The solar wind's proton velocity distribution should be roughly Maxwellian — a thermal core plus a beam component streaming outward from the Sun. Deviations from this shape are common but were cataloged as curiosities: kappa distributions, field-aligned beams, temperature anisotropies. The hammerhead distribution — a proton velocity distribution with a constricted gap between core and beam, shaped like the head of a hammerhead shark — was one such curiosity. Rare, unexplained, possibly instrumental.
Parker Solar Probe data across twenty encounters (arXiv:2603.11329) reveals that hammerhead distributions are not rare. They are systematically concentrated near the heliospheric current sheet — the warped surface where the Sun's magnetic polarity reverses. As the solar cycle progresses, the concentration tightens: hammerheads become more localized around the current sheet, not less. The pattern intensifies with solar activity.
What looked like noise turns out to be a signal. The hammerhead shape traces a specific energization process operating at the heliospheric current sheet. The constricted gap between core and beam is not a measurement artifact but a physical signature — evidence that protons near the current sheet undergo a distinct acceleration mechanism that reshapes their velocity distribution in a characteristic way.
The concentration toward the current sheet with advancing solar cycle suggests the energization process strengthens as the current sheet itself intensifies. The hammerhead is not caused by a local, one-time event. It is maintained by ongoing dynamics at the polarity reversal surface — a steady signature of a steady process.
Twenty encounters. Consistent pattern. The distribution that was cataloged as an anomaly is the cleanest tracer of the heliosphere's most important magnetic boundary.