A Possible Mechanism for Enriching Heavy Ions in
3He-rich Solar Energetic Particle Events
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(a) dE/dx energy loss rates for O and Fe in hydrogen gas at ambient (solid lines) and 300,000 K (dashed lines). (b) E-3 differential energy spectra before (green line) and after passing through 200 micro-g/cm2 of H; arrows show lowest incident energy that penetrates the gas for Fe (blue) and O (red). (c) blue points: ratios of elements normalized to O after passing through 200 micro-g/cm2 of H gas for original spectral indices -3 and -4; red points are observed enhancements in 3He-rich SEP events (Mason et al. 2004)
Extreme enrichments (>10,000x) of 3He in small SEP events have long been taken as evidence of a process that favorably heats 3He due to its unique charge-to-mass (Q/M) ratio. Heavy ions up through Fe are routinely also enhanced proportional to their mass, but at a relatively modest level (~8 for Fe) and, surprisingly, uncorrelated with the 3He enhancement. The discovery by advanced instruments on Wind and ACE that the heavy ion enhancement extends into the ultra-heavy range (UH, 78-220 amu), and reaches factors of 100-200 above 100 amu, does not fit easily into models depending on Q/M since the wide UH mass range covers a very broad range of values for Q/M. So there is still no satisfactory model for heavy ion enrichments in 3He-rich SEP events.
A different process for creating a seed population enriched in heavy ions could arise from the fact that heavier ions have a greater affinity for electron pickup as they lose energy by ionization. Panel (a) shows dE/dx rates for O and Fe in hydrogen gas; below the Bragg peak at a few hundred keV/nucleon the Fe picks up more electrons and so its dE/dx rate is closer to O than above the Bragg peak. This allows Fe very near the end of its range to pass more easily through the material. An example of this property is shown in panel (b), where identical E-3 differential energy spectra for O and Fe are passed through 200 micro-g/cm2 of H gas. The blue and red arrows mark the energies below which the particles are stopped: 85 keV/nuc for Fe, and 230 keV/nuc for O. The spectra after passage through the material are shown by the solid blue and red curves, where the Fe enhancement over O is clearly visible.
Integrating the 2 spectra in panel (b) after passing through the material, the number of Fe ions is enhanced by a factor of ~7 compared to O. Because of the steepness of the spectra, particles above a few hundred keV/nucleon do not contribute significantly. Panel (c) shows enhancements of 19 species from He through U passing through 200 micro-g/cm2 of H gas calculated in the same manner. The 2 blue lines plot results for original E-3 and E-4 spectra, showing that steeper original spectra result in larger enhancements, but that for elements below Fe the effect is not large. The outlier point in panel (c) is for Ni, which arises from its dE/dx rate falling below Fe between ~20-200 keV/nucleon. At solar coronal temperatures the H gas is ionized, resulting in a higher dE/dx as shown by the dashed lines in panel (a). However, the greater affinity for electrons persists so that the Fe and O dE/dx rates are closer together at low energies than above the Bragg peak, thereby preserving the enhancement mechanism. As an example, for 200,000 K, the calculated enhancement through Fe is roughly similar to panel (c), but with a factor of 10 less material thickness required (20 micro-g/cm2) due to the elevated low energy dE/dx rates at high temperatures. We presume that a further acceleration step energizes the enhanced heavy ion seed population to the energies observed in 3He-rich SEP events. For additional details and references see Mason and Klecker (ApJ, 862:7, 2018; http://doi.org/10.3847/1538-4357/aac94c).
This item was contributed by Glenn M. Mason (APL) and Berndt Klecker (Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany). Address questions and comments to
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Last modified 14 Aug 2018.