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Solar Wind Compressibility

The solar wind's compressibility describes how much it "puffs and squashes" as it flows off the Sun—i.e., how strongly the particle density swings around its typical value. When those density swings are small, the wind behaves more like an Alfvénic, wave-dominated fluid and looks fairly steady; when the swings are larger, the wind carries more compressive structures (like slow modes or pressure-balanced packets). Both types show up across all speeds, but the usual rule of thumb is that density variations of roughly 10–15% or more indicate a compressive interval, while very small variations indicate an incompressible one.Why it matters: helium behaves differently in these two regimes. Above a characteristic "saturation" speed, incompressible (more Alfvénic) wind tends to keep helium levels modest and fairly stable, whereas compressive, less-Alfvénic wind can drive helium to higher and more variable levels, with a much stronger rise as speed increases. Practically, that means averages can be misleading: to reveal clean trends, it's best to analyze solar-wind data by conditioning on both compressibility (how big the density swings are) and Alfvénicity (how wave-like the flow is), using simple thresholds like 10–15% density variation and a moderate Alfvénicity cutoff.

Figure showing the changes in the solar wind compressibility with solar wind source region.
A heat map of the solar wind's compressibility () as a function of the normalized cross helicity or Alfvénicity () helium abundance (). Low compressibility () signals typical solar wind, while a higher compressibility likely indicates interplanetary coronal mass ejections (ICMEs). The ambient solar wind's compressibility changes depending on where on the Sun it was born (see Sources of the Solar Wind). When , the solar wind was likely born in coronal holes, while a compressibility typically indicates solar wind born in other, more variable sources (see Space Weather). From Alterman & D'Amicis (2025), arXiv e-prints.