Helium Abundance

Tracing solar wind origins through composition

Helium abundance in the solar wind varies dramatically with speed, rising from near-zero in the slowest wind to a saturation value of 4.19% above 433 km/s. This compositional fingerprint, locked in at the chromosphere, provides a powerful diagnostic for distinguishing solar wind from source regions with intermittently and continuously open magnetic field topologies.

Helium abundance rises with speed then saturates

This heatmap shows how frequently the solar wind contains different amounts of helium (vertical axis) at each speed (horizontal axis), with brighter warm colors indicating more common combinations. A blue line traces the average helium at each speed, and a dashed green line shows a two-segment fit. At low speeds, helium rises steeply from near zero. Above about 430 km/s, the average helium levels off at roughly 4.2 percent, forming a plateau the authors call saturation. Each column is normalized to its own maximum so that the less frequently observed fast wind remains visible.

helium_abundancesolar_wind_speedsaturation_pointbilinear_fitslow_windfast_windWind_spacecrafthelium abundancesaturationsaturation speedsolar wind speedcompositionopen fieldclosed fieldphotospheric abundance

Related Figures

Fifteen colored lines trace how helium abundance (vertical axis) changes with solar wind speed (horizontal axis), each representing a different compressibility level from low (dark blue) to high (pink).

Compressible wind drives helium far above the plateau

When sorted by hydrogen density fluctuation level, two starkly different behaviors emerge above saturation.

Shows that compressibility, not Alfvénicity, is the primary regulator of helium saturation

The solar minimum wind speed distribution is shown as a solid black curve on a logarithmic vertical axis, with five color-coded segments highlighting characteristic speed ranges.

Five characteristic speeds span the fast/slow transition zone

The five characteristic speeds derived throughout this paper map onto consecutive portions of the solar minimum speed distribution.

Maps all five characteristic speeds onto the wind speed distribution, revealing that the fast/slow boundary is not a single threshold but a sequence of overlapping helium-related transitions spanning ~130 km/s

A logarithmic chart shows hydrogen (solid green line) and helium (orange dash-dot line) particle density versus solar wind speed from 300 to 800 km/s.

Helium density peaks near the closed-to-open source transition

While hydrogen density simply decreases with speed, helium density reaches a distinct peak at 409 km/s, right where the transition between closed and open magnetic source regions occurs.

Shows that helium density peaks at 409 km/s, right where the transition between closed and open magnetic source regions occurs, providing independent confirmation of the saturation-based source-region boundary

Saturation speed and abundance anticorrelate with wave activity

The saturation speed decreases by 23 km/s (from 433 to 410) while the saturation abundance increases by 0.26 percentage points (from 3.87% to 4.13%) as wave activity increases.

Quantifies the anticorrelation between saturation speed and saturation abundance as a function of Alfvénicity, providing the key empirical relationship for understanding helium behavior

Fifteen helium-versus-speed curves, one per wave-activity bin, rescaled so each curve's saturation point maps to the coordinate (1, 1), marked by a green dot.

Helium depletion below saturation follows one universal curve

Rescaling reveals that helium depletion in slow wind follows a single universal process that is completely independent of wave activity.

Reveals that helium abundance follows a universal scaling law below saturation regardless of Alfvénicity, but diverges into Alfvénicity-dependent behavior above saturation

Fifteen colored lines trace helium abundance versus solar wind speed, colored from dark blue (lowest wave activity) through yellow to dark red (highest wave activity).

Higher wave activity shifts helium saturation to lower speeds

The speed at which helium saturates depends systematically on wave activity.

Demonstrates that the helium saturation point is not fixed but shifts systematically with Alfvénicity or cross helicity, revealing an unexpected enhancement in fast wind helium abundance when the Alfvénicity is low in fast wind. Alterman & D'Amicis (2025b, ApJL) resolves the contradiction implied by this observation.

Fifteen helium-speed curves, one per compressibility bin, rescaled so that each saturation point sits at coordinates (1, 1).

Incompressible curves collapse onto a universal shape

When each curve is normalized to its own saturation point, the incompressible curves collapse onto a single universal shape.

Demonstrates that limiting compressibility < 15% excludes confounding impacts and enables reliable source region identification

Source

Cross Helicity and the Helium Abundance as an In Situ Metric of Solar Wind Acceleration

The Astrophysical Journal Letters (2025)

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