Mean speed exceeds saturation only in Alfvenic wind
What We See
This panel maps mean solar wind speed (color) across wave activity (horizontal axis) and compressibility (vertical axis, logarithmic). Two black contours mark 433 and 450 km/s, the fastest saturation speeds from the companion paper and from incompressible wind in this paper. The fastest mean speeds (warmest colors, above 450 km/s) appear only in the Alfvenic, incompressible region at lower right. The compressible region at top shows moderate mean speeds that do not exceed the saturation speed contours, despite the high helium found there in Figure 15.
The Finding
Mean solar wind speed exceeds the largest incompressible saturation speed only in the Alfvenic, incompressible region. Compressible wind, despite carrying helium comparable to fast wind, has mean speeds that remain below the saturation speed threshold. Contours of constant average speed show an anticorrelation between wave activity and compressibility, meaning the same average speed can be achieved by either Alfvenic or compressible wind through different physical processes.
Why It Matters
This panel demonstrates that speed-based source region classification is fundamentally limited. The compressible population carries fast-wind-like helium but at slower average speeds than the Alfvenic population. Space weather models that rely on speed alone to classify source regions will systematically misidentify compressible wind, which may carry different magnetic field configurations and particle populations than genuine coronal hole wind.
Appears In
Alterman 2026 ApJL 996 L12 · fig 16a