Heavy Ion Composition

Universal behavior below the fast-slow transition

When normalized to their saturation points, all heavy ion species follow an identical trajectory in slow solar wind, revealing universal physics that doesn't discriminate by mass, charge, or ionization potential. Above saturation, this universality breaks down as element-specific fractionation takes over in fast wind.

Ten element abundance-versus-speed curves, rescaled so that each element's transition point sits at the coordinate (1, 1).

Slow wind composition is universal; fast wind fractionates by mass

Ten element abundance-versus-speed curves are rescaled so that each element's transition point sits at the coordinate (1, 1). The horizontal axis shows speed divided by saturation speed (0.6 to 2.0); the vertical axis shows abundance divided by saturation abundance (0 to 2.2). Below the (1, 1) point, all ten curves collapse onto one rising line, overlapping tightly. Above it, the curves fan apart: nitrogen (orange X markers) and carbon (blue squares) climb highest to about 2.0, oxygen (green pluses) reaches roughly 1.8, neon through sulfur form an intermediate cluster near 1.3 to 1.5, and iron (pink stars) and helium barely rise above 1.0 to 1.15.

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Related Figures

A scatter plot shows each element's transition speed plotted against its first ionization potential, the energy needed to strip its outermost electron.

Transition speed does not depend on ionization energy

The fast-slow transition speed is essentially the same for all heavy elements regardless of how easily they are ionized.

Proves saturation speed is independent of first ionization potential—all heavy elements cluster at 327 km/s

A stepped blue histogram on a logarithmic vertical scale shows the probability density of solar wind speed observations, spanning roughly 200 to 800 km/s.

Heavy and helium transition speeds bracket the most common wind

The heavy-element transition speed sits just below the most commonly observed solar wind speed, while helium's transition speed sits above it.

Context showing the saturation speeds straddle the solar wind's peak distribution, making the discrepancy operationally important

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.

Demonstrates that slow-wind heavy element abundances dip at solar minimum, confirming the solar cycle modulation seen in helium extends across the periodic table

A scatter plot shows how much each element's abundance at the fastest observed speed (592 km/s) exceeds its transition-point abundance, plotted against element mass in atomic mass units.

Fast wind reveals an unexplained mass-dependent fractionation

In fast solar wind, lighter heavy elements are enhanced more above their transition values than heavier ones, forming a clear mass-dependent trend.

Fast-wind enhancement shows mass dependence above saturation, motivating the charge-state analysis

Ten colored curves track the photospheric-normalized abundance of different chemical elements against solar wind speed, spanning roughly 300 to 650 km/s.

Heavy ions transition to fast wind 75 km/s slower than helium

Every element follows the same two-regime pattern: a steep rise at lower speeds and a shallower trend at higher speeds.

Shows the raw data before normalization, revealing the 75 km/s gap between heavy-ion and helium saturation speeds

A column-normalized density map shows helium abundance (vertical axis, 0-10%) versus solar wind speed (horizontal axis, 200-800 km/s).

Helium abundance saturates at 4.19% above 433 km/s

Helium abundance undergoes a sharp transition at 433 km/s.

Links to Helium Abundance page; helium shows similar saturation behavior but at different speed threshold

A scatter plot shows each element's abundance at the transition speed, plotted against its first ionization potential.

Transition abundances confirm expected FIP fractionation pattern

The roughly two-fold enhancement of low-FIP elements over high-FIP elements at the fast-slow transition point matches the well-established FIP effect pattern.

Saturation abundances show expected FIP pattern, revealing where chromospheric fractionation sets composition

A scatter plot shows fast-wind abundance ratios for ten elements, plotted against each element's typical electrical charge in the solar wind.

Charge state organizes fast wind fractionation into a tight trend

The fast-wind fractionation correlates far more tightly with charge state than with mass, FIP, or mass-to-charge ratio, as measured by a robust coefficient of determination (R-squared of 0.95 for charge state versus below 0.55 for all other quantities tested).

Explains the divergence above saturation—charge state (R²=0.95), not mass, controls fast-wind fractionation

Source

The transition from slow to fast wind as observed in composition observations

Astronomy and Astrophysics (2025)

View Paper