Slow-wind helium swings with the sunspot cycle over 23 years
What We See
Ten colored lines track the percentage of helium in the solar wind from 1995 to 2018, each representing a different wind speed bin (slowest in blue, fastest in lavender). A dashed black curve shows the sunspot count on the right axis, rising and falling through two peaks around 2001 and 2014. The slowest wind (blue triangles at 322 km/s) swings dramatically from below 1% during quiet Sun periods up to about 4% during active periods. Faster wind lines sit progressively higher and flatter, with the fastest (542 km/s) hovering around 4-5% throughout. A legend lists each speed bin alongside its Spearman correlation coefficient, ranging from 0.90-0.91 in the slowest bins down to 0.57 in the fastest.
The Finding
Over a full 23-year span encompassing two complete solar activity cycles and one full magnetic reversal cycle of the Sun, helium abundance faithfully rises and falls with sunspot number at every wind speed. The correlation is strongest in slow wind (0.90-0.91) and weakest in the fastest bin (0.57). A visible time shift between the colored helium lines and the dashed sunspot curve hints that helium responds to solar activity changes after a delay. Despite cycle 24 being weaker than cycle 23 in sunspot count, helium reaches comparable peak values in both maxima.
Why It Matters
This 23-year record from a single instrument on the Wind spacecraft provides the first continuous measurement of helium across an entire Hale cycle, during which the Sun's magnetic dipole flipped and returned to its starting orientation. The dataset establishes that helium's connection to solar activity persists across all wind speeds and multiple cycles, setting the stage for discovering that the apparent time offset between helium and sunspots depends systematically on wind speed.
Appears In
Alterman 2019 ApJL 879 L6 · fig 1