is the helium saturation point with saturation speed and saturation abundance . The diffences for speeds above and below are likely due to differences in how slow and fast wind are born when the coronal plasma is accelerated into the solar wind. From Alterman & D'Amicis (2025), The Astrophysical Journal Letters.