Suprathermal Ions

Remnants of past energetic events

Suprathermal ions are particles with energies above the thermal solar wind but below fully accelerated cosmic rays. These remnant populations exhibit a universal spectral index of 2.5 that remains invariant across nine different elements and two full solar cycles, revealing a fundamental constraint on particle acceleration in the heliosphere.

Two decades of measurements converge on a single spectral slope of 2.5

A stacked histogram collects every spectral-slope measurement from all nine elements across every year from 1998 to 2019. The horizontal axis runs from 2.0 to 3.2 in spectral index; the vertical axis counts summed measurements per bin, reaching about 28 at the peak. Each element occupies its own color layer stacked from bottom to top: carbon (blue), nitrogen (orange), oxygen (green), neon (red), magnesium (brown), silicon (pink), sulfur (gray), calcium (yellow-green), and iron (cyan). The resulting shape is a clear bell curve peaking near 2.4 to 2.6. A white open circle near the base of the histogram marks the overall mean at 2.5, with horizontal bars extending one standard deviation (0.3) in each direction.

spectral indexhistogramdistributionsuprathermal ionsuniversal valueseed populationpower lawremnant particlesgamma 2.5universal behaviorheavy ionssuprathermal particles

Related Figures

A color bar at the top maps years from 1997 (dark blue-purple) through 2021 (dark red), with a dashed black curve showing 13-month smoothed sunspot number peaking around 2000 and 2014.

Quiet-time thresholds remain consistent across two solar cycles

Despite spanning two complete solar cycles with dramatically different activity levels, the quiet-time thresholds cluster within a narrow range.

Validates that the quiet-time selection method produces consistent thresholds across two full solar cycles, ensuring that downstream suprathermal abundance measurements reflect real physical changes rather than methodological drift

Two stacked panels spanning 1997 to 2021 showing hourly suprathermal ion intensity colored by quiet or non-quiet classification (top) and annual quiet-hour counts alongside the sunspot number cycle (bottom).

Quiet hours peak when sunspot activity drops to its lowest

The number of quiet hours strongly anticorrelates with sunspot number at a Pearson correlation of -0.95.

Confirms that quiet hours anticorrelate with sunspot number at -0.95, validating the quiet-time identification as physically meaningful and establishing the 22-year baseline for studying suprathermal composition evolution

A log-log plot titled '2004' shows how particle intensity drops with increasing energy for nine chemical elements measured during quiet periods.

Nine heavy-ion species fall along identical power-law spectra

Despite spanning a factor of nearly five in atomic mass — from carbon (mass 12) to iron (mass 56) — every element produces essentially the same spectral shape during quiet times.

Shows all 9 elements following the same spectral slope

Power-law exponents from quiet-time abundance variability fits are plotted against SEP mass-per-charge ratio (M/Q) on the horizontal axis, with element labels along the top (C, N, O, Ne, Mg, Si, S, Ca, Fe).

Suprathermal variability increases with mass-per-charge, matching GSEP trends

The variability of quiet-time suprathermal abundances increases systematically with mass-per-charge ratio across all solar cycle phases.

Connects suprathermals to their role as seed population for solar energetic particle events

A log-log plot shows eight heavy element abundances (C through Fe, vertically offset by labeled scale factors) plotted against the iron-to-carbon ratio.

All heavy ion abundances follow power-law trends with Fe/C ratio

Fe/C serves as a sensitive single-number indicator of the overall heavy-ion enrichment state in quiet-time suprathermals.

Reduces the complex behavior of eight suprathermal elements to power-law relationships with a single enrichment indicator (Fe/C), revealing cyclic composition loops tied to solar activity phase

A dual-axis time-series plot spanning 1998 to about 2020 showing annual oxygen spectral indices with error bars (left axis) alongside the sunspot number cycle (right axis).

Spectral slope holds steady as sunspot activity rises and falls

Over more than two decades, the oxygen spectral slope fluctuates around a value of roughly 2.5 without tracking the dramatic swings in sunspot activity.

Demonstrates independence from solar activity

Panel (a) shows eight colored time series from 1998 to 2019, each tracing a different element's abundance relative to oxygen on a logarithmic scale.

Heavy ion abundances shift between CIR and SEP values with the solar cycle

Heavy element abundances in quiet-time suprathermal ions change systematically with the solar cycle.

Proves suprathermals are remnants from CIRs during solar minimum and CME-driven shocks during solar maximum

Source

Quiet-time Spectra of Suprathermal Heavy Ions near 1 au in Solar Cycles 23 and 24

The Astrophysical Journal Letters (2024)

View Paper