Helium depletes sharply before each solar cycle begins
What We See
Panel A spans 1974 to 2020 and shows ten colored lines with distinct markers, each tracking helium percentage in the solar wind at a different wind speed (330 to 568 km/s, labeled in the top legend). The lines rise and fall together over roughly 11-year intervals. A dashed black curve on the right axis traces the 13-month smoothed sunspot number. Vertical purple dash-dotted lines mark the official start of each solar cycle (minima 21 through 25); vertical magenta dotted lines with cyan uncertainty bands mark moments when helium suddenly dips. These dips consistently appear to the left of the purple lines, indicating they occur earlier. Panel B shows Lyman-alpha irradiance (solid blue) and F10.7 radio emission (dashed orange) over the same period, both lagging the sunspot minima.
The Finding
Just before each new solar cycle begins, helium in the solar wind undergoes a rapid depletion and recovery that occurs at approximately the same time across all wind speeds. This helium shutoff consistently precedes the official sunspot-defined solar minimum by roughly 10 months, making it an earlier herald of the new cycle than sunspot counting or other standard activity indicators like Lyman-alpha and F10.7. The pattern repeats across all five solar minima observed in this 45-year dataset.
Why It Matters
Predicting when a new solar cycle begins matters for forecasting space weather that affects satellites, power grids, and astronauts. The discovery that helium provides advance warning of solar cycle transitions offers a new forecasting tool. Because the shutoff occurs simultaneously across wind speeds, it likely reflects a fundamental change deep in the Sun rather than differences in how the solar wind is accelerated at the surface. This points to a universal mechanism tied to the Sun's internal magnetic cycle.
Appears In
s11207-021-01801-9 · fig 1