Solar Activity

How the 11-year cycle reshapes solar wind composition

Solar wind composition breathes in sync with the Sun's 11-year activity cycle. Fast wind from coronal holes maintains stable abundances regardless of solar activity, while slow wind abundances rise during solar maximum and crash during solar minimum—with heavier elements showing stronger modulation due to gravitational settling.

$A time series spanning 1994–2022 plots the fraction of helium in the solar wind relative to the Sun's surface value.$

Slow-wind helium swings with the sunspot cycle

A time series spanning 1994–2022 plots the fraction of helium in the solar wind relative to the Sun's surface value. Each point averages roughly 250 days of Wind/SWE measurements. Light blue open hexagons trace helium in slower wind; orange filled hexagons show faster wind. A dashed black curve on the right axis marks the normalized sunspot number, cycling approximately every 11 years. Fast-wind helium hovers near a dotted horizontal line at about half the photospheric value, enclosed in a gray uncertainty band. Slow-wind helium swings dramatically in step with the sunspot curve. A cyan bar at the bottom marks when SWICS data are also available.

solar windhelium abundancesolar cyclesunspot numberfast windslow windsource regionsphotospheric abundancetime seriestemporal evolution22-year observationssource region variability

Related Figures

Two series of colored data points trace the strength of the helium-sunspot connection at each wind speed.

Delaying sunspot number lifts the helium correlation at all speeds

Accounting for the time delay between solar activity changes and helium's response dramatically improves the helium-sunspot correlation, especially in faster wind.

Shows that accounting for the time delay between solar activity and helium's response dramatically improves the helium-sunspot correlation at all speeds, overturning the conclusion that this was a purely slow-wind phenomenon

Colored data points with error bars march upward from left to right across the plot, showing the time delay in days between a change in sunspot number and the corresponding change in helium abundance.

Faster solar wind waits longer to respond to solar activity

The delay between solar activity and helium's response increases at a rate of about 1.1 days per additional kilometer per second of wind speed.

Reveals that helium doesn't respond instantaneously—delay increases linearly with speed, providing a window into source region response times

Two panels spanning 1998–2012 track nine element abundances (He through Fe) relative to their photospheric values on a logarithmic vertical scale.

Slow-wind heavy element abundances dip at solar minimum

In the slow wind, all heavy element abundances except carbon track the solar cycle strongly, dropping during the deep solar minimum around 2008–2009 and recovering as activity rises.

Extends the helium finding across the periodic table, confirming the fast/slow dichotomy is universal

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).

Slow-wind helium swings with the sunspot cycle over 23 years

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.

Establishes the 23-year baseline showing helium abundance rising and falling with sunspot number at every wind speed across two complete solar cycles

Each element's slow-wind correlation coefficient with solar activity is plotted against its atomic mass, from helium (~4 amu) on the left to iron (~56 amu) on the right.

Correlation with solar activity grows with element mass

For elements heavier than helium, the correlation between slow-wind abundance and solar activity rises monotonically with atomic mass—from carbon (~0.51–0.56) to iron (~0.82–0.84).

Reveals the physical mechanism: gravitational settling preferentially depletes heavier elements during solar minimum

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).

Helium depletes sharply before each solar cycle begins

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.

Demonstrates predictive capability: helium shutoff consistently precedes solar minimum by ~10 months across four cycles

Two stacked panels for 1995-2021 showing solar wind helium abundance at different wind speeds with sunspot number overlay (top) and EUV brightpoint density as a function of latitude and time (bottom).

Helium shutoff coincides with new-cycle magnetic emergence

Helium shutoff occurs at approximately the same time that new-cycle brightpoints emerge at mid-latitudes, particularly in the hemisphere that leads the new cycle.

Shows that helium shutoff coincides with new-cycle magnetic emergence at mid-latitudes, linking the compositional transition to the underlying magnetic cycle

Source

Heavy ion abundances evolve with solar activity

Astronomy and Astrophysics (2025)

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